Epithelium-specific MyD88 signaling, but not DCs or macrophages, control acute intestinal infection with Clostridium difficile

Infection with Clostridium difficile is one of the major causes of health care acquired diarrhea and colitis. Signaling though MyD88 downstream of TLRs is critical for initiating the early protective host response in mouse models of C. difficile infection (CDI). In the intestine, MyD88 is expressed in various tissues and cell types, such as the intestinal epithelium and mononuclear phagocytes (MNP), including DC or macrophages. Using a genetic gain-of-function system, we demonstrate here that restricting functional MyD88 signaling to the intestinal epithelium, but also to MNPs is sufficient to protect mice during acute CDI by upregulation of the intestinal barrier function and recruitment of neutrophils. Nevertheless, we also show that mice depleted for CD11c-expressing MNPs in the intestine display no major defects in mounting an effective inflammatory response, indicating that the absence of these cells is irrelevant for inducing host protection during acute infection. Together, our results highlight the importance of epithelial-specific MyD88 signaling and demonstrate that although functional MyD88 signaling in DC and macrophages alone is sufficient to correct the phenotype of MyD88-deficiency, these cells do not seem to be essential for host protection in MyD88-sufficient animals during acute infection with C. difficile.

Activation of Both TLR and NOD Signaling Confers Host Innate Immunity-Mediated Protection Against Microbial Infection

The detection of microbial pathogens relies on the recognition of highly conserved microbial structures by the membrane sensor Toll-like receptors (TLRs) and cytosolic sensor NOD-like receptors (NLRs). Upon detection, these sensors trigger innate immune responses to eradicate the invaded microbial pathogens. However, it is unclear whether TLR and NOD signaling are both critical for innate immunity to initiate inflammatory and antimicrobial responses against microbial infection. Here we report that activation of both TLR and NOD signaling resulted in an augmented inflammatory response and the crosstalk between TLR and NOD led to an amplified downstream NF-κB activation with increased nuclear transactivation of p65 at both TNF-α and IL-6 promoters. Furthermore, co-stimulation of macrophages with TLR and NOD agonists maximized antimicrobial activity with accelerated phagosome maturation. Importantly, administration of both TLR and NOD agonists protected mice against polymicrobial sepsis-associated lethality with increased serum levels of inflammatory cytokines and accelerated clearance of bacteria from the circulation and visceral organs. These results demonstrate that activation of both TLR and NOD signaling synergizes to induce efficient inflammatory and antimicrobial responses, thus conferring protection against microbial 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.

Apoptosis of intestinal epithelial cells restricts Clostridium difficile infection in a model of pseudomembranous colitis

Clostridium difficile is the leading cause of pseudomembranous colitis in hospitalized patients. C. difficile enterotoxins TcdA and TcdB promote this inflammatory condition via a cytotoxic response on intestinal epithelial cells (IECs), but the underlying mechanisms are incompletely understood. Additionally, TcdA and TcdB engage the Pyrin inflammasome in macrophages, but whether Pyrin modulates CDI pathophysiology is unknown. Here we show that the Pyrin inflammasome is not functional in IECs and that Pyrin signaling is dispensable for CDI-associated IEC death and for in vivo pathogenesis. Instead, our studies establish that C. difficile enterotoxins induce activation of executioner caspases 3/7 via the intrinsic apoptosis pathway, and demonstrate that caspase-3/7-mediated IEC apoptosis is critical for in vivo host defense during early stages of CDI. In conclusion, our findings dismiss a critical role for inflammasomes in CDI pathogenesis, and identify IEC apoptosis as a host defense mechanism that restricts C. difficile infection in vivo.

Fecal Host Transcriptomics for Non-Invasive Human Mucosal Immune Profiling: Proof of Concept in Clostridium Difficile Infection

Background:

Host factors play an important role in pathogenesis and disease outcome in Clostridium difficile infection (CDI), and characterization of these responses could uncover potential host biomarkers to complement existing microbe-based diagnostics.

Methods:

We extracted RNA from fecal samples of patients with CDI and profiled human mRNA using amplicon-based next-generation sequencing (NGS). We compared the fecal host mRNA transcript expression profiles of patients with CDI to controls with non-CDI diarrhea.

Results:

We found that the ratio of human actin gamma 1 (ACTG1) to 16S ribosomal RNA (rRNA) was highly correlated with NGS quality as measured by percentage of reads on target. Patients with CDI could be differentiated from those with non-CDI diarrhea based on their fecal mRNA expression profiles using principal component analysis. Among the most differentially expressed genes were ones related to immune response (IL23A, IL34) and actin-cytoskeleton function (TNNT1, MYL4, SMTN, MYBPC3, all adjusted P-values < 1 x 10^-3).

Conclusions:

In this proof-of-concept study, we used host fecal transcriptomics for non-invasive profiling of the mucosal immune response in CDI. We identified differentially expressed genes with biological plausibility based on animal and cell culture models. This demonstrates the potential of fecal transcriptomics to uncover host-based biomarkers for enteric infections.

Nucleotide-binding oligomerization domain-containing protein 1 (NOD1) in Asian seabass, Lates calcarifer: Cloning, ontogeny and expression analysis following bacterial infection or ligand stimulation

NOD1 (Nucleotide-binding oligomerization domain-containing protein 1) is one of the most prominent intracellular Nod-like receptors (NLRs), responsible for detecting different microbial components and products arising from tissue injury. Here, we have identified and cloned NOD1 transcript in the Asian seabass, Lates calcarifer (AsNOD1), which consists of 3749 nucleotides and encodes for a predicted putative protein of 900 AA. The AsNOD1 possesses the typical structure of NLR family, consisting of N-terminal CARD domain, centrally located NACHT domain and C-terminal LRRs. The AsNOD1 showed ubiquitous tissue expression in 11 different tissues of healthy animals tested with high levels of expression in hindgut and gill. From the ontogenetic expression profile of AsNOD1, it is quite evident that this gene might follow a maternally-transferred trend in euryhaline teleosts, as it is highly abundant in embryonic developmental stages. The constitutive immunomodulation of AsNOD1 in terms of expression level was clearly evident in the different tissues of Asian seabass-injected either with Vibrio alginolyticus or poly I:C. However, injection with Staphylococcus aureus did not elicit similar immunomodulation except for the up-regulation noticed at few time-points in some tissues. SISK-cell line induced with different ligands such as poly I:C, LPS and PGN also showed up-regulation of AsNOD1 in certain time-points in vitro. Based on the results obtained in the present study, it can be inferred that the AsNOD1 might play an immunoregulatory role upon exposure to different bacterial as well as viral PAMPs and also might be an important component of innate immune element during embryonic and larval development in the euryhaline teleost Asian seabass.

