Inhibition of melanoma growth by homeostatic T cell proliferation. (a) Nonirradiated (circles) or sublethally irradiated (squares) C57BL/6 mice were challenged subcutaneously with 5 × 105 B78D14 melanoma cells. Groups of irradiated mice were transfused with 5 × 106 (inverted triangles) or 5 × 107 (triangles) syngeneic B6.PL LN cells. Means and standard deviation of tumor growth are indicated (all groups, n = 7). (b) Proliferation profiles of gated Thy1.1+CD4+ and Thy1.1+CD4– (CD8+) cells in secondary lymphoid organs shown as CFSE histograms. Upper panel: nonirradiated C57BL/6 mice (Thy1.2+) transfused with 107 B6.PL (Thy1.1+) cells; middle panel: C57BL/6 mice irradiated and transfused with 5 × 106 B6.PL LN cells; lower panel: C57BL/6 mice irradiated and transfused with 5 × 107 B6.PL LN cells. FACS dot plots of host lymphoid organs are depicted to the right. (c) Total number of proliferating (one or more cell divisions) donor CD4+ and CD8+ cells in C57BL/6 recipients of 5 × 106 or 5 × 107 B6.PL LN cells calculated from data presented in b.