Transgenic major histocompatibility complex class I antigen expressed in mouse trophoblast affects maternal immature B cells

B cells: roles in physiology and pathology of pregnancy

B cells constitute a diverse and adaptable immune cell population with functions that can vary according to the environment and circumstances. The involvement of B cells in pregnancy, as well as the associated molecular pathways, has yet to be investigated. This review consolidates current knowledge on B cell activities and regulation during pregnancy, with a particular focus on the roles of various B cell subsets and the effects of B cell-derived factors on pregnancy outcomes. Moreover, the review examines the significance of B cell-associated autoantibodies, cytokines, and signaling pathways in relation to pregnancy complications such as pregnancy loss, preeclampsia, and preterm birth.

Isolation and characterization of uterine leukocytes collected using a uterine swab technique

PROBLEM

Leukocytes from the maternal-fetal interface are a valuable tool to study local changes in immune function during pregnancy; however, sampling can be challenging due to inadequate tissue availability and the invasive nature of placental bed biopsy. Here, we aim to purify and characterize leukocytes from paired peripheral and uterine blood samples to assess whether a less invasive method of uterine blood collection could yield a population of enriched uterine leukocytes suitable for ex vivo and in vitro analyses.

METHOD OF STUDY

Human peripheral blood mononuclear cells (PBMC) and uterine blood mononuclear cells (UBMC) expressed from surgical gauze post C-section were isolated, and immunophenotypic information was acquired by multi-parameter flow cytometry. PBMC and UBMC were stained for markers used to define T and B lymphocytes, macrophages, regulatory T (T_Reg ) cells, and natural killer (NK) cells. Prime flow was performed to check expression and analysis of CD16^- CD56⁺⁺ and CD16^- CD56++ NK transcripts in PBMC and UBMC samples.

RESULTS

Immunophenotyping revealed that over 95% of both live PBMC and UBMC consisted of CD45⁺ leukocytes. Higher percentages of CD16^- CD56⁺⁺ , characterized as uterine NK (uNK) cells, were observed in UBMC samples as compared to PBMC samples (18.41% of CD45⁺ CD3^- vs. 2.73%, respectively), suggesting that CD16^- CD56⁺⁺ cells were enriched in these samples. In UBMC, 49.64% of CD3-negative cells were of peripheral NK phenotype (CD16⁺ CD56⁺⁺ ), suggesting infiltration of maternal peripheral NK (pNK) cell in the uterine interface.

CONCLUSION

Intrauterine leukocytes, especially CD16^- CD56⁺⁺ NK cells, can be collected in sufficient numbers with increased purity by sampling the uterine cavity postdelivery with surgical gauze. Our results suggest that this non-invasive protocol is a useful sampling technique for isolating CD16^- CD56⁺⁺ cells, however, due to peripheral blood contamination, the NK cell yield could be lower compared to actual decidual or endometrial samples post-partum which is more invasive.

Fetal‐placental antigens and the maternal immune system: Reproductive immunology comes of age

Reproductive physiology and immunology as scientific disciplines each have rich, largely independent histories. The physicians and philosophers of ancient Greece made remarkable observations and inferences to explain regeneration as well as illness and immunity. The scientific enlightenment of the renaissance and the technological advances of the past century have led to the explosion of knowledge that we are experiencing today. Breakthroughs in transplantation, immunology, and reproduction eventually culminated with Medawar’s discovery of acquired immunological tolerance, which helped to explain the transplantation success and failure. Medawar’s musings also keenly pointed out that the fetus apparently breaks these newly discovered rules, and with this, the field of reproductive immunology was launched. As a result of having stemmed from transplantation immunology, scientist still analogizes the fetus to a successful allograft. Although we now know of the fundamental differences between the two, this analogy remains a useful tool to understand how the fetus thrives despite its immunological disparity with the mother. Here, we review the history of reproductive immunology, and how major and minor histocompatibility antigens, blood group antigens, and tissue‐specific “self” antigens from the fetus and transplanted organs parallel and differ.

