Interleukin-22 and CD160 play additive roles in the host mucosal response to Clostridium difficile infection in mice

New treatment approaches for Clostridioides difficile infections: alternatives to antibiotics and fecal microbiota transplantation

Clostridioides difficile causes a range of debilitating intestinal symptoms that may be fatal. It is particularly problematic as a hospital-acquired infection, causing significant costs to the health care system. Antibiotics, such as vancomycin and fidaxomicin, are still the drugs of choice for C. difficile infections, but their effectiveness is limited, and microbial interventions are emerging as a new treatment option. This paper focuses on alternative treatment approaches, which are currently in various stages of development and can be divided into four therapeutic strategies. Direct killing of C. difficile (i) includes beside established antibiotics, less studied bacteriophages, and their derivatives, such as endolysins and tailocins. Restoration of microbiota composition and function (ii) is achieved with fecal microbiota transplantation, which has recently been approved, with standardized defined microbial mixtures, and with probiotics, which have been administered with moderate success. Prevention of deleterious effects of antibiotics on microbiota is achieved with agents for the neutralization of antibiotics that act in the gut and are nearing regulatory approval. Neutralization of C. difficile toxins (iii) which are crucial virulence factors is achieved with antibodies/antibody fragments or alternative binding proteins. Of these, the monoclonal antibody bezlotoxumab is already in clinical use. Immunomodulation (iv) can help eliminate or prevent C. difficile infection by interfering with cytokine signaling. Small-molecule agents without bacteriolytic activity are usually selected by drug repurposing and can act via a variety of mechanisms. The multiple treatment options described in this article provide optimism for the future treatment of C. difficile infection.

Polyamines and hypusination are important for Clostridioides difficile toxin B (TcdB)-mediated activation of group 3 innate lymphocytes (ILC3s)

Clostridioides difficile is the most common cause of nosocomial gastrointestinal tract bacterial infections. We lack fully effective reliable treatments for this pathogen, and there is a critical need to better understand how C. difficile interacts with our immune system. Group 3 innate lymphocytes (ILC3s) are rare immune cells localized within mucosal tissues that protect against bacterial infections. Upon activation, ILC3s secrete high levels of the cytokine interleukin-22 (IL-22), which is a critical regulator of tissue responses during infection. C. difficile toxin B (TcdB), the major virulence factor, directly activates ILC3s, resulting in high IL-22 levels. We previously reported that polyamines are important in the activation of ILC3s by the innate cytokine interleukin-23 (IL-23) but did not identify a specific mechanism. In this study, we examine how a pathogen impacts a metabolic pathway important for immune cell function and hypothesized that polyamines are important in TcdB-mediated ILC3 activation. We show that TcdB upregulates the polyamine biosynthesis pathway, and the inhibition of the pathway decreases TcdB-mediated ILC3 activation. Two polyamines, putrescine and spermidine, are involved. Spermidine is the key polyamine in the hypusination of eukaryotic initiation factor 5A (eIF5A), and the inhibition of eIF5A reduced ILC3 activation. Thus, there is potential to leverage polyamines in ILC3s to promote activation of ILC3s during C. difficile infection and other bacterial infections where ILC3s serve a protective role.

Peptides, Exopolysaccharides, and Short-Chain Fatty Acids from Fermented Milk and Perspectives on Inflammatory Bowel Diseases

Crohn's disease and ulcerative colitis are characterized by chronic inflammatory processes and an imbalanced immune response along the gastrointestinal (GI) tract. Pharmacological treatments have been widely used, although their long-term application has adverse side effects. On the other hand, milks fermented with specific lactic acid bacteria (LAB) have been shown to be useful as alternative or complementary aids. Many metabolites such as peptides, exopolysaccharides, and short-chain fatty acids are produced during milk fermentation. These components have been shown to change the pH of the gastrointestinal lumen, aid intestine mucosal recovery, modulate the microbiota, and reduce the inflammatory response (innate and adaptive immune system), both in vitro and in vivo. Therefore, the objective of the present review is to describe how these bioactive compounds from fermented milk by specific LAB can decrease the deleterious symptoms of inflammatory bowel disease.

