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This chapter should be cited as follows:
Hyde, K, Andelin, C, et al, Glob. libr. women's med.,
(ISSN: 1756-2228) 2014; DOI 10.3843/GLOWM.10320
This chapter was last updated:
July 2014

Immunology of Isolated and Recurrent Spontaneous Pregnancy Loss

Authors

INTRODUCTION

Spontaneous pregnancy loss is the most common complication of pregnancy: approximately 70% of human conceptions fail to achieve viability, and an estimated 50% are lost before the first missed menstrual period.1 Recurrent pregnancy loss (RPL) or recurrent spontaneous abortion is much less common, occurring in about 1 in 100 pregnant women.2

Historically, recurrent abortion was defined as three or more clinically recognized pregnancy losses before 20 weeks of gestation. Using this definition, RPL occurs in approximately 1 in 300 pregnancies.3 However, many recommend that clinical investigation and intervention be initiated after two consecutive spontaneous abortions, especially if any of the following are coexistent: fetal heart activity was identified before any of the pregnancy losses, fetal karyotyping of pregnancy tissues revealed normal chromosome content, the woman is older than 35 years of age, or the couple also shows subfertility. 

The etiologies of RPL and their respective prevalences are controversial. The only undisputed causes of RPL are parental chromosomal abnormalities and thrombotic complications of the antiphospholipid syndrome (APS). Most investigators consider the following conditions to be associated with RPL: uterine anatomic abnormalities (congenital or acquired); endocrine abnormalities; infections; and immunologic factors, including those associated with APS.4, 5, 6 Other factors, such as environmental exposures, have been implicated but probably account for a small fraction of cases. The exact proportion of patients affected by each of these factors differs depending on the population being studied. Etiologic proportions may also be affected by referral bias and therefore may differ based on the interests of the investigator conducting a particular study. 

After a thorough evaluation, the potential cause for RPL remains unexplained in approximately 45–50% of patients.4, 5, 6 Investigators have frequently suggested that a substantial proportion of these otherwise unexplained RPL cases may have an immunologic component. Many cases of either isolated or recurrent spontaneous pregnancy loss may have more than one potential etiology, and immunologic mechanisms have been frequently invoked as contributory. This is certainly proposed when infection is associated with pregnancy loss.

Reproductive tract infections with bacterial, viral, parasitic, zoonotic, and fungal organisms have been theoretically linked to both isolated and recurrent pregnancy loss. However, the etiologic mechanisms linking specific organisms to either isolated or recurrent pregnancy loss remain unclear and must certainly differ among infectious organisms.7, 8, 9 For instance, some viral organisms, such as herpes simplex virus10 and human cytomegalovirus,11 have been shown to infect the placenta/fetus directly, and pregnancy loss related to these infections may be the result of direct damage to the developing fetus by the infectious organism. Alternatively, pregnancy disruption might be an untoward effect of the immune response to the infectious agent. This mechanism may be responsible for some adverse infection-associated events later in gestation, such as intrauterine growth restriction,12 premature rupture of membranes, and preterm parturition.13 Conversely, there is evidence that some of the same mechanisms that potentially protect the fetus from autoimmune rejection may also protect virally infected placental cells from recognition and clearance. Some of the fetoprotective immunoregulatory events that occur at the maternal–fetal interface could thereby make the implanting fetus particularly vulnerable to certain infectious agents, including herpes, cytomegalovirus, and HIV.14

IMMUNOLOGIC PHENOMENA

The number of investigations involving reproductive immunology and its relationship to both isolated and recurrent pregnancy loss has expanded exponentially during the past few decades. Many of these studies, however, have been poorly designed or poorly controlled or have involved insufficient numbers of patients to warrant their conclusions. The clinical study of the immunology of early pregnancy is also hampered by a variety of intrinsic factors. First, a diversity of distinct immune alterations may result in isolated or recurrent pregnancy loss.15 Each of these may act alone or in combination with additional immune or non-immune factors to result in pregnancy loss. Second, many RPL patients present after their current pregnancy has expired but before its expulsion. Diagnosis of an immunologic cause for that index pregnancy loss is therefore difficult because the expected physiologic immune reaction to the presence of nonviable tissue may mask any alternative underlying immune causes for the demise itself. Prior studies are also compromised because of their failure to correct for embryo and fetal chromosomal abnormalities, which are independent causes of pregnancy loss. Perhaps the most important intrinsic factor is the reassuring fact that for most patients with RPL, the possibility that their next pregnancy will result in a term delivery is quite high. Therefore, very large studies are needed to detect therapeutic effects, including those related to immunomodulatory interventions.

BASIC IMMUNOLOGIC CONCEPTS

A few basic immunologic concepts should be re-examined before a more directed discussion of the involvement of the immune system in pregnancy maintenance.16 Those concepts include the following immunologic subcategorizations: innate versus acquired immune responses; cellular versus humoral immune responses; antigen presentation by major histocompatibility complex (MHC) class I versus MHC class II molecules; and peripheral versus mucosal immune systems.  Each of these subdivisions is related to the others, and each is useful in understanding particular aspects of investigations on the immune interactions at the maternal–fetal interface. 

Innate Versus Acquired Immune Responses 

Immune responses are classically divided into innate and acquired. Innate responses represent the body's first line of defense against pathogenic invasion. Innate responses are rapid and are not antigen-specific. Cell types and mechanisms typically considered vital to innate immunity include complement activation, phagocytosis by macrophages, and lysis by natural killer (NK) cells and possibly by T cells expressing the γδ T-cell receptor (TCRγδ) (see below). Acquired immune responses, in contrast, are antigen-specific and are largely mediated by classical T and B cells. Acquired responses can be classified as either primary (a response associated with initial antigen contact) or secondary (a rapid and powerful amnestic response associated with repeated contact to the same antigen). 

Cellular Versus Humoral Immune Responses 

Further subdivision of acquired immunity into either cellular or humoral responses is useful, even though it grossly oversimplifies the complexity of immune interactions. By definition, the ability to mount a cellular immune response to a particular antigen can be transferred to a naïve (nonimmunized) individual via the lymphocytes (but not plasma or serum) from a separate immunized subject. Conversely, humoral immune responsiveness to a particular antigen can be transferred to a naïve subject using only the plasma or serum from an immunized individual. No cells need be transferred, and the response is known to be dependent on the presence of antibodies in the immunizing sera. Put simply, cellular responses require cell-to-cell interactions, whereas humoral responses are antibody-mediated. Like innate and acquired immunity, these cellular and humoral responses are intricately intertwined. 

Antigen Presentation by MHC Class I Versus MHC Class II Molecules 

Cellular and humoral immune responses are largely dependent on the presence of two sets of genes in the MHC that play major roles in determining antigen specificity. These genetic loci encode the MHC class I and MHC class II products, as well as many of the supporting effector molecules involved in antigen presentation. Both MHC class I and MHC class II molecules help to alert the immune system to alterations that would require an immune response. MHC class I molecules (HLA-A, -B, and -C) are present on the surface of nearly every cell in the human body and are important in defense against intracellular pathogens, such as viruses, and against oncogenic transformation. MHC class I molecules act as important ligands for the T-cell receptor on CD8+ cytotoxic/suppressor T cells and for a variety of receptors on NK cells.17 MHC class I-mediated intercellular interactions generally activate the cellular portion of the immune response and result in killing of the antigen-presenting (i.e. virally infected or cancerous) cell expressing the class I molecule. 

In contrast, MHC class II molecules (HLA-DR, HLA-DP, and HLA-DQ) are present on the surface of a limited number of 'professional' antigen-presenting cells. These include dendritic cells, macrophages, monocytes, B cells, and tissue-specific antigen presenters (e.g. Langerhans cells in the skin). MHC class II molecules are important in defense against extracellular pathogens, such as bacterial invaders. The major ligand for MHC class II is the T cell receptor on CD4+ T-helper cells. MHC class II-mediated interactions generally result in the modulation of humoral immune responses. 

Peripheral Versus Mucosal Immune Systems 

Perhaps the immunologic categorization most relevant to the discussion of pregnancy maintenance is that which divides the immune system into two distinct compartments: peripheral and mucosal. In general, basic immunologic principles have been well-tested and -described for the immune effector cells populating only one of these compartments—the peripheral immune system. Until recently, very little was known about the mucosal immune compartment, and its description continues to lag far behind that of the periphery. 

The peripheral immune system consists of the spleen and peripheral blood, and it is generally responsible for protection against blood-borne pathogens. In contrast, the mucosal immune system is typically the first to encounter those pathogens that enter via the extensive surface areas of the lacrimal ducts, respiratory and gastrointestinal tracts, mammary ducts, and genitourinary tracts. Some effector cells within mucosal sites such as the gastrointestinal tract and reproductive tract appear to have been educated outside of the thymus—a process referred to as extrathymic education.18 Thus, the mucosal immune system appears to be responsible for providing at least initial immunologic protection against the majority of exogenous pathogens and may play a role in its own effector cell education.

IMMUNOLOGIC INTERACTIONS AT THE MATERNAL–FETAL INTERFACE

Immune interactions at the maternal–fetal interface differ from those of the peripheral immune system and from those ar other mucosal immune sites.19 The first of the mucosal immune sites to be extensively studied was the gastrointestinal tract, and investigations revealed its immunology to be quite distinct from that of the periphery.20 Studies extending to the reproductive tract documented immunologic distinctions from both the periphery and the gastrointestinal tract.19 Further application to immunologic interactions at the site of embryo implantation have revealed additional unique immunologic characteristics. For instance, descriptions of the immune cells that populate both the female reproductive tract and the peri-implantation decidua implicate a particularly important role for 'innate-like' immune interactions during implantation and early pregnancy.21, 22

IMMUNE CELLS POPULATING THE FEMALE REPRODUCTIVE TRACT

Investigations on the immune cells of the female reproductive tract, particularly those populating the decidua at the time of implantation and during early pregnancy, have identified at least four potentially important lines of investigation and four unusual immune cell types: decidual granular lymphocytes, NKT cells, TCRγδ+ cells, and dicidual macrophages. 

Decidual Granular Lymphocytes 

Whereas the human endometrium is normally populated by T cells, macrophages, NK-like cells, and a very limited number of B cells,21, 22, 23 the relative proportions of these resident cells show menstrual cyclicity. The most dramatic changes are noted during the late luteal phase and in early pregnancy, when the proportion of one unusual type of immune cell rises to nearly 70–90% of the total endometrial lymphocyte populations.21, 22, 23 These unusual cells have been variably called decidual granular lymphocytes (DGLs), large granular lymphocytes (LGLs), and decidual NK cells.24 The origin, functional capabilities, and physiologic purpose of these cells remain enigmatic, but their abundance at sites of implantation compels further study.25, 26 Although the effector functions of peripheral NK cells are known to be regulated by interactions between target-cell expressed classical MHC class I molecules (HLA-A and B) and activating and inhibitory receptors on the peripheral NK cell surface, the NK-like cells of the decidua exhibit receptors encoded by the killer immunoglobulin-like receptor (KIR) gene family that have been shown to react preferentially with trophoblast HLA-C molecules and possibly with nonclassical MHC class I products.27, 28 Decidual NK cells come into direct contact with the allogeneic fetally derived placental cells responsible for uterine artery remodelling during the first trimester of pregnancy. Here, they may serve to support trophoblast invasion, vascular remodelling, and vascular growth in the decidua through the production of a variety of cytokines and other angiogenic factors rather than serving the cytotoxic functions more typical of their peripheral counterparts.27, 28, 29, 30  Although this particular cell type differs from similar cells isolated from the periphery, most believe it to be an NK cell variant. If so, the implantation site represents the largest accumulation of NK cells in any state of human health or disease. Investigations have shown that, rather than being derivatives of peripheral NK cells, decidual NK cells likely derive from endometrial NK cells that undergo proliferation during pregnancy.31 Given this relationship, it is unlikely that measures of peripheral NK cells would be good predictors of decidual NK cell competency or their effects on pregnancy outcomes.

NKT Cells 

Peripheral immune cells have been described that have characteristics of both NK cells and T cells.32 These NKT cells and their ligands (e.g. CD1) have been shown to be present in the decidua of animals33, 34, 35 and have been implicated in some forms of pregnancy loss.33 In addition, the presence of NKT cells at the implantation sites of murine pregnancies has been shown to be mediated by interactions with fetally expressed MHC class I or class I-like products.34 Initial investigations in humans have also demonstrated the presence of NKT cells within the endometrium and decidua.36 The NK-like phenotype of these cells suggests innate immune function. 

TCR γ δ+ Cells and Decidual Macrophages

In the peripheral immune compartment, the vast majority of T cells express a T-cell receptor composed of an αβ heterodimer (TCRαβ+). In addition to TCRαβ+ T cells, the human reproductive tract is also populated by a subset of T cells with a distinctive T-cell receptor, the γδ heterodimer (TCRγδ+ cells). The number of these cells in the decidua increases in early pregnancy.37, 38, 39 TCRγδ+ T cells appear to fulfill functions quite distinct from their TCRαβ+ counterparts; these functions may include direct, non-MHC-restricted recognition of antigens within tissues.40 TCRγδ+ T cells may fill a protective niche missed or poorly addressed by B cells and TCRαβ+ T cells. By secreting Il (interleukin)-10 and TGF (transforming growth factor)-β, TCRγδ+ T cells support trophoblast invasion and survival and may therefore play a very important role in the maintenance of early pregnancy.41 Similar to the vast array of functions that macrophages exert in the periphery, decidual macrophages are responsible for a variety of activities at the maternal–fetal interface, many of which are linked to pregnancy maintenance. Through their production of IL-10 and indoleamine 2,3-dioxygenase (IDO) activity, decidual macrophages have been shown to possess a predominately immunosuppressive phenotype, deemed M2 polarization. They aid in maternal immune tolerance to the semiallogenic fetus as well as transformation of uterine vasculature.42

 

In conclusion, very characteristic immune effector cells populate the human decidua. To date, insufficient patient numbers have largely hampered investigations into whether alterations in these cellular populations (including T cells, decidual NK cells, and NKT cells) determine pregnancy outcome. Efforts made to target NK cells to treat infertility and pregnancy loss have been ineffective.43 Still, the general consensus of these studies is that these populations are altered in RPL patients.44, 45, 46, 47, 48, 49

IMMUNE TOLERANCE

Immune Cell Education and Homing to the Female Reproductive Tract

One very important concept in immunology that has particular application to pregnancy is that of immune tolerance. The concept is very well described for the immune cells of the peripheral immune system.16 Those immune cells arising in the bone marrow and destined to become circulating T lymphocytes typically pass through the fetal thymus, where they undergo a process termed thymic education. During thymic education, T cells 'mature' from CD4/CD8 double-positive cells to single-positive (CD4+ or CD8+) cells. This maturation occurs by a two-step process that spares only those T cells capable of recognizing one's own MHC class I or class II molecules but incapable of recognizing other self peptides. This education promotes T-cell tolerance to self by allowing selection and survival only of T cells that will not react against the organism's own peptides. 