Trophoblast antigens, fetal blood cell antigens, and the paradox of fetomaternal tolerance

The paradox of fetomaternal tolerance has puzzled immunologists and reproductive biologists alike for almost 70 yr. Even the idea that the conceptus evokes a uniformly tolerogenic immune response in the mother is contradicted by the long-appreciated ability of pregnant women to mount robust antibody responses to paternal HLA molecules and RBC alloantigens such as Rh(D). Synthesizing these older observations with more recent work in mice, we discuss how the decision between tolerance or immunity to a given fetoplacental antigen appears to be a function of whether the antigen is trophoblast derived-and thus decorated with immunosuppressive glycans-or fetal blood cell derived.

Immune Cells at the Fetomaternal Interface: How the Microenvironment Modulates Immune Cells To Foster Fetal Development

Immune cells adapt their phenotypic and functional characteristics in response to the tissue microenvironment within which they traffic and reside. The fetomaternal interface, consisting of placental trophoblasts and the maternal decidua, is a highly specialized tissue with a unique and time-limited function: to nourish and support development of the semiallogeneic fetus and protect it from inflammatory or immune-mediated injury. It is therefore important to understand how immune cells within these tissues are educated and adapt to fulfill their biological functions. This review article focuses on the local regulatory mechanisms ensuring that both innate and adaptive immune cells appropriately support the early events of implantation and placental development through direct involvement in promoting immune tolerance of fetal alloantigens, suppressing inflammation, and remodeling of maternal uterine vessels to facilitate optimal placental function and fetal growth.

Plasma Cell Alloantigen 1 and IL-10 Secretion Define Two Distinct Peritoneal B1a B Cell Subsets With Opposite Functions, PC1high Cells Being Protective and PC1low Cells Harmful for the Growing Fetus

B cells possess various immuno regulatory functions. However, research about their participation in tolerance induction toward the fetus is just emerging. Accumulating evidence supports the idea that B cells can play seemingly conflicting roles during pregnancy, either protecting or harming the fetus. Previous findings indicated the presence of two different peritoneal B cell subsets, defined by the expression of the plasma cell alloantigen 1 (PC1) and with distinct immune modulatory functions. Here, we aimed to study the participation of these two B cell subsets, on pregnancy outcome in a murine model of disturbed fetal tolerance. The frequencies and cell numbers of peritoneal and splenic CD19⁺IL-10⁺ and CD19⁺CD5⁺IL-10⁺PC1⁺ cells were assessed in virgin as well as normal pregnant (NP) and abortion-prone (AP) females during the course of gestation. Peritoneal PC1^low or PC1^high B1a B cells were sorted, analyzed for their ability to secrete IL-10 and adoptively transferred into NP or AP females. On gestation day (gd) 12, the abortion rate as well as the frequencies and cell numbers of regulatory T cells, TH1 and TH17 cells were determined in spleens and decidua. In addition, mRNA expression of IL-10, TGF-β, IFN-γ, and TNF-α was analyzed in decidual tissue. Peritoneal CD19⁺IL-10⁺ and CD19⁺CD5⁺IL-10⁺PC1⁺ frequencies fluctuated during the progression of normal pregnancies while no significant changes were observed in spleen. AP females showed significantly reduced frequencies of both B cell populations and exhibited an altered peritoneal PC1^high/PC1^low ratio at gd10. Adoptive transfers of PC1^low B1a B cells into NP females increased the abortion rate in association with a reduced splenic regulatory T/TH17 ratio. By contrast, the transfer of PC1^high B1a B cells into AP females significantly diminished the fetal rejection rate and significantly reduced the numbers of splenic TH17 cells. Our results suggest that the peritoneum harbors two distinct B1a B cell subsets that can be distinguished by their PC1 expression. Whereas PC1^high B1a B cells seem to support fetal survival, PC1^low cells B1a B cells may compromise fetal well-being.

