Analysis of MHC class I expression in equine trophoblast cells using in situ hybridization

Down-regulation of major histocompatibility complex (MHC) genes by trophoblast cells is considered to be a primary mechanism preventing maternal immune rejection of the fetal-placental unit in mammalian pregnancy by rendering these cells, which form the primary barrier between mother and fetus, relatively non-antigenic. In situ hybridization with probes encoding human and horse MHC class I genes was used to characterize 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 observed in the descendent endometrial cup trophoblast cells. In the non-invasive trophoblast cells of the allantochorion, no hybridization signals specific for horse MHC class I mRNA transcripts were consistently detected. In parallel to the in vivo results, strong hybridization signals were observed in the small, mononuclear cells present in chorionic girdle cell explant cultures, but not in the population of large binucleate cells corresponding to endometrial cup cells. The results obtained using in situ hybridization are consistent with the hypothesis that expression of MHC class I genes may be controlled at the transcriptional level in horse invasive and non-invasive trophoblast cells, and suggest that down-regulation of MHC class I antigen expression in endometrial cup cells may be accomplished by the same mechanisms in vivo and in vitro.