Tumor-associated macrophages share in vitro growth characteristics with resident but not elicited macrophages

Recent studies have shown that tissue macrophages are capable of proliferation and that this capability is enhanced by various cytokines, including macrophage colony-stimulating factor (M-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF). Tumor-associated macrophages (TAM) have been demonstrated to proliferate in vitro, but no information is currently available on the ability of M-CSF and GM-CSF to enhance this response. To address this problem, limiting dilution analysis was utilized to examine the proliferative ability of macrophages isolated from two murine tumors of distinct origin following growth in secondary hosts. As a means of comparison, resident peritoneal macrophages (RPM) and thioglycolate-elicited macrophages (TEM) were also analyzed. Results indicate that a rare subset of TAM and RPM is capable of proliferation and that M-CSF and GM-CSF enhance the frequency of TAM and RPM which proliferate, but do not enhance the growth of TEM.