A mixture of functionally oligoclonal humanized monoclonal antibodies that neutralize Clostridium difficile TcdA and TcdB with high levels of in vitro potency shows in vivo protection in a hamster infection model. Clin Vaccine Immunol. 2013;20:377-390. [PMID: 23324518 DOI: 10.1128/cvi.00625-12]

Immunization Strategies Against Clostridioides difficile

Clostridioides difficile (C. difficile) infection (CDI) is an important healthcare but also a community-associated disease. CDI is considered a public health threat and an economic burden. A major problem is the high rate of recurrences. Besides classical antibiotic treatments, new therapeutic strategies are needed to prevent infection, to treat patients, and to prevent recurrences. If fecal transplantation has been recommended to treat recurrences, another key approach is to elicit immunity against C. difficile and its virulence factors. Here, after a summary concerning the virulence factors, the host immune response against C. difficile, and its role in the outcome of disease, we review the different approaches of passive immunotherapies and vaccines developed against CDI. Passive immunization strategies are designed in function of the target antigen, the antibody-based product, and its administration route. Similarly, for active immunization strategies, vaccine antigens can target toxins or surface proteins, and immunization can be performed by parenteral or mucosal routes. For passive immunization and vaccination as well, we first present immunization assays performed in animal models and second in humans and associated clinical trials. The different studies are presented according to the mode of administration either parenteral or mucosal and the target antigens and either toxins or colonization factors.

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

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

Development and characterization of phage display-derived anti-toxin antibodies neutralizing TcdA and TcdB of Clostridioides difficile

TcdA and TcdB are known as the major virulence attributes of Clostridioides difficile. Hence, neutralizing the TcdA and TcdB activities can be considered as an efficient therapeutic approach against C. difficile infection (CDI). In this work, we utilized phage display technique to select single-chain fragment variable (scFv) fragments as recombinant antibodies displayed on the surface of phages, which specifically target native TcdA, or TcdB (nTcdA and nTcdB), and their recombinant C-terminal combined repetitive oligopeptide (CROP) domains (rTcdA and rTcdB). After three rounds of biopanning, abundance of phage clones displaying high reactivity with TcdA or TcdB was quantified through enzyme-linked immunosorbent assay (ELISA). Furthermore, selected scFvs were characterized by cell viability and neutralization assays. The gene expression of immunological markers, IL-8 and TNF-α, was examined in treated Caco-2 cells by RT-qPCR. The epitopes of neutralizing scFvs were also identified by molecular docking. Totally, 18 scFv antibodies (seven for TcdA and 11 for TcdB) were identified by ELISA. Among selected scFvs, two clones for TcdA (rA-C2, A-C9) and three clones for TcdB (rB-B4, B-F5, B-F11) exhibited the highest neutralizing activity in Caco-2 and Vero cells. Moreover, the cocktail of anti-TcdA and anti-TcdB antibodies notably decreased the mRNA expression of TNF-α and IL-8 in Caco-2 cells. Molecular docking revealed that the interaction between scFv and toxin was mostly restricted to CROP domain of TcdA or TcdB. Our results collectively provided more insights for the development of neutralizing scFvs against C. difficile toxins using phage display. Further research is needed to meticulously evaluate the potential of scFvs as an alternative treatment for CDI using animal models and clinical trials.IMPORTANCETargeting the major toxins of Clostridioides difficile by neutralizing antibodies is a novel therapeutic approach for CDI. Here, we report a panel of new anti-TcdA (rA-C2, A-C9) and anti-TcdB (rB-B4, B-F5, and B-F11) recombinant antibody fragments (scFvs) isolated from Tomlinson I and J libraries using phage display technique. These scFv antibodies were capable of neutralizing their respective toxin and showed promise as potential therapeutics against TcdA and TcdB of C. difficile in different in vitro models. In addition, in silico analysis showed that at least two neutralization mechanisms, including inhibiting cell surface binding of toxins and inhibiting toxin internalization can be proposed for the isolated scFvs in this work. These findings provide more insights for the applicability of specific scFvs toward C. difficile toxins at in vitro level. However, further research is required to evaluate the potential application of these scFvs as therapeutic agents for CDI treatment in clinical setting.

Biofilm Formation of Clostridioides difficile, Toxin Production and Alternatives to Conventional Antibiotics in the Treatment of CDI

Clostridioides difficile is considered a nosocomial pathogen that flares up in patients exposed to antibiotic treatment. However, four out of ten patients diagnosed with C. difficile infection (CDI) acquired the infection from non-hospitalized individuals, many of whom have not been treated with antibiotics. Treatment of recurrent CDI (rCDI) with antibiotics, especially vancomycin (VAN) and metronidazole (MNZ), increases the risk of experiencing a relapse by as much as 70%. Fidaxomicin, on the other hand, proved more effective than VAN and MNZ by preventing the initial transcription of RNA toxin genes. Alternative forms of treatment include quorum quenching (QQ) that blocks toxin synthesis, binding of small anion molecules such as tolevamer to toxins, monoclonal antibodies, such as bezlotoxumab and actoxumab, bacteriophage therapy, probiotics, and fecal microbial transplants (FMTs). This review summarizes factors that affect the colonization of C. difficile and the pathogenicity of toxins TcdA and TcdB. The different approaches experimented with in the destruction of C. difficile and treatment of CDI are evaluated.

Evaluation of a panel of therapeutic antibody clinical candidates for efficacy against SARS-CoV-2 in Syrian hamsters

The COVID-19 pandemic spurred the rapid development of a range of therapeutic antibody treatments. As part of the US government's COVID-19 therapeutic response, a research team was assembled to support assay and animal model development to assess activity for therapeutics candidates against SARS-CoV-2. Candidate treatments included monoclonal antibodies, antibody cocktails, and products derived from blood donated by convalescent patients. Sixteen candidate antibody products were obtained directly from manufacturers and evaluated for neutralization activity against the WA-01 isolate of SARS-CoV-2. Products were further tested in the Syrian hamster model using prophylactic (-24 h) or therapeutic (+8 h) treatment approaches relative to intranasal SARS-CoV-2 exposure. In vivo assessments included daily clinical scores and body weights. Viral RNA and viable virus titers were quantified in serum and lung tissue with histopathology performed at 3d and 7d post-virus-exposure. Sham-treated, virus-exposed hamsters showed consistent clinical signs with concomitant weight loss and had detectable viral RNA and viable virus in lung tissue. Histopathologically, interstitial pneumonia with consolidation was present. Therapeutic efficacy was identified in treated hamsters by the absence or diminution of clinical scores, body weight loss, viral loads, and improved semiquantitative lung histopathology scores. This work serves as a model for the rapid, systematic in vitro and in vivo assessment of the efficacy of candidate therapeutics at various stages of clinical development. These efforts provided preclinical efficacy data for therapeutic candidates. Furthermore, these studies were invaluable for the phenotypic characterization of SARS CoV-2 disease in hamsters and of utility to the broader scientific community.

