Impact of Immunoglobulin Isotype and Epitope on the Functional Properties of Vibrio cholerae O-Specific Polysaccharide-Specific Monoclonal Antibodies

Antibody responses in Klebsiella pneumoniae bloodstream infection: a cohort study

Background

Klebsiella pneumonia (Kpn) is the fourth leading cause of infection-related deaths globally, yet little is known about human antibody responses to invasive Kpn. In this study, we sought to determine whether the O-specific polysaccharide (OPS) antigen, a vaccine candidate, is immunogenic in humans with Kpn bloodstream infection (BSI). We also sought to define the cross-reactivity of human antibody responses among structurally related Kpn OPS subtypes and to assess the impact of capsule production on OPS-targeted antibody binding and function.

Methods

We measured plasma antibody responses to OPS (and MrkA, a fimbrial protein) in a cohort of patients with Kpn BSI and compared these with controls, including a cohort of healthy individuals and a cohort of individuals with Enterococcus BSI. We performed flow cytometry to measure the impact of Kpn capsule production on whole cell antibody binding and complement deposition, utilizing patient isolates with variable levels of capsule production and isogenic capsule-deficient strains derived from these isolates.

Findings

We enrolled 69 patients with Kpn BSI. Common OPS serotypes accounted for 57/69 (83%) of infections. OPS was highly immunogenic in patients with Kpn BSI, and peak OPS-IgG antibody responses in patients were 10 to 30-fold higher than antibody levels detected in healthy controls, depending on the serotype. There was significant cross-reactivity among structurally similar OPS subtypes, including the O1v1/O1v2, O2v1/O2v2 and O3/O3b subtypes. Physiological amounts of capsule produced by both hyperencapsulated and non-hyperencapsulated Kpn significantly inhibited OPS-targeted antibody binding and function.

Interpretation

OPS was highly immunogenic in patients with Kpn BSI, supporting its potential as a candidate vaccine antigen. The strong cross-reactivity observed between similar OPS subtypes in humans with Kpn BSI suggests that it may not be necessary to include all subtypes in an OPS-based vaccine. However, these observations are tempered by the fact that capsule production, even in non-highly encapsulated strains, has the potential to interfere with OPS antibody binding. This may limit the effectiveness of vaccines that exclusively target OPS.

Funding

National Institute of Allergy and Infectious Diseases at the National Institutes of Health.

Research in Context

Evidence before this study: Despite the potential of O-specific polysaccharide (OPS) as a vaccine antigen against Klebsiella pneumoniae (Kpn), the immunogenicity of OPS in humans remains largely unstudied, creating a significant knowledge gap with regard to vaccine development. A search of PubMed for publications up to March 18, 2024, using the terms " Klebsiella pneumoniae " and "O-specific polysaccharide" or "O-antigen" or "lipopolysaccharide" revealed no prior studies addressing OPS antibody responses in humans with Kpn bloodstream infections (BSI). One prior study 1 evaluated antibody response to a single lipopolysaccharide (which contains one subtype of OPS) in humans with invasive Kpn infection; however, in this study OPS typing of the infecting strains and target antigen were not described. Added value of this study: Our investigation into OPS immunogenicity in a human cohort marks a significant advance. Analyzing plasma antibody responses in 69 patients with Kpn BSI, we found OPS to be broadly immunogenic across all the types and subtypes examined, and there was significant cross-reactivity among structurally related OPS antigens. We also demonstrated that Kpn capsule production inhibit OPS antibody binding and the activation of complement on the bacterial surface, even in classical Kpn strains expressing lower levels of capsule.Implications of all the available evidence: While the immunogenicity and broad cross-reactivity of OPS in humans with Kpn BSI suggests it is a promising vaccine candidate, the obstruction of OPS antibody binding and engagement by physiologic levels of Kpn capsule underscores the potential limitations of an exclusively OPS-antigen based vaccine for Kpn. Our study provides insights for the strategic development of vaccines aimed at combating Kpn infections, an important antimicrobial resistant pathogen.

