Interleukin-7 mediates the homeostasis of naive and memory CD8 T cells in vivo

The naive and memory T lymphocyte pools are maintained through poorly understood homeostatic mechanisms that may include signaling via cytokine receptors. We show that interleukin-7 (IL-7) plays multiple roles in regulating homeostasis of CD8(+) T cells. We found that IL-7 was required for homeostatic expansion of naive CD8(+) and CD4(+) T cells in lymphopenic hosts and for CD8(+) T cell survival in normal hosts. In contrast, IL-7 was not necessary for growth of CD8(+)T cells in response to a virus infection but was critical for generating T cell memory. Up-regulation of Bcl-2 in the absence of IL-7 signaling was impaired after activation in vivo. Homeostatic proliferation of memory cells was also partially dependent on IL-7. These results point to IL-7 as a pivotal cytokine in T cell homeostasis.