Application of recombinant antibodies for treatment of Clostridioides difficile infection: Current status and future perspective

Clostridioides difficile (C. difficile), known as the major cause of antibiotic-associated diarrhea, is regarded as one of the most common healthcare-associated bacterial infections worldwide. Due to the emergence of hypervirulent strains, development of new therapeutic methods for C. difficile infection (CDI) has become crucially important. In this context, antibodies have been introduced as valuable tools in the research and clinical environments, as far as the effectiveness of antibody therapy for CDI was reported in several clinical investigations. Hence, production of high-performance antibodies for treatment of CDI would be precious. Traditional approaches of antibody generation are based on hybridoma technology. Today, application of in vitro technologies for generating recombinant antibodies, like phage display, is considered as an appropriate alternative to hybridoma technology. These techniques can circumvent the limitations of the immune system and they can be exploited for production of antibodies against different types of biomolecules in particular active toxins. Additionally, DNA encoding antibodies is directly accessible in in vitro technologies, which enables the application of antibody engineering in order to increase their sensitivity and specificity. Here, we review the application of antibodies for CDI treatment with an emphasis on recombinant fragment antibodies. Also, this review highlights the current and future prospects of the aforementioned approaches for antibody-mediated therapy of CDI.

Novel structural insights for a pair of monoclonal antibodies recognizing non-overlapping epitopes of the glucosyltransferase domain of Clostridium difficile toxin B

Clostridium difficile toxins are the primary causative agents for hospital-acquired diarrhea and pseudomembranous colitis. Numerous monoclonal antibodies (mAbs) targeting different domains of Clostridium difficile toxin have been reported. Here we report the crystal structures of two mAbs, B1 and B2, in complex with the glycosyltransferase domain (GTD) of the Clostridium difficile toxin B (TcdB). B2 bound to the N-terminal 4 helix bundle of the GTD, a conserved membrane localization domain (MLD) found in the large clostridial glycosylating toxin family implicated in targeting plasma membrane. B1 bound to a distinct epitope at the hinge region between the MLD and the catalytic subdomain of the GTD. Functional studies revealed the potency of these mAbs in vitro and in vivo to be synergistic when given in combination.

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Identified 2 novel potent mAbs B1 and B2 targeting the TcdB GTD domain and synergistic effects were observed when combined.

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Novel non-overlapped epitopes were identified for B1 and B2 through X-ray crystallography.

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B2 epitope belongs to a conserved MLD (membrane localization domain) in the large clostridial glycosylating toxin family.

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B2 was shown to target the key regions (Loop 1 and Loop 3) of MLD proposed to be essential for membrane localization.

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B1 epitope was found to be at the hinge region between the GTD catalytic domain and the MLD of GTD.

A probiotic yeast-based immunotherapy against Clostridioides difficile infection

Antibiotic-resistant Clostridioides difficile is an anaerobic Gram-positive bacterium that colonizes the colon and is responsible for more than 29,000 deaths in the United States each year. Hence, C. difficile infection (CDI) poses an urgent threat to public health. Antibody-mediated neutralization of TcdA and TcdB toxins, the major virulence factors of CDI, represents an effective strategy to combat the disease without invoking antibiotic resistance. However, current antitoxin approaches are mostly based on parenteral infusion of monoclonal antibodies that are costly, narrow spectrum, and not optimized against the intestinal disease. Here, we engineered probiotic Saccharomyces boulardii to constitutively secrete a single tetra-specific antibody that potently and broadly neutralized both toxins and demonstrated protection against primary and recurrent CDI in both prophylactic and therapeutic mouse models of disease. This yeast immunotherapy is orally administered, can be used concurrently with antibiotics, and may have potential as a prophylactic against CDI risk and as a therapeutic for patients with CDI.

The Impact of Actotoxumab Treatment of Gnotobiotic Piglets Infected With Different Clostridium difficile Isogenic Mutants

Nosocomial infections with Clostridium difficile are on the rise in the Unites States, attributed to emergence of antibiotic-resistant and hypervirulent strains associated with greater likelihood of recurrent infections. In addition to antibiotics, treatment with Merck anti-toxin B (TcdB) antibody bezlotoxumab is reported to reduce recurrent infections. However, treatment with anti-toxin A (TcdA) antibody actotoxumab was associated with dramatically increased disease severity and mortality rates in humans and gnotobiotic piglets. Using isogenic mutants of C. difficile strain NAPI/BI/027 deficient in TcdA (A-B+) or TcdB (A+B-), and the wild type, we investigated how and why treatment of infected animals with anti-TcdA dramatically increased disease severity. Contrary to the hypothesis, among piglets treated with anti-TcdA, those with A+B- infection were disease free, in contrast to the disease enhancement seen in those with wild-type or A-B+ infection. It seems that the lack of TcdA, through either deletion or neutralization with anti-TcdA, reduces a competitive pressure, allowing TcdB to freely exert its profound effect, leading to increased mucosal injury and disease severity.

Neutralizing Monoclonal Antibodies against the Gn and the Gc of the Andes Virus Glycoprotein Spike Complex Protect from Virus Challenge in a Preclinical Hamster Model

Hantaviruses are the etiological agent of hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS). The latter is associated with case fatality rates ranging from 30% to 50%. HCPS cases are rare, with approximately 300 recorded annually in the Americas. Recently, an HCPS outbreak of unprecedented size has been occurring in and around Epuyén, in the southwestern Argentinian state of Chubut. Since November of 2018, at least 29 cases have been laboratory confirmed, and human-to-human transmission is suspected. Despite posing a significant threat to public health, no treatment or vaccine is available for hantaviral disease. Here, we describe an effort to identify, characterize, and develop neutralizing and protective antibodies against the glycoprotein complex (Gn and Gc) of Andes virus (ANDV), the causative agent of the Epuyén outbreak. Using murine hybridoma technology, we generated 19 distinct monoclonal antibodies (MAbs) against ANDV GnGc. When tested for neutralization against a recombinant vesicular stomatitis virus expressing the Andes glycoprotein (GP) (VSV-ANDV), 12 MAbs showed potent neutralization and 8 showed activity in an antibody-dependent cellular cytotoxicity reporter assay. Escape mutant analysis revealed that neutralizing MAbs targeted both the Gn and the Gc. Four MAbs that bound different epitopes were selected for preclinical studies and were found to be 100% protective against lethality in a Syrian hamster model of ANDV infection. These data suggest the existence of a wide array of neutralizing antibody epitopes on hantavirus GnGc with unique properties and mechanisms of action.IMPORTANCE Infections with New World hantaviruses are associated with high case fatality rates, and no specific vaccine or treatment options exist. Furthermore, the biology of the hantaviral GnGc complex, its antigenicity, and its fusion machinery are poorly understood. Protective monoclonal antibodies against GnGc have the potential to be developed into therapeutics against hantaviral disease and are also great tools to elucidate the biology of the glycoprotein complex.

