Intestinal microbiota programming of alveolar macrophages influences severity of respiratory viral infection.

Susceptibility to respiratory virus infections (RVIs) varies widely across individuals. Because the gut microbiome impacts immune function, we investigated the influence of intestinal microbiota composition on RVI and determined that segmented filamentous bacteria (SFB), naturally acquired or exogenously administered, protected mice against influenza virus (IAV) infection. Such protection, which also applied to respiratory syncytial virus and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), was independent of interferon and adaptive immunity but required basally resident alveolar macrophages (AMs). In SFB-negative mice, AMs were quickly depleted as RVI progressed. In contrast, AMs from SFB-colonized mice were intrinsically altered to resist IAV-induced depletion and inflammatory signaling. Yet, AMs from SFB-colonized mice were not quiescent. Rather, they directly disabled IAV via enhanced complement production and phagocytosis. Accordingly, transfer of SFB-transformed AMs into SFB-free hosts recapitulated SFB-mediated protection against IAV. These findings uncover complex interactions that mechanistically link the intestinal microbiota with AM functionality and RVI severity.

Intestinal microbiota programming of alveolar macrophages influences severity of respiratory viral infection.

Susceptibility to respiratory virus infections (RVIs) varies widely across individuals. Because the gut microbiome impacts immune function, we investigated the influence of intestinal microbiota composition on RVI and determined that segmented filamentous bacteria (SFB), naturally acquired or exogenously administered, protected mice against influenza virus (IAV) infection. Such protection, which also applied to respiratory syncytial virus and SARS-CoV-2, was independent of interferon and adaptive immunity but required basally resident alveolar macrophages (AM). In SFB-negative mice, AM were quickly depleted as RVI progressed. In contrast, AM from SFB-colonized mice were intrinsically altered to resist IAV-induced depletion and inflammatory signaling. Yet, AM from SFB-colonized mice were not quiescent. Rather, they directly disabled IAV via enhanced complement production and phagocytosis. Accordingly, transfer of SFB-transformed AM into SFB-free hosts recapitulated SFB-mediated protection against IAV. These findings uncover complex interactions that mechanistically link the intestinal microbiota with AM functionality and RVI severity. One sentence summary: Intestinal segmented filamentous bacteria reprogram alveolar macrophages promoting nonphlogistic defense against respiratory viruses.

Psyllium fiber protects mice against western diet-induced metabolic syndrome via the gut microbiota-dependent mechanism.

Impacts of dietary fiber on intestinal inflammation are complex, but some specific semi-purified fibers, particularly psyllium, can protect humans and rodents against colitis. Mechanisms underlying such protection are not fully understood but may involve activation of the FXR bile acid receptor. Obesity and its associated consequences, referred to as metabolic syndrome, are associated with, and promoted by, low-grade inflammation in a variety of tissues including the intestine. Hence, we examined whether psyllium might ameliorate the low-grade intestinal inflammation that occurs in diet-induced obesity and, moreover, the extent to which it might ameliorate adiposity and/or dysglycemia in this disease model. We observed that enriching a high-fat diet with psyllium provided strong protection against the low-grade gut inflammation and metabolic consequences that were otherwise induced by the obesogenic diet. Such protection was fully maintained in FXR-deficient mice, indicating that distinct mechanisms mediate psyllium's protection against colitis and metabolic syndrome. Nor did psyllium's protection associate with, or require, fermentation or IL-22 production, both of which are key mediators of beneficial impacts of some other dietary fibers. Psyllium's beneficial impacts were not evident in germfree mice but were observed in Altered Schaedler Flora mice, in which psyllium modestly altered relative and absolute abundance of the small number of taxa present in these gnotobiotic mice. Thus, psyllium protects mice against diet-induced obesity/metabolic syndrome by a mechanism independent of FXR and fermentation but nonetheless requires the presence of at least a minimal microbiota.

Psyllium Fiber Protects Against Colitis Via Activation of Bile Acid Sensor Farnesoid X Receptor.

