). Expression of the anti-apoptotic genes Bcl-2 and Bcl-xL was markedly decreased in the T-cell-specific Stat3-deleted group compared with the control group (Fig. 6a). Stat3, Bcl-2 and Bcl-xL protein levels were reduced; accordingly, the expression of cleaved caspase-3 was enhanced in purified T cells from T-cell-specific Stat3-deficient mice (Fig. 6b). Furthermore,

expression of both Bcl-2 and Bcl-xL in splenic T cells was considerably reduced in Stat3-deficient mice, as shown by flow cytometry analyses and immunofluorescence assays (Fig. 6c,d). These data collectively suggest that Stat3 plays a crucial role in maintenance of the T-cell population by inducing Bcl-2 and Bcl-xL expression. We demonstrated that Stat3 contributes to T-cell homeostasis by inducing the expression of Bcl-2 family genes. Stat3 deficiency may

enhance the susceptibility of check details T cells to apoptosis by attenuating the expression of Bcl-2 and Bcl-xL, resulting in the breakdown find more of T-cell homeostasis in lymphoid organs. In the present study, we successfully generated T-cell-specific Stat3-deficient mice, as described previously[17] (Fig. 1). These mice were born healthy and presented no obvious abnormalities. The study in which T-cell-specific Stat3-deficient mice were first generated showed that these mice had no abnormalities in T-cell development.[17] Instead, it demonstrated that Stat3-deficient T lymphocytes have impaired proliferation in response to IL-6 treatment and defective IL-2-mediated Endonuclease IL-2 receptor α chain expression.[16, 17] However, a recent study reported that the spleen and lymph nodes of T-cell-specific Stat3-deficient mice were smaller than those of wild-type littermates.[18] We also found that the spleens of T-cell-specific Stat3-deficient mice were considerably smaller and that their cell numbers were reduced (Figs 1c,d, and 2b). These findings were attributable to the deficiency of T lymphocytes, rather than non-T

cells, in Stat3-knockout mice (Fig. 2a,c). We demonstrated that both the per cent population and absolute numbers of CD4+ and CD8+ T cells were reduced in Stat3-deficient mice (Fig. 2d–f). The maintenance of Foxp3+ regulatory T cells has also been reported to be attributable to the common γ chain cytokine signalling in which Stat3 and Stat5 are involved.[24, 25] Consistently, the population and the number of cells of CD4+ Foxp3+ T cells were notably decreased in Stat3-deficient mice when compared with the control group (Fig. 2g,h). Next, we investigated whether the reduction of CD4+ or CD8+ T lymphocytes was mainly a result of the decrease of naive or memory/effector T cells. The population of CD44low CD62Lhigh naive cells in both CD4+ and CD8+ T lymphocytes was significantly reduced in splenocytes and lymph node cells from Stat3-deficient mice, whereas that of CD44high CD62Llow effector/memory cells was unchanged (Fig. 3a–c).