Susceptibility to NK cell lysis is abolished in tumor cells by a factor which restores their contact inhibited growth.

It is well recognized that physical contact between natural killer (NK) cells and tumor targets is necessary for cell lyses. Therefore, any modulation of the tumor cell surface that alters intercellular contact could affect NK cell cytotoxicity. To examine this hypothesis, a contact inhibitory factor (CIF), which had been shown to restore contact inhibition of growth to several malignant cell lines was tested for its ability to render such cells immune to recognition by NK cells. When three NK-sensitive melanoma and two NK-sensitive colon carcinoma targets were cultured with CIF, they did not only change morphologically, but also showed a 70% to 95% reduction in their sensitivity to lysis by NK cells. In addition, K562 cells, which grow in suspension and do not permit a morphologic evaluation of the CIF effect, also became resistant to lysis by NK cells after culture with CIF. CIF did not reduce the viability nor the cytotoxicity of NK cells. GIF did not contain interferon nor did the GIF-treated targets induce the production of interferon during the cytotoxicity assay. It is concluded that restoration of contact inhibition of growth and resistance to NK cell lysis are cell surface phenomena that may run in parallel.