Protective antibody responses against Clostridium difficile elicited by a DNA vaccine expressing the enzymatic domain of toxin B

Enhancing bezlotoxumab binding to C. difficile toxin B2: insights from computational simulations and mutational analyses for antibody design

Clostridioides difficile infection (CDI) is a significant concern caused by widespread antibiotic use, resulting in diarrhea and inflammation from the gram-positive anaerobic bacterium C. difficile. Although bezlotoxumab (Bez), a monoclonal antibody (mAb), was developed to address CDI recurrences, the recurrence rate remains high, partly due to reduced neutralization efficiency against toxin B2. In this study, we aimed to enhance the binding of Bez to C. difficile toxin B2 by combining computational simulations and mutational analyses. We identified specific mutations in Bez, including S28R, S31W/K, Y32R, S56W and G103D/S in the heavy chain (Hc), and S32F/H/R/W/Y in the light chain (Lc), which significantly improved binding to toxin B2 and formed critical protein-protein interactions. Through molecular dynamics simulations, several single mutations, such as HcS28R, LcS32H, LcS32R, LcS32W and LcS32Y, exhibited superior binding affinities to toxin B2 compared to Bez wild-type (WT), primarily attributed to Coulombic interactions. Combining the HcS28R mutation with four different mutations at residue LcS32 led to even greater binding affinities in double mutants (MTs), particularly HcS28R/LcS32H, HcS28R/LcS32R and HcS28R/LcS32Y, reinforcing protein-protein binding. Analysis of per-residue decomposition free energy highlighted key residues contributing significantly to enhanced binding interactions, emphasizing the role of electrostatic interactions. These findings offer insights into rational Bez MT design for improved toxin B2 binding, providing a foundation for developing more effective antibodies to neutralize toxin B2 and combat-related infections.Communicated by Ramaswamy H. Sarma.

Elucidation of bezlotoxumab binding specificity to toxin B in Clostridioides difficile

C. difficile or Clostridioides difficile infection (CDI) is currently one of the major causes of epidemics worldwide. Toxin B from Clostridioides difficile toxin B (TcdB) infection is the main target protein inhibiting CDI recurrence. Clinical research suggested that bezlotoxumab's (Bez) efficiency is significantly reduced in neutralizing the B2 strain compared to the B1 strain. The monoclonal antibody (mAb) functions by binding to the epitope 1 and 2 regions in the combined repetitive oligopeptide (CROP) domain. Some binding residues are distinctively different between B1 and B2 strains. In this work, we aimed to elucidate and compare insights into the interaction of toxins B1 and B2 in complex with Bez by using all-atom molecular dynamics (MD) simulations and binding free energy calculations. The predicted ΔGbinding values suggested that the antibody (Ab) could bind to toxin B1 significantly better than B2, supported by higher salt bridge and hydrogen bonding (H-bonding) interactions, as well as the number of contact residues between the two focused proteins. The toxin B1 residues important for binding with Bez were E1878, T1901, E1902, F1905, N1941, V1946, N2031, T2032, E2033, V2076, V2077, and E2092. The lower susceptibility of Bez towards toxin B2 was primarily due to a change of residue E2033 from glutamate to alanine (A2033) and the loss of E1878 and E1902 contributions, as determined by the intermolecular interaction changes from the dynamic residue interaction network (dRIN) analysis. The obtained data strengthen our understanding of Bez/toxin B binding.

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.

Novel immunoassay for diagnosis of ongoing Clostridioides difficile infections using serum and medium enriched for newly synthesized antibodies (MENSA)

BACKGROUND

Clostridioides difficile infections (CDI) have been a challenging and increasingly serious concern in recent years. While early and accurate diagnosis is crucial, available assays have frustrating limitations.

OBJECTIVE

Develop a simple, blood-based immunoassay to accurately diagnose patients suffering from active CDI.

MATERIALS AND METHODS

Uninfected controls (N = 95) and CDI patients (N = 167) were recruited from Atlanta area hospitals. Blood samples were collected from patients within twelve days of a positive CDI test and processed to yield serum and PBMCs cultured to yield medium enriched for newly synthesized antibodies (MENSA). Multiplex immunoassays measured Ig responses to ten recombinant C. difficile antigens.

