Host responses with solid tumors. I. Monocytic effector cells within rat sarcomas.

Morphologic studies of two unrelated progressively growing sarcomas in rats revealed the presence of a large proportion of host cells distinct from tumor cells. Enzymatically or mechanically dispersed tumor suspensions were fractionated by the velocity sedimentation method, which resulted in a separation of host and tumor cells. The host cells had the typical morphology of macrophages and also were adherent to a plastic surface after a short incubation period. The proportion of host cells was greater in a young (10 or 12 days) than in an older tumor (35 days). These macrophage-like host cells isolated from sarcomas 12 and 35 days after implantation inhibited the colony formation of tumor cells in vitro in the nonspecific manner ascribed to activated marcrophages. In addition to these effector cells, the host fraction contained macrophage precursors able to proliferate in vitro but apparently inhibited by the presence of tumor cells. The results indicated that these macrophage-like effector cells may be the predominant host reaction within some rat sarcomas, and also that thymus- and bone marrow-derived lymphocytes act only indirectly if they are involved.