Macrophage Content of Spontaneous Métastases at Different Stages of Growth1

The macrophage content of spontaneous mA©tastaseshas been quanti fied morphometrically for a panel of rodent tumors at different stages of metastatic tumor growth. Using a histochemical technique to selectively stain macrophages, we have evaluated the relative content of macrophages in spontaneous pulmonary mA©tastasesfrom the 13762NF MTLn3 rat mammary adenocareinoma and the B16-BL6 mouse melanoma, as well as in spontaneous hepatic mA©tastasesfrom the M5076 mouse reticulum cell sarcoma and from autochthonous reticulum cell sarcomas in SJL/J mice. Between 112 and 254 separate, individual mA©tastaseswere evalu ated for each of these tumors. The data show that the relative macrophage content of very small mA©tastasesis high. However, as mA©tastasesgrow the relative macrophage content falls, reaching uniformly low levels by the time the mA©tastasesare 0.5 mm in diameter. These data are very similar to our previous observations on experimental mA©tastaseswhere the same pattern of high macrophage content in small mA©tastaseswas seen. Finding the same pattern in more slowly growing, spontaneous mA©tastasesof tumors derived from several different tissues and in two species suggests that the fall in relative macrophage content is not a phenomenon isolated to experimental mA©tastases,a particular site, or a tissue of origin for the tumor. The relative decrease in macrophage content may thus be a general phenomenon with important implications for immunotherapy directed to enhancing the tumoricidal activity of macrophages.