Suppression of natural cytotoxicity in tumor-bearing mice and inhibition of the suppression by D-mannose.

Natural cytotoxic activity of spleen cells from normal and tumor-bearing BALB/c and C57BL/6 mice was assayed using Meth A, MH136 and MM2 ascites tumor cells as the target by 51Cr-release during 18 h incubation. The cytolytic activity of the cells from tumor-bearers, especially in the early stage of tumor progression, was considerably lower than that of cells from normal controls. When Meth A and MH134 target cells were used, it was found that such change was not due to decrease of effectors but was caused by increase of activity of cells suppressive to the lytic reaction. The suppressive cells were recovered in nylon wool-adherent fraction of the spleen cells. The suppression was mediated, at least partly, by a very stable soluble factor. The suppression by the cell and that by the factor were both inhibited by addition of D-mannose. When MM2 was used as the target, such inhibition of the suppressive activity by D-mannose was not observed.