Nave CD8 T cells require both IL-7 signaling and TCR signaling through the interaction with self peptides and MHC complex for T cell homeostasis. However nobody knows why these cells require two signals or why IL-7 signaling is not enough, since IL-7 signaling by itself can provide strong survival signals. Because IL-7 signaling can down-regulate IL-7R expression, we hypothesizes that down-regulation of IL-7R causes a problem for nave CD8 T cell homeostasis and survival. To address this question, we used T cells expressing transgenic IL-7R under the control of the human CD2 promoter which is not down-regulated by IL-7 stimulation. We found that IL-7R Tg nave CD8 T cells can proliferate dramatically in vitro in response to IL-7. This suggests that the maintenance of IL-7Ra expression is important for the proliferation by IL-7. HY TCR Tg female T cells cannot undergo homeostatic proliferation because their TCR affinity to self ligand is too low. We found that their IL-7R expression is very low and can be up-regulated by stronger TCR signaling. Interestingly, introducing an IL-7Ra Tg to HY T cells rescued their homeostatic proliferation. Consequently, we have discovered a new mechanism of T cell homeostasis in which homeostatic TCR signaling mediates the maintenance of IL-7R expression. Recently, we have discovered that, in the absence of interruption by TCR engagements, persistent IL-7 signaling is toxic to mature CD8 T cells in the periphery by inducing T cells to secrete IFNgamma which signals IFNgamma-induced cell death. Thus, we have discovered that the importance of TCR signaling during periperhal T cell homeostasis is specifically to prevent IL-7 from continuously signaling and causing cell death.
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