RORγt and commensal microflora are required for the differentiation of mucosal interleukin 22–producing NKp46+ cells

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Abstract
Mouse lymphoid tissue–inducer (LTi) cells require the transcription factor RORγt. Cupedo's group identifies RORγt+ human LTi cell equivalents as committed natural killer cell precursors, and teams led by Vivier and Diefenbach describe RORγt-expressing interleukin 22–producing natural killer cells in mouse gut. The mucosal immune system of the intestine is separated from a vast array of microbes by a single layer of epithelial cells. Cues from the commensal microflora are needed to maintain epithelial homeostasis, but the molecular and cellular identities of these cues are unclear. Here we provide evidence that signals from the commensal microflora contribute to the differentiation of a lymphocyte population coexpressing stimulatory natural killer cell receptors and the transcription factor RORγt that produced interleukin 22 (IL-22). The emergence of these IL-22-producing RORγthiNKp46+NK1.1int cells depended on RORγt expression, which indicated that these cells may have been derived from lymphoid tissue–inducer cells. IL-22 released by these cells promoted the production of antimicrobial molecules important in the maintenance of mucosal homeostasis.