Maternal Serum B-Cell Activating Factor Levels

Hypertensive disorders of pregnancy are a leading cause of maternal and perinatal morbidity and mortality. Early suppression of B-cell lymphopoiesis is necessary for a normal pregnancy. Dysregulation of factors critical to B-cell survival may result in pregnancy complications, including hypertension. In this prospective observational study at a single medical center, serum levels of BAFF (B-cell activating factor) were measured in pregnant participants at each trimester, at delivery, and postpartum and in nonpregnant controls at a single time point. Comparisons were made between nonpregnant and pregnant subjects and between time periods of pregnancy. First-trimester serum BAFF levels were further tested for association with hypertensive disorders of pregnancy. The study included 149 healthy pregnant women, 25 pregnant women with chronic hypertension, and 48 nonpregnant controls. Median first-trimester serum BAFF level (ng/mL) for healthy women (0.90) was lower than median serum BAFF levels for women with chronic hypertension (0.96; P =0.013) and controls (1.00; P =0.002). Serum BAFF levels steadily declined throughout pregnancy, with the median second-trimester level lower than the corresponding first-trimester level (0.77; P =0.003) and the median third-trimester level lower than the corresponding second-trimester level (0.72; P =0.025). The median first-trimester serum BAFF level was elevated in women who subsequently developed hypertension compared with women who remained normotensive (1.02 versus 0.85; P =0.012), with the area under the receiver operating characteristic curve being 0.709. First-trimester serum BAFF level may be an early and clinically useful predictor of hypertensive disorders of pregnancy.

Maternal vaccination: moving the science forward

BACKGROUND

Infections remain one of the leading causes of morbidity in pregnant women and newborns, with vaccine-preventable infections contributing significantly to the burden of disease. In the past decade, maternal vaccination has emerged as a promising public health strategy to prevent and combat maternal, fetal and neonatal infections. Despite a number of universally recommended maternal vaccines, the development and evaluation of safe and effective maternal vaccines and their wide acceptance are hampered by the lack of thorough understanding of the efficacy and safety in the pregnant women and the offspring.

METHODS

An outline was synthesized based on the current status and major gaps in the knowledge of maternal vaccination. A systematic literature search in PUBMED was undertaken using the key words in each section title of the outline to retrieve articles relevant to pregnancy. Articles cited were selected based on relevance and quality. On the basis of the reviewed information, a perspective on the future directions of maternal vaccination research was formulated.

RESULTS

Maternal vaccination can generate active immune protection in the mother and elicit systemic immunoglobulin G (IgG) and mucosal IgG, IgA and IgM responses to confer neonatal protection. The maternal immune system undergoes significant modulation during pregnancy, which influences responsiveness to vaccines. Significant gaps exist in our knowledge of the efficacy and safety of maternal vaccines, and no maternal vaccines against a large number of old and emerging pathogens are available. Public acceptance of maternal vaccination has been low.

CONCLUSIONS

To tackle the scientific challenges of maternal vaccination and to provide the public with informed vaccination choices, scientists and clinicians in different disciplines must work closely and have a mechanistic understanding of the systemic, reproductive and mammary mucosal immune responses to vaccines. The use of animal models should be coupled with human studies in an iterative manner for maternal vaccine experimentation, evaluation and optimization. Systems biology approaches should be adopted to improve the speed, accuracy and safety of maternal vaccine targeting.

The immune privilege of testis and gravid uterus: same difference?

The fetus in the gravid uterus and the developing spermatogenic cells in the adult testis both comprise special challenges for the host immune system. Protection of the neoantigens of the fetus and male germ cells from immune attack, defined as immune privilege, is fundamental for the propagation of species. Immune privilege is not simply the absence of leukocytes, but involves immune and non-immune cells acting synergistically together at multiple levels to create a unique tolerogenic environment. A number of the pathways are shared by the testis and gravid uterus. Amongst them steroid hormones, namely testosterone in the male and progesterone in the female, seem to function as key molecules that govern the local production of immunoregulatory factors which finally control the overall immune environment.

