Differing contribution of various effector cells in the elimination of syngeneic or allogeneic cells

The growth of Meth A (MA) tumors was suppressed in tumor-antigen-specific manner in BALB/c mice immunized with mitomycin C-treated MA (MMC-MA) cells in saline or in Freund's complete adjuvant (FCA). The antitumor activity of their peritoneal exudate cells (PEC) was detected by in vivo neutralization test and in vitro cytostasis assay but not by in vitro cytolysis assay. Positive delayed footpad reaction was elicited by footpad injection of MMC-MA cells in such immunized mice, before or after the inoculation of viable MA cells. Expression of cytostatic activity in PEC required the interaction of non-adherent and adherent cells. Normal PEC could exert the cytostatic activity in the presence of a nonadherent population of immune PEC. These findings suggest that cytostatic macrophages are activated after the interaction between sensitized lymphocytes and tumor-specific antigens and that they play an important role in the principal effector mechanism in this syngeneic system. On the other hand, immunization with MMC-MA in FCA or viable MA cells also induced PEC the antitumor activity detected by in vivo neutralization test in allogeneic C57BL/6 hosts. Immunization with viable MA cells induced not only cytolytic but also cytostatic activity, whereas, immunization with MMC-MA in FCA induced cytostatic activity but not cytolytic activity. In contrast to a syngeneic system, cytolytic activity was effectively induced against allogeneic MA cells together with cytostatic activity. We conclude that there are various effector cells contributing to the elimination of syngeneic or allogeneic murine tumor cells.