The mechanisms by which various leukocyte subpopulations elicited by an immunogenic, nontumorigenic subclone (C3471) of B16 melanoma caused rejection of the tumorigenic parental melanoma (B559), were investigated. Leukocytes from C3471-immune mice were co-injected with B559 tumor cells in Winn assays into normal syngeneic recipients. Tumor formation by B559 cells was prevented when C3471-immune (a) unfractionated peritoneal leukocytes, or (b) glass-adherent peritoneal cells (90% macrophages), or (c) nylon wool purified nonadherent cells (95% Thy-1.2+) were used in the Winn assays. If the C3471-immunized mice were treated with antithymocyte serum before harvest of their peritoneal cells, none of these leukocyte populations were effective in the Winn assay. However, macrophages from these immunologically compromised donors regained their tumoricidal activity after incubation in vitro with T lymphocytes from untreated C3471-immune donors; similarly, C3471-immune lymphocytes rendered normal resident peritoneal macrophages tumoricidal in Winn assays. When C3471-immunized mice were irradiated or treated with antithymocyte serum before direct challenge with B559 cells, melanomas developed, thus providing additional evidence for the need for intact T cell function to establish immunity against the melanoma. Furthermore, when Winn assay recipients were treated with antithymocyte serum, neither C3471-immune macrophages nor T cells were able to prevent tumor formation. These findings indicate that antithymocyte serum-sensitive (Thy-1.2+) lymphocytes are necessary both for the generation of tumoricidal leukocytes in C3471-immunized mice, and for the rejection of B559 melanoma by demonstrably tumoricidal macrophages in Winn assay recipients. In addition, long-lasting immunity developed in 50% of the normal mice that had received both C3471-immune peritoneal cells and B559 tumor cells, as manifested by their capacity to reject a second challenge with B559 cells 40-60 d later.

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