During tumor development, growth factors may act in autocrine manner stimulating cell proliferation, or in paracrine manner affecting the microenvironment of the tumor and modulating the immune system. Murine mammary adenocarcinoma M3 tumor bearers develop lung metastases and leukocytosis during its evolution. Previously we described that M3 conditioned media enhanced metastasis incidence, when it was inoculated in tumor-operated mice. In the present study we determine that spleen cells from M3 tumor operated mice treated with M3 conditioned media, were able to transfer the capacity to enhance metastasis to other tumor operated mice. Spleen cells have immune suppressor activity that could be reversed by cyclophosfamide treatment. M3 tumor cells secrete GM-CSF, which is able to promote in vitro proliferation of M3 cells as well as spleen cells. This proliferation could be abrogated by the addition of anti-GM-CSF. We report that the GM-CSF secreted by M3 tumor cells had stimulatory activity on M3 tumor cell and lymphocyte proliferation.

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