If one considers the implanting blastocyst as the most common example of a successful allograft, one must question how the concepts of tolerance apply at the maternal–fetal interface against tissue that is most certainly not self. A few concepts require attention. First, are there characteristics other than those phenotypic differences already described that promote tolerance to fetal antigens among the resident immune cells in the maternal decidua? Both animal and human data suggest that the immune cells at the maternal–fetal interface are selected and maintained in ways that differ both from peripheral immune cells and from cells populating other mucosal sites, such as the intestine.19, 50 Second, if the cells populating the maternal–fetal interface have distinctive phenotypes and antifetal reactivity, how are these particular cells recruited to the site of implantation? Cellular recruitment (homing) to the intestine has been shown to rely on the presence of receptors (integrins) on the surface of particular immune cell subpopulations that specifically interact with ligands on the surface of the endothelial cells of blood vessels within the intestine (e.g. selectins, VCAM, MECAs).51, 52 Similar homing characteristics also appear to define immune cell types and vascular structures within the reproductive tract.51, 52, 53, 54 

Control of Decidual Immune Cell Immunoreactivity 

Because safeguarding the fetus from maternal immune rejection is obviously crucial for successful reproduction, it is not surprising that evolution has selected a wide variety of overlapping and partially redundant immune adaptations at the maternal–fetal interface. In addition to distinctions in phenotype, homing, and antigenic education, the immune cells that populate the maternal decidua also appear to be affected by local immunomodulation. We know from both human and animal studies that immune responses to fetal antigens, including proinflammatory responses and those involved in transformation of the uterine vasculature, can be detected.55, 56, 57, 58 Thus, the regulation of these responses at the maternal–fetal interface may be critical for pregnancy maintenance. The concept that successful pregnancy requires some form of suppression of the maternal immune response is supported by reports that failure to downregulate maternal responses to recall antigens, such as tetanus toxoid and influenza, is associated with poor pregnancy outcome among RPL patients.57 Some of the mechanisms hypothesized to be important in this localized immune regulation include alterations in localized cytokine profiles, the immune effects of pregnancy-associated hormones, and the in situ tolerance-promoting characteristics of tryptophan metabolism.

Cytokine Dysregulation

An essential role for isolated cytokines in reproductive success or failure is not well supported by animal studies. To date, although most cytokines have been gene-deleted in separate animal models, few of these factors appears to be essential to pregnancy maintenance. Two cytokines of the IL-6 family, leukemia inhibitory factor (LIF) and IL11, are required for blastocyst implantation,59 but not for subsequent embryogenesis.60, 61 The production of these two cytokines is distinct in both space and time; LIF is produced by the endometrial epithelium during the blastocyst attachment/adhesion phase of implantation, whereas IL11 levels reflect decidualized stromal cell function postimplantation.62 Although this does not argue that other cytokines and soluble immunoregulatory factors are inconsequential, there certainly seems to be significant redundancy among these substances at the site of implantation.

It may not be particularly relevant to examine these factors in isolation. Rather, groups of cytokines may best reflect immune responses.63, 64 The division of antigen-stimulated immune responses involving CD4+ T cells into T helper 1 (Th1) responses and T helper 2 (Th2) responses appears to be useful, albeit somewhat oversimplified. This subclassification is defined both by the characteristics of the CD4+ cells present and by the cytokines these cells secrete. In general, the characteristics of the cytokine response depends on the environment in which naïve CD4+ (Th0) cells mature and differentiate: Th0 cells exposed to interferon (IFN)-γ become Th1-type cells, and those exposed to interleukin (IL)-4 become Th2-type cells.63, 64 Th1 responses are associated with inflammation and are typically characterized by the release of IFN-γ, IL-12, IL-2, and tumor necrosis factor (TNF)-β. Th2 cell responses are associated with antibody production and the cytokines IL-10, IL-4 and IL-5. 65, 66, 67 Although TNF-α can be secreted by both Th1 and Th2 cells, it is most often associated with a Th1 response.67, 68 Th1 and Th2 responses are intimately interdigitated, with each type of response promoting itself while limiting the alternative response.69, 70, 71, 72

The human endometrium and decidua are replete with immune and inflammatory cells capable of cytokine secretion.73, 74, 75, 76, 77 Extension of the Th1/Th2 paradigm to pregnancy has generated provocative hypotheses. Most women with normal pregnancies appear to have a predominant Th2 immune response to undefined trophoblast antigens.78 In contrast, some patients with recurrent pregnancy loss exhibit a dysregulation of their T-helper response to antigens at the site of implantation, with shifts toward Th1 inflammatory responses.78, 79, 80, 81 Further, Th1-type cytokines have been shown to be harmful to an implanting embryo.82, 83, 84 Depending on the individual series, 15–20% of nonpregnant women with a history of otherwise unexplained recurrent spontaneous abortion have been found to have evidence of abnormal in vitro Th1 cellular immune responses to trophoblast antigens. Fewer than 3% of women with normal reproductive histories have these responses.78, 79, 83, 85 

Yet there is much more to the story. For instance, two additional T helper cell subsets have garnered significant attention in the field of pregnancy maintenance: Th17 and T regulatory (Treg) cells. Treg cells have an important role in the development of pregnancy-specific immune responses in mouse models.86 In humans, Tregs purified from pregnant women suppress autologous peripheral blood mononuclear cell (PMBC) secretion of IFN-γ in response to paternal or unrelated PMBCs in mixed culture. These cells also suppress IL-4 secretion against paternal but not unrelated alloantigens during pregnancy.87 Furthermore, studies have shown that Treg cells make up about 14% of the CD4+ T cells in the decidua and increases in the frequencies of Treg cells in the peripheral blood of pregnant women are seen during the first and second trimesters when compared to non-pregnant women.88 An absence of Treg expansion has been found in decidual tissues from women suffering miscarriage, as well preeclampsia and preterm labor, suggesting that Treg cell function is important in the maintenance of pregnancy and in the development of a healthy placenta.89, 90 Some propose that peripheral Treg levels may even be used to predict the risk of pregnancy loss in newly pregnant women with a history of miscarriage.91 This has not been studied fully and should not be included in standard testing at this time.

Many of the autoimmune effects attributed to Th1 cells in the past are now being attributed to Th17 cells.92 The recent identification of these cells has implicated yet another set of mechanisms that may contribute to recurrent pregnancy loss. In normal pregnancy, Th17 cell proportions in the peripheral blood do not change.93 In contrast, Th17 cell proportions increase in the peripheral blood and decidua of women with recurrent pregnancy loss. Furthermore, the levels of interleukins related to Th17 cells, such as pro-inflammatory IL17 and IL23, were also found to be significantly increased in both the peripheral blood and decidua of women with histories of RPL when compared to normal, early pregnant women.94 Later reports examining the relationship between Th17 and Treg cells found an inverse correlation between the populations of these two cells types in the peripheral blood and decidua of women with pregnancy loss and demonstrated an elevated ratio of Th17/Treg cells in these women compared to normal pregnant and non-pregnant women.95, 96 Serum levels of IL17 have also been positively correlated with increased levels of Th17 cells and an increased Th17/Treg ratio. This suggests that Treg cells may function to inhibit IL17 production, and this suppressive effect may be impaired in women with recurrent pregnancy loss.96, 97

The literature on reproductive immunology is often frustratingly inconsistent. Part of the frustration that arises when one attempts to synthesize the data surrounding cytokine dysregulation and poor reproductive outcome lies in the variations among study designs. Methods for the documentation of cytokine dysregulation among RPL patients have included confirmation of the abnormality within the endometrium98, 99, 100 and the decidua.101 Other investigators use peripheral lymphocytes from women with a history of RPL and stimulate them in vitro with trophoblast antigens.78, 102 Can these two types of studies be compared? Whether peripheral cytokine levels reflect T-helper cell dysregulation at the maternal–fetal interface and whether this localized dysregulation affects peripheral as well as local immune responses during pregnancy remain controversial.103, 104, 105 

 

Reproductive Hormones

Notable gender differences in immune responsiveness106, 107, 108 likely reflect the fact that reproductive hormones have dramatic effects on peripheral cell-mediated immunity. Human chorionic gonadotropin (hCG), estrogen and progesterone have been shown to be potent immunomodulators, although the latter has received the most attention in terms of implications for the maintenance of the semiallogeneic implanting conceptus.109 It is known that elevations in both estrogen and progesterone are seen early in pregnancy and these high levels are maintained throughout the course of the pregnancy.110In vitro evidence supports progesterone's immunosuppressive attributes by demonstrating an inhibition of mitogen-induced proliferation of and cytokine secretion by CD8+ T cells.111 By playing a role in the modulation of inflammatory cytokine expression and Treg cell homing, hCG has also been implicated in immune tolerance towards the fetus in early pregnancy.

The significant effects of reproductive hormones at the maternal-fetal interface may be due to their increased concentration at this location when compared to concentrations found in the periphery.112 Progesterone may promote the development of a cytokine microenvironment favoring pregnancy maintenance. Progesterone-mediated changes in T cell gene expression have been associated with Th2-type T helper cell responses and with increased expression of leukemia inhibitory factor.101, 113 High levels of progesterone stimulate the production of a protein called progesterone-induced binding factor (PIBF) by lymphocytes; the levels of PIBF continue to rise throughout gestation and drop after delivery.114 These elevated concentrations of PIBF have been shown to not only promote the differentiation of CD4+ T cells into Th2 cells, but also to increase Th1 type cytokine production. Furthermore, decreased levels of PIBF have been linked to preterm labor, miscarriage, and preeclampsia.115, 116 Because a shift in the intrauterine immune environment from Th2 to Th1 has been linked with early pregnancy loss,101, 117 the elevated concentrations of progesterone and PIBF characteristic of early pregnancy may promote an immune environment favoring pregnancy maintenance. Of course, progesterone is not present in isolation at the maternal-fetal interface, so in vitro studies addressing isolated effects on immunity present an incomplete picture of what actually occurs in vivo.

Although not specifically addressing pregnancy, several reports have focused attention on the general immunomodulatory effects of estrogens. Estrogens have been shown to improve immune responses in men after significant trauma/hemorrhage,118 to suppress cell-mediated immunity after thermal injury,119 and to protect against chronic renal allograft rejection,120 all in animal models. In vitro, estrogens downregulate delayed-type hypersensitivity reactions and promote the development of Th2-type immune responses, particularly when present in high, and often supraphysiologic, concentrations.121, 122, 123 T reg cells numbers are known to increase during human pregnancy124 and their levels are lower in women with spontaneous pregnancy loss.125 This is thought to be the result of exposure to pregnancy hormones. Estrogen exposure potentiates suppressive function and proliferation of T reg cells in humans.126  

Of course, one obvious question poses itself when hypotheses on progesterone/estrogen immunomodulation and their role in pregnancy maintenance are examined. If elevated levels of progesterone and estrogen are sufficient to promote tolerance to the implanting fetus, are pregnant women also systemically immunosuppressed? The answer to this question is complex. Some forms of immunoreactivity are indeed systemically suppressed in pregnant women. Examples of these effects include the symptomatic remission of such Th1-mediated autoimmune disorders as rheumatoid arthritis and multiple sclerosis among pregnant women with disease resurgence postpartum.127, 128 In contrast, Th2-mediated autoimmune disorders such as systemic lupus erythematosus tend to worsen during pregnancy.109 Further, parasitic diseases and some viral diseases, including varicella, are particularly aggressive when first encountered during pregnancy.129, 130 Finally, although the levels of potentially immunosuppressive hormones are systemically elevated in pregnant women, their concentrations at the maternal–fetal interface appear to be significantly higher than in the maternal circulation.112 This may help to delimit many immune effects to the site of implantation. To this point, it is important to note that pregnant women still have protection against the vast majority of infectious diseases and are subject to the consequences of the vast majority of autoimmune disorders.

Tryptophan Metabolism

A novel immunoregulatory pathway involving the amino acid tryptophan and its catabolizing enzyme, indoleamine 2,3 dioxygenase (IDO), has been proposed as a mechanism for maternal tolerance to the fetal allograft. T cells require tryptophan for activation and proliferation.131, 132 Local alterations in tryptophan supply or in tryptophan metabolism at the maternal–fetal interface would therefore be predicted to modulate immune cell function at this site. Evidence implicating tryptophan metabolism in maternal–fetal immune interactions arose initially in animal models. The inhibition of IDO in mice has been shown to promote loss of allogeneic but not syngeneic fetuses, an effect mediated by lymphocytes.133 IDO activity has also been shown to aid in the suppressive role of Treg cells in murine studies.134 Further, hamsters fed diets high in tryptophan have increased rates of fetal wastage.135 IDO has been reported to be expressed in human uterine decidua136 and alterations in serum tryptophan levels with increasing gestational age during human pregnancy have also been noted.137

MHC Antigens in the Placenta 

Using available reagents and investigative techniques, early investigators failed to detect MHC-encoded transplantation antigens in the placenta. It was already known that these MHC class I and class II antigens promoted the rejection of transplanted allogeneic tissue in nonreproductive systems. Therefore, their proposed absence on the fetal semiallograft fostered the hypothesis that the maternal immune cells were tolerant of an implanting pregnancy because the invading tissues simply were not recognized as foreign. This hypothesis, and the data on which it was based, are partially correct. 

Indeed, MHC class II molecules are not expressed on the surface of placental trophoblast cells,138, 139 nor are the classical MHC class I transplantation antigens HLA-A and -B. However, a group of unique MHC class I products—HLA-C, HLA-E, and HLA-G—are expressed on a subpopulation of placental cells called extravillous cytotrophoblast cells.139, 140, 141, 142, 143, 144, 145 Extravillous cytotrophoblast cells are derived from the villous core of the placental cotyledon. During the course of early placental maturation, these cells take on invasive qualities. Invasion of extravillous cytotrophoblast cells may be regulated by a number of factors, including MHC expression patterns, integrin expression patterns/integrin switching,146 and in situ oxygen tension. Invasion is characterized initially by movement of the extravillous cytotrophoblast cells from the tips of the placental anchoring villae toward positions deep within the maternal decidua. When they are present in the decidual vasculature, they can replace cells within the walls of arterial decidual vessels.147, 148 At this site, these fetally derived extravillous cytotrophoblast cells intimately contact maternal immune effector cells, thereby exposing the fetus to potential MHC-restricted recognition as nonself.

