Pronounced antitumor effect of LAK-like cells induced in the peritoneal cavity of mice after intraperitoneal injection of OK-432, a killed streptococcal preparation.

More than 80% of BALB/c mice bearing BAMC-1 ascites tumor were completely cured after five consecutive (once every 2 days) i.p. injections of a 0.1 mg dose of OK-432, beginning on day 2 after tumor implantation. The antitumor effect of OK-432 was abolished in athymic nu/nu mice and in anti-thymocyte globulin-treated euthymic BALB/c mice, so although OK-432 treatment did increase the length of survival, all animals eventually died as a result of tumor growth. When peritoneal exudate cells (PEC), obtained on day 12 from OK-432-treated BAMC-1-bearing euthymic mice were evaluated for in vivo tumor neutralization activity, all mice receiving an i.p. injection of the admixture of the nonadherent PEC (1 X 10(7) cells) with BAMC-1 cells (1 X 10(5)) survived for more than 60 days. When the same nonadherent PEC (1 X 10(7) cells) were i.p. transferred adoptively 1 day after the inoculation of 1 X 10(5) BAMC-1 tumor cells, again all mice survived. When these in vivo active PEC were tested for cytotoxicity in vitro against fresh BAMC-1 tumor cells, natural killer (NK) sensitive syngeneic RL male 1, NK-sensitive allogeneic YAC-1 cells, NK-resistant syngeneic Meth-A cells, allogeneic tumor cells (EL4, B16, and P815) and xenogenic human cells, the PEC were found to be capable of lysing BAMC-1 tumor cells together with almost all of the other tumor cells, including NK-resistant cells. Nonadherent PEC contained at least two subpopulations of killer cells. One, directed to syngeneic BAMC-1 cells, was both Thy1.2 and asialo GM1 positive, and another, directed to allogeneic YAC-1 cells, was asialo GM1 positive but Thy1.2 negative. A cold target inhibition assay also suggested the presence of more than two subpopulations. These results indicate that T cells play a determined role in the immunotherapeutic effect of OK-432 on BALB/c mice bearing BAMC-1 tumor, although the participation of activated macrophages could not be excluded. The cells responsible for killing BAMC-1 and other tumor cells appearing in the PEC on day 12 were characterized as containing at least two kinds of lymphokine-activated killer cells.