Epigenetic Checkpoint Regulation of Naive T-cell Quiescence and Peripheral Tolerance

Epigenetic checkpoint regulation of naive T-cell quiescence and peripheral tolerance Maintaining the quiescence of autoreactive naïve T cells and preventing them from reacting to self-antigens are crucial for T-cell peripheral tolerance, contributing to immune homeostasis and suppression of autoimmune diseases. We seek to understand how this process is regulated by T-cell-extrinsic and -intrinsic mechanisms. The former includes regulatory-T-cell-mediated immunosuppression, and the latter covers checkpoint regulators of conventional naïve T-cell activation and effector differentiation. These cell-intrinsic negative mechanisms re- main to be fully explored, given that causes of numerous autoimmune disorders are still elusive, despite signifi- cant progress in our understanding of regulatory T cells and the final effector stage of inflammation and damage. Immunogenetic studies suggest that environmental factors, such as nutrients, significantly impact the im- mune system. However, how T-cell homeostasis and immune tolerance are programmed by nutritional cues is not well defined. Although routinely considered permissive factors, they can profoundly affect T-cell metabolism, signal transduction, and gene regulation, raising questions about how they shape T-cell peripheral tolerance. To explore this, we searched for cell-intrinsic checkpoint regulators of conventional T-cell reactivity to self-antigens. This led to the discovery of the ascorbate transporter Slc23a2 as a negative regulator of the activation and effector differentiation of naïve T cells, independent of regulatory T cells. We confirmed that Slc23a2 contributes to high levels of T-cell ascorbate. Although ascorbate plays several biochemical roles, we propose its activity via TET methylcytosine dioxygenases (or DNA demethylases) in maintaining naïve T-cell quiescence and restricting activation and differentiation to effector T cells. We also observed reduced ascorbate and increased DNA meth- ylation in naïve T cells from aged animals and humans, suggesting this axis prevents spontaneous T-cell acti- vation and effector differentiation during aging that cause inflammaging. We plan to test the hypothesis that the ascorbate-TET axis acts as a checkpoint regulator, restricting the activation and effector differentiation of autoreactive naïve T cells in response to self-antigen stimulation. We will approach this from several angles. First, we will determine how ascorbate deficiency in T cells causes low-grade chronic autoimmunity, akin to inflammaging. Then, we will establish the role of the ascorbate-TET axis in regu- lating T-cell activation and effector differentiation. Finally, we will unveil how the ascorbate-TET axis acts step- wise to suppress naïve T-cell activation and differentiation into effector T cells upon TCR stimulation. Successful completion of our study will reveal a crucial nutrient-epigenetic axis involving cell-intrinsic ascor- bate and TET-DNA demethylation in maintaining naïve T-cell quiescence and peripheral tolerance, particularly in suppressing T-cell inflammaging-like autoimmunity. The mechanistic insights gained in this project could en- hance the diagnosis, prevention, or treatment of autoimmune diseases, especially during aging.