EXPRESSION OF HLA CLASS-I GENES IN MEIOTIC AND POSTMEIOTIC HUMAN SPERMATOGENIC CELLS

In human spermatogenic cells, in contrast to somatic cells, expression of major histocompatibility complex (MHC) class I molecules is undete ctable. This lack of expression may contribute to the absence of femal e immune reaction against spermatozoa and may be necessary for gamete fusion. Among the molecular repressor mechanisms that may be used at t he DNA level, we investigated 5' CpG methylation of the different clas s la and class Ib loci in meiotic pachytene spermatocytes and postmeio tic round spermatids, which had been purified from human testes by cen trifugal elutriation. These results were compared with those obtained with mature spermatozoa and peripheral blood mononuclear cells. Using methylation-sensitive restriction enzymes and DNA locus-specific probe s, we found that HLA-A, HLA-B/C, and HLA-E loci were similarly unmethy lated in the germ and somatic cells tested, whereas HLA-F and HLA-G we re even less methylated in the former cells. Together with the observa tion that spermatozoon DNA contains class I genes that are transfectab le and able to direct transcription and protein synthesis in murine L cells, these data suggest that HLA class I genes are in an active conf ormation in male germ cells. We indeed found that both spermatocytes a nd spermatids contained low levels' of class Ia and class Ib mRNA. Usi ng reverse transcriptase-polymerase chain reaction, followed by DNA se quencing, we also detected three HLA-G transcriptional isoforms, resul ting from alternative splicings, which suggested that this class Ib ge ne may have a potential function in these germ cells. Although intrace llular expression of beta(2)-microglobulin (the light chain that assoc iates with HLA class I heavy chains) was found in spermatocytes but no t in round spermatids, no membrane-bound nor intracellular translated HLA class I heavy chain was detected in either germ cell type, when mo nomorphic anti-HLA class I monoclonal antibodies were used. Thus, lack of expression of HLA class I proteins in the male germ line is likely to involve post-transcriptional mechanisms of regulation.