TPL2 Is a Key Regulator of Intestinal Inflammation in Clostridium difficile Infection

Tumor progression locus 2 (TPL2), a serine/threonine protein kinase, is a major inflammatory mediator in immune cells. The predominant inflammatory actions of TPL2 depend on the activation of mitogen-activated protein kinases (MAPK) and the upregulated production of the cytokines tumor necrosis factor alpha (TNF-α) and interleukin 1β (IL-1β) in macrophages and dendritic cells in response to lipopolysaccharide (LPS). Significant increases in TNF-α, IL-6, IL-β, and IL-8 levels in patients with Clostridium difficile infection (CDI) have been reported. Both TNF-α and IL-6 have been postulated to play key roles in the systemic inflammatory response in CDI, and IL-8 is essential for the development of local intestinal inflammatory responses in CDI. The objective of this study was to elucidate the role of TPL2 in the pathogenesis of CDI. We found that TPL2 was significantly activated in human and mouse intestinal tissues upon C. difficile toxin exposure or CDI. We further demonstrated that TPL2 knockout (TPL2-KO) mice were significantly more resistant to CDI than wild-type mice, with significantly reduced production of TNF-α, IL-6, IL-1β, KC (a mouse homologue of IL-8), and myeloperoxidase (MPO) in the ceca and colons of TPL2-KO mice. Finally, we found that TPL2 inhibition by a specific inhibitor or TPL2 gene ablation significantly reduced TcdB-induced production of TNF-α, IL-6, IL-β, and KC by inhibiting the activation of p38, extracellular signal-regulated kinase (ERK), and c-Jun NH₂-terminal kinase (JNK). Taken together, our data suggest that TPL2 represents a potential therapeutic target for CDI treatment.

The NFκB signaling pathway serves an important regulatory role in Klebsiella pneumoniae liver abscesses

The incidence of Klebsiella pneumoniae liver abscess (KPLA) has increased in a number of Asian countries over the past 30 years. Diabetes mellitus (DM) is a risk factor for KPLA. The prevalence and clinical features of KPLA in patients with and without DM have been well described; however, the underlying molecular mechanism responsible for the increased incidence of KPLA in patients with DM remains unclear. In the present study, a mouse model of DM was constructed and mice were infected with K. pneumoniae. Tissues were harvested for immunohistochemical and inflammatory factor expression analyses. The results revealed that the number of liver abscesses in mice with DM was greater than that observed in normal mice. The expression of interleukin (IL)-1β, IL-2, IL-6, macrophage inflammatory protein-1α and tumor necrosis factor-α in the liver tissues of mice with DM was significantly higher compared with normal mice. Western blotting results revealed that the expression of phosphorylated (p)-inhibitor of nuclear factor κB (NFκB) kinase subunit β, p-NFκB and p-inhibitor of NFκB was significantly increased in the liver tissue of mice with DM compared with that of normal mice. These results suggest that activation of the NFκB signaling pathways has a regulatory effect on the pathogenesis of K. pneumoniae bacteria liver abscesses and that high glucose conditions may promote the activation of NFκB signaling.

The ATP-P2X7 Signaling Axis Is an Essential Sentinel for Intracellular Clostridium difficile Pathogen-Induced Inflammasome Activation

Clostridium difficile infection (CDI) is the leading cause of nosocomial infection in hospitalized patients receiving long-term antibiotic treatment. An excessive host inflammatory response is believed to be the major mechanism underlying the pathogenesis of C. difficile infection, and various proinflammatory cytokines such as IL-1β are detected in patients with C. difficile infection. IL-1β is known to be processed by caspase-1, a cysteine protease that is regulated by a protein complex called the inflammasome, which leads to a specialized form of cell death called pyroptosis. The function of inflammasome activation-induced pyroptosis is to clear or limit the spread of invading pathogens via infiltrated neutrophils. Here, we focused on inflammasome activation induced by intact C. difficile to re-evaluate the nature of inflammasome activation in CDI pathogenesis, which could provide information that leads to an alternative therapeutic strategy for the treatment of this condition in humans. First, we found that caspase-1-dependent IL-1β production was induced by C. difficile pathogens in macrophages and increased in a time-dependent manner. Moreover, intracellular toxigenic C. difficile was essential for ATP-P2X₇ pathway of inflammasome activation and subsequent caspase-1-dependent pyroptotic cell death, leading to the loss of membrane integrity and release of intracellular contents such as LDH. Notably, we also observed that bacterial components such as surface layer proteins (SLPs) were released from pyroptotic cells. In addition, pro-IL-1β production was completely MyD88 and partially TLR2 dependent. Finally, to investigate the role of the caspase-1-dependent inflammasome in host defense, we found that colonic inflammasome activation was also induced by CDI and that caspase-1 inhibition by Ac-YVAD-CMK led to increased disease progression and C. difficile load. Taken together, the present results suggest that MyD88 and TLR2 are critical component in pro-IL-1β production and intracellular C. difficile following the ATP-P2X₇ pathway of inflammasome activation and pyroptosis, which play important roles in host defense through the utilization of inflammation-mediated bacterial clearance mechanisms during C. difficile infection.

Preterm Life in Sterile Conditions: A Study on Preterm, Germ-Free Piglets

Preterm infants born with immature organ systems, which can impede normal development, can also be highly sensitive to different biological and/or environmental factors. Animal models could aid in investigating and understanding the effects of different conditions on the health of these immunocompromised infants. The epitheliochorial placentation of the pig prevents the prenatal transfer of protective colostral immunoglobulins. Surgical colostrum-deprived piglets are free of maternal immunoglobulins, and the cells that are normally provided via colostrum. We bred preterm germ-free piglets in sterile conditions and compared them with their term counterparts. Enterocyte development and intestinal morphology, tight junction proteins claudin-1 and occludin, pattern-recognizing receptors, adaptor molecules and coreceptors (RAGE, TLR2, TLR4, TLR9, MyD88, TRIF, MD2, and CD14), and inflammasome NLRP3 transcription were all evaluated. The production of inflammatory mediators IFN-α, IL-4, IL-6, IL-8, IL-10, IL-12/23 p40, TNF-α, IFN-γ, and high mobility group box 1 (HMGB1) in the intestine of germ-free piglets was also assessed. In the preterm germ-free piglets, the ileum showed decreased lamina propria cellularity, reduced villous height, and thinner and less distinct stratification - especially muscle layer, in comparison with their term counterparts. Claudin-1 transcription increased in the intestine of the preterm piglets. The transcription levels of pattern-recognizing receptors and adaptor molecules showed ambiguous trends between the groups. The levels of IL-6, IL-8, IL-10, and TNF-α were increased in the preterm ileum numerically (though not significantly), with statistically significant increases in the colon. Additionally, IL-12/23 p40 and IFN-γ were statistically significantly higher in the preterm colon. Both blood plasma and intestinal HMGB1 levels were nonsignificantly higher in the preterm group. We propose that the intestine of the preterm germ-free piglets showed "mild inflammation in sterile conditions." This model, which establishes preterm, hysterectomy-derived germ-free piglets, without protective maternal immunoglobulins, can be used to study influences of microbiota, nutrition, and therapeutic interventions on the development and health of vulnerable immunocompromised preterm infants.

