Defining the Peripheral and Thymic Niche of Regulatory T Cells

To be effective in recognizing a wide variety of pathogens, the adaptive immune system must generate a diverse repertoire of antigen receptors. However, the cost of this diversity is that some receptors can recognize self-antigens and cause autoimmunity. For T cells, two complementary mechanisms keep self-reactive T cells in check--one is the deletion of self-reactive developing thymocytes or negative selection, and the other is the generation of Foxp3+ regulatory T cells (Tregs), which can arise both in the thymus and in the periphery. Previous sequencing studies using fixed TCR-beta chain transgenic mice have shown that despite a slight overlap between the freely rearranging alpha chains found on conventional T cells (Tconv) and Tregs, their TCR repertoires are largely unique. In order to further investigate the role of TCR specificity in thymic Treg development and their subsequent maintenance in the periphery, we generated transgenic mice bearing either Treg or naïve Tconv receptors. We were surprised to find that Treg TCR transgenic mice generated very few thymic Tregs when crossed to the Rag1-deficient background. We resolved this apparent paradox by carefully titrating the TCR transgenic bone marrow with wild-type filler, when generating mixed bone marrow chimeras. We discovered that upon thymic reconstitution, Treg development for these cells are efficient only if the precursor frequency is low, suggesting that thymic Treg development is subject to intraclonal competition. We\u27ve extended this study to determine if despite this limiting niche, mechanisms are in place to maximize the efficiency of thymic Treg induction. We found that self-reactive CD4SP thymocytes, including Treg precursors, undergo developmental arrest so they are exported much later than Tconv cells, giving them more time to encounter the selecting antigen that could trigger their deletion or induce Foxp3. Finally, using the TCR transgenic mice we\u27ve generated, we also explored the niche occupied by Tregs in the periphery. We found that this niche reflects the competitive fitness of Tregs and is very sensitive to the availability of IL-2 and of self- antigen. Overall, our studies indicate that TCR specificity is important not only for the development of Tregs, but for their maintenance in the periphery as well