IL-21 promotes T lymphocyte survival by activating the phosphatidylinositol-3 kinase signaling cascade.

IL-21 is a Type I cytokine, which uses the common gamma chain (gamma(c)) in its receptor. As members of the gamma(c) cytokine/cytokine receptors family play crucial role in the differentiation, activation, and survival of lymphocytes, we have investigated if IL-21 could promote T cell survival and thus, contribute to T cell homeostasis and expansion. Unlike most gamma(c) cytokine receptors, we report that IL-21R is constitutively expressed by all mature T lymphocytes and that stromal cells of lymphoid organs are a constitutive source of IL-21. These observations are reminiscent of what is observed for IL-7/IL-7R, which control T cell survival and homeostasis and suggest a role for IL-21 in T cell homeostasis. Indeed, our results show that IL-21 is a survival factor for resting and activated T cells. Moreover, the ability of IL-21 to costimulate T cell proliferation is mediated by enhancing T cell viability. Further investigation of how IL-21R signaling induces T cell survival shows for the first time that IL-21 binding to its receptor activates the PI-3K signaling pathway and induces Bcl-2 expression. Moreover, the activation of the PI-3K signaling pathway is essential for IL-21-mediated T cell survival. Our data provide a new role for IL-21 in the immune system, which might be used to improve T cell homeostasis in immunocompromised patients.

Overexpression of IL-21 promotes massive CD8+ memory T cell accumulation.

The ability of IL-21 to promote in vitro T cell survival led us to investigate its biological activity in vivo. We report that overexpression of IL-21 in transgenic mice drives CD8(+) memory T cell accumulation with a concomitant reduction in naive T cell numbers. These memory T cells are functional, given their ability to rapidly produce IFN-gamma and proliferate following stimulation. Since the homeostasis of naive and memory T cells is controlled by cytokines, we evaluated whether IL-21 influences cytokine receptor expression. We show that IL-21 inhibits IL-7R expression on naive T cells in vitro, suggesting impaired IL-7-mediated naive T cell survival in IL-21-transgenic mice. In contrast, IL-7R expression on CD4(+) memory T cells is not affected, allowing their IL-7-dependent survival in IL-21-transgenic mice. Although IL-21 decreases IL-7R expression on CD8(+) memory T cells, this has no impact on their survival since their maintenance in the T cell pool is IL-7-independent. Rather, we demonstrate that CD8(+) memory T cells are receptive to IL-21 survival signals allowing for their accumulation in IL-21-transgenic mice. This study identifies new roles for IL-21 in T cell homeostasis and in the regulation of T cell responses to cytokines.