Epithelial HVEM maintains intraepithelial T cell survival and contributes to host protection

Intraepithelial T cells (IETs) are in close contact with intestinal epithelial cells and the underlying basement membrane, and they detect invasive pathogens. How intestinal epithelial cells and basement membrane influence IET survival and function, at steady state or after infection, is unclear. The herpes virus entry mediator (HVEM), a member of the TNF receptor superfamily, is constitutively expressed by intestinal epithelial cells and is important for protection from pathogenic bacteria. Here, we showed that at steady-state LIGHT, an HVEM ligand, binding to epithelial HVEM promoted the survival of small intestine IETs. RNA-seq and addition of HVEM ligands to epithelial organoids indicated that HVEM increased epithelial synthesis of basement membrane proteins, including collagen IV, which bound to β₁ integrins expressed by IETs. Therefore, we proposed that IET survival depended on β₁ integrin binding to collagen IV and showed that β₁ integrin-collagen IV interactions supported IET survival in vitro. Moreover, the absence of β₁ integrin expression by T lymphocytes decreased TCR αβ⁺ IETs in vivo. Intravital microscopy showed that the patrolling movement of IETs was reduced without epithelial HVEM. As likely consequences of decreased number and movement, protective responses to Salmonella enterica were reduced in mice lacking either epithelial HVEM, HVEM ligands, or β₁ integrins. Therefore, IETs, at steady state and after infection, depended on HVEM expressed by epithelial cells for the synthesis of collagen IV by epithelial cells. Collagen IV engaged β₁ integrins on IETs that were important for their maintenance and for their protective function in mucosal immunity.

Microbiota-mediated protection against antibiotic-resistant pathogens

Colonization by the microbiota provides one of our most effective barriers against infection by pathogenic microbes. The microbiota protects against infection by priming immune defenses, by metabolic exclusion of pathogens from their preferred niches, and through direct antimicrobial antagonism. Disruption of the microbiota, especially by antibiotics, is a major risk factor for bacterial pathogen colonization. Restoration of the microbiota through microbiota transplantation has been shown to be an effective way to reduce pathogen burden in the intestine but comes with a number of drawbacks, including the possibility of transferring other pathogens into the host, lack of standardization, and potential disruption to host metabolism. More refined methods to exploit the power of the microbiota would allow us to utilize its protective power without the drawbacks of fecal microbiota transplantation. To achieve this requires detailed understanding of which members of the microbiota protect against specific pathogens and the mechanistic basis for their effects. In this review, we will discuss the clinical and experimental evidence that has begun to reveal which members of the microbiota protect against some of the most troublesome antibiotic-resistant pathogens: Klebsiella pneumoniae, vancomycin-resistant enterococci, and Clostridioides difficile.

Acetate coordinates neutrophil and ILC3 responses against C. difficile through FFAR2

Microbiota-derived acetate coordinates innate immune responses during intestinal Clostridium difficile infection through its cognate receptor FFAR2. Acetate accelerates early neutrophil recruitment and increases ILC3 expression of the IL-1 receptor, boosting ILC3 production of IL-22 in response to neutrophil-derived IL-1β.

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.

Systemic Inflammatory Mediators Are Effective Biomarkers for Predicting Adverse Outcomes in Clostridioides difficile Infection

Each year in the United States, Clostridioides difficile causes nearly 500,000 gastrointestinal infections that range from mild diarrhea to severe colitis and death. The ability to identify patients at increased risk for severe disease or mortality at the time of diagnosis of C. difficile infection (CDI) would allow clinicians to effectively allocate disease modifying therapies. In this study, we developed models consisting of only a small number of serum biomarkers that are capable of predicting both 30-day all-cause mortality and adverse outcomes of patients at time of CDI diagnosis. We were able to validate these models through experimental mouse infection. This provides evidence that the biomarkers reflect the underlying pathophysiology and that our mouse model of CDI reflects the pathogenesis of human infection. Predictive models can not only assist clinicians in identifying patients at risk for severe CDI but also be utilized for targeted enrollment in clinical trials aimed at reduction of adverse outcomes from severe CDI.

