ANALYSIS OF MHC CLASS-I EXPRESSION IN EQUINE TROPHOBLAST CELLS USING IN-SITU HYBRIDIZATION

Down-regulation of major histocompatibility complex (MHC) genes by tro phoblast cells is considered to be a primary mechanism preventing mate rnal immune rejection of the fetal-placental unit in mammalian pregnan cy by rendering these cells, which form the primary barrier between mo ther and fetus, relatively non-antigenic. In situ hybridization with p robes encoding human and horse MHC class I genes was used to character ize the pattern of MHC class I mRNA expression in the various forms of horse trophoblast. Strong hybridization signals were observed in the invasive trophoblast cells of chorionic girdle tissue. In contrast, no hybridization signal specific for MHC class I mRNA transcripts was ob served in the descendent endometrial cup trophoblast cells. In the non -invasive trophoblast cells of the allantochorion, no hybridization si gnals specific for horse MHC class I mRNA transcripts were consistentl y detected. In parallel to the in vivo results, strong hybridization s ignals were observed in the small, mononuclear cells present in chorio nic girdle cell explant cultures, but not in the population of large b inucleate cells corresponding to endometrial cup cells. The results ob tained using in situ hybridization are consistent with the hypothesis that expression of MHC class I genes may be controlled at the transcri ptional level in horse invasive and non-invasive trophoblast cells, an d suggest that down-regulation of MHC class I antigen expression in en dometrial cup cells may be accomplished by the same mechanisms in vivo and in vitro. (C) 1996 W. B. Saunders Company Ltd