BACKGROUND & AIMS: Fiber-rich foods promote health, but mechanisms by which they do so remain poorly defined. Screening fiber types, in mice, revealed psyllium had unique ability to ameliorate 2 chronic inflammatory states, namely, metabolic syndrome and colitis. We sought to determine the mechanism of action of the latter. METHODS: Mice were fed grain-based chow, which is naturally rich in fiber or compositionally defined diets enriched with semi-purified fibers. Mice were studied basally and in models of chemical-induced and T-cell transfer colitis. RESULTS: Relative to all diets tested, mice consuming psyllium-enriched compositionally defined diets were markedly protected against both dextran sulfate sodium- and T-cell transfer-induced colitis, as revealed by clinical-type, histopathologic, morphologic, and immunologic parameters. Such protection associated with stark basal changes in the gut microbiome but was independent of fermentation and, moreover, maintained in mice harboring a minimal microbiota (ie, Altered Schaedler Flora). Transcriptomic analysis revealed psyllium induced expression of genes mediating bile acids (BA) secretion, suggesting that psyllium's known ability to bind BA might contribute to its ability to prevent colitis. As expected, psyllium resulted in elevated level of fecal BA, reflecting their removal from enterohepatic circulation but, in stark contrast to the BA sequestrant cholestyramine, increased serum BA levels. Moreover, the use of BA mimetics that activate the farnesoid X receptor (FXR), as well as the use of FXR-knockout mice, suggested that activation of FXR plays a central role in psyllium's protection against colitis. CONCLUSIONS: Psyllium protects against colitis via altering BA metabolism resulting in activation of FXR, which suppresses pro-inflammatory signaling.

Segmented filamentous bacteria impede rotavirus infection via retinoic acid receptor-mediated signaling.

Prevention of rotavirus (RV) infection by gut-resident segmented filamentous bacteria (SFB) is an example of the influence of gut microbiota composition on enteric viral infection. Yet, the mechanism by which SFB prevents RV infection is poorly understood. A recent report that SFB colonization of germfree mice generates retinoic acid (RA) thus activating RA receptor (RAR) signaling, which protected against Citrobacter rodentium infection, prompted us to investigate whether this pathway might contribute to SFB's protection against RV infection. Colonization of conventional mice by SFB indeed increased intestinal RA levels and direct administration of RA partially mimicked the protection against RV infection conferred by SFB. Moreover, blockade of RAR signaling eliminated SFB's protection against RV infection. Blockade of RAR signaling did not impact RV infection in the absence of SFB, nor did it alter the protection against RV infection conferred by bacterial flagellin, which in contrast to SFB, is dependent upon IL-22 signaling. SFB/RA-mediated prevention of RV infection was associated with an RA-dependent increase in enterocyte migration, consistent with the notion that enhanced anoikis is the ultimate means by which SFB, IL-22, and RA impede RV infection.

Maternal fiber deprivation alters microbiota in offspring, resulting in low-grade inflammation and predisposition to obesity.

Diet, especially fiber content, plays an important role in sustaining a healthy gut microbiota, which promotes intestinal and metabolic health. Another major determinant of microbiota composition is the specific microbes that are acquired early in life, especially maternally. Consequently, we hypothesized that alterations in maternal diet during lactation might lastingly impact the microbiota composition and health status of offspring. Accordingly, we observed that feeding lactating dams low-fiber diets resulted in offspring with lasting microbiota dysbiosis, including reduced taxonomic diversity and increased abundance of Proteobacteria species, despite the offspring consuming a fiber-rich diet. Such microbiota dysbiosis was associated with increased encroachment of bacteria into inner mucus layers, low-grade gut inflammation, and a dramatically exacerbated microbiota-dependent increase in adiposity following exposure to an obesogenic diet. Thus, maternal diet is a critical long-lasting determinant of offspring microbiota composition, impacting gut health and proneness to obesity and its associated disorders.

Microbiota as a potentially-modifiable factor influencing COVID-19.

Impacts of respiratory tract viruses have long been appreciated to highly heterogeneous both between and within various populations. The SARS-CoV-2 pandemic, which is the first time that a pathogen's spread across the globe has been extensively monitored by direct detection of the pathogen itself rather just than the morbidity left in its wake, indicates such heterogeneity is not limited to outcomes of infections but whether infection of a particular host occurs at all. This suggests an important role for yet to be discovered environmental (i.e. non-genetic) factors that influence whether an exposure to the virus initiates a productive infection and, moreover, the severity of disease that results. This article discusses the emerging hypothesis that the composition of a host's commensal microbial communities, that is, its 'microbiome', may be one such determinant that influences outcomes following encounters with respiratory viral pathogens in general and SARS-CoV-2 in particular. Specifically, we will review the rationales and evidence that supports this hypothesis and, moreover, speculate as to possible approaches to manipulate microbiota to ameliorate disease induced by respiratory viral pathogens.

