HLA-G expression in human melanoma cells: protection from NK cytolysis

Expression of the non-classical HLA-G class I antigen is physiologically restricted to a limited number of tissues including trophoblasts, and is thought to play a role in establishing tolerance of the fetus by the maternal immune system. We investigated whether ectopic expression of HLA-G could also be detected in tumor cells and confer them the ability to escape immune cytotoxic responses. High levels of all alternatively spliced HLA-G transcripts could be detected in melanoma cells by RT-PCR. Analysis of biopsies from a melanoma patient revealed a higher HLA-G transcription level in skin metastasis as compared to healthy skin, while specific amplification of the HLA-G5 transcript was only observable in the tumor. HLA-G protein expression could also be detected in two melanoma cell lines. HLA-G-positive tumors inhibit cytotoxic lysis by the NK cell line YT2C2-PR. This inhibition is not observed with B-EBV cell lines bearing matched class I specificities, and is thought to occur through interaction of HLA-G with inhibitory receptors that are distinct from known KIRs interacting with HLA-E or classical class I molecules. Together, these results confirm that HLA-G expression at the surface of tumor cells can participate in the evasion of antitumoral immune responses and favor tumor progression.