Adherent neoplastic cells grown at confluence downregulate HLA class I expression and enhance their susceptibility to lysis mediated by natural killer cells.

The number of HLA class I molecules and the susceptibility to lysis mediated by natural killer (NK) cells were evaluated on cell targets obtained from confluent and sparsely plated cultures of both normal and tumor cell lines. Sparsely plated proliferating cells expressed high amounts of HLA class I molecules and were more resistant to NK cell-mediated lysis than confluent cells, which expressed low amounts of HLA class I antigens. This characteristic could be involved in the control of cancer progression and could also explain the wide variability of assays of lymphocyte-mediated cytotoxic activity.