Human peptide α-defensin-1 interferes with Clostridioides difficile toxins TcdA, TcdB, and CDT

The human pathogenic bacterium Clostridioides difficile produces two exotoxins TcdA and TcdB, which inactivate Rho GTPases thereby causing C. difficile-associated diseases (CDAD) including life-threatening pseudomembranous colitis. Hypervirulent strains produce additionally the binary actin ADP-ribosylating toxin CDT. These strains are hallmarked by more severe forms of CDAD and increased frequency and severity. Once in the cytosol, the toxins act as enzymes resulting in the typical clinical symptoms. Therefore, targeting and inactivation of the released toxins are of peculiar interest. Prompted by earlier findings that human α-defensin-1 neutralizes TcdB, we investigated the effects of the defensin on all three C. difficile toxins. Inhibition of TcdA, TcdB, and CDT was demonstrated by analyzing toxin-induced changes in cell morphology, substrate modification, and decrease in transepithelial electrical resistance. Application of α-defensin-1 protected cells and human intestinal organoids from the cytotoxic effects of TcdA, TcdB, CDT, and their combination which is attributed to a direct interaction between the toxins and α-defensin-1. In mice, the application of α-defensin-1 reduced the TcdA-induced damage of intestinal loops in vivo. In conclusion, human α-defensin-1 is a specific and potent inhibitor of the C. difficile toxins and a promising agent to develop novel therapeutic options against C. difficile infections.

Bacterial Manipulation of Wnt Signaling: A Host-Pathogen Tug-of-Wnt

The host-pathogen interface is a crucial battleground during bacterial infection in which host defenses are met with an array of bacterial counter-mechanisms whereby the invader aims to make the host environment more favorable to survival and dissemination. Interestingly, the eukaryotic Wnt signaling pathway has emerged as a key player in the host and pathogen tug-of-war. Although studied for decades as a regulator of embryogenesis, stem cell maintenance, bone formation, and organogenesis, Wnt signaling has recently been shown to control processes related to bacterial infection in the human host. Wnt signaling pathways contribute to cell cycle control, cytoskeleton reorganization during phagocytosis and cell migration, autophagy, apoptosis, and a number of inflammation-related events. Unsurprisingly, bacterial pathogens have evolved strategies to manipulate these Wnt-associated processes in order to enhance infection and survival within the human host. In this review, we examine the different ways human bacterial pathogens with distinct host cell tropisms and lifestyles exploit Wnt signaling for infection and address the potential of harnessing Wnt-related mechanisms to combat infectious disease.

Recent advances in the treatment of C. difficile using biotherapeutic agents

Clostridium difficile (C. difficile) is rapidly becoming one of the most prevalent health care–associated bacterial infections in the developed world. The emergence of new, more virulent strains has led to greater morbidity and resistance to standard therapies. The bacterium is readily transmitted between people where it can asymptomatically colonize the gut environment, and clinical manifestations ranging from frequent watery diarrhea to toxic megacolon can arise depending on the age of the individual or their state of gut dysbiosis. Several inexpensive approaches are shown to be effective against virulent C. difficile in research settings such as probiotics, fecal microbiota transfer and immunotherapies. This review aims to highlight the current advantages and limitations of the aforementioned approaches with an emphasis on recent studies.

Neutralization of Clostridium difficile toxin B with VHH-Fc fusions targeting the delivery and CROPs domains

An increasing number of antibody-based therapies are being considered for controlling bacterial infections, including Clostridium difficile by targeting toxins A and B. In an effort to develop novel C. difficile immunotherapeutics, we previously isolated several single-domain antibodies (V_HHs) capable of toxin A neutralization through recognition of the extreme C-terminal combined repetitive oligopeptides (CROPs) domain, but failed at identifying neutralizing V_HHs that bound a similar region on toxin B. Here we report the isolation of a panel of 29 V_HHs targeting at least seven unique epitopes on a toxin B immunogen composed of a portion of the central delivery domain and the entire CROPs domain. Despite monovalent affinities as high as K_D = 70 pM, none of the V_HHs tested were capable of toxin B neutralization; however, modest toxin B inhibition was observed with V_HH-V_HH dimers and to a much greater extent with V_HH-Fc fusions, reaching the neutralizing potency of the recently approved anti-toxin B monoclonal antibody bezlotoxumab in in vitro assays. Epitope binning revealed that several V_HH-Fcs bound toxin B at sites distinct from the region recognized by bezlotoxumab, while other V_HH-Fcs partially competed with bezlotoxumab for toxin binding. Therefore, the V_HHs described here are effective at toxin B neutralization when formatted as bivalent V_HH-Fc fusions by targeting toxin B at regions both similar and distinct from the bezlotoxumab binding site.

Clostridioides (Clostridium) difficile infection: current and alternative therapeutic strategies

Clostridioides difficile (C. difficile) has become a pathogen of worldwide importance considering that epidemic strains are disseminated in hospitals of several countries, where community-acquired infections act as a constant source of new C. difficile strains into hospitals. Despite the advances in the treatment of infections, more effective therapies against C. difficile are needed but, at the same time, these therapies should be less harmful to the resident gastrointestinal microbiota. The purpose of this review is to present a description of issues associated to C. difficile infection, a summary of current therapies and those in developmental stage, and a discussion of potential combinations that may lead to an increased efficacy of C. difficile infection treatment.

Immunization Strategies Against Clostridium difficile

C. difficile infection (CDI) is an important healthcare- but also community-associated disease. CDI is considered a public health threat and an economic burden. A major problem is the high rate of recurrences. Besides classical antibiotic treatments, new therapeutic strategies are needed to prevent infection, to treat patients and prevent recurrences. If fecal transplantation has been recommended to treat recurrences, another key approach is to restore immunity against C. difficile and its virulence factors. Here, after a summary concerning the virulence factors, the host immune response against C. difficile and its role in the outcome of disease, we review the different approaches of passive immunotherapies and vaccines developed against CDI. Passive immunization strategies are designed in function of the target antigen, the antibody-based product and its administration route. Similarly, for active immunization strategies, vaccine antigens can target toxins or surface proteins and immunization can be performed by parenteral or mucosal routes. For passive immunization and vaccination as well, we first present immunization assays performed in animal models and second in humans and associated clinical trials. The different studies are presented according to the mode of administration either parenteral or mucosal and the target antigens, either toxins or colonization factors.

