Forkhead box P3 (FOXP3)-positive Treg cells are crucial for maintaining immune homeostasis. FOXP3 cooperates with its binding partners to elicit Treg cells' signature and function, but the molecular mechanisms underlying the modulation of the FOXP3 complex remain unclear. Here we report that Deleted in breast cancer 1 (DBC1) is a key subunit of the FOXP3 complex. We found that DBC1 interacts physically with FOXP3, and depletion of DBC1 attenuates FOXP3 degradation in inflammatory conditions. Treg cells from Dbc1-deficient mice were more resistant to inflammationmediated abrogation of Foxp3 expression and function and delayed the onset and severity of experimental autoimmune encephalomyelitis and colitis in mice. These findings establish a previously unidentified mechanism regulating FOXP3 stability during inflammation and reveal a pathway for potential therapeutic modulation and intervention in inflammatory diseases.+ Treg cells are actively engaged in the prevention of autoimmunity and the mitigation of aberrant or excessive immune responses (1-3). The transcription factor Forkhead box P3 (denoted "FOXP3" in humans, and "Foxp3" in mice) is a well-characterized marker of Treg cells, and its expression typically is considered a requisite for Tregcell differentiation and function (4, 5). FOXP3 deficiency leads to the scurfy phenotype in mice and to the immune dysregulation, polyendocrinopathy, and enteropathy, X-linked syndrome in humans (6). Moreover, Treg-cell function is impaired in several autoimmune and inflammatory diseases, including colitis, rheumatoid arthritis, multiple sclerosis, and systemic lupus erythematosus (7). Thus, the manipulation of Treg-cell function might provide a practical approach to the treatment of autoimmune and inflammatory diseases.Despite the central role of FOXP3 in Treg cells, many questions remain regarding the molecular mechanisms by which FOXP3 regulates Treg-cell function. Foxp3 protein is expressed transiently in CD4 +
CD25− effector T cells upon T-cell receptor (TCR) stimulation but does not generate T cells with suppressive activity (8, 9). It has become evident that Foxp3 alone is insufficient to reproduce completely the differentiation and functional characteristics of Treg cells (10-12). FOXP3 binds with its partners to form multiple positive and negative feedback loops to regulate Treg-cell function subtly (13). FOXP3 interacts with FOXP1 to form heterodimers that promote FOXP3-mediated repression of IL-2 production (14). FOXP3 also binds with several nuclear factors, such as GATA3 (11, 15), RORγt (16), Eos (17), and RUNX1 (18), to drive the Treg cells' genetic program. FOXP3 function also is regulated at the posttranslational level. FOXP3 has been shown to interact with the acetyltransferase Tat-interaction protein 60 kDa (TIP60) to promote FOXP3 acetylation, which is required for Treg cells' suppressive function (19). P300 also regulates (11); therefore, many questions remain regarding the differential modulation of this complex and its effect on Treg-cell differenti...