Faecalibacterium prausnitzii and a Prebiotic Protect Intestinal Health in a Mouse Model of Antibiotic and Clostridium difficile Exposure

BACKGROUND

Clostridium difficile (CD) infection (CDI) increases patient morbidity, mortality and healthcare costs. Antibiotic treatment induces gut dysbiosis and is both a major risk factor for CD colonization and treatment of CDI. Probiotics have been trialed to support commensal gut microbiota and reduce CDI. This study investigated commensal microbe Faecalibacterium prausnitzii (FP) and a prebiotic, both known to yield butyrate and be anti-inflammatory and immunomodulatory, on CD colonization and gut integrity in mice.

METHODS

Mice were randomly grouped and supplemented daily with FP, prebiotic, FP + prebiotic, FP/prebiotic supernatant, or saline throughout the entire study. Following treatment with clindamycin for 3 days, mice were exposed to CD. Feces were collected at baseline, the day after antibiotic, and 1, 3, and 5 days after CD exposure and cultured for bacterial overgrowth and CD colonization. On days 1 and 5 after CD exposure, mice were randomly euthanized, and proximal colon was dissected for histological analysis and preparation of RNA for analysis of proinflammatory and anti-inflammatory cytokines.

RESULTS

Although all mice exhibited bacterial overgrowth and CD colonization, bacterial burden resolved quicker in the FP + prebiotic group. This was associated with induction and resolution of innate immune responses, anion exchanger, and tight junction protein preservation in proximal colon. CD toxin virulence potential was questionable as expression of CD toxin B receptor was depleted in the FP + prebiotic group.

CONCLUSION

Supplementation with anti-inflammatory butyrate-supporting commensal bacteria and prebiotic may support innate immune responses and minimize bacterial burden and negative effects during antibiotic and CD exposure.

Tackling the recurrence of Clostridium difficile infection

The pathogenesis of recurrent Clostridium difficile infection (CDI) is still poorly understood. The risk of recurrence is approximately 20% after an initial CDI episode and dramatically increases with subsequent CDI recurrences. Several factors may play a role in recurrent CDI (rCDI), including conditions influencing germination, metabolic pathways that influence toxin production of C. difficile, and the microbiota composition offering protection against colonization and disease caused by C. difficile. Paradoxically, the currently recommended treatment for acute symptomatic CDI, i.e. metronidazole or vancomycin, can cause modification of the intestinal flora. Indeed, administration of anti-CDI antibiotics leads to suppression of C. difficile, along with collateral damage of the protective intestinal microbiota and opening of a "window of vulnerability" for recurrence. Host factors also have a prominent role, including innate and acquired humoral immunity, i.e. passive antibodies administration or active vaccination as a prevention strategy. They play a crucial role in the protection against severe and recurrent CDI. The assessment of risk factors of recurrence and modeling prediction scores could help in preventing the troublesome experience of CDI recurrence. Six studies have methodologically assessed prediction scores for rCDI. However, the definition of recurrence was heterogeneous, external validation was often not performed, and immunological factors were often not considered. There is a need for further studies on the pathophysiology of recurrence to design models for prediction that are sound and applicable in clinical practice.

NOD1 and NOD2: Molecular targets in prevention and treatment of infectious diseases

Nucleotide-binding oligomerization domain (NOD) 1 and NOD2 are pattern-recognition receptors responsible for sensing fragments of bacterial peptidoglycan known as muropeptides. Stimulation of innate immunity by systemic or local administration of NOD1 and NOD2 agonists is an attractive means to prevent and treat infectious diseases. In this review, we discuss novel data concerning structural features of selective and non-selective (dual) NOD1 and NOD2 agonists, main signaling pathways and biological effects induced by NOD1 and NOD2 stimulation, including induction of pro-inflammatory cytokines, type I interferons and antimicrobial peptides, induction of autophagy, alterations of metabolism. We also discuss interactions between NOD1/NOD2 and Toll-like receptor agonists in terms of synergy and cross-tolerance. Finally, we review available animal data on the role of NOD1 and NOD2 in protection against infections, and discuss how these data could be applied in human infectious diseases.

Is Clostridium difficile infection a risk factor for subsequent bloodstream infection?

BACKGROUND

Clostridium difficile infection (CDI) is a common nosocomial diarrheal illness increasingly associated with mortality in United States. The underlying factors and mechanisms behind the recent increases in morbidity from CDI have not been fully elucidated. Murine models suggest a mucosal barrier breakdown leads to bacterial translocation and subsequent bloodstream infection (BSI). This study tests the hypothesis that CDI is associated with subsequent BSI in humans.

METHODS

We conducted a retrospective cohort study on 1132 inpatients hospitalized >72 h with available stool test results for toxigenic C. difficile. The primary outcome was BSI following CDI. Secondary outcomes included 30-day mortality, colectomy, readmission, and ICU admission. Unadjusted and adjusted logistic regression models were developed.

RESULTS

CDI occurred in 570 of 1132 patients (50.4%). BSI occurred in 86 (7.6%) patients. Enterococcus (14%) and Klebsiella (14%) species were the most common organisms. Patients with BSI had higher comorbidity scores and were more likely to be male, on immunosuppression, critically ill, and have a central venous catheter in place. Of the patients with BSI, 36 (42%) had CDI. CDI was not associated with subsequent BSI (OR 0.69; 95% CI 0.44-1.08; P = 0.103) in unadjusted analysis. In multivariable modeling, CDI appeared protective against subsequent BSI (OR 0.57; 95% CI 0.34-0.96; P = 0.036). Interaction modeling suggests a complicated relationship among CDI, BSI, antibiotic exposure, and central venous catheter use.

CONCLUSIONS

In this cohort of inpatients that underwent testing for CDI, CDI was not a risk factor for developing subsequent BSI.

The role of toxins in Clostridium difficile infection

Clostridium difficile is a bacterial pathogen that is the leading cause of nosocomial antibiotic-associated diarrhea and pseudomembranous colitis worldwide. The incidence, severity, mortality and healthcare costs associated with C. difficile infection (CDI) are rising, making C. difficile a major threat to public health. Traditional treatments for CDI involve use of antibiotics such as metronidazole and vancomycin, but disease recurrence occurs in about 30% of patients, highlighting the need for new therapies. The pathogenesis of C. difficile is primarily mediated by the actions of two large clostridial glucosylating toxins, toxin A (TcdA) and toxin B (TcdB). Some strains produce a third toxin, the binary toxin C. difficile transferase, which can also contribute to C. difficile virulence and disease. These toxins act on the colonic epithelium and immune cells and induce a complex cascade of cellular events that result in fluid secretion, inflammation and tissue damage, which are the hallmark features of the disease. In this review, we summarize our current understanding of the structure and mechanism of action of the C. difficile toxins and their role in disease.

