Role of the interferon-stimulated response element in HLA-B downregulation in human melanoma cell lines

Tumor cells are thought to escape immune surveillance from T cells by suppressing expression of major histocompatibility complex (MHC) class I molecules at their cell surface. Human MHC class I molecules are encoded by three different loci (HLA-A, -B, and -C). In primary human melanomas as well as melanoma cell lines, HLA class I expression is frequently downregulated in a B locus-specific manner. To study the involvement of promoter elements in HLA-B locus-specific downregulation, a series of reporter constructs containing 5'-flanking sequences of the HLA-A2 and -B7 genes were transfected into melanoma cell lines expressing high and low levels of HLA-B antigens. It is shown that enhancer A, which is generally believed to be a potent enhancer in HLA class I gene transcription, only weakly activates transcription in melanoma cell lines. In contrast, the interferon-stimulated response element (ISRE), known to induce MHC class I expression in response to IFNs, as well as a region comprising site alpha/enhancer B significantly stimulate constitutive transcription of HLA class I genes. Although none of the promoter elements tested could be demonstrated to mediate HLA-B locus-specific downregulation, high and low HLA-B melanoma cell lines do differ in ISRE activity as well as in ISRE-binding nuclear factors. The finding that high and low HLA-B melanoma cell lines contain different transcription factors binding to elements not actively involved in the process of HLA-B locus abrogation suggests that these cell lines originate from distinct types of melanocyte precursor cells expressing a different set of transcription factors.