Abstract
Low-dose IL-2 is being advanced as a therapy for autoimmunity based on selectivity toward Tregs. Regarding human CD4+ Treg and effector memory (TEM) T cells, the IL-2 sensitivity of Tregs is 10- and 100-fold greater for proximal signaling and down-stream gene activation, respectively. To further investigate the efficiency of IL-2R signaling in Tregs, we monitored the persistence of a single wave of pSTAT5 and assessed if this supported IL-2-dependent transcription. When Treg and TEM cells were pulsed with a high dose of IL-2 to normalize initial STAT5 activation, pSTAT5 persisted for >2 hr only in Tregs. Lower amounts of pulsed-IL-2 also led to persistent pSTAT5 in Tregs. The short persistence of pSTAT5 in TEM cells is not cell intrinsic because pulsing TEM cells with IL-7 also led to lasting pSTAT5. Thus, the high expression of CD25 and CD127 in Treg and TEM cells likely contributed to persistent pSTAT5. This conclusion implies that a component of proximal STAT5-dependent signaling is limiting in comparison to the numbers of bound receptors. RNA Prime Flow demonstrated that transient pulse of IL-2 or IL-7 in Treg and TEM cells caused upregulation of CISH mRNA. These results indicate that a limited exposure of T cells expressing high amounts of the IL-2R and IL-7R are linked to a mechanism to sustain proximal signaling to support gene transcription. This mechanism may fine tune gene expression enabling cells to maximize utilization of limiting cytokines in vivo.