Prevalence of binary-toxin genes (cdtA and cdtB) among clinical strains of Clostridium difficile isolated from diarrheal patients in Iran

AIM

In this study we investigated the prevalence of binary toxin genes, cdtA and cdtB, in clinical isolates of C. difficile from hospitalized patients with diarrhea.

BACKGROUND

C. difficile binary toxin (CDT) is an action-specific ADP-ribosyltransferase that is produced by some strains of C. difficile. Co-expression of this toxin with tcdA and tcdB can lead to more severe disease in CDI patients.

METHODS

Totally, 930 patients suspected of having CDI was included in this study. All samples were treated with methanol and cultured on selective C. difficile agar plates. The C. difficile isolates were further identified by PCR. Presence of tcdA, tcdB, cdtA, and cdtB genes among the strains were examined by PCR.

RESULTS

Analysis of the PCR results showed a prevalence of 85.2% (144/169) for toxigenic C. diffidile. Toxin genotyping of the strains for tcdA and tcdB genes revealed the toxin profiles of A+B+, A+B-, A-B+ accounting for 86.1% (124/144), 7.6% (11/144), 6.2% (9/144) among the strains, respectively. Totally, 12.4% (21/169) of the C. difficile strains were binary toxin-positive. cdtA-B+, cdtA+‏B‏+ and cdtA+B- were detected in 43% (9/21), 38% (8/21) and 19% (4/21) of the strains, respectively. Interestingly, 12% (3/25) of nontoxigenic C. difficile strains (tcdA-B-) had either cdtA+‏B‏+ or cdtA-B+ profiles.

CONCLUSION

This is the first report for the prevalence of binary toxin genes in C. difficile strains isolated from Iran. Further studies are required to investigate the exact role of binary toxins in the pathogenesis of C. difficile particularly in patients with chronic diarrhea among Iranian populations.

Nonantimicrobial drug targets for Clostridium difficile infections

Clostridium difficile infection (CDI) is a major public health problem worldwide. Treatment has become complicated due to the emergence of strains with increased toxigenicity and sporulation rate, together with rampant antibiotics use that disrupts colonization resistance of the colonic microbiota. As a result, there is a critical need for nonantibiotic treatments. Therapies based on inhibiting the toxins, bacterial structures responsible for colonization, virulence and restoration of the gut microbiota are the most important nonantibiotic targets to combat CDI. This report outlines these targets and how they could become the focus of future therapeutic agents. Inhibiting colonization and virulence factors during CDI will disrupt pathogen persistence and decrease exposure to the inflammatory toxins, allowing the immune system to clear the infection.

High prevalence of subclass-specific binding and neutralizing antibodies against Clostridium difficile toxins in adult cystic fibrosis sera: possible mode of immunoprotection against symptomatic C. difficile infection

Objectives

Despite multiple risk factors and a high rate of colonization for Clostridium difficile, the occurrence of C. difficile infection in patients with cystic fibrosis is rare. The aim of this study was to compare the prevalence of binding C. difficile toxin-specific immunoglobulin (Ig)A, IgG and anti-toxin neutralizing antibodies in the sera of adults with cystic fibrosis, symptomatic C. difficile infection (without cystic fibrosis) and healthy controls.

Methods

Subclass-specific IgA and IgG responses to highly purified whole C. difficile toxins A and B (toxinotype 0, strain VPI 10463, ribotype 087), toxin B from a C. difficile toxin-B-only expressing strain (CCUG 20309) and precursor form of B fragment of binary toxin, pCDTb, were determined by protein microarray. Neutralizing antibodies to C. difficile toxins A and B were evaluated using a Caco-2 cell-based neutralization assay.

Results

Serum IgA anti-toxin A and B levels and neutralizing antibodies against toxin A were significantly higher in adult cystic fibrosis patients (n=16) compared with healthy controls (n=17) and patients with symptomatic C. difficile infection (n=16); p≤0.05. The same pattern of response prevailed for IgG, except that there was no difference in anti-toxin A IgG levels between the groups. Compared with healthy controls (toxins A and B) and patients with C. difficile infection (toxin A), sera from cystic fibrosis patients exhibited significantly stronger protective anti-toxin neutralizing antibody responses.

Conclusion

A superior ability to generate robust humoral immunity to C. difficile toxins in the cystic fibrosis population is likely to confer protection against symptomatic C. difficile infection. This protection may be lost in the post-transplantation setting, where sera monitoring of anti-C. difficile toxin antibody titers may be of clinical value.

Bezlotoxumab: anti-toxin B monoclonal antibody to prevent recurrence of Clostridium difficile infection

Clostridium difficile infection (CDI) is the most common nosocomial infection in the U.S. 25% of CDI patients go on to develop recurrent CDI (rCDI) following current standard of care (SOC) therapy, leading to morbidity, mortality and economic loss. The first passive immunotherapy drug targeting C.difficile toxin B (bezlotoxumab) has been approved recently by the FDA and EMA for prevention of rCDI. Areas covered: A body of key studies was selected and reviewed by the authors. The unmet needs in CDI care were ascertained with emphasis in rCDI, including the epidemiology, pathophysiology and current management. The current knowledge about the immune response to C. difficile toxins and how this knowledge led to the development and the clinical use of bezlotoxumab is described. Current and potential future competitors to the drug were examined. Expert commentary: A single 10 mg/kg intravenous infusion of bezlotoxumab has been shown to decrease rCDI by ~40% (absolute reduction ~10%) in patients being treated for primary CDI or rCDI with SOC antibiotics. Targeting C.difficile toxins by passive immunotherapy is a novel mechanism for prevention of C.difficile infection. Bezlotoxumab will be a valuable adjunctive therapy to reduce the burden of CDI.

