Antibody-Based Correlates of Protection Against Cholera Analysis of a Challenge Study in a Cholera-Naïve Population

Comparative analysis of cholera serum vibriocidal antibodies from Convalescent and vaccinated adults in Zambia

Cholera is responsible for 1.3 to 4.0 million cholera cases globally and poses a significant threat, with Zambia reporting 17,169 cases as of 4th February 2024. Recognizing the crucial link between natural cholera infections and vaccine protection, this study aimed to assess immune responses post cholera infection and vaccination. This was a comparative study consisting of 50 participants enrolled during a cholera outbreak in Zambia's Eastern Province and an additional 56 participants who received oral cholera vaccinations in Zambia's Central Province. Vibriocidal antibodies were plotted as geometric mean titres in the naturally infected and vaccinated individuals. A significant difference (p < 0.047) emerged when comparing naturally infected to fully vaccinated individuals (2 doses) on day 28 against V. cholerae Ogawa. Those who received two doses of the oral cholera vaccine had higher antibody titres than those who were naturally infected. Notably, the lowest titres occurred between 0-9 days post onset, contrasting with peak responses at 10-19 days. This study addresses a critical knowledge gap in understanding cholera immunity dynamics, emphasizing the potential superiority of vaccination-induced immune responses. We recommend post infection vaccination after 40 days for sustained immunity and prolonged protection, especially in cholera hotspots.

Population-Based Serologic Survey of Vibrio cholerae Antibody Titers before Cholera Outbreak, Haiti, 2022

A Vibrio cholerae O1 outbreak emerged in Haiti in October 2022 after years of cholera absence. In samples from a 2021 serosurvey, we found lower circulating antibodies against V. cholerae lipopolysaccharide in children <5 years of age and no vibriocidal antibodies, suggesting high susceptibility to cholera, especially among young children.

Vibrio cholerae, classification, pathogenesis, immune response, and trends in vaccine development

Vibrio cholerae is the causative agent of cholera, a highly contagious diarrheal disease affecting millions worldwide each year. Cholera is a major public health problem, primarily in countries with poor sanitary conditions and regions affected by natural disasters, where access to safe drinking water is limited. In this narrative review, we aim to summarize the current understanding of the evolution of virulence and pathogenesis of V. cholerae as well as provide an overview of the immune response against this pathogen. We highlight that V. cholerae has a remarkable ability to adapt and evolve, which is a global concern because it increases the risk of cholera outbreaks and the spread of the disease to new regions, making its control even more challenging. Furthermore, we show that this pathogen expresses several virulence factors enabling it to efficiently colonize the human intestine and cause cholera. A cumulative body of work also shows that V. cholerae infection triggers an inflammatory response that influences the development of immune memory against cholera. Lastly, we reviewed the status of licensed cholera vaccines, those undergoing clinical evaluation, and recent progress in developing next-generation vaccines. This review offers a comprehensive view of V. cholerae and identifies knowledge gaps that must be addressed to develop more effective cholera vaccines.

Immunogenicity of partial doses of live oral cholera vaccine CVD 103-HgR in children in the United States

In a phase 4, placebo-controlled, double-blind, multi-center study performed to assess the immunogenicity of a single oral dose of live, attenuated cholera vaccine, volunteers aged 2-17 years were randomized 6:1 to receive 1 × 10⁹ colony forming units of PXVX0200 or placebo. In the subset of subjects who consumed < 80 % of the vaccine dose, seroconversion rates were calculated and stratified by amount consumed. Of 468 subjects dosed, a subset of 33 (7 %) received < 80 % of the vaccine dose. SVA seroconversion occurred in 75.8 % of these subjects, including 100 % (7/7) of those who took 50-80 % and 69.2 % (18/26) of those who took < 50 %, versus 98.5 % of those who consumed 80 % or more. Vaccination with PXVX0200 produced an immune response in most children who received partial dosing. Since SVA seroconversion is a strong correlate of protection, PXVX0200 may protect against cholera infection in children who ingest only part of the vaccine dose.

Building an integrated serosurveillance platform to inform public health interventions: Insights from an experts' meeting on serum biomarkers

The use of biomarkers to measure immune responses in serum is crucial for understanding population-level exposure and susceptibility to human pathogens. Advances in sample collection, multiplex testing, and computational modeling are transforming serosurveillance into a powerful tool for public health program design and response to infectious threats. In July 2018, 70 scientists from 16 countries met to perform a landscape analysis of approaches that support an integrated serosurveillance platform, including the consideration of issues for successful implementation. Here, we summarize the group's insights and proposed roadmap for implementation, including objectives, technical requirements, ethical issues, logistical considerations, and monitoring and evaluation.

A subset of follicular helper-like MAIT cells can provide B cell help and support antibody production in the mucosa

[Figure: see text].

