We examined the effect of exogenous IL-10 on the sensitivity of rat W14 and W31 tumor cells to natural killer (NK) cell-mediated cytotoxicity in relation to previously identified NK target structure (NKTS) expressed on these cells. We also examined the effect of endogenous interleukin-10 (IL-10) on rat IL-10 cDNA-introduced W31 cells (W31T-H, a high-IL-10-producer clone; W31T-L, a low-IL-10-producer clone). Both exogenous and endogenous IL-10 had no effect on the proliferative activity of these cells, but incubation of cells with recombinant human (rh) IL-10 resulted in a dose-dependent decrease in the expression of NKTS recognized by mAb 109. The expression level of NKTS on W31T-H cells was dramatically decreased compared with that on W31T-L cells and parental W31 cells. In addition, treatment of W31 cells with the culture supernatants of W31T-H cells could downregulate the expression of the NKTS. Moreover, NK sensitivity of W31T-H was suppressed down to a level equivalent to that of W31 cells pretreated with exogenous rhIL-10, and cytolysis could no longer be inhibited by mAb 109. We previously demonstrated that IL-10 downregulated MHC class I expression in this model. Nevertheless, NK susceptibility was also decreased. Taken together, these results suggest that the IL-10-mediated decrease in NKTS expression has a larger effect than decreased MHC class I expression on NK sensitivity. Thus, our data reveal a novel mechanism for an IL-10-mediated escape of tumor cells from host immune surveillance by downregulation of NKTS expression.