RESULTS

Sixty-six percent of CDI patients produced measurable responses to C. difficile antigens in their serum or MENSA within twelve days of a positive CDI test. Fifty-two of the 167 CDI patients (31%) were detectable in both serum and MENSA, but 32/167 (19%) were detectable only in MENSA, and 27/167 (16%) were detectable only in serum.

DISCUSSION

We describe the results of a multiplex immunoassay for the diagnosis of ongoing CDI in hospitalized patients. Our assay resolved patients into four categories: MENSA-positive only, serum-positive only, MENSA- and serum-positive, and MENSA- and serum-negative. The 30% of patients who were MENSA-positive only may be accounted for by nascent antibody secretion prior to seroconversion. Conversely, the serum-positive only subset may have been more advanced in their disease course. Immunocompromise and misdiagnosis may have contributed to the 34% of CDI patients who were not identified using MENSA or serum immunoassays.

IMPORTANCE

While there was considerable overlap between patients identified through MENSA and serum, each method detected a distinctive patient group. The combined use of both MENSA and serum to detect CDI patients resulted in the greatest identification of CDI patients. Together, longitudinal analysis of MENSA and serum will provide a more accurate evaluation of successful host humoral immune responses in CDI patients.

Development and verification of an enzyme-linked immunosorbent assay for the quantification of toxoid A and toxoid B from Clostridioides difficile

Clostridioides difficile (C. difficile) is the most common cause of nosocomial antibiotic associated diarrhoea. The incidence of C. difficile infection (CDI) has been rising worldwide over the last 20 years with consequent rises in morbidity, mortality and healthcare costs, although the incidence has fallen in the UK over the last few years. Confirmation of diagnosis and early intervention are critical to the management of CDI. The standard treatment for CDI is the administration of antibiotics. However, vaccination has been recognized as the most cost-effective treatment for the prevention and possible long-term protection against CDI episode. There are several promising vaccine candidates in various stages of development. Many of these vaccines have displayed good efficacy for CDI under laboratory conditions or in clinical trials. With the emergence of vaccines against C. difficile, here we describe the development and verification of an Enzyme Linked Immunosorbent Assay (ELISA) that can be used for the quality control testing of candidate vaccines against C. difficile through the measurement of vaccine antigen content. Verification of the assay was performed by assessment of specificity, sensitivity, intermediate precision and relative accuracy. The ELISAs were specific for the toxoids being detected and the detection limit of the assay for toxoid A was 4.88 ng/mL and 3.91 ng/mL for toxoid B. The geometric coefficients of variation for intermediate precision did not exceed 25% and relative accuracy was within 77-130%. We therefore conclude that the ELISA described here is sufficiently sensitive, specific, precise and accurate for use for the quality control testing of candidate C. difficile vaccines.

Designed Ankyrin Repeat Protein (DARPin) Neutralizers of TcdB from Clostridium difficile Ribotype 027

Clostridium difficile infection (CDI) is a leading cause of hospital-acquired diarrhea. In recent decades, the emergence of the "hypervirulent" BI/NAP1/027 strains of C. difficile significantly increased the morbidity and mortality of CDI. The pathogenesis of CDI is primarily mediated by the action of two toxins, TcdA and TcdB, with TcdB being the major virulent factor in humans. In this report, we describe the engineering of a panel of designed ankyrin repeat proteins (DARPins) that potently neutralize TcdB from the BI/NAP1/027 strains (e.g., TcdBUK1). The most effective DARPin, D16, inhibits TcdBUK1 with a 50% effective concentration (EC50) of 0.5 nM, which is >66-fold lower than that of the FDA-approved anti-TcdB antibody bezlotoxumab (EC50, ∼33 nM). Competitive enzyme-linked immunosorbent assays (ELISAs) showed that D16 blocks interactions between TcdB and its receptor, chondroitin sulfate proteoglycan 4 (CSPG4). The dimeric DARPin U3D16, which pairs D16 with DARPin U3, a disrupter of the interaction of TcdB with Frizzled 1/2/7 receptor, exhibits 10-fold-to-20-fold-enhanced neutralization potency against TcdB from C. difficile strains VPI 10463 (laboratory strain) and M68 (CF/NAP9/017) but identical activity against TcdBUK1 relative to D16. Subsequent ELISAs revealed that TcdBUK1 did not significantly interact with Frizzled 1/2/7. Computation modeling revealed 4 key differences at the Frizzled 1/2/7 binding interface which are likely responsible for the significantly reduced binding affinity.IMPORTANCE We report the engineering and characterization of designed ankyrin proteins as potent neutralizers of TcdB toxin secreted by a hypervirulent ribotype 027 strain of Clostridium difficile We further show that although TcdB toxins from both ribotype 027 and VPI 10461 interact efficiently with TcdB receptors CSPG4 and Pvrl3, TcdB027 lacks significant ability to bind the only known physiologically relevant TcdB receptor, Frizzled 1/2/7.