Update on CVD 103-HgR single-dose, live oral cholera vaccine

Cholera remains endemic in >50 countries, putting millions at risk, especially young children for whom killed vaccines offer limited protection. An oral, live attenuated vaccine - CVD 103-HgR (Vaxchora vaccine) - was licensed by the US FDA in 2016 for adults aged 18-64 years traveling to endemic regions, based on clinical trials in human volunteers showing the vaccine was well tolerated and conferred 90% efficacy within 10 days. The evidence base for Vaxchora vaccine has expanded with additional clinical trial data, in older adults (aged 46-64 years) and children (aged 2-17 years), demonstrating that the vaccine produces a strong vibriocidal antibody response. Over 68,000 doses have been administered in the United States, with no new safety signals. The dose volume has been reduced in children to improve acceptability, and cold chain requirements are less st ringent, at +2°C─+8°C. The vaccine has recently been licensed in the Untied States for children aged 2-17 years, in Europe for individuals aged ≥2 years, and for home administration in Europe. Next steps include a Phase 4 study in infants (6-23 months). Additional information is needed regarding duration of immunity, the need for and timing of revaccination, and efficacy data from lower-middle-income countries.

A dysbiotic gut microbiome suppresses antibody mediated-protection against Vibrio cholerae

Cholera is a severe diarrheal disease that places a significant burden on global health. Cholera's high morbidity demands effective prophylactic strategies, but oral cholera vaccines exhibit variable efficacy in human populations. One contributor of variance in human populations is the gut microbiome, which in cholera-endemic areas is modulated by malnutrition, cholera, and non-cholera diarrhea. We conducted fecal transplants from healthy human donors and model communities of either human gut microbes that resemble healthy individuals or those of individuals recovering from diarrhea in various mouse models. We show microbiome-specific effects on host antibody responses against Vibrio cholerae, and that dysbiotic human gut microbiomes representative of cholera-endemic areas suppress the immune response against V. cholerae via CD4+ lymphocytes. Our findings suggest that gut microbiome composition at time of infection or vaccination may be pivotal for providing robust mucosal immunity, and suggest a target for improved prophylactic and therapeutic strategies for cholera.

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.

Correlates of Protection for Cholera

A correlate of protection (CoP) is a measured adaptive immune response to vaccination or infection that is associated with protection against disease. However, the degree to which a CoP can serve as a surrogate end point for vaccine efficacy should depend on the robustness of this association. While cholera toxin is a dominant target of the human antibody response to Vibrio cholerae infection, antitoxin responses are not associated with long-term immunity, and are not effective CoPs for cholera. Instead, protection appears to be mediated by functional antibodies that target the O-polysaccharide coated V. cholerae outer membrane. Vibriocidal antibodies, which are complement-dependent bactericidal antibodies, remain the most accepted CoP for cholera and are used as surrogate end points in some vaccine studies. However, the association between vibriocidal antibody titers and immunity is not absolute, and they are unlikely to reflect a mechanistic correlate of protection against cholera.

Longitudinal analysis of human humoral responses after vaccination with a live attenuated V. cholerae vaccine

Vibrio cholerae is a bacterial pathogen which causes the severe acute diarrheal disease cholera. Given that a symptomatic incident of cholera can lead to long term protection, a thorough understanding of the immune response to this pathogen is needed to identify parameters critical to the generation and durability of immunity. To approach this, we utilized a live attenuated cholera vaccine to model the response to V. cholerae infection in 12 naïve subjects. We found that this live attenuated vaccine induced durable vibriocidal antibody titers that were maintained at least one year after vaccination. Similar to what we previously reported in infected patients from Bangladesh, we found that vaccination induced plasmablast responses were primarily specific to the two immunodominant antigens lipopolysaccharide (LPS) and cholera toxin (CT). Interestingly, the magnitude of the early plasmablast response at day 7 predicted the serological outcome of vaccination at day 30. However, this correlation was no longer present at later timepoints. The acute responses displayed preferential immunoglobulin isotype usage, with LPS specific cells being largely IgM or IgA producing, while cholera toxin responses were predominantly IgG. Finally, CCR9 was highly expressed on vaccine induced plasmablasts, especially on IgM and IgA producing cells, suggesting a role in migration to the gastrointestinal tract. Collectively, these findings demonstrate that the use of a live attenuated cholera vaccine is an effective tool to examine the primary and long-term immune response following V. cholerae exposure. Additionally, it provides insight into the phenotype and specificity of the cells which likely return to and mediate immunity at the intestinal mucosa. A thorough understanding of these properties both in peripheral blood and in the intestinal mucosae will inform future vaccine development against both cholera and other mucosal pathogens. Trial Registration: NCT03251495.