The MHC class I molecules on the extravillous cytotrophoblast cell are represented by the classical MHC class I product HLA-C and the nonclassical HLA-E and HLA-G products. Several lines of investigation have been pursued to determine why all placental cells downregulate expression of the classical transplantation antigens (HLA-A and HLA-B), whereas invasive extravillous cytotrophoblasts express HLA-C, HLA-E, and HLA-G.21, 25, 132, 148 There may be functional similarities among HLA-C, HLA-E, and HLA-G that promote recognition by immune cells at the maternal–fetal interface and downregulation of antifetal responses. Because NK cells recognize and kill cells lacking MHC I products, interactions of these class I products with the abundant NK-like immune cells at the maternal–fetal interface may avert NK cell receptor-mediated killing of extravillous cytotrophoblast.2 Interactions of trophoblast HLA-C, HLA-E, and/or HLA-G with surface receptors on either NK cells or other maternal immune effector cells may modulate immune cell cytokine expression profiles.25, 26 Supporting the latter hypothesis, aberrant cytokine secretion has been documented when peripheral blood lymphocytes from RPL patients are stimulated by HLA-G-bearing cells in vitro.149 Further, both decidual and peripheral immune cells have been shown to alter their cytokine secretion patterns when exposed to HLA-G.150 The expression by fetus-derived cells of any or all of the trophoblast MHC class I products could also promote essential decidual and vascular invasion, spiral artery remodelling, and angiogenesis necessary for reproductive success.58, 151, 152 Altered trophoblast expression of HLA-G has been linked to disorders of placental invasion, such as pre-eclampsia.151, 153 Finally, the effects of MHC class I products on maternal reactivity toward the implanting pregnancy could be indirect. Secretion of soluble forms of HLA-C, HLA-E, and/or HLA-G into the maternal circulation may have pro-tolerance immune inductive effects at sites distant from the maternal–fetal interface.154

Although associations between HLA typing and recurrent pregnancy loss have been suggested (see below), few investigations have specifically addressed the role of trophoblast MHC class I products (HLA-C, HLA-E, and HLA-G) in pregnancy loss and these studies have provided conflicting results. Two small studies investigating a possible association between polymorphisms in these MHC genes and the occurrence of RPL were negative.155, 156  Newer studies have suggested that isolated polymorphisms of the HLA class Ib, particulary HLA-G, and KIR/classical HLA combinations, are indeed associated with reduced fertility and early pregnancy loss.157, 158, Furthermore, when investigating the interplay between inhibitory and activating MHC receptor functions, some have proposed that a balance shift towards the activating state may predispose to recurrent miscarriage.159, 160 Others have addressed the possibility that because trophoblast cells do not spontaneously express either classical HLA-A and HLA-B products nor MHC class II products, an aberrant expression of these molecules might result in an adverse maternal immune response to the implanting fetus. Data supporting this hypothesis remain limited.161, 162, 163

Humoral Immune Mechanisms 

The literature concerning the study of humoral/antibody-mediated immune responses and isolated or recurrent spontaneous pregnancy loss is controversial. In fact, much of this literature has now become obsolete. Responses to pregnancy-specific antigens certainly occur, and patients with RPL can display altered humoral responses to endometrial antigens.164 Still, investigators continue to debate whether recognition of fetal antigens is essential/protective or harmful. 

Immunoprotection: Blocking Antibody Deficiency

One hypothesis promoting maternal recognition of the implanting pregnancy as essential rests on the supposition that humoral immune-mediated pregnancy loss results from a deficiency in the maternal production of protective (blocking) factors. The production of these protective factors (presumably antibodies) was thought to be the result of immune recognition of fetus-derived (paternal) alloantigens. It was suggested that these protective factors, termed 'blocking antibodies', would prevent the maternal, cell-mediated, antifetal immune response that was believed to occur in all pregnancies. The production of 'blocking antibodies' was therefore deemed crucial; in the absence of these factors, abortion would occur.165

A deficiency in 'blocking antibodies' was demonstrated in vitro by maternal hyporesponsiveness in mixed lymphocyte culture with paternal stimulator cells.165 Hyporesponsiveness in these assays was associated with RPL;57, 166, 167 however, this association is now thought to represent the immune effects of repeated losses rather than the cause of pregnancy loss. Further evidence refuting a role for the blocking antibody hypothesis in immune-mediated pregnancy loss comes from reports of successful pregnancies among women who do not produce serum factors capable of mixed lymphocyte culture inhibition165 and among women who do not produce antipaternal cytotoxic antibodies.166 

Trophoblast-Lymphocyte Cross-Reactive Antigens

Out of the blocking antibody theory and its associated investigations came a number of related propositions that are no longer invoked. One of these involved the description of what was thought to be a novel HLA-linked alloantigen system called trophoblast-lymphocyte cross-reactive antigens, or TLX. The presence of TLX was first suggested by reports that polyclonal rabbit antisera could recognize both lymphocytes and trophoblast cells.168 A number of initial reports then linked TLX to maternal blocking antibody deficiency and recurrent pregnancy loss.169, 170 TLX, however, was subsequently found to be identical to CD46, a complement receptor that is thought to protect the placenta from complement-mediated attack.171 CD46 is not a novel alloantigen; it is not even an alloantigen. CD46 can be found on a wide variety of cells and has no significant role in distinguishing self from nonself. 

HLA Sharing

Linked to the hypothesis that maternal recognition of the fetal antigens was essential to pregnancy maintenance were studies suggesting that parental HLA (classical MHC class I) sharing predisposed a couple with RPL to blocking antibody deficiency.172, 173 According to this theory, if maternal and paternal HLA products were too similar, fetally expressed paternal alloantigens would not elicit a maternal immune response to produce 'blocking antibodies'. The two largest studies addressing this possibility used a small religious sect, the Hutterite community, as their study population. The first study was prospective, with population-based controls, and demonstrated that HLA heterogeneity was not essential for successful pregnancy among the Hutterites.174 A follow-up study, however, reported contrasting results. In fact, among the Hutterite population studied in this more recent, 10-year, prospective trial, complete sharing of the entire HLA region was associated with an increase of spontaneous pregnancy loss.175 The conclusions drawn by these authors should be considered carefully, because the incidence of complete sharing of the HLA region between sexual partners in outbred populations is exceedingly rare; only isolated and significantly inbred populations have a chance for such HLA homogeneity. Therefore, the authors conclude that HLA typing is of no clinical utility in outbred populations. A similar link between MHC class II typing and recurrent pregnancy loss has also been reported,176, 177  but its significance is unclear.

Bystander Effect: Autoimmunity and Pregnancy Loss

Antibody-mediated mechanisms for recurrent abortion in which humoral immune reactivity might directly result in adverse antifetal effects have been proposed. These include reports on associations between RPL and the presence of antisperm and antitrophoblast antibodies; however, each has been subsequently shown to have minimal clinical relevance.7, 162 In contrast, demonstrated associations between pregnancy loss and autoantibodies directed against nonreproductive tissues provide indirect evidence for humoral immune-mediated pregnancy loss. For instance, antithyroid antibodies have been linked to adverse pregnancy outcomes. One large, retrospective study reported an increased prevalence of these antibodies among women with a history of RPL, even in the absence of thyroid endocrinologic abnormalities.178 Others have reported similar findings.179 In contrast, two studies addressing pregnancy outcome and the presence of serum antithyroid antibodies in maternal blood have demonstrated neither an association with RPL nor a need for antithyroid antibody testing among RPL patients.180, 181 The differences between these findings are most likely related to differences in study designs. For example, older studies used a TSH value of 4.5 or 5.0 mlU/ml to define the upper limits of normal, but the American Endocrine Society now defines hypothyroidism as a TSH greater than 2.5 mlU/ml. As a result, it is possible that many of the “euthyroid” women examined in older studies were actually hypothyroid; therefore, it was clinical hypothyroidism, and not anti-thyroid antibodies, that caused the high prevalence of recurrent pregnancy loss.182 This is further supported by studies showing that the presence of antithyroid antibodies in euthyroid women does not affect pregnancy outcomes.179, 183

Antiphospholipid Antibodies 

More consistent, and more therapeutically relevant, have been reports linking organ-nonspecific autoantibodies associated with APS to adverse pregnancy outcomes. Clinical and laboratory evidence for antiphospholipid antibodies were originally based in vivo on thrombosis and in vitro on prolongation in one of the phospholipid-dependent coagulation tests such as the activated partial thromboplastin time or the Russell viper venom time. With time, patterns among the thrombotic complications associated with antiphospholipid antibodies were noted and criteria proposed that defined APS. Although many of the complications of APS are systemic, some are pregnancy-specific, including spontaneous abortion, premature labor, premature rupture of membranes, stillbirth, intrauterine growth restriction, and pre-eclampsia.181 The importance of pregnancy-specific complications in the diagnosis of APS gained renewed prominence when the defining criteria for the disorder were reassessed in 1998 (the Sapporo criteria) and again in 2006 (Table 1, see below).184, 185, 186 

Table 1. Criteria for diagnosing the antiphospholipid antibody syndrome 

One or more of the following clinical and one or more laboratory criteria must be present in the same patient:
Clinical
 1.  One or more confirmed episodes of vascular thrombosis of any type, including:
 Venous
 Arterial
 Small vessel
 2.  Pregnancy complications, including:

Three or more consecutive spontaneous pregnancy losses at less than 10 weeks of gestation, with exclusion of maternal anatomic and hormonal abnormalities and exclusion of paternal and maternal chromosomal abnormalities

One or more unexplained deaths of a morphologically normal fetus at or beyond 10 weeks gestation (normal fetal morphology documented by ultrasound or direct examination of the fetus)
One or more premature births of a morphologically normal neonate at or before 34 weeks gestation secondary to severe pre-eclampsia or placental insufficiency
 
 Laboratory (testing must be positive on two or more occasions, 12 weeks or more apart)
 1.  Positive plasma levels of anticardiolipin antibodies of the IgG or IgM isotypes at medium to high levels
 2.  Positive plasma levels of lupus anticoagulant
 3.  Positive plasma levels of the anti-β 2 glycoprotein 1 antibody of the IgG or IgM isotypes in titers greater than the 99th percentile

(Adapted from Miyakis et al.186

 

Clinical data have linked antiphospholipid antibodies such as anticardiolipin or lupus anticoagulant to adverse pregnancy outcome.187, 188 In a large series of couples with recurrent abortion, the incidence of APS was 3–5%.7 Pathologic data, however, do not consistently demonstrate causal involvement of APS in pregnancy loss. For instance, although placental infarction, abruption, and hemorrhage are considered to be defining placental lesions in APS, they are often missing in women with antiphospholipid antibodies.189 Conversely, women with recurrent abortion but no biochemical evidence of antiphospholipid antibodies often demonstrate placental findings consistent with the diagnosis of APS.190 

Antiphospholipid antibodies, however, have mechanistic links to placental thrombotic changes and subsequent adverse pregnancy outcome. Antiphospholipid antibodies may indirectly promote thrombosis. Associations between antiphospholipid antibodies and alterations in prostacyclin/thromboxane metabolism have been demonstrated. Local alterations in these pathways at the level of the maternal–fetal interface could promote vascular constriction, platelet adhesion, and placental infarction.191, 192, 193, 194 Similarly, antiphospholipid antibody positivity has been linked to reductions in the levels of placental antithrombotic products, such as annexin V, among women with RPL.195 Antiphospholipid antibodies may promote localized atherosclerosis. Supporting this concept, atherosclerosis has been shown to develop rapidly in the spiral arteries of patients positive for antiphospholipid antibodies.196 

In contrast to the focus on the thrombophilic qualities of antiphospholipid antibodies, newer studies have supported a primarily autoimmune effect of these antibodies. In fact, low thrombotic event rates can be demonstrated in women with APS and past histories of early loss. Rather than the finding of blood clots in placentas following spontaneous miscarriage, evidence has been found supporting defective endovascular trophoblastic invasion, proliferation, chemotaxis, maturation, and differentiation.197 To illustrate, IgM against phosphatidylserine inhibits the trophoblast syncytialization198 in cell cultures and sera from antibody-positive RPL patients inhibits trophoblast adhesion to endothelial cells in vitro.199 Other studies have linked antiphospholipid antibodies to decreases in the release of hCG from human placental explants and the induction of an inflammatory response secondary to complement activation on the trophoblast surface.200

Attention in the field of antiphospholipid antibodies and pregnancy loss has turned partially away from the antigens cardiolipin and phosphatidylserine but toward a protein cofactor called β2 glycoprotein 1.201, 202 This glycoprotein acts as an essential cofactor in antiphospholipid antibody reactivity against membrane phospholipid and, in many instances, may provide the true antigenic determinant for a given antiphospholipid antibody.201, 202 Because both extravillous cytotrophoblast and syncytiotrophoblast cells have been demonstrated to synthesize β2 glycoprotein 1,203 the placental microenvironment has the potential for robust antigenicity. Anti- β2 glycoprotein 1 antibodies are an independent risk factor for thrombotic events and for pregnancy loss in APS patients.204, 205, 206 They are the sole antibody detected in 3–10% of APS patients.207, 208, 209

IMMUNOLOGIC TESTING

Although much is now known about the immunology of the human genital tract, many questions remain. Immunologic interactions at the maternal–fetal interface are particularly intriguing, but in most cases, basic immunologic findings in the laboratory have yet to be fully applied toward diagnostic or therapeutic use at the bedside. Much progress will undoubtedly be made during the next decade; however, in this era of evidence-based medicine, very few immunologic tests or therapies are presently warranted in the diagnosis and treatment of patients with isolated or recurrent spontaneous pregnancy loss. Most of these tests remain experimental at best and demand further investigation before they can be clinically applied. Still others are of historical interest only. 

Antibody Testing 

As mentioned above, the 1998 reassessment of the criteria used for the diagnosis of APS made complications of pregnancy defining clinical criteria.184 A brief review of the 2006 revised Sapporo criteria (see Table 1) demonstrates that although most patients with an isolated spontaneous pregnancy loss do not warrant testing for APS, many patients with recurrent losses do require further laboratory assessment. Screening for the lupus anticoagulant (activated partial thromboplastin time or Russell viper venom testing) and assessment for the presence of anticardiolipin antibodies and anti- β2 glycoprotein 1 antibodies is indicated for most patients with RPL but few with isolated losses. 

Although investigators have suggested testing for a large number of isolated organ-specific and nonspecific autoantibodies among patients with RPL, testing for antibodies other than the lupus anticoagulant, anticardiolipin antibodies and anti- β2 glycoprotein 1 antibodies remains controversial.186, 210, 211 For example, although some have shown the prevalence of the organ-specific antithyroid antibodies to be increased among patients with a history of RPL,178, 180 others have found no association between the presence of antithyroid antibodies and recurrent loss.179 Even if the prevalence of antithyroid antibodies is increased among these patients, their clinical and mechanistic significance remains unclear.212 

There is no consistent evidence supporting the use of extensive panels of serum or site-specific autoantibodies or alloantibodies (including antinuclear antibodies and antipaternal cytotoxic antibodies) in the evaluation of either isolated or recurrent pregnancy loss. Use of these panels often serves only to verify the statistical tenet that if the number of tests performed reaches a critical limit, at least one will be positive in every patient. 

Cytokine Testing 

The hypothesis that cytokine dysregulation (Th1/Th2 imbalance) at the maternal–fetal interface may affect pregnancy maintenance is gaining momentum, although the number of cytokines involved and the complexity of their local interactions makes interpretable study in humans a daunting undertaking. Although there exists evidence that these effects may be clinically important in some cases of pregnancy loss.21, 213 Our ability to diagnose such dysregulation, however, has been hampered by a number of variables, many of which are difficult to circumvent. We have herein described that the immune cells populating the maternal–fetal interface are unique and characteristic. Therefore, although Th1/Th2 dysregulation may be occurring within the immune microenvironment at the site of implantation, peripheral immune cells may be spared. Safe methods to assess the immune characteristics at the site of an ongoing and desired pregnancy do not presently exist. Can significant alterations in cytokine profiles that are isolated within the uterus be detected using peripheral serum testing? This remains unclear. Some small studies have documented peripheral shifts toward Th1 cytokine profiles in RPL patients who subsequently lose their pregnancy, but not in similar patients who have a successful pregnancy outcome.105 Other larger studies have failed to show any association between peripheral cytokine alterations and pregnancy outcome among patients with a history of recurrent losses.214 One additional caveat is that pregnancy demise is itself accompanied by dramatic immunologic alterations within the uterus directed toward the presence of necrotic tissues. Therefore, studies on cytokine dysregulation, including those reporting a peripheral shift toward Th1 profiles at the time of fetal demise among patients with RPL,104 may not be able to differentiate cause from effect.