Novel Chimeric Protein Vaccines Against Clostridium difficile Infection

Clostridium difficile infection (CDI) is the leading cause of world-wide nosocomial acquired diarrhea in adults. Active vaccination is generally accepted as a logical and cost-effective approach to prevent CDI. In this paper, we have generated two novel chimeric proteins; one designated Tcd169, comprised of the glucosyltransferase domain (GT), the cysteine proteinase domain (CPD), and receptor binding domain (RBD) of TcdB, and the RBD of TcdA; the other designated Tcd169FI, which contains Salmonella typhimurium flagellin (sFliC) and Tcd169. Both proteins were expressed in and purified from Bacillus megaterium. Point mutations were made in the GT (W102A, D288N) and CPD (C698) of TcdB to ensure that Tcd169 and Tcd169FI were atoxic. Immunization with Tcd169 or Tcd169Fl induced protective immunity against TcdA/TcdB challenge through intraperitoneal injection, also provided mice full protection against infection with a hyper-virulent C. difficile strain (BI/NAP1/027). In addition, inclusion of sFlic in the fusion protein (Tcd169Fl) enhanced its protective immunity against toxin challenge, reduced C. difficile numbers in feces from Tcd169Fl-immunized mice infected C. difficile. Our data show that Tcd169 and Tcd169FI fusion proteins may represent alternative vaccine candidates against CDI.

Vaccines against major ICU pathogens: where do we stand?

PURPOSE OF REVIEW

Multidrug resistance of bacterial pathogens has confronted physicians around the world with the threat of inefficacy of the antibiotic regime, which is particularly important for patients with sepsis. Antibiotic resistance has revived search for alternative nonantibiotic strategies. Among them, prophylaxis by vaccination is an appealing concept.

RECENT FINDINGS

This review provides a compact overview on available vaccines against community-acquired pathogens such as pneumococci (in synergy with influenza) and meningococci and provides an overview on the ongoing developments of vaccines targeting typical nosocomial pathogens such as Clostridium difficile, Staphylococcus aureus, Acintetobacter baumannii, Klebsiella pneumonia, and Pseudomonas aeruginosa.

SUMMARY

The effects achieved by some conjugated vaccines (e.g. against Haemophilus influenzae B and Streptococcus pneumoniae) are encouraging. Their widespread use has resulted in a decrease or almost elimination of invasive diseases by the covered pneumococcal serotypes or Haemophilus influenzae B, respectively. These vaccines confer not only individual protection but also exploit herd protection effects. However, a multitude of failures reflects the obstacles on the way to effective and well tolerated bacterial vaccines. Regional differences in strain prevalence and variability of antigens that limit cross-protectivity remain major obstacles. However, promising candidates are in clinical development.

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.

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.

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.

Conventional and alternative treatment approaches for Clostridium difficile infection

Clostridium difficile-associated disease continues to be one of the leading health concerns worldwide. C. difficile is considered as a causative agent of nosocomial diarrhea that causes serious infection, which may result in death. The incidences of C. difficile infection (CDI) in developed countries have become increasingly high which may be attributed to the emergence of newer epidemic strains, extensive use of antibiotics, and limited alternative therapies. The available treatment options against CDI are expensive and promote resistance. Therefore, there is urgent need for new approaches to meet these challenges. This review discusses the current understanding of CDI, the existing clinical treatment strategies and future potential options as antidifficile agents based on the available published works.

A DNA vaccine targeting TcdA and TcdB induces protective immunity against Clostridium difficile

Background

Clostridium difficile-associated disease (CDAD) constitutes a great majority of hospital diarrhea cases in industrialized countries and is induced by two types of large toxin molecules: toxin A (TcdA) and toxin B (TcdB). Development of immunotherapeutic approaches, either active or passive, has seen a resurgence in recent years. Studies have described vaccine plasmids that express either TcdA and/or TcdB receptor binding domain (RBD). However, the effectiveness of one vector encoding both toxin RBDs against CDAD has not been evaluated.

