Naturally derived regulatory T (Treg) cells are characterized by stable expression of the transcription factor Foxp3 and characteristic epigenetic imprinting at the Foxp3 gene locus. Here, we found that enhancing nuclear factor (NF)-κB activity via a constitutive active inhibitor of κB kinase β (IKKβ) transgene in T cells led to increased number of Foxp3+ cells in the thymus and can rescue Foxp3 expression in thymocytes deficient in other pleiotropic signaling molecules. Enhancing the signal strength of the NF-κB pathway also induced Foxp3 expression in otherwise conventionally selected T cells. NF-κB directly promoted the transcription of Foxp3, and upon T cell receptor (TCR) stimulation, c-Rel, a NF-κB family member, bound to Foxp3 enhancer region, which is specifically demethylated in natural Treg cells. Hence, NF-κB signaling pathway is a key regulator of Foxp3 expression during natural Treg cell development.