Protective antibodies against Clostridium difficile are present in intravenous immunoglobulin and are retained in humans following its administration

The prevalence of serum antibodies against Clostridium difficile (CD) toxins A and B in healthy populations have prompted interest in evaluating the therapeutic activity of intravenous immunoglobulin (IVIg) in individuals experiencing severe or recurrent C. difficile infection (CDI). Despite some promising case reports, a definitive clinical role for IVIg in CDI remains unclear. Contradictory results may be attributed to a lack of consensus regarding optimal dose, timing of administration and patient selection as well as variability in specific antibody content between commercial preparations. The purpose of this study was to investigate retrospectively the efficacy of three commercial preparations of IVIg for treating severe or recurrent CDI. In subsequent mechanistic studies using protein microarray and toxin neutralization assays, all IVIg preparations were analysed for specific binding and neutralizing antibodies (NAb) to CD antigens in vitro and the presence of anti-toxin NAbs in vivo following IVIg infusion. A therapeutic response to IVIg was observed in 41% (10 of 17) of the CDI patients. Significant variability in multi-isotype specific antibodies to a 7-plex panel of CD antigens and toxin neutralization efficacies were observed between IVIg preparations and also in patient sera before and after IVIg administration. These results extend our current understanding of population immunity to CD and support the inclusion of surface layer proteins and binary toxin antigens in CD vaccines. Future strategies could enhance IVIg treatment response rates by using protein microarray to preselect donor plasma/serum with the highest levels of anti-CD antibodies and/or anti-toxin neutralizing capacities prior to fractionation.

Emerging monoclonal antibodies against Clostridium difficile infection

INTRODUCTION

Clostridium difficile infections are characterized by a high recurrence rate despite antibiotic treatments and there is an urgent need to develop new treatments such as fecal transplantation and immonotherapy. Besides active immunotherapy with vaccines, passive immunotherapy has shown promise, especially with monoclonal antibodies. Areas covered: Herein, the authors review the different assays performed with monoclonal antibodies against C. difficile toxins and surface proteins to treat or prevent primary or recurrent episodes of C. difficile infection in animal models and in clinical trials as well. Notably, the authors lay emphasis on the phase III clinical trial (MODIFY II), which allowed bezlotoxumab to be approved by the Food and Drug Administration and the European Medicines Agency. They also review new strategies for producing single domain antibodies and nanobodies against C. difficile and new approaches to deliver them in the digestive tract. Expert opinion: Only two human Mabs against TcdA and TcdB have been tested alone or in combination in clinical trials. However, many animal model studies have provided rationale for the use of Mabs and nanobodies in C. difficile infection and pave the way for further clinical investigation.

A Review of Experimental and Off-Label Therapies for Clostridium difficile Infection

In spite of increased awareness and the efforts taken to optimize Clostridium difficile infection (CDI) management, with the limited number of currently available antibiotics for C. difficile the halt of this increasing epidemic remains out of reach. There are, however, close to 80 alternative treatment methods with controversial anti-clostridial efficacy or in experimental phase today. Indeed, some of these therapies are expected to become acknowledged members of the recommended anti-CDI arsenal within the next few years. None of these alternative treatment methods can respond in itself to all the major challenges of CDI management, which are primary prophylaxis in the susceptible population, clinical cure of severe cases, prevention of recurrences, and forestallment of asymptomatic C. difficile carriage and in-hospital spread. Yet, the greater the variety of treatment choices on hand, the better combination strategies can be developed to reach these goals in the future. The aim of this article is to provide a comprehensive summary of these experimental and currently off-label therapeutic options.

Epitopes and Mechanism of Action of the Clostridium difficile Toxin A-Neutralizing Antibody Actoxumab

The exotoxins toxin A (TcdA) and toxin B (TcdB) are produced by the bacterial pathogen Clostridium difficile and are responsible for the pathology associated with C. difficile infection (CDI). The antitoxin antibodies actoxumab and bezlotoxumab bind to and neutralize TcdA and TcdB, respectively. Bezlotoxumab was recently approved by the FDA for reducing the recurrence of CDI. We have previously shown that a single molecule of bezlotoxumab binds to two distinct epitopes within the TcdB combined repetitive oligopeptide (CROP) domain, preventing toxin binding to host cells. In this study, we characterize the binding of actoxumab to TcdA and examine its mechanism of toxin neutralization. Using a combination of approaches including a number of biophysical techniques, we show that there are two distinct actoxumab binding sites within the CROP domain of TcdA centered on identical amino acid sequences at residues 2162-2189 and 2410-2437. Actoxumab binding caused the aggregation of TcdA especially at higher antibody:toxin concentration ratios. Actoxumab prevented the association of TcdA with target cells demonstrating that actoxumab neutralizes toxin activity by inhibiting the first step of the intoxication cascade. This mechanism of neutralization is similar to that observed with bezlotoxumab and TcdB. Comparisons of the putative TcdA epitope sequences across several C. difficile ribotypes and homologous repeat sequences within TcdA suggest a structural basis for observed differences in actoxumab binding and/or neutralization potency. These data provide a mechanistic basis for the protective effects of the antibody in vitro and in vivo, including in various preclinical models of CDI.

Targeted Fcγ Receptor (FcγR)-mediated Clearance by a Biparatopic Bispecific Antibody

Soluble ligands have commonly been targeted by antibody therapeutics for cancers and other diseases. Although monoclonal antibodies targeting such ligands can block their interactions with their cognate receptors, they can also significantly increase the half-life of their ligands by FcRn-mediated antibody recycling, thereby evading ligand renal clearance and requiring increasingly high antibody doses to neutralize the increasing pool of target. To overcome this issue, we generated a bispecific/biparatopic antibody (BiSAb) that targets two different epitopes on IL-6 to block IL-6-mediated signaling. The BiSAb formed large immune complexes with IL-6 that can bind Fcγ receptors on phagocytic cells and are rapidly internalized. In addition, rapid clearance of the BiSAb·IL-6 complex was observed in mice while the parental antibodies prolonged the serum half-life of IL-6. Intravital imaging of the liver in mice confirmed that the rapid clearance of these large immune complexes was associated with Fcγ receptor-dependent binding to Kupffer cells in the liver. The approach described here provides a general strategy for therapeutic antibodies with the ability to not only neutralize but also actively drive clearance of their soluble antigens.

An update on antibody-based immunotherapies for Clostridium difficile infection

Clostridium difficile continues to be one of the most prevalent hospital-acquired bacterial infections in the developed world, despite the recent introduction of a novel and effective antibiotic agent (fidaxomicin). Alternative approaches under investigation to combat the anaerobic Gram-positive bacteria include fecal transplantation therapy, vaccines, and antibody-based immunotherapies. In this review, we catalog the recent advances in antibody-based approaches under development and in the clinic for the treatment of C. difficile infection. By and large, inhibitory antibodies that recognize the primary C. difficile virulence factors, toxin A and toxin B, are the most popular passive immunotherapies under investigation. We provide a detailed summary of the toxin epitopes recognized by various antitoxin antibodies and discuss general trends on toxin inhibition efficacy. In addition, antibodies to other C. difficile targets, such as surface-layer proteins, binary toxin, motility factors, and adherence and colonization factors, are introduced in this review.

