Augmentation of specific cell-mediated immune responses to tumor cells in tumor-bearing rats pretreated wih the antileukemia drug busulfan.

Lethal growth of a syngeneic transplanted tumor (KMT-17) was inhibited in inbred WKA rats pretreated with the antileukemia drug busulfan (BU). However, the lethal growth of KMT-17 was not inhibited by pretreatment with cyclophosphamide, adriamycin, or ftorafur. With the Winn assay, spleen cells from BU-pretreated KMT-17-bearing rats (TBR) inhibited the growth of admixed KMT-17 cells more strongly than did spleen cells from BU-untreated TBR. The augmented tumor inhibitory activity of spleen cells was KMT-17-specific, and this activity was abrogated by in vitro treatment of spleen cells with anti-T serum and guinea pig complement. Augmentation of the immune response to KMT-17-associated antigen(s) in BU-pretreated TBR was also demonstrated in lymphocyte-mediated cytotoxicity, as detected by a 51Cr release assay and by a delayed-type hypersensitivity with a radioisotope footpad assay. Tumor regression in BU-pretreated rats was demonstrated to be mediated by the augmentation of T-cell-mediated immune responses to tumor-associated antigens. The tumor inhibitory effect of BU was abrogated by adoptive transfer with thymus cells from normal rats but not with those from BU-pretreated rats 1 day before tumor inoculation. The augmentation oif the antitumor immune responses by pretreatment with BU was suggested to be due to the fact that BU selectively inhibited the suppressor cells of their precursors.