Antibiotic-induced dysbiosis is a key predisposing factor for Clostridium difficile infections (CDIs), which cause intestinal disease ranging from mild diarrhea to pseudomembranous colitis. Here, we examined the impact of a microbiota-derived metabolite, short-chain fatty acid acetate, on an acute mouse model of CDI. We found that administration of acetate is remarkably beneficial in ameliorating disease. Mechanistically, we show that acetate enhances innate immune responses by acting on both neutrophils and ILC3s through its cognate receptor free fatty acid receptor 2 (FFAR2). In neutrophils, acetate-FFAR2 signaling accelerates their recruitment to the inflammatory sites, facilitates inflammasome activation, and promotes the release of IL-1β; in ILC3s, acetate-FFAR2 augments expression of the IL-1 receptor, which boosts IL-22 secretion in response to IL-1β. We conclude that microbiota-derived acetate promotes host innate responses to C. difficile through coordinate action on neutrophils and ILC3s.

Although innate lymphoid cells (ILCs) functionally analogous to T helper type 1 (Th1), Th2, and Th17 cells are well characterized, an ILC subset strictly equivalent to IL-10–secreting regulatory T cells has only recently been proposed. Here, we report the absence of an intestinal regulatory ILC population distinct from group 1 ILCs (ILC1s), ILC2s, and ILC3s in (1) mice bred in our animal facility; (2) mice from The Jackson Laboratory, Taconic Biosciences, and Charles River Laboratories; and (3) mice subjected to intestinal inflammation. Instead, a low percentage of intestinal ILC2s produced IL-10 at steady state. A screen for putative IL-10 elicitors revealed that IL-2, IL-4, IL-27, IL-10, and neuromedin U (NMU) increased IL-10 production in activated intestinal ILC2s, while TL1A suppressed IL-10 production. Secreted IL-10 further induced IL-10 production in ILC2s through a positive feedback loop. In summary, ILC2s provide an inducible source of IL-10 in the gastrointestinal tract, whereas ILCregs are not a generalizable immune cell population in mice.

Clock protein REV-ERBα influences development of type 3 innate lymphoid cells in a circadian-independent manner (see the related Focus by Mortha and Research Article by Teng et al. ).

The gut microbiota and tumor-associated macrophages (TAMs) affect tumor responses to anti–programmed cell death protein 1 (PD-1) immune checkpoint blockade. Reprogramming TAM by either blocking or deleting the macrophage receptor triggering receptor on myeloid cells 2 (TREM2) attenuates tumor growth, and lack of functional TREM2 enhances tumor elimination by anti–PD-1. Here, we found that anti–PD-1 treatment combined with TREM2 deficiency in mice induces proinflammatory programs in intestinal macrophages and a concomitant expansion of Ruminococcus gnavus in the gut microbiota. Gavage of wild-type mice with R. gnavus enhanced anti–PD-1–mediated tumor elimination, recapitulating the effect occurring in the absence of TREM2. A proinflammatory intestinal environment coincided with expansion, increased circulation, and migration of TNF-producing CD4 ⁺ T cells to the tumor bed. Thus, TREM2 remotely controls anti–PD-1 immune checkpoint blockade through modulation of the intestinal immune environment and microbiota, with R. gnavus emerging as a potential probiotic agent for increasing responsiveness to anti-PD-1.

Here, we explore the relationship between dietary fibers, colonic epithelium major histocompatibility complex class II (MHC-II) expression, and immune cell interactions in regulating susceptibility to Clostridioides difficile infection (CDI). We find that a low-fiber diet increases MHC-II expression in the colonic epithelium, which, in turn, worsens CDI by promoting the development of pathogenic CD4⁺ intraepithelial lymphocytes (IELs). The influence of dietary fibers on MHC-II expression is mediated by its metabolic product, acetate, and its receptor, free fatty acid receptor 2 (FFAR2). While acetate activation of FFAR2 on epithelial cells helps resist CDI, it does not directly regulate MHC-II expression. Instead, MHC-II is regulated by FFAR2 in type 3 innate lymphoid cells (ILC3s). Acetate enhances interleukin-22 (IL-22) production by ILC3s, which then suppresses MHC-II expression on the colonic epithelium. In conclusion, a low-fiber diet reduces acetate-induced IL-22 production by ILC3s, leading to increased MHC-II on the colonic epithelium. This change affects recovery from CDI by expanding the population of pathogenic CD4⁺ IELs.

Hyperbaric oxygen (HBO) therapy is a well-established method for improving tissue oxygenation and is typically used for the treatment of various inflammatory conditions, including infectious diseases. However, its effect on the intestinal mucosa, a microenvironment known to be physiologically hypoxic, remains unclear. Here, we demonstrated that daily treatment with hyperbaric oxygen affects gut microbiome composition, worsening antibiotic-induced dysbiosis. Accordingly, HBO-treated mice were more susceptible to Clostridioides difficile infection (CDI), an enteric pathogen highly associated with antibiotic-induced colitis. These observations were closely linked with a decline in the level of microbiota-derived short-chain fatty acids (SCFAs). Butyrate, a SCFA produced primarily by anaerobic microbial species, mitigated HBO-induced susceptibility to CDI and increased epithelial barrier integrity by improving group 3 innate lymphoid cell (ILC3) responses. Mice displaying tissue-specific deletion of HIF-1 in RORγt-positive cells exhibited no protective effect of butyrate during CDI. In contrast, the reinforcement of HIF-1 signaling in RORγt-positive cells through the conditional deletion of VHL mitigated disease outcome, even after HBO therapy. Taken together, we conclude that HBO induces intestinal dysbiosis and impairs the production of SCFAs affecting the HIF-1α-IL-22 axis in ILC3 and worsening the response of mice to subsequent C. difficile infection.