The forkhead/winged-helix transcription factor FOXP3 confers suppressive ability to CD4(+)FOXP3(+) regulatory T (Treg) cells. Human Treg cells express several different isoforms of FOXP3 that differ in function. However, the regulation and functional consequences of FOXP3 isoform expression remains poorly understood. In order to study the function of the FOXP3Δ2Δ7 isoform in vivo we generated mice that exclusively expressed a Foxp3 isoform lacking exon 2 and 7. These mice exhibited multi-organ inflammation, increased cytokine production, global T cell activation, activation of antigen-presenting cells and B cell developmental defects, all features that are shared with mice completely deficient in FOXP3. Our results demonstrate that the mouse counterpart of human FOXP3Δ2Δ7 is unable to confer suppressive ability to Treg cells.