Novel Clostridium difficile Anti-Toxin (TcdA and TcdB) Humanized Monoclonal Antibodies Demonstrate In Vitro Neutralization across a Broad Spectrum of Clinical Strains and In Vivo Potency in a Hamster Spore Challenge Model

Clostridium difficile (C. difficile) infection (CDI) is the main cause of nosocomial antibiotic-associated colitis and increased incidence of community-associated diarrhea in industrialized countries. At present, the primary treatment of CDI is antibiotic administration, which is effective but often associated with recurrence, especially in the elderly. Pathogenic strains produce enterotoxin, toxin A (TcdA), and cytotoxin, toxin B (TcdB), which are necessary for C. difficile induced diarrhea and gut pathological changes. Administration of anti-toxin antibodies provides an alternative approach to treat CDI, and has shown promising results in preclinical and clinical studies. In the current study, several humanized anti-TcdA and anti-TcdB monoclonal antibodies were generated and their protective potency was characterized in a hamster infection model. The humanized anti-TcdA (CANmAbA4) and anti-TcdB (CANmAbB4 and CANmAbB1) antibodies showed broad spectrum in vitro neutralization of toxins from clinical strains and neutralization in a mouse toxin challenge model. Moreover, co-administration of humanized antibodies (CANmAbA4 and CANmAbB4 cocktail) provided a high level of protection in a dose dependent manner (85% versus 57% survival at day 22 for 50 mg/kg and 20 mg/kg doses, respectively) in a hamster gastrointestinal infection (GI) model. This study describes the protective effects conferred by novel neutralizing anti-toxin monoclonal antibodies against C. difficile toxins and their potential as therapeutic agents in treating CDI.

Effect of passive immunotherapy against Clostridium difficile infection: a systematic review and meta-analysis

This systematic review aimed to determine the effect of antibody therapy against Clostridium difficile infection (CDI) using meta-analysis. In total, 28 studies (animals - 12; human - 17) were identified from the database on the basis of inclusion criteria; then selected studies were systematically reviewed and statistically analyzed. In animal experiments, the pooled relative risk of eight potential studies suggested that the antibody treatment could reduce the risk of CDI. However, the methodological heterogeneity was moderately higher. In human subjects, the majority of reports demonstrated the beneficial effect of passive immunotherapy against CDI. However, this systematic review and meta-analysis recommends that more intensive controlled studies are indispensable for legitimate confirmation.

Clostridium difficile infection: A brief update on emerging therapies

PURPOSE

Established and investigational antibiotic, monoclonal antibody, vaccine, and microbe-based approaches to the prevention and treatment of Clostridium difficile infection (CDI) are reviewed.

SUMMARY

CDI is increasingly prevalent in the United States and other countries, particularly among hospitalized patients and the elderly, who are at high risk for potentially fatal CDI-related enterotoxic diarrhea. Established therapies for CDI such as vancomycin and metronidazole (an off-label use) are limited by poor efficacy and high recurrence rates. An investigational antibiotic with potent in vitro activity against all C. difficile strains (including the hypervirulent BI/NAP1/027 strain) has yielded encouraging results in early clinical trials. Another promising approach involves the use of monoclonal antibodies with selective activity against toxins responsible for CDI-associated diarrhea; in a small Phase II clinical trial, a single monoclonal antibody infusion in combination with vancomycin or metronidazole therapy was more effective than antibiotic therapy alone in preventing CDI relapse. Other emerging approaches to CDI treatment and prophylaxis include the use of vaccines against C. difficile toxins (several C. difficile-targeted vaccines are under development in Europe and the United States); microbe-based strategies such as fecal microbiota transplants, "microbial ecosystem therapeutics," and probiotic supplements; and an investigational encapsulated form of β-lactamase designed to prevent C. difficile colonization from progressing to CDI.

CONCLUSION

The current antibiotic therapies for CDI, mainly vancomycin and (off-label) metronidazole and the newer agent fidaxomicin, have limitations with respect to efficacy, recurrence rates, and adverse effects, but a variety of promising approaches are emerging.

Recombinant lipoprotein-based vaccine candidates against C. difficile infections

Background

Opportunistically nosocomial infections in hospitalized patients are often related to Clostridium difficile infections (CDI) due to disruption of the intestinal micro-flora by antibiotic therapies during hospitalization. Clostridial exotoxins A and B (TcdA and TcdB) specifically bind to unknown glycoprotein(s) in the host intestine, disrupt the intestinal barrier leading to acute inflammation and diarrhea. The C-terminal receptor binding domain of TcdA (A-rRBD) has been shown to elicit antibody responses that neutralize TcdA toxicity in Vero cell cytotoxicity assays, but not effectively protect hamsters against a lethal dose challenge of C. difficile spores. To develop an effective recombinant subunit vaccine against CDI, A-rRBD was lipidated (rlipoA-RBD) as a rational design to contain an intrinsic adjuvant, a toll-like receptor 2 agonist and expressed in Escherichia coli.

Results

The purified rlipoA-RBD was characterized immunologically and found to have the following properties: (a) mice, hamsters and rabbits vaccinated with 3 μg of rlipoA-RBD produced strong antibody responses that neutralized TcdA toxicity in Vero cell cytotoxicity assays; furthermore, the neutralization titer was comparable to those obtained from antisera immunized either with 10 μg of TcdA toxoid or 30 μg of A-rRBD; (b) rlipoA-RBD elicited immune responses and protected mice from TcdA challenge, but offered insignificant protection (10 to 20 %) against C. difficile spores challenge in hamster models; (c) only rlipoA-RBD formulated with B-rRBD consistently confers protection (90 to 100 %) in the hamster challenge model; and (d) rlipoA-RBD was found to be 10-fold more potent than A-rRBD as an adjuvant to enhancing immune responses against a poor antigen such as ovalbumin.

Conclusion

These results indicate that rlipoA-RBD formulated with B-rRBD could be an excellent vaccine candidate for preclinical studies and future clinical trials.

