Lysis and growth stimulation of a murine melanoma determined by density of macrophage populations

Activated macrophages were found to induce opposite effects on the growth of the F10 variant of B16 melanoma in both in vivo and in vitro systems. Low macrophage numbers had lytic effects and high numbers stimulated the growth of the melanoma. In fact, the number of macrophages per area (namely, their density) was found to determine whether lysis or growth stimulation would occur. Low density macrophages were found to be structurally and functionally different from high density macrophages. Macrophages in sparse cultures were round, separate, and lysed tumor cells, while crowded macrophages appeared spread, with pseudopodia, and caused enhancement of tumor growth.