Tuft cell-derived acetylcholine regulates epithelial fluid secretion and helminth clearance

Abstract

Helminth, or parasitic worm, infection afflicts nearly one third of humans worldwide, resulting in massive suffering and comorbidity. The arm of the immune system responsible for combatting helminth infection, called “Type 2” immunity, is also responsible for causing a variety of allergic diseases. Understanding how Type 2 immunity is regulated has the potential to inform both better treatment for helminth infection as well as allergic disease.Mucosal barrier tissues are the site of exposure to Type 2 stimuli and the locus of the ensuing immune response. Both the airways and the gastrointestinal tract are protected by a gradient of mucus, antimicrobial peptides, and fluid secreted by epithelial cells that contribute to host defense and enable key physiological functions of the tissues. The Type 2 immune response triggers epithelial remodeling that increases fluid and mucus secretion as well as contractility of underlying smooth muscle in order to flush away helminths and allergens. Acetylcholine is a key regulator of both epithelial secretion and muscle contraction. Collectively this response is known as “weep and sweep” and familiar to anyone who experiences seasonal allergies. While the effects of Type 2 immunity have been well understood for decades, the initiation of the response remained a mystery until recently, when tuft cells were discovered as the key sensors of intestinal helminth infection. Tuft cells are epithelial cells that possess chemosensory machinery linking sensing of lumenal stimuli via a suite of receptors to basolateral secretion of immune- and neuro-modulating factors including IL-25, leukotrienes, and acetylcholine (ACh). In response to small intestinal (SI) helminth infection or succinate, a metabolite produced by protist and bacterial colonization, tuft cells secrete IL-25 and leukotrienes to activate Group 2 innate lymphoid cells (ILC2s) that produce the hallmark Type 2 cytokines IL-5 and IL-13. IL-13 signals on the intestinal stem cell compartment to drive preferential differentiation of mucus-producing goblet cells and, intriguingly, tuft cells, which rapidly remodels the epithelium in a matter of days. This remodeling, and thus tuft cells, are required for helminth clearance. Additional functions of SI tuft cells or tuft-derived ACh are not known. We show that in response to sensing of succinate or direct activation of the chemosensory ion channel TRPM5, SI tuft cells secrete ACh to induce epithelial fluid secretion in the intestine and airways, independently of neurons. Unlike other tissues where nearly 100% of tuft cells express the enzyme Chat required for ACh synthesis, the frequency of Chat+ SI tuft cells occurred in a gradient from the proximal to distal SI, increasing from approximately 40% to 80% of tuft cells. Succinate-induced fluid secretion was restricted to the distal SI where Chat+ frequency and SUCNR1 expression was highest. Consistent with their high expression of Sucnr1, tuft cells in the trachea also responded to succinate by inducing ACh-dependent fluid secretion. In the proximal SI and colon tuft cell activation via the TRPM5 agonist Class 8 induced fluid secretion, but the exact ligands sensed by tuft cells in these tissues remains to be determined. Oral administration of Class 8 induced fluid secretion in vivo as measured by fecal water content. During Type 2 tissue remodeling, Chat+ tuft cells increase in number, enhancing the fluid secretion response. Upon helminth infection, mice with Chat-deficient tuft cells experience delayed helminth clearance despite normal tuft-ILC2 circuit activation. We conclude that tuft cell-derived ACh regulates epithelial fluid secretion, and that this effector function can contribute to Type 2 immune responses during helminth infection. By coupling chemosensing to rapid epithelial fluid secretion, tuft cells coordinate an epithelium-intrinsic effector unit that can flush offending agents away from the tissue.