Polyclonal homeostatic expansion of CD8+ T cells is sufficient for melanoma growth inhibition. (a) Nonirradiated (circles) and irradiated (triangles) B6.RAG–/– mice challenged with melanoma cells show no difference in tumor growth (n = 5 for each group). (b) Transfusion of 5 × 106 syngeneic B6.LN cells to nonirradiated B6.RAG–/– mice leads to significant melanoma growth inhibition (triangles) compared with nonirradiated, nontransfused controls (circles). Adoptive transfer of purified C57BL/6 CD8+ T cells is sufficient to exert an antitumor effect (squares) equal to that of whole LN cells (all groups, n = 7). (c) Adoptive transfer of 5 × 106 2C CD8+ TCR transgenic cells into irradiated B6 hosts (triangles) did not enhance the antitumor effect achieved by irradiation alone (squares), despite the ability of 2C cells to undergo homeostatic proliferation (inset). Controls included nonirradiated, nontransfused C57BL/6 mice (circles) and irradiated, transfused (5 × 106 B6 LN cells) C57BL/6 mice (inverted triangles) (all groups, n = 6).