Clostridioides difficile infection (CDI) can result in severe disease and death, with no accurate models that allow for early prediction of adverse outcomes. To address this need, we sought to develop serum-based biomarker models to predict CDI outcomes. We prospectively collected sera ≤48 h after diagnosis of CDI in two cohorts. Biomarkers were measured with a custom multiplex bead array assay. Patients were classified using IDSA severity criteria and the development of disease-related complications (DRCs), which were defined as ICU admission, colectomy, and/or death attributed to CDI. Unadjusted and adjusted models were built using logistic and elastic net modeling. The best model for severity included procalcitonin (PCT) and hepatocyte growth factor (HGF) with an area (AUC) under the receiver operating characteristic (ROC) curve of 0.74 (95% confidence interval, 0.67 to 0.81). The best model for 30-day mortality included interleukin-8 (IL-8), PCT, CXCL-5, IP-10, and IL-2Rα with an AUC of 0.89 (0.84 to 0.95). The best model for DRCs included IL-8, procalcitonin, HGF, and IL-2Rα with an AUC of 0.84 (0.73 to 0.94). To validate our models, we employed experimental infection of mice with C. difficile. Antibiotic-treated mice were challenged with C. difficile and a similar panel of serum biomarkers was measured. Applying each model to the mouse cohort of severe and nonsevere CDI revealed AUCs of 0.59 (0.44 to 0.74), 0.96 (0.90 to 1.0), and 0.89 (0.81 to 0.97). In both human and murine CDI, models based on serum biomarkers predicted adverse CDI outcomes. Our results support the use of serum-based biomarker panels to inform Clostridioides difficile infection treatment.

The role of innate lymphoid cells in response to microbes at mucosal surfaces

Innate lymphoid cells (ILCs) are a lymphocyte population that is mostly resident at mucosal surfaces. They help to induce an appropriate immune response to the microbiome at homeostasis. In healthy people, the mucosal immune system works symbiotically with organisms that make up the microbiota. ILCs play a critical role in orchestrating this balance, as they can both influence and in turn be influenced by the microbiome. ILCs also are important regulators of the early response to infections by diverse types of pathogenic microbes at mucosal barriers. Their rapid responses initiate inflammatory programs, production of antimicrobial products and repair processes. This review will focus on the role of ILCs in response to the microbiota and to microbial infections of the lung and intestine.

Interleukin-22-mediated host glycosylation prevents Clostridioides difficile infection by modulating the metabolic activity of the gut microbiota

The involvement of host immunity in the gut microbiota-mediated colonization resistance to Clostridioides difficile infection (CDI) is incompletely understood. Here, we show that interleukin (IL)-22, induced by colonization of the gut microbiota, is crucial for the prevention of CDI in human microbiota-associated (HMA) mice. IL-22 signaling in HMA mice regulated host glycosylation, which enabled the growth of succinate-consuming bacteria Phascolarctobacterium spp. within the gut microbiome. Phascolarctobacterium reduced the availability of luminal succinate, a crucial metabolite for the growth of C. difficile, and therefore prevented the growth of C. difficile. IL-22-mediated host N-glycosylation is likely impaired in patients with ulcerative colitis (UC) and renders UC-HMA mice more susceptible to CDI. Transplantation of healthy human-derived microbiota or Phascolarctobacterium reduced luminal succinate levels and restored colonization resistance in UC-HMA mice. IL-22-mediated host glycosylation thus fosters the growth of commensal bacteria that compete with C. difficile for the nutritional niche.

Type 3 Immunity during Clostridioides difficile Infection: Too Much of a Good Thing?

