Suppression of IL7Rα transcription by IL-7 and other prosurvival cytokines:: A novel mechanism for maximizing IL-7-dependent T cell survival

Survival of naive T cells is dependent upon IL-7, which is present in vivo in limiting amounts with the result that naive T cells must compete for IL-7-mediated survival signals. It would seem imperative during T cell homeostasis that limiting IL-7 be shared by the greatest possible number of T cells. We now describe a novel regulatory mechanism that specifically suppresses IL7Ralpha transcription in response to IL-7 and other prosurvival cytokines (IL-2, IL-4, IL-6, and IL-15). Consequently, IL7R expression is reduced on T cells that have received cytokine-mediated survival signals so they do not compete with unsignaled T cells for remaining IL-7. Interestingly, cytokine-mediated suppression of IL7Ralpha transcription involves different molecular mechanisms in CD4(+) and CD8(+) T cells, as CD8(+) T cells utilize the transcriptional repressor GFI1 while CD4(+) T cells do not. We suggest that this homeostatic regulatory mechanism promotes survival of the maximum possible number of T cells for the amount of IL-7 available.