Humoral immune responses in murine pregnancy. I. Anti-paternal alloantibody levels in maternal serum

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.

Toward an understanding of allogeneic conflict in pregnancy and transplantation

Pregnancy is recognized as a spontaneously acquired state of immunological tolerance by the mother to her semi-allogeneic fetus, but it is a major cause of allosensitization in candidates for organ transplantation. This sensitization, assessed by the presence of anti-HLA IgG, contributes to sex disparity in access to transplantation and increases the risk for rejection and graft loss. Understanding this dual tolerance/sensitization conundrum may lead to new strategies for equalizing access to transplantation among sexes and improving transplant outcomes in parous women. Here, we review the clinical evidence that pregnancy results in humoral sensitization and query whether T cell responses are sensitized. Furthermore, we summarize preclinical evidence on the effects of pregnancy on fetus-specific CD4⁺ conventional, regulatory, and CD8⁺ T cells, and humoral responses. We end with a discussion on the impact of the divergent effects that pregnancy has upon alloantigen re-encounter in the context of solid organ transplantation, and how these insights point to a therapeutic roadmap for controlling pregnancy-dependent allosensitization.

The effect of skin allografting on the equine endometrial cup reaction

This research tested the hypothesis that immunological sensitization of mares by skin allografting, followed by the establishment of pregnancy using semen from the skin-graft donor, would give rise to secondary immune responses to the developing horse conceptus, resulting in an earlier demise of the fetally derived endometrial cups. Maiden mares received skin allografts from a stallion homozygous for Major Histocompatibility Complex (MHC) antigens and/or equivalent autografts and were subsequently mated to the skin-graft donor stallion during the next two breeding seasons. Mares that had been immunologically primed to the foreign MHC class I antigens of the skin-graft donor stallion developed strong secondary antibody responses early in their first pregnancies, whereas autografted mares made weak primary antibody responses in their first pregnancies and strong secondary responses in their second pregnancies. In contrast, histological examination of the endometrial cups after surgical pregnancy termination at Day 60 of gestation revealed no discernible differences between allografted and autografted mares, and there were no significant differences in the concentrations and/or duration of secretion of the endometrial cup-specific hormone, equine chorionic gonadotrophin (eCG), between allografted and autografted mares, nor in either group between first and second pregnancies. The vigorous antibody response observed in the pregnant allografted mares supported the first part of our hypothesis, providing evidence of systemic immunological priming. However, there was a lack of an equivalent heightened cellular response to the endometrial cups. These findings provided strong evidence for an asymmetric immune response to the conceptus, characterized by strong humoral immunity and a dampened cellular response.

Modulation of Allospecific CTL Responses During Pregnancy in Equids: An Immunological Barrier to Interspecies Matings?

Maternal immune recognition of the developing conceptus in equine pregnancy is characterized by the strongest and most consistent alloantibody response described in any species, a response directed almost exclusively against paternal MHC class I Ags. This work investigated the cellular immune response to paternal MHC Ags in pregnant and nonpregnant horses and donkeys, and in horses carrying interspecies hybrid mule conceptuses. We observed profound decreases in classical, MHC-restricted, CTL activity to allogeneic paternal cells in peripheral blood lymphocytes from both horse mares and donkey jennets carrying intraspecies pregnancies, compared with cells from nonpregnant controls. This is the first evidence in a randomly bred species for a generalized systemic shift of immune reactivity away from cellular and toward humoral immunity during pregnancy. Surprisingly, mares carrying interspecies hybrid mule conceptuses did not exhibit this transient, pregnancy-associated decrease in CTL activity. The failure of interspecies pregnancy to down-regulate cellular immune responses may be a heretofore-unrecognized, subtle barrier to reproductive success between species.

Fetal-maternal immune interaction: blocking antibody and survival of the fetus

In the late 1940s it became clear that the homograft reaction was essentially the result of an immune response. Subsequently, Medawar commented on the apparent paradox of the survival of the mammalian fetus in the face of such a potential (cell-mediated) immune response. In an outbred population the fetal-placental unit will be antigenically different to the mother by virtue of its complement of paternal genes and additionally there may be developmental or stage-specific gene products that are immunogenic. Many mechanisms have been proposed to account for the survival of the fetus in the face of a potential immune attack and, while many of these have been investigated in considerable detail, there has been no clear-cut indication that any one plays a predominant role. Either control of immune rejection of the fetus is exercised by an as yet undiscovered mechanism or, more probably, by a combination of some or all of the mechanisms that have been proposed by many workers over the last three decades. Potential controlling processes, which will be reviewed briefly, include: systemic and local modification of maternal responsiveness; altered expression of MHC antigens on extra-embryonic tissues; the placenta as a barrier; and blocking antibody responses. We discuss some of our recent studies in which we have started to look for potential blocking antibodies in a mouse model system. Cells secreting immunoglobulins M and G, characterized in hemolytic plaque assays, have been mapped to areas close to the midgestation mouse embryo, using an immunocryohistological technique. A scaled-down version of hybridoma technology has been used as an analytical probe of the specificity and isotype of immunoglobulin secreted by cells originating either from close to the embryo/fetus or from the para-aortic lymph nodes (PALN). So far monoclonal (IgG1) antibodies with specificity for embryonic cells have been derived together with some monoclonal immunoglobulins with as yet uncharacterized antibody specificity.

