Contrasting effects of activated and nonactivated macrophages and macrophages from tumor-bearing mice on tumor growth in vivo.

The effect of macrophages from normal and tumor-bearing mice on tumor growth was investigated with the use of an in vivo neutralization test. Macrophages from unstimulated and thioglycollate-stimulated peritoneal cavities (nonactivated macrophages) of normal mice enhanced growth of various syngeneic tumors [a 3-methylcholanthrene-induced transplantable fibrosarcoma from inbred C3HeB mice, a spontaneously originated transplantable melanoma (B16) from inbred C57BL/6 mice, and a radiation-induced lymphoma from inbred BALB/c mice]. This enhancing effect was not destroyed by irradiation of macrophages and was apparently mediated by macrophage secretory products. The effect appeared to be unrelated to immunosuppression and may have reflected direct stimulation of tumor cells. In contrast, Corynebacterium parvum-activated macrophages markedly inhibited tumor growth. Peritoneal macrophages from fibrosarcoma-bearing mice, which possessed tumor-inhibitory T-lymphocytes, enhanced tumor growth and abolished the effects of the tumor-inhibitory lymphocytes. Clearly, under certain conditions nonactivated macrophages interfered with the mechanisms of T-cell-mediated antitumor resistance in tumor-bearing mice.