Inhibition of tumor growth in vivo and in vitro by macrophages from rats treated with a streptococcal preparation, OK-432.

The role of macrophages from rats treated with a streptococcal preparation (OK-432) for inhibition of syngeneic tumor growth was examined both in vivo and in vitro. Treatment of rats with OK-432 intraperitoneally resulted in a transient inhibition of ascites tumor growth and concomitant increase in survival time. Peritoneal exudate cells from these rats completely suppressed tumor growth when admixed with tumor cells and transferred subcutaneously to syngeneic recipient rats. No such effect was observed with tumor cells from saline-treated control rats. The macrophages were responsible for the neutralization of tumor growth, since the removal of adherent cells from peritoneal exudate cells of rats treated with OK-432 resulted in the abrogation of their antitumor activity. To further investigate the antitumor activity of macrophages, an in vitro cytotoxicity assay was carried out on several tumor cell lines. A complete target cell destruction was brought about by macrophages from the rats treated with OK-432 but not from the control animals. The tumor cytotoxicity mediated by macrophages activated with OK-432 was regarded as non-specific because they could damage all the target cell lines tested.