[Anticancer effects of OK-432 (2). Anticancer actions of M phi activated by OK-432]

Mouse spleen cells (DDI, BALB/c, 10 weeks old), either pretreated in vitro with 100 u/ml of OK-432-induced IFN gamma for 18 hr or obtained from mice 24 or 48 hr after iv injection of OK-432 (100 micrograms/mouse), were examined for their antitumor effect by Winn assay against Meth-A tumor cells in BALB/c mice. Both of these spleen cells preparations clearly inhibited the growth of admixed Meth-A cells. In order to determine the effector subpopulation, these spleen cells were treated with either anti-Thy-1 monoclonal antibody plus complement, anti-asialo GM1 serum plus complement or in combination of adherence on plastic plates followed by Sephadex G-10 column treatment. As a result, the effector activity in Winn assay was lost only after the removal of macrophages through plastic plate adherence and Sephadex G-10 column treatment, but not after anti-Thy-1 or anti-asialo GM1 treatment, with either spleen cell populations. Moreover, after anti-Thy-1 treatment, the effector activity of spleen cells increased, suggesting the presence of suppressor T cells in these spleen cell populations. The growth of Meth-A cells was not only inhibited by these activated macrophages in Winn assay but also by adoptive transfer of OK-432-induced cytotoxic spleen macrophages, intralegionally 4 days after the implantation of 1 X 10(6) Meth-A cells. In conclusion, the effector cells which appeared in mouse spleens after iv injection of OK-432 were found to be cytotoxic macrophages, which inhibited the growth of Meth-A cells, both in vitro and in vivo. IFN gamma induced by OK-432 in mouse spleen cell cultures produced the same cytotoxic macrophages when added to the spleen cell cultures, at a dose as low as 100 u/ml. All of our evidence suggests that the systemic action of OK-432 can be explained by the effect of IFN gamma induction.