Biosynthesis of HLA-C heavy chains in melanoma cells with multiple defectsin the expression of HLA-A, -B, -C molecules

Recent investigations have shown that malignant transformation may down-reg ulate the expression of class I HLA molecules, beta(2)-microglobulin (beta( 2)m) and members of the antigen-processing machinery. In the present study, we HLA-genotyped and identified at a biochemical level the three (HLA-A25, -B8, -Cw7) class I alleles expressed by the previously described [D'Urso C M et al (1992) J Clin Invest 87: 284-292] beta(2)m-defective human melanoma FO-1 cell line and tested their ability to interact with calnexin, calreti culin and the TAP (transporter associated with antigen processing) complex. Ail these alleles were found to bind calnexin, but not calreticulin or the poorly expressed TAP complex, both in parental and beta(2)m-transfected FO -1 cells, demonstrating a complex defect of class I expression in FO-1 cell s. In these conditions, Cw7 heavy chains interacted with calnexin more stro ngly than A25 and B8, and preferentially accumulated in the endoplasmic ret iculum, in both a calnexin-associated and a calnexin-free form. In addition , they could be transported to the cell surface at low levels even in the a bsence of beta(2)m, without undergoing terminal glycosylation. These result s establish a parallel between HLA-C and the murine D-b and L-d molecules w hich have been found to be surface expressed and functional in beta(2)m-def ective cells. They also demonstrate distinctive features of HLA-C molecules . We propose that the accumulation of several assembly intermediates of HLA -C might favour the binding of peptide antigens not readily bound by HLA-A and -B molecules in neoplastic cells with suboptimal class I expression.