Inulin Fermentable Fiber Ameliorates Type I Diabetes via IL22 and Short-Chain Fatty Acids in Experimental Models.

BACKGROUND & AIMS: Nourishment of gut microbiota via consumption of fermentable fiber promotes gut health and guards against metabolic syndrome. In contrast, how dietary fiber impacts type 1 diabetes is less clear. METHODS: To examine impact of dietary fibers on development of type 1 diabetes in the streptozotocin (STZ)-induced and spontaneous non-obese diabetes (NOD) models, mice were fed grain-based chow (GBC) or compositionally defined diets enriched with a fermentable fiber (inulin) or an insoluble fiber (cellulose). Spontaneous (NOD mice) or STZ-induced (wild-type mice) diabetes was monitored. RESULTS: Relative to GBC, low-fiber diets exacerbated STZ-induced diabetes, whereas diets enriched with inulin, but not cellulose, strongly protected against or treated it. Inulin's restoration of glycemic control prevented loss of adipose depots, while reducing food and water consumption. Inulin normalized pancreatic function and markedly enhanced insulin sensitivity. Such amelioration of diabetes was associated with alterations in gut microbiota composition and was eliminated by antibiotic administration. Pharmacologic blockade of fermentation reduced inulin's beneficial impact on glycemic control, indicating a role for short-chain fatty acids (SCFA). Furthermore, inulin's microbiota-dependent anti-diabetic effect associated with SCFA-independent restoration of interleukin 22, which was necessary and sufficient to ameliorate STZ-induced diabetes. Inulin-enriched diets significantly delayed diabetes in NOD mice. CONCLUSIONS: Fermentable fiber confers microbiota-dependent increases in SCFA and interleukin 22 that, together, may have potential to prevent and/or treat type 1 diabetes.

IL-36 cytokines and gut immunity.

Interleukin 36 (IL-36) constitutes a group of cytokines that belong to the IL-1 superfamily. Emerging evidence has suggested a role of IL-36 in the pathogenesis of many inflammatory disorders. Intriguingly, in the gastrointestinal tract, IL-36 has a rather complex function. IL-36 receptor ligands are overexpressed in both animal colitis models and human IBD patients and may play both pathogenic and protective roles, depending on the context. IL-36 cytokines comprise three receptor agonists: IL-36α, IL-36ß and IL-36γ, and two receptor antagonists: IL-36Ra and IL-38. All IL-36 receptor agonists bind to the IL-36R complex and exert pleiotropic effects during inflammatory settings. Here, we first briefly review the processing and secretion of IL-36 cytokines. We then focus on the current understanding of the immunology effects of IL-36 in gut immunity. In addition, we also discuss the ongoing trials that aim to blockage IL-36R signalling for treating chronic intestinal inflammation and present some unexplored questions regarding IL-36 research.

Self and microbiota-derived epitopes induce CD4+ T cell anergy and conversion into CD4+Foxp3+ regulatory cells.

The physiological role of T cell anergy induction as a key mechanism supporting self-tolerance remains undefined, and natural antigens that induce anergy are largely unknown. In this report, we used TCR sequencing to show that the recruitment of CD4+CD44+Foxp3-CD73+FR4+ anergic (Tan) cells expands the CD4+Foxp3+ (Tregs) repertoire. Next, we report that blockade in peripherally-induced Tregs (pTregs) formation due to mutation in CNS1 region of Foxp3 or chronic exposure to a selecting self-peptide result in an accumulation of Tan cells. Finally, we show that microbial antigens from Akkermansia muciniphila commensal bacteria can induce anergy and drive conversion of naive CD4+CD44-Foxp3- T (Tn) cells to the Treg lineage. Overall, data presented here suggest that Tan induction helps the Treg repertoire to become optimally balanced to provide tolerance toward ubiquitous and microbiome-derived epitopes, improving host ability to avert systemic autoimmunity and intestinal inflammation.

