In vivo physiological and transcriptional profiling reveals host responses to Clostridium difficile toxin A and toxin B

Clostridioides difficile major toxins remodel the intestinal epithelia, affecting spore adherence/internalization into intestinal tissue and their association with gut vitronectin

The most common cause of healthcare-associated diarrhea and colitis in the U.S., is Clostridioides difficile, a spore-forming pathogen. Two toxins, TcdA and TcdB, are major virulence factors essential for disease manifestations, while C. difficile spores are essential for disease transmission and recurrence. Both toxins cause major damage to the epithelial barrier, trigger massive inflammation, and reshape the microbiome and metabolic composition, facilitating C. difficile colonization. C. difficile spores, essential for transmission and recurrence of the disease, persist adhered and internalized in the intestinal epithelia. Studies have suggested that toxin-neutralization in combination with antibiotic during CDI treatment in humans significantly reduces disease recurrence, suggesting a link between toxin-mediated damage and spore persistence. Here, we show that TcdA/TcdB-intoxication of intestinal epithelial Caco-2 cells leads to remodeling of accessible levels of fibronectin (Fn) and vitronectin (Vn) and their cognate alpha-integrin subunits. While TcdB-intoxication of intestinal tissue had no impact in accessible levels of Fn and Vn, but significantly increased levels of intracellular Vn. We observed that Fn and Vn released to the supernatant readily bind to C. difficile spores in vitro, while TcdB-intoxication of intestinal tissue led to increased association of C. difficile spores with gut Vn. Toxin-intoxication of the intestinal tissue also contributes to increased adherence and internalization of C. difficile spores. However, TcdB-intoxicated ligated loops infected of mice treated with Bezlotoxumanb (monoclonal anti-TcdB antibodies) did not prevent TcdB-mediated increased spore adherence and internalization into intestinal tissue. This study highlights the importance of studying the impact of C. difficile toxins of host tissues has in C. difficile interaction with host surfaces that may contribute to increased persistence and disease recurrence.

Dual RNA-seq study of the dynamics of coding and non-coding RNA expression during Clostridioides difficile infection in a mouse model

Clostridioides difficile is the leading cause of healthcare-associated diarrhea in industrialized countries. Many questions remain to be answered about the mechanisms governing its interaction with the host during infection. Non-coding RNAs (ncRNAs) contribute to shape virulence in many pathogens and modulate host responses; however, their role in C. difficile infection (CDI) has not been explored. To better understand the dynamics of ncRNA expression contributing to C. difficile infectious cycle and host response, we used a dual RNA-seq approach in a conventional murine model. From the pathogen side, this transcriptomic analysis revealed the upregulation of virulence factors, metabolism, and sporulation genes, as well as the identification of 61 ncRNAs differentially expressed during infection that correlated with the analysis of available raw RNA-seq data sets from two independent studies. From these data, we identified 118 potential new transcripts in C. difficile, including 106 new ncRNA genes. From the host side, we observed the induction of several pro-inflammatory pathways, and among the 185 differentially expressed ncRNAs, the overexpression of microRNAs (miRNAs) previously associated to inflammatory responses or unknown long ncRNAs and miRNAs. A particular host gene expression profile could be associated to the symptomatic infection. In accordance, the metatranscriptomic analysis revealed specific microbiota changes accompanying CDI and specific species associated with symptomatic infection in mice. This first adaptation of in vivo dual RNA-seq to C. difficile contributes to unravelling the regulatory networks involved in C. difficile infectious cycle and host response and provides valuable resources for further studies of RNA-based mechanisms during CDI.IMPORTANCEClostridioides difficile is a major cause of nosocomial infections associated with antibiotic therapy classified as an urgent antibiotic resistance threat. This pathogen interacts with host and gut microbial communities during infection, but the mechanisms of these interactions remain largely to be uncovered. Noncoding RNAs contribute to bacterial virulence and host responses, but their expression has not been explored during C. difficile infection. We took advantage of the conventional mouse model of C. difficile infection to look simultaneously to the dynamics of gene expression in pathogen, its host, and gut microbiota composition, providing valuable resources for future studies. We identified a number of ncRNAs that could mediate the adaptation of C. difficile inside the host and the crosstalk with the host immune response. Promising inflammation markers and potential therapeutic targets emerged from this work open new directions for RNA-based and microbiota-modulatory strategies to improve the efficiency of C. difficile infection treatments.

Bacterial toxins induce non-canonical migracytosis to aggravate acute inflammation

Migracytosis is a recently described cellular process that generates and releases membrane-bound pomegranate-like organelles called migrasomes. Migracytosis normally occurs during cell migration, participating in various intercellular biological functions. Here, we report a new type of migracytosis induced by small GTPase-targeting toxins. Unlike classic migracytosis, toxin-induced migrasome formation does not rely on cell migration and thus can occur in both mobile and immobile cells. Such non-canonical migracytosis allows the cells to promptly respond to microbial stimuli such as bacterial toxins and effectors and release informative cellular contents in bulk. We demonstrated that C. difficile TcdB3 induces liver endothelial cells and Kupffer cells to produce migrasomes in vivo. Moreover, the migracytosis-defective Tspan9‒/‒ mice show less acute inflammation and lower lethality rate in the toxin challenge assay. Therefore, we propose that the non-canonical migracytosis acts as a new mechanism for mammalian species to sense and exacerbate early immune response upon microbial infections.

