Stimulation of proliferation in mixed cultures of mouse tumor cells and nonimmune peritoneal cells. II. Stimulation of tumor cells by macrophages and of lymphocytes by a tumor cell product.

Stimulation of [methyl-3H]thymidine incorporation in mixed cultures of mouse peritoneal cells (PC) and tumor cells required a large adherent PC. Flow cytometric analysis, as well as autoradiography, showed that the stimulation involved increased proliferation of tumor cells. The stimulation required contact between tumor and PC; it was partly inhibited by Trasylol (aprotinin) and potentiated by dexamethasone. In addition, there was minor stimulation of PC by a soluble product of tumor cells; this stimulation was not affected by Trasylol or dexamethasone. It is suggested that macrophages can stimulate tumor cell proliferation and that such stimulation may be an important factor in host-tumor relationships.