Environmental conditions favorable for tumor progression in peritoneal cavity induced by peritoneal cells without tumor selectivity.

The incidence of the lethal growth of 10(1) L1210 murine leukemia cells in mice was higher in intraperitoneal (i.p.) (97%) than in intradermal (i.d.) (17%) inoculation, and survival time of mice was shorter in i.p. than i.d. inoculation. It was supposed that resident peritoneal cells (PC) enhanced tumor progression. I.d. inoculation of 10(1) L1210 cells mixed with 10(6) PC induced a lethal tumor growth at higher incidence than that of 10(1) L1210 cells alone or the mixture of 10(1) L1210 cells and 10(4) peripheral blood mononuclear cells (PBM) did. Furthermore, co-inoculation of a tumorigenic number of L1210 cells (10(3] with 10(6) PC resulted in marked shortening of median survival time of mice. Similar growth enhancing effect of PC was observed in Meth 1 fibrosarcoma. Meth A fibrosarcoma and colon carcinoma 26 (C26). Further study showed that PC, intact or X-rayed, helped the in vitro tumor growth under the conditions in which L1210 alone did not grow at all, whereas PBM had no enhancing effect to L1210 growth. We characterized the cells involved in tumor growth enhancement by the in vivo and in vitro tests. Plastic dish adherent cells of PC which were Mac-1 positive, large in size and resistant to X-ray, enhanced L1210 growth, whereas non-adherent cells which were Mac-1 negative and small in size, did not. These data suggest that the cells responsible for enhancing activity of tumor progression in the peritoneal cavity were macrophages (M phi).