Maternal CD4⁺ and CD8⁺ T cell tolerance towards a fetal minor histocompatibility antigen in T cell receptor transgenic mice

Tolerance of the maternal immune system in pregnancy is important for successful pregnancy because the semiallogeneic fetus may be subject to antifetal responses. We examined maternal tolerance to the fetus using a murine system in which a model paternally inherited antigen, ovalbumin (OVA), is expressed exclusively in the fetus and placenta. By employing T cell receptor (TCR) transgenic mice specific for major histocompatibility complex class I- or class II-restricted epitopes of OVA (OT-I and OT-II) as mothers, we investigated the fate of fetus-specific CD8⁺ and CD4⁺ T cells, respectively, during gestation. Both OVA-specific CD8⁺ and CD4⁺ T cells displayed an activated phenotype in the peripheral lymphoid tissues of OVA-bred OT-I and OT-II mice, consistent with their encounter of fetal antigen. Whereas a small percentage of OVA-specific CD4⁺ T cells were deleted in the periphery and thymus of OVA-bred OT-II mice, with evidence of TCR downregulation in the remaining T cells, deletion and TCR downregulation were not observed in OVA-bred OT-I mice. Both CD4⁺ and CD8⁺ T cells upregulated inducible costimulator expression in response to the fetal antigen, but only CD4⁺ T cells consistently upregulated the inhibitory receptors programmed cell death 1 and cytotoxic T lymphocyte antigen-4. More regulatory T cells (Tregs) were present in pregnant OVA-bred than in WT-bred OT-II mice, revealing that Tregs expanded specifically in response to the fetal antigen. These data indicate that several mechanisms tolerize fetal antigen-specific maternal CD4⁺ T cells, whereas tolerance of fetal antigen-specific CD8⁺ T cells is less effective. The importance of these mechanisms is underscored by the finding that fetal loss occurs in OVA-bred OT-I but not OT-II mice.

To B or not to B cells-mediate a healthy start to life

Maternal immune responses during pregnancy are critical in programming the future health of a newborn. The maternal immune system is required to accommodate fetal immune tolerance as well as to provide a protective defence against infections for the immunocompromised mother and her baby during gestation and lactation. Natural immunity and antibody production by maternal B cells play a significant role in providing such immunoprotection. However, aberrations in the B cell compartment as a consequence of maternal autoimmunity can pose serious risks to both the mother and her baby. Despite their potential implication in shaping pregnancy outcomes, the role of B cells in human pregnancy has been poorly studied. This review focuses on the role of B cells and the implications of B cell depletion therapy in pregnancy. It highlights the evidence of an association between aberrant B cell compartment and obstetric conditions. It also alludes to the potential mechanisms that amplify these B cell aberrances and thereby contribute to exacerbation of some maternal autoimmune conditions and poor neonatal outcomes. Clinical and experimental evidence suggests strongly that maternal autoantibodies contribute directly to the pathologies of obstetric and neonatal conditions that have significant implications for the lifelong health of a newborn. The evidence for clinical benefit and safety of B cell depletion therapies in pregnancy is reviewed, and an argument is mounted for further clinical evaluation of B cell-targeted therapies in high-risk pregnancy, with an emphasis on improving neonatal outcomes and prevention of neonatal conditions such as congenital heart block and fetal/neonatal alloimmune thrombocytopenia.

Impact of mouse pregnancy on thymic T lymphocyte subsets

It has been reported that fetal lymphoid progenitor cells are acquired during gestation and are able to develop in the maternal mouse thymus into functional T cells. Moreover, previous pregnancies increase the number of fetal cells in the mother. In the present study, we investigated whether mouse pregnancy induces changes in T lymphocyte subsets in the maternal thymus. We determined the T lymphocyte subsets in two allogeneic cross-breedings, namely CBA/J × BALB/c (normal) and CBA/J × DBA/2 (abortion prone), and investigated the effects of the age and parity of the female, as well as pregnancy outcome, on thymocyte populations. In addition, hormonal effects were evaluated in a syngeneic combination (CBA/J × CBA/J). We found that during pregnancy both hormonal and allogeneic stimuli induced a reduction in the CD4+CD8+ subset with an increase in the CD4+CD8– population. Only young females of the normal combination exhibited an increase in the CD4–CD8+ population. All young mice showed an increase in CD4+CD25+FoxP3+ T cells. Interestingly, the γδT thymus pool was increased in all females of the normal allogeneic pregnancy only, suggesting the participation of this pool in the observed beneficial effect of multiparity in this cross-breeding. Our results demonstrate that allogeneic pregnancies induce important variations in maternal thymocyte subpopulations depending on the age of the female and the male component of the cross-breeding.

