CIITA B-cell-specific promoter suppression in MHC class II-silenced cell hybrids

In this study, various sets of somatic cell hybrids, generated by the fusion of epithelial cell lines with B-lymphoblastoid cell lines, were analyzed for the expression of major histocompatibility complex (MHC) class II antigens. We first demonstrate, in human and mouse intraspecies hybrids, the coordinate suppression of MHC class II, Ii (invariant chain) and HLA-DM gene transcription, and the release of the silencing by the addition of interferon gamma. Using interspecies hybrids, the segregation of human chromosomes allowed us to establish that MHC class II extinction is linked to the presence in the hybrids of the chromosomes from the epithelial fusion partner. Moreover, our data provide evidence that the expression pattern of MHC class II mRNA is correlated with that of the class II transactivator (CIITA), suggesting that CIITA is the actual target of the silencing. To gain further insight into the suppression phenomenon we performed luciferase assays which show that silencing affects the activity of the B-cell-specific promoter of CIITA. These results therefore demonstrate that the MHC class II gene silencing in somatic cell hybrids is due to an active suppression of one of the promoters of the CIITA gene, mediated by the epithelial cell fusion partner.