The liver-brain-gut neural arc maintains the Treg cell niche in the gut.

The gut–brain axis, a reciprocal interaction between the central nervous system (CNS) and peripheral intestinal functions, is conceptually feasible from recent clinical and experimental evidence showing mutual interactions between the CNS and gut microbiota that are closely associated with the bidirectional effects of inflammatory bowel diseases (IBDs) and CNS disorders1–4. Despite recent advances in our understanding of neuroimmune interactions, it remains unclear how the gut and brain communicate to maintain gut immune homeostasis, including induction and maintenance of peripheral regulatory T cells (pTreg cells) and what environmental cues prompt the host to protect host from development of IBDs. Here, we report a novel liver-brain-gut neural arc that ensures proper differentiation and maintenance of pTreg cells in the gut. The hepatic vagal sensory afferents were responsible for indirectly sensing the gut microenvironment and relaying the sensory inputs to the nucleus tractus solitarius of the brainstem, and ultimately to the vagal parasympathetic nerves and enteric neurons. Surgical and chemical perturbation of the vagal sensory afferents at the hepatic afferent level significantly impaired colonic pTreg cells, which was attributed to impairment of aldehyde dehydrogenase (ALDH) expression and retinoic acid (RA) synthesis by intestinal antigen-presenting cells (APCs). Muscarinic Ach receptor (mAChR) activation directly induced ALDH gene expression both in human and mouse colonic APCs, whereas genetic ablation of mAChRs abolished APC excitement in vitro. Disruption of left vagal sensory afferents from the liver to the brainstem in colitis models reduced the colonic pTreg pool, resulting in increased susceptibility to colitis. These results demonstrate that the novel vago-vagal liver–brain–gut reflex arc tunes the number of pTreg cells and maintains the gut homeostasis. Intervening in this autonomic feedback feed-forward system could help develop new therapeutic strategies to treat or prevent immunological disorders of the gut.