Clostridioides (formerly known as Clostridium) difficile is the leading cause of hospital-acquired gastrointestinal infections in the United States and one of three urgent health care threats identified by the Centers for Disease Control and Prevention. C. difficile disease is mediated by the production of toxins that disrupt the epithelial barrier and cause a robust host inflammatory response. Studies in humans as well as animal models of disease have shown that the type of immune response generated against the infection dictates the outcome of disease, often irrespective of bacterial burden. Much of the focus on immunity during C. difficile infection (CDI) has been on type 3 immunity because of the established role for this arm of the immune system in other gastrointestinal inflammatory conditions such as inflammatory bowel disease (IBD). For example, interleukin-22 (IL-22) production by group 3 innate lymphoid cells (ILC3s) protects against pathobionts translocating across the epithelium during CDI. On the other hand, interleukin-17 (IL-17) production by Th17 cells increases CDI-associated mortality. Additionally, neutropenia has been associated with increased susceptibility to CDI in humans, but increased neutrophilia in mouse models correlates with host pathology. Taking the data together, these findings suggest dual roles for type 3 immune responses during infection. Here, we review the complex role of type 3 immunity during CDI and delineate what is known about innate and adaptive cellular immunity as well as the downstream effector cytokines known to be important during this infection.

Aging impairs protective host defenses against Clostridioides (Clostridium) difficile infection in mice by suppressing neutrophil and IL-22 mediated immunity

BACKGROUND

Morbidity and mortality associated with Clostridioides (formerly Clostridium) difficile infection (CDI) rises progressively with advanced age (≥65 years) due in part to perturbations of the gut microbiota and immune dysfunction. Epidemiological data of community-acquired CDI suggests increased susceptibility may begin earlier during middle-age (45-64 years) but the causation remains unknown.

METHODS

Middle-aged (12-14 months) and young (2-4 months) adult mice were infected with C. difficile, and disease severity, gut microbiome and innate immune response were compared. Cytokine reconstitution studies were performed in infected middle-aged mice.

RESULTS

Infection of middle-aged mice with C. difficile led to greater disease compared to young controls, which was associated with increases in C. difficile burden and toxin titers, and elevated bacterial translocation. With the exception of an expansion of C. difficile in middle-aged mice, microbiome analysis revealed no age-related differences. In contrast, middle-aged mice displayed a significant defect in neutrophil recruitment to the colon, with diminished levels of innate immune cytokines IL-6, IL-23 and IL-22. Importantly, recombinant IL-22 administration during CDI reduced morbidity and prevented death in middle-aged mice.

CONCLUSION

Increased susceptibility to C. difficile occurs in middle-aged mice modeling the community-acquired CDI demographics and is driven by an impaired innate immune response.

CD160 Stimulates CD8+ T Cell Responses and Is Required for Optimal Protective Immunity to Listeria monocytogenes

CD160 promotes NK cell cytotoxicity and IFN-γ production, but the function of CD160 on CD8⁺ T cells remains unclear with some studies supporting a coinhibitory role and others a costimulatory role. In this study, we demonstrate that CD160 has a costimulatory role in promoting CD8⁺ T cell effector functions needed for optimal clearance of oral Listeria monocytogenes infection. CD160^-/- mice did not clear oral L. monocytogenes as efficiently as wild type (WT) littermates. WT RAG^-/- and CD160^-/- RAG^-/- mice similarly cleared L. monocytogenes, indicating that CD160 on NK cells does not contribute to impaired L. monocytogenes clearance. Defective L. monocytogenes clearance is due to compromised intraepithelial lymphocytes and CD8⁺ T cell functions. There was a reduction in the frequencies of granzyme B-expressing intraepithelial lymphocytes in L. monocytogenes-infected CD160^-/- mice as compared with WT littermate controls. Similarly, the frequencies of granzyme B-expressing splenic CD8⁺ T cells and IFN-γ and TNF-α double-producer CD8⁺ T cells were significantly reduced in L. monocytogenes-infected CD160^-/- mice compared with WT littermates. Adoptive transfer studies showed that RAG^-/- recipients receiving CD160^-/- CD8⁺ T cells had a higher mortality, exhibited more weight loss, and had a higher bacterial burden compared with RAG^-/- recipients receiving WT CD8⁺ T cells. These findings demonstrate that CD160 provides costimulatory signals to CD8⁺ T cells needed for optimal CD8⁺ T cell responses and protective immunity during an acute mucosal bacterial infection.