Textile azo dye, Sudan Black B, inducing hepatotoxicity demonstrated in in vivo zebrafish larval model

Environmental pollution, particularly from textile industry effluents, raises concerns globally. The aim of this study is to investigate the hepatotoxicity of Sudan Black B (SBB), a commonly used textile azo dye, on embryonic zebrafish. SBB exposure led to concentration-dependent mortality, reaching 100% at 0.8 mM, accompanied by growth retardation and diverse malformations in zebrafish. Biochemical marker analysis indicated adaptive responses to SBB, including increased SOD, CAT, NO, and LDH, alongside decreased GSH levels. Liver morphology analysis unveiled significant alterations, impacting metabolism and detoxification. Also, glucose level was declined and lipid level elevated in SBB-exposed in vivo zebrafish. Inflammatory gene expressions (TNF-α, IL-10, and INOS) showcased a complex regulatory interplay, suggesting an organismal attempt to counteract pro-inflammatory states during SBB exposure. The increased apoptosis revealed a robust hepatic cellular response due to SBB, aligning with observed liver tissue damage and inflammatory events. This multidimensional study highlights the intricate web of responses due to SBB exposure, which is emphasizing the need for comprehensive understanding and targeted mitigation strategies. The findings bear the implications for both aquatic ecosystems and potentially parallels to human health, underscoring the imperative for sustained research in this critical domain.

Dual RNA-seq identifies genes and pathways modulated during Clostridioides difficile colonization

IMPORTANCE

The initial interactions between the colonic epithelium and the bacterium are likely critical in the establishment of Clostridioides difficile infection, one of the major causes of hospital-acquired diarrhea worldwide. Molecular interactions between C. difficile and human gut cells have not been well defined mainly due to the technical challenges of studying cellular host-pathogen interactions with this anaerobe. Here we have examined transcriptional changes occurring in the pathogen and host cells during the initial 24 hours of infection. Our data indicate several changes in metabolic pathways and virulence-associated factors during the initial bacterium-host cell contact and early stages of infection. We describe canonical pathways enriched based on the expression profiles of a dual RNA sequencing in the host and bacterium, and functions of bacterial factors that are modulated during infection. This study thus provides fresh insight into the early C. difficile infection process.

Network analysis of toxin production in Clostridioides difficile identifies key metabolic dependencies

Clostridioides difficile pathogenesis is mediated through its two toxin proteins, TcdA and TcdB, which induce intestinal epithelial cell death and inflammation. It is possible to alter C. difficile toxin production by changing various metabolite concentrations within the extracellular environment. However, it is unknown which intracellular metabolic pathways are involved and how they regulate toxin production. To investigate the response of intracellular metabolic pathways to diverse nutritional environments and toxin production states, we use previously published genome-scale metabolic models of C. difficile strains CD630 and CDR20291 (iCdG709 and iCdR703). We integrated publicly available transcriptomic data with the models using the RIPTiDe algorithm to create 16 unique contextualized C. difficile models representing a range of nutritional environments and toxin states. We used Random Forest with flux sampling and shadow pricing analyses to identify metabolic patterns correlated with toxin states and environment. Specifically, we found that arginine and ornithine uptake is particularly active in low toxin states. Additionally, uptake of arginine and ornithine is highly dependent on intracellular fatty acid and large polymer metabolite pools. We also applied the metabolic transformation algorithm (MTA) to identify model perturbations that shift metabolism from a high toxin state to a low toxin state. This analysis expands our understanding of toxin production in C. difficile and identifies metabolic dependencies that could be leveraged to mitigate disease severity.

Clostridioides difficile toxin A-mediated Caco-2 cell barrier damage was attenuated by insect-derived fractions and corresponded to increased gene transcription of cell junctional and proliferation proteins

Pathogenesis of C. difficile in the intestine is associated with the secretion of toxins which can damage the intestinal epithelial layer and result in diseases such as diarrhoea. Treatment for C. difficile infections consists of antibiotics which, however, have non-specific microbiocidal effects and may cause intestinal dysbiosis which results in subsequent health issues. Therefore, alternative treatments to C. difficile infections are required. We investigated whether different black soldier fly- and mealworm-derived fractions, after applying the INFOGEST digestion protocol, could counteract C. difficile toxin A-mediated barrier damage of small intestinal Caco-2 cells. Treatment and pre-treatment with insect-derived fractions significantly (p < 0.05) mitigated the decrease of the transepithelial electrical resistance (TEER) of Caco-2 cells induced by C. difficile toxin A. In relation to these effects, RNA sequencing data showed an increased transcription of cell junctional and proliferation protein genes in Caco-2 cells. Furthermore, the transcription of genes regulating immune signalling was also increased. To identify whether this resulted in immune activation we used a Caco-2/THP-1 co-culture model where the cells were only separated by a permeable membrane. However, the insect-derived fractions did not change the basolateral secreted IL-8 levels in this model. To conclude, our findings suggest that black soldier fly- and mealworm-derived fractions can attenuate C. difficile induced intestinal barrier disruption and they might be promising tools to reduce the symptoms of C. difficile infections.

