a, Schematic of the T cell-transfer experiment. The inset shows the gating strategy for flow cytometry that was used to select memory T cells. T cell-donor mice received CD45–APC intravenously (IV) three minutes before the collection of lymph nodes to label and exclude the circulating immune cells. Note that the control experiment in which mice were vaccinated with parvovirus was done in parallel with the MmuPV1 challenge, and the SCC primary tumour growth experiment was done in parallel with the infection of back skin with MmuPV1. b, Right, representative images of the warts on the back skin of mice three weeks after infection with MmuPV1. Scale bar, 1 cm. Left, flow cytometry demonstrates the presence of CD4+ and CD8+ T cells in the peripheral blood of the recipient mice, indicating successful adoptive transfer of T cells (n = 4 per group). Wt, wild type. c, Growth of subcutaneously injected DMBA–TPA-induced primary SCC tumour cells in wild-type mice (n = 9), Cd4−/−Cd8−/− mice (n = 5) and Cd4−/−Cd8−/− mice that received T cells from MmuPV1-immune donors (Cd4−/−Cd8−/− + test T cells) (n = 4). Two-tailed Mann–Whitney U test; *P < 0.05 compared with the wild-type group. Data are mean + s.d.