NATURAL RESISTANCE AGAINST TUMORS GRAFTED INTO THE BRAIN IN ASSOCIATION WITH HISTOCOMPATIBILITY-CLASS-I-ANTIGEN EXPRESSION

The role of MHC-class-I-antigen expression in intracerebral anti-tumor natural resistance was examined using MHCpositive Lym+ and MHC-negative Lym- lymphoma cell lines. Lym+ was sensitive to MHC-class-I-restricted CTL-mediated lysis, while lym- was resistant. Both lines were susceptible to NK-cell-mediated lysis. There was no difference in in vitro growth rate or in vivo intraperitoneal tumorigenicity between them. Inoculation of Lym+ cells into the brain caused upregulation of the intracellular MHC mRNA to the same level as after treatment with interferon-gamma, resulting in an increase in cell-surface MHC expression. Although inoculated Lym- cells also underwent an increase in cytosolic MHC mRNA, the cell-surface MHC expression remained negative. Immunoprecipitation revealed that the terminal glycosylation did not occur normally in Lym-. An in vivo intracerebral tumorigenicity assay, using 2 groups of untreated and NK-cell-depleted syngeneic mice, showed that Lym+ was less tumorigenic than Lym-. In T-cell-depleted mice, however, no difference was detected between them. In addition, when Lym+ and Lym- cells were inoculated into the brain of allogeneic or syngeneic preimmunized mice (immunized with tumor cells), Lym+ was rejected, while Lym- was accepted. When allogeneic mice had received treatment for T-cell depletion before intracerebral inoculation, no rejection was observed in Lym+. On the other hand, Lym- cells, when injected i.p. into NK-depleted mice, had greater killing activity than Lym+ cells, while in T-cell-depleted mice Lym- was less tumorigenic than Lym+. These results suggest that MHC-positive tumor cells grafted into the brain may be rejected by CTL in an MHC-dependent manner, whereas MHC-negative tumor cells can escape from T-cell-mediated immunosurveillance and grow progressively in the brain, due to absence of intracerebral natural resistance mediated by NK cells.