Background: The gut is densely colonized with harmless microbes that reside in close proximity to host cells, highlighting the need for active immune tolerance to commensal microbes. Reciprocally, fractured tolerance to commensal microbiota is increasingly recognized in the pathogenesis of inflammatory bowel disease. Fundamental knowledge gaps remain as to how commensal tolerance is achieved.

Methods and Results: To fill these knowledge gaps, an instructive preclinical model to precisely identify CD4+ T cells with surrogate commensal specificity was developed by establishing intestinal colonization with recombinant Candida albicans. Tracking endogenous CD4+ T cells with defined commensal specificity using MHC class II tetramers (Fig. 1A) and RNA profiling (Fig. 1B, C) shows nearly half of the cells that expand in response to microbiota stimulation are unified by expression of the zinc finger transcription factor Kruppel-like factor-2 (KLF2). Mice with conditional deletion of KLF2 in CD4+ T cells develop spontaneous colitis with rectal prolapse and increased Th17 inflammation (Fig. 2A-D). These phenotypes reflect microbiota driven inflammation since each is overturned after eradication of commensal microbes (Fig. 2A-D). Purified KLF2+ cells suppress responder T cell proliferation and intestinal inflammation in classical T cell driven colitis models. Importantly, suppression by KLF2+ CD4 T cells is independent from classical FOXP3+ regulatory T cells. KLF2+ T cells selectively produce the pleotropic immune suppressive cytokine IL10. IL10 production by KLF2+ T cells is essential for their suppressive function in vitro and protection against intestinal inflammation in vivo (Fig. 2E, F). These preclinical observations dovetail nicely with selectively reduced accumulation of KLF2+ CD4+ T cells, but normal levels of FOXP3+ regulatory T cells, in intestinal biopsies of Crohn’s disease patients (Fig. 2G).

Conclusions: KLF2 identifies a new suppressive lineage of IL10 producing CD4 T cells that sustain commensal tolerance and are naturally reduced in human Crohn’s disease. These findings significantly expand our knowledge of how commensal tolerance works by identifying a previously undefined subset of suppressive CD4 T cells that produce IL10 in response to commensal microbiota stimulation. In turn, naturally reduced accumulation of this population in human Crohn’s disease sheds important new light on the immune-pathogenesis inflammatory bowel disease and innovative therapeutic approaches for suppressing aberrant intestinal inflammation.

Tzu-yu Shao

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