C. difficile exploits a host metabolite produced during toxin-mediated disease

Several enteric pathogens can gain specific metabolic advantages over other members of the microbiota by inducing host pathology and inflammation. The pathogen Clostridium difficile is responsible for a toxin-mediated colitis that causes 450,000 infections and 15,000 deaths in the United States each year¹; however, the molecular mechanisms by which C. difficile benefits from this pathology remain unclear. To understand how the metabolism of C. difficile adapts to the inflammatory conditions that its toxins induce, here we use RNA sequencing to define, in a mouse model, the metabolic states of wild-type C. difficile and of an isogenic mutant that lacks toxins. By combining bacterial and mouse genetics, we demonstrate that C. difficile uses sorbitol derived from both diet and host. Host-derived sorbitol is produced by the enzyme aldose reductase, which is expressed by diverse immune cells and is upregulated during inflammation-including during toxin-mediated disease induced by C. difficile. This work highlights a mechanism by which C. difficile can use a host-derived nutrient that is generated during toxin-induced disease by an enzyme that has not previously been associated with infection.

The role of the globular heads of the C1q receptor in TcdA-induced human colonic epithelial cell apoptosis via a mitochondria-dependent pathway

BACKGROUND

Clostridioides (formerly Clostridium) difficile infection is the leading cause of antibiotic-associated colitis. Studies have demonstrated that C. difficile toxin A (TcdA) can cause apoptosis of many human cell types. The purpose of this study was to investigate the relationships among exposure to TcdA, the role of the receptor for the globular heads of C1q (gC1qR) gene and the underlying intracellular apoptotic mechanism in human colonic epithelial cells (NCM 460). In this study, gC1qR expression was examined using real-time polymerase chain reaction (PCR), western blotting and immunohistochemical staining. Cell viability was assessed by the water-soluble tetrazolium salt (WST-1) assay, and cell apoptosis was assessed by flow cytometry and the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling (TUNEL) assay. Mitochondrial function was assessed based on reactive oxygen species (ROS) generation, changes in the mitochondrial membrane potential (ΔΨm) and the content of ATP.

RESULTS

Our study demonstrated that increasing the concentration of TcdA from 10 ng/ml to 20 ng/ml inhibited cell viability and induced cell apoptosis (p < 0.01). Moreover, the TcdA-induced gC1qR expression and enhanced expression of gC1qR caused mitochondrial dysfunction (including production of ROS and decreases in the ΔΨm and the content of ATP) and cell apoptosis. However, silencing of the gC1qR gene reversed TcdA-induced cell apoptosis and mitochondrial dysfunction.

CONCLUSION

These data support a mechanism by which gC1qR plays a crucial role in TcdA-induced apoptosis of human colonic epithelial cells in a mitochondria-dependent manner.

Clostridium difficile toxins or infection induce upregulation of adenosine receptors and IL-6 with early pro-inflammatory and late anti-inflammatory pattern

Clostridium difficile causes intestinal inflammation, which increases adenosine. We compared the expression of adenosine receptors (AR) subtypes A1, A2A, A2B, and A3 in HCT-8, IEC-6 cells, and isolated intestinal epithelial cells, challenged or not with Clostridium difficile toxin A and B (TcdA and TcdB) or infection (CDI). In HCT-8, TcdB induced an early A2BR expression at 6 h and a late A2AR expression at 6 and 24 h. In addition, both TcdA and TcdB increased IL-6 expression at all time-points (peak at 6 h) and PSB603, an A2BR antagonist, decreased IL-6 expression and production. In isolated cecum epithelial cells, TcdA induced an early expression of A2BR at 2s and 6 h, followed by a late expression of A2AR at 6 and 24 h and of A1R at 24 h. In CDI, A2AR and A2BR expressions were increased at day 3, but not at day 7. ARs play a role in regulating inflammation during CDI by inducing an early pro-inflammatory and a late anti-inflammatory response. The timing of interventions with AR antagonist or agonists may be of relevance in treatment of CDI.

Enterotoxic Clostridia: Clostridioides difficile Infections

Clostridioides difficile is a Gram-positive, anaerobic, spore forming pathogen of both humans and animals and is the most common identifiable infectious agent of nosocomial antibiotic-associated diarrhea. Infection can occur following the ingestion and germination of spores, often concurrently with a disruption to the gastrointestinal microbiota, with the resulting disease presenting as a spectrum, ranging from mild and self-limiting diarrhea to severe diarrhea that may progress to life-threating syndromes that include toxic megacolon and pseudomembranous colitis. Disease is induced through the activity of the C. difficile toxins TcdA and TcdB, both of which disrupt the Rho family of GTPases in host cells, causing cell rounding and death and leading to fluid loss and diarrhea. These toxins, despite their functional and structural similarity, do not contribute to disease equally. C. difficile infection (CDI) is made more complex by a high level of strain diversity and the emergence of epidemic strains, including ribotype 027-strains which induce more severe disease in patients. With the changing epidemiology of CDI, our understanding of C. difficile disease, diagnosis, and pathogenesis continues to evolve. This article provides an overview of the current diagnostic tests available for CDI, strain typing, the major toxins C. difficile produces and their mode of action, the host immune response to each toxin and during infection, animal models of disease, and the current treatment and prevention strategies for CDI.

