Elicitation of the maternal antibody response to the fetus by a broadly shared MHC class I antigenic determinant

Polymorphism of thePaGene

PROBLEM

This study was undertaken to identify the number of alleles of the Pa gene at the DNA level and to correlate the presence of the different alleles with the ability of a strain to elicit an anti-Pa antibody response when mated with a WF female. The Pa gene is present in both Pa+ and Pa- strains of rats, but it has unique restriction fragment length polymorphisms (RFLP) in the two types of strains: 1.7 kb in Pa- and 1.8 kb in Pa+XbaI digests using a probe derived from the Pa gene.

RESULTS

Examination of DNA from a variety of strains using different enzymes showed that there were characteristic RFLP patterns for Pa+ and Pa- strains. Strains of the b haplotype, however, had an intermediate RFLP pattern, and all of these strains had a relatively low level of reactivity with anti-Pa antibody.

CONCLUSIONS

Thus, there are three alleles of the Pa gene based on their level of expression: Paa, high; Pab, low; and Pa-, none.

A rat model of pregnancy-induced sensitization to transplants

Previous pregnancy is a known risk factor for alloantibody production and graft rejection in clinical transplantation. However, in previous rat models, immune responses to RT1.A antigens induced by allogeneic pregnancy resulted in prolonged survival of subsequent allografts. This study was designed to investigate the effects of a previous pregnancy on alloantibody response, complement activation, and allograft survival in a highly immunogenic rat strain combination. C6-sufficient and -deficient female PVG.1U (RT1.A(u)B(u)) rats were mated with allogeneic PVG.R8 (RT1.A(a)B(u)) males or control isogeneic PVG.1U (RT1.A(u)B(u)) males. Three weeks after parturition, experimental and control females received cardiac allografts from female PVG.R8 donors. A low dose of cyclosporine (CsA, 5 mg/kg on alternate days) was used for immunosuppression after transplantation. Allogeneic, but not control isogeneic, pregnancy elicited a weak, transient IgG alloantibody response that declined before transplantation. Experimental female recipients produced a rapid, vigorous IgM and IgG alloantibody response to the transplant despite CsA treatment. C6-sufficient recipients rejected their transplants at an accelerated rate (5 days, n = 6) compared with control animals (7 days, n = 5). In contrast, allografts to C6-deficient recipients functioned until sacrifice at 90 days in both the experimental group (n = 7) and control group (n = 4). Most experimental C6-deficient recipients continued to produce strong IgG alloantibodies for 90 days. Complement activation resulting from the alloantibody response was evidenced by the diffuse deposition of C3d on the vascular endothelium of the grafts. In summary, previous pregnancy leads to memory alloantibody responses that accelerate allograft rejection even with immunosuppression. Membrane attack complex is required for accelerated rejection induced by previous pregnancy.

Co-dominant expression of the HLA-G gene and various forms of alternatively spliced HLA-G mRNA in human first trimester trophoblast

Genes may be silenced at the transcriptional level by 'genomic imprinting' in such a way that only one of the parental alleles is expressed. Imprinting may be tissue-specific and in some cases it seems also to be time-dependent during development. The phenomenon has been studied in pre- and post-implantation developmental processes. Animal studies of genomic imprinting of major histocompatibility complex (MHC) antigens in the placenta have shown discordant results. To address this issue in the human placenta, we examined the expression of the non-classical human leukocyte antigen (HLA) class I gene, HLA-G. Genomic imprinting of the HLA-G locus could have implications for the interaction in the feto-maternal relationship. Restriction Fragment Length Polymorphism (RFLP), allele-specific amplification and Single Strand Conformation Polymorphism (SSCP) analysis followed by DNA sequencing were performed on Reverse Transcription (RT) Polymerase Chain Reaction (PCR) products of HLA-G mRNA to examine the expression of maternal and paternal alleles. Our results demonstrate that HLA-G is co-dominantly expressed in first trimester trophoblast cells. A "new" non-synonymous base substitution in exon 4 was detected. We also investigated the different alternatively spliced forms of HLA-G mRNA in first trimester trophoblast and found the full-length transcript to be the far most abundant.