Immune Cell Evaluation 

Efforts have been made to evaluate both the proportions and activity of immune cells isolated from patients with a history of spontaneous pregnancy loss. These studies are also limited both by the inability to sample sites of implantation during desired pregnancies and by the tenuous association between most decidual and peripheral immune characteristics. Diagnostic use of mixed lymphocyte cultures, originally proposed to evaluate the presence of 'blocking antibody' activity, has not been consistently validated. The identification of large numbers of NK-like cells at sites of implantation led to the hypothesis that alterations in the prevalence and activity of these cells might significantly affect pregnancy maintenance. Although the rationale may be questioned, this hypothesis has been extended to promote testing of the prevalence and activity of peripheral NK cells among patients with RPL.215, 216 Again, because decidual and peripheral NK cells are markedly different, one must resolve whether intrauterine events are reflected using NK cells isolated from the periphery of these women. Two small studies reported that the evaluation of these peripheral NK cell characteristics predicts prognosis and assists in the counseling of patients with RPL.215, 216 These studies have not been adequately substantiated and, in fact, are contradicted by a 2011 comprehensive review of over 780 publications. In this review, the authors did not find consistent evidence linking peripheral NK cell testing results with pregnancy outcomes among women with a history of RPL.217 Their conclusions are further supported by recent flow cytometric analyses demonstrating distinct differences between the phenotypes of peripheral and endometrial NK cells in women with RPL.218 In summary, the weight of evidence does not support testing for peripheral NK cell activity or basing treatments on the results of peripheral NK assays in women with recurrent pregnancy loss.219

HLA Screening

Testing for parental HLA profiles is not indicated in outbred populations. Those reports demonstrating that HLA sharing is associated with poor pregnancy outcomes were strictly limited to the specific populations studied.174, 175 In the most recent and definitive of these investigations,175 only patients with complete sharing of the entire HLA locus demonstrated an increased risk for adverse pregnancy outcomes. As discussed, the chance of discovering complete sharing of the entire HLA region among partners in outbred populations is infinitesimally small. Therefore, generalized HLA screening of patients with either isolated or recurrent spontaneous pregnancy loss cannot be rationalized.

 

 

 

THERAPY

Like immunologic testing, 'immunologic' therapies for patients with presumed immunologic pregnancy losses have been best substantiated among patients with concomitant hypercoagulability. Treatment regimens for patients with adverse pregnancy outcomes and APS is appropriately discussed alongside other proposed therapies for immune-mediated pregnancy loss, even though the treatments are themselves best subcategorized as antithrombotic rather than immunomodulating. That said, the antithrombotic agent heparin has been shown to bind to antiphospholipid antibodies in vitro, an effect that suggests that it possesses distinct immunoregulatory properties distinct from its anthithrombotic effects.220 In addition, heparin promotes trophoblast invasion,221 inhibits trophoblast apoptosis,222 restores placental hCG production221, 223 and directly alters cellular immune responses.224

Antithrombotic Therapy 

Aspirin and Unfractionated Heparin

The safety and efficacy of the combined use of aspirin (ASA) and heparin has been best studied among pregnant women who carry the diagnosis of APS. Although variations have been described,225, 226, 227 a typical medical regimen for the antithrombotic management of these patients is shown in Table 2.

Table 2. Antithrombotic treatment of antiphospholipid antibody syndrome during pregnancy

Aspirin 75–85 mg, orally QDBegin with attempts at conception
Unfractionated sodium5,000–10,000 IU, BID, subcutaneouslyBegin with confirmation of pregnancy
MonitoringCheck activated partial thromboplastin time each week; adjust dosage to maintain anticoagulation 

 

Therapy with combinations of ASA and unfractionated heparin in this patient population is not without associated risks. Increases in the incidence of pregnancy-specific complications, including premature labor, premature rupture of the membranes, intrauterine growth restriction, intrauterine fetal demise, pre-eclampsia, and abruptio placenta have all been reported.226, 227, 228  Specific maternal risks include gastric bleeding and osteopenia. Pregnant patients with APS should be considered high risk and managed in conjunction with a perinatologist. Despite these risks, a recent systematic literature review and meta-analysis examining different aspirin and heparin treatment regimens confirmed the finding of improved live birth rates in women with antiphospholipid syndrome treated with unfractionated heparin plus aspirin compared with aspirin monotherapy.229 This treatment combination, however, has not been found to be beneficial in women with recurrent miscarriage but without APS.230, 231

Low-Molecular-Weight Heparin

The inconvenient dosing schedules and unacceptable side effect profiles associated with unfractionated heparin use led to the development of low-molecular-weight heparin (LMWH). When compared with unfractionated heparin, LMWH has an increased antithrombotic ratio, allowing treatment of inappropriate clotting with fewer bleeding side effects.232 The incidence of thrombocytopenia and osteoporosis are similarly lower using LMWH. The prolonged half-life of LMWH compared with its unfractionated counterpart permits less frequent dosing and less frequent thrombotic monitoring. Both promote patient acceptance and compliance. 

Whereas LMWH has been shown to be safe, effective, and convenient in the treatment of many clotting disorders unrelated to reproduction,232 their use in pregnancy is only now being comprehensively evaluated. Still, specific use of LMWH (in combination with low-dose ASA) among patients with RPL and thrombotic disorders has been quickly adopted clinically. To date, the safety and efficacy of LMWH use have been suggested by interventional trials conducted among RPL patients with APS226, 233 and among those with activated protein C resistance associated with factor V Leiden.234, 235, 236 The same meta-analysis that supported combined therapy with aspirin and unfractionated heparin, however, did not find significant efficacy for a combination of low molecular weight heparin and aspirin, but supported the need for larger controlled trials.229,

 

Aspirin Prophylaxis

Patients seeking therapy for reproductive disorders, including those experiencing isolated and recurrent spontaneous pregnancy losses, tend to be well-informed. They are therefore quick to apply the well-publicized cardiovascular benefits and infrequent side effects of daily prophylactic low-dose ASA to their own medical condition. Many patients with histories of recurrent loss are now either self-prescribing this therapy or inquiring about its efficacy. To summarize the data addressing prophylactic ASA use among patients with RPL, current data does not support its clinical application either among patients with unexplained RPL with losses at less than 10 weeks or in those with coagulopathies.. Although ASA/heparin has been demonstrated to be effective in the prevention and treatment of adverse pregnancy outcomes among patients with APS (see above), the sole use of low-dose ASA among patients with RPL and APS appears ineffective.225, 237 Other studies have reached similar conclusions against the efficacy of aspirin in preventing future pregnancy loss. In one large, prospective interventional trial, the use of prophylactic low-dose ASA could not be recommended for the treatment of patients with unexplained recurrent early pregnancy losses.238 Its use among women with unexplained late pregnancy losses (later than 13 weeks gestation) did, however, appear promising.238 Most recently, a multicenter, double-blind, placebo-controlled trial was conducted to more adequately assess the use of aspirin in preventing pregnancy loss in women with one loss at less than 20 weeks’ gestation during the previous year and in women with one to two previous losses with no restriction in gestational age at the time of loss. In the group of women with one to two previous losses, preconception-initiated low-dose aspirin did not increase the rate of live births or reduce rates of pregnancy loss. The group of women with a single loss at less than 20 weeks’ gestation in the previous year, however, did experience a higher pregnancy rate and live birth rate. Overall the authors concluded from these somewhat confusing results that aspirin could not be recommended to decrease the rate of pregnancy loss.239

 

Immunomodulating Therapies 

Other than the antithrombotic approaches, all alternative therapies for immune-mediated spontaneous pregnancy loss can be classified as immunoregulatory. Historical controversy over whether an immune response to the implanting pregnancy was essential or detrimental has led to the development and use of both immunosuppressive and immunostimulating therapies for the treatment of these patients. Although a large number of immune interventions have been proposed, these therapies remain experimental, and many have been proven ineffective; some even appear to be unsafe. 

Although immune interactions seem to be involved in some instances of isolated spontaneous pregnancy loss, most of the published interventional studies involving immunotherapy for the prevention of subsequent pregnancy loss have been limited to patients with recurrent losses. Even these studies, however, have been limited by poor study design, small sample size, lack of patient prestratification by maternal age and number of prior losses before randomization, and a variety of methodologic and statistical inaccuracies. Another glaring omission in most studies has been the inconsistent analysis of aborted tissues for chromosomal abnormalities, because approximately 60% of individual losses are undoubtedly due to numeric chromosomal aberrations.240  Immunotherapy for RPL has been addressed in a number of excellent reviews6, 241, 242 and will be briefly summarized here. 

Immunosuppressive Therapies 

The hypothesis that inappropriate cellular or humoral immune responses to the implanting fetus causes pregnancy loss fostered the development of several immunosuppressive therapeutic approaches to the treatment of patients with immune-mediated RPL. These suppressive therapies have included the use of standard immunosuppressive drugs, immunosuppressive reproductive hormones, and intravenous immunoglobulins. 

Standard Immunosuppressive Therapies

Several immunoregulatory regimens, including the use of cyclosporine, pentoxifylline, and nifedipine, have been suggested based on related efficacy trials in the autoimmunity and transplantation literature. Extension of most of these interventions to patients with RPL, however, has been hampered by known or unknown maternal and fetal risks. For instance, plasmapheresis has been used for the prevention of thrombotic complications associated with the presence of antiphospholipid antibodies, and only one isolated study has examined its efficacy among patients with recurrent abortion and antiphospholipid antibodies.243 The best-studied of the standard immunosuppressive approaches has involved the use of corticosteroid therapy. Although there exists evidence that prednisone is useful in the treatment of patients with recurrent pregnancy loss and APS,244 antithrombotic therapy with heparin and aspirin has been shown to be as or more effective and safer in this same patient population. A large, randomized, placebo-controlled trial addressing the use of prednisone in combination with low-dose ASA among women with autoantibodies and RPL may relegate this therapy to historical interest only. In this investigation, pregnancy outcomes for treated and control patients were similar, but the incidence of maternal diabetes and hypertension and the risk of premature delivery were all increased among those treated with prednisone and ASA.245 

Immunosuppressive Reproductive Hormones

Both estrogen and progesterone have immunomodulating effects.106, 109, 111, 246, 118, 119, 120, 121, 122, 123, 127, 247 The well-described and marked elevations of these reproductive hormones during pregnancy make them attractive candidates for use as immunosuppressive agents among patients with presumed immune-mediated pregnancy losses. As mentioned previously, in vitro evidence suggests that progesterone alters cytokine responses,111, 246, 248, 249 which might promote its appropriate use in patients with RPL and cytokine dysregulation at the maternal–fetal interface. Documenting such local dysregulation, however, is problematic. Small studies on empiric progesterone use in patients with unexplained RPL have presented conflicting results. However, a meta-analysis has advocated its use in patients with recurrent, but not isolated pregnancy losses.250 Vaginal administration of progesterone may increase local, intrauterine concentrations of the hormone better than systemic administration and may therefore provide improved local immunosuppression while averting systemic side effects.  

Intravenous Immunoglobulin

The pooling of the immunoglobulin proteins isolated from the serum of a large number of volunteer blood donors results in a blood product known as intravenous immunoglobulin (IVIg). The use of IVIg may be accurately categorized as immunosuppressive therapy, although the mechanistic basis for IVIg immunoregulation is poorly understood. Still, the list of potential immunoregulatory mechanisms is long and includes: decreased autoantibody production and increased autoantibody clearance, T-cell and Fc receptor regulation,251 complement inactivation,252 enhanced T-cell suppressor function, decreased T-cell adhesion to the extracellular matrix,253 and downregulation of Th1 cytokine synthesis.254 Because IVIg is a blood product, its use is associated with a variety of real and potential risks. Like other blood products, IVIg is screened for transmissible agents such as HIV and hepatitis. However, up to 200 different donors are required to manufacture one vial of IVIg, and the transmission of prion-related disorders (e.g. 'mad cow disease', Jakob-Creutzfeldt disease) and other disorders that are not readily detected in routine screening should be considered a potential risk of therapy. Side effects of IVIg therapy include nausea, headache, myalgias, hypotension, and in some instances anaphylaxis.255 

Therapy with IVIg for recurrent pregnancy loss is expensive, invasive, and time-consuming, requiring multiple intravenous infusions over the course of pregnancy. Although the list of financial and medical costs appears daunting, does treatment outcome using IVIg among patients with RPL justify the expense, side effects, and risks of this immunointervention? The data addressing the use of IVIg in patients with RPL is fairly extensive and, until recently, the answer to this question remained unclear. Although a number of relatively small studies using a variety of treatment protocols produced conflicting results, larger and more consistent studies have been unable to conclusively justify the use of IVIg in patients with unexplained (and presumed immunologic) RPL.256, 257, 258, 259, 260, 261, 262, 263, 264 Still, some may argue that the use of IVIg may be indicated among those specific patients with autoimmune-mediated pregnancy loss associated with APS.265, 266 Although they would be useful, no safety and efficacy studies have been performed comparing the use of IVIg with that of unfractionated heparin/ASA or LMWH/ASA in these patients.  The results of a large, randomized, multicentered placebo-controlled trial evaluated treatment with IVIg to saline placebo in women with idiopathic secondary recurrent miscarriage, defined as “a history of at least one prior ongoing pregnancy followed by three or more consecutive unexplained miscarriages” and concluded that no significant effect was found in patients treated with IVIg; this is the largest randomized controlled trial on this topic to date.267

Immunostimulating Therapies 

The development of immunostimulating therapies for the prevention of spontaneous pregnancy losses was based on the premise that an appropriate immune recognition and reaction to the implanting fetus was essential to subsequent pregnancy maintenance. There may be significant problems with this underling premise. First, although we know that pregnancies are specifically recognized and anti-paternal immune reactions mounted by the pregnant woman,164 a requirement for these responses has never been definitively demonstrated. Further, the fetal/paternal alloantigens recognized by the maternal immune system have never been appropriately isolated and identified. This explains the variety of antigen preparations and routes of administration that have been promoted during the development of immunostimulating 'therapies' for RPL. For instance, syncytiotrophoblast microvillus plasma membrane vesicles have been prepared and administered intravenously to mimic the fetal cell contact with maternal blood that normally occurs in pregnancy.268 The erroneous belief that TLX was part of an idiotype/anti-idiotype control system269 led to the use of third-party seminal plasma suppositories for immunostimulation in women with histories thought to be consistent with immune-mediated RPL.270 Neither therapy was proven effective. 