Innate Immune Response and Outcome of Clostridium difficile Infection Are Dependent on Fecal Bacterial Composition in the Aged Host

Background

Clostridium difficile infection (CDI) is a serious threat for an aging population. Using an aged mouse model, we evaluated the effect of age and the roles of innate immunity and intestinal microbiota.

Methods

Aged (18 months) and young (8 weeks) mice were infected with C difficile, and disease severity, immune response, and intestinal microbiome were compared. The same experiment was repeated with intestinal microbiota exchange between aged and young mice before infection.

Results

Higher mortality was observed in aged mice with weaker neutrophilic mobilization in blood and intestinal tissue and depressed proinflammatory cytokines in early infection. Microbiota exchange improved survival and early immune response in aged mice. Microbiome analysis revealed that aged mice have significant deficiencies in Bacteroidetes phylum and, specifically, Bacteroides, Alistipes, and rc4-4 genera, which were replenished by cage switching.

Conclusions

Microbiota-dependent alteration in innate immune response early on during infection may explain poor outcome in aged host with CDI.

Clostridium difficile Toxoid Vaccine Candidate Confers Broad Protection against a Range of Prevalent Circulating Strains in a Nonclinical Setting

Clostridium difficile infection (CDI) is a leading cause of nosocomial and antibiotic-associated diarrhea. A vaccine, based on formalin-inactivated toxins A and B purified from anaerobic cultures of C. difficile strain VPI 10463 (toxinotype 0), has been in development for the prevention of symptomatic CDI. We evaluated the breadth of protection conferred by this C. difficile toxoid vaccine in cross-neutralization assessments using sera from vaccinated hamsters against a collection of 165 clinical isolates. Hamster antisera raised against the C. difficile toxoid vaccine neutralized the cytotoxic activity of culture supernatants from several toxinotype 0 strains and heterologous strains from 10 different toxinotypes. Further assessments performed with purified toxins confirmed that vaccine-elicited antibodies can neutralize both A and B toxins from a variety of toxinotypes. In the hamster challenge model, the vaccine conferred significant cross-protection against disease symptoms and death caused by heterologous C. difficile strains from the most common phylogenetic clades, including the most prevalent toxinotypes.

Structural basis for recognition of frizzled proteins by Clostridium difficile toxin B

Clostridium difficile infection is the most common cause of antibiotic-associated diarrhea in developed countries. The major virulence factor, C. difficile toxin B (TcdB), targets colonic epithelia by binding to the frizzled (FZD) family of Wnt receptors, but how TcdB recognizes FZDs is unclear. Here, we present the crystal structure of a TcdB fragment in complex with the cysteine-rich domain of human FZD2 at 2.5-angstrom resolution, which reveals an endogenous FZD-bound fatty acid acting as a co-receptor for TcdB binding. This lipid occupies the binding site for Wnt-adducted palmitoleic acid in FZDs. TcdB binding locks the lipid in place, preventing Wnt from engaging FZDs and signaling. Our findings establish a central role of fatty acids in FZD-mediated TcdB pathogenesis and suggest strategies to modulate Wnt signaling.

Effects of Clostridium difficile toxin A on K562/A02 cell proliferation, apoptosis and multi-drug resistance

The aim of the present study was to investigate the cytotoxic effect and multi-drug resistance (MDR) of Clostridium difficile toxin A (TcdA) on K562/A02 cells, and understand its underlying molecular pathways. K562/A02 cells were treated with TcdA at different concentrations for 24, 48 and 72 h, and the inhibition effect and drug resistance of TcdA on K562/A02 cell proliferation was assessed by methyl thiazolyl tetrazolium colorimetric assay. Furthermore, cell cycle-apoptosis was analyzed by flow cytometry, P-glycoprotein (P-gp) expression was determined by western blot analysis and caspase-3 activity was measured using a caspase-3 activity kit. TcdA inhibited K562/A02 cell proliferation in a time- and dose-dependent manner. The inhibition rate of K562/A02 cells reached 8.76±0.76, 28.55±0.43, 47.89±0.27, 58.08±0.06 and 57.70±0.79% following treatment with 50, 100, 200, 400 and 800 ng/ml TcdA, respectively, for 24 h. K562/A02 cells in the G0/G1 phase increased and cells in the S phase decreased following treatment with TcdA (P<0.05), and the apoptotic rates in the 200 and 400 ng/ml concentration groups were 14.05 and 22.89%, respectively. In addition, TcdA (50 ng/ml) significantly inhibited the proliferation of K562/A02 cells and reduced the half maximal inhibitory concentration of these drugs in combination with chemotherapy drugs. The reversal folds were 3.09, 2.89 and 2.79, respectively. Furthermore, TcdA treatment was associated with the upregulation of P-gp in K562/A02 cells, and caspase-3 activity was observed to increase in K562/A02 cells following TcdA treatment, when compared with untreated controls (P<0.05). These findings suggested that TcdA may be able to inhibit K562/A02 cell growth, induce cell apoptosis by decreasing P-gp levels and caspase-3 activation, and partially reverse MDR. Further studies are required to evaluate the potential of TcdA as a candidate for the chemotherapy of hematologic malignancies.

