Preprint Club
A cross-institutional Journal Club Initiative

Keywords

• Tuft cells

• Microbiota

• Type 2 immunity

Main Findings

Intestinal homeostasis is heavily influenced by the microbiota and its interactions with the local immune system. Previous research has shown how succinate, a citric acid cycle intermediate and an intermediate in propionate production by microbiota, modulates type 2 immunity in the gut. In the context of infection, Howitt et al. described the mechanism of how parasite-derived succinate sensed by taste-chemosensory tuft cells (TCs) triggers a type 2 response that ultimately results in the expulsion of the parasite (“weep and sweep” response)1. As the microbiota is capable of secreting substantial amounts of succinate, could a similar regulation exist in an organ predominantly populated by microbiota – the colon? Kellogg et al. asked if succinate production by microbiota can be protective in a C. difficile infection via a similar mechanism of succinate sensing by TCs and a downstream protective type 2 immune response. The authors show that vancomycin treatment depleted succinate-consuming Gram-negative bacteria and led to increased IL-25 levels and TCs numbers in the proximal colon. To understand what microbial species could be responsible for these effects, Kellogg et al. correlated enriched bacterial species in vancomycin-treated microbiota with IL-25 production. One of the identified candidates was B. thetaiotaomicron, a succinate-producing bacterium. Next experiments showed that colonization with B. thetaiotaomicron and two other succinate-producing species (B. vulgatus, P. copri) leads to an increase in specific type 2 cytokines and TCs abundance. To confirm that the observed effects are mediated by tuft cells, the authors also utilize a Pou2f3 mouse model deficient in TCs; colonization of Pou2f3-/- mice with succinate producers did not activate a type 2 response. Similar results are observed upon administration of bacteria deficient in fumarate reductase. The culminating experiment in this study shows that succinate producers given two weeks before C. difficile infection can rescue the mice from infection-associated mortality, associating microbiota-derived succinate with a protective type 2 immune response.

Limitations

• This study would benefit from having histopathology scores of the colon tissue during C. difficile infection and succinate-producing bacteria supplementation. Adding to that, the imaging of how succinate producers affect tuft cells morphology would strengthen the findings.

• Besides histopathology, C. difficile experiment needs quantification of type 2 cytokines to confirm the protective effects of these responses.

• To strengthen the findings in Figure 5A/B, what is the protection provided against C. difficile infection when mice are colonized with non-succinate producers? Furthermore, does supplementation with succinate alone reduce C. difficile mortality?

• It would be interesting to see how colonization with succinate producers changes the compartment of immune cells that secretes type 2 cytokines. What happens to the type 2 innate lymphoid cells?

• In Figure 4, the authors could add an untreated control in the panel D, as well as succinate levels measurement in B. theta or B. theta Δfrd treated mice.

• It was a bit surprising to see changes only in the succinate levels in the metabolomics experiments, as such large perturbation of the microbiome would drive more differences. We hope that this could be explored more in the future research, as a recent study by Eshleman et al. suggests butyrate as another regulator of tuft cell expansion2.

Minor comment: the resolution of the figures could be improved.

Significance/Novelty

This study revealed for the first time (to these reviewer’s knowledge) that tuft cells provided protection against C. difficile infection. Furthermore, the authors determined that tuft cell expansion in the colon is dependent on succinate-producing members of the microbiota. These results can have implications in directing treatment for C. difficile infection (for instance, providing succinate-producing bacteria to provide colonization resistance against C. difficile). 

Credit

Reviewed by anonymous reviewers as part of a cross-institutional journal club between the Vanderbilt University Medical Center (VUMC), the Max-Delbrück Center Berlin, the Ragon Institute Boston (Mass General, MIT, Harvard), the Medical University of Vienna and other life science institutes in Vienna.

The author declares no conflict of interests in relation to their involvement in the review.

References:

1. Howitt, M. R. et al. Tuft cells, taste-chemosensory cells, orchestrate parasite type 2 immunity in the gut. Science 351, 1329–1333 (2016).

2. Eshleman, E. M. et al. Microbiota-derived butyrate restricts tuft cell differentiation via histone deacetylase 3 to modulate intestinal type 2 immunity. Immunity 57, 319-332.e6 (2024).