Tuft cells are specialized epithelial cells that are distributed across many barrier surfaces, but are particularly involved in the detection, amplification, and effector functions of the immune response to parasite infections in the gut ^[1][2][3][1,2,3]. In the case of intestinal helminth (worm) parasites, tuft cells detect and respond to their presence by releasing the alarmin interleukin-25 (IL-25), which activates group 2 innate lymphoid cells (ILC2s) to initiate the anti-helminth immune response through type 2 helper T cells (Th2) ^[4][5][6][4,5,6]. Hence, tuft cells are a critical component of the pathway through which innate immunity triggers and expands the adaptive immune system [7].

Th2 activation leads to the production of type 2 cytokines, such as interleukin-4 (IL-4), interleukin-5 (IL-5), and interleukin-13 (IL-13), which promote the expulsion of parasites from the gut [8]. Notably, IL-4 and IL-13 act on intestinal stem cells to promote differentiation of secretory goblet and tuft cells which expand in numbers during helminth infection. This expansion allows intestinal tuft cells to serve as effectors by secreting small pharmacologically active molecules, including leukotrienes, prostaglandins, and acetylcholine [9].

Therefore, tuft cells are important players in the host–parasite interaction in the gut and represent potential targets for the development of novel therapies against parasitic infections. For example, priming animals to expand tuft cell activity or number prior to parasite exposure, with molecules such as succinate, might be an effective strategy in the control of gastrointestinal helminths, particularly for livestock in which helminth infections are a global problem due to widespread and increasing resistance to current anthelmintic drugs [10].

Tuft cells in the different barrier tissues show important anatomical and physiological differences; for example, in the airways they are directly innervated, which may facilitate an immediate response to the entry of noxious substances. In the gut, where all epithelial cell types are replaced with rapidity, nervous system interactions may be mediated indirectly (for example by ILC2 neuromedin). Further heterogeneity is seen within the intestinal tuft cell populations, with proximal–distal gradients in receptor expression (in the case of succinate receptor) or a dichotomy between CD45– and CD45+ tuft cells that has been proposed to demarcate “Tuft-1” and “Tuft-2” cells, associated with a more neuronal or lymphoid gene expression pattern, respectively [11].

In pathogen-free laboratory animals, intestinal tuft cells are found at a low frequency, but rapidly increase in number in response to molecular cues or luminal signals such as pathogen colonization as a result of preferential differentiation from intestinal stem cells in the epithelial crypts. While most specialized gut secretory cells, including goblet cells and Paneth cells, require the Atoh1 transcription factor ^[12][13][18,19], its role in tuft cell formation is less well defined, with studies reporting both absent and increased tuft cells upon Atoh1 deletion, with disparate results between embryonic versus adult animals, and between small and large intestinal tissues ^{[14][15][16][17]}[20,21,22,23]. A likely explanation is that there are both Atoh1-dependent and -independent pathways that are initiated in a temporal- and tissue-specific manner [2].

As tuft cells mature, they also require a POU domain, class 2, transcription factor 3 (Pou2f3) [4], without which mice remain tuft-cell deficient; other characteristic genes in mice include Transient Receptor Potential Cation Channel Subfamily M Member 5 (Trpm5) ^[18][24], and Doublecortin-like kinase-1 (Dclk1). A suite of key genes are also expressed uniquely by tuft cells in the intestinal tract, such as Gnat3 and Gfi1b, as well as Il17rb (encoding IL-25R), choline acetyltransferase (Chat), and arachidonic acid metabolism genes (Cox1, Ptgds1, Alox5) ^[11][14][11,20]. Tuft cells also express the succinate receptor (Sucnr1).

TRPM5 mediates a pivotal step in the taste signal transduction pathway ^[19][25], closely linking tuft cell function to taste sensation; tuft cells also express a set of G-protein coupled (GPCR) taste receptors, and, as detailed below, a loss of TRPM5 can ablate function in vivo [6] as is also found with the loss of certain taste receptor proteins such as Tas1R3 and Tas2r. However, taste receptors and TRPM5 are expressed by other cell subsets. In mice, enteroendocrine cells also express functional TRPM5, meaning that this product is not a specific marker for tuft cells ^[19][20][25,26].