Leukocyte Immunization 

The best-studied immunostimulation therapy to be promoted for use among patients with immune RPL involved the intravenous administration of either paternal or pooled donor leukocytes. Because leukocytes from either source supply innumerable alloantigens, it is not surprising that maternal immune responses would be stimulated. Immune mechanisms responsible for the transfer of these systemic responses to potential fetoprotective effects were suggested271, 272, 273, 274 but never proven. Like IVIg, paternal or donor leukocytes represent blood products, and their administration to a third party carries the same risks as with other blood products. Reported risks specific to the use of leukocyte immunization in pregnant women encompass a variety of adverse fetal and maternal effects, including graft-versus-host disease, severe intrauterine growth restriction, and potentially fatal fetal thrombocytopenia.7, 261, 275, 276, 277, 278, 279 Again, as with the use of IVIg in RPL patients, the question arises of whether the treatment outcome with the use of leukocyte immunization among patients with RPL justifies the expense, side effects, and risks of this immunointervention. The data addressing this question are similarly heterogeneous and similarly confusing.275, 276, 280, 281, 282, 283 However, the most recent and largest trial evaluating the efficacy of leukocyte immunization in patients with unexplained RPL was a part of the Recurrent Miscarriage Study (REMIS).283 This investigation involved more than 90 patients per treatment arm and was prospective, placebo-controlled, randomized, and double-blinded. The study indicated that paternal leukocyte immunization was not efficacious in couples with unexplained RPL; indeed, women receiving leukocyte immunization were more likely to experience a repeat loss than women receiving the placebo.

 

CONCLUSIONS

Immunologic interactions are complex. To those studying the immune interactions surrounding implantation, it is sobering to note that pregnancy does not appear to require an intact maternal immune system. To this point, women and animals who lack immunoglobulins (agammaglobulinemic) successfully reproduce.284 Women with severe immune deficiencies, mice that lack T and B cells (SCID mice), and mice with congenital absence of their thymus (nude mice) also carry pregnancies to term deliveries.285 Defining the role of a particular immune factor in pregnancy maintenance is therefore likely to be neither simple nor expedient. 

The prognosis for a patient with RPL is good. Most women with a history of RPL is more likely to carry their next pregnancy to term than to miscarry. In fact, for patients with a history of RPL, the risk of subsequent pregnancy loss is estimated to be 24% after two clinically recognized losses, 30% after three losses, and 40–50% after four losses.286 These prognostic data, while helpful, still frustrate attempts to document the efficacy of interventional therapy. Unless interventions are remarkably effective, treatment groups must be almost prohibitively large to demonstrate alterations in outcome. 

Clinical studies on immune-mediated pregnancy loss have been regrettably difficult to interpret. The disease itself is hard to accurately diagnose, typically relying solely on the exclusion of other etiologies. RPL represents a number of different specific disorders. With inconsistent clinical definition, trials involving these patients are difficult to compare and evaluate. Trial design is frequently substandard, with lack of rationale, lack of appropriate controls, and poor statistical analysis limiting the ability to draw rational conclusions from reported results. 

The future, however, is not bleak. The rapid expansion in our understanding of the molecular and cellular immune environment at the maternal–fetal interface will surely stimulate clinical applications, and clinical insight will translate into therapeutic advances. The next several decades will undoubtedly witness significant inroads into the diagnosis and treatment of immune-mediated isolated and recurrent spontaneous pregnancy loss.

REFERENCES

1

Edmonds, D.K., et al., Early embryonic mortality in women. Fertility and Sterility, 1982. 38: p. 447-453.

2

Alberman, E., The epidemiology of repeated abortion, in Early Pregnancy Loss: Mechanisms and Treatment, R.W. Beard and F. Sharp, Editors. 1988, Springer-Verlag: New York. p. 9-17.

3

Wilcox, A.J., et al., Incidence of early loss of pregnancy. New England Journal of Medicine, 1988. 319(4): p. 189-94.

4

Hill, J.A., Recurrent pregnancy loss, in Maternal Fetal Medicine, R.K. Creasy and R. Resnik, Editors. 2004, W.B. Saunders: New York. p. 579-587.

5

Stephenson, M., Frequency of factors associated with habitual abortion in 197 couples. Fertil Steril, 1996. 66: p. 24-29.

6

Stephenson, M. and W. Kutteh, Evaluation and management of recurrent early pregnancy loss. Clin Obstet Gynecol, 2007. 50(1): p. 132-45.

7

Hill, J.A., Sporadic and recurrent spontaneous abortion. Current Problems in Obstetrics, Gynecology and Fertility, 1994. 17(4): p. 114-162.

8

Summers, P.R., Microbiology relevant to recurrent miscarriage. Clinics in Obstetrics and Gynecology, 1994. 37: p. 722-729.

9

Sugiura-Ogasawara, M., et al., Pregnancy outcome in recurrent aborters is not influenced by Chlamydia IgA and/or G. Am J Reprod Immunol, 2005. 53(1): p. 50-3.

10

Robb, J.A., K. Benirschke, and R. Barmeyer, Intrauterine latent herpes simplex virus infection: I. Spontaneous abortion. Hum Pathol, 1986. 17(12): p. 1196-209.

11

Altshuler, G., Immunologic competence of the immature human fetus. Morphologic evidence from intrauterine Cytomegalovirus infection. Obstet Gynecol, 1974. 43(6): p. 811-6.

12

Heyborne, K.D., S.S. Witkin, and J.A. McGregor, Tumor necrosis factor-alpha in midtrimester amniotic fluid is associated with impaired intrauterine fetal growth. Am J Obstet Gynecol, 1992. 167(4 Pt 1): p. 920-5.

13

Romero R, Mazor M, Sepulueda W et al: Tumor necrosis factor in preterm and term labor. Am J Obstet Gynecol 166: 1576– 1587, 1992

14

Furman, M., H. Ploegh, and D. Schust, Can viruses help us to understand and classify the MHC class I molecules at the maternal-fetal interface? Hum Immunol, 2000. 61: p. 1169-76.

15

Thellin O, Coumans B, Zorzi W et al: Tolerance of the feto-placental “graft”: Ten ways to support a child for nine months. Curr Opin Immunol 12: 731– 737, 2000

16

Abbas AK, Lichtman AH, Pober JS: Cellular and Molecular Immunology. 4th ed. Philadelphia, WB Saunders, 2000

17

Lanier LL: Activating and inhibitory NK cell receptors. Ann Rev Immunol 452: 13– 18, 1998

18

Terra, R., N. Labrecque, and C. Perreault, Thymic and extrathymic T cell development pathways follow different rules. J Immunol, 2002. 169(2): p. 684-92.

19

Gould DS, Ploegh HL, Schust DJ: Murine female reproductive tract intraepithelial lymphocytes display selection characteristics distinct from both peripheral and other mucosal T cells. J Reprod Immunol 52: 85, 2001 Loke YW, King A: Immunological aspects of human implantation. J Reprod Fertil Steril 55: 83– 90, 2000

20

Lefrancois, L., et al., On the front lines: intraepithelial lymphocytes as primary effectors of intestinal immunity. Springer Semin Immunopathol, 1997. 18: p. 463-75.

21

Loke, Y. and A. King, Immunological aspects of human implantation. J Reprod Fertil Steril, 2000. 55: p. 83-90.

22

Johnson PM, Christmas PE, Vince GS: Immunological aspects of implantation and implantation failure. Hum Reprod 14 (suppl 2): 26– 36, 1999 Loke YW, King A: Decidual natural-killer-cell interaction with trophoblast: Cytolysis or cytokine production? Biochem Soc Trans 28: 196– 198, 2000

23

Vince, G. and P. Johnson, Leukocyte populations and cytokine regulation in human uteroplacental tissues. Biochem Soc Trans, 2000. 28: p. 191-5.

24

Croy, B., et al., Decidual natural killer cells: key regulators of placental development (a review). J Reprod Immunol, 2002. 57: p. 151-68.

25

Loke, Y. and A. King, Decidual natural-killer-cell interaction with trophoblast: cytolysis or cytokine production? Biochem Soc Trans, 2000. 28: p. 196-8.

26

King, A., et al., Recognition of trophoblast HLA class I molecules by decidual NK cell receptors--a review. Placenta, 2000. 21 Suppl A: p. S81-5.

27

Male V, Trundley A, Gardner L, et al: Natural killer cells in human pregnancy. Methods Mol Biol, 2010. 612: p. 447-463.

28

Chazara O, Xiong S, Moffett A: Maternal KIR and fetal HLA-C: a fine balance. J Leukoc Biol, 2011. 90(4): p. 703-716.

29

Kopcow HD, Allan DS, Chen X, et al: Human decidual NK cells form immature activating synapses and are not cytotoxic. Pro Natl Acad Sci USA, 2005. 102(43): p. 15563-15568.

30

Hanna J, Goldman-Wohl D, Hamani Y, Avraham I, Greenfield C, Natanson-Yaron S, et al. Decidual NK cells regulate key developmental processes at the human fetal-maternal interface. Nat Med, 2006. 12(9): p. 1065-74.

31

Manaster I, Mizrahi S, Goldman-Wohl D, et al: Endometrial NK cells are special immature cells that await pregnancy. J Immunol, 2008. 181(3): p. 1869-1876.

32

Godfrey, D., et al., NKT cells: facts, functions, and fallacies. Immunol Today, 2000. 21: p. 573-83.

33

Ito, K., et al., Involvement of decidual Valpha14 NKT cells in abortion. Proc Natl Acad Sci U S A, 2000. 97(2): p. 740-4.

34

Dang, Y. and K. Heyborne, Cutting edge: regulation of uterine NKT cells by a fetal class I molecule other than CD1. J Immunol, 2001. 166(6): p. 3641-4.

35

Dang, Y., et al., Natural killer 1.1(+) alpha beta T cells in the periimplantation uterus. Immunology, 2000. 101(1): p. 484-91.

36

Boyson, J., et al., CD1d and invariant NKT cells at the human maternal-fetal interface. Proc Natl Acad Sci U S A, 2002. 99(21): p. 13741-6.

37

Mincheva-Nilsson, L., et al., Immunomorphologic studies of human decidua-associated lymphoid cells in normal early pregnancy. Adv Exp Med Biol, 1995. 371A: p. 367-71.

38

Christmas, S., et al., Extensive TCR junctional diversity of V gamma 9/V delta 2 clones from human female reproductive tissues. J Immunol, 1995. 155(5): p. 2453-8.

39

Vassiliadou, N. and J. Bulmer, Characterization of endometrial T lymphocyte subpopulations in spontaneous early pregnancy loss. Hum Reprod, 1998. 13(1): p. 44-7.

40

Hayday, A.C., [gamma][delta] cells: a right time and a right place for a conserved third way of protection. Annu Rev Immunol, 2000. 18: p. 975-1026.

41

Fan DX, Duan J, Li MQ, et al: The decidual gamma-delta T cell upregulates the biological functions of trophoblasts via IL-10 secretion in early human pregnancy. Clin Immunol, 2011. 141(3): p. 284-292.

42

Nagamatsu T, Schust DJ: The immunomodulatory roles of macrophages at the maternal-fetal interface. Reprod Sci. 2010. 17(3): p. 209-218.

43

Moffett, A., L. Regan, and P. Braude, Natural killer cells, miscarriage, and infertility. Bmj, 2004. 329(7477): p. 1283-5.

44

Clifford, K., A.M. Flanagan, and L. Regan, Endometrial CD56+ natural killer cells in women with recurrent miscarriage: a histomorphometric study. Hum Reprod, 1999. 14(11): p. 2727-30.

45

Quenby, S., et al., Pre-implantation endometrial leukocytes in women with recurrent miscarriage. Hum Reprod, 1999. 14(9): p. 2386-91.

46

Yamamoto T, Takahashi Y, Kase N, Mori H: Proportion of CD56+3+ T cells in decidual and peripheral lymphocytes of normal pregnancy and spontaneous abortion with and without history of recurrent abortion. Am J Reprod Immunol 42: 355– 360, 1999

47

Yamamoto T, Takahashi Y, Kase N, Mori H: Decidual natural killer cells in recurrent spontaneous abortion with normal chromosomal content. Am J Reprod Immunol 41: 337– 342, 1999

48

Lachapelle, M.H., et al., Endometrial T, B, and NK cells in patients with recurrent spontaneous abortion. Altered profile and pregnancy outcome. J Immunol, 1996. 156(10): p. 4027-34.

49

Kwak-Kim, J. and A. Gilman-Sachs, Clinical implication of natural killer cells and reproduction. Am J Reprod Immunol, 2008. 59(5): p. 388-400.

50

Pudney, J., A.J. Quayle, and D.J. Anderson, Immunological microenvironments in the human vagina and cervix: mediators of cellular immunity are concentrated in the cervical transformation zone. Biol Reprod, 2005. 73(6): p. 1253-63.

51

Schon, M.P., et al., Mucosal T lymphocyte numbers are selectively reduced in integrin alpha E (CD103)-deficient mice. J Immunol, 1999. 162(11): p. 6641-9.

52

Kruse, A., R. Hallmann, and E.C. Butcher, Specialized patterns of vascular differentiation antigens in the pregnant mouse uterus and the placenta. Biol Reprod, 1999. 61(6): p. 1393-401.

53

Perry, L.L., et al., Distinct homing pathways direct T lymphocytes to the genital and intestinal mucosae in Chlamydia-infected mice. J Immunol, 1998. 160(6): p. 2905-14.

54

Pudney, J. and D.J. Anderson, Immunobiology of the human penile urethra. Am J Pathol, 1995. 147(1): p. 155-65.

55

Billington, W., M. Davies, and S. Bell, Maternal antibody to foetal histocompatibility and trophoblast-specific antigens. Ann Immunol (Paris), 1984. 135D(3): p. 331-5.

56

Tafuri, A., et al., T cell awareness of paternal alloantigens during pregnancy. Science, 1995. 270: p. 630-3.

57

Sargent, I.L., T. Wilkins, and C.W.G. Redman, Maternal immune responses to the fetus in early pregnancy and recurrent miscarriage. Lancet, 1994. 2: p. 1099-1104.

58

Madeja Z, Yadi H, Apps R, et al: Paternal MHC expression on mouse trophoblast affects uterine vascularization and fetal growth. Proc Natl Acad Sci USA, 2011. 108(10): p. 4012-4017.

59

Stewart, C., et al., Blastocyst implantation depends on maternal expression of leukaemia inhibitory factor. Nature, 1992. 359(6390): p. 76-9.

60

Chen, J., et al., Leukemia inhibitory factor can substitute for nidatory estrogen and is essential to inducing a receptive uterus for implantation but is not essential for subsequent embryogenesis. Endocrinology, 2000. 141(12): p. 4365-72.

61

Robb L, Li R, Hartley L, et al: Infertility in female mice lacking the receptor for interleukin 11 is due to a defective uterine response to implantation. Nat Med, 1998. 4(3): p. 303-308.

62

Dimitriadis E, Menkhorst E, Salamonsen LA, et al: Review: LIF and IL11 in trophoblast-endometrial interactions during the establishment of pregnancy. Placenta, 2010. 31 (Suppl): p. S99-104.

63

Asnagli, H. and K.M. Murphy, Stability and commitment in T helper cell development. Curr Opin Immunol, 2001. 13(2): p. 242-7.

64

Palmer, E.M. and G.A. van Seventer, Human T helper cell differentiation is regulated by the combined action of cytokines and accessory cell-dependent costimulatory signals. J Immunol, 1997. 158(6): p. 2654-62.