A Combination of Three Fully Human Toxin A- and Toxin B-Specific Monoclonal Antibodies Protects against Challenge with Highly Virulent Epidemic Strains of Clostridium difficile in the Hamster Model

Clostridium difficile infection (CDI) is the principal cause of nosocomial diarrhea and pseudomembranous colitis associated with antibiotic therapy. Recent increases in the number of outbreaks attributed to highly virulent antibiotic-resistant strains underscore the importance of identifying efficacious alternatives to antibiotics to control this infection. CDI is mediated by two large exotoxins, toxins A and B. Strong humoral toxin-specific immune responses are associated with recovery and a lack of disease recurrence, whereas insufficient humoral responses are associated with recurrent CDI. Multiple approaches targeting these toxins, including intravenous immunoglobulin, neutralizing polymers, active vaccines, and, most recently, monoclonal antibodies (MAbs), have been explored, with various degrees of success. In this study, we describe the characterization of the first MAbs isolated from healthy human donors using a high-throughput B-cell cloning strategy. The MAbs were selected based on their ability to inhibit the actions of toxins A and B in vitro and because of their in vivo efficacy in a hamster challenge model. A potent 2-MAb cocktail was identified and then further potentiated by the addition of a second anti-toxin B MAb. This 3-MAb combination protected animals against mortality and also reduced the severity and duration of diarrhea associated with challenge with highly virulent strains of C. difficile toxinotypes 0 and III. This highly efficacious cocktail consists of one MAb specific to the receptor binding domain of toxin A and two MAbs specific to nonoverlapping regions of the glucosyltransferase domain of toxin B. This MAb combination offers great potential as a nonantibiotic treatment for the prevention of recurrent CDI.

Identification of toxemia in patients with Clostridium difficile infection

Toxemia can develop in Clostridium difficile-infected animals, and correlates with severe and fulminant disease outcomes. Circumstantial evidence suggests that toxemia may occur in patients with C. difficile infection (CDI), but positive diagnosis is extremely rare. We analyzed the potential for C. difficile toxemia in patients, determined its characteristics, and assessed challenges. C. difficile toxins in serum from patients were tested using an ultrasensitive cell-based assay and further confirmed by Rac1 glucosylation assay. The factors that hinder a diagnosis of toxemia were assessed, including investigation of toxin stability, the level of toxins-specific neutralizing antibodies in sera and its effect on diagnosis limits. CDI patients develop detectable toxemia in some cases (2.3%). Toxins were relatively stable in stored sera. Neutralizing anti-toxin antibodies were present during infection and positively correlated with the diagnosis limits. Thus, the masking effect of toxin-specific neutralizing antibodies is the major obstacle in diagnosing C. difficile toxemia using cell-based bioassays.

The prospect for vaccines to prevent Clostridium difficile infection

Clostridium difficile is a spore-forming anaerobic gram-positive organism that is the leading cause of antibiotic-associated nosocomial infectious diarrhea in the Western world. This article describes the evolving epidemiology of C difficile infection (CDI) in the twenty-first century, evaluates the importance of vaccines against the disease, and defines the roles of both innate and adaptive host immune responses in CDI. The effects of passive immunotherapy and active vaccination against CDI in both humans and animals are also discussed.

Investigational new treatments for Clostridium difficile infection

Significant progress has been made by industry and academia in the past two years to address the medical threats posed by Clostridium difficile infection. These developments provide an excellent example of how patient need has driven a surge of innovation in drug discovery. Indeed, only two drugs were approved for the infection in the past 30 years but there are 13 treatment candidates in clinical trials today. What makes the latter number even more remarkable is the diversity in the strategies represented (antibiotics, microbiota supplements, vaccines, antibiotic quenchers and passive immunization). In this review, we provide a snapshot of the current stage of these breakthroughs and argue that there is still room for further innovation in treating C. difficile infection.

Broad coverage of genetically diverse strains of Clostridium difficile by actoxumab and bezlotoxumab predicted by in vitro neutralization and epitope modeling

Clostridium difficile infections (CDIs) are the leading cause of hospital-acquired infectious diarrhea and primarily involve two exotoxins, TcdA and TcdB. Actoxumab and bezlotoxumab are human monoclonal antibodies that neutralize the cytotoxic/cytopathic effects of TcdA and TcdB, respectively. In a phase II clinical study, the actoxumab-bezlotoxumab combination reduced the rate of CDI recurrence in patients who were also treated with standard-of-care antibiotics. However, it is not known whether the antibody combination will be effective against a broad range of C. difficile strains. As a first step toward addressing this, we tested the ability of actoxumab and bezlotoxumab to neutralize the activities of toxins from a number of clinically relevant and geographically diverse strains of C. difficile. Neutralization potencies, as measured in a cell growth/survival assay with purified toxins from various C. difficile strains, correlated well with antibody/toxin binding affinities. Actoxumab and bezlotoxumab neutralized toxins from culture supernatants of all clinical isolates tested, including multiple isolates of the BI/NAP1/027 and BK/NAP7/078 strains, at antibody concentrations well below plasma levels observed in humans. We compared the bezlotoxumab epitopes in the TcdB receptor binding domain across known TcdB sequences and found that key substitutions within the bezlotoxumab epitopes correlated with the relative differences in potencies of bezlotoxumab against TcdB of some strains, including ribotypes 027 and 078. Combined with in vitro neutralization data, epitope modeling will enhance our ability to predict the coverage of new and emerging strains by actoxumab-bezlotoxumab in the clinic.

Study of the frequency of Clostridium difficile tcdA, tcdB, cdtA and cdtB genes in feces of Calves in south west of Iran

Background

Clostridium difficile (C. difficile) is a gram-positive, toxin-producing bacillus which is an intestinal pathogen in both humans and animals and causes a range of digestive disorders including inflammation of the bowel, abdominal pain, fever and diarrhea. C. difficile toxins include enterotoxin (Toxin A), cytotoxin (Toxin B) and a binary toxin. Two large protein toxins A and B are encoded by separate genes, tcdA and tcdB. Clostridium difficile infection (CDI) mainly caused by the activity of the genes tcdA and tcdB. The binary toxin is encoded by the genes cdtA and cdtB. The binary toxin caused increased adherence of bacteria to intestinal epithelium. The aim of the present study was isolation of C. difficile from feces of calves, and study of the frequency of C. difficile virulence genes.

Methods

150 samples of fresh feces from calves were collected and C. difficile was isolated from feces of calves using bacterial culture methods. DNA was extracted by a genomic DNA purification kit. Then PCR method was used for definitive diagnosis of C. difficile. Multiplex PCR method performed for identification of tcdA, tcdB, cdtA and cdtB genes. In the final stage antimicrobial resistance determining was carried out by standard Bauer-Kirby disk diffusion method.

Results

C. difficile was isolated from 90 samples (60%). The tcdA was observed in 8 isolates (8.8%), tcdB in 16 isolates (17.7%), cdtA in 8 isolates (8.8%) and cdtB in 14 isolates (15.5%). Only 1 isolated (1.1%) was containing all four genes tcdA, tcdB, cdtA and cdtB, 2 isolates (2.2%) only had both tcdA and tcdB genes, and there was no sample positive only for both cdtA and cdtB. The highest rate of drug resistance was against clindamycin (100%) and the highest rate of drug sensitivity was against ciprofloxacin (50%).