Review: Fetal antigens--identity, origins, and influences on the maternal immune system

Pregnancy induces priming of the maternal cellular and humoral immune systems. The paternally-inherited fetal antigens that influence maternal T and B cells include both major and minor histocompatibility antigens - the same antigens that are problematic in allotransplantation. Animal models have facilitated our understanding of the lymphocyte responses to fetal antigens, and our appreciation of the parallel response in pregnant women is increasing. The physiologic properties of the placenta as well as trafficking of cells between mother and fetus allow ample opportunity for sampling of fetal proteins by the maternal immune system. Here, the current state of knowledge of fetal antigen-specific lymphocyte responses in pregnancy is reviewed.

Split immunological tolerance to trophoblast

Split immunological tolerance refers to states in which an individual is capable of mounting certain types of immune responses to a particular antigenic challenge, but is tolerant of the same antigen in other compartments of the immune system. This concept is applicable to the immunological relationship between mother and fetus, and particularly relevant in equine pregnancy. In pregnant mares, antibody responses to paternal foreign Major Histocompatibility Complex class I antigens are robust, while anti-paternal cytotoxic T cell responses are diminished compared to those mounted by non-pregnant mares. Here, we compared the distribution of the major lymphocyte subsets, the percentage of lymphocytes expressing Interferon Gamma (IFNG) and Interleukin 4 (IL4) and the level of expression of the immunoregulatory transcription factor FOXP3 between pregnant and non-pregnant mares, and between peripheral blood and the endometrium during pregnancy. In a cohort of mares in which peripheral blood lymphocytes were tested during early pregnancy and in the non-pregnant state, there were only slight changes observed during pregnancy. In contrast, comparison of peripheral blood lymphocytes with lymphocytes isolated from the endometrial cups of pregnant mares revealed striking differences in lymphocyte sub-populations. The endometrial cups contained higher numbers of IFNG+ lymphocytes, and lower numbers of lymphocytes expressing IL4. The endometrial cup lymphocytes also had higher numbers of FOXP3+ cells compared to peripheral blood lymphocytes. Taken together, these results strengthen the evidence for a state of split tolerance to trophoblast, and furthermore define sharp differences in immune reactivity during equine pregnancy between peripheral blood lymphocytes and lymphocytes at the maternal-fetal interface.

The hidden maternal-fetal interface: events involving the lymphoid organs in maternal-fetal tolerance

The genetic disparity between the mother and fetus has long enticed immunologists to search for mechanisms of maternal tolerance to fetal antigens. The study of antigen-specific tolerance in murine and human pregnancy has gained new momentum in recent years through the focus on antigen-presenting cells, uterine lymphatics and fetal antigen-specific maternal T cell responses. In mice, we now know that these responses occur within the secondary lymphoid structures as they can be conveniently tracked through the use of defined, often transgenic fetal antigens and maternal T cell receptors. Although the secondary lymphoid organs are sites of both immunization and tolerization to antigens, the immunological processes that occur in response to fetal antigens during the healthy pregnancy must invariably lead to tolerance. The molecular properties of these maternal-fetal tolerogenic interactions are still being unraveled, and are likely to be greatly influenced by tissue-specific microenvironments and the hormonal milieu of pregnancy. In this article, we discuss the events leading to antigen-specific maternal tolerance, including the trafficking of fetal antigens to secondary lymphoid organs, the properties of the antigen-presenting cells that display them to maternal T lymphocytes, and the nature of the ensuing tolerogenic response. Experimental data generated from human biological specimens as well as murine transgenic models are considered.