65

O'Garra, A. and N. Arai, The molecular basis of T helper 1 and T helper 2 cell differentiation. Trends Cell Biol, 2000. 10(12): p. 542-50.

66

Kurt-Jones, E., et al., Heterogeneity of helper/inducer T lymphocytes. I. Lymphokine production and lymphokine responsiveness. J Exp Med, 1987. 166(6): p. 1774-87.

67

Romagnani, S., Human TH1 and TH2 subsets: regulation of differentiation and role in protection and immunopathology. Int Arch Allergy Immunol, 1992. 98(4): p. 279-85.

68

Mosmann, T.R. and R.L. Coffman, Heterogeneity of cytokine secretion patterns and functions of helper T cells. Adv Immunol, 1989. 46: p. 111-47.

69

Mosmann, T. and R. Coffman, TH1 and TH2 cells: different patterns of lymphokine secretion lead to different functional properties. Annu Rev Immunol., 1989. 7: p. 145-73.

70

Maggi, E., et al., Reciprocal regulatory effects of IFN-gamma and IL-4 on the in vitro development of human Th1 and Th2 clones. J Immunol, 1992. 148(7): p. 2142-7.

71

Fiorentino, D., M. Bond, and T. Mosmann, Two types of mouse T helper cell. IV. Th2 clones secrete a factor that inhibits cytokine production by Th1 clones. J Exp Med, 1989. 170(6): p. 2081-95.

72

Mosmann, T. and K. Moore, The role of IL-10 in crossregulation of TH1 and TH2 responses. Immunol Today, 1991. 12(3): p. A49-53.

73

Sen, D. and H. Fox, The lymphoid tissue of the endometrium. Gynecol Pathol, 1967. 163: p. 371.

74

Kearns, M. and P. Lala, Bone marrow origin of decidual cell precursors in the pseudopregnant mouse uterus. J Exp Med, 1982. 155(5): p. 1537-54.

75

Bulmer, J. and C. Sunderland, Immunohistological characterization of lymphoid cell populations in the early human placental bed. Immunology, 1984. 52(2): p. 349-57.

76

Tabibzadeh, S., Human endometrium: an active site of cytokine production and action. Endocr Rev, 1991. 12(3): p. 272-90.

77

Klentzeris, L., et al., Endometrial lymphoid tissue in the timed endometrial biopsy: morphometric and immunohistochemical aspects. Am J Obstet Gynecol, 1992. 167(3): p. 667-74.

78

Hill, J., K. Polgar, and D. Anderson, T-helper 1-type immunity to trophoblast in women with recurrent spontaneous abortion. JAMA, 1995. 273(24): p. 1933-6.

79

Hill, J., et al., Evidence of embryo- and trophoblast-toxic cellular immune response(s) in women with recurrent spontaneous abortion. Am J Obstet Gynecol, 1992. 166(4): p. 1044-52.

80

Mallmann, P., A. Werner, and D. Krebs, Serum levels of interleukin-2 and tumor necrosis factor-alpha in women with recurrent abortion. Am J Obstet Gynecol, 1990. 163: p. 1367.

81

Yamada, H., K. Polgar, and J. Hill, Cell-mediated immunity to trophoblast antigens in women with recurrent spontaneous abortion. Am J Obstet Gynecol, 1994. 170(5 pt 1): p. 1339-44.

82

Berkowitz, R., et al., Effects of products of activated leukocytes (lymphokines and monokines) on the growth of malignant trophoblast cells in vitro. Am J Obstet Gynecol, 1988. 158(1): p. 199-203.

83

Hunt, J., et al., Products of lipopolysaccharide-activated macrophages (tumor necrosis factor-alpha, transforming growth factor-beta) but not lipopolysaccharide modify DNA synthesis by rat trophoblast cells exhibiting the 80-kDa lipopolysaccharide-binding protein. J Immunol, 1989. 143(5): p. 1606-13.

84

Hill, J., F. Haimovici, and D. Anderson, Products of activated lymphocytes and macrophages inhibit mouse embryo development in vitro. J Immunol, 1987. 139(7): p. 2250-4.

85

Ecker, J.L., M.R. Laufer, and J.A. Hill, Measurement of embryotoxic factors is predictive of pregnancy outcome in women with a history of recurrent abortion. Obstetrics and Gynecology, 1993. 81: p. 84-87.

86

Zenclussen, A.C., et al., Abnormal T-cell reactivity against paternal antigens in spontaneous abortion: adoptive transfer of pregnancy-induced CD4+CD25+ T regulatory cells prevents fetal rejection in a murine abortion model. Am J Pathol, 2005. 166(3): p. 811-22.

87

Mjosberg, J., et al., CD4+ CD25+ regulatory T cells in human pregnancy: development of a Treg-MLC-ELISPOT suppression assay and indications of paternal specific Tregs. Immunology, 2007. 120(4): p. 456-66.

88

Heikkinen J, M Heikkinen J, MÖTtÖNen M, Alanen A, Lassila O. Phenotypic characterization of regulatory T cells in the human decidua. Clinical & Experimental Immunology, 2004. 136(2): p. 373-8.

89

Jin L-P, Chen Q-Y, Zhang T, Guo P-F, Li D-J. The CD4+CD25bright regulatory T cells and CTLA-4 expression in peripheral and decidual lymphocytes are down-regulated in human miscarriage. Clinical Immunology, 2009. 133(3): p. 402-10.

90

Steinborn A, Schmitt E, Kisielewicz A, Rechenberg S, Seissler N, Mahnke K, et al. Pregnancy-associated diseases are characterized by the composition of the systemic regulatory T cell (Treg) pool with distinct subsets of Tregs. Clin Exp Immunol, 2012. 167(1): p. 84-98.

91

Winger EE, Reed JL. Low Circulating CD4+ CD25+ Foxp3+ T Regulatory Cell Levels Predict Miscarriage Risk in Newly Pregnant Women with a History of Failure. American Journal of Reproductive Immunology, 2011. 66(4): p. 320-8.

92

Nakae, S., et al., Phenotypic differences between Th1 and Th17 cells and negative regulation of Th1 cell differentiation by IL-17. J Leukoc Biol, 2007. 81(5): p. 1258-68.

93

Nakashima A, Ito M, Yoneda S, Shiozaki A, Hidaka T, Saito S: Circulating and decidual Th17 cell levels in healthy pregnancy. Am J Reprod Immunol, 2010. 63: p. 104-109.

94

Wang WJ, Hao CF, Yi L, Yin GJ, Bao SH, Qiu LH, Lin QD: Increased prevalence of T helper 17 (Th17) cells in the peripheral blood and decidua in unexplained recurrent spontaneous abortion patients. J Reprod Immunol, 2010. 84: p. 164-170.

95

Wang WJ, Hao CF, Qu QL, Wang X, Qiu LH, Lin QD: The deregulation of regulatory T cells on interleukin-17-producing T helper cells in patients with unexplained early recurrent miscarriage. Hum Reprod, 2010. 25: p. 2591-2596.

96

Liu YS, Wu L, Tong XH, Wu LM, He GP, Zhou GX, Luo LH, Luan HB: Study on the relationship between TH17 cells and unexplained recurrent spontaneous abortion. Am J Reprod Immunol, 2011. 65: p. 503-511

97

Lee SK, Kim JY, Lee M, Gilman-Sachs A, Kwak-Kim J. Th17 and regulatory T cells in women with recurrent pregnancy loss. Am J Reprod Immunol, 2012. 67(4): p. 311-818.

98

Lea, R.G., M. Tulppala, and H.O. Critchley, Deficient syncytiotrophoblast tumour necrosis factor-alpha characterizes failing first trimester pregnancies in a subgroup of recurrent miscarriage patients. Hum Reprod, 1997. 12(6): p. 1313-20.

99

von Wolff, M., et al., Regulated expression of cytokines in human endometrium throughout the menstrual cycle: dysregulation in habitual abortion. Mol Hum Reprod, 2000. 6(7): p. 627-34.

100

Lim, K.J., et al., The role of T-helper cytokines in human reproduction. Fertil Steril, 2000. 73(1): p. 136-42.

101

Piccinni, M., et al., Defective production of both leukemia inhibitory factor and type 2 T-helper cytokines by decidual T cells in unexplained recurrent abortions. Nat Med, 1998. 4(9): p. 1020-4.

102

Raghupathy, R., et al., Cytokine production by maternal lymphocytes during normal human pregnancy and in unexplained recurrent spontaneous abortion. Hum Reprod, 2000. 15(3): p. 713-8.

103

Raghupathy, R., et al., Maternal Th1- and Th2-type reactivity to placental antigens in normal human pregnancy and unexplained recurrent spontaneous abortions. Cell Immunol, 1999. 196(2): p. 122-30.

104

Makhseed, M., et al., Circulating cytokines and CD30 in normal human pregnancy and recurrent spontaneous abortions. Hum Reprod, 2000. 15(9): p. 2011-7.

105

Jenkins, C., et al., Evidence of a T(H) 1 type response associated with recurrent miscarriage. Fertil Steril, 2000. 73(6): p. 1206-8.

106

Grossman, C., Possible underlying mechanisms of sexual dimorphism in the immune response: fact and hypothesis. J Steroid Biochem, 1989. 34: p. 241-51.

107

Yokoyama, Y., et al., Gender dimorphism in immune responses following trauma and hemorrhage. Immunol Res, 2002. 26(1-3): p. 63-76.

108

Oertelt-Prigione S: The influence of sex and gender on the immune response. Autoimmun Rev, 2012. 11(6-7): p. A479-A485.

109

Siiteri, P., et al., Progesterone and maintenance of pregnancy: is progesterone nature's immunosuppressant? Ann N Y Acad Sci, 1977. 286: p. 384-97.

110

Jackson DL, Schust DJ: The Role of the Placenta in Autoimmune Disease and Early Pregnancy Loss. In Kay HH, Nelson DM, Wang Y, editors: The Placenta: From Development to Disease, Hoboken, NJ, 2011, Wiley-Blackwell, p. 215-221.

111

Vassiliadou, N., L. Tucker, and D. Anderson, Progesterone-induced inhibition of chemokine receptor expression on peripheral blood mononuclear cells correlates with reduced HIV-1 infectability in vitro. J Immunol, 1999. 162(12): p. 7510-8.

112

Runnebaum, B., I. Stober, and J. Zander, Progesterone, 20 alpha-dihydroprogesterone and 20 beta-dihydroprogesterone in mother and child at birth. Acta Endocrinol (Copenh), 1975. 80(3): p. 569-76

113

Hunt, J., et al., Female steroid hormones regulate production of pro-inflammatory molecules in uterine leukocytes. J Reprod Immunol, 1997. 35(2): p. 87-99

114

Szekeres-Bartho J, Polgar B. PIBF: The Double Edged Sword. Pregnancy and Tumor. American Journal of Reproductive Immunology, 2010. 64(2): p. 77-86.

115

Robinson DP, Klein SL. Pregnancy and pregnancy-associated hormones alter immune responses and disease pathogenesis. Hormones and Behavior, 2012. 62(3): p. 263-71.

116

Polgár B, Nagy E, Mikó É, Varga P, Szekeres-Barthó J. Urinary Progesterone-Induced Blocking Factor Concentration Is Related to Pregnancy Outcome. Biology of Reproduction, 2004. 71(5): p. 1699-705.

117

Hill, J.A., K. Polgar, and D.J. Anderson, T-helper 1-type immunity to trophoblast in women with recurrent spontaneous abortion. Jama, 1995. 273(24): p. 1933-6

118

Knoferi, M., et al., Do female sex steroids adversely or beneficially affect the depressed immune responses in males after trauma-hemorrhage? Arch Surg, 2000. 135(4): p. 425-33.

119

Gregory, M., et al., Estrogen mediates the sex difference in post-burn immunosuppression. J Endocrinol, 2000. 164(2): p. 129-38.

120

Muller, V., et al., Sex hormones and gender-related differences: their influence on chronic renal allograft rejection. Kidney Int, 1999. 55(5): p. 2011-20.

121

Salem, M., et al., beta-estradiol suppresses T cell-mediated delayed-type hypersensitivity through suppression of antigen-presenting cell function and Th1 induction. Int Arch Allergy Immunol, 2000. 121(2): p. 161-9.

122

Correale, J., M. Arias, and W. Gilmore, Steroid hormone regulation of cytokine secretion by proteolipid protein-specific CD4+ T cell clones isolated from multiple sclerosis patients and normal control subjects. J Immunol, 1998. 161(7): p. 3365-74.

123

Cutolo, M. and R. Wilder, Different roles for androgens and estrogens in the susceptibility to autoimmune rheumatic diseases. Rheum Dis Clin North Am, 2000. 26(4): p. 825-39.

124

Somerset, D.A., et al., Normal human pregnancy is associated with an elevation in the immune suppressive CD25+ CD4+ regulatory T-cell subset. Immunology, 2004. 112(1): p. 38-43.

125

Sasaki, Y., et al., Decidual and peripheral blood CD4+CD25+ regulatory T cells in early pregnancy subjects and spontaneous abortion cases. Mol Hum Reprod, 2004. 10(5): p. 347-53.

126

Prieto, G.A. and Y. Rosenstein, Oestradiol potentiates the suppressive function of human CD4 CD25 regulatory T cells by promoting their proliferation. Immunology, 2006. 118(1): p. 58-65.

127

Kanik, K. and R. Wilder, Hormonal alterations in rheumatoid arthritis, including the effects of pregnancy. Rheum Dis Clin North Am, 2000. 26(4): p. 805-23.

128

Zaffaroni, M. and A. Ghezzi, The prognostic value of age, gender, pregnancy and endocrine factors in multiple sclerosis. Neurol Sci, 2000. 21(4 Suppl 2): p. S857-60

129

Chapman, S., Varicella in pregnancy. Semin Perinatol, 1998. 22: p. 339-46

130

Varfas-Villavicencio JA, De Leon-Nava MA, Morales-Montor J: Immunoendocrine mechanisms associated with resistance or susceptibility to parasitic diseases during pregnancy, Neuroimmunomodulation, 2009. 16(2): p. 114-121.

131

Munn, D., et al., Inhibition of T cell proliferation by macrophage tryptophan catabolism. J Exp Med, 1999. 189(9): p. 1363-72.

132

Mellor, A.L. and D.H. Munn, Immunology at the maternal-fetal interface: lessons for T cell tolerance and suppression. Annu. Rev. Immunol. 2000, 2000. 18: p. 367-391.

133

Munn, D., et al., Prevention of allogeneic fetal rejection by tryptophan catabolism. Science, 1998. 281(5380): p. 1191-3.

134

Wilczynski JR, Radwan M, Kalinka J: The characterization and role of regulatory T cells in immune reactions. Front Biosci, 2008. 13: p. 2266-2274.

135

Meier, A.H. and J.M. Wilson, Tryptophan feeding adversely influences pregnancy. Life Sci, 1983. 32(11): p. 1193-6.

136

Kamimura, S., et al., Localization and developmental change of indoleamine 2,3-dioxygenase activity in the human placenta. Acta Med Okayama, 1991. 45(3): p. 135-9.

137

Schrocksnadel, H., et al., Decreased plasma tryptophan in pregnancy. Obstet Gynecol, 1996. 88(1): p. 47-50.

