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Tumor Immunity in Perforin-Deficient Mice: A Role for CD95 (Fas/APO-1)¹

Dalia Rosen^*, Jie-Hui Li^*, Sergey Keidar, Ilya Markon, Ruben Orda and Gideon Berke^2,*

^* Department of Immunology, The Weizmann Institute of Science, Rehovot, Israel; and Department of Surgery "A," Assaf Harofeh Medical Center, Tel-Aviv University, Zerifin, Israel

CTL and NK cells use two distinct cytocidal pathways: 1) perforin and granzyme based and 2) CD95L/CD95 mediated. The former requires perforin expression by the effectors (CTL or NK), whereas the latter requires CD95 (Fas/APO-1) expression by the target. We have investigated how these two factors contribute to tumor immune surveillance by studying the immunity of perforin-deficient mice against the progressor C57BL/6 Lewis lung carcinoma 3LL, which expresses no CD95 when cultured in vitro. Unexpectedly, the results indicated that the perforin-independent CD95L/CD95 pathway of CTL/NK plays a role in acting against D122 and K^b39.5 (39.5) high and low metastatic sublines, respectively, derived from the 3LL tumor. Although no membrane-bound CD95 was detected on cultured D122 and 39.5 cells, surface CD95 expression on both D122 and 39.5 was considerably up-regulated when the tumors were grown in vivo. A similarly enhanced expression of CD95 was observed with three additional tumors; LF^-, BW, and P815, injected into syngeneic and allogeneic mice. The finding of up-regulated CD95 expression on tumor cells placed in vivo suggests that a CD95-based mechanism plays a role in tumor immunity at early stages of tumor growth. Consequently, the progressive down-regulation of CD95 expression during tumor progression may indeed be an escape mechanism as previously reported. Together, these results suggest a role for CD95-dependent, perforin-independent immunity against certain tumors.