Expression of HLA G in human tumors is not a frequent event

Expression of HLA G may be a way for tumor cells to escape immuno-surveilla nce. HLA G is selectively expressed by extravillous trophoblast in the huma n placenta, a tissue that does not express HLA A or B molecules. It is temp ting to propose that tumor cells resemble this unique HLA class I phenotype as they frequently lose classical HLA A, B and C class I expression. Such peculiar HLA class I distribution would in theory allow tumor cells to esca pe from T- and NK-cell cytotoxicity, To determine whether HLA G is expresse d on tumor cells, we studied HLA G mRNA levers using RT-PCR and HLA G cell- surface expression by immunohistological techniques in a panel of 50 human solid tumor tissues, 31 tumor cell lines of different origin, 4 autologous mucosa samples and 3 peripheral white cell samples. We found mRNA transcrip ts of different HLA G isoforms in most of the samples studied. However, we did not detect cell-surface expression of HLA G using 3 specific monoclonal antibodies (MAbs; 87G, 01G and G223), HLA G was detected only in the U937 myelomanocytic cell line after stimulation with IFN-gamma. We favor the hyp othesis that HLA G plays a minor role, if any, in providing an inhibitory s ignal to NK cells to escape immunosurveillance. We cannot, however, exclude the possibility that some other HLA G isoforms may he expressed in some tu mors. (C) 1999 Wiley-Liss. Inc.