Artificial surface labelling of Escherichia coli with StrepTagII antigen to study how monoclonal antibodies drive complement-mediated killing

Antibodies play a key role in the immune defence against Gram-negative bacteria. After binding to bacterial surface antigens, IgG and IgM can activate the complement system and trigger formation of lytic membrane attack complex (MAC) pores. Molecular studies to compare functional activity of antibodies on bacteria are hampered by the limited availability of well-defined antibodies against bacterial surface antigens. Therefore, we genetically engineered E. coli by expressing the StrepTagII antigen into outer membrane protein X (OmpX) and validated that these engineered bacteria were recognised by anti-StrepTagII antibodies. We then combined this antigen-antibody system with a purified complement assay to avoid interference of serum components and directly compare MAC-mediated bacterial killing via IgG1 and pentameric IgM. While both IgG1 and IgM could induce MAC-mediated killing, we show that IgM has an increased capacity to induce complement-mediated killing of E. coli compared to IgG1. While Fc mutations that enhance IgG clustering after target binding could not improve MAC formation, mutations that cause formation of pre-assembled IgG hexamers enhanced the complement activating capacity of IgG1. Altogether, we here present a system to study antibody-dependent complement activation on E. coli and show IgM's enhanced capacity over IgG to induce complement-mediated lysis of E. coli.

Comparison of O-specific polysaccharide responses in patients following infection with Vibrio cholerae O139 versus vaccination with a bivalent (O1/O139) oral killed cholera vaccine in Bangladesh

Cholera caused by Vibrio cholerae O139 emerged in the early 1990s and spread rapidly to 11 Asian countries before receding for unclear reasons. Protection against cholera is serogroup-specific, which is defined by the O-specific polysaccharide (OSP) component of lipopolysaccharide (LPS). V. cholerae O139 also expresses the OSP-capsule. We, therefore, assessed antibody responses targeting V. cholerae O139 OSP, LPS, capsule, and vibriocidal responses in patients in Bangladesh with cholera caused by V. cholerae O139. We compared these responses to those of age-gender-blood group-matched recipients of the bivalent oral cholera vaccine (OCV O1/O139). We found prominent OSP, LPS, and vibriocidal responses in patients, with a high correlation between these responses. OSP responses primarily targeted the terminal tetrasaccharide of OSP. Vaccinees developed OSP, LPS, and vibriocidal antibody responses, but of significantly lower magnitude and responder frequency (RF) than matched patients. We separately analyzed responses in pediatric vaccinees born after V. cholerae O139 had receded in Bangladesh. We found that OSP responses were boosted in children who had previously received a single dose of bivalent OCV 3 yr previously but not in vaccinated immunologically naïve children. Our results suggest that OSP-specific responses occur during cholera caused by V. cholerae O139 despite the presence of capsules, that vaccination with bivalent OCV is poorly immunogenic in the short term in immunologically naïve individuals, but that OSP-specific immune responses can be primed by previous exposure, although whether such responses can protect against O139 cholera is uncertain. IMPORTANCE Cholera is a severe dehydrating illness in humans caused by Vibrio cholerae serogroups O1 or O139. Protection against cholera is serogroup-specific, which is defined by the O-specific polysaccharide (OSP) of V. cholerae LPS. Yet, little is known about immunity to O139 OSP. In this study, we assessed immune responses targeting OSP in patients from an endemic region with cholera caused by V. cholerae O139. We compared these responses to those of the age-gender-blood group-matched recipients of the bivalent oral cholera vaccine. Our results suggest that OSP-specific responses occur during cholera caused by V. cholerae O139 and that the OSP responses primarily target the terminal tetrasaccharide of OSP. Our results further suggest that vaccination with the bivalent vaccine is poorly immunogenic in the short term for inducing O139-specific OSP responses in immunologically naïve individuals, but OSP-specific immune responses can be primed by previous exposure or vaccination.

