The relationship between nonspecific cytotoxic activity of spleen cells and the resistance against the graft challenge of a human hepatoma cell line (HCC-M) was investigated in nude mice. Two administrations of an immunopotentiator, OK-432 or human interleukin-2, prior to the subcutaneous inoculation of HCC-M cells, which was performed 24 h after the last administration, significantly inhibited the tumor development in terms of rate of tumor take and tumor size. This effect was abrogated by simultaneous administration of an anti-asialo GM1 (ASGM1) antiserum. There was a significant inverse correlation between tumor volume and spleen cell cytotoxicity which was determined at the time of HCC-M cell inoculation against a YAC-1 or HCC-M target. Spleen cell cytotoxicity enhanced by these immunopotentiators could not completely be abolished by in vitro treatment with ASGM1 and complement. This result suggests that effector cells of the enhanced cytotoxicity consist of heterogeneous cells including both ASGM1+ natural killer cells and other nonselective cytotoxic cells. These results suggest that nonspecific cytotoxic cells play crucial roles in the resistance against tumor cell challenge and that the total level of cytotoxic activity of these cells at the time of tumor cell challenge is a key factor which determines tumor development.
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