Tuft cells exhibit significant heterogeneity, as highlighted by a single-cell RNA (scRNA) sequencing (scRNA-Seq) analysis [11] and antibody probing by immunofluorescence ^[21][27]. In the mouse small intestinal epithelium, two distinct populations of mature tuft cells, designated as tuft-1 and tuft-2, were distinguished by scRNA analysis. Both clusters exhibited Dclk1 expression; however, the tuft-2 cluster showed enrichment in immune-related genes, including Ptprc, which encodes the pan-immune marker CD45 [11]. This unexpected finding was confirmed using single-molecule FISH, revealing coexpression of Dclk1 and Ptprc mRNA in certain tuft cells. Furthermore, the tuft-1 cluster displayed an enrichment of neuronal genes, suggesting that these cells mediate nervous system interactions as found in airway tuft cell populations ^[21][22][27,28]. In contrast, significant levels of the type 2-promoting cytokine TSLP were exclusively expressed in tuft-2 cells [11]. In addition, a novel subset of tuft cells showed immunoreactivity for 5HT (serotonin) localized to their apical surface, although not expressing the tryptophan hydroxylase (TPH) which is considered the enzyme required for 5HT synthesis ^[23][29].

As previously mentioned, the intestinal epithelium plays a crucial role in initiating and executing immune responses regulated by immune-specific cytokines such as IFNγ, IL-13, and IL-22 ^[4][5][6][4,5,6]. Notably, IL-13 induces BMP signalling, which functions as a negative feedback loop in limiting tuft cell hyperplasia driven by immune type 2 responses ^[24][30]. This feedback loop involves the inhibition of SOX4 expression to regulate the tuft cell progenitor population. Moreover, blocking BMP signalling with the ALK2 inhibitor DMH1 disrupts the feedback loop and increases tuft cell numbers in both in vitro and in vivo settings ^[24][30]. Overall, these novel insights into cytokine effector responses highlight the unexpected and crucial role of BMP signalling in type 2 immunity, offering potential opportunities for tailored epithelial immune responses.

The critical functional role of tuft cells was not appreciated until two studies in 2016 demonstrated that they sense and respond to intestinal helminth infection through the release of IL-25 which primes protective type 2 immune responses in the gut ^[4][5][4,5]. IL-25 acts as an alarmin to activate type 2 innate lymphoid cells (ILC2s) to produce IL-13; this key cytokine induces epithelial stem cells to differentiate into tuft cells (as well as goblet cells) in a positive feedback loop. The infection of mice with parasites that elicit type 2 immune responses, such as the intestinal nematode Nippostrongylus brasiliensis, drives tuft cell hyperplasia ^[4][5][4,5] concomitant with increased Sox4 expression ^[15][21]. Significantly, Pou2f3-deficient mice were unable to expel the N. brasiliensis infection [4], as were Sox4 deficient mice ^[15][21].

In addition, infections with an enteric protozoan (Tritrichomonas muris) can induce the expansion of tuft cells and corresponding activation of the type 2 response [6]. The activation of tuft cells during Trit. muris infection is known to be via the SucnR1 receptor ^[25][26][31,32] and also requires TRPM5 ^[27][33]. Another parasitic helminth, Trichinella spiralis, activates a Tas2R-mediated signalling pathway in intestinal tuft cells ^[28][34], while, in the case of Trit. muris infection, Tas1R3 regulates small intestinal tuft cell homeostasis as well as the Sucnr1 [6].

Downstream of tuft cell differentiation and expansion, there is an enhanced production of acetylcholine and arachidonic acid metabolites such as leukotrienes and prostaglandins [9]. Leukotrienes synergise with IL-25 in the activation of ILC2 cells, thereby contributing to anti-helminth immunity ^[29][35]. Notably, both acetylcholine and prostaglandins are also produced by airway tuft cells ^[30][31][36,37], indicating a general role in inflammatory responses to exogenous threats.

However, observations of tuft cell expansion are not always reproduced across other parasitic infections. The helminth Heligmosomoides polygyrus establishes a chronic intestinal infection in mice, accompanied by a relatively weak tuft cell response ^[32][38]. Notably, infection with H. polygyrus renders mice less responsive to N. brasiliensis with a subdued level of tuft cell activation. This has been attributed to secretory proteins released by H. polygyrus that target the intestinal stem cell differentiation programme.

