Paradoxically Increased FOXP3+T Cells in IBD Do Not Preferentially Express the Isoform of FOXP3 Lacking Exon 2

Forkhead box P3 (FOXP3)+ regulatory T cells (Tregs) are critical for controlling inflammation in the gastrointestinal tract. There is a paradoxical increase of mucosal FOXP3+ T cells in patients with inflammatory bowel disease (IBD). These FOXP3+ cells were recently shown to include interleukin (IL)-17A-producing cells in Crohn's disease, resembling Th17 cells implicated in autoimmune diseases. FOXP3 inhibits IL-17A production, but a naturally occurring splice variant of FOXP3 lacking exon 2 (Delta exon2) cannot. We hypothesized that IBD patients preferentially express the Delta exon2 variant of FOXP3 so the paradoxically increased mucosal Tregs in IBD could represent cells expressing only Delta exon2. We used antibodies and primers that can distinguish between the full-length and Delta exon2 splice variant of FOXP3 to evaluate expression of these isoforms in human intestinal tissue by immunohistochemistry and quantitative polymerase chain reaction (PCR), respectively. No difference in the expression pattern of Delta exon2 relative to full-length FOXP3 was seen in ulcerative colitis or Crohn's disease versus non-IBD controls. By immunofluorescence microscopy and flow cytometry, we also did not find individual cells which expressed FOXP3 protein exclusively in the Delta exon2 isoform in either IBD or control tissue. FOXP3+ mucosal CD4+ T cells from both IBD and control specimens were able to make IL-17A in vitro after phorbol myristate acetate (PMA) and ionomycin stimulation, but these cells did not preferentially express Delta exon2. Our data do not support the hypothesis that selective expression of FOXP3 in the Delta exon2 isoform accounts for the inability of copious FOXP3+ T cells to inhibit inflammation or IL-17 expression in IBD.