Clostridium difficile infection: new approaches to prevention, non-antimicrobial treatment, and stewardship

INTRODUCTION

Despite the large amount of scientific publications exploring the epidemiology and the clinical management of Clostridium difficile (CD) infection, some issues remain unsolved or need further studies. The aim of this review is to give an update on the hot topics on CD prevention, including stewardship programs, and on the non-microbiological treatment of CD infection. Areas covered: This article will review the importance of minimizing the CD spore shedding in the healthcare environment for potentially reducing CD transmission. Moreover, antimicrobial stewardship programs aimed to reduce CD incidence will be reviewed. Finally, new strategies for reducing CD infection recurrence will be described. Expert commentary: Besides the basic infection control and prevention practices, including hand hygiene, contact isolation and environmental cleaning, in the prevention of CD infection other issues should be addressed including minimizing the spread of CD in the healthcare setting, and implementing the best strategy for reducing CD infection occurrence, including tailored antimicrobial stewardship programs. Regarding new advancements in treatment and management of CDI episodes, non-antimicrobial approaches seem to be promising in reducing and managing recurrent CD infection.

NOD1 deficiency impairs CD44a/Lck as well as PI3K/Akt pathway

Pattern recognition receptors (PRRs) are crucial for host defense and tissue homeostasis against infecting pathogens. PRRs are highly conserved cross species, suggesting their key roles in fundamental biological processes. Though much have been learned for NOD1 receptor in the innate and adaptive immune responses, the roles of NOD1 during embryonic and larval stages remain poorly understood. Here, we report that NOD1 is necessary for the modulation of PI3K-Akt pathway and larval survival in zebrafish. Transcriptome analysis revealed that the significantly enriched pathways in NOD1 -/- zebrafish larvae were mainly involved in metabolism and immune system processes. Biochemical analysis demonstrated that NOD1 was required for the expression of CD44a that, in turn, activated the PI3K-Akt pathway during larval development. Conversely, over-expression of CD44a in NOD1-deficient zebrafish restored the modulation of the PI3K-Akt pathway and improved larval survival. Collectively, our work indicates that NOD1 plays a previously undetected protective role in larval survival through CD44a-mediated activation of the PI3K-Akt signaling.

Use of a Collagen Membrane to Enhance the Survival of Primary Intestinal Epithelial Cells

Intestinal epithelial cell culture is important for biological, functional, and immunological studies. Since enterocytes have a short in vivo life span due to anoikis, we aimed to establish a novel and reproducible method to prolong the survival of mouse and human cells. Cells were isolated following a standard procedure, and cultured on ordered-cow's collagen membranes. A prolonged cell life span was achieved; cells covered the complete surface of bio-membranes and showed a classical enterocyte morphology with high expression of enzymes supporting the possibility of cryopreservation. Apoptosis was dramatically reduced and cultured enterocytes expressed cytokeratin and LGR5 (low frequency). Cells exposed to LPS or flagellin showed the induction of TLR4 and TLR5 expression and a functional phenotype upon exposure to the probiotic Bifidobacterium bifidum or the pathogenic Clostridium difficile. The secretion of the homeostatic (IL-25 and TSLP), inhibitory (IL-10 and TGF-β), or pro-inflammatory mediators (IL-1β and TNF) were induced. In conclusion, this novel protocol using cow's collagen-ordered membrane provides a simple and reproducible method to maintain intestinal epithelial cells functional for cell-microorganism interaction studies and stem cell expansion. J. Cell. Physiol. 232: 2489-2496, 2017. © 2016 Wiley Periodicals, Inc.

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.

Nucleotide-binding oligomerization domain 1 and gastrointestinal disorders

Nucleotide-binding oligomerization domain 1 (NOD1) is an intracellular sensor that detects small peptides derived from the cell wall component of intestinal microflora. NOD1 is expressed in both non-hematopoietic cells such as epithelial cells and hematopoietic cells such as antigen-presenting cells. Detection of its ligand by NOD1 leads to innate immune responses through activation of nuclear factor kappa B and type I interferon as well as induction of autophagy. Innate immune responses through NOD1 activation play an indispensable role both in host defense against microbial infection and in the development of gastrointestinal disorders. Of particular importance, NOD1-mediated innate immune responses are associated with mucosal host defenses against Helicobacter pylori (H. pylori) infection of the stomach and with the development of pancreatitis. In this review, we discuss the molecular mechanisms by which NOD1 activation leads to the development of H. pylori-related gastric diseases and pancreatitis.

Familial Mediterranean fever mutations lift the obligatory requirement for microtubules in Pyrin inflammasome activation

Familial Mediterranean fever (FMF) is the most common monogenic autoinflammatory disease worldwide. It is caused by mutations in the inflammasome adaptor Pyrin, but how FMF mutations alter signaling in FMF patients is unknown. Herein, we establish Clostridium difficile and its enterotoxin A (TcdA) as Pyrin-activating agents and show that wild-type and FMF Pyrin are differentially controlled by microtubules. Diverse microtubule assembly inhibitors prevented Pyrin-mediated caspase-1 activation and secretion of IL-1β and IL-18 from mouse macrophages and human peripheral blood mononuclear cells (PBMCs). Remarkably, Pyrin inflammasome activation persisted upon microtubule disassembly in PBMCs of FMF patients but not in cells of patients afflicted with other autoinflammatory diseases. We further demonstrate that microtubules control Pyrin activation downstream of Pyrin dephosphorylation and that FMF mutations enable microtubule-independent assembly of apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) micrometer-sized perinuclear structures (specks). The discovery that Pyrin mutations remove the obligatory requirement for microtubules in inflammasome activation provides a conceptual framework for understanding FMF and enables immunological screening of FMF mutations.

[Clostridium difficile infection : What is currently available for treatment?]

Clostridium difficile (C. difficile) is an anaerobic, Gram-positive, spore-forming, toxin-secreting bacillus. It is transmitted via a fecal-oral route and can be found in 1-3 % of the healthy population. Symptoms caused by C. difficile range from uncomplicated diarrhea to a toxic megacolon. The incidence, frequency of recurrence, and mortality rate of C. difficile infections (CDIs) have increased significantly over the past few decades. The most important risk factor is antibiotic treatment in elderly patients and patients with severe comorbidities. There is a screening test available to detect C. difficile-specific glutamate dehydrogenase (GDH), which is produced by both toxigenic and non-toxigenic strains. To confirm CDIs, it is necessary to test for toxins in a fresh, liquid stool sample via polymerase chain reaction or an enzyme-coupled immune adsorption test. If CDIs are diagnosed, then ongoing antibiotic treatment should be ended. Metronidazole is used to treat mild cases, and vancomycin is recommended for severe cases. Vancomycin or fidaxomicin should be used to treat recurrences (10-25 % of patients). In cases with several recurrences, a treatment option is fecal microbiome transfer (FMT). The cure rate following FMT is approximately 80 %. The treatment of severe and complicated CDI with a threatening toxic megacolon remains problematic. The degree of evidence of medicated treatment in this situation is low; the significance of metronidazole i. v. as an additional therapeutic measure is controversial. Tigecycline i. v. is an alternative option. Surgical treatment must be considered in patients with a toxic megacolon or an acute abdomen.