Analysis of the expression and immunostimulatory capacity of class I major histocompatibility antigens on rat trophoblast cell lines

In rat strains expressing the a and other major histocompatibility complex (MHC) haplotypes, subpopulations of placental trophoblast cells synthesize the nonclassical class I Pa antigen in preference to the classical RT1.Aa antigen. In this study, a rat trophoblast cell line, R8RP.3, which was derived from midgestation placentas of PVG.R8 (RT1.Aa) rats, was shown to express class I antigens similarly to those of trophoblast cells in situ. Both unstimulated and IFN-gamma-exposed metabolically labeled R8RP.3 cells synthesized more Pa than RT1.Aa antigen. The reverse was true for labeled spleen cells from PVG.R8 rats. The R8RP.3 cells failed to stimulate allogeneic lymphocyte proliferation even when high levels of both classical and nonclassical class I MHC antigens were expressed on their membranes after incubation with IFN-gamma. These experiments thus supply the first evidence that the inductive phase of the immune response is not promoted by trophoblast cell class I MHC antigens, which could explain the failure of mothers to mount immune responses to class I MHC positive trophoblast cells.

Morphologic changes in the rat uterus following natural mating and embryo transfer

In order to gain some insight into the putative immune suppression that may be induced at the placental implantation sites, the morphological changes at these sites following natural matings and following the transfer of embryos fertilized in vivo were studied. The only histologic parameter that showed a significant difference was the number of granulated metrial gland (GMG) cells. More GMG cells were present in allogeneic than in syngeneic pregnancies, and more GMG cells were present following embryo transfer into an allogeneic female than following the comparable natural mating. The role of the GMG cells in pregnancy is, however, still unresolved.

Expression and analysis of the rat placental class I cDNA clone encoding the Pa antigen

The previously sequenced cDNA clone pARI.5 was recloned into the mammalian expression vector pcEXV3, and transient and permanent transfectants were prepared in COS7 green monkey kidney fibroblasts. The transfectants were analyzed by indirect immunofluorescence using monoclonal and polyclonal antibodies raised in specifically selected rat strain combinations. These studies showed that pARI.5 encodes the Pa antigen and that the Pa molecule is distinct from the Aa molecule. Probes were derived from the pARI.5 clone and used to study the genomic DNA from Pa-positive and Pa-negative strains. Two probes derived from the 3' untranslated region (3'apARI.5 and 3'bpARI.5) and one probe derived from the 5' region (5'pARI.5) hybridized nonspecifically in all strains under moderate stringency conditions. By contrast, an Xba I restriction fragment unique to the Pa gene was detected with the 5'pARI.5 probe under high stringency conditions. This probe hybridized with a 1.8 kilobase (kb) fragment in the Pa-positive strains and with a 1.7 kb band in the Pa-negative strains. These studies suggest that the gene encoding the Pa antigen, or a fragment thereof, is present in both Pa-positive and Pa-negative strains but may not be expressed in the latter.

Gene imprinting and major histocompatibility complex class I antigen expression in the rat placenta

Ultrastructural immunocytochemical studies of the expression of major histocompatibility complex class I antigens in the placentas of inbred rats were performed using placentas derived from natural matings and from embryo transfers into females made pseudopregnant by mechanical stimulation. The studies utilized the WF (u) and DA (a) strains and monoclonal antibodies to all of the class I antigens involved. All four mating combinations of the two strains showed that only paternal antigens were expressed in the placenta and that they were limited to the basal trophoblast. This conclusion was confirmed using embryo transfer experiments. In allogeneic natural matings, the allele-specific class I transplantation antigens were not expressed on the membrane of the basal trophoblast but they were expressed in embryo transfers involving embryos of the same genotype. In both types of pregnancies, the pregnancy-associated (Pa) antigen was present on the membrane of the basal trophoblast. The antibody response to DA X DA and to WF X DA embryos transferred into pseudopregnant WF females was against the allele-specific RT1.Aa antigen and the Pa antigen, whereas the antibody response to the heterozygous embryo in the natural WF (female) X DA (male) mating was against the Pa antigen only. These results are consistent with the hypothesis that the suppression of the expression of the allele-specific major histocompatibility complex class I antigens occurs shortly after fertilization and that it requires the uterine environment of a natural mating.