LIGHT-HVEM Signaling in Innate Lymphoid Cell Subsets Protects Against Enteric Bacterial Infection

Innate lymphoid cells (ILCs) are important regulators of early infection at mucosal barriers. ILCs are divided into three groups based on expression profiles, and are activated by cytokines and neuropeptides. Yet, it remains unknown if ILCs integrate other signals in providing protection. We show that signaling through herpes virus entry mediator (HVEM), a member of the tumor necrosis factor (TNF) receptor superfamily, in ILC3 is important for host defense against oral infection with the bacterial pathogen Yersinia enterocolitica. HVEM stimulates protective interferon-γ (IFN-γ) secretion from ILCs, and mice with HVEM-deficient ILC3 exhibit reduced IFN-γ production, higher bacterial burdens and increased mortality. In addition, IFN-γ production is critical as adoptive transfer of wild-type but not IFN-γ-deficient ILC3 can restore protection to mice lacking ILCs. We identify the TNF superfamily member, LIGHT, as the ligand inducing HVEM signals in ILCs. Thus HVEM signaling mediated by LIGHT plays a critical role in regulating ILC3-derived IFN-γ production for protection following infection. VIDEO ABSTRACT.

Systemic Inflammatory Responses in Ulcerative Colitis Patients and Clostridium difficile Infection

BACKGROUND/AIMS

Finding differences in systemic inflammatory response in ulcerative colitis (UC), UC with Clostridium difficile infection (CDI), and CDI could lead to a better ability to differentiate between UC with symptomatic CDI and UC with C. difficile colonization, and could identify specific inflammatory pathways for UC or CDI, which could be therapeutic targets.

METHODS

We prospectively collected sera from symptomatic UC patients whose stools were tested for toxigenic C. difficile, and from CDI patients who did not have UC (CDI-noUC). The UC patients with positive tests (UC-CDI) were further categorized into responders to CDI treatment (UC-CDI-R) and non-responders (UC-CDI-NR). We compared serum inflammatory mediators among groups using unadjusted and adjusted multivariable statistics.

RESULTS

We included 117 UC [27 UC-CDI, 90 UC without CDI (UC-noCDI)] and 16 CDI-noUC patients. Principal component analysis (PCA) did not reveal significant differences either between UC-CDI and UC-noCDI groups, or between UC-CDI-R and UC-CDI-NR groups. In contrast, the PCA showed significant separation between the UC and CDI-noUC groups (P = 0.002). In these two groups, hepatocyte growth factor (HGF) and chemokine (C-C motif) ligand 2 (CCL2) levels were significantly lower and IL-23 levels were higher in UC patients in multivariable analyses. The model to distinguish UC from CDI including IL-23, HGF, CCL2, age, gender, and HGB had an AuROC of 0.93.

CONCLUSION

Inflammatory profiles could not distinguish UC-CDI from UC-noCDI, and UC-CDI-R from UC-CDI-NR. However, the UC and CDI-noUC groups were significantly different. Future work should examine whether therapeutic agents inhibiting IL-23 or stimulating HGF can treat UC.

Role of the leukocyte response in normal and immunocompromised host after Clostridium difficile infection

Clostridium difficile is the leading cause of healthcare-associated infections in the United States. Clinically, C. difficile-associated disease can present as asymptomatic colonization, self-limited diarrheal illness or severe colitis (that may result in death). This variability in disease course and outcomes suggests that host factors play an important role as key determinants of disease severity. Currently, there are several scoring indices to estimate severity of C. difficile-associated disease. Leukocytosis and renal failure are considered to be the most important predictors of C. difficile disease severity in hosts with a normal immune system. The degree of leukocytosis which is considered significant for severe disease and how it is scored vary amongst scoring indices. None of the scores have been prospectively validated, and while total WBC count is useful to estimate the magnitude of the host response in most patient populations, in immune-compromised patients like those receiving chemotherapy, solid organ transplant patients or hematopoietic stem cell transplants the WBC response can be variable or even absent making this marker of severity difficult to interpret. Other cellular subsets like neutrophils, eosinophils and lymphocytes provide important information about the host immune status and play an important role in the immune response against C. difficile infection. However, under the current scoring systems the role of these cellular subsets have been underestimated and only total white blood cell counts are taken into account. In this review we highlight the role of host leukocyte response to C. difficile challenge in the normal and immunocompromised host, and propose possible ways that would allow for a better representation of the different immune cell subsets (neutrophils, lymphocytes and eosinophils) in the current scoring indices.

