Differential regulation of human IL-7 receptor alpha expression by IL-7 and TCR signaling

IL-7Ralpha is essential for the development and homeostatic maintenance of mature T cells. Studies in humans and mice have shown that IL-7Ralpha expression is reduced by its cognate cytokine, IL-7, and Ag, suggesting that active regulation of IL-7 responsiveness is necessary to balance T cell numbers. We show that IL-7- or TCR/CD28-mediated signaling induced a rapid down-regulation of IL-7Ralpha expression on naive T cells on the mRNA and protein level, with a mild (10-fold) or strong (50-fold) gene suppression, respectively. In both situations, the down-regulation of IL-7Ralpha was blocked by cyclohexamide and actinomycin D, indicating the involvement of an active mechanism dependent on new transcription and protein synthesis. Upon IL-7 withdrawal, IL-7Ralpha mRNA and surface protein reappeared in a transcription-dependent manner within 7 h. Yet, IL-7Ralpha was hardly re-expressed during the same period after TCR/CD28-activation. Likewise, T cells that were activated through CMV in vivo did not re-express IL-7Ralpha after in vitro culture. Functionally, IL-7-induced down-regulation of IL-7Ralpha did not hinder the responsiveness of naive T cells to IL-7. Conversely, down-regulation of IL-7Ralpha on TCR/CD28-activated cells limited IL-7 responsiveness. Strikingly, ectopic expression of IL-7Ralpha cells on TCR/CD28-activated cells conferred a selective advantage in the response to IL-7. In conclusion, our data show that IL-7- and TCR/CD28-mediated signaling differentially regulate IL-7Ralpha expression on human T cells with a transient and chronic effect, respectively. The stringent and active regulation of IL-7Ralpha may constitute a homeostatic mechanism to curtail unwarranted T cell expansion.