Augmentation of cell-mediated cytotoxicity to a rat lymphoma. II. Characterization of the non-T cytotoxic cells stimulated in vivo by tumour cells as natural killer cells.

A variety of tumours injected into rats were found to rapidly stimulate cytotoxicity which was similar to naturally-occurring cytotoxicity of normal rats. Cytotoxic cells from the spleen and peritoneal cavity closely resembled NK cells in their lytic specificity and cell-surface characteristics. Thus, although cytotoxicity could be stimulated "non-specifically" with tumours which were resistant to lysis in vitro by NK cells, the cytotoxic cells exhibited patterns of specificity against a panel of target cells in direct lysis or competitive inhibition assays which were similar to those of NK cells from normal rats. These cells also closely resembled NK cells in being largely non-adherent, non-T cells, and in exhibiting a similar heterogeneity in the expression of Fc receptors. Thus, cytotoxicity which was augmented shortly after tumour inoculation appeared to be attributable to NK cells. However, whilst the majority of NK cells from normal or tumour-inoculated rats shared these properties, significant heterogeneity was observed. Minor populations of cytotoxic cells were adherent, were lysed by a heterologous anti-T-cell antiserum and complement and did not express an Fc receptor, although it was not determined whether the same subpopulation possessed all three characteristics.