TGF-β production by eosinophils drives the expansion of peripherally induced neuropilin RORγt regulatory T-cells during bacterial and allergen challenge.

Eosinophils are best known for their effector functions in settings of parasitic infection or allergen challenge, but have also increasingly been implicated in immune regulation at mucosal sites. Here, we show using bacterial infection and antigen challenge models that extrathymic Foxp3 regulatory T-cells that arise de novo in the context of bacterial infection require an intact eosinophil compartment. Mouse strains with a constitutive or conditional eosinophil deficiency, or with an eosinophil-specific ablation of , lack bacterially induced neuropilin-negative, RORγt-positive gastrointestinal Treg populations in models of , and infection, as well as in the steady state colon and upon oral ovalbumin challenge. Treg priming in lymph nodes appears not to be impaired. Eosinophil-dependent tissue-resident Tregs express CTLA4, ICOS, CD39 and T-bet in addition to RORγt. Eosinophils reside in close proximity to Tregs in infected tissues, and specifically induce the expansion of newly formed Tregs, but not conventional T-cells in vivo and in vitro. TGF-β expression in eosinophils is induced by bacterial contact and during allergen exposure. Specific ablation in eosinophils and the associated Treg defects result in excessive T-cell responses in the examined Th2- but not Th1-polarized settings. [Image: see text]