Constitutive Expression of MHC Class II Genes in Melanoma Cell Lines Results from the Transcription of Class II Transactivator Abnormally Initiated from Its B Cell-Specific Promoter

摘要

In melanoma cell lines, two different patterns of MHC class II expression have been described, either an IFN y-inducible ex- pression of HLA-DR and HLA-DP, with a faint or nun expression of HLA-DQ, resembling that described for melanocytes, or a constitutive expression, i.e., IFN-y independent, of an three HLA-D isotypes. As this latter phenotype has been associated with a more rapid progression of melanoma tumors, we have analyzed in different melanoma cell lines the molecular mechanisms leading to this abnormal pattern of MHC class II expression. In agreement with the evidence of a coordinate transcription of the HLA-D genes in these cell lines, we have shown the constitutive expression of CIITA (class II transactivator) transcripts, CIITA being known as the master switch of MHC class II expression. Unexpectedly, these transcripts initiate from promoter III of the CIITA gene, a promoter that is mainly used constitutively in B lymphocytes. This expression was further shown to occur through factor(s) acting on the enhancer located upstream of CIITA promoter 111, which was previously described in epithelioid cells as an IFN- y-response sequence. The hypothesis of a general abnormality of the IFN-y transduction pathway was dismissed. Constitutive transcription of CIIT A from promoter III having been observed in unrelated melanoma cell lines, we propose the hypothesis that this phenomenon might not be a random event, but could be linked to the neoplasic state of the melanoma cells.