A reassessment of the role of B7-1 expression in tumor rejection

Introduction of the B7-1 gene into murine tumor cells can result in rejection of the B7-1 transductants and, in some cases, systemic immunity to subsequent challenge with the nontransduced tumor cells. These effects have been largely attributed to the function of B7-1 as a co-stimulator in directly activating tumor specific, major histocompatibility complex I- restricted CD8⁺ T cells. We examined the role of B7-1 expression in the direct rejection as well as in the induction of systemic immunity to a nonimmunogenic murine tumor. B-16 melanoma cells with high levels of B7-1 expression did not grow in C57BL/6 recipient mice, while wild-type B-16 cells and cells with low B7-1 expression grew progressively within 21 d. In mixing experiments with B7-1(hi) and wild-type B-16 cells, tumors grew out in vivo even when a minority of cells were B7-1^-. Furthermore, the occasional tumors that grew out after injection of 100% B-16 B7-1(hi) cells showed markedly decreased B7-1 expression. In vivo antibody depletions showed that NK1-1 and CD8⁺ T cells, but not CD4⁺ T cells, were essential for the in vivo rejection of tumors. Animals that rejected B-16 B7-1(hi) tumors did not develop enhanced systemic immunity against challenge with wild-type B-16 cells. These results suggest that a major role of B7-1 expression by tumors is to mediate direct recognition and killing by natural killer cells. With an intrinsically nonimmunogenic tumor, this direct killing does not lead to enhanced systemic immunity.