Absence of Leucine Zipper in the Natural FOXP3Δ2Δ7 Isoform Does Not Affect Dimerization but Abrogates Suppressive Capacity

Background: Phenotype and function of regulatory T cells (Treg) largely depend on the presence of the transcription factor FOXP3. In contrast to mice, human Treg cells express isoforms of this protein. Besides the full length version (FOXP3fl), an isoform lacking the exon 2 (FOXP3 Delta 2) is co-expressed in comparable amounts. Recently, a third splice variant has been described that in addition to exon 2 also misses exon 7 (FOXP3 Delta 2 Delta 7). Exon 7 encodes for a leucine zipper motif commonly used as structural dimerization element. Mutations in exon 7 have been linked to IPEX, a severe autoimmune disease suggested to be caused by impaired dimerization of the FOXP3 protein. Principal Findings: This study shows that the lack of exon 7 does not affect (homo-) dimerization. Moreover, the interaction of FOXP3 Delta 2 Delta 7 to RUNX1, NFAT and NF-kB appeared to be unchanged in co-immunoprecipitation experiments and reporter gene assays, when compared to FOXP3fl and FOXP3 Delta 2. Nevertheless, retroviral transduction with FOXP3 Delta 2 Delta 7 failed to induce the typical Treg-associated phenotype. The expression of FOXP3-induced surface molecules such as CD25 and CTLA-4 were not enhanced in FOXP3 Delta 2 Delta 7 transduced CD4+ T cells, which also failed to exhibit any suppressive capacity. Notably, however, co-expression of FOXP3fl with FOXP3 Delta 2 Delta 7 resulted in a reduction of CD25 expression by a dominant negative effect. Conclusions: The leucine zipper of FOXP3 does not mediate dimerization or interaction with NFAT, NF-kB and RUNX1, but is indispensable for the characteristic phenotype and function in Treg cells. FOXP3 Delta 2 Delta 7 could play a role in regulating the function of the other FOXP3 isoforms and may be involved in cancer pathogenesis, as it is overexpressed by certain malignant cells.