A Cellular MicroRNA Facilitates Regulatory T Lymphocyte Development by Targeting the FOXP3 Promoter TATA-Box Motif

The CD4(+)CD25(+)FOXP3(+) regulatory T cells (Tregs) mediate immunological self-tolerance and suppress various immune responses. FOXP3 is a key transcriptional factor for the generation and development of Tregs. Its expression is regulated by various cytokines including TGF-beta, IL-2, and IL-10. It is important to further identify the regulatory factors for Tregs. Given that many microRNAs (miRNAs) could specifically interact with the core promoter region and specifically enhance the transcription of many target genes, we searched for any possible miRNA(s) targeting the core promoter region of the FOXP3 gene. We found that miR-4281, an miRNA specifically expressed in hominids, can potently and specifically upregulate FOXP3 expression by directly interacting with the TATA-box motif in the human FOXP3 promoter. Consequently, miR-4281 significantly accelerated the differentiation of human naive cells to induced Tregs (iTregs) that possess immune suppressor functions and weaken the development of graft-versus-host disease in a humanized mouse model. Interestingly, iTregs induced by the combination of TGF-beta, IL-2, and chemically synthesized miR-4281 were more stable and functional than those induced by TGF-beta and IL-2 alone. Moreover, we found that the IL-2/STAT5 signal transduction upregulates FOXP3 expression not only through the classical pathway, but also by enhancing the expression of the miR-4281 precursor gene (SNCB) and, correspondingly, miR-4281. This study reveals a novel mechanism regulating FOXP3 expression and human iTreg development and, therefore, offers a new therapeutic target to manipulate immunosuppressive system.