c-REL and IκB NS Govern Common and Independent Steps of Regulatory T Cell Development from Novel CD122-Expressing Pre-Precursors

Foxp3-expressing regulatory T cells (Tregs) are essential regulators of immune homeostasis and, thus, are prime targets for therapeutic interventions of diseases such as cancer and autoimmunity. c-REL and IκB NS are important regulators of Foxp3 induction in Treg precursors upon γ-chain cytokine stimulation. In c-REL/IκB NS double-deficient mice, Treg numbers were dramatically reduced, indicating that together, c-REL and IκB NS are pivotal for Treg development. However, despite the highly reduced Treg compartment, double-deficient mice did not develop autoimmunity even when aged to more than 1 y, suggesting that c-REL and IκB NS are required for T cell effector function as well. Analyzing Treg development in more detail, we identified a CD122 + subset within the CD25 − Foxp3 − precursor population, which gave rise to classical CD25 + Foxp3 − Treg precursors. Importantly, c-REL, but not IκB NS , controlled the generation of classical CD25 + Foxp3 − precursors via direct binding to the Cd25 locus. Thus, we propose that CD4 + GITR + CD122 + CD25 − Foxp3 − cells represent a Treg pre-precursor population, whose transition into Treg precursors is mediated via c-REL.