Lack of correlation between the progressive growth of spontaneous metastases and their content of infiltrating macrophages.

The macrophage content of spontaneously occurring metastases was examined to determine whether metastases represent selected tumor populations that are not infiltrated by macrophages to the same extent as their parent tumors. In this study, we used three different spontaneously metastatic mouse tumors, the B16 melanoma in C57BL/6 mice, and the UV-2237 and UV-1422 fibrosarcomas in C3H/HeN (MTV-) mice. All lung and lymph node metastases in these three tumor systems contained macrophages. Some metastases had fewer, and others had the same or even more infiltrating macrophages than their respective parent tumors. Therefore, in the tumor systems we studied, the survival and growth of metastases are unlikely to be contingent upon the absence of infiltrating macrophages.