Cysteine Protease-Mediated Autocleavage of Clostridium difficile Toxins Regulates Their Proinflammatory Activity

BACKGROUND & AIMS

Clostridium difficile toxin A (TcdA) and C difficile toxin toxin B (TcdB), the major virulence factors of the bacterium, cause intestinal tissue damage and inflammation. Although the 2 toxins are homologous and share a similar domain structure, TcdA is generally more inflammatory whereas TcdB is more cytotoxic. The functional domain of the toxins that govern the proinflammatory activities of the 2 toxins is unknown.

METHODS

Here, we investigated toxin domain functions that regulate the proinflammatory activity of C difficile toxins. By using a mouse ilea loop model, human tissues, and immune cells, we examined the inflammatory responses to a series of chimeric toxins or toxin mutants deficient in specific domain functions.

RESULTS

Blocking autoprocessing of TcdB by mutagenesis or chemical inhibition, while reducing cytotoxicity of the toxin, significantly enhanced its proinflammatory activities in the animal model. Furthermore, a noncleavable mutant TcdB was significantly more potent than the wild-type toxin in the induction of proinflammatory cytokines in human colonic tissues and immune cells.

CONCLUSIONS

In this study, we identified a novel mechanism of regulating the biological activities of C difficile toxins in that cysteine protease-mediated autoprocessing regulates toxins' proinflammatory activities. Our findings provide new insight into the pathogenesis of C difficile infection and the design of therapeutics against the disease.

The role of purified Clostridium difficile glucosylating toxins in disease pathogenesis utilizing a murine cecum injection model

Most pathogenic Clostridium difficile produce two major exotoxins TcdA and TcdB, in the absence of which the bacterium is non-pathogenic. While it is important to investigate the role of each toxin in the pathogenesis of C. difficile infection (CDI) using isogenic strains, it is impossible to precisely control the expression levels of individual toxins and exclude bacterial factors that may contribute to the toxins' effects during infection. In this study, we utilized an acute intestinal disease model by injecting purified toxins directly into mouse cecum after a midline laparotomy. We evaluated the physical condition of mice by clinical score and survival, and the intestinal tissue damage and inflammation by histology. Depending on the dose of the toxins, mice developed mild to severe colitis, experienced diarrhea or rapidly died. We found that both purified TcdA and TcdB were able to induce clinical disease, intestinal inflammation, and tissue damage that resembled CDI. TcdA was significantly faster in inducing intestinal inflammation and tissue damage, and was approximately five times more potent than TcdB in terms of inducing severe gut disease and death outcomes in mice. Moreover, we found that the two toxins had significant synergistic effects on disease induction. Comparison of the in vivo toxicity of TcdB from clinical strains revealed that TcdB from an epidemic RT 027 strain was more toxic than the others. Our study thus demonstrates that both TcdA and TcdB, independent of other factors from C. difficile bacterium, are able to cause disease that resembles CDI and highlights the importance of targeting both toxins for vaccines and therapeutics against the disease.

Cryptosporidium Priming Is More Effective than Vaccine for Protection against Cryptosporidiosis in a Murine Protein Malnutrition Model

Cryptosporidium is a major cause of severe diarrhea, especially in malnourished children. Using a murine model of C. parvum oocyst challenge that recapitulates clinical features of severe cryptosporidiosis during malnutrition, we interrogated the effect of protein malnutrition (PM) on primary and secondary responses to C. parvum challenge, and tested the differential ability of mucosal priming strategies to overcome the PM-induced susceptibility. We determined that while PM fundamentally alters systemic and mucosal primary immune responses to Cryptosporidium, priming with C. parvum (10⁶ oocysts) provides robust protective immunity against re-challenge despite ongoing PM. C. parvum priming restores mucosal Th1-type effectors (CD3⁺CD8⁺CD103⁺ T-cells) and cytokines (IFNγ, and IL12p40) that otherwise decrease with ongoing PM. Vaccination strategies with Cryptosporidium antigens expressed in the S. Typhi vector 908htr, however, do not enhance Th1-type responses to C. parvum challenge during PM, even though vaccination strongly boosts immunity in challenged fully nourished hosts. Remote non-specific exposures to the attenuated S. Typhi vector alone or the TLR9 agonist CpG ODN-1668 can partially attenuate C. parvum severity during PM, but neither as effectively as viable C. parvum priming. We conclude that although PM interferes with basal and vaccine-boosted immune responses to C. parvum, sustained reductions in disease severity are possible through mucosal activators of host defenses, and specifically C. parvum priming can elicit impressively robust Th1-type protective immunity despite ongoing protein malnutrition. These findings add insight into potential correlates of Cryptosporidium immunity and future vaccine strategies in malnourished children.