Protection Efficacy of Monoclonal Antibodies Targeting Different Regions of Specific SzM Protein from Swine-Isolated Streptococcus equi ssp. zooepidemicus Strains

Streptococcus equi subsp. zooepidemicus (SEZ) has a wide host spectrum, including humans and domestic animals. The SEZ-caused swine streptococcicosis outbreak has occurred in several countries, and the swine-isolated strains usually have specific S. zooepidemicus M-like (szm) gene types. In this study, we found that the production of this specific szm gene (SzM protein) was an effective vaccine candidate. It could provide better protection with a 7-day interval immune procedure than the traditional vaccine strain ST171 and attenuate the strain ΔsezV against swine-isolated hypervirulent SEZ infections. According to this outcome, we developed monoclonal antibodies (McAbs) targeting the variable and conserved regions of this SzM protein, respectively. These McAbs all belong to the IgG1 isotype with a κ type light chain and have opsonophagocytic activity rather than agglutination or complement activation functions. We estimated the protection efficiency of the McAbs with 3 different passive immunotherapy programs. The anti-conserved region McAb can provide effective protection against swine-isolated SEZ infections with only the inconvenient immunotherapy program. It also partially works in preventing infection by other SEZ strains. In contrast, the anti-variable region McAb is only adapted to protect the host against a specific szm type SEZ strain isolated from pigs, but it is flexible for different immunotherapy programs. These data provide further information to guide the development of derived, genetically engineered McAbs that have potential applications in protecting hosts against swine-isolated, hypervirulent SEZ infections in the future. IMPORTANCE The swine-isolated SEZ, with its specific szm gene sequence, has impacted the pig feeding industry in China and North America and has led to serious economic loss. Though the SzM protein of SEZ has been proven to be an effective vaccine in preventing infection, most previous studies focused on horse-isolated strains, which have different szm gene types compared to swine-isolated strains. In this study, we developed the McAbs targeting the conserved and variable regions of this SzM protein from the swine-isolated hypervirulent strains and evaluated their protection efficiency. Our research provided information for the development of chimeric McAbs or other genetically engineered McAbs that have potential applications in protecting pigs against hypervirulent SEZ infections in the future.

Cholera

Cholera was first described in the areas around the Bay of Bengal and spread globally, resulting in seven pandemics during the past two centuries. It is caused by toxigenic Vibrio cholerae O1 or O139 bacteria. Cholera is characterised by mild to potentially fatal acute watery diarrhoeal disease. Prompt rehydration therapy is the cornerstone of management. We present an overview of cholera and its pathogenesis, natural history, bacteriology, and epidemiology, while highlighting advances over the past 10 years in molecular epidemiology, immunology, and vaccine development and deployment. Since 2014, the Global Task Force on Cholera Control, a WHO coordinated network of partners, has been working with several countries to develop national cholera control strategies. The global roadmap for cholera control focuses on stopping transmission in cholera hotspots through vaccination and improved water, sanitation, and hygiene, with the aim to reduce cholera deaths by 90% and eliminate local transmission in at least 20 countries by 2030.

Virus-like Particle Display of Vibrio choleraeO-Specific Polysaccharide as a Potential Vaccine against Cholera

Vibrio cholerae, a noninvasive mucosal pathogen, is endemic in more than 50 countries. Oral cholera vaccines, based on killed whole-cell strains of Vibrio cholerae, can provide significant protection in adults and children for 2-5 years. However, they have relatively limited direct protection in young children. To overcome current challenges, in this study, a potential conjugate vaccine was developed by linking O-specific polysaccharide (OSP) antigen purified from V. cholerae O1 El Tor Inaba strain PIC018 with Qβ virus-like particles efficiently via squarate chemistry. The Qβ-OSP conjugate was characterized with mass photometry (MP) on the whole particle level. Pertinent immunologic display of OSP was confirmed by immunoreactivity of the conjugate with convalescent phase samples from humans with cholera. Mouse immunization with the Qβ-OSP conjugate showed that the construct generated prominent and long-lasting IgG antibody responses against OSP, and the resulting antibodies could recognize the native lipopolysaccharide from Vibrio cholerae O1 Inaba. This was the first time that Qβ was conjugated with a bacterial polysaccharide for vaccine development, broadening the scope of this powerful carrier.

