Localization of the mRNA regulatory element in HLA-C genes

Previous studies showed that HLA-C genes have lower mRNA levels and a shorter mRNA half-life than HLA-B genes. To investigate the region(s) associated with lowered mRNA levels I generated a panel of chimeric HLA-B/C molecules by exchanging exons with surrounding intronic sequences of HLA-B7 for HLA-Cw3 and the converse. The chimeric pairs include exchanging exons: 4 and 5 (nomenclature refers to exon number) of B7 and Cw3, exons 6 and 7 of B7 and Cw3, exons 6 through 8 of B7 and Cw3, exon 8 of B7 and Cw3, and the promoter/enhancer region and exon 8 of B7 and Cw3. Each chimera and the parental B7 and Cw3 genes were transfected separately into 721.221, an EBV transformed B lymphocyte cell line with no endogenous classical HLA class I genes. The transfectants were analyzed by indirect flow cytometry to assess cell surface protein expression levels and Northern analysis to assess mRNA levels. The results show that cell surface expression of B7/Cw3 chimeras decreases relative to parental B7 in chimeras containing HLA-C sequences for exons 6 and 7, encoding the cytoplasmic tail and or exons 6, 7 and 8, including the 3′ untranslated region. The converse is also true, the Cw3/B7 chimeras containing HLA-B7 sequences for exons 6 and 7, 6 through 8, or exon 8 alone increase relative to parental Cw3 cell surface expression. The implications of these findings will be discussed.