To test this possibility in vivo, we implanted p53−/− and WT mice with the OVA-transfected syngenic mouse thymoma cell line EG.7. EG.7 or its parent cell line EL4 has been shown to induce protective T-cell immune responses in cbl-b−/− mice and are thus immunogenic 34, 35. Mice were injected with 106 EG.7 tumors subcutaneously learn more in the flanks and their growth was monitored. In one of the p53−/− mice a very small tumor was detected around day 7, but was cleared very rapidly (Fig. 5A). In three other p53−/− mice, a palpable tumor was present on day 7, became undetectable around day 21. In contrast,

in all the WT mice (n=6) the tumor kept growing (>250 mm2 after days 21) (Fig. 5A), suggesting the p53−/− mice are resistant to transplanted tumors. To test the hypothesis that more effective effector T-cell responses against EG.7 were responsible for rejection of EG.7 in p53−/− mice, OVA-specific CTL activity

in WT and p53−/− mice after EG.7 implantation Sorafenib was measured. At 21 days after EG.7 implantation, mice were injected with a mixture of CFSEhigh labeled SIINFEKL peptide (OVA peptide 257–264)-loaded and CFSElow labeled (not loaded with SIINFEKL) syngeneic spleen cells and 4 h later the ratio of CFSElow and CFSEhigh cells were determined in the spleen of recipients. As a control, naïve C57BL/6 mice also received the mixture of CFSEhigh labeled SIINFEKL loaded and CFSElow labeled syngeneic spleen cells. Compared to naïve C57BL/6 mice, EG.7 implanted WT mice did not exhibit any killing of the SIINFEKL-labeled targets (0.33±0.85% specific killing). In sharp contrast, EG.7 transplanted p53−/− mice exhibited significantly higher levels of in vivo CTL activity (11.7±2% specific killing) (Fig. 5B). Collectively these data show that p53−/− mice mounted a robust and effective immune response against immunogenic tumors leading to their rejection. T cells undergo activation, proliferation and differentiation into effector cells after encounter with Ag. TCR stimulation of naïve T cells induces

SSR128129E both T-cell proliferation and apoptosis. Our results demonstrate that following TCR stimulation p53-deficient T cells are hyperproliferative and less apoptotic. A previous study by Ohkusu-Tsukada 36 showed two findings: (i) compared to WT mice, p53−/− mice showed enhanced generation of memory T cells (both spontaneously and after immunization with sheep red blood cells), and (ii) young p53−/− mice showed comparable anti-CD3-induced proliferation of T cells, while older mice showed significantly less proliferation than WT counterparts. The first observation may be explained by our finding, i.e. hyperproliferation of p53-deficient T cells. The use of total T cells by Ohkusu-Tsukada et al., which will contain Treg may have resulted in a different outcome than that observed in the current study with sorted CD4+CD25 or CD8+ T cells.