Local proliferation of mononuclear phagocytes in tumors

The fraction of macrophages and tumor cells in the DNA synthetic phase of the cell cycle was determined for the transplantable fibrosarcomas, RIF and KHT tumors, the mammary tumors EMT6, S102, and 67A, the B 16 melanoma, and the Lewis lung carcinoma. Thirty minutes after intraperitoneal injection of 3HTdR (tritiated thymidine), 10% to 28% of the macrophages were labeled. A similar proportion of tumor cells was labeled. This represents a high degree of local proliferation for macrophages in situ. Thus, macrophages can proliferate in growing tumors, and this local proliferation may represent an important source of these host defense cells.