Methods

In the study, we constructed highly optimized plasmids to express the receptor binding domains of both TcdA and TcdB from a single vector. The DNA vaccine was evaluated in two animal models for its immunogenicity and protective effects.

Results

The DNA vaccine induced high levels of serum antibodies to toxin A and/or B and demonstrated neutralizing activity in both in vitro and in vivo systems. In a C. difficile hamster infection model, immunization with the DNA vaccine reduced infection severity and conferred significant protection against a lethal C. difficile strain.

Conclusions

This study has demonstrated a single plasmid encoding the RBD domains of C. difficile TcdA and TcdB as a DNA vaccine that could provide protection from C. difficile disease.

A chimeric protein comprising the glucosyltransferase and cysteine proteinase domains of toxin B and the receptor binding domain of toxin A induces protective immunity against Clostridium difficile infection in mice and hamsters

Clostridium difficile is the major cause of hospital-acquired infectious diarrhea and colitis in developed countries. The pathogenicity of C. difficile is mainly mediated by the release of 2 large potent exotoxins, toxin A (TcdA) and toxin B (TcdB), both of which require neutralization to prevent disease occurrence. We have generated a novel chimeric protein, designated mTcd138, comprised of the glucosyltransferase and cysteine proteinase domains of TcdB and the receptor binding domain of TcdA and expressed it in Bacillus megaterium. To ensure that mTcd138 is atoxic, 2 point mutations were introduced to the glucosyltransferase domain of TcdB, which essentially eliminates toxicity of mTcd138. Parenteral immunizations of mice and hamsters with mTcd138 induced protective antibodies to both toxins and provided protection against infection with the hyper-virulent C. difficile strain UK6.

Progress and problems in vaccination against necrotic enteritis in broiler chickens

Necrotic enteritis in broilers is caused by Clostridium perfringens type A strains that produce the NetB toxin. Necrotic enteritis is one of the gastrointestinal diseases in poultry that has gained worldwide importance during the last decade due to efforts to improve broiler performance. Prevention strategies include avoiding predisposing factors, such as coccidiosis, and in-feed supplementation with a variety of feed additives. However, vaccination with modified toxin or other secreted immunogenic proteins seems a logical preventive tool for protection against a toxin-producing bacterium. Formalin-inactivated crude supernatant has been used initially for vaccination. Several studies have been carried out recently to identify the most important immunogenic and protective proteins that can be used for vaccination. These include the NetB toxin, as well as a number of other proteins. There is evidence that immunization with single proteins is not protective against severe challenge and that combinations of different antigens are needed. Most published studies have used multiple dosage vaccination regimens that are not relevant for practical use in the broiler industry. Single vaccination regimens for 1-day-old chicks appear to be non-protective. This review describes the history of vaccination strategies against necrotic enteritis in broilers and gives an update on future vaccination strategies that are applicable in the field. These may include breeder hen vaccination, in ovo vaccination and live attenuated vectors to be used in feed or in drinking water.

The potential for emerging therapeutic options for Clostridium difficile infection

Clostridium difficile is mainly a nosocomial pathogen and is a significant cause of antibiotic-associated diarrhea. It is also implicated in the majority of cases of pseudomembranous colitis. Recently, advancements in next generation sequencing technology (NGS) have highlighted the extent of damage to the gut microbiota caused by broad-spectrum antibiotics, often resulting in C. difficile infection (CDI). Currently the treatment of choice for CDI involves the use of metronidazole and vancomycin. However, recurrence and relapse of CDI, even after rounds of metronidazole/vancomycin administration is a problem that must be addressed. The efficacy of alternative antibiotics such as fidaxomicin, rifaximin, nitazoxanide, ramoplanin and tigecycline, as well as faecal microbiota transplantation has been assessed and some have yielded positive outcomes against C. difficile. Some bacteriocins have also shown promising effects against C. difficile in recent years. In light of this, the potential for emerging treatment options and efficacy of anti-C. difficile vaccines are discussed in this review.

