Transient local depletion of Foxp3+ regulatory T cells during recovery from colitis via Fas/Fas ligand-induced death

Regulatory T cells (Tregs) play a fundamental role in regulating the immune system in health and disease. Considerable evidence exists demonstrating that transfer of Tregs can cure colitis and a variety of other inflammatory disorders. However, little is known about the effects of inflammation on resident Tregs. Mice (BALB/c or C57BL/6) treated with an intrarectal instillation of the haptenizing agent 2,4-dinitrobenzene sulfonic acid (DNBS) develop an acute inflammatory disease, the histopathology of which peaks at 3 days posttreatment and resolves spontaneously thereafter. In this study we demonstrate that DNBS (or oxazolone)-induced colitis causes a depletion of colonic Foxp3+ Tregs 8 days posttreatment, while the proportion of Foxp3+ cells in the ileum, mesenteric lymph nodes, and spleen remains unchanged. Replenishment of the colonic Treg population was associated with the reappearance of mucosal homing (alpha4beta7+) CD4+Foxp3+ Tregs. Assessing the mechanism of local Treg depletion, we found no evidence to implicate cytokine-induced phenotypic switching in the Foxp3+ population or increased SMAD7 expression despite the essential role that TGF-beta has in Foxp3+ Treg biology. Increased Fas ligand (FasL) expression was observed in the colon of colitic mice and in vitro stimulation with a Fas cross-linking Ab resulted in apoptosis of CD4+Foxp3+ but not CD4+Foxp3- cells. Furthermore, DNBS-induced colitis in Fas/FasL-deficient mice did not result in depletion of colonic Tregs. Finally, adoptively transferred synergic Fas-/- but not Fas+/+ Tregs were protected from depletion in the colon 8 days post-DNBS treatment, thus substantiating the hypothesis that inflammation-induced local depletion of Foxp3+ Tregs in the colon of mice occurs via Fas/FasL-mediated death.