in vitro proliferative responses of spleen lymphocytes from rats bearing progressive or regressive tumors induced by cell variants of a syngeneic colon carcinoma

From one colonic carcinoma chemically induced in the rat, 2 sublines of tumor cells have been cloned, one (PROb) inducing progressive tumors, the other (REGb) generating tumors that regress a few weeks after s.c. injection into syngeneic hosts. Our study was aimed at comparing cellular immunity between animals bearing PROb or REGb tumors. Spleen cells were first tested for in vitro proliferation in response to mitomycin-treated PROb or REGb cells. Only spleen cells from rats injected with REGb cells proliferated significantly when mixed with PROb or REGb cells. The proliferative response induced by REGb cells was considerably higher than the response to PROb cells. When spleen cells from rats bearing REGb tumors were cultured with a mixture of REGb and PROb cells at various PROb/REGb cell ratios, PROb cells significantly suppressed the strong proliferative response generated by the same number of REGb cells alone. REGb-immune spleen cells, after in vitro stimulation by PROb or REGb cells, were not cytotoxic for either cell variant. REGb-immune spleen cells did not differ in their content of T lymphocytes expressing CD4 or CD8 markers when they were stimulated by PROb or REGb cells in vitro, but REGb cells induced a larger number of activated lymphocytes expressing the IL-2 receptor. Our results indicate that, compared to REGb cells, PROb cells are poorer stimulators of proliferation of tumor-immune spleen cells, and that they are able to suppress the proliferative response induced by REGb cells.