Peripheral deletion of mature alloreactive B cells induced by costimulation blockade

Alloreactive B cells can contribute to graft rejection. Anti-CD154 treatment together with donor-specific transfusion (DST) results in the long-term survival of MHC-mismatched mouse heart grafts and inhibition of alloantibody production. To characterize the mechanism of B cell tolerance induced by the anti-CD154 and DST, we used 3-83Igi mice, on BALB/c (H-2K(d)) background, that express a B cell receptor that reacts with MHC class I antigens H-2K(b). Transplanting C57BL/6 (H-2K(b)) hearts into 3-83Igi mice, followed by tolerance induction, resulted in the peripheral deletion of mature but not immature 3-83 B cells. The sustained deletion of mature alloreactive B cells required the presence of the allograft and can be explained by the absence of T cell help.

Maternal acceptance of the fetus: true human tolerance

Induction and maintenance of immunologic tolerance in humans remains a desirable but elusive goal. Therefore, understanding the physiologic mechanisms of regulation of immune responses is highly clinically relevant for immune-mediated diseases (e.g., autoimmunity and asthma/allergy) and for cell and organ transplantation. Acceptance of the fetus, which expresses paternally inherited alloantigens, by the mother during pregnancy is a unique example of how the immune system reshapes a destructive alloimmune response to a state of tolerance. Understanding the complex mechanisms of fetomaternal tolerance has important implications for developing novel strategies to induce immunologic tolerance in humans in general and for prevention of spontaneous abortion in at-risk populations in particular.

Bi-directional cell trafficking between mother and fetus in mouse placenta

It is now well established that cells are exchanged between mother and fetus during gestation. It has been proposed that some of these exchanges take place in the placenta, but it has never been demonstrated. Here, we made use of EGFP (Enhanced Green Fluorescent Protein) transgenic mice to precisely visualize the juxtaposition of maternal and fetal tissues at the implantation site, as well as to describe the bi-directional cell trafficking between mother and fetus at different stages of gestation. The influence of genetic differences between mother and fetus on the cell migration was also addressed by studying various types of matings: syngeneic, allogeneic and outbred. The frequency of maternal-fetal cell exchanges within the placenta is much higher in syngeneic and allogeneic gestations than in outbred ones. Maternal cells were mainly localized in the labyrinth where they were scattered or sometimes grouped in or near blood spaces. Groups of maternal cells could also be observed in maternal blood sinuses of the spongiotrophoblast. Conversely, fetal cells were organized in rings surrounding maternal blood sinuses in the decidua at 10-12 days of gestation. After day 13, they invaded the decidua. Fetal cells could also be detected in maternal peripheral blood and organs by nested PCR and fluorescence microscopy on cryosections, respectively. This suggests a role in the establishment and maintenance of the maternal tolerance to the fetus.

Immune interactions at the maternal–fetal interface

Models of murine allogeneic pregnancy have established that maternal T cells recognize fetal alloantigens and are normally suppressed or deleted. While the precise cellular interactions and mechanisms involved in maternal lymphocyte tolerance are not yet clear, the identity of some of the critical factors are beginning to be uncovered. Signals that have been shown in mice to have an obligatory role in immunological survival of the semiallogeneic fetus include, but are probably not limited to, indoleamine-2,3-dioxygenase and the newly discovered B7 family protein, B7-H1. Whether these proteins have intersecting functions is unknown, but it is possible that both are involved in the control of maternal T regulatory cells, which are also strictly required for successful allogeneic pregnancy in mice. Additional factors that are involved include trophoblast and/or maternally derived FasL, and in humans, class Ib HLA molecules. The potency of these mechanisms in protecting the fetal allograft is underscored by the scarcity of knockout and transgenic models in which pregnancy is immunologically compromised. Here, the current understanding of mechanisms of specific suppression of maternal lymphocytes is reviewed.

Nonhuman primate placental MHC expression: a model for exploring mechanisms of human maternal-fetal immune tolerance

Placental contributions to the establishment of maternal-fetal immune tolerance, and placental influences on maturation and vascular development of the endometrium in the human have been difficult to explore directly. Although significant differences exist in organization and relevant gene expression between human and nonprimate placentas, the nonhuman primate has substantial potential to provide insights into the physiology of human pregnancy and maternal-fetal immune tolerance. In this report, we will summarize major histocompatability complex class I gene expression in the nonhuman primate placenta and present progress in characterizing the immune cells resident in the primate endometrium. Finally, we will outline new experimental approaches for modifying placental function now available to move research forward in this field.