The microbiota and immune response during Clostridium difficile infection

Clostridium difficile is a gram-positive, spore forming anaerobe that infects the gut when the normal microbiota has been disrupted. C. difficile infection (CDI) is the most common cause of hospital acquired infection in the United States, and the leading cause of death due to gastroenteritis. Patients suffering from CDI have varying symptoms which range from mild diarrhea to pseudomembranous colitis and death. The involvement of the immune response to influence disease severity is just beginning to be investigated. There is evidence that the immune response can facilitate either protective or pathogenic phenotypes, suggesting it plays a multifaceted role during CDI. In addition to the immune response, the microbiota is pivotal in dictating the pathogenesis to CDI. A healthy microbiota effectively inhibits infection by restricting the ability of C. difficile to expand in the colon. Thus, understanding which immune mediators and components of the microbiota play beneficial roles during CDI will be important to future therapeutic developments. This review outlines how the microbiota can modulate specific immune mediators, such as IL-23 and others, to influence disease outcome.

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.

Older Is Not Wiser, Immunologically Speaking: Effect of Aging on Host Response to Clostridium difficile Infections

Clostridium difficile infection (CDI) is the most common cause of antibiotic-associated diarrhea and a significant burden on the health care system. Aging has been identified in the literature as a risk factor for CDI as well as adverse outcome from CDI. Although this effect of advanced age on CDI could be partially explained by clinical factors associated with aging, biologic factors are important. Innate immune system, responsible for immediate response to acute infections, plays a major role in CDI pathogenesis. Impairment in function of innate immunity with aging, demonstrated in other infection models, may lead to worse outcome with CDI. C. difficile toxin-specific antibody response protects the host against initial and recurrent infections as shown in observational studies and clinical trial. Effect of aging on antibody response to CDI has not been demonstrated, but the results from vaccine studies in other infections suggest a negative effect on humoral immunity from aging. Although intestinal microbiota from healthy people confers resistance to CDI by preventing C. difficile colonization, changes in composition of microbiota with aging may affect that resistance and increase risk for CDI. There are also age-associated changes in physiology, especially of the gastrointestinal tract, that may play a role in CDI risk and outcomes. In this review, we will first discuss the epidemiology of CDI in the elderly people, then the alteration in innate immunity, humoral response, and microbiota that increases susceptibility to CDI and severe disease and lastly, the physiological and functional changes that may modify outcomes of infection.

Endogenous IL-17 as a factor determining the severity of Clostridium difficile infection in mice

Clostridium difficile infection (CDI) is a toxin-mediated intestinal disease. Toxin A, toxin B and binary toxin are believed to be responsible for the pathogenesis of CDI, which is characterized by massive infiltration of neutrophils at the infected intestinal mucosa. IL-17 is one of the cytokines that play critical roles in several inflammatory and immunological diseases through various actions, including promoting neutrophil recruitment. The aim of this study was to examine the role of this cytokine in CDI by employing IL-17 A and F double knockout (IL-17 KO) mice for the CDI model. We demonstrated that IL-17 KO mice were more resistant to CDI than WT mice using several factors, such as diarrhoea score, weight change and survival rate. Although the bacterial numbers of C. difficile in faeces were not different, the inflammatory mediator levels at the large intestine on day 3 post-infection were attenuated in IL-17 KO mice. Finally, we showed that infiltration of neutrophils, but not macrophages, in the large intestine was significantly decreased in IL-17 KO mice compared to WT mice. In conclusion, the data demonstrate that endogenous IL-17 may be a factor determining the severity of CDI in mice. Although the mechanism is totally unknown, IL-17-mediated inflammatory responses, such as cytokine/chemokine production and neutrophil accumulation, may be plausible targets for future investigations.

Invasive Streptococcus mutans induces inflammatory cytokine production in human aortic endothelial cells via regulation of intracellular toll-like receptor 2 and nucleotide-binding oligomerization domain 2

Streptococcus mutans, the primary etiologic agent of dental caries, can gain access to the bloodstream and has been associated with cardiovascular disease. However, the roles of S. mutans in inflammation in cardiovascular disease remain unclear. The aim of this study was to examine cytokine production induced by S. mutans in human aortic endothelial cells (HAECs) and to evaluate the participation of toll-like receptors (TLRs) and cytoplasmic nucleotide-binding oligomerization domain (NOD) -like receptors in HAECs. Cytokine production by HAECs was determined using enzyme-linked immunosorbent assays, and the expression of TLRs and NOD-like receptors was evaluated by real-time polymerase chain reaction, flow cytometry and immunocytochemistry. The involvement of TLR2 and NOD2 in cytokine production by invaded HAECs was examined using RNA interference. The invasion efficiencies of S. mutans strains were evaluated by means of antibiotic protection assays. Five of six strains of S. mutans of various serotypes induced interleukin-6, interleukin-8 and monocyte chemoattractant protein-1 production by HAECs. All S. mutans strains upregulated TLR2 and NOD2 mRNA levels in HAECs. Streptococcus mutans Xc upregulated the intracellular TLR2 and NOD2 protein levels in HAECs. Silencing of the TLR2 and NOD2 genes in HAECs invaded by S. mutans Xc led to a reduction in interleukin-6, interleukin-8 and monocyte chemoattractant protein-1 production. Cytokine production induced by invasive S. mutans via intracellular TLR2 and NOD2 in HAECs may be associated with inflammation in cardiovascular disease.

Clostridium difficile infection

Infection of the colon with the Gram-positive bacterium Clostridium difficile is potentially life threatening, especially in elderly people and in patients who have dysbiosis of the gut microbiota following antimicrobial drug exposure. C. difficile is the leading cause of health-care-associated infective diarrhoea. The life cycle of C. difficile is influenced by antimicrobial agents, the host immune system, and the host microbiota and its associated metabolites. The primary mediators of inflammation in C. difficile infection (CDI) are large clostridial toxins, toxin A (TcdA) and toxin B (TcdB), and, in some bacterial strains, the binary toxin CDT. The toxins trigger a complex cascade of host cellular responses to cause diarrhoea, inflammation and tissue necrosis - the major symptoms of CDI. The factors responsible for the epidemic of some C. difficile strains are poorly understood. Recurrent infections are common and can be debilitating. Toxin detection for diagnosis is important for accurate epidemiological study, and for optimal management and prevention strategies. Infections are commonly treated with specific antimicrobial agents, but faecal microbiota transplants have shown promise for recurrent infections. Future biotherapies for C. difficile infections are likely to involve defined combinations of key gut microbiota.