Long-Term Immunogenicity of Live Oral Cholera Vaccine CVD 103-HgR in Adolescents Aged 12-17 Years in the United States

As part of a phase 4, randomized, double-blind, placebo-controlled trial to assess the immunogenicity and safety of PXVX0200 in children and adolescents aged 2-17 years, a subset of 73 adolescent subjects aged 12-17 years was followed for 2 years after vaccination and had blood collected for antibody assays on days 1, 11, 29, 91, 181, 365, 547, and 730. Endpoints included serum vibriocidal antibody (SVA) seroconversion, defined as a 4-fold or greater rise in antibody titer over baseline; geometric mean titers (GMTs); and geometric mean fold increase (GMFI) over baseline. Serum vibriocidal antibody seroconversion persisted in most subjects, with a rate of 64.5% noted at day 730. Geometric mean titers and GMFI both peaked at day 11 and remained greater than baseline at all time points, including day 730. Vaccination with PXVX0200 produces an immune response which persists for at least 2 years in adolescents aged 12-17 years.

Vibrio cholerae Sialidase-Specific Immune Responses Are Associated with Protection against Cholera

Cholera remains a major public health problem in resource-limited countries. Vaccination is an important strategy to prevent cholera, but currently available vaccines provide only 3 to 5 years of protection. Understanding immune responses to cholera antigens in naturally infected individuals may elucidate which of these are key to longer-term protection seen following infection. We recently identified Vibrio cholerae O1 sialidase, a neuraminidase that facilitates binding of cholera toxin to intestinal epithelial cells, as immunogenic following infection in two recent high-throughput screens. Here, we present systemic, mucosal, and memory immune responses to sialidase in cholera index cases and evaluated whether systemic responses to sialidase correlated with protection using a cohort of household contacts. Overall, we found age-related differences in antisialidase immune response following cholera. Adults developed significant plasma anti-sialidase IgA, IgG, and IgM responses following infection, whereas older children (≥5 years) developed both IgG and IgM responses, and younger children only developed IgM responses. Neither older children nor younger children had a rise in IgA responses over the convalescent phase of infection (day 7/day 30). On evaluation of mucosal responses and memory B-cell responses to sialidase, we found adults developed IgA antibody-secreting cell (ASC) and memory B-cell responses. Finally, in household contacts, the presence of serum anti-sialidase IgA, IgG, and IgM antibodies at enrollment was associated with a decrease in the risk of subsequent infection. These data show cholera patients develop age-related immune responses against sialidase and suggest that immune responses that target sialidase may contribute to protective immunity against cholera.IMPORTANCE Cholera infection can result in severe dehydration that may lead to death within a short period of time if not treated immediately. Vaccination is an important strategy to prevent the disease. Oral cholera vaccines provide 3 to 5 years of protection, with 60% protective efficacy, while natural infection provides longer-term protection than vaccination. Understanding the immune responses after natural infection is important to better understand immune responses to antigens that mediate longer-term protection. Sialidase is a neuraminidase that facilitates binding of cholera toxin to intestinal epithelial cells. We show here that patients with cholera develop systemic, mucosal, and memory B-cell immune responses to the sialidase antigen of Vibrio cholerae O1 and that plasma responses targeting this antigen correlate with protection.

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

Vibrio cholerae causes the severe diarrheal disease cholera. Clinical disease and current oral cholera vaccines generate antibody responses associated with protection. Immunity is thought to be largely mediated by lipopolysaccharide (LPS)-specific antibodies, primarily targeting the O-antigen. However, the properties and protective mechanism of functionally relevant antibodies have not been well defined. We previously reported on the early B cell response to cholera in a cohort of Bangladeshi patients, from which we characterized a panel of human monoclonal antibodies (MAbs) isolated from acutely induced plasmablasts. All antibodies in that previous study were expressed in an IgG1 backbone irrespective of their original isotype. To clearly determine the impact of affinity, immunoglobulin isotype and subclass on the functional properties of these MAbs, we re-engineered a subset of low- and high-affinity antibodies in different isotype and subclass immunoglobulin backbones and characterized the impact of these changes on binding, vibriocidal, agglutination, and motility inhibition activity. While the high-affinity antibodies bound similarly to O-antigen, irrespective of isotype, the low-affinity antibodies displayed significant avidity differences. Interestingly, despite exhibiting lower binding properties, variants derived from the low-affinity MAbs had comparable agglutination and motility inhibition properties to the potently binding antibodies, suggesting that how the MAb binds to the O-antigen may be critical to function. In addition, not only pentameric IgM and dimeric IgA, but also monomeric IgA, was remarkably more potent than their IgG counterparts at inhibiting motility. Finally, analyzing highly purified F(ab) versions of these antibodies, we show that LPS cross-linking is essential for motility inhibition.IMPORTANCE Immunity to the severe diarrheal disease cholera is largely mediated by lipopolysaccharide (LPS)-specific antibodies. However, the properties and protective mechanisms of functionally relevant antibodies have not been well defined. Here, we have engineered low and high-affinity LPS-specific antibodies in different immunoglobulin backbones in order to assess the impact of affinity, immunoglobulin isotype, and subclass on binding, vibriocidal, agglutination, and motility inhibition functional properties. Importantly, we found that affinity did not directly dictate functional potency since variants derived from the low-affinity MAbs had comparable agglutination and motility inhibition properties to the potently binding antibodies. This suggests that how the antibody binds sterically may be critical to function. In addition, not only pentameric IgM and dimeric IgA, but also monomeric IgA, was remarkably more potent than their IgG counterparts at inhibiting motility. Finally, analyzing highly purified F(ab) versions of these antibodies, we show that LPS cross-linking is essential for motility inhibition.

