Isoform-specific inhibition of RORα-mediated transcriptional activation by human FOXP3

FOXP3 is a forkhead family transcriptional repressor important for the development and function of CD4(+)CD25(+) regulatory T cells. In humans, FOXP3 is expressed as two isoforms, a full-length form and a smaller form lacking exon 2. These two isoforms are expressed in approximately equal amounts in circulating regulatory T cells, and are induced equally in freshly activated CD4(+)CD25(-) T cells. Herein, we show that FOXP3 interacts with retinoic acid receptor-related orphan receptor (ROR)alpha, and that this interaction inhibits transcriptional activation mediated by ROR alpha. Full-length FOXP3, but not the isoform lacking exon 2, interacts with ROR alpha, and the region of FOXP3 involved in the interaction is encoded by exon 2. Mutation of the LxxLL motif in FOXP3, located in exon 2, abolished interaction and repression by FOXP3. Additionally, the inhibition of ROR alpha by FOXP3 does not require an intact forkhead domain, demonstrating a mode of FOXP3 function that is independent of DNA binding. Interestingly, expression of ROR alpha in T cells leads to the expression of genes that define Th17 cells, and the expression of each of these gene was inhibited by coexpression of full-length, but not Delta Ex2, FOXP3. These data expand the possible targets of FOXP3-mediated repression and demonstrate functional differences between FOXP3 isoforms.