Tuft Cells-Systemically Dispersed Sensory Epithelia Integrating Immune and Neural Circuitry

Abstract

Tuft cells-rare solitary chemosensory cells in mucosal epithelia-are undergoing intense scientific scrutiny fueled by recent discovery of unsuspected connections to type 2 immunity. These cells constitute a conduit by which ligands from the external space are sensed via taste-like signaling pathways to generate outputs unique among epithelial cells: the cytokine IL-25, eicosanoids associated with allergic immunity, and the neurotransmitter acetylcholine. The classic type II taste cell transcription factor POU2F3 is lineage defining, suggesting a conceptualization of these cells as widely distributed environmental sensors with effector functions interfacing type 2 immunity and neural circuits. Increasingly refined single-cell analytics have revealed diversity among tuft cells that extends from nasal epithelia and type II taste cells to ex-Aire-expressing medullary thymic cells and small-intestine cells that mediate tissue remodeling in response to colonizing helminths and protists.

Keywords: IL-25; POU2F3; TRPM5; chemosensation; tuft cells; type 2 immunity.

Figure 1:

Tuft cells have been found in diverse epithelia where it is thought they play chemosensory and/or immunomodulatory roles enabled by expression of multiple GPCRs to sense luminal stimuli. Tuft cells in the small intestine, for example, are known to respond to luminal helminths and protists like Tritrichomonas. Presence of the latter is sensed by detection of a major metabolic endproduct, succinate, via a tuft cell GPCR, SUCNR1. Subsequently, tuft cells activate tissue resident ILC2s in a TRPM5- and IL-25-dependent fashion to produce effector cytokines like IL-13. Other tuft cell-derived immune mediators, including ALOX5-dependent eicosanoids, may be involved. IL-13 acts on epithelial progenitor cells in the intestinal crypt, promoting dynamic increases in tuft and goblet cell output. In other tissues such as the olfactory epithelium, tuft cells appear to respond to bitter taste ligands, although the physiological ligands are unknown. Here, tuft cells may enact a neuronal effector program, perhaps through secretion of ACh which could act on efferent neurons, smooth muscle, or epithelial progenitor cells. Additional tuft cell effectors, including HPGDS-dependent prostanoids like PGD₂, may contribute. Inset: We propose that tuft cells exhibit immunologic and/or neuronal effector states. These effector phenotypes could be mutually exclusive, discreet cell populations, or could represent tuning of fully differentiated cells along a spectrum of function dependent on mutable luminal or niche signals.

Figure 2:

Recently published RNA-seq data (9) was reanalyzed to isolate heterogeneity of tuft cells among distinct tissues. Samples are sorted tuft cells (CD45^low EPCAM⁺ IL-25⁺) from various tissues of IL-25 reporter mice, with mRNA prepared and analyzed as described in Nadjsombati et al. The following processing steps were applied in this order: exclusion of very large outliers, minimal cumulative read count per transcript cut off at 150, log 10 transformed, missing values set to minimum value of the sample, scaled to a mean of 0 with standard deviation of 1. (a) Bayesian principle component analysis with two components was performed on processed and filtered data, with each dot representing one biological replicate (see Nadjsombati et al.). (b) Hierarchical clustering was performed on the processed data using average linkage and Euclidean distance. The dendogram is not shown for clarity. The colored side bar represents gene membership in clusters determined by K-means (k = 5).

Figure 3:

(a) Small intestinal epithelia comprise absorptive enterocytes , M cells, and secretory lineages, including tuft, goblet, enteroendocrine and Paneth cells, which derive from progenitor cells in the intestinal crypt. In the presence of type 2 immune stimulation, IL-4/13-dependent tuft and goblet cell hyperplasia is induced (not to scale). (b) In addition to the increased proportion of tuft cells, extrapolation from existing literature suggests that the tuft cells arising in this IL4Rα-dependent fashion could engage an alternative path of differentiation, schematized on the right, developing in a SOX4-dependent, ATOH1-independent fashion from an unknown epithelial progenitor, and in contrast to a basal and fetal trajectories involving an ATOH1-dependent development similar to other secretory lineages. Thus, observed tuft cell heterogeneity could be programmed early in lineage specification by impacts on epithelial progenitor cells.

Figure 4:

Expression of IL-25 (in red) in lingual taste buds of IL-25-RFP reporter mice; DAPI in blue. Taste bud structure is outlined. Scale bar, 20 μm.