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Augmentation of cell‐mediated cytotoxicity to a rat lymphoma. III. In vitro stimulation of natural killer cells by a soluble factor

The in vitro cytotoxicity of rat natural killer cells was augmented by the incubation of normal spleen cells with a soluble factor derived from the 24‐h culture of spleen cells and various tumour cells. Cytotoxicity was augmented approximately 4‐, 3‐ or 2‐fold depending on whether the spleen cells were incubated with the factor for 18 or 3 h or during the course of the cytotoxicity assay respectively. The factor was produced in cultures containing syngeneic, allogeneic or xenogeneic tumours and no correlation was observed between the ability of tumour cells to induce the production of the factor and their susceptibility to lysis in the 4‐h cytotoxicity assay. The factor was produced in cultures of spleen cells of several rat strains, and it stimulated the cytotoxicity of syngeneic and allogeneic spleen cells. The cells responsive to stimulation closely resembled NK cells of normal rats. Culture supernatants containing the stimulatory factor possessed anti‐viral activity, and evidence suggested that NK cells were stimulated by a factor resembling a type‐1 interferon. The relevance of these findings to the stimulation of NK cells by tumours in vivo is discussed.