TCR signal strength during positive selection shapes CD25 expression patterns on thymically derived regulatory T cells.

Regulatory T cells (Tregs) are a suppressive subset of CD4 T cells that maintain immune homeostasis. Most Tregs develop in the thymus but how thymic selection impacts peripheral Treg fate remains unknown. Here we show that the strength of TCR signal experienced during positive selection in the thymic cortex impacts how CD4 SP thymocytes respond to Treg inducing signals in the medulla. Thymocytes that experienced weak signals during positive selection (identified as CD5LO CD4 SP) tend to produce Foxp3+ cells that lack the classic Treg marker CD25 and show a greater dependence on TGFβ than IL-2 for Treg induction in vitro. Moreover, CD4 clones that give rise to CD25- thymic Tregs also produce CD25- peripheral Tregs. These data indicate that positive selection provides critical context for how CD4 T cells respond to TCR agonist and cytokine signals, leading to an alternative lineage of Tregs that lacks constitutive CD25 expression.