An update on antibody-based immunotherapies for Clostridium difficile infection

Current and Ongoing Developments in Targeting Clostridioides difficile Infection and Recurrence

Clostridioides difficile is a Gram-positive, spore-forming anaerobic bacterial pathogen that causes severe gastrointestinal infection in humans. This review provides background information on C. difficile infection and the pathogenesis and toxigenicity of C. difficile. The risk factors, causes, and the problem of recurrence of disease and current therapeutic treatments are also discussed. Recent therapeutic developments are reviewed including small molecules that inhibit toxin formation, disrupt the cell membrane, inhibit the sporulation process, and activate the host immune system in cells. Other treatments discussed include faecal microbiota treatment, antibody-based immunotherapies, probiotics, vaccines, and violet-blue light disinfection.

Probiotics for Prevention and Treatment of Clostridium difficile Infection

Probiotics have been claimed as a valuable tool to restore the balance in the intestinal microbiota following a dysbiosis caused by, among other factors, antibiotic therapy. This perturbed environment could favor the overgrowth of Clostridium difficile, and in fact, the occurrence of C. difficile-associated infections (CDI) is increasing in recent years. In spite of the high number of probiotics able to in vitro inhibit the growth and/or toxicity of this pathogen, its application for treatment or prevention of CDI is still scarce since there are not enough well-defined clinical studies supporting efficacy. Only a few strains, such as Lactobacillus rhamnosus GG and Saccharomyces boulardii, have been studied in more extent. The increasing knowledge about the probiotic mechanisms of action against C. difficile, some of them reviewed here, makes promising the application of these live biotherapeutic agents against CDI. Nevertheless, more effort must be paid to standardize the clinical studies conducted to evaluate probiotic products, in combination with antibiotics, in order to select the best candidate for C. difficile infections.

Delivery of a therapeutic antibody to the lower gastrointestinal tract for the treatment of Clostridium difficile infection (CDI)

The colonic delivery system of toxin neutralizing antibody is a promising method for treating Clostridium difficile infection (CDI) and has some advantages over the parental administration of a neutralizing antibody. However, colonic delivery of biologics presents several challenges, including instability of biologics during encapsulation into the delivery system and harsh conditions in the upper GI tract. In this work, we described a multi-particulate delivery system encapsulating a tetra-valent antibody ABAB-IgG1 with the potential to treat CDI. This work first approved that the cecum injection of ABAB-IgG1 into the lower GI tract of mice could relieve the symptoms, enhance the clinical score, and improve the survival rate of mice during CDI. Then, the antibody was spray layered onto mannitol beads and then enteric coated with pH-sensitive polymers to achieve colon-targeting release. The in vitro release of antibody from the multi-particulate system and the pH-sensitive release of antibody was monitored. The in vivo efficacy of this system was further examined and confirmed in mice and hamsters. In summary, the findings of this study should provide practical information and potential treatment options for CDI through colonic delivery of antibody therapeutics to the lower GI tract using a multi-particulate delivery system.

Structure-guided design of a potent Clostridiodes difficile toxin A inhibitor

Crystal structures of camelid heavy-chain antibody variable domains (V_HHs) bound to fragments of the combined repetitive oligopeptides domain of Clostridiodes difficile toxin A (TcdA) reveal that the C-terminus of V_HH A20 was located 30 Å away from the N-terminus of V_HH A26. Based on this observation, we generated a biparatopic fusion protein with A20 at the N-terminus, followed by a (GS)₆ linker and A26 at the C-terminus. This A20-A26 fusion protein shows an improvement in binding affinity and a dramatic increase in TcdA neutralization potency (>330-fold [IC ₅₀]; ≥2,700-fold [IC ₉₉]) when compared to the unfused A20 and A26 V_HHs. A20-A26 also shows much higher binding affinity and neutralization potency when compared to a series of control antibody constructs that include fusions of two A20 V_HHs, fusions of two A26 V_HHs, a biparatopic fusion with A26 at the N-terminus and A20 at the C-terminus (A26-A20), and actoxumab. In particular, A20-A26 displays a 310-fold (IC ₅₀) to 29,000-fold (IC ₉₉) higher neutralization potency than A26-A20. Size-exclusion chromatography-multiangle light scattering (SEC-MALS) analyses further reveal that A20-A26 binds to TcdA with 1:1 stoichiometry and simultaneous engagement of both A20 and A26 epitopes as expected based on the biparatopic design inspired by the crystal structures of TcdA bound to A20 and A26. In contrast, the control constructs show varied and heterogeneous binding modes. These results highlight the importance of molecular geometric constraints in generating highly potent antibody-based reagents capable of exploiting the simultaneous binding of more than one paratope to an antigen.