Role of interferon-γ and inflammatory monocytes in driving colonic inflammation during acute Clostridium difficile infection in mice

The inflammatory response to the colonic pathogen Clostridium difficile is characterized by the induction of inflammatory cytokines including Interleukin-23 (IL-23) and interferon-γ (IFN-γ) and the recruitment of myeloid cells including Ly6C^High monocytes. IL-23 knockout mice showed reduced expression of the monocyte chemokines Ccl4 and Ccl7, but not Ccl2, as well as reduced Ly6C^High Ly6G^Mid monocyte recruitment to the colon in response to C. difficile colitis. Clostridium difficile-infected CCR2^-/- (CCR2 KO) mice showed a significant defect in Ly6C^High Ly6G^Mid monocyte recruitment to the colon in response to C. difficile. Although there was no decrease in expression of the inflammatory cytokines Il1b, Il6 or Tnf or reduction in the severity of colonic histopathology associated with ablation of monocyte recruitment, Slpi and Inos expression was significantly reduced in the colons of these animals. Additionally, neutralization of IFN-γ through the administration of anti-IFN-γ monoclonal antibody resulted in a significant reduction in the expression of the IFN-γ-inducible chemokines Cxcl9 and Cxcl10, but not a reduction in the neutrophil chemokines Cxcl1, Cxcl2 and Ccl3 or the monocyte chemokine Ccl2. Consistently, monocyte and neutrophil recruitment were unchanged following anti-IFN-γ treatment. Additionally, Inos and Slpi expression were unchanged following anti-IFN-γ treatment, suggesting that Inos and Slpi regulation is independent of IFN-γ during C. difficile colitis. Taken together, these data strongly suggest that IL-23 and CCR2 signalling are required for monocyte recruitment during C. difficile colitis. Additionally, these studies also suggest that monocytes, but not IFN-γ, are necessary for full expression of Inos and Slpi in the colon.

Immune responses induced by Clostridium difficile

The spectrum of Clostridium difficile infections is highly variable, ranging from asymptomatic carriage to fatal colitis depending on the strain virulence and on the host, its gut microbiota and its immune response. After disruption of the gut microbiota, C. difficile pathogenesis can be divided into three steps: 1) contamination by spores and their germination; 2) multiplication of vegetative cells and intestinal colonization using colonization factors; 3) production of the toxins TcdA and TcdB, and for some strains, the binary toxin, which are responsible for the clinical signs. Three lines of defense counteract C. difficile. The first line is the epithelial barrier, which is breached by the toxins. Then, a rapid innate immune response follows, which forms the second line of defense. It provides very quick defense reactions against C. difficile but is non-specific and does not confer memory. C. difficile and its virulence factors, the toxins and colonization factors, induce a highly pro-inflammatory response, which can be either beneficial or harmful, but triggers the adaptive immunity as the third line of defense required to control the infectious process. Adaptive immunity provides a highly specific immune response against C. difficile with memory and long lasting immunity. The innate and adaptive immune responses against the toxins and surface components are analyzed as well as their role in disease susceptibility, severity and recurrences.

Clostridium difficile colitis: pathogenesis and host defence

Clostridium difficile is a major cause of intestinal infection and diarrhoea in individuals following antibiotic treatment. Recent studies have begun to elucidate the mechanisms that induce spore formation and germination and have determined the roles of C. difficile toxins in disease pathogenesis. Exciting progress has also been made in defining the role of the microbiome, specific commensal bacterial species and host immunity in defence against infection with C. difficile. This Review will summarize the recent discoveries and developments in our understanding of C. difficile infection and pathogenesis.

