CTL memory development and persistence.

(A) SDF-tumor rejection supports the development of antitumor memory T cells. C57BL/6 mice (10 mice/group) were challenged intravenously with 2 × 10⁵ wild-type C1498 cells 3 (●) or 4 months (▴) after the rejection of live SDF-C1498 cells. Naive C57BL/6 mice were used as controls (▪). Both groups had delayed tumor growth, as compared to control animals, and 40% (3 months) and 50% (4 months) of the mice in the 2 groups were resistant to the challenge (P = .0001 versus control). The graph is representative of 2 independent experiments. (B) ⁵¹Cr release CTL assays. Spleens were collected from mice 11 weeks after SDF-B16F1 tumor inoculation/rejection and splenocytes were cocultured with irradiated B16F1 cells as described in “Materials and methods.” Six days later, splenocytes were harvested and used as effector cells in CTL assays. ⁵¹Cr-labeled B16F1 (H-2^d) or control allogeneic TSA (H-2^b) tumor cells were used as targets in the standard 4-hour CTL assays. Splenocytes from mice that had rejected SDF-B16F1 tumors lysed syngeneic B16F1 but not allogeneic TSA cells. The results are representative of 2 independent experiments. (C) CD4⁺ cells are indispensable for SDF-mediated tumor rejection. C57BL/6 mice were depleted of CD4⁺ (●) or CD8⁺ (▴) T cells, as described in “Materials and methods.” Control mice were treated with PBS (▪). Three days after the last injection the mice were injected intravenously with 2 × 10⁵ live SDF-C1498 cells and antibody injections continued every 5 days for 3 weeks. All the mice treated with PBS and 80% of the mice treated with anti-CD8⁺ mAb rejected the SDF-C1498 cells and did not develop any signs of leukemia. Depletion of CD4⁺ T cells resulted in 100% lethal leukemia (P = .0001 versus control PBS).