Differentiation and activation of tuft cells tunes type 2 immunity in the small intestine

Abstract

Type 2 immune responses are elicited by parasitic worm, helminth, infections and are the cause of many allergic diseases. Both helminth infection and allergy are significant global health issues with high prevalence worldwide. Therefore, a better understanding of type 2 immunity will provide novel insights into how we can treat both types of conditions. Type 2 immune responses are most often elicited at barrier tissues like the lung, skin and gut. However, unlike for bacteria, viruses and fungi, our understanding of how the immune system senses the presence of helminths or allergens is much more limited. Tuft cells, a specialized lineage of epithelial cells, have been identified as critical in initiating immune responses to helminths and protists in the small intestine. Tuft cells secrete interleukin 25 (IL-25) and other effector molecules to activate group 2 innate lymphoid cells (ILC2s) in the lamina propria. ILC2s produce canonical type 2 cytokines to promote a multifaceted immune response. IL-13 in particular signals to epithelial stem cells, biasing their lineage commitment toward tuft and goblet cells, resulting in hyperplasia of both cell types and activation of a feed-forward circuit, which we refer to as the tuft-ILC2 circuit. The identification of a role for tuft cells provided a major advance in our understanding of how type 2 immune responses are initiated in the small intestine. However, the mechanism by which tuft cells sense the presence of helminths or protists in the intestinal lumen remained unclear. Studying tuft cell activation mechanisms, we identified succinate as the first intestinal tuft cell ligand. We find that administering succinate in the drinking water of mice is sufficient to activate the tuft-ILC2 circuit and elicit a multifaceted type 2 immune response. This response is mediated by binding of succinate to the succinate receptor (SUCNR1) on tuft cells. Yet activation of the tuft-ILC2 circuit by the helminth, N. brasiliensis, is SUCNR1-independent. In contrast, detection of Tritrichomonas protist colonization requires SUCNR1. These findings uncover a novel paradigm in which the type 2 immune response monitors microbial metabolism in the small intestine. Additionally, they highlight how tuft cell sensing of pathogens or commensals is context specific and likely involves multiple different signaling pathways. The importance of the tuft-ILC2 circuit has been demonstrated in numerous contexts, but how epithelial progenitors become tuft cells and how the circuit differs between mouse strains remains poorly understood. To address these questions, we characterized the succinate response in C57BL/6J (B6) and Balb/cJ (Balb) mice. Compared to B6, Balb mice have fewer tuft cells at baseline and do not develop tuft cell hyperplasia when treated with succinate. Through quantitative trait locus mapping we found a single locus on chromosome 9 associated with these phenotypes. Congenic Balb mice carrying the B6 chromosome 9 locus (Balb.Chr9B6/B6) have elevated baseline number of tuft cells and develop tuft cell hyperplasia when treated with succinate. Within this chromosome 9 locus is Pou2af2, a transcriptional cofactor essential for tuft cell differentiation. There are two isoforms of Pou2af2, of which only the long isoform encodes a functional protein. The ratio of long isoform to total Pou2af2 transcript is significantly decreased in Balb tuft cell progenitors. Finally, we find Balb mice maintain effective activation of the tuft-ILC2 circuit when infected with helminths but do not activate the circuit when colonized with innocuous Tritrichomonas protists. In sum, we identify a genetic locus that regulates tuft cell differentiation and sets the threshold of tuft-ILC2 circuit activation between different strains of mice. Together, these findings advance our knowledge about how tuft cells sense luminal signals to initiate downstream type 2 immunity and how regulation of tuft cell differentiation can impact these immunological outcomes in the small intestine. These novel insights into tuft cell biology have important implications for our understanding of intestinal homeostasis and inflammation.