Cryptosporidium attributable morbidities in malnourished children are increasingly recognized. Exactly how malnutrition interferes with host mucosal immunity to diarrheal pathogens and mucosal vaccine responses remains unclear. Dissecting these interactions in an experimental model of cryptosporidiosis can uncover new insights into novel therapeutic approaches against a pathogen for which effective therapies and vaccines are currently unavailable. We demonstrate that although malnutrition diminishes baseline (primary) Th1-type mucosal immunity these deficits can be partially overcome via non-specific mucosal strategies (S. Typhi and CpG) and completely restored after a sub-clinical (low-dose) exposure to viable C. parvum. These results add insight into preventive strategies to help alleviate Cryptosporidium-specific diarrhea in children in low-resource settings and abrogate prolonged post-infection sequelae.

Glucosylation Drives the Innate Inflammatory Response to Clostridium difficile Toxin A

Clostridium difficile is a major, life-threatening hospital-acquired pathogen that causes mild to severe colitis in infected individuals. The tissue destruction and inflammation which characterize C. difficile infection (CDI) are primarily due to the Rho-glucosylating toxins A and B. These toxins cause epithelial cell death and induce robust inflammatory signaling by activating the transcription factor NF-κB, leading to chemokine and cytokine secretion. The toxins also activate the inflammasome complex, which leads to secretion of the pyrogenic cytokine IL-1β. In this study, we utilized glucosylation-deficient toxin A to show that activation of the inflammasome by this toxin is dependent on Rho glucosylation, confirming similar findings reported for toxin B. We also demonstrated that tissue destruction and in vivo inflammatory cytokine production are critically dependent on the enzymatic activity of toxin A, suggesting that inhibiting toxin glucosyltransferase activity may be effective in combating this refractory disease.

Screening of Bifidobacteria and Lactobacilli Able to Antagonize the Cytotoxic Effect of Clostridium difficile upon Intestinal Epithelial HT29 Monolayer

Clostridium difficile is an opportunistic pathogen inhabiting the human gut, often being the aetiological agent of infections after a microbiota dysbiosis following, for example, an antibiotic treatment. C. difficile infections (CDI) constitute a growing health problem with increasing rates of morbidity and mortality at groups of risk, such as elderly and hospitalized patients, but also in populations traditionally considered low-risk. This could be related to the occurrence of virulent strains which, among other factors, have high-level of resistance to fluoroquinolones, more efficient sporulation and markedly high toxin production. Several novel intervention strategies against CDI are currently under study, such as the use of probiotics to counteract the growth and/or toxigenic activity of C. difficile. In this work, we have analyzed the capability of twenty Bifidobacterium and Lactobacillus strains, from human intestinal origin, to counteract the toxic effect of C. difficile LMG21717 upon the human intestinal epithelial cell line HT29. For this purpose, we incubated the bacteria together with toxigenic supernatants obtained from C. difficile. After this co-incubation new supernatants were collected in order to quantify the remnant A and B toxins, as well as to determine their residual toxic effect upon HT29 monolayers. To this end, the real time cell analyser (RTCA) model, recently developed in our group to monitor C. difficile toxic effect, was used. Results obtained showed that strains of Bifidobacterium longum and B. breve were able to reduce the toxic effect of the pathogen upon HT29, the RTCA normalized cell-index values being inversely correlated with the amount of remnant toxin in the supernatant. The strain B. longum IPLA20022 showed the highest ability to counteract the cytotoxic effect of C. difficile acting directly against the toxin, also having the highest capability for removing the toxins from the clostridial toxigenic supernatant. Image analysis showed that this strain prevents HT29 cell rounding; this was achieved by preserving the F-actin microstructure and tight-junctions between adjacent cells, thus keeping the typical epithelium-like morphology. Besides, preliminary evidence showed that the viability of B. longum IPLA20022 is needed to exert the protective effect and that secreted factors seems to have anti-toxin activity.

Pathogenesis--Thoughts from the Front Line

The field of bacterial pathogenesis has advanced dramatically in the last decade. High throughput molecular technologies have empowered scientists as never before. However, there remain some limitations, misconceptions and ambiguities in the field that may bedevil even the experienced investigator. Here, I consider some of the unanswered questions that are not readily tractable to even the most powerful technology.

Pathogenic effects of glucosyltransferase from Clostridium difficile toxins

The glucosyltransferase domain ofClostridium difficiletoxins modifies guanine nucleotide-binding proteins of Rho family. It is the major virulent domain of the holotoxins. Various pathogenic effects ofC. difficiletoxins in response to Rho glucosylation have been investigated including cytoskeleton damage, cell death and inflammation. The most recent studies have revealed some significant characteristics of the holotoxins that are independent of glucosylating activity. These findings arouse discussion about the role of glucosyltransferase activity in toxin pathogenesis and open up new insights for toxin mechanism study. In this review, we summarize the pathogenic effects of glucosyltransferase domain of the toxins in the past years.

Vancomycin Treatment Alters Humoral Immunity and Intestinal Microbiota in an Aged Mouse Model of Clostridium difficile Infection

BACKGROUND

The elderly host is highly susceptible to severe disease and treatment failure in Clostridium difficile infection (CDI). We investigated how treatment with vancomycin in the aged host influences systemic and intestinal humoral responses and select intestinal microbiota.