IL-36R signaling integrates innate and adaptive immune-mediated protection against enteropathogenic bacteria.

Enteropathogenic bacterial infections are a global health issue associated with high mortality, particularly in developing countries. Efficient host protection against enteropathogenic bacterial infection is characterized by coordinated responses between immune and nonimmune cells. In response to infection in mice, innate immune cells are activated to produce interleukin (IL)-23 and IL-22, which promote antimicrobial peptide (AMP) production and bacterial clearance. IL-36 cytokines are proinflammatory IL-1 superfamily members, yet their role in enteropathogenic bacterial infection remains poorly defined. Using the enteric mouse pathogen, C.rodentium, we demonstrate that signaling via IL-36 receptor (IL-36R) orchestrates a crucial innate-adaptive immune link to control bacterial infection. IL-36R-deficient mice (Il1rl2-/- ) exhibited significant impairment in expression of IL-22 and AMPs, increased intestinal damage, and failed to contain C. rodentium compared to controls. These defects were associated with failure to induce IL-23 and IL-6, two key IL-22 inducers in the early and late phases of infection, respectively. Treatment of Il1rl2-/- mice with IL-23 during the early phase of C. rodentium infection rescued IL-22 production from group 3 innate lymphoid cells (ILCs), whereas IL-6 administration during the late phase rescued IL-22-mediated production from CD4+ T cell, and both treatments protected Il1rl2-/- mice from uncontained infection. Furthermore, IL-36R-mediated IL-22 production by CD4+ T cells was dependent upon NFκB-p65 and IL-6 expression in dendritic cells (DCs), as well as aryl hydrocarbon receptor (AhR) expression by CD4+ T cells. Collectively, these data demonstrate that the IL-36 signaling pathway integrates innate and adaptive immunity leading to host defense against enteropathogenic bacterial infection.

Internalization of Garlic-Derived Nanovesicles on Liver Cells is Triggered by Interaction With CD98.

The mechanism of how plant-derived nanovesicles are uptaken by cells remains unknown. In this study, the garlic-derived nanovesicles (GDVs) were isolated and digested with trypsin to remove all surface proteins. Digested GDVs showed less uptake compared to undigested GDVs, confirming that the surface proteins played a role in the endocytosis. On the cell side (HepG2), interestingly, blocking the CD98 receptors significantly reduced the uptake of GDVs. During the cellular internalization of GDVs, we observed that some surface proteins of GDVs were co-localized with CD98. A total lysate of the GDV surface showed a high presence of a mannose-specific binding protein, II lectin. Blocking GDV II lectin (using mannose preincubation) highly reduced the GDV internalization, which supports that direct interaction between II lectin and CD98 plays an important role in internalization. The GDVs also exhibited in vitro anti-inflammatory effect by downregulating proinflammatory factors on the HepG2 cells. This work contributes to understanding a part of the GDV internalization process and the cellular anti-inflammatory effects of garlic.

Erythroid differentiation regulator-1 induced by microbiota in early life drives intestinal stem cell proliferation and regeneration.

Gut microbiota and their metabolites are instrumental in regulating intestinal homeostasis. However, early-life microbiota associated influences on intestinal development remain incompletely understood. Here we demonstrate that co-housing of germ-free (GF) mice with specific-pathogen free (SPF) mice at weaning (exGF) results in altered intestinal gene expression. Our results reveal that one highly differentially expressed gene, erythroid differentiation regulator-1 (Erdr1), is induced during development in SPF but not GF or exGF mice and localizes to Lgr5+ stem cells and transit amplifying (TA) cells. Erdr1 functions to induce Wnt signaling in epithelial cells, increase Lgr5+ stem cell expansion, and promote intestinal organoid growth. Additionally, Erdr1 accelerates scratch-wound closure in vitro, increases Lgr5+ intestinal stem cell regeneration following radiation-induced injury in vivo, and enhances recovery from dextran sodium sulfate (DSS)-induced colonic damage. Collectively, our findings indicate that early-life microbiota controls Erdr1-mediated intestinal epithelial proliferation and regeneration in response to mucosal damage.

Flagellin-expressing virus-like particles exhibit adjuvant effects on promoting IgG isotype-switched long-lasting antibody induction and protection of influenza vaccines in CD4-deficient mice.

