Human leukocyte antigen (HLA) class I molecules on hematopoietic cancers and melanomas inhibit attack by natural killer lymphocytes, but previous studies have not consistently demonstrated that carcinoma cells are protected by HLA class I expression. We investigated whether HLA class I molecules protect oral and pharyngeal squamous cell carcinoma cells from natural killer lymphocyte attack. Squamous cell carcinoma cell lines expressed varying levels of HLA class I, which correlated inversely with cytolysis by natural killer-enriched polyclonal lymphocytes. Cytolysis was increased by the presence of anti-HLA class I blocking monoclonal antibody (mAb). Subclones of the NK-92 human natural killer lymphoma cell line were derived by treatment with 5-aza-2'-deoxycytidine and limiting dilution cloning. NK-92 subclones expressed distinct sets of HLA class I-specific receptors. Some NK-92 subclones differentially lysed hematopoietic cells and squamous cell carcinoma cells, even in the presence of anti-HLA class I blocking mAb. This suggests that natural killer cells recognize different non-HLA ligands on hematopoietic and squamous cell carcinoma cells. In the presence of anti-HLA class I monoclonal antibody, other NK-92 subclones increased cytolysis of squamous cell carcinoma cells with moderate-to-high HLA class I levels. Anti- HLA class I mAb also increased natural killer cell attack of squamous cell carcinoma cells that were adherent to plastic. These data suggest that natural killer cell recognition of squamous cell carcinoma cells depends upon the balance of stimulatory and inhibitory ligands.