Antibody-Based Immunotherapies as a Tool for Tackling Multidrug-Resistant Bacterial Infections

The discovery of antimicrobials is an outstanding achievement of mankind that led to the development of modern medicine. However, increasing antimicrobial resistance observed worldwide is rendering commercially available antimicrobials ineffective. This problem results from the bacterial ability to adapt to selective pressure, leading to the development or acquisition of multiple types of resistance mechanisms that can severely affect the efficacy of antimicrobials. The misuse, over-prescription, and poor treatment adherence by patients are factors strongly aggravating this issue, with an epidemic of infections untreatable by first-line therapies occurring over decades. Alternatives are required to tackle this problem, and immunotherapies are emerging as pathogen-specific and nonresistance-generating alternatives to antimicrobials. In this work, four types of antibody formats and their potential for the development of antibody-based immunotherapies against bacteria are discussed. These antibody isotypes include conventional mammalian polyclonal antibodies that are used for the neutralization of toxins; conventional mammalian monoclonal antibodies that currently have 100 IgG mAbs approved for therapeutic use; immunoglobulin Y found in birds and an excellent source of high-quality polyclonal antibodies able to be purified noninvasively from egg yolks; and single domain antibodies (also known as nanobodies), a recently discovered antibody format (found in camelids and nurse sharks) that allows for a low-cost synthesis in microbial systems, access to hidden or hard-to-reach epitopes, and exhibits a high modularity for the development of complex structures.

Proteomic profiling of precipitated Clostridioides difficile toxin A and B antibodies

Clostridioides difficile infection is the leading cause of nosocomial diarrhoea globally. Immune responses to toxins produced by C. difficile are important in disease progression and outcome. Here, we analysed the anti-toxin A and anti-toxin B serum antibody proteomes following natural infection or vaccination with a C. difficile toxoid A/toxoid B vaccine using a modified miniaturised proteomic approach based on de novo mass spectrometric sequencing. Analysis of immunoglobulin variable region (IgV) subfamily expression in immunoprecipitated toxin A and toxin B antibodies from four and seven participants of a vaccine trial, respectively, revealed a polyclonal proteome with restricted IGHV, IGKV and IGLV subfamily usage. No dominant IGHV subfamily was observed in the toxin A response, however the dominant anti-toxin B heavy (H)-chain was encoded by IGHV3-23. Light (L)-chain usage was convergent for both anti-toxin A and anti-toxin B proteomes with IGKV3-11, 3-15, 3-20 and 4-1 shared among all subjects in both cohorts. Peptide mapping of common IgV families showed extensive public and private amino acid substitutions. The cohort responses to toxin A and toxin B showed limited similarity in shared IGHV subfamilies. L-chain subfamily usage was more similar in the anti-toxin A and anti-toxin B responses, however the mutational signatures for each subfamily were toxin-dependent. Samples taken both post vaccination (n = 5) or at baseline, indicating previous exposure (n = 2), showed similar anti-toxin B IgV subfamily usage and mutational profiles. In summary, this study provides the first sequence-based proteomic analysis of the antibody response to the major disease-mediating toxins of C. difficile, toxin A and toxin B, and demonstrates that despite the potential for extreme diversity, the immunoglobulin repertoire can raise convergent responses to specific pathogens whether through natural infection or following vaccination.

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.

Are Clostridium difficile toxins nephrotoxic?

Clostridium difficile-associated disease (CDAD) occurs along a spectrum from simple uncomplicated enteritis to a multi-system disease which may include nephropathy. Pathology is attributed to bacterial toxins, but it is unclear if the latter are directly nephrotoxic. Anecdotes of renal disease from human biopsy findings suggest a variation of histopathologies, but data are relatively limited. Acute renal failure does occur in patients with advanced morbidity. CDAD can complicate chronic renal failure. Kidney tissue culture cytotoxicity has long been known. Kidney function alterations among animal models or diseased humans are relatively uncommon in mild to moderate enteritis. Rare findings of toxinemia are reported. Some have proposed that renal dysfunction arises more from pre-renal compromises. Direct toxin studies on whole kidney are sparse. The role of direct toxin-associated renal disease is worthy of further investigation given the current impetus towards the development of protective and therapeutic passive and active immunity. Hypotheses of toxin-direct or pre-renal toxin compromise of renal function prevail.

Bezlotoxumab: A Review in Preventing Clostridium difficile Infection Recurrence

Bezlotoxumab (Zinplava™) is a fully human monoclonal antibody against Clostridium difficile toxin B indicated for the prevention of C. difficile infection (CDI) recurrence in patients with a high recurrence risk. It is the first agent approved for recurrence prevention and is administered as a single intravenous infusion in conjunction with standard-of-care (SoC) antibacterial treatment for CDI. In well-designed, placebo-controlled, phase 3 trials (MODIFY 1 and 2), a single infusion of bezlotoxumab, given in combination with SoC antibacterial therapy for CDI in adults, was effective in reducing CDI recurrence in the 12 weeks post-treatment, with this benefit being seen mainly in the patients at high recurrence risk. Bezlotoxumab did not impact the efficacy of the antibacterials being used to treat the CDI and, consistent with its benefits on CDI recurrence, appeared to reduce the need for subsequent antibacterials, thus minimizing further gut microbiota disruption. Longer term, there were no further CDI recurrences over 12 months' follow-up among patients who had received bezlotoxumab in MODIFY 2 and entered an extension substudy. Bezlotoxumab has low immunogenicity and is generally well tolerated, although the potential for heart failure in some patients requires consideration; cost-effectiveness data for bezlotoxumab are awaited with interest. Thus, a single intravenous infusion of bezlotoxumab during SoC antibacterial treatment for CDI is an emerging option for reducing CDI recurrence in adults at high risk of recurrence.