METHODS

Young (age, 2 months) and aged (age, 18 months) C57BL/6 mice were infected with VPI 10463 after exposure to broad-spectrum antibiotics. Vancomycin was given 24 hours after infection, and treatment was continued for 5 days. At select time points, specimens of serum and intestinal tissue and contents were collected for histopathologic analysis, to measure antibody levels and the pathogen burden, and to determine the presence and levels of select intestinal microbiota and C. difficile toxin.

RESULTS

Levels of disease severity, relapse, and mortality were increased, and recovery from infection was slower in aged mice compared to young mice. Serum levels of immunoglobulin M, immunoglobulin A, and immunoglobulin G against C. difficile toxin A were depressed in aged mice, and vancomycin treatment reduced antibody responses in both age groups. While baseline levels of total bacterial load, Bacteroidetes, Firmicutes, and Enterobacteriaceae were mostly similar, aged mice had a significant change in the Firmicutes to Bacteroidetes ratio with vancomycin treatment.

CONCLUSIONS

Vancomycin treatment decreases the systemic humoral response to CDI. Increased mortality from and recurrence of CDI in the aged host are associated with an impaired humoral response and a greater susceptibility to vancomycin-induced alteration of intestinal microbiota.

Glucosyltransferase activity of Clostridium difficile Toxin B is essential for disease pathogenesis

Clostridium difficile TcdB harbors a glucosyltransferase that targets host Rho GTPases. However, the role of the enzyme activity in the induction of host intestinal disease has not been demonstrated. In this study, we established a mouse acute intestinal disease model by cecum injection of wild type and glucosyltransferase-deficient TcdB and a chronic model by delivering toxin intraluminally via engineered surrogate host Bacillus megaterium. We demonstrated, for the first time, that the glucosyltransferase activity of TcdB is essential for inducing disease symptoms and intestinal pathological responses that resemble human disease, highlighting the importance of targeting toxin glucosyltransferase activity for future therapy.

Defining the Roles of TcdA and TcdB in Localized Gastrointestinal Disease, Systemic Organ Damage, and the Host Response during Clostridium difficile Infections

Clostridium difficile is a leading cause of antibiotic-associated diarrhea, a significant animal pathogen, and a worldwide public health burden. Most disease-causing strains secrete two exotoxins, TcdA and TcdB, which are considered to be the primary virulence factors. Understanding the role that these toxins play in disease is essential for the rational design of urgently needed new therapeutics. However, their relative contributions to disease remain contentious. Using three different animal models, we show that TcdA⁺ TcdB⁻ mutants are attenuated in virulence in comparison to the wild-type (TcdA⁺ TcdB⁺) strain, whereas TcdA⁻ TcdB⁺ mutants are fully virulent. We also show for the first time that TcdB alone is associated with both severe localized intestinal damage and systemic organ damage, suggesting that this toxin might be responsible for the onset of multiple organ dysfunction syndrome (MODS), a poorly characterized but often fatal complication of C. difficile infection (CDI). Finally, we show that TcdB is the primary factor responsible for inducing the in vivo host innate immune and inflammatory responses. Surprisingly, the animal infection model used was found to profoundly influence disease outcomes, a finding which has important ramifications for the validation of new therapeutics and future disease pathogenesis studies. Overall, our results show unequivocally that TcdB is the major virulence factor of C. difficile and provide new insights into the host response to C. difficile during infection. The results also highlight the critical nature of using appropriate and, when possible, multiple animal infection models when studying bacterial virulence mechanisms.

Clostridium difficile is a leading cause of antibiotic-associated diarrhea and an important hospital pathogen. TcdA and TcdB are thought to be the primary virulence factors responsible for disease symptoms of C. difficile infections (CDI). However, the individual contributions of these toxins to disease remain contentious. Using three different animal models of infection, we show for the first time that TcdB alone causes severe damage to the gut, as well as systemic organ damage, suggesting that this toxin might be responsible for MODS, a serious but poorly understood complication of CDI. These findings provide important new insights into the host response to C. difficile during infection and should guide the rational development of urgently required nonantibiotic therapeutics for the treatment of CDI.

High temporal resolution of glucosyltransferase dependent and independent effects of Clostridium difficile toxins across multiple cell types

BACKGROUND

Clostridium difficile toxins A and B (TcdA and TcdB), considered to be essential for C. difficile infection, affect the morphology of several cell types with different potencies and timing. However, morphological changes over various time scales are poorly characterized. The toxins' glucosyltransferase domains are critical to their deleterious effects, and cell responses to glucosyltransferase-independent activities are incompletely understood. By tracking morphological changes of multiple cell types to C. difficile toxins with high temporal resolution, cellular responses to TcdA, TcdB, and a glucosyltransferase-deficient TcdB (gdTcdB) are elucidated.

