ABSTRACT: The mutualistic relationship of gut-resident microbiota and cells of the host immune system promotes homeostasis that ensures maintenance of the microbial community and of a poised, but largely non-aggressive, immune cell compartment1. Consequences of disturbing this balance, by environmental or genetic factors, include proximal inflammatory conditions, like Crohn’s disease, and systemic illnesses, both metabolic and autoimmune. One of the means by which this equilibrium is achieved is through induction of both effector and suppressor or regulatory arms of the adaptive immune system. In mice, Helicobacter species induce regulatory (iTreg) and follicular helper (Tfh) T cells in the colon-draining mesenteric lymph nodes under homeostatic conditions, but can instead induce inflammatory Th17 cells when iTreg cells are compromised3,4. How Helicobacter hepaticus and other gut bacteria direct T cells to adopt distinct functions remains poorly understood. Here, we investigated which cells and molecular components are required to convey the microbial instruction for the iTreg differentiation program. We found that antigen presentation by cells expressing RORgt, rather than by classical dendritic cells, was both required and sufficient for iTreg induction. These RORgt+ cells, likely to be type 3 innate lymphoid cells (ILC3) or a recently-described population of Aire+ cells termed Janus cells5, require the MHC class II antigen presentation machinery, the chemokine receptor CCR7, and av integrin, which activates TGF-β, for iTreg cell differentiation. In the absence of any of these, instead of iTreg cells there was expansion of microbiota-specific pathogenic Th17 cells, which were induced by other antigen presenting cells (APCs) that did not require CCR7. Thus, intestinal commensal microbes and their products target multiple APCs with pre-determined features suited to directing appropriate T cell differentiation programs, rather than a common APC that they endow with appropriate functions.
