Role of host MHC class II, and of host NK cells, in CD4-mediated tumor rejection. (A) Role of host MHC class II. RAG.KO (H-2^b) mice (squares) and CD3KO/γcKO H-2^b (circles) or H-2^k (triangles) mice were challenged with MB49 cells. Approximately half of the mice per group were either untreated (left panel) or received 10⁶ Marilyn cells 1 day later (right panel). To overcome the need for class II (A^b) for initiating and maintaining CD4 T-cell activation, mice receiving Marilyn cells were immunized with male splenocytes at days 2, 5, 7, 9, 13, and 17 after tumor challenge. The average tumor size (6-12 mice per group) from a total of 3 individual experiments is shown. (B) Role of NK cells. Percentage of RAG.KO/γcKO recipient mice (left panel) or RAG.KO mice (right panel) alive after receiving MB49 challenge and left untreated (broken line) or treated with 10⁶ Marilyn cells (triangles). Some of the RAG.KO mice received Marilyn cells that had been depleted of NK cells (to deplete donor NK cells only [closed diamonds]), Marilyn cells plus either anti-NK monoclonal antibodies (to deplete both host and donor NK cells [closed circles]), or control antibodies (open circles) as described in “Materials and methods.” Data pooled from 3 independent experiments. (C) NK cells are present in the tumor. MB49 tumors were collected from Marilyn-treated RAG.KO mice 49 days after challenge and analyzed for the presence of various cell populations by FACS, gating on living 7-AAD⁻ cells. Numbers represent the percentage of CD4 or NK cells within the CD45⁻ population found at the tumor site. For comparison, tumors growing in untreated RAG.KO mice were stained for NK in the same way cells, 28 days after tumor challenge. Data are representative of 1 of 2 mice.