138

Mattson, R., The non- expression of MHC class II in trophoblast cells. Am J Reprod Immunol, 1998. 40: p. 385-94.

139

Murphy, S. and T. Tomasi, Absence of MHC class II antigen expression in trophoblast cells results from a lack of class II transactivator (CIITA) gene expression. Mol Reprod Dev, 1998. 51(1): p. 1-12.

140

Kovats, S., et al., A class I antigen, HLA-G, expressed in human trophoblasts. Science, 1990. 248(4952): p. 220-3.

141

Ellis, S., M. Palmer, and A. McMichael, Human trophoblast and the choriocarcinoma cell line BeWo express a truncated HLA Class I molecule. J Immunol, 1990. 144(2): p. 731-5.

142

Wei, X. and H. Orr, Differential expression of HLA-E, HLA-F, and HLA-G transcripts in human tissue. Hum Immunol, 1990. 29(2): p. 131-42.

143

Sernee, M., H. Ploegh, and D. Schust, Why certain antibodies cross-react with HLA-A and HLA-G: epitope mapping of two common MHC class I reagents. Mol Immunol, 1998. 35(3): p. 177-88.

144

King, A., et al., Surface expression of HLA-C antigen by human extravillous trophoblast. Placenta, 2000. 21(4): p. 376-87.

145

King, A., et al., HLA-E is expressed on trophoblast and interacts with CD94/NKG2 receptors on decidual NK cells. Eur J Immunol, 2000. 30(6): p. 1623-31.

146

Zhou, Y., et al., Human cytotrophoblasts adopt a vascular phenotype as they differentiate. A strategy for successful endovascular invasion? J Clin Invest., 1997. 99(9): p. 2139-51.

147

Kam, E., et al., The role of trophoblast in the physiological change in decidual spiral arteries. Hum Reprod, 1999. 14(8): p. 2131-8.

148

Damsky, C. and S. Fisher, Trophoblast pseudo-vasculogenesis: faking it with endothelial adhesion receptors. Curr Opin Cell Biol, 1998. 10(5): p. 660-6.

149

Hamai, Y., et al., Peripheral blood mononuclear cells from women with recurrent abortion exhibit an aberrant reaction to release cytokines upon the direct contact of human leukocyte antigen-G-expressing cells. Am J Reprod Immunol, 1998. 40(6): p. 408-13.

150

Kanai, T., et al., Human leukocyte antigen-G-expressing cells differently modulate the release of cytokines from mononuclear cells present in the decidua versus peripheral blood. Am J Reprod Immunol, 2001. 45(2): p. 94-9.

151

Lim, K., et al., Human cytotrophoblast differentiation/invasion is abnormal in pre-eclampsia. Am J Pathol, 1997. 151(6): p. 1809-18.

152

Trowsdale J, Moffett A: NK receptor interactions with MHC class I molecules in pregnancy. Semin Immunol, 2008. 20(6): p. 317-320.

153

Goldman-Wohl, D., et al., Lack of human leukocyte antigen-G expression in extravillous trophoblasts is associated with pre-eclampsia. Mol Hum Reprod, 2000. 6(1): p. 88-95.

154

Hunt, J., et al., Soluble HLA-G circulates in maternal blood during pregnancy. Am J Obstet Gynecol, 2000. 183(3): p. 682-8.

155

Yamashita, T., et al., Analysis of human leukocyte antigen-G polymorphism including intron 4 in Japanese couples with habitual abortion. Am J Reprod Immunol, 1999. 41(2): p. 159-63.

156

Steffensen, R., et al., HLA-E polymorphism in patients with recurrent spontaneous abortion. Tissue Antigens, 1998. 52(6): p. 569-72.

157

Dahl M, Hviid TV: Human leucocyte antigen class Ib molecules in pregnancy success and early pregnancy loss. Hum Reprodu Update, 2012. 18(1): p. 92-109.

158

Campbell KS, Purdy AK: Structure/function of human killer cell immunoglobulin-like receptors: lessons from polymorphisms, evolution, crystal structures and mutations. Immunology, 2011. 132(3): p. 315-325.

159

Carrington M, Martin MP: The impact of variation at the KIR gene cluster on human disease. Curr Top Microbiol Immunol, 2006. 298: p. 225-257.

160

Faridi RM, DasV, Tripthi G, et al: Influence of activating and inhibitory killer immunoglobulin-like receptors on predisposition to recurrent miscarriages. Hum Reprod, 2009. 24 (7): p. 1758-1764.

161

Feinman, M.A., H.J. Kliman, and E.K. Main, HLA antigen expression and induction by gamma-interferon in cultured human trophoblasts. Am J Obstet Gynecol, 1987. 157(6): p. 1429-34.

162

Hill, J.A., Immunological mechanisms of pregnancy maintenance and failure: a critique of theories and therapy. Am J Reprod Immunol, 1990. 22(1-2): p. 33-41.

163

Hill, J.A., G.C. Melling, and P.M. Johnson, Immunohistochemical studies of human uteroplacental tissues from first-trimester spontaneous abortion. Am J Obstet Gynecol, 1995. 173(1): p. 90-6.

164

Eblen, A.C., et al., Alterations in humoral immune responses associated with recurrent pregnancy loss. Fertil Steril, 2000. 73(2): p. 305-13.

165

Rocklin, R.E., J.L. Kitzmiller, and M.R. Garvoy, Maternal-fetal relation. II. Further characterization of an immunologic blocking factor that develops during pregnancy. Clin Immunol Immunopathol, 1982. 22(3): p. 305-15.

166

Amos, D.B. and D.D. Kostyu, HLA--a central immunological agency of man. Adv Hum Genet, 1980. 10: p. 137-208, 385-6.

167

Coulam, C.B., Immunologic tests in the evaluation of reproductive disorders: a critical review. Am J Obstet Gynecol, 1992. 167(6): p. 1844-51.

168

McIntyre, J.A., et al., Human trophoblast-lymphocyte cross-reactive (TLX) antigens define a new alloantigen system. Science, 1983. 222(4628): p. 1135-7.

169

Roumen, G., et al., A role for TLX antigens in pregnancy. Acta Eur Fertil, 1991. 22(3): p. 181-7.

170

Bjercke, S., Recurrent abortions and lymphocyte transfusions. Acta Obstet Gynecol Scand, 1994. 73(5): p. 373-6.

171

Purcell, D.F., et al., The human cell-surface glycoproteins HuLy-m5, membrane co-factor protein (MCP) of the complement system, and trophoblast leucocyte-common (TLX) antigen, are CD46. Immunology, 1990. 70(2): p. 155-61.

172

Beer, A.E., et al., Major histocompatibility complex antigens, maternal and paternal immune responses, and chronic habitual abortions in humans. Am J Obstet Gynecol, 1981. 141(8): p. 987-99.

173

McIntyre, J.A. and W.P. Faulk, Recurrent spontaneous abortion in human pregnancy: results of immunogenetical, cellular, and humoral studies. Am J Reprod Immunol, 1983. 4(4): p. 165-70.

174

Ober, C., et al., Shared HLA antigens and reproductive performance among Hutterites. Am J Hum Genet, 1983. 35(5): p. 994-1004.

175

Ober, C., et al., Human leukocyte antigen matching and fetal loss: results of a 10 year prospective study. Hum Reprod, 1998. 13(1): p. 33-8.

176

Christiansen, O.B., et al., Maternal HLA class II allogenotypes are markers for the predisposition to fetal losses in families of women with unexplained recurrent fetal loss. Eur J Immunogenet, 1995. 22(4): p. 323-34.

177

Christiansen, O.B., et al., HLA class II alleles confer susceptibility to recurrent fetal losses in Danish women. Tissue Antigens, 1994. 44(4): p. 225-33.

178

Kutteh, W., et al., Increased prevalence of antithyroid antibodies identified in women with recurrent pregnancy loss but not in women undergoing assisted reproduction. Fertil Steril, 1999. 71(5): p. 843-8.

179

Bussen, S. and T. Steck, Thyroid antibodies and their relation to antithrombin antibodies, anticardiolipin antibodies and lupus anticoagulant in women with recurrent spontaneous abortion (antithyroid, anticardiolipin and antithrombin antibodies and lupus anticoagulant in habitual aborters). Eur J Obstet Gyn Reprod Biol, 1997. 74(2): p. 139-43.

180

Esplin, M., et al., Thyroid autoantibodies are not associated with recurrent pregnancy loss. Am J Obstet Gynecol, 1998. 179(6 Pt 1): p. 1583-6.

181

Harris, N., Syndrome of the Black Swan. Br J Rheumatol, 1987. 26: p. 324-6.

182

Negro R, Schwartz A, Gismondi R, et al: Increased pregnancy loss rate in thyroid antibody negative women with TSH levels between 2.5 and 5.0 in the first trimester of pregnancy. J Clin Endocrinol Metab, 2010. 95(9): p. E44-E48.

183

Rushworth, F., et al., Prospective pregnancy outcome in untreated recurrent miscarriers with thyroid autoantibodies. Hum Reprod, 2000. 15(1637-39).

184

Wilson, W., et al., International consensus statement on preliminary classification criteria for definite antiphospholipid syndrome: report of an international workshop. Arthritis Rheum, 1999. 42(7): p. 1309-11.

185

Lockshin, M.D., L.R. Sammaritano, and S. Schwartzman, Validation of the Sapporo criteria for antiphospholipid syndrome. Arthritis Rheum, 2000. 43(2): p. 440-3.

186

Miyakis, S., et al., International consensus statement on an update of the classification criteria for definite antiphospholipid syndrome (APS). J Thromb Haemost, 2006. 4(2): p. 295-306.

187

Out, H.J., et al., A prospective, controlled multicenter study on the obstetric risks of pregnant women with antiphospholipid antibodies. Am J Obstet Gynecol, 1992. 167(1): p. 26-32.

188

Kutteh, W.H., et al., Association of anticardiolipin antibodies and pregnancy loss in women with systemic lupus erythematosus. Fertil Steril, 1993. 60(3): p. 449-55.

189

Hanly, J.G., et al., Lupus pregnancy. A prospective study of placental changes. Arthritis Rheum, 1988. 31(3): p. 358-66.

190

Lockshin, M.D., et al., Antibody to cardiolipin predicts placental insufficiency in pregnant patients with systemic lupus erythematosus. New England Journal of Medicine, 1985. 313: p. 152-156.

191

Harris, E.N., et al., Thrombocytopenia in SLE and related autoimmune disorders: association with anticardiolipin antibody. Br J Haematol, 1985. 59(2): p. 227-30.

192

Cariou, R., et al., Inhibition of protein C activation by endothelial cells in the presence of lupus anticoagulant. N Engl J Med, 1986. 314(18): p. 1193-4.

193

Freyssinet, J.M., et al., An IgM lupus anticoagulant that neutralizes the enhancing effect of phospholipid on purified endothelial thrombomodulin activity--a mechanism for thrombosis. Thromb Haemost, 1986. 55(3): p. 309-13.

194

Gris, J., et al., Antiphospholipid and antiprotein syndromes in non-thrombotic, non-autoimmune women with unexplained recurrent primary early foetal loss. The Nimes Obstetricians and Haematologists Study--NOHA. Thromb Haemost, 2000. 84(2): p. 228-36.

195

Rand, J.H., et al., Reduction of annexin-V (placental anticoagulant protein-I) on placental villi of women with antiphospholipid antibodies and recurrent spontaneous abortion. Am J Obstet Gynecol, 1994. 171(6): p. 1566-72.

196

Rand, J.H., et al., Pregnancy loss in the antiphospholipid-antibody syndrome--a possible thrombogenic mechanism. N Engl J Med, 1997. 337(3): p. 154-60.

197

Cervera R, Balasch J: Autoimmunity and recurrent pregnancy losses. Clin Rev Allergy Immunol, 2010. 39(3): p. 148-152.

198

Lyden, T.W., A.K. Ng, and N.S. Rote, Modulation of phosphatidylserine epitope expression by BeWo cells during forskolin treatment. Placenta, 1993. 14(2): p. 177-86.

199

Bulla, R., et al., Inhibition of trophoblast adhesion to endothelial cells by the sera of women with recurrent spontaneous abortions. Am J Reprod Immunol, 1999. 42(2): p. 116-23.

200

Ernest JM, Marshburn PB, Kutteh WH: Obstetric antiphospholipid syndrome: an update on pathophysiology and management. Semin Reprod Med, 2011. 29(6): p. 522-539.

201

Galli, M., et al., Anticardiolipin antibodies (ACA) directed not to cardiolipin but to a plasma protein cofactor [see comments]. Lancet, 1990. 335(8705): p. 1544-7.

202

McNeil, H.P., et al., Anti-phospholipid antibodies are directed against a complex antigen that includes a lipid-binding inhibitor of coagulation: beta 2- glycoprotein I (apolipoprotein H). Proc Natl Acad Sci U S A, 1990. 87(11): p. 4120-4

203

Chamley, L., J. Allen, and P. Johnson, Synthesis of beta2 glycoprotein 1 by the human placenta. Placenta, 1997. 18(5-6): p. 403-10.

204

Galli, M., et al., Anti-beta 2-glycoprotein I, antiprothrombin antibodies, and the risk of thrombosis in the antiphospholipid syndrome. Blood, 2003. 102(8): p. 2717-23.

205

Bates SM, Ginsberg JS: Anticoagulation in pregnancy. Pharm Pract Manag Q 19: 51– 60, 1999 Mezzesimi, A., et al., The detection of anti-beta2-glycoprotein I antibodies is associated with increased risk of pregnancy loss in women with threatened abortion in the first trimester. Eur J Obstet Gynecol Reprod Biol, 2007. 133(2): p. 164-8.

206

Pengo, V., et al., Antibody profiles for the diagnosis of antiphospholipid syndrome. Thromb Haemost, 2005. 93(6): p. 1147-52.

207

Ebeling, F., et al., Beta-2-glycoprotein I antibodies in patients with thrombosis. Scand J Clin Lab Invest, 2003. 63(2): p. 111-8.

208

Lee, E.Y., et al., Does the anti-beta2-glycoprotein I antibody provide additional information in patients with thrombosis? Thromb Res, 2003. 111(1-2): p. 29-32.

209

Lee, R., et al., Anti-beta2-glycoprotein 1 antibodies in women with recurrent spontaneous abortion, unexplained fetal death, and antiphospholipid syndrome. Am J Obstet Gynecol, 1999. 181(3): p. 642-8.

210

Yetman, D. and W. Kutteh, Antiphospholipid antibody panels and recurrent pregnancy loss: prevalence of anticardiolipin antibodies compared with other antiphospholipid antibodes. Fertil Steril, 1996. 66: p. 540-6.

211

Branch, D., et al., Antiphospholipid antibodies other than lupus anticoagulant and anticardiolipin antibodies in women with recurrent pregnancy loss, fertile controls, and antiphospholipid syndrome. Obstet Gynecol, 1997. 89(4): p. 549-55

212

Rushworth, F., et al., Prospective pregnancy oiutcome in untreated recurrent miscarriers with thyroid autoantibodies. Hum Reprod, 2000. 15(1637-39).

213

Hill, J.A. and B.C. Choi, Maternal immunological aspects of pregnancy success and failure. J Reprod Fertil Suppl, 2000. 55: p. 91-7.

