Cell-Intrinsic NF-κB Activation Is Critical for the Development of Natural Regulatory T Cells in Mice

Background: Naturally occurring CD4 + CD25 + Foxp3 + regulatory T (Treg) cells develop in the thymus and represent a mature T cell subpopulation critically involved in maintaining peripheral tolerance. The differentiation of Treg cells in the thymus requires T cell receptor (TCR)/CD28 stimulation along with cytokine-promoted Foxp3 induction. TCR-mediated nuclear factor kappa B (NF-kB) activation seems to be involved in differentiation of Treg cells because deletion of components of the NF-kB signaling pathway, as well as of NF-kB transcription factors, leads to markedly decreased Treg cell numbers in thymus and periphery. Methodology/Principal Findings: To investigate if Treg cell-intrinsic NF-kB activation is required for thymic development and peripheral homeostasis of Treg cells we used transgenic (Tg) mice with thymocyte-specific expression of a stable IkBa mutant to inhibit NF-kB activation solely within the T cell lineage. Here we show that Treg cell-intrinsic NF-kB activation is important for the generation of cytokine-responsive Foxp3 2 thymic Treg precursors and their further differentiation into mature Treg cells. Treg cell development could neither be completely rescued by the addition of exogenous Interleukin 2 (IL-2) nor by the presence of wild-type derived cells in adoptive transfer experiments. However, peripheral NF-kB activation appears to be required for IL-2 production by conventional T cells, thereby participating in Treg cell homeostasis. Moreover, pharmacological NF-kB inhibition via the IkB kinase b (IKKb) inhibitor AS602868 led to markedly diminished thymic and peripheral Treg cell frequencies. Conclusion/Significance: Our results indicate that Treg cell-intrinsic NF-kB activation is essential for thymic Treg cell differentiation, and further suggest pharmacological NF-kB inhibition as a potential therapeutic approach for manipulating this process.