TCR triggering modulates the responsiveness and homeostatic proliferation of CD4+ thymic emigrants to IL-7 therapy.

Interleukin-7 (IL-7) is currently used in clinical trials to augment T-cell counts. Paradoxically, elevated systemic IL-7 found in lymphopenic humans is typically insufficient for CD4(+) T-cell regeneration, and thymopoiesis becomes critical in this process. Here we show that the proliferative effect of IL-7 is more pronounced on CD4(+)CD8(-) thymocytes compared with peripheral CD4(+) T cells. These cells express miR181a at higher levels and respond to lower concentrations of IL-7. As single-positive CD4(+) thymocytes (CD4(+)(SPT)) exit the thymus, they rapidly diminish their proliferation to IL-7 therapy, and this is mediated, at least in part, by major histocompatibility complex class II distribution outside the thymus. Interestingly, increasing T-cell receptor (TCR) stimulation augments IL-7 responsiveness and proliferation of peripheral CD4(+) T cells, whereas failure to stimulate TCR abrogates proliferation induced by IL-7. Finally, we demonstrated that IL-7 enhances the proliferation of CD4(+) T cells that undergo "slow proliferation" in lymphopenic hosts. To date, our results indicate that TCR signaling is a major controlling factor for CD4 responsiveness and proliferation to IL-7 therapy.