Major histocompatibility antigen expression on the bovine placenta: its relationship to abnormal pregnancies and retained placenta

In viviparous animals, regulation of expression of major histocompatibility complex (MHC) class I antigens by the trophoblast cells, which constitute the outermost layer of the placenta, seems to be critical for maternal immunological acceptance of an allogeneic fetus. Cattle are unusual in this regard, since the bovine trophoblast cells, in specific regions of the uterine/placental interface, normally express MHC class I antigens during the third trimester of gestation. This expression appears to be biologically relevant as MHC class I compatibility between a cow and her fetus has been associated with an increased incidence of placental retention. We have found significant differences in lymphocyte populations, cytokine production, and trophoblast cell apoptosis in the placentomes of MHC-compatible and -incompatible pregnancies at parturition. This suggests that maternal immunological recognition of fetal MHC class I proteins triggers an immune/inflammatory response that contributes to placental separation at parturition in cattle. Early in pregnancy, a complete shutdown of MHC class I expression by trophoblast cells appears to be critical for normal placental development and fetal survival. In bovine somatic cell nuclear transfer (SCNT) pregnancies, there is an extremely high rate of fetal loss between days 30 and 90 of pregnancy. We have shown that in bovine SCNT pregnancies, between days 34 and 63 of gestation, there is both abnormal expression of MHC class I antigens by trophoblast cells and an abnormal accumulation of lymphocytes within the uterine stroma. Consequently, it is likely that activation of the maternal mucosal immune system, within the uterus at the same time when placentomes are being established, interferes with the process of placentome development and leads to immune-mediated abortion. Our data suggest that bovine MHC-compatible pregnancies provide a unique model for studying regulation of the uterine immune system, as well as immune-mediated placental rejection.

Pregnancy initiation in the rhesus macaque: towards functional manipulation of the maternal-fetal interface

Nonhuman primates provide an important opportunity to define the mechanisms that contribute to the success of early pregnancy. We have focused for several years now on defining the expression of novel placental major histocompatibility complex (MHC) class I molecules. In parallel, we have used reagents against human immune cell markers to characterize the leukocyte population in the decidua and have demonstrated dynamic changes in these cell populations during the first 5 weeks of gestation. The challenge is to identify the possible role(s) of placental MHC class I in modifying/directing the maternal endometrial or systemic immune system in the post-implantation period. Foremost among the challenges is the difficulty in modifying placental function. In the instance of trophoblast surface proteins, passive immunization studies are feasible, although limitations include the empirical nature of this approach, as well as the inability to modify intracellular function. We have shown that using lentiviral vectors to effect preimplantation gene transfer for transgene expression in the placenta is not only feasible, but of good efficiency. In addition to transgene overexpression, robust approaches for knocking down/knocking out placental gene expression are essential. Recent developments in RNA interference approaches may allow "transient knockout" experiments. While the rhesus monkey has been our model of choice, currently there are limitations in the number of available female rhesus monkeys of reproductive age for research in early pregnancy. It is critical that the technologies for advanced study move forward in other species. The baboon has been used significantly in reproductive tract biology and early pregnancy research and important models have been developed for manipulation of the maternal-fetal interface. Additional characterization of other species, such as the cynomolgus and African green (vervet) monkey is critical. Given the limitations on antigen recognition when using human reagents, we also propose that the development of panels of primate-specific anti-leukocyte antibodies is essential for moving forward nonhuman primate reproductive research.

Tolerance of the fetus by the maternal immune system: role of inflammatory mediators at the feto-maternal interface

The adaptive immune system of placental mammals has evolved to tolerate the fetus. Rejection of the fetus by adaptive immune responses is therefore a rare event, with abortion being caused more frequently by inflammation in the placenta. This review will cover recent aspects of immune privilege and the innate immune system at the feto-maternal interface, citing examples of the role played by microbial infections in fetal demise.