Major histocompatibility complex class I antigens expressed on rat trophoblast cells

There is controversy about the size of the major histocompatibility complex antigens of trophoblast cells from placenta. There are some reports that the heavy chains of these molecules are smaller (39-43 kd) than those of the classical class I antigens (45-46 kd), while there are others which show that both the light and the heavy forms of class I antigens occur in the trophoblast cells. In order to investigate this problem, we studied the classical class I antigen (RT1.Aa) and the pregnancy-associated class I antigen (Pa) of the rat from 125I-labeled basal trophoblast cells, isolated from the placenta of WF females pregnant by DA males, using very mild conditions. These antigens were compared with those of the syngeneic (DA x DA) trophoblast cells or paternal (DA) lymphocytes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Both the Aa and Pa antigens, precipitated from the two trophoblast preparations, showed a heavy chain of 46 kd associated with a 12 kd beta 2-microglobulin component, as did the same molecules precipitated from the lymphocytes. Heavy chains in the range of 39-43 kd could not be detected in any of the samples. The results suggest that the smaller molecular weight heavy chains are methodological artifacts and could arise from loss of a glycan(s) during isolation.

The rat as an experimental animal

The development and characterization of many inbred, congenic, and recombinant strains of rats in recent years has led to the detailed genetic description of this species, especially in regard to its major histocompatibility complex. This information has contributed substantially to the study of comparative genetics and has greatly enhanced the utility of the rat in a variety of areas of biomedical research. This article focuses on the use of the rat in immunogenetics, transplantation, cancer-risk assessment, cardiovascular diseases, and behavior.

Characterisation of the humoral immune response during murine pregnancy

Blood samples from female C57BL/10 mice mated with CBA/Ca males were obtained before, during and after both first and second pregnancies. A cellular enzyme-linked immunospecific assay (CELISA) was used to detect maternal antibodies against antigens on paternal splenocytes. Alloantibodies were detected in 48% of mice during or 9 days after a first pregnancy and in 82% of mice by the ninth day after the second pregnancy; these antibodies were first observed on day 10 of the first pregnancy. In two of four active multigravid sera tested, an increase in IgG1 concentration was detected; the level of all other isotypes remained within normal limits. Weak binding of alloantibody to an antigen of approximate molecular weight 44,000 was detected on CBA/Ca splenocytes by immunoblotting sera from multiparous animals. These sera also recognised an antigen of similar molecular weight on H-2b identical 129J splenocytes but not on splenocytes from the maternal strain. These results provide further information on the maternal humoral immune response during murine pregnancy.

Isolation and characterization of rat trophoblast cells

Previous immunohistochemical studies of the rat placenta using specific alloantisera and/or monoclonal antibodies showed that the basal zone trophoblasts stained for Pa and Aa class I major histocompatibility complex (MHC) antigens and for the human SP1-related antigen. In an effort to isolate the basal zone trophoblast cells from the rat placenta, we used these markers to assess the degree of purification of the cells separated by density gradient centrifugation using either Ficoll-Hypaque or Percoll as the gradient medium. The cells were put either on the top or at the bottom of discontinuous density gradients in the range of 1.005-1.10 g/ml. The cell separation profiles for the two media were different. With Percoll, most of the trophoblast cells (80-95%) were collected at the density gradients 1.04/1.06 and 1.06/1.08, whereas with Ficoll-Hypaque, these gradients separated only a small fraction (4-23%) of the trophoblast cells, and most of them pelleted at the bottom of the tube. The trophoblast cells separated by Ficoll-Hypaque, however, showed fewer contaminant cells than those separated by the Percoll gradients.

Electron microscopic localization of the Pa and RT1.Aa antigens on the placenta of the rat

A major factor in the ability of the placenta to avoid allograft rejection is the differential expression of MHC class I antigens on its surface. Using monoclonal antibodies and the electron microscopic immunogold technique, we have demonstrated that only the pregnancy-associated (Pa) antigen, which carries a broadly shared antigenic determinant, is expressed on the placental surface in the rat, whereas the allele-specific classical transplantation antigens are not. Both types of antigens are, however, present in the cytoplasm of the basal trophoblast but completely absent from the labyrinthine trophoblast.

