Proliferation and immunohistochemistry of tumor-infiltrating T cells. (a) Groups of C57BL/6 mice (Thy1.2⁺) were sublethally irradiated, challenged with melanoma cells, and transfused with 5 × 10⁶ CFSE-labeled syngeneic B6.PL (Thy1.1⁺) LN cells. Four weeks later, single cell suspensions of host LN, spleen, and tumor tissue were analyzed for division characteristics by flow cytometry of gated Thy1.1⁺CD4⁺ and Thy1.1⁺CD4^– (CD8⁺) donor cells. (b–g) Groups of C57BL/6 mice were untreated or sublethally irradiated and challenged with melanoma cells. The irradiated mice were then transfused with 5 × 10⁷ syngeneic B6.PL LN cells. Immunohistochemistry of residual tumors in transfused (b, d, f) mice compared with large tumors of untreated controls (c, e, g) is shown at day 60. Staining with an anti-Thy1.1 Ab (OX-7) reveals donor cells in the regressed tumor tissue of the LN cell–transfused irradiated host (b), but not in the progressing melanoma tumor of transfused, nonirradiated animals (c). Staining for CD4⁺ cells demonstrates considerable infiltration in the irradiated reconstituted group (d), but minor infiltration in the nonirradiated, nonreconstituted mice (e). Similarly, CD8⁺ cells are found in higher numbers in irradiated and transfused (f) than in nonirradiated, nontransfused (g) mice. Staining for CD56 did not reveal appearance of NK cells (not shown).