Metabolic signaling dictates context-dependent Treg cell functional heterogeneity

ABSTRACT Regulatory T (Treg) cells play central roles in the establishment of immune tolerance but are barriers to anti-tumor immunity and immunotherapy. In response to immune- and microenvironment-derived signals, Treg cells undergo activation and differentiation from quiescent central Treg (cTreg) cells into an activated Treg cell population called effector Treg (eTreg) cells with heightened suppressive function. How Treg cells are fated for eTreg cell differentiation and functional programming in different immune contexts and tissue microenvironments remains poorly defined. Further, although the concept of Treg cell heterogeneity (beyond the cTreg versus eTreg cell paradigm) is evolving, the molecular processes shaping such heterogeneity and the functional effects remain poorly understood. Metabolic reprogramming is emerging as a regulator of Treg cell identity, differentiation, and function. Our lab has established mTORC1-orchestrated nutrient signaling and metabolic rewiring, including mitochondrial metabolic fitness, as important drivers of eTreg cell differentiation or maintenance. Although these effects are attributed to reduced mTORC1 signaling, hyperactivation of mTORC1 in Treg cells drives impaired eTreg cell lineage stability and survival, associated with a loss of immune tolerance while simultaneously improving anti-tumor responses. Thus, metabolic processes act as a rheostat to control eTreg cell functional fitness in diverse contexts, and uncovering upstream signals that regulate intracellular metabolic reprogramming in Treg cells may reveal novel ways to boost their function in autoimmune or inflammatory diseases or limit their activity in the tumor microenvironment (TME). In our preliminary studies, using genetic models of Treg cell-specific gene deletion in vivo, we uncovered a role for metabolic signaling complexes in shaping the generation of distinct eTreg cell functional states that establish tumor immunosuppression versus tissue tolerance. Specifically, we applied Treg- specific genetic targeting approaches to disrupt multiple components of autophagy- and endosome-associated pathways. Our preliminary data show that Treg cell-specific deletion of such genes led to markedly disrupted immune homeostasis, causing mice to develop a fatal Scurfy-like inflammatory disorder in conjunction with improved anti-tumor responses. Such effects were associated with altered eTreg cell accumulation and metabolic fitness and mechanistically were partly attributed to autophagy but not to endosome activity. My central hypothesis is that metabolism-associated signaling and processes downstream of autophagy initiation and endosome maturation facilitate context-dependent Treg cell functional adaptation and heterogeneity. Further, targeting such molecules may represent a novel means to tune Treg cell function in autoimmune or inflammatory diseases and cancer. Through the completion of three complementary but independent Aims, I will establish the cellular and molecular mechanisms of eTreg cell accumulation and function that are mediated by autophagy- related molecules i) under steady state and ii) within the tumor microenvironment, as well as expand my studies to iii) determine the role of endosome-associated molecules in the maintenance of Treg cell functional fitness.