Differential expression of MHC class I antigens on the placenta of the rat. A mechanism for the survival of the fetal allograft

In some mating combinations in rats, there is a maternal antibody response to the maternal antigenic components of the placenta without any previous immunization of the mother. The highest response occurs in the WF (u) female mated to the DA (a) male, and it is against a unique MHC-encoded class I antigen, the Pa antigen, and not against the major allele-specific transplantation antigen of the DA strain, RT1.Aa. The development of mAbs to the Pa and Aa antigens allowed us to localize these antigens on the placenta and to explore the reason for the differential antibody response to them using immunohistochemical and biochemical techniques. Both antibodies reacted with the WF X DA placenta and stained the endovascular and interstitial trophoblast of the decidua, the basal trophoblast, Reichert's membrane, and the yolk sac epithelium, but they did not stain the labyrinthine trophoblast. Blocking studies showed that each antibody reacted with a separate molecule in the placenta. Anti-class II mAbs reactive with the a or u haplotype did not stain the WF X DA, DA X DA, or WF X WF placenta; hence, there are no class II antigens in the placenta. Electron microscopic studies of the semiallogeneic WF X DA placenta using the immunogold technique with both single- and double-labeling showed that only the Pa antigen was expressed on the surface of the basal trophoblast, but that both the Pa and Aa antigens were in the cytoplasm of these cells; neither antigen was found in the labyrinthine trophoblast. By contrast, the placenta from the syngeneic DA X DA mating expressed both the Pa and Aa antigens on the surface of the basal trophoblast as well as in the cytoplasm; neither antigen was found in the labyrinthine trophoblast. These observations were quantified morphometrically using electron photomicrographs of single-labeled tissues. Both the Pa and Aa antigens isolated from the plasma membrane of lymphocytes have heavy chains of 46 kD, but those antigens isolated from the plasma membrane of basal trophoblast cells have heavy chains of 43 kD. Based on densitometric measurements of autoradiographs, the Pa/Aa ratio in the basal trophoblast membrane is 23.5, whereas it is 0.46 in lymphocyte membranes. These studies show that there is differential regulation of the expression of class I antigens on basal trophoblast cells in semiallogeneic pregnancies, but not in syngeneic pregnancies, such that the major allele-specific transplantation antigen is scarcely expressed on the surface of the basal trophoblast.(ABSTRACT TRUNCATED AT 400 WORDS)

Alterations in immunological reactivity during pregnancy in mice determined in vitro by lymphoproliferation tests

Primiparous and multiparous mice, either syngeneically or allogeneically pregnant, were sacrificed at various stages of pregnancy, and the immune reactivity of cells from different lymphatic organs was analyzed by mixed lymphocyte reaction (MLR) and concanavalin A (Con A)-induced lymphocyte proliferation. In the MLR, spleen cells and cells from the axillary lymph nodes of pregnant animals showed similar changes in their response to allogeneic cells during the course of gestation. In comparison to age-matched virgin controls they had an increased reactivity between the 7th and 11th day of pregnancy. During the preimplantation period and the last week of pregnancy, their alloreactivity was comparable to that of the controls. Cells from the para-aortic lymph nodes, which drain the uterus, also showed an increased reactivity at mid-gestation, but in the preimplantation period and third week of pregnancy their alloreactivity was even decreased in comparison to the control animals. When T cell immunocompetence was measured by means of lymphocyte transformation induced with Con A, the pattern of reactivity was completely different. Lymphoid cells from all the above-mentioned tissues showed the highest response to Con A during the preimplantation period, constantly weakening towards term.

