Regulation of immune responses by tuft cells

Abstract

Tuft cells are rare, secretory epithelial cells that generated scant immunological interest until contemporaneous reports in 2016 linked tuft cells with type 2 immunity in the small intestine. Tuft cells have the capacity to produce an unusual spectrum of biological effector molecules, including IL-25, eicosanoids implicated in allergy (such as cysteinyl leukotrienes and prostaglandin D₂) and the neurotransmitter acetylcholine. In most cases, the extracellular signals controlling tuft cell effector function are unknown, but signal transduction is thought to proceed via canonical, G protein-coupled receptor-dependent pathways involving components of the signalling pathway used by type II taste bud cells to sense sweet, bitter and umami compounds. Tuft cells are ideally positioned as chemosensory sentinels that can detect and relay information from diverse luminal substances via what appear to be stereotyped outputs to initiate both positive and aversive responses through populations of immune and neuronal cells. Despite recent insights, numerous questions remain regarding tuft cell lineage, diversity and effector mechanisms and how tuft cells interface with the immunological niche in the tissues where they reside.

Figure 1.. The small intestinal tuft cell ILC2 circuit.

Rare gustatory epithelial tuft cells detect the presence of luminal pathosymbionts, including the protist Tritrichomonas and intestinal helminths. Tuft cells activate IL-17RB-expressing (IL-25R competent) ILC2s in the lamina propria in an IL-25-dependent manner, which involves taste sensation components; this, along with potential uncharacterized mechanisms, results in release of IL-25 from tuft cells. Activated ILC2s increase IL-13 expression, which acts on epithelial progenitor(s) to promote lineage specification towards tuft and goblet cells, thereby creating a feed-forward circuit through expansion of IL-25-expressing tuft cells.The metabolite succinate is sufficient for circuit activation; elevated succinate in vivo occurs downstream of colonization with Tritrichomonas, which are anaerobic protists that excrete succinate as a metabolic end-product. The identity of other tuft cell-activating ligands remains unknown, as succinate sensing is not required for circuit activation in response to helminths. Rapid epithelial replenishment, tuft cell gustatory signal transduction and rapid IL-13 expression by poised tissue-resident ILC2s contribute to the high sensitivity and dynamic nature of this circuit. Beyond the acute phases of increased tuft cell abundance and IL-13 production, more durable effects of circuit activation include smooth muscle hyperplasia, adaptive remodeling of epithelial and lamina propria compartments, and concomitant immunity, by which subsequent infections become attenuated. Additional ILC2-activating signals may contribute to modulating this circuit and might include tuft cell effectors (discussed in the text), IL-33 and neuropeptides, such a neuromedin U.

Figure 2.. GPCR signaling in tuft and tuft-like cells.

A) Through the use of knockout mice, it has been shown that effector functions (IL-25) of tuft cells downstream of succinate receptor ligand engagement are dependent on TRPM5 and the Gα protein α-gustducin (Gα_gust). This suggests that typical taste receptor signaling pathways are involved in succinate responses, possibly including a depolarization-dependent mechanism for secretion of tuft cell effector molecules like IL-25 and/or other effector molecules. B) Taste receptor engagement on Type II taste cells activates a stereotypical taste transduction cascade to facilitate release of ATP which acts on afferent sensory neurons^,. C) Taste receptor/GPCR ligand binding induces release of the Gα protein from the trimeric G protein complex. Gβγ activates PLCβ2 cleavage of PIP2 into IP3 and DAG. IP3 binds the IP3R on the endoplasmic reticulum, releasing intracellular calcium stores into the cytosol. Increased cytosolic calcium activates the inwardly rectifying sodium channel, TRPM5. Sodium influx depolarizes the cell body, and ATP is released via a CALHM1/3 channel. Additional signaling pathways engaged include DAG/calcium activation of PKC, Gα signaling, and additional calcium-dependent processes including activation of PLA.

Figure 3.. Biosynthetic pathways for putative tuft cell effector molecules.

A) Biosynthesis of acetylcholine (ACh) requires extracellular choline, imported via the high affinity choline transporter, CHT1, and mitochondrial acetyl-CoA. Highly expressed in tuft cells, choline acetyltransferase (ChAT) produces ACh, which is then packaged into vesicles via the vesicular transporter VAChT. Sequencing data suggests that tuft cells in all tissues assayed to date lack expression of CHT1; VAChT expression may be inducible as noted in the text. B-C) Tuft cells express the critical enzymes for conversion of arachidonic acid into leukotrienes and PGD₂, but do not express the synthase required for PGE₂ production (B). While COX-1 is broadly expressed, among the small intestinal epithelium specific tuft cell staining has been observed. COX-2 and HPGDS have also been validated as tuft cell markers by immunostaining, which may support constitutive activity of this pathway in tuft cells. C) Similarly, tuft cells appear transcriptionally competent to produce leukotriene B₄ (LTB₄) and the cysteinyl leukotrienes leukotriene C₄, D₄ and E₄ (LTC₄, LTD₄ and LTE₄), arachidonic acid derivatives with multiple roles in immune function and inflammation^,,. Although likely, secretion of ACh, PGD_2, and leukotriene has yet to be conclusively demonstrated from tuft cells.