Incorporation of membrane-anchored flagellin molecules into the surfaces of influenza virus-like particles (VLP) was previously reported to promote T helper (Th) 1-biased IgG antibody production and protective efficacy of co-presented vaccine antigens. Herein, we investigated the potential adjuvant effects and mechanisms of flagellin-expressing VLP (FliC-VLP) as an independent component on influenza vaccination in wild-type and mutant mouse models. FliC-VLP adjuvanted influenza vaccination was highly effective in promoting the induction of Th1-biased IgG isotype switched antibodies, enhanced protection, and long-lasting IgG antibody responses in both wild-type and CD4-knockout mice. In contrast, the adjuvant effects of soluble flagellin were Th2-biased and required CD4 T helper cells. The adjuvant effects of FliC-VLP were less dependent on CD4 T cells and flagellin-mediated innate immune signaling pathways. The results suggest that FliC-VLP might play an effective adjuvant role in an immune competent condition as well as in a defect of CD4 T cells.

Combined IL-2 Immunocomplex and Anti-IL-5 mAb Treatment Expands Foxp3+ Treg Cells in the Absence of Eosinophilia and Ameliorates Experimental Colitis.

Interleukin (IL)-2 is expressed during T cell activation and induces the proliferation and differentiation of T cells. CD4+Foxp3+ regulatory T cells (Tregs) constitutively express the high affinity IL-2 receptor (CD25/IL-2Rα) and rapidly respond to IL-2 to elaborate numerous suppressive mechanisms that limit immune-mediated pathologies. Accumulating evidence supports the concept that an aberrant balance between Tregs and Teff contribute to the pathology of intestinal inflammation and that the IL-2/Treg axis is a potential pathway to exploit for the treatment of inflammatory bowel disease (IBD). Here, we show that treatment of mice with IL-2/IL-2 antibody (JES6-1) immunocomplex during DSS-induced colitis induced Foxp3+ Treg expansion, but also potently stimulated GATA3+ type 2 innate lymphoid cell (ILC2) proliferation and high-level expression of IL-5. Furthermore, IL-2/JES6-1 treatment resulted in massive eosinophil accumulation and activation in the inflamed colon, and afforded only modest protection from colitis. In light of these findings, we observed that combined IL-2/JES6-1 and anti-IL-5 mAb treatment was most effective at ameliorating DSS-induced colitis compared to either treatment alone and that this regimen allowed for Foxp3+ Treg expansion without concomitant eosinophilia. Collectively, our findings provide insight into how blockade of IL-5 may aid in optimizing IL-2 immunotherapy for the treatment of intestinal inflammation.

A cytokine network involving IL-36γ, IL-23, and IL-22 promotes antimicrobial defense and recovery from intestinal barrier damage.

The gut epithelium acts to separate host immune cells from unrestricted interactions with the microbiota and other environmental stimuli. In response to epithelial damage or dysfunction, immune cells are activated to produce interleukin (IL)-22, which is involved in repair and protection of barrier surfaces. However, the specific pathways leading to IL-22 and associated antimicrobial peptide (AMP) production in response to intestinal tissue damage remain incompletely understood. Here, we define a critical IL-36/IL-23/IL-22 cytokine network that is instrumental for AMP production and host defense. Using a murine model of intestinal damage and repair, we show that IL-36γ is a potent inducer of IL-23 both in vitro and in vivo. IL-36γ-induced IL-23 required Notch2-dependent (CD11b+CD103+) dendritic cells (DCs), but not Batf3-dependent (CD11b-CD103+) DCs or CSF1R-dependent macrophages. The intracellular signaling cascade linking IL-36 receptor (IL-36R) to IL-23 production by DCs involved MyD88 and the NF-κB subunits c-Rel and p50. Consistent with in vitro observations, IL-36R- and IL-36γ-deficient mice exhibited dramatically reduced IL-23, IL-22, and AMP levels, and consequently failed to recover from acute intestinal damage. Interestingly, impaired recovery of mice deficient in IL-36R or IL-36γ could be rescued by treatment with exogenous IL-23. This recovery was accompanied by a restoration of IL-22 and AMP expression in the colon. Collectively, these data define a cytokine network involving IL-36γ, IL-23, and IL-22 that is activated in response to intestinal barrier damage and involved in providing critical host defense.