Tuft cells have been identified across a range of mammalian species from mice and ruminants to pigs and humans, generally under steady-state conditions rather than in the setting of infection. However, a detailed study of the presence and gene expression profile of tuft cells in sheep, following nematode infections using immunohistochemistry and single-cell RNA-sequencing, was recently published ^[33][39]. Tuft cells were characterised in the ovine abomasum, the true stomach of ruminants, and a significant increase in their numbers was observed after infection with the globally important nematodes Teladorsagia circumcincta and Haemonchus contortus. These ovine tuft cells have an enrichment of classical tuft cell gene markers such as Pou2F3, Gfi1B, and Trpm5, as well as genes associated with signalling and inflammatory pathways. Interestingly, it was also found that while murine tuft cells express the succinate receptor SucnR1 and free fatty acid receptor Ffar3 as “sensors”, these receptors were not found to be expressed in ovine tuft cells. Instead, an enrichment of taste receptor Tas2R16 and mechanosensory receptor Adgrg6 in ovine tuft cells was observed ^[33][39]. With progress in gastrointestinal organoid cultures from ruminant species, showing differentiation of the specialized cell types, further advances in defining these pathways are to be expected ^[34][35][40,41].

While less well understood in terms of their gene expression profile, tuft cells have also been identified in pig intestines as DCLK1⁺ epithelial cells, and have been shown to increase in number following infection with the porcine whipworm, Trichuris muris ^[36][42]. 

Much of the current information on tuft cells inevitably derives from murine studies, but, at least in the steady state, human intestinal tuft cells show similar patterns of gene expression to those from other mammalian species ^[37][38][44,45] and so may be predicted to respond to parasite infections in a similar manner. However, in one study which has been recently published, patients with chronic infections with the parasitic flatworm Schistosoma did not show an increased number of tuft cells in the large intestine compared to uninfected controls, although an increase in mucus production by large intestinal goblet cells was manifest ^[39][46]. Although this observation could suggest a lower abundance of tuft cells in humans, it should be noted that schistosomiasis patients have long-standing infections during which parasite-mediated down-modulation of host immunity may have taken place.

Although tuft cell activation is most strongly associated with parasite infections, there are also key interactions with certain bacterial organisms. Although tuft cells are not activated by pathogenic bacteria such as Salmonella enterica [11], they do sense other bacteria, such as Bifidobacterium species, in a succinate-dependent manner ^[40][47]. On the other hand, in the colon, tuft cells are highly sensitive to intestinal bacteria and respond to changes in the microbiome ^[41][48]. Antibiotic-mediated depletion of the gut microbiome can affect tuft cell populations, and the presence of bacteria can alter tuft cell gene expression and expansion ^[21][27]; equally, tuft cell activation through succinate can change the spectrum of antimicrobial peptide expression, resulting in a significant change in the microbiome ^[42][49].

In the context of viral infections, tuft cells can be directly infected by certain viruses, such as murine norovirus (MNV) ^[41][48] and murine rotavirus ^[43][50]. In the case of MNV, tuft cells are the primary target for infection, and the virus can exploit the immune-privileged niche of tuft cells to evade immune responses. Norovirus targets tuft cells via the CD300lf receptor, while also promoting expression of type 2 cytokines, which act to counteract the antiviral response and thereby amplify norovirus infection ^[41][44][48,51]. This scenario is supported by further work showing that an active H. polygyrus infection exacerbated West Nile Virus (WNV) pathology by promoting Type 2 responses in a tuft cell-dependent manner, while pathology was ameliorated in either IL4R- or tuft cell-deficient mice ^[45][52]. In the rotavirus setting, tuft cells also became directly infected but showed no numerical expansion, and, in contrast to events during helminth infection, down-regulated IL-25 and leukotriene production, indicating a conventional type 1 antiviral state may be induced ^[43][50].

Overall, tuft cells play diverse roles in the immune response to parasites, bacteria, and viruses in different tissues ^[44][51]. They are also pivotal in the regulation of the commensal microbiota; for example, through sensing of succinate levels in the intestinal tract, tuft cells control Paneth cell gene expression, and thereby the level of anti-microbial peptides (AMPs) that differentially control microbial taxa in the intestine ^[42][46][49,53]. Thus, there is a continuum of tuft cell activation and secretion phenotypes that vary depending on the type of micro-organism or parasite involved and their location within the gastrointestinal tract. It will be fascinating to further explore how tuft cells contribute to the regulation of bacterial microflora, and how these interactions are associated with allergy and autoimmunity, given the correlation between reduced helminth prevalence, dysregulated microbiomes, and increased incidence of inflammatory disorders ^[47][54].