RESULTS

Human umbilical vein endothelial cells, J774 macrophage-like cells, and four epithelial cell lines (HCT8, T84, CHO, and immortalized mouse cecal epithelial cells) were treated with TcdA, TcdB, gdTcdB. Impedance across cell cultures was measured to track changes in cell morphology. Metrics from impedance data, developed to quantify rapid and long-lasting responses, produced standard curves with wide dynamic ranges that defined cell line sensitivities. Except for T84 cells, all cell lines were most sensitive to TcdB. J774 macrophages stretched and increased in size in response to TcdA and TcdB but not gdTcdB. High concentrations of TcdB and gdTcdB (>10 ng/ml) greatly reduced macrophage viability. In HCT8 cells, gdTcdB did not induce a rapid cytopathic effect, yet it delayed TcdA and TcdB's rapid effects. gdTcdB did not clearly delay TcdA or TcdB's toxin-induced effects on macrophages.

CONCLUSIONS

Epithelial and endothelial cells have similar responses to toxins yet differ in timing and degree. Relative potencies of TcdA and TcdB in mouse epithelial cells in vitro do not correlate with potencies in vivo. TcdB requires glucosyltransferase activity to cause macrophages to spread, but cell death from high TcdB concentrations is glucosyltransferase-independent. Competition experiments with gdTcdB in epithelial cells confirm common TcdA and TcdB mechanisms, yet different responses of macrophages to TcdA and TcdB suggest different, additional mechanisms or targets in these cells. This first-time, precise quantification of the response of multiple cell lines to TcdA and TcdB provides a comparative framework for delineating the roles of different cell types and toxin-host interactions.

Zinc deficiency alters host response and pathogen virulence in a mouse model of enteroaggregative Escherichia coli-induced diarrhea

Enteroaggregative Escherichia coli (EAEC) is increasingly recognized as a major cause of diarrheal disease globally. In the current study, we investigated the impact of zinc deficiency on the host and pathogenesis of EAEC. Several outcomes of EAEC infection were investigated including weight loss, EAEC shedding and tissue burden, leukocyte recruitment, intestinal cytokine expression, and virulence expression of the pathogen in vivo. Mice fed a protein source defined zinc deficient diet (dZD) had an 80% reduction of serum zinc and a 50% reduction of zinc in luminal contents of the bowel compared to mice fed a protein source defined control diet (dC). When challenged with EAEC, dZD mice had significantly greater weight loss, stool shedding, mucus production, and, most notably, diarrhea compared to dC mice. Zinc deficient mice had reduced infiltration of leukocytes into the ileum in response to infection suggesting an impaired immune response. Interestingly, expression of several EAEC virulence factors were increased in luminal contents of dZD mice. These data show a dual effect of dietary zinc in benefitting the host while impairing virulence of the pathogen. The study demonstrates the critical importance of zinc and may help elucidate the benefits of zinc supplementation in cases of childhood diarrhea and malnutrition.

Interleukin-22 regulates the complement system to promote resistance against pathobionts after pathogen-induced intestinal damage

Pathobionts play a critical role in disease development, but the immune mechanisms against pathobionts remain poorly understood. Here, we report a critical role for interleukin-22 (IL-22) in systemic protection against bacterial pathobionts that translocate into the circulation after infection with the pathogen Clostridium difficile. Infection with C. difficile induced IL-22, and infected Il22(-/-) mice harbored high numbers of pathobionts in extraintestinal organs despite comparable pathogen load and intestinal damage in mutant and wild-type mice. Pathobionts exhibited increased resistant against complement-mediated phagocytosis, and their intravenous administration resulted in high animal mortality. Selective removal of translocated commensals rescued Il22(-/-) mice, and IL-22 administration enhanced the elimination of pathobionts. Mechanistically, IL-22 augmented bacterial phagocytosis by increasing the expression and bacterial binding of complement C3. Our study demonstrates an unexpected role for IL-22 in controlling the elimination of pathobionts that enter the systemic circulation through the regulation of the complement system.

Host recognition of Clostridium difficile and the innate immune response

Clostridium difficile is a Gram-positive, spore forming bacillus and the most common cause of antibiotic-associated diarrhea in the United States. Clinical outcomes of C. difficile infection (CDI) range from asymptomatic colonization to pseudomembranous colitis, sepsis and death. Disease is primarily mediated by the action of the Rho-glucosylating toxins A and B, which induce potent pro-inflammatory signaling within the host. The role of this inflammatory response during infection is just beginning to be appreciated, with recent data suggesting inflammatory markers correlate closely with disease severity. In addition to the toxins, multiple innate immune signaling pathways have been implicated in establishing an inflammatory response during infection. In intoxication-based models of disease, inflammation typically enhances pathogenesis, while protection from infection seems to require some level of inflammatory response. Thus, the host immune response plays a key role in shaping the course of infection and a balanced inflammatory response which eradicates infection without damaging host tissues is likely required for successful resolution of disease.

Antibodies for treatment of Clostridium difficile infection

Antibodies for the treatment of Clostridium difficile infection (CDI) have been demonstrated to be effective in the research and clinical environments. Early uncertainties about molecular and treatment modalities now appear to have converged upon the systemic dosing of mixtures of human IgG1. Although multiple examples of high-potency monoclonal antibodies (MAbs) exist, significant difficulties were initially encountered in their discovery. This minireview describes historical and contemporary MAbs and highlights differences between the most potent MAbs, which may offer insight into the pathogenesis and treatment of CDI.