Conclusion

The results showed high incidence of C. difficile and also high antibiotic resistance of this bacterium, but frequency of strains containing virulence genes (tcdA, tcdB, cdtA and cdtB) was low.

Variations in virulence and molecular biology among emerging strains of Clostridium difficile

Clostridium difficile is a Gram-positive, spore-forming organism which infects and colonizes the large intestine, produces potent toxins, triggers inflammation, and causes significant systemic complications. Treating C. difficile infection (CDI) has always been difficult, because the disease is both caused and resolved by antibiotic treatment. For three and a half decades, C. difficile has presented a treatment challenge to clinicians, and the situation took a turn for the worse about 10 years ago. An increase in epidemic outbreaks related to CDI was first noticed around 2003, and these outbreaks correlated with a sudden increase in the mortality rate of this illness. Further studies discovered that these changes in CDI epidemiology were associated with the rapid emergence of hypervirulent strains of C. difficile, now collectively referred to as NAP1/BI/027 strains. The discovery of new epidemic strains of C. difficile has provided a unique opportunity for retrospective and prospective studies that have sought to understand how these strains have essentially replaced more historical strains as a major cause of CDI. Moreover, detailed studies on the pathogenesis of NAP1/BI/027 strains are leading to new hypotheses on how this emerging strain causes severe disease and is more commonly associated with epidemics. In this review, we provide an overview of CDI, discuss critical mechanisms of C. difficile virulence, and explain how differences in virulence-associated factors between historical and newly emerging strains might explain the hypervirulence exhibited by this pathogen during the past decade.

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.

Antibody engineering and therapeutics, The Annual Meeting of the Antibody Society: December 8-12, 2013, Huntington Beach, CA

The 24^th Antibody Engineering & Therapeutics meeting brought together a broad range of participants who were updated on the latest advances in antibody research and development. Organized by IBC Life Sciences, the gathering is the annual meeting of The Antibody Society, which serves as the scientific sponsor. Preconference workshops on 3D modeling and delineation of clonal lineages were featured, and the conference included sessions on a wide variety of topics relevant to researchers, including systems biology; antibody deep sequencing and repertoires; the effects of antibody gene variation and usage on antibody response; directed evolution; knowledge-based design; antibodies in a complex environment; polyreactive antibodies and polyspecificity; the interface between antibody therapy and cellular immunity in cancer; antibodies in cardiometabolic medicine; antibody pharmacokinetics, distribution and off-target toxicity; optimizing antibody formats for immunotherapy; polyclonals, oligoclonals and bispecifics; antibody discovery platforms; and antibody-drug conjugates.

Clinical update for the diagnosis and treatment of Clostridium difficile infection

Clostridium difficile infection (CDI) presents a rapidly evolving challenge in the battle against hospital-acquired infections. Recent advances in CDI diagnosis and management include rapid changes in diagnostic approach with the introduction of newer tests, such as detection of glutamate dehydrogenase in stool and polymerase chain reaction to detect the gene for toxin production, which will soon revolutionize the diagnostic approach to CDI. New medications and multiple medical society guidelines have introduced changing concepts in the definitions of severity of CDI and the choice of therapeutic agents, while rapid expansion of data on the efficacy of fecal microbiota transplantation heralds a revolutionary change in the management of patients suffering multiple relapses of CDI. Through a comprehensive review of current medical literature, this article aims to offer an intensive review of the current state of CDI diagnosis, discuss the strengths and limitations of available laboratory tests, compare both current and future treatments options and offer recommendations for best practice strategies.

Structural basis for antibody recognition in the receptor-binding domains of toxins A and B from Clostridium difficile

Clostridium difficile infection is a serious and highly prevalent nosocomial disease in which the two large, Rho-glucosylating toxins TcdA and TcdB are the main virulence factors. We report for the first time crystal structures revealing how neutralizing and non-neutralizing single-domain antibodies (sdAbs) recognize the receptor-binding domains (RBDs) of TcdA and TcdB. Surprisingly, the complexes formed by two neutralizing antibodies recognizing TcdA do not show direct interference with the previously identified carbohydrate-binding sites, suggesting that neutralization of toxin activity may be mediated by mechanisms distinct from steric blockage of receptor binding. A camelid sdAb complex also reveals the molecular structure of the TcdB RBD for the first time, facilitating the crystallization of a strongly negatively charged protein fragment that has resisted previous attempts at crystallization and structure determination. Electrospray ionization mass spectrometry measurements confirm the stoichiometries of sdAbs observed in the crystal structures. These studies indicate how key epitopes in the RBDs from TcdA and TcdB are recognized by sdAbs, providing molecular insights into toxin structure and function and providing for the first time a basis for the design of highly specific toxin-specific therapeutic and diagnostic agents.

The P2Y6 receptor mediates Clostridium difficile toxin-induced CXCL8/IL-8 production and intestinal epithelial barrier dysfunction

C. difficile is a Gram-positive spore-forming anaerobic bacterium that is the leading cause of nosocomial diarrhea in the developed world. The pathogenesis of C. difficile infections (CDI) is driven by toxin A (TcdA) and toxin B (TcdB), secreted factors that trigger the release of inflammatory mediators and contribute to disruption of the intestinal epithelial barrier. Neutrophils play a key role in the inflammatory response and the induction of pseudomembranous colitis in CDI. TcdA and TcdB alter cytoskeletal signaling and trigger the release of CXCL8/IL-8, a potent neutrophil chemoattractant, from intestinal epithelial cells; however, little is known about the surface receptor(s) that mediate these events. In the current study, we sought to assess whether toxin-induced CXCL8/IL-8 release and barrier dysfunction are driven by the activation of the P2Y₆ receptor following the release of UDP, a danger signal, from intoxicated Caco-2 cells. Caco-2 cells express a functional P2Y₆ receptor and release measurable amounts of UDP upon exposure to TcdA/B. Toxin-induced CXCL8/IL-8 production and release were attenuated in the presence of a selective P2Y₆ inhibitor (MRS2578). This was associated with inhibition of TcdA/B-induced activation of NFκB. Blockade of the P2Y₆ receptor also attenuated toxin-induced barrier dysfunction in polarized Caco-2 cells. Lastly, pretreating mice with the P2Y₆ receptor antagonists (MSR2578) attenuated TcdA/B-induced inflammation and intestinal permeability in an intrarectal toxin exposure model. Taken together these data outline a novel role for the P2Y₆ receptor in the induction of CXCL8/IL-8 production and barrier dysfunction in response to C. difficile toxin exposure and may provide a new therapeutic target for the treatment of CDI.