Antitumor cells found in tumor-bearing mice given ubenimex

Antitumor effector cells in spleens of tumor-bearing mice given ubenimex were investigated. The administration of ubenimex, starting 7 days after the tumor inoculation, was effective in inhibiting growth of IMC carcinoma. Spleen cells taken from these mice showed a marked suppression of the tumor growth by the WINN assay. The antitumor activity of spleen cells was reduced by treatment to remove T cells or NK cells, whereas spleen cell preparations enriched for T cells showed the strongest antitumor activity. Moreover, NK activity against YAC-1 cells was found in the spleen. These results indicate that the administration of ubenimex to IMC carcinoma-bearing mice generates cytotoxic T cells and NK cells in the spleen. The antitumor effect of ubenimex was not observed in X-ray-irradiated and in anti-asialo GM1 serum-treated mice.