Interactions between the Prophage 919TP and Its Vibrio cholerae Host: Implications of gmd Mutation for Phage Resistance, Cell Auto-Aggregation, and Motility

Prophage 919TP is widely distributed among Vibrio cholera and is induced to produce free φ919TP phage particles. However, the interactions between prophage φ919TP, the induced phage particle, and its host remain unknown. In particular, phage resistance mechanisms and potential fitness trade-offs, resulting from phage resistance, are unresolved. In this study, we examined a prophage 919TP-deleted variant of V. cholerae and its interaction with a modified lytic variant of the induced prophage (φ919TP cI^-). Specifically, the phage-resistant mutant was isolated by challenging a prophage-deleted variant with lytic phage φ919TP cI^-. Further, the comparative genomic analysis of wild-type and φ919TP cI^--resistant mutant predicted that phage φ919TP cI^- selects for phage-resistant mutants harboring a mutation in key steps of lipopolysaccharide (LPS) O-antigen biosynthesis, causing a single-base-pair deletion in gene gmd. Our study showed that the gmd-mediated O-antigen defect can cause pleiotropic phenotypes, e.g., cell autoaggregation and reduced swarming motility, emphasizing the role of phage-driven diversification in V. cholerae. The developed approach assists in the identification of genetic determinants of host specificity and is used to explore the molecular mechanism underlying phage-host interactions. Our findings contribute to the understanding of prophage-facilitated horizontal gene transfer and emphasize the potential for developing new strategies to optimize the use of phages in bacterial pathogen control.

Scalable production and immunogenicity of a cholera conjugate vaccine

There is a need to develop cholera vaccines that are protective in young children under 5 years of age, which induce long-term immunity, and which can be incorporated into the Expanded Programme of Immunization (EPI) in cholera-endemic countries. The degree of protection afforded by currently available oral cholera vaccines (OCV) to young children is significantly lower than that induced by vaccination of older vaccine recipients. Immune responses that protect against cholera target the O-specific polysaccharide (OSP) of Vibrio cholerae, and young children have poor immunological responses to bacterial polysaccharides, which are T cell independent antigens. To overcome this, we have developed a cholera conjugate vaccine (CCV) containing the OSP of V. cholerae O1, the main cause of endemic and epidemic cholera. Here, we describe production of CCV through a scalable manufacturing process and preclinical evaluation of immunogenicity in the presence and absence of aluminum phosphate (alum) as an adjuvant. The vaccine displays V. cholerae O1 Inaba OSP in sun-burst display via single point attachment of core oligosaccharide to a recombinant tetanus toxoid heavy chain fragment (rTTHc). Two different pilot-scale production batches of non-GMP CCV were manufactured and characterized in terms of physico-chemical properties and immunogenicity. In preclinical testing, the vaccine induced OSP- and lipopolysaccharide (LPS)-specific IgG and IgM responses, vibriocidal responses, memory B cell responses, and protection in a V. cholerae O1 challenge model. The addition of alum to the administered vaccine increased OSP-specific immune responses. These results support evaluation of CCV in humans.

Systemic, Mucosal, and Memory Immune Responses following Cholera

Vibrio cholerae O1, the major causative agent of cholera, remains a significant public health threat. Although there are available vaccines for cholera, the protection provided by killed whole-cell cholera vaccines in young children is poor. An obstacle to the development of improved cholera vaccines is the need for a better understanding of the primary mechanisms of cholera immunity and identification of improved correlates of protection. Considerable progress has been made over the last decade in understanding the adaptive and innate immune responses to cholera disease as well as V. cholerae infection. This review will assess what is currently known about the systemic, mucosal, memory, and innate immune responses to clinical cholera, as well as recent advances in our understanding of the mechanisms and correlates of protection against V. cholerae O1 infection.