TOR Complex 1 and TOR Complex 2 Differentially Regulate Effector and Regulatory T Cell Differentiation

Effector T cell lineage commitment is determined by the integration of multiple and sometimes opposing signals. Our lab has identified mTOR, an evolutionarily conserved serine/threonine kinase, as a crucial protein dictating the outcome of T cell fate in response to antigen. To do this we utilized a Cre-lox system to conditionally delete mTOR in T cells. In such mice, although mTOR is deleted in the double-positive stage, lymphocyte populations in the spleen and the periphery are comparable to wild-type mice. T cells lacking mTOR proliferate more slowly but secrete appropriate levels of IL-2 upon initial stimulation. However, such cells fail to differentiate into Th1, Th2 or Th17 effector T cells under the appropriate skewing conditions. This failure to differentiate is the result of decreases in appropriate STAT activation and the concomitant lack of upregulation in lineage specific transcription factors. Notably, under normally activating conditions, T cells lacking mTOR preferentially differentiate into Foxp3⁺ regulatory cells. Supporting this observation, mTOR deficient T cells display hyperactive Smad3 activation, even in the absence of exogenous TGF-β.

mTOR signals through two known signaling complexes, TORC1 and TORC2. TORC1 contains Rheb, mTOR, GβL, and raptor, while TORC2 contains mSin1, mTOR, GβL, and rictor. In order to determine the specific role of TORC1 in T cell lineage commitment we conditionally deleted Rheb in T cells. Upon activation such cells fail to phosphorylate the TORC1 substrate S6K-1 while demonstrating normal TORC2 activity. As was the case for the mTOR^−/− T cells, Rheb^−/− T cells fail to differentiate into Th1 and Th17 cells when skewed in vitro. However, unlike mTOR^−/− T cells, the Rheb deficient T cells are capable of becoming Th2 cells. In spite of lacking TORC1 activity, T cells lacking Rheb do not spontaneously develop into Foxp3⁺ cells. Such observations implicate a specific and novel role for Rheb in regulating T cell lineage commitment. Overall, our data identify mTOR as a regulator of T cell lineage commitment through which TORC1 and TORC2 signaling differentially regulate T cell fate. These findings support a novel paradigm whereby T cell activation induces a default pathway of differentiation to regulatory T cells and that TORC2 signaling is required to divert differentiation to appropriately programmed effector lineages.