Dual T cell receptor expression provides intrinsic advantage for agonist selection of thymic FoxP3+ regulatory T cells (Tregs) in the neonatal thymus.

Co-expression of 2 T cell receptor (TCR) clonotypes due to allelic inclusion occurs in ∼16% of peripheral blood T cells in mice and humans. Evidence indicates dual TCR expression can affect thymic development and peripheral function. Notably, dual TCR cells demonstrate increased reactivity against self-antigens and heightened TCR signaling, properties that could favor development of FoxP3+ regulatory CD4+ T cells (Tregs). However, the impact of dual TCR expression on Treg development remains controversial. We hypothesize that this discrepancy arises from 2 opposing factors: While dual TCR expression enhances TCR signal strength and promotes agonist selection into the Treg lineage, the capacity of the thymic Treg niche may impose extrinsic limitations that obscure this effect. To test this hypothesis, we utilized the B6.TCRA-GFP/RFP reporter model, which enables precise identification of single and dual TCRα-expressing cells. We found that dual TCR expression is associated with increased frequencies of thymic Tregs in neonatal (day 10) mice, an age characterized by robust Treg output, but not in adults. In both age groups, dual TCR Tregs exhibited elevated expression of Nur77, consistent with stronger TCR signaling during selection. However, in adult mice, the presence of recirculating peripheral Tregs reduces niche availability, thereby diminishing the selection advantage conferred by dual TCR expression. These findings reveal a previously underappreciated role for dual TCR expression in shaping thymic Treg development and provide insight into mechanisms of self-tolerance in neonates and in settings such as hematopoietic stem cell transplantation.