Clostridium difficile virulence factors: Insights into an anaerobic spore-forming pathogen

The worldwide emergence of epidemic strains of Clostridium difficile linked to increased disease severity and mortality has resulted in greater research efforts toward determining the virulence factors and pathogenesis mechanisms used by this organism to cause disease. C. difficile is an opportunist pathogen that employs many factors to infect and damage the host, often with devastating consequences. This review will focus on the role of the 2 major virulence factors, toxin A (TcdA) and toxin B (TcdB), as well as the role of other putative virulence factors, such as binary toxin, in C. difficile-mediated infection. Consideration is given to the importance of spores in both the initiation of disease and disease recurrence and also to the role that surface proteins play in host interactions.

The host immune response to Clostridium difficile infection

Clostridium difficile infection (CDI) is the most common infectious cause of healthcare-acquired diarrhoea. Outcomes of C. difficile colonization are varied, from asymptomatic carriage to fulminant colitis and death, due in part to the interplay between the pathogenic virulence factors of the bacterium and the counteractive immune responses of the host. Secreted toxins A and B are the major virulence factors of C. difficile and induce a profound inflammatory response by intoxicating intestinal epithelial cells causing proinflammatory cytokine release. Host cell necrosis, vascular permeability and neutrophil infiltration lead to an elevated white cell count, profuse diarrhoea and in severe cases, dehydration, hypoalbuminaemia and toxic megacolon. Other bacterial virulence factors, including surface layer proteins and flagella proteins, are detected by host cell surface signal molecules that trigger downstream cell-mediated immune pathways. Human studies have identified a role for serum and faecal immunoglobulin levels in protection from disease, but the recent development of a mouse model of CDI has enabled studies into the precise molecular interactions that trigger the immune response during infection. Key effector molecules have been identified that can drive towards a protective anti-inflammatory response or a damaging proinflammatory response. The limitations of current antimicrobial therapies for CDI have led to the development of both active and passive immunotherapies, none of which have, as yet been formally approved for CDI. However, recent advances in our understanding of the molecular basis of host immune protection against CDI may provide an exciting opportunity for novel therapeutic developments in the future.

An optimized, synthetic DNA vaccine encoding the toxin A and toxin B receptor binding domains of Clostridium difficile induces protective antibody responses in vivo

Clostridium difficile-associated disease (CDAD) constitutes a large majority of nosocomial diarrhea cases in industrialized nations and is mediated by the effects of two secreted toxins, toxin A (TcdA) and toxin B (TcdB). Patients who develop strong antitoxin antibody responses can clear C. difficile infection and remain disease free. Key toxin-neutralizing epitopes have been found within the carboxy-terminal receptor binding domains (RBDs) of TcdA and TcdB, which has generated interest in developing the RBD as a viable vaccine target. While numerous platforms have been studied, very little data describes the potential of DNA vaccination against CDAD. Therefore, we created highly optimized plasmids encoding the RBDs from TcdA and TcdB in which any putative N-linked glycosylation sites were altered. Mice and nonhuman primates were immunized intramuscularly, followed by in vivo electroporation, and in these animal models, vaccination induced significant levels of both anti-RBD antibodies (blood and stool) and RBD-specific antibody-secreting cells. Further characterization revealed that sera from immunized mice and nonhuman primates could detect RBD protein from transfected cells, as well as neutralize purified toxins in an in vitro cytotoxicity assay. Mice that were immunized with plasmids or given nonhuman-primate sera were protected from a lethal challenge with purified TcdA and/or TcdB. Moreover, immunized mice were significantly protected when challenged with C. difficile spores from homologous (VPI 10463) and heterologous, epidemic (UK1) strains. These data demonstrate the robust immunogenicity and efficacy of a TcdA/B RBD-based DNA vaccine in preclinical models of acute toxin-associated and intragastric, spore-induced colonic 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.

Vaccines against Clostridium difficile

Clostridium difficile infection (CDI) is recognized as a major cause of nosocomial diseases ranging from antibiotic related diarrhea to fulminant colitis. Emergence during the last 2 decades of C. difficile strains associated with high incidence, severity and lethal outcomes has increased the challenges for CDI treatment. A limited number of drugs have proven to be effective against CDI and concerns about antibiotic resistance as well as recurring disease solicited the search for novel therapeutic strategies. Active vaccination provides the attractive opportunity to prevent CDI, and intense research in recent years led to development of experimental vaccines, 3 of which are currently under clinical evaluation. This review summarizes recent achievements and remaining challenges in the field of C. difficile vaccines, and discusses future perspectives in view of newly-identified candidate antigens.

