Characterisation of Foxp3 splice variants in human CD4+ and CD8+ T cells--identification of Foxp3Δ7 in human regulatory T cells.

Foxp3 is proposed to play a critical role in the development and function of regulatory T cells. Functional and transgenic studies in mice propose Foxp3 as a "regulatory T cell lineage specification factor" but conflicting data exist in humans. Expression of multiple Foxp3 splice variants in humans represents an additional layer of complexity for this transcription factor and acts as a possible mechanism of regulating protein diversity. We report the identification of a novel splice variant of Foxp3, called Foxp3Δ7, in ex vivo CD4+CD25+ T cells and CD8+ regulatory T cell clones. Foxp3Δ7 lacks the 81bp region that encodes exon 7 of Foxp3, which is a part of the leucine zipper domain of the protein. The three splice variants of Foxp3 namely Foxp3FL, Foxp3Δ2 and Foxp3Δ7 are co-expressed in ex vivo human CD4+CD25+ T cells and CD8+ Treg clones. Stimulation of freshly isolated CD4+CD25+ T cells with anti-CD3 and anti-CD28 antibodies leads to a 140-fold upregulation of Foxp3Δ7 within 24h of stimulation, which is ∼10-fold greater than that observed in stimulated CD4+CD25- T cells. In addition, resting CD8+ Treg cells have decreased expression of Foxp3FL and Foxp3Δ2; however they have a 10-fold higher expression of Foxp3Δ7, in comparison to ex vivo CD4+CD25+ T cells. In order to assess the functional effects of these Foxp3 isoforms, we carried out lentivirus expression studies. All three isoforms were capable of inducing increased levels of CD25 expression in primary human CD4+ T cells, along with a tendency to decreased levels of CD127. Further investigation into pathways that alter the relative proportions of Foxp3 isoforms, and hence their interaction with other transcriptional co-regulators, will help to define the role of Foxp3 isoforms in immune regulation.