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.

Probiotics for Prevention and Treatment of Clostridium difficile Infection

Probiotics have been claimed as a valuable tool to restore the balance in the intestinal microbiota following a dysbiosis caused by, among other factors, antibiotic therapy. This perturbed environment could favor the overgrowth of Clostridium difficile and, in fact, the occurrence of C. difficile-associated infections (CDI) is being increasing in recent years. In spite of the high number of probiotics able to in vitro inhibit the growth and/or toxicity of this pathogen, its application for treatment or prevention of CDI is still scarce since there are not enough well-defined clinical studies supporting efficacy. Only a few strains, such as Lactobacillus rhamnosus GG and Saccharomyces boulardii have been studied in more extent. The increasing knowledge about the probiotic mechanisms of action against C. difficile, some of them reviewed here, makes promising the application of these live biotherapeutic agents against CDI. Nevertheless, more effort must be paid to standardize the clinical studied conducted to evaluate probiotic products, in combination with antibiotics, in order to select the best candidate for C. difficile infections.

Application of Assisted Design of Antibody and Protein Therapeutics (ADAPT) improves efficacy of a Clostridium difficile toxin A single-domain antibody

Assisted Design of Antibody and Protein Therapeutics (ADAPT) is an affinity maturation platform interleaving predictions and testing that was previously validated on monoclonal antibodies (mAbs). This study expands the applicability of ADAPT to single-domain antibodies (sdAbs), a promising class of recombinant antibody-based biologics. As a test case, we used the camelid sdAb A26.8, a V_HH that binds Clostridium difficile toxin A (TcdA) relatively weakly but displays a reasonable level of TcdA neutralization. ADAPT-guided A26.8 affinity maturation resulted in an improvement of one order of magnitude by point mutations only, reaching an equilibrium dissociation constant (K_D) of 2 nM, with the best binding mutants having similar or improved stabilities relative to the parent sdAb. This affinity improvement generated a 6-fold enhancement of efficacy at the cellular level; the A26.8 double-mutant T56R,T103R neutralizes TcdA cytotoxicity with an IC₅₀ of 12 nM. The designed mutants with increased affinities are predicted to establish novel electrostatic interactions with the antigen. Almost full additivity of mutation effects is observed, except for positively charged residues introduced at adjacent positions. Furthermore, analysis of false-positive predictions points to general directions for improving the ADAPT platform. ADAPT guided the efficacy enhancement of an anti-toxin sdAb, an alternative therapeutic modality for C. difficile.

Exploring ways to improve CDI outcomes

Clostridium difficile is an anaerobic spore-forming Gram-positive bacillus recognized as an evolving international health problem. Metronidazole and vancomycin were - until recently - the only drugs available to treat C. difficile infection (CDI). Better knowledge of the pathophysiology and the development of new drugs completely modified the management of initial episodes and recurrences of CDI. Fidaxomicin significantly reduced recurrences compared with vancomycin. New drugs are also currently evaluated (cadazolid, surotomycin, ridinilazole, rifaximin). Gut microbiota homeostasis was clearly shown to be a key determinant in recurrences as demonstrated by the development of gut microbiota transplantation and alternative microbiota substitution. Passive immunotherapy and vaccinal approaches are also currently being evaluated. In conclusion, CDI treatment has evolved with the development of new therapeutic pathways which now need to be implemented in international guidelines.

The role of toxins in Clostridium difficile infection

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

Ribotypes associated with Clostridium difficile outbreaks in Brazil display distinct surface protein profiles

Clostridium difficile is a spore-forming anaerobic intestinal pathogen that causes Clostridium difficile infection (CDI). C. difficile is the leading cause of toxin-mediated nosocomial antibiotic-associated diarrhea. The pathogenesis of CDI is attributed to two major virulence factors, TcdA and TcdB toxins, that cause the symptomatic infection. C. difficile also expresses a number of key proteins, including cell wall proteins (CWPs). S-layer proteins (SLPs) are CWPs that form a paracrystalline surface array that coats the surface of the bacterium. SLPs have a role in C. difficile binding to the gastrointestinal tract, but their importance in virulence need to be better elucidated. Here, we describe bottom-up proteomics analysis of surface-enriched proteins fractions obtained through glycine extraction of five C. difficile clinical isolates from Brazil using gel-based and gel-free approaches. We were able to identify approximately 250 proteins for each strain, among them SlpA, Cwp2, Cwp6, CwpV and Cwp84. Identified CWPs presented different amino acid coverage, which might suggest differences in post-translational modifications. Proteomic analysis of SLPs from ribotype 133, agent of C. difficile outbreaks in Brazil, revealed unique proteins and provided additional information towards in depth characterization of the strains causing CDI in Brazil.

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.