Inhibiton of tumor cell proliferation: second role for suppressor cells?

An antimitotic factor for mouse tumor cells was isolated from the supernatant fluids of antigen-stimulated ovalbumin-immune mouse spleen cells. A similar antimitotic factor was obtained from the supernatant fluids of phytohemagglutinin-stimulated non-immune spleen cells. The inhibitor prevented the multiplication of mouse L929 fibroblast cells in vitro and the in vivo proliferation of Ehrlich ascites tumor cells. Inhibition was not due to cytotoxic effects and the antimitotic effects were reversible. The antimitotic activity may be species-specific since the purified factor obtained from mouse spleen cells had no effect on human or monkey cell lines. This factor appears to be the same as the T lymphocyte-dependent suppressor for antibody production that we described previously. Several implications for the production of the suppressor in response to tumor cells that may be beneficial or harmful to the host are discussed.