IMMUNOBIOLOGY FoxP3 maintains Treg unresponsiveness by selectively inhibiting the promoter DNA-binding activity of AP-1

Regulatory T cells (Tregs) have been shown to play a crucial role in maintain-ing self-tolerance and suppressing autoimmunity. The forkhead transcription factor, FoxP3, is a key molecule neces-sary and sufficient for Tregs development and function. However, the molecular mechanisms by which FoxP3 regulates the phenotypic (anergic) and the func-tional (suppressive) characteristics of Tregs are not well defined. Here we found that the promoter DNA-binding activity of AP-1 transcription factors is selectively inhibited in the naturally occurring CD4 CD25 Tregs from mice. The impaired AP-1 DNA binding is not the result of the decreased nuclear translocation of AP-1 family transcription factors, including c-Jun, JunB, and c-Fos. FoxP3 significantly suppresses both the transcriptional activ-ity and promoter DNA-binding of AP-1 by interacting with c-Jun. The N-terminus of FoxP3, but not its C-terminus forkhead domain, specifically interacts with phos-phorylated c-Jun and alters c-Jun sub-nuclear distribution. This N-terminus of FoxP3 with nuclear localization signals (FoxP3N/NLS) is able to suppress AP-1 transcriptional activity. Ectopic expres-sion of FoxP3N/NLS sufficiently induces the unresponsiveness of mouse primary CD4 CD25 T cells, whereas the full-length FoxP3 is required for the suppres-sive functions of Tregs. These findings uncover one of the mechanisms underly-ing how FoxP3 maintains the unrespon