Abstract
Human tumor cells downregulate their MHC class I molecules to escape T cell attack but do not elicit natural killer (NK) cell response as predicted by the “missing-self” hypothesis established in mouse tumor model systems. Here we examined whether this surprising finding reflects lack of functional expression by tumor cells of ligands for NK-activating receptor NKG2D. Four tumor cell lines with marked HLA class I downregulation or loss were analyzed for their sensitivity to NK cell cytotoxicity and NKG2D ligand MICA expression. The renal carcinoma cell (RCC) RCC52-E3D4 and RCC52-M1G7 were sensitive to NK cell-mediated cytolysis, while the breast carcinoma cell line MDA-MB-157 and melanoma cell line COPA-159 were resistant. MICA protein was not detected in all four cell lines, suggesting that the two RCC cell lines were killed through an NKG2D-independent mechanism. MICA mRNA was detected in all four cell lines, though COPA-159 cells expressing at a lower level, suggesting that MICA protein and mRNA expression was not coordinated. RCC cells carry homozygous defective MICA*010 alleles (6Arg→Pro), while MDA-MB-157 and COPA-159 cells express MICA*018 and MICA*012, respectively. In COPA-159 cells, lack of MICA protein expression in spite of mRNA expression was likely to be caused by MICA mRNA 3’UTR, since the level of MICA expressed by MICA*012-3’UTR construct was markedly lower than MICA*012 coding region only construct. This possibility was supported by the presence of two MICA-specific microRNA in COPA-159 cells. Reconstitution with high-level MICA expression on MDA-MB-157 and COPA-159 cells resulted in restored sensitivity to NKG2D-dependent NK cell-mediated cytolysis only in MDA-MB-157 cells. These results indicate the role of MICA in stimulating NK cell cytotoxicity for MDA-MB-157, but suggest that reconstituted MICA on COPA-159 cells is not functional. Taken together, lack of NK cell response is attributed to lack of MICA expression in some tumor cells. The mechanisms underlying non-functional MICA expression on COPA-159 cells is under investigation.