Possible participation of tumoricidal macrophages in the therapeutic effect of bleomycin on a transplantable rat fibrosarcoma.

When bleomycin (BLM) (5 mg/kg/day) was administered i.p. to WKA rats for 5 days from the eighth day after KMT-17 implantation, the therapeutic effects of BLM were demonstrated by complete tumor regression in 50% of the cases and prolongation of the mean survival time of the remainder [survival days, 44.3 +/- 13.6 (SD)]. The combined administration of an antimacrophage agent, carrageenan, with BLM significantly inhibited the therapeutic effects of BLM (cure rate, 33%; survival days, 29.8 +/- 5.8). By means of a Winn assay, the tumor neutralizing activity of both spleen cells and peritoneal cells (PC) against KMT-17 cells was found to be augmented in BLM-treated tumor bearing rats as compared with that in nontreated tumor bearing rats. The enhanced tumor neutralizing activity of spleen cells was not abolished by an anti-rat T-cell serum plus complement treatment and was present in an adherent macrophage enriched population. Similarly, an in vitro [125I]iododeoxyuridine release test enabled us to observe the enhancement of the cytotoxic activity of adherent spleen cells and adherent PC in BLM-treated rats. The cytotoxic activity of the PC of BLM-treated rats was not specific to KMT-17 cells alone but was also observed to operate against antigenically different tumor cells such as WFT-2N, KST-20, and K562 cells. An in vitro carrageenan treatment of PC taken from BLM-treated rats reduced their cytotoxic activity. At the same time, the combined administration of carrageenan and BLM also reduced the cytotoxic activity of PC. These results suggest that the tumoricidal activity of macrophages in tumor bearing rats is augmented after BLM therapy and that the activated tumoricidal macrophages may participate in the host-mediated antitumor effects of BLM.