214

Schust, D.J., D.J. Anderson, and J.A. Hill, Progesterone-induced immunosuppression is not mediated through the progesterone receptor. Hum Reprod, 1996. 11(5): p. 980-5.

215

Aoki, K., et al., Pre-conceptional natural-killer-cell activity as a predictor of miscarriage. Lancet, 1995. 345(8961): p. 1340-2.

216

Emmer, P., et al., Peripheral natural killer cytotoxicity and CD56(pos)CD16(pos) cells increase duringearly pregnancy in women with a history of recurrent spontaneous abortion. Hum Reprod, 2000. 15(5): p. 1163-9.

217

Tang AW, Alfirevic Z, Quenby S: Natural killer cells and pregnancy outcomes in women with recurrent miscarriage and infertility: a systematic review. Hum Reprod, 2011. 26(8): p. 1971-1980.

218

Laird SM, Mariee N, Wei L, et al: Measurements of CD56+ cells in peripheral blood and endometrium by flow cytometry and immunohistochemical staining in situ. Hum Reprod, 2011. 26(6): p. 1331-1337.

219

Laird SM, Tuckerman EM, Cork BA, et al: A review of immune cells and molecules in women with recurrent miscarriage. Hum Reprod Update, 2003. 9(2): p. 163-174.

220

Ermel, L.D., P.B. Marshburn, and W.H. Kutteh, Interaction of heparin with antiphospholipid antibodies (APA) from the sera of women with recurrent pregnancy loss (RPL). Am J Reprod Immunol, 1995. 33(1): p. 14-20.

221

Di Simone, N., et al., Low-molecular weight heparin restores in-vitro trophoblast invasiveness and differentiation in presence of immunoglobulin G fractions obtained from patients with antiphospholipid syndrome. Hum Reprod, 1999. 14(2): p. 489-95.

222

Bose, P., et al., Adverse effects of lupus anticoagulant positive blood sera on placental viability can be prevented by heparin in vitro. Am J Obstet Gynecol, 2004. 191(6): p. 2125-31.

223

Di Simone, N., et al., Heparin and low-dose aspirin restore placental human chorionic gonadotrophin secretion abolished by antiphospholipid antibody-containing sera. Hum Reprod, 1997. 12(9): p. 2061-5.

224

Lider, O., et al., Suppression of experimental autoimmune diseases and prolongation of allograft survival by treatment of animals with low doses of heparins. J Clin Invest, 1989. 83(3): p. 752-6.

225

Kutteh, W.H., Antiphospholipid antibody-associated recurrent pregnancy loss: treatment with heparin and low-dose aspirin is superior to low-dose aspirin alone. Am J Obstet Gynecol, 1996. 174(5): p. 1584-9.

226

Thornton CA, Ballow M: Safety of intravenous immunoglobulin. Arch Neurol 50: 135– 136, 1993 Lima, F., et al., A study of sixty pregnancies in patients with the antiphospholipid syndrome. Clin Exp Rheumatol, 1996. 14(2): p. 131-6.

227

Rai, R., et al., Randomized controlled trial of aspirin and aspirin plus heparin in pregnant women with recurrent miscarriage associated with phospholipid antibodies. Br Med J, 1997. 314: p. 253-7.

228

Bates, S.M., et al., Venous thromboembolism, thrombophilia, antithrombotic therapy, and pregnancy: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). Chest, 2008. 133(6 Suppl): p. 844S-886S.

229

Ziakas PD, Pavlou M, Voulgarelis M: Heparin treatment in antiphospholipid syndrome with recurrent pregnancy loss: a systematic review and meta-analysis. Obstet Gynecol, 2010. 115(6): p. 1256-1262.

230

Kaandorp S, Di Nisio M, Goddijn M, et al: Aspirin or anticoagulants for treating recurrent miscarriage in women without antiphospholipid syndrome. Cochran Database Syst Rev, 2009. 1: p. CD004734.

231

Kaandorp SP, Goddijn M, van der Post JA, et al: Aspirin plus heparin or aspirin alone in women with recurrent miscarriage. N Engl J Med, 2010. 362(17): p. 1586-1596.

232

Bijsterveld, N.R., et al., Low-molecular weight heparins in venous and arterial thrombotic disease. Thromb Haemost, 1999. 82 Suppl 1: p. 139-47.

233

Noble, L.S., et al., Antiphospholipid antibodies associated with recurrent pregnancy loss: prospective, multicenter, controlled pilot study comparing treatment with low-molecular-weight heparin versus unfractionated heparin. Fertil Steril, 2005. 83(3): p. 684-90.

234

Bar, J., et al., Low-molecular-weight heparin for thrombophilia in pregnant women. Int J Gynaecol Obstet, 2000. 69(3): p. 209-13.

235

Younis, J.S., et al., The effect of thrombophylaxis on pregnancy outcome in patients with recurrent pregnancy loss associated with factor V Leiden mutation. Bjog, 2000. 107(3): p. 415-9.

236

Brenner, B., et al., Gestational outcome in thrombophilic women with recurrent pregnancy loss treated by enoxaparin. Thromb Haemost, 2000. 83(5): p. 693-7.

237

Pattison, N., et al., Does aspirin have a role in improving pregnancy outcome for women with antiphospholipid syndrome? A randomized controlled trial. Am J Obstet Gynecol, 2000. 183: p. 1008-12.

238

Rai, R., et al., Recurrent miscarriage--an aspirin a day? Hum Reprod, 2000. 15(10): p. 2220-3.

239

Schisterman, Enrique F., et al. Preconception low-dose aspirin and pregnancy outcomes: results from the EAGeR randomised trial. The Lancet, 2014.

240

Boue, J., A. Bou, and P. Lazar, Retrospective and prospective epidemiological studies of 1500 karyotyped spontaneous human abortions. Teratology, 1975. 12(1): p. 11-26.

241

Hill, J.A., Immunotherapy for recurrent pregnancy loss: "Standard of care or buyer beware". Journal of the Society for Gynecologic Investigation, 1997. 4(6): p. 267-273.

242

Stovall, D.W. and B.J. Van Voorhis, Immunologic tests and treatments in patients with unexplained infertility, IVF-ET, and recurrent pregnancy loss. Clin Obstet Gynecol, 1999. 42(4): p. 979-1000.

243

Ferro, D., et al., Successful removal of antiphospholipid antibodies using repeated plasma exchanges and prednisone. Clin Exp Rheumatol, 1989. 7(1): p. 103-4.

244

Lubbe, W.F., et al., Fetal survival after prednisone suppression of maternal lupus-anticoagulant. Lancet, 1983. 1(8338): p. 1361-3.

245

Laskin, C.A., et al., Prednisone and aspirin in women with autoantibodies and unexplained recurrent fetal loss [see comments]. N Engl J Med, 1997. 337(3): p. 148-53.

246

Hunt, J., et al., Female steroid hormones regulate production of pro-inflammatory molecules in uterine leukocytes. J Reprod Immunol, 1997. 35(2): p. 87-99.

247

Zaffaroni, M. and A. Ghezzi, The prognostic value of age, gender, pregnancy and endocrine factors in multiple sclerosis. Neurol Sci, 2000. 21(4 Suppl 2): p. S857-60.

248

Piccinni, M.P., et al., Defective production of both leukemia inhibitory factor and type 2 T-helper cytokines by decidual T cells in unexplained recurrent abortions. Nat Med, 1998. 4(9): p. 1020-4.

249

Choi, B.C., et al., Progesterone inhibits in-vitro embryotoxic Th1 cytokine production to trophoblast in women with recurrent pregnancy loss. Hum Reprod, 2000. 15 Suppl 1: p. 46-59.

250

Oates-Whitehead, R.M., D.M. Haas, and J.A. Carrier, Progestogen for preventing miscarriage. Cochrane Database Syst Rev, 2003(4): p. CD003511

251

Samuelsson, A., T.L. Towers, and J.V. Ravetch, Anti-inflammatory activity of IVIG mediated through the inhibitory Fc receptor. Science, 2001. 291(5503): p. 484-6.

252

Mollnes, T.E., et al., High-dose intravenous immunoglobulin treatment activates complement in vivo. Scand J Immunol, 1998. 48(3): p. 312-7.

253

Jerzak, M., T. Rechberger, and A. Gorski, Intravenous immunoglobulin therapy influences T cell adhesion to extracellular matrix in women with a history of recurrent spontaneous abortions. Am J Reprod Immunol, 2000. 44(6): p. 336-41.

254

Dwyer, J.M., Manipulating the immune system with immune globulin. N Engl J Med, 1992. 326(2): p. 107-16.

255

Thornton, C.A. and M. Ballow, Safety of intravenous immunoglobulin. Arch Neurol, 1993. 50(2): p. 135-6.

256

Mueller-Eckhardt, G., O. Heine, and B. Polten, IVIG to prevent recurrent spontaneous abortion. Lancet, 1991. 337(8738): p. 424-5.

257

Stricker, R.B., et al., Successful treatment of immunologic abortion with low-dose intravenous immunoglobulin. Fertil Steril, 2000. 73(3): p. 536-40.

258

Jablonowska, B., et al., Prevention of recurrent spontaneous abortion by intravenous immunoglobulin: a double-blind placebo-controlled study. Hum Reprod, 1999. 14(3): p. 838-41.

259

Daya, S., et al., Critical analysis of intravenous immunoglobulin therapy for recurrent miscarriage. Hum Reprod Update, 1999. 5(5): p. 475-82.

260

Stephenson, M.D., et al., Prevention of unexplained recurrent spontaneous abortion using intravenous immunoglobulin: a prospective, randomized, double-blinded, placebo-controlled trial. Am J Reprod Immunol, 1998. 39(2): p. 82-8.

261

Christiansen, O.B., Intravenous immunoglobulin in the prevention of recurrent spontaneous abortion: the European experience. Am J Reprod Immunol, 1998. 39(2): p. 77-81.

262

Perino, A., et al., Short-term therapy for recurrent abortion using intravenous immunoglobulins: results of a double-blind placebo-controlled Italian study. Hum Reprod, 1997. 12(11): p. 2388-92.

263

Coulam, C.B., et al., Intravenous immunoglobulin for treatment of recurrent pregnancy loss. Am J Reprod Immunol, 1995. 34(6): p. 333-7.

264

Branch, D., et al., A multi-center, placebo-controlled pilot study of intravenous immune globulin treatment of antiphospholipid syndrome during pregnancy. The Pregnancy Loss Study Group. Am J Obstet Gynecol, 2000. 182(1 Pt 1): p. 122-7.

265

Vaquero, E., et al., Pregnancy outcome in recurrent spontaneous abortion associated with antiphospholipid antibodies: a comparative study of intravenous immunoglobulin versus prednisone plus low-dose aspirin. Am J Reprod Immunol, 2001. 45(3): p. 174-9.

266

Harris, E.N. and S.S. Pierangeli, Utilization of intravenous immunoglobulin therapy to treat recurrent pregnancy loss in the antiphospholipid syndrome: a review. Scand J Rheumatol Suppl, 1998. 107: p. 97-102.

267

Stephenson MD, Kutteh WH, Purkiss S, et al: Intravenous immunoglobulin and idiopathic secondary recurrent miscarriage: a multicentered randomized placebo-controlled trial. Hum Reprod, 2010. 25(9): p. 2203-2209.

268

Johnson, P.M. and G.H. Ramsden, Recurrent miscarriage. Ballieres Clin Immunol Allergy, 1992. 2: p. 607-624.

269

Thaler, C.J., Immunological role for seminal plasma in insemination and pregnancy. Am J Reprod Immunol, 1989. 21(3-4): p. 147-50.

270

Coulam, C.B. and J.J. Stern, Seminal plasma treatment of recurrent spontaneous abortion, in Reproductive Immunology. Serono Symposia, F. Dondero and P.M. Johnson, Editors. 1993, Raven Press: New York. p. 257-262.

271

Agrawal, S., M.K. Pandey, and A. Pandey, Prevalence of MLR blocking antibodies before and after immunotherapy. J Hematother Stem Cell Res, 2000. 9(2): p. 257-62

272

Prigoshin, N., et al., Microchimerism and blocking activity in women with recurrent spontaneous abortion (RSA) after alloimmunization with the partner's lymphocytes. J Reprod Immunol, 1999. 44(1-2): p. 41-54.

273

Ito, K., et al., Possible mechanisms of immunotherapy for maintaining pregnancy in recurrent spontaneous aborters: analysis of anti-idiotypic antibodies directed against autologous T-cell receptors. Hum Reprod, 1999. 14(3): p. 650-5.

274

Gafter, U., et al., Suppressed cell-mediated immunity and monocyte and natural killer cell activity following allogeneic immunization of women with spontaneous recurrent abortion. J Clin Immunol, 1997. 17(5): p. 408-19.

275

Fraser, E.J., D.A. Grimes, and K.F. Schulz, Immunization as therapy for recurrent spontaneous abortion: a review and meta-analysis. Obstet Gynecol, 1993. 82(5): p. 854-9.

276

Group, T.R.M.I.T., Worldwide collaborative observational study and meta-analysis on allogenic leukocyte therapy for recurrent spontaneous abortion. American Journal of Reproductive Immunology, 1994. 32: p. 55-72.

277

Hill, J.A. and D.J. Anderson, Blood transfusions for recurrent abortion: is the treatment worse than the disease? [letter]. Fertil Steril, 1986. 46(1): p. 152-4.

278

Hofmeyr, G.J., et al., Immunologic investigation of recurrent pregnancy loss and consequences of immunization with husbands' leukocytes. Fertil Steril, 1987. 48(4): p. 681-4.

279

Katz, I., et al., Cutaneous graft-versus-host-like reaction after paternal lymphocyte immunization for prevention of recurrent abortion. Fertil Steril, 1992. 57(4): p. 927-9.

280

Mowbray, J.F., et al., Controlled trial of treatment of recurrent spontaneous abortion by immunisation with paternal cells. Lancet, 1985. 1(8435): p. 941-3.

281

Daya, S. and J. Gunby, The effectiveness of allogeneic leukocyte immunization in unexplained primary recurrent spontaneous abortion. Recurrent Miscarriage Immunotherapy Trialists Group [see comments]. Am J Reprod Immunol, 1994. 32(4): p. 294-302.

282

Ober, C., et al., Mononuclear-cell immunication in prevention of recurrent miscarriage: a randomized trial. Lancet, 1999. 354: p. 365-9.

283

Rodger, J.C., Lack of a requirement for a maternal humoral immune response to establish or maintain successful allogeneic pregnancy. Transplantation, 1985. 40(4): p. 372-5.

284

Ossa, J.E., A.P. Cadavid, and J.G. Maldonado, Is the immune system necessary for placental reproduction? A hypothesis on the mechanisms of alloimmunotherapy in recurrent spontaneous abortion. Med Hypotheses, 1994. 42(3): p. 193-7.

285

Croy, B.A., The application of scid mouse technology to questions in reproductive biology. Lab Anim Sci, 1993. 43(2): p. 123-6.

286

Regan, L., P.R. Braude, and P.L. Trembath, Influence of past reproductive performance on risk of spontaneous abortion. British Medical Journal, 1989. 299: p. 541-545.