Immune reactivity of allogeneically pregnant mice to paternal MHC antigens on fetal and placental cells assessed by second set rejection of ascites tumor cells

In vivo immunogenicity of fetus- and placenta-derived cells as well as the immune reactivity of pregnant mice to fetal cells were examined for graft rejecting response (GRR). Systemic administration of small numbers of fetal cells but not placental cells from allogeneically pregnant mice (10(6) cells per mouse) or adult allogeneic spleen cells (10(4) cells) sensitized mice for second-set rejection of an ascitic tumor bearing paternal major histocompatibility complex (MHC) antigens. Despite this fact and the known positive humoral response, pregnant and parous mice are not even minimally sensitized with fetal MHC antigens for GRR transplacentally. Nevertheless, any pregnancy-related systemically active control, which would selectively prevent the mother from being sensitized for GRR by limiting numbers of semi-allogeneic fetal cells, was not demonstrable in either allogeneically or syngeneically pregnant mice. Irrespective of pregnancy, mice did not, however, respond to repeated administration of very small numbers of allogeneic spleen cells (5 X 10(2) cells per mouse) for graft rejection. These findings support the notion that deviation of maternal immunity to fetal antigens away from harmful GRR is mediated principally by local mechanisms which inhibit fetal cells from gaining access to the mother for GRR, and additionally by the innate inability of mice to respond to very small numbers of allogeneic cells that might escape past the local maternal-fetal barrier.

Influence of paternal immunity on idiotype expression of offspring

The immune response of the rat to group A streptococcal carbohydrate (SACHO) and an associated idiotype, Id-1, was used to examine the effect of paternal immunity on Id-1 and SACHO-specific antibody expression by the offspring. First litters, conceived before immunization of the father, had significantly higher Id-1 levels than litters conceived by the same parental pairs after hyperimmunization of the father (P greater than 0.01). Total anti-SACHO levels were not affected. The effect appeared to be independent of the level of Id-1 expressed by the father or grandfather. No significant difference in Id-1 production was found between offspring of actively immune, neonatally Id-1 suppressed fathers and fathers expressing high levels of Id-1. We suggest that the paternal immunoregulatory influence acts via the maternal immune system to modify the idiotype repertoire expressed in the immune response of the offspring, and is not the result of genetic transmission of a trait acquired by the father. Some possible mechanisms of transmission are discussed.

Humoral immune responses in murine pregnancy. IV. Strain dependency and alloantibody specificity

The relationship between the ability of female inbred mice to produce an anti-paternal humoral immune response to allogeneic multiparity and the genotype of the female and male strains has been investigated. Only three, all H-2b haplotype strains, were 'responder' strains and produced anti-paternal alloantibody which did not exhibit C'-dependent cytotoxicity. 'Non-responder' strains produced no alloantibody in spite of multiple pregnancies with H-2 and non-H-2 incompatible male strains. However, even responder strains did not produce alloantibody with all incompatible male strains. The absence of a response in a responder strain mated with a male strain differing at only the H-2 locus implicated a role for non-H-2 influences. A study of the specificity of pregnancy-induced alloantibody suggested that this represented only a fraction of the total alloantibody population induced by conventional immunization in the same strain combination. It is suggested that in pregnancy only the anti-non-H-2 humoral responses remain similar to those induced by immunization, whereas anti-H-2 humoral responses are either absent or are restricted to a fraction of the total foreign H-2 specificities presented. These observations are discussed in relation to the nature of the immunogenic stimulus in pregnancy and the expression of histocompatibility antigens on the feto-placental unit.

In vitro susceptibility of mouse placental trophoblast to cytotoxic effector cells

The susceptibility of mouse placental cells to hyperimmune cell killing directed against paternal combined H-2 and non-H-2 antigens was investigated using [3H]uridine as a terminal label in an in vitro cell-mediated microcytotoxicity test. The stage of development of the conceptus from which the short-term placental cell monolayers were prepared determined their susceptibility to immune cell lysis. Cultures from whole placentae taken at 9 days post-coitum (p.c.) were not significantly affected whereas similar monolayers prepared at 10.5 days p.c. or later underwent extensive destruction. Embryonic fibroblasts were susceptible at all stages examined from 9-16 days p.c. The onset of susceptibility correlates with the reported appearance of H-2 antigens on the placenta during ontogeny. All cultures of dissected populations of 13-day p.c. placental spongiotrophoblast were susceptible but only 70% of those of labyrinthine trophoblast. It is suggested that of the two major trophoblastic components of the mouse placenta the spongiotrophoblast expresses antigens that render it vulnerable to maternal immune attack whilst the labyrinthine trophoblast is only weakly or non-antigenic, with the observed killing being due largely to contamination of these cultures by antigenic foetal mesenchymal elements. Since failure to express appropriate target antigens cannot be the reason for the in vivo survival of the spongiotrophoblast it must be presumed that some form of immunoregulatory mechanism(s) is responsible for the maintenance of the foeto-placental allograft.