DNA immunization

DNA immunization was discovered in early 1990s, and its use has been expanded from vaccine studies to a broader range of biomedical research areas, such as the generation of high-quality polyclonal and monoclonal antibodies as research reagents. In this unit, three common DNA immunization methods are described: needle injection, electroporation, and gene gun. In addition, several common considerations related to DNA immunization are discussed.

Potent monoclonal antibodies against Clostridium difficile toxin A elicited by DNA immunization

Recent studies have demonstrated that DNA immunization is effective in eliciting antigen-specific antibody responses against a wide range of infectious disease targets. The polyclonal antibodies elicited by DNA vaccination exhibit high sensitivity to conformational epitopes and high avidity. However, there have been limited reports in literature on the production of monoclonal antibodies (mAb) by DNA immunization. Here, by using Clostridium difficile (C. diff) toxin A as a model antigen, we demonstrated that DNA immunization was effective in producing a panel of mAb that are protective against toxin A challenge and can also be used as sensitive reagents to detect toxin A from various testing samples. The immunoglobulin (Ig) gene usage for such mAb was also investigated. Further studies should be conducted to fully establish DNA immunization as a unique platform to produce mAb in various hosts.

Systemic antibody responses induced by a two-component Clostridium difficile toxoid vaccine protect against C. difficile-associated disease in hamsters

Clostridium difficile infection (CDI) has been identified as the leading cause of nosocomial diarrhoea and pseudomembranous colitis associated with antibiotic therapy. Recent epidemiological changes as well as increases in the number of outbreaks of strains associated with increased virulence and higher mortality rates underscore the importance of identifying alternatives to antibiotics to manage this important disease. Animal studies have clearly demonstrated the roles that toxins A and B play in gut inflammation as well as diarrhoea; therefore it is not surprising that serum anti-toxin A and B IgG are associated with protection against recurrent CDI. In humans, strong humoral toxin-specific immune responses elicited by natural C. difficile infection is associated with recovery and lack of disease recurrence, whereas insufficient humoral responses are associated with recurrent CDI. The first generation of C. difficile vaccine that contained inactivated toxin A and B was found to be completely protective against death and diarrhoea in the hamster C. difficile challenge model. When tested in young healthy volunteers in Phase I clinical trials, this investigational vaccine was shown to be safe and immunogenic. Moreover, in a separate study this vaccine was able to prevent further relapses in three out of three patients who had previously suffered from chronic relapsing C. difficile-associated diarrhoea. Herein we examined the immunogenicity and protective activity of a next-generation Sanofi Pasteur two-component highly purified toxoid vaccine in a C. difficile hamster model. This model is widely recognized as a stringent and relevant choice for the evaluation of novel treatment strategies against C. difficile and was used in preclinical testing of the first-generation vaccine candidate. Intramuscular (i.m.) immunizations with increasing doses of this adjuvanted toxoid vaccine protected hamsters from mortality and disease symptoms in a dose-dependent manner. ELISA measurements of pre-challenge sera showed that the median anti-toxin A and anti-toxin B IgG titres in the group of surviving animals were significantly higher than the median values in the group of animals that did not survive challenge. Assessment of the neutralizing activity of these sera revealed a statistically significant difference between the levels of both toxin A and toxin B neutralizing titres in protected versus unprotected animals as the median anti-toxin A and anti-toxin B neutralizing titres from surviving animals were higher than the median values from animals that succumbed to challenge. Statistically significant correlations between the toxin-specific binding titres and toxin neutralizing titres were seen for both toxin A and toxin B responses. The role of circulating anti-toxin antibodies in immunity against disease was evaluated by passive transfer of immune sera against C. difficile toxoids to naïve hamsters. Passively immunized animals were protected against morbidity and mortality associated with C. difficile challenge. Taken together, these results indicate the ability of i.m. immunization with inactivated toxins A and B to induce robust dose-dependent anti-toxin A and anti-toxin B IgG responses, the principal role of circulating anti-toxin antibody in immunity against disease and that antibody toxin binding and neutralization titres can serve as correlates of protection in the hamster challenge model of C. difficile.