Immunological and genetic factors influencing development and susceptibility to cancer

The results of a variety of studies on the genetic and immunological aspects of reproduction can be integrated into a hypothesis about the factors that regulate implantation and development and that may also cause an increased susceptibility to cancer. The primary condition for successful reproduction is genetic compatibility between the mating partners: there must be no recessive lethal genes that could act alone or epistatically to cause embryonic or fetal death. Such recessive lethal genes have been identified in the mouse (t-haplotypes) and in the rat (grc), and there is some evidence that they also exist in humans. Immunological factors may modulate the implantation of the fertilized ovum under some circumstances after the genetic condition has been met. The same genetic factors that affect development may also affect susceptibility to cancer. This part of the hypothesis is supported by a number of clinical correlations between congenital defects and a higher incidence of cancer and by the demonstration of an increased susceptibility to the effects of chemical carcinogens in rats carrying the grc.

Antibody isotype produced by pregnant mice immunised with fetal allogeneic lymphocytes is mainly IgG1

Virgin and Pregnant BALB/c mice were immunized three times with (BALB/c X CBA)F1 fetal or adult lymphocytes. An enzyme linked immunosorbent assay (ELISA) was developed for analysing the isotypes and specificities of the alloantibody produced. Pregnant mice immunized with fetal lymphocytes showed an immunodeviation towards IgG1 production compared with other groups.

Ontogeny of expression of Pa and RT1.Aa antigens on rat placenta and on fetal tissues

The unique pregnancy-associated (Pa) antigen, which is a class I antigen encoded by the major histocompatibility complex (MHC), elicits a nondestructive maternal antibody response. By contrast, the class I transplantation antigen RT1.Aa elicits a destructive antibody response in tissue transplantation but not during pregnancy. With the use of the avidin-biotin complex (ABC) immunohistochemical method, the Pa and RT1.Aa antigens were localized on the basophilic and giant cells of the basal zone trophoblast, the endovascular trophoblast and decidual interstitial trophoblast, and the chorioallantoic membrane but not on the labyrinthine zone trophoblast as early as the 12th day of gestation. These two antigens were also expressed on the epidermis, hair follicles, spleen, thymic medulla, bronchial epithelium, intestinal epithelium, the hepatic Kupffer cells, endocardium, endothelium of blood vessels, renal tubular cells and glomeruli, and renal pelvis and ureter of fetal and adult rat tissues. Absorption studies with placental tissue confirmed the presence of these two antigens in the rat placenta, and antibody-blocking studies confirmed their unique specificities.

Comparison of rat MHC class I antigens by peptide mapping

Monoclonal antibodies specific for the rat major histocompatibility complex (MHC) class I antigens RT1.An, RT1.Au, and RT1.Eu were used for immunoprecipitation of antigens biosynthetically radiolabeled with 14C- or 3H-labeled arginine, lysine, and tyrosine; with arginine or tyrosine alone; and with or without tunicamycin in the culture medium. Heavy chains of the glycosylated and unglycosylated antigens were purified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and their tryptic and chymotryptic peptides were compared by high performance liquid chromatography. The antigens coded by the same locus in two different haplotypes (An and Au) differed by 30%, whereas the products of two different loci in the same haplotype (Au and Eu) differed only by 1-3%. Comparative analysis of the data for samples labeled with single amino acids indicated that two amino acids in Au have been substituted by an arginine and probably by a tyrosine residue, respectively, in Eu. The high degree of homology between the products of the A and E loci in the same haplotype accounts for the difficulty in detecting recombinational events within the MHC of the rat by classical serological approaches.

Antigen expression by trophoblast populations in the human placenta and their possible immunobiological relevance

Antigen expression by villous and extravillous human trophoblast populations at discrete anatomical sites has been reviewed. The various different antigenic phenotypes have been highlighted using a panel of monoclonal antibodies reactive with characteristic trophoblast membrane antigens, a trophoblast-leucocyte common antigen, class I MHC antigens, epithelial cell cytokeratin and epithelial membrane markers. This approach has allowed three separate fetal trophoblast populations to be identified within term amniochorionic membranes, and also has facilitated further definition of trophoblast populations in maternal uterine tissues. Furthermore, antigenic alterations have been noted in the maternal uterine gland epithelium in pregnancy leading to the expression of a trophoblastic phenotype, thereby suggesting a mechanism of extrinsic regulation of gene expression in these tissues. The possible involvement in the immunoregulatory control of maternal responses in pregnancy of MHC-linked gene products expressed by trophoblast has been discussed.