Localization of fetal major histocompatibility complex antigens and maternal leukocytes in murine placenta. Implications for maternal-fetal immunological relationship.

An immunohistochemical study of the days 14-19 murine placenta and uterus using a panel of antibodies specific for maternal and paternal class I major histocompatibility complex (MHC) antigens, specific leukocyte subsets, and other lineage-specific markers was performed to elucidate the nature of the maternal tolerance of the fetal implant in the uterus. Fetally derived trophoblast lining maternal blood spaces lacked MHC antigens, but other interstitial trophoblasts expressed fetally derived polymorphic class I MHC determinants. The latter class I MHC-bearing trophoblasts were most prominent in the maternal decidua basalis where they were mixed with maternal cells. Our results identify two factors that may be relevant for understanding why these alloantigen-bearing fetal cells are not rejected by the mother: a paucity of la-bearing antigen presenting cells, macrophages, and mature T and B lymphocytes and the presence of large numbers of Thy-1+ asialo-GM-1+ CD4- CD8- bone marrow-derived (CLA/Ly5+) cells of maternal origin in the decidua basalis. These latter cells correspond to previously described granulated metrial gland cells that may be involved in local immunoregulation.