The Interface of Vibrio cholerae and the Gut Microbiome

The bacterium Vibrio cholerae is the etiologic agent of the severe human diarrheal disease cholera. The gut microbiome, or the native community of microorganisms found in the human gastrointestinal tract, is increasingly being recognized as a factor in driving susceptibility to infection, in vivo fitness, and host interactions of this pathogen. Here, we review a subset of the emerging studies in how gut microbiome structure and microbial function are able to drive V. cholerae virulence gene regulation, metabolism, and modulate host immune responses to cholera infection and vaccination. Improved mechanistic understanding of commensal-pathogen interactions offers new perspectives in the design of prophylactic and therapeutic approaches for cholera control.

Safety and Immunogenicity of Live Oral Cholera Vaccine CVD 103-HgR in Children Aged 2-5 Years in the United States

In a phase 4, randomized, placebo-controlled, double-blind, multicenter study, to assess the safety and immunogenicity of live, attenuated cholera vaccine PXVX0200 in children aged 2-5 years in the United States, 172 volunteers were randomized 6:1 to receive a single dose of 1 × 10⁹ CFU of PXVX0200 or placebo. Immunogenicity endpoints included serum vibriocidal antibody (SVA) levels on days 1, 11, and 29. Safety was assessed by comparing solicited signs and symptoms on days 1-8, unsolicited adverse events through day 29, and serious adverse events (SAEs) through day 181. The SVA seroconversion rates 10 days after immunization were 98.1% and 0% in vaccine and placebo recipients, respectively, and the vaccine seroconversion rate was non-inferior to the 93.5% rate seen in the bridging population of adults aged 18-45 years from a lot consistency study. Most reactogenicity was mild to moderate, and there were no study-related SAEs. PXVX0200 appears safe and immunogenic in children aged 2-5 years.

Estimating cholera incidence with cross-sectional serology

The development of new approaches to cholera control relies on an accurate understanding of cholera epidemiology. However, most information on cholera incidence lacks laboratory confirmation and instead relies on surveillance systems reporting medically attended acute watery diarrhea. If recent infections could be identified using serological markers, cross-sectional serosurveys would offer an alternative approach to measuring incidence. Here, we used 1569 serologic samples from a cohort of cholera cases and their uninfected contacts in Bangladesh to train machine learning models to identify recent Vibrio cholerae O1 infections. We found that an individual’s antibody profile contains information on the timing of V. cholerae O1 infections in the previous year. Our models using six serological markers accurately identified individuals in the Bangladesh cohort infected within the last year [cross-validated area under the curve (AUC), 93.4%; 95% confidence interval (CI), 92.1 to 94.7%], with a marginal performance decrease using models based on two markers (cross-validated AUC, 91.0%; 95% CI, 89.2 to 92.7%). We validated the performance of the two-marker model on data from a cohort of North American volunteers challenged with V. cholerae O1 (AUC range, 88.4 to 98.4%). In simulated serosurveys, our models accurately estimated annual incidence in both endemic and epidemic settings, even with sample sizes as small as 500 and annual incidence as low as two infections per 1000 individuals. Crosssectional serosurveys may be a viable approach to estimating cholera incidence.

Safety and Immunogenicity of Live Oral Cholera Vaccine CVD 103-HgR in Children and Adolescents Aged 6-17 Years

The attenuated recombinant Vibrio cholerae O1 vaccine strain CVD 103-HgR, redeveloped as PXVX0200, elicits a rapid serum vibriocidal antibody (SVA) response and protects against cholera-induced diarrhea in adult volunteer challenge trials but has not been studied in children in developed countries. We performed a phase 4, placebo-controlled, double-blind, multicenter study to assess the safety, immunogenicity, and tolerability of a single, oral dose of PXVX0200 in children and adolescents aged 6-17 years in the United States and bridged immunogenicity to adults aged 18-45 years from a separate lot consistency study. Volunteers were randomized to receive a single dose of 1 × 109 colony forming units (CFU) of PXVX0200