Neutrophil-mediated inflammation in the pathogenesis of Clostridium difficile infections

Clostridium difficile is the most important cause of nosocomial infectious diarrhea in the western world. C. difficile infections are a major healthcare burden with approximately 500,000 new cases every year and an estimated annual cost of nearly $1 billion in the U.S. Furthermore, the infections are no longer restricted to health care facilities, and recent studies indicate spread of C. difficile infection to the community as well. The clinical spectrum of C. difficile infection ranges from asymptomatic colonization to severe diarrhea, fulminant colitis and death. This spectrum results from a complex interplay between bacterial virulence factors, the colonic microbiome and the host inflammatory response. The overall vigor of host inflammatory response is believed to be an important determinant of C. difficile disease severity, and a more robust immune response is associated with worse outcomes. Neutrophils are the primary cells that respond to C. difficile invasion and neutrophilic inflammation is the hallmark of C. difficile-associated disease. In this review, we will focus on the role of neutrophils (infiltration to infected tissue, pathogen clearance and resolution of inflammation) in the immuno-pathogenesis of C. difficile-associated disease (CDAD).

Interleukin-23 (IL-23), independent of IL-17 and IL-22, drives neutrophil recruitment and innate inflammation during Clostridium difficile colitis in mice

Our objective was to determine the role of the inflammatory cytokine interleukin-23 (IL-23) in promoting neutrophil recruitment, inflammatory cytokine expression and intestinal histopathology in response to Clostridium difficile infection. Wild-type (WT) and p19(-/-) (IL-23KO) mice were pre-treated with cefoperazone in their drinking water for 5 days, and after a 2-day recovery period were challenged with spores from C. difficile strain VPI 10463. Interleukin-23 deficiency was associated with significant defects in both the recruitment of CD11b(High) Ly6G(H) (igh) neutrophils to the colon and the expression of neutrophil chemoattractants and stabilization factors including Cxcl1, Cxcl2, Ccl3 and Csf3 within the colonic mucosa as compared with WT animals. Furthermore, the expression of inflammatory cytokines including Il33, Tnf and Il6 was significantly reduced in IL-23-deficient animals. There was also a trend towards less severe colonic histopathology in the absence of IL-23. The induction of Il17a and Il22 was also significantly abrogated in IL-23KO mice. Inflammatory cytokine expression and neutrophilic inflammation were not reduced in IL-17a-deficient mice or in mice treated with anti-IL-22 depleting monoclonal antibody. However, induction of RegIIIg was significantly reduced in animals treated with anti-IL-22 antibody. Taken together, these data indicate that IL-23, but not IL-17a or IL-22, promotes neutrophil recruitment and inflammatory cytokine and chemokine expression in the colon in response to C. difficile infection.

Innate Immune Defenses Mediated by Two ILC Subsets Are Critical for Protection against Acute Clostridium difficile Infection

Infection with the opportunistic enteric pathogen Clostridium difficile is an increasingly common clinical complication that follows antibiotic treatment-induced gut microbiota perturbation. Innate lymphoid cells (ILCs) are early responders to enteric pathogens; however, their role during C. difficile infection is undefined. To identify immune pathways that mediate recovery from C. difficile infection, we challenged C57BL/6, Rag1(-/-) (which lack T and B cells), and Rag2(-/-)Il2rg(-/-) (Ragγc(-/-)) mice (which additionally lack ILCs) with C. difficile. In contrast to Rag1(-/-) mice, ILC-deficient Ragγc(-/-) mice rapidly succumbed to infection. Rag1(-/-) but not Ragγc(-/-) mice upregulate expression of ILC1- or ILC3-associated proteins following C. difficile infection. Protection against infection was restored by transferring ILCs into Ragγc(-/-) mice. While ILC3s made a minor contribution to resistance, loss of IFN-γ or T-bet-expressing ILC1s in Rag1(-/-) mice increased susceptibility to C. difficile. These data demonstrate a critical role for ILC1s in defense against C. difficile.