Ontogeny and distribution of cells expressing HLA-B locus-specific determinants in the placenta and extraplacental membranes

Immunology of normal pregnancy

Since Medawar's initial contemplations in 1953 on the mechanisms of immune evasion allowing for the survival of the allogeneic conceptus in an immunologically competent mother, physicians and immunologists alike have struggled to understand the immunological paradox of pregnancy. Ultimately, our attempts to define the immunology of normal pregnancy have broadened our appreciation of the myriad mechanisms at play that enable the promotion of implantation and maintenance of pregnancy. In this review, we summarise what is known regarding the immunology of normal pregnancy, with special emphasis on the relation to common disorders of pregnancy.

Antigenic heterogeneity of human cytotrophoblast and evidence for the transient expression of MHC class I antigens distinct from HLA-G

Expression of MHC class I antigens on trophoblast populations in first trimester human chorionic villous tissue was assessed by immunohistology. Antibodies used were W6/32 which recognizes a non-polymorphic framework determinant of HLA- A, -B, -C, MHM5 specific for HLA-B, C and 4E and B23.1 which are specific for HLA-B. Syncytiotrophoblast and villous cytotrophoblast were negative with all the anti (HLA class I) antibodies tested. Interstitial trophoblast cells within the maternal decidua were identified with a new antibody, NDOG5, which is specific for extravillous cytotrophoblast. Double labelling showed that they bind W6/32 but not 4E, MHM5 or B23.1; consistent with the expression of the monomorphic HLA-G. In contrast the cytotrophoblast cells of the cell islands and cytotrophoblast shell, which also express the NDOG5 antigen, were positive with W6/32, 4E, MHM5 and B23.1. Cell column cytotrophoblast cells were negative with all four MHC class I antibodies. These results suggest that differentiation of cytotrophoblast from noninvasive to invasive forms is associated with transient expression of class I antigens other than HLA-G on cytotrophoblast shell and cell island cytotrophoblast.

Detection of class I MHC mRNA in subpopulations of first trimester cytotrophoblast cells by in situ hybridization

Class I MHC mRNA has been identified in both villous and extravillous cytotrophoblast cells in first trimester placentas by in situ hybridization. In this report, we expand those observations to additional morphologically and anatomically distinct subpopulations of trophoblast cells in early placentas using the same experimental approach. In the transition zone of first trimester placental villi, where cytotrophoblast cells are proliferating to form new villi or to migrate into adjacent tissue, both cytotrophoblast cells beneath the uninterrupted syncytium and the proliferating cytotrophoblast cells contained class I mRNA whereas a layer of cytotrophoblast cells proximal to the villus core did not contain class I mRNA. In the placental bed, migrating cytotrophoblast cells contained high levels of class I mRNA as determined by the intensity of staining. In contrast, multinucleated giant trophoblast cells contained little specific message. Alterations in levels of class I mRNA seem therefore to be associated with trophoblast proliferation, migration and differentiation.

Evasive strategies of trophoblast cells: selective expression of membrane antigens

Contemporary experimental tools, particularly those of molecular biology, have allowed some insights on the selective display of membrane antigens by trophoblast cells. The unique observations offered by reproduction immunologists may have wider application than expected inasmuch as tumor cells may use some of the same mechanisms to avoid recognition and destruction by host immune cells and antibodies. Thus, further pursuit of the conditions that prevail in pregnancy may lead to a better understanding of the mechanisms used by "foreign" cells to coexist peaceably with host cells, bypassing immune mechanisms designed for their destruction.

Allogeneic recognition and rejection reactions in the placenta

One of the major questions in medicine is how human extraembryonic tissues escape rejection by maternal immune responses, because these tissues are allogeneic and should stimulate allogeneic recognition and rejection reactions. Evidence for maternal allogeneic recognition of extraembryonic tissues in chorionic villi, basal plate, and spiral arteries is presented. Extraembryonic membranes seem always to stimulate maternal allogeneic recognition and rejection reactions, and abnormal pregnancies appear to be associated with a simple quantitative increase of the same placental immunopathological lesions found in normal pregnancies.

Amniochorion: immunologic aspects--a review

Exploration of trophoblast cell gene expression may assist in elucidating the mechanisms responsible for allowing genetically disparate maternal and fetal cells to coexist during pregnancy. In the extraplacental membranes, chorionic cytotrophoblast cells are in direct contact with maternal cells. In theory, paternally derived major histocompatibility antigens (HLA) expressed by the chorion cells should stimulate a graft rejection response by the mother, yet there is no evidence for lymphocytic infiltration of the membranes. The results of recent in situ hybridization, Northern blotting, and other molecular studies suggest that failure of maternal immune cells to attack the membranes may be due to the ability of chorion cells selectively to transcribe class I HLA genes and/or to process the products of those genes differently from other types of cells. Inasmuch as some tumor cells exhibit patterns of class I HLA that are similar to those of trophoblast cells, regulation of class I HLA expression may be a general mechanism used by cells expressing non-self antigens (paternally derived HLA, tumor-specific antigens) to establish residency in host tissues.