NOD-Like Receptors: Guardians of Intestinal Mucosal Barriers

The NOD-like receptors (NLRs) are cytosolic pattern-recognition receptors, which are critically involved in mucosal immune defense. The association of the NLR, NOD2, with inflammatory bowel disease first pointed to the NLRs potential function as guardians of the intestinal barrier. Since then, several studies have emphasized the importance of NLRs in maintaining gut homeostasis and intestinal infections, and in shaping the microbiota. In this review, we will highlight the function of NLRs in intestinal inflammation.

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).

Molecular cloning and functional analysis of nucleotide-binding oligomerization domain-containing protein 1 in rainbow trout, Oncorhynchus mykiss

NOD1 has important roles in innate immunity as sensor of microbial components derived from bacterial peptidoglycan. In this study, we identified genes encoding components of the NOD1 signaling pathway, including NOD1 (OmNOD1) and RIP2 (OmRIP2) from rainbow trout, Oncorhynchus mykiss, and investigated whether OmNOD1 has immunomodulating activity in a rainbow trout hepatoma cell line RTH-149 treated with NOD1-specific ligand (iE-DAP). The deduced amino acid sequence of OmNOD1 contained conserved CARD, NOD and LRR domains. Loss-of-function and gain-of-function experiments indicated that OmNOD1 is involved in the expression of pro-inflammatory cytokines. Silencing of OmNOD1 in RTH-149 cells treated with iE-DAP decreased the expression of IL-1β, IL-6, IL-8 and TNF-α. Conversely, overexpression of OmNOD1 resulted in up-regulation of IL-1β, IL-6, IL-8 and TNF-α expression. In addition, RIP2 inhibitor (gefitinib) significantly decreased the expression of these pro-inflammatory cytokines induced by iE-DAP in RTH-149 cells. These findings highlight the important role of NOD1 signaling pathway in fish in eliciting innate immune response.

The role of the innate immune system in destruction of pancreatic beta cells in NOD mice and humans with type I diabetes

Type 1 diabetes (T1D) is an organ-specific autoimmune disease characterized by T cell-mediated destruction of the insulin-producing pancreatic β cells. A combination of genetic and environmental factors eventually leads to the loss of functional β cell mass and hyperglycemia. Both innate and adaptive immunity are involved in the development of T1D. In this review, we have highlighted the most recent findings on the role of innate immunity, especially the pattern recognition receptors (PRRs), in disease development. In murine models and human studies, different PRRs, such as toll-like receptors (TLRs) and nucleotide-binding domain, leucine-rich repeat-containing (or Nod-like) receptors (NLRs), have different roles in the pathogenesis of T1D. These PRRs play a critical role in defending against infection by sensing specific ligands derived from exogenous microorganisms to induce innate immune responses and shape adaptive immunity. Animal studies have shown that TLR7, TLR9, MyD88 and NLPR3 play a disease-predisposing role in T1D, while controversial results have been found with other PRRs, such as TLR2, TLR3, TLR4, TLR5 and others. Human studies also shown that TLR2, TLR3 and TLR4 are expressed in either islet β cells or infiltrated immune cells, indicating the innate immunity plays a role in β cell autoimmunity. Furthermore, some human genetic studies showed a possible association of TLR3, TLR7, TLR8 or NLRP3 genes, at single nucleotide polymorphism (SNP) level, with human T1D. Increasing evidence suggest that the innate immunity modulates β cell autoimmunity. Thus, targeting pathways of innate immunity may provide novel therapeutic strategies to fight this disease.

Transcription Profiling of NOD-like Receptors in the Human Cornea with Disease

PURPOSE

To investigate the expression of nucleotide-binding oligomerization domain-like receptors (NLRs) in human corneas with disease and corneal cells.

METHODS

The expression of NOD1, NOD2, NLRP1, and NLRP3 was analyzed using real-time RT-PCR in (1) corneas with active infection, history of herpetic stromal keratitis (HSK), chronic allograft rejection, and limbal stem cell deficiency (LSCD), and (2) human corneal cells after lipopolysaccharide (LPS) stimulation. Healthy corneas and cells without LPS served as controls.

RESULTS

The mRNA levels of NOD2 and NLRP3 were increased in corneas with infection and HSK. Conversely, the levels of NOD1, NOD2, NLRP1, and NLRP3 transcripts were decreased in corneas with LSCD. In corneas with rejection, the expression of NOD1 and NLRP1 was downregulated. Corneal endothelial cells upregulated the expression of NOD2 and NLRP3 upon LPS.

CONCLUSIONS

The changes in the NLR expression may reflect different susceptibility to infectious and non-infectious injuries in corneas with various diseases.

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.

Inflammatory responses to a Clostridium perfringens type A strain and α-toxin in primary intestinal epithelial cells of chicken embryos

The causative pathogen of necrotic enteritis is the Gram-positive bacterium Clostridium perfringens. Its main cell wall component, peptidoglycan (PGN), can be recognized by Toll-like receptor 2 and nucleotide-binding oligomerization domain (NOD). Consequently, the immune response is initiated via activation of nuclear factor kappa B (NF-κB) signalling pathway. An in vitro study was conducted to investigate chicken intestinal inflammatory responses to C. perfringens type A and one of its virulence factors, α-toxin. In primary intestinal epithelial cells, C. perfringens as well as commercially available PGN and α-toxin challenge upregulated mRNA expression of interleukin (IL)-6, IL-8 and inducible nitric oxide synthase (iNOS) with a dosage-dependent manner at 3 h post infection (p.i.; P ≤ 0.001). Time-course effects of three stimulators at high concentration were further examined. C. perfringens infection elevated IL-6, IL-8 and iNOS levels from 1 h to 9 h p.i., while PGN treatment increased IL-6 and IL-8 expression at 1 h and 3 h p.i. (P < 0.05). Bacterial and PGN treatments induced NOD1 expression at 6 h p.i. and only bacterial infection boosted NF-κB p65 expression at 6 h and 9 h p.i. (P < 0.05). α-Toxin treatment upregulated IL-6 and IL-8 expression throughout infection, as well as iNOS, TNF-α and NF-κB p65 expression at later hours p.i. (P < 0.05). In conclusion, both C. perfringens and α-toxin challenge induced intense cytokine expression associated with NF-κB activation in chicken intestinal epithelial cells. The receptors for the recognition of PGN component of C. perfringens need further investigation.

Disruptions of Host Immunity and Inflammation by Giardia Duodenalis: Potential Consequences for Co-Infections in the Gastro-Intestinal Tract

Giardia duodenalis (syn. G. intestinalis, or G. lamblia) is a leading cause of waterborne diarrheal disease that infects hundreds of millions of people annually. Research on Giardia has greatly expanded within the last few years, and our understanding of the pathophysiology and immunology on this parasite is ever increasing. At peak infection, Giardia trophozoites induce pathophysiological responses that culminate in the development of diarrheal disease. However, human data has suggested that the intestinal mucosa of Giardia-infected individuals is devoid of signs of overt intestinal inflammation, an observation that is reproduced in animal models. Thus, our understanding of host inflammatory responses to the parasite remain incompletely understood and human studies and experimental data have produced conflicting results. It is now also apparent that certain Giardia infections contain mechanisms capable of modulating their host's immune responses. As the oral route of Giardia infection is shared with many other gastrointestinal (GI) pathogens, co-infections may often occur, especially in places with poor sanitation and/or improper treatment of drinking water. Moreover, Giardia infections may modulate host immune responses and have been found to protect against the development of diarrheal disease in developing countries. The following review summarizes our current understanding of the immunomodulatory mechanisms of Giardia infections and their consequences for the host, and highlights areas for future research. Potential implications of these immunomodulatory effects during GI co-infection are also discussed.

Interrelatedness between dysbiosis in the gut microbiota due to immunodeficiency and disease penetrance of colitis

The composition of the microbiome in health and disease has only recently become a major research focus. Although it is clear that an imbalance or dysbiosis in the microbiota is associated with disease, its interrelatedness to disease penetrance is largely unknown. Inflammatory bowel disease (IBD) is an excellent disease in which to explore these questions because of the extensive genetic studies identifying disease susceptibility loci and the ability to easily sample the intestinal microbiota in IBD patients due to the accessibility of stool samples. In addition, mouse models of IBD have contributed to our understanding of the interrelatedness of the gut microbiota and genes associated with IBD. The power of the mouse studies is that multiple colitis models exist that can be used in combination with genetically modified mice that harbour deficiencies in IBD susceptibility genes. Collectively, these studies revealed that bacterial dysbiosis does occur in human IBD and in mouse colitis models. In addition, with an emphasis on immune genes, the mouse studies provided evidence that specific immune regulatory proteins associated with IBD influence the gut microbiota in a manner consistent with disease penetrance. In this review, we will discuss studies in both humans and mice that demonstrate the impact of immunodeficiences in interleukin-10, interleukin-17, nucleotide-binding oligomerization domain (NOD) 2, NOD-like receptor proteins 3 and 6, Toll-like receptor or IgA have on the interrelatedness between the composition of the gut microbiota and disease penetrance of IBD and its mouse models.

Colonization Resistance of the Gut Microbiota against Clostridium difficile

Antibiotics strongly disrupt the human gut microbiota, which in consequence loses its colonization resistance capacity, allowing infection by opportunistic pathogens such as Clostridium difficile. This bacterium is the main cause of antibiotic-associated diarrhea and a current problem in developed countries, since its incidence and severity have increased during the last years. Furthermore, the emergence of antibiotic resistance strains has reduced the efficiency of the standard treatment with antibiotics, leading to a higher rate of relapses. Here, we review recent efforts focused on the impact of antibiotics in the gut microbiome and their relationship with C. difficile colonization, as well as, in the identification of bacteria and mechanisms involved in the protection against C. difficile infection. Since a healthy gut microbiota is able to avoid pathogen colonization, restoration of the gut microbiota seems to be the most promising approach to face C. difficile infection, especially for recurrent cases. Therefore, it would be possible to design probiotics for patients undergoing antimicrobial therapies in order to prevent or fight the expansion of the pathogen in the gut ecosystem.

Pathogenesis of Clostridium difficile Infection and Its Potential Role in Inflammatory Bowel Disease

Colonization with toxigenic Clostridium difficile may be associated with a wide spectrum of clinical presentation ranging from asymptomatic carriage to mild diarrhea to life-threatening colitis. Over the last 15 years, there has been a marked increase in the incidence of C. difficile infection, which predominantly affects elderly patients on antibiotics. More recently, there has been significant interest in the association between inflammatory bowel disease (IBD) and C. difficile infection. This review article discusses in some detail current knowledge of the mechanisms by which C. difficile toxins may mediate mucosal inflammation, together with the role of cell wall components of the microorganism in disease pathogenesis. Innate and adaptive host responses to C. difficile toxins and other components are described and include consideration of the potential role of known mucosal changes in IBD that may lead to an enhanced inflammatory response in the presence of C. difficile infection. Recent studies, which have characterized resident microbiota that may mediate protection against colonization by C. difficile, including their mechanisms of action, are also discussed. This includes the role of bile acids and 7α-dehydroxylase-expressing bacteria, such as Clostridium scindens. Recent studies suggest a higher carriage rate of C. difficile in patients with IBD. It is anticipated that future studies will determine the role of dysbiosis in IBD in predisposing to colonization with C. difficile.

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.

The role of Gr-1(+) cells and tumour necrosis factor-α signalling during Clostridium difficile colitis in mice

The host response to Clostridium difficile infection in antibiotic-treated mice is characterized by robust recruitment of Gr-1(+) cells, increased expression of inflammatory cytokines including tumour necrosis factor-α (TNF-α), and the development of severe epithelial damage. To investigate the role of Gr-1(+) cells and TNF-α during C. difficile colitis, we treated infected mice with monoclonal antibodies against Gr-1 or TNF-α. Mice were challenged with vegetative cells of C. difficile strain VPI 10463 following treatment with the third-generation cephalosporin ceftriaxone. Ceftriaxone treatment alone was associated with significant changes in cytokine expression within the colonic mucosa but not overt inflammatory histopathological changes. In comparison, C. difficile infection following ceftriaxone treatment was associated with increased expression of inflammatory cytokines and chemokines including Cxcl1, Cxcl2, Il1b, Il17f and Tnfa, as well as robust recruitment of Ly6C(Mid)  Gr-1(High) neutrophils and Ly6C(High) Gr-1(Mid) monocytes and the development of severe colonic histopathology. Anti-Gr-1 antibody treatment resulted in effective depletion of both Ly6C(Mid) Gr-1(High) neutrophils and Ly6C(High) Gr-1(Mid) monocytes: however, we observed no protection from the development of severe pathology or reduction in expression of the pro-inflammatory cytokines Il1b, Il6, Il33 and Tnfa following anti-Gr-1 treatment. By contrast, anti-TNF-α treatment did not affect Gr-1(+) cell recruitment, but was associated with increased expression of Il6 and Il1b. Additionally, Ffar2, Ffar3, Tslp, Tff and Ang4 expression was significantly reduced in anti-TNF-α-treated animals, in association with marked intestinal histopathology. These studies raise the possibility that TNF-α may play a role in restraining inflammation and protecting the epithelium during C. difficile infection.

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

Our previous work has shown the significant up-regulation of Il22 and increased phosphorylation of signal transducer and activator of transcription 3 (STAT3) as part of the mucosal inflammatory response to Clostridium difficile infection in mice. Others have shown that phosphorylation of STAT3 at mucosal surfaces includes interleukin-22 (IL-22) and CD160-mediated components. The current study sought to determine the potential role(s) of IL-22 and/or CD160 in the mucosal response to C. difficile infection. Clostridium difficile-infected mice treated with anti-IL-22, anti-CD160 or a combination of the two showed significantly reduced STAT3 phosphorylation in comparison to C. difficile-infected mice that had not received either antibody. In addition, C. difficile-infected mice treated with anti-IL-22/CD160 induced a smaller set of genes, and at significantly lower levels than the untreated C. difficile-infected mice. The affected genes included pro-inflammatory chemokines and cytokines, and anti-microbial peptides. Furthermore, histopathological and flow cytometric assessments both showed a significantly reduced influx of neutrophils in C. difficile-infected mice treated with anti-IL-22/CD160. These data demonstrate that IL-22 and CD160 are together responsible for a significant fraction of the colonic STAT3 phosphorylation in C. difficile infection. They also underscore the additive effects of IL-22 and CD160 in mediating both the pro-inflammatory and pro-survival aspects of the host mucosal response in this infection.

Inflammasome activation contributes to interleukin-23 production in response to Clostridium difficile

Clostridium difficile is the most common hospital-acquired pathogen, causing antibiotic-associated diarrhea in over 250,000 patients annually in the United States. Disease is primarily mediated by toxins A and B, which induce potent proinflammatory signaling in host cells and can activate an ASC-containing inflammasome. Recent findings suggest that the intensity of the host response to infection correlates with disease severity. Our lab has identified the proinflammatory cytokine interleukin-23 (IL-23) as a pathogenic mediator during C. difficile infection (CDI). The mechanisms by which C. difficile induces IL-23, however, are not well understood, and the role of toxins A and B in this process is unclear. Here, we show that toxins A and B alone are not sufficient for IL-23 production but synergistically increase the amount of IL-23 produced in response to MyD88-dependent danger signals, including pathogen-associated molecular patterns (PAMPs) and host-derived damage associated molecular patterns (DAMPs). Danger signals also enhanced the secretion of IL-1β in response to toxins A and B, and subsequent IL-1 receptor signaling accounted for the majority of the increase in IL-23 that occurred in the presence of the toxins. Inhibition of inflammasome activation in the presence of extracellular K⁺ likewise decreased IL-23 production. Finally, we found that IL-1β was increased in the serum of patients with CDI, suggesting that this systemic response could influence downstream production of pathogenic IL-23. Identification of the synergy of danger signals with toxins A and B via inflammasome signaling represents a novel finding in the mechanistic understanding of C. difficile-induced inflammation.

Clostridium difficile is among the leading causes of death due to health care-associated infection, and factors determining disease severity are not well understood. C. difficile secretes toxins A and B, which cause inflammation and tissue damage, and recent findings suggest that some of this tissue damage may be due to an inappropriate host immune response. We have found that toxins A and B, in combination with both bacterium- and host-derived danger signals, can induce expression of the proinflammatory cytokines IL-1β and IL-23. Our results demonstrate that IL-1β signaling enhances IL-23 production and could lead to increased pathogenic inflammation during CDI.

New perspectives in Clostridium difficile disease pathogenesis

Clostridium difficile is associated with a spectrum of clinical manifestations ranging from asymptomatic carriage to severe life-threatening pseudomembranous colitis. Current perspectives indicate that C difficile pathogenesis is a multifactorial disease process dictated by pathogenic toxin production, gut microbial dysbiosis, and altered host inflammatory responses. This article summarizes recent findings underpinning the cellular and molecular mechanisms regulating bacterial virulence and sheds new light on the critical roles of the host immune response, intestinal microbiota, and metabolome in mediating disease pathogenesis.

New role for human α-defensin 5 in the fight against hypervirulent Clostridium difficile strains

Clostridium difficile infection (CDI), one of the most common hospital-acquired infections, is increasing in incidence and severity with the emergence and diffusion of hypervirulent strains. CDI is precipitated by antibiotic treatment that destroys the equilibrium of the gut microbiota. Human α-defensin 5 (HD5), the most abundant enteric antimicrobial peptide, is a key regulator of gut microbiota homeostasis, yet it is still unknown if C. difficile, which successfully evades killing by other host microbicidal peptides, is susceptible to HD5. We evaluated, by means of viability assay, fluorescence-activated cell sorter (FACS) analysis, and electron microscopy, the antimicrobial activities of α-defensins 1 and 5 against a panel of C. difficile strains encompassing the most prevalent epidemic and hypervirulent PCR ribotypes in Europe (012, 014/020, 106, 018, 027, and 078). Here we show that (i) concentrations of HD5 within the intestinal physiological range produced massive C. difficile cell killing; (ii) HD5 bactericidal activity was mediated by membrane depolarization and bacterial fragmentation with a pattern of damage peculiar to C. difficile bacilli, compared to commensals like Escherichia coli and Enterococcus faecalis; and (iii) unexpectedly, hypervirulent ribotypes were among the most susceptible to both defensins. These results support the notion that HD5, naturally present at very high concentrations in the mucosa of the small intestine, could indeed control the very early steps of CDI by killing C. difficile bacilli at their germination site. As a consequence, HD5 can be regarded as a good candidate for the containment of hypervirulent C. difficile strains, and it could be exploited in the therapy of CDI and relapsing C. difficile-associated disease.

NOD1 and NOD2: signaling, host defense, and inflammatory disease

The nucleotide-binding oligomerization domain (NOD) proteins NOD1 and NOD2, the founding members of the intracellular NOD-like receptor family, sense conserved motifs in bacterial peptidoglycan and induce proinflammatory and antimicrobial responses. Here, we discuss recent developments about the mechanisms by which NOD1 and NOD2 are activated by bacterial ligands, the regulation of their signaling pathways, and their role in host defense and inflammatory disease. Several routes for the entry of peptidoglycan ligands to the host cytosol to trigger activation of NOD1 and NOD2 have been elucidated. Furthermore, genetic screens and biochemical analyses have revealed mechanisms that regulate NOD1 and NOD2 signaling. Finally, recent studies have suggested several mechanisms to account for the link between NOD2 variants and susceptibility to Crohn's disease. Further understanding of NOD1 and NOD2 should provide new insight into the pathogenesis of disease and the development of new strategies to treat inflammatory and infectious disorders.