Signals Controlling Regulatory T Cell Differentiation and Homeostasis

CD4+Foxp3+ regulatory T cells (Treg)s are essential for the prevention of autoimmunity. Treg lineage commitment requires T cell receptor (TCR) interactions that induce expression of foxp3, whose protein product enforces Treg fate. Treg homeostasis is critical for self-tolerance and is achieved through both Treg generation and maintenance. Treg maintenance occurs in part through a process of self-renewing cell division of existing Tregs. This self-renewing Treg division has been shown to be TCR dependent. Despite the crucial role of the TCR in Treg generation and maintenance, neither the specific signaling pathways that control Treg generation nor the nature of the TCR signals required for their division in the periphery are well understood. Here, we demonstrated that dendritic cells (DC)s coordinate Treg division in vitro. DCs elicit interleukin-2 (IL-2) production from conventional CD4+ T cells (Tconv)s in a major histocompatibility complex class II (MHCII)-dependent fashion. Tconv-derived IL-2 cooperates with contact-dependent signals from DCs to induce Treg division. Contrary to prior studies, we showed that in the presence of exogenous IL-2, Treg division becomes MHCII-independent in vitro. Treg division required only MHCII-independent DC-derived signals and a source of IL-2. Next, we found that peripheral Tregs can divide in the absence of TCR signaling in vivo if exogenous IL-2 receptor (IL-2R) agonists are administered. Furthermore, activation of the IL-2-induced STAT5 pathway is minimally sufficient to support Treg division independent of TCR signaling. These data suggest that depending on the degree of concomitant IL-2 receptor/STAT5 activation, a range of TCR signals can sustain Treg division. In addition to Treg division, we also investigated the TCR signals that promote Treg development. Our preliminary experiments showed that diacylglycerol (DAG) signaling promotes Treg differentiation. Deletion of diacylglycerol kinase ζ, a negative regulator of this pathway, resulted in augmented DAG signaling and enhanced Treg generation. We hypothesize that DAG signaling enhances NF-κB activation through c-Rel, a transcription factor known to promote Treg differentiation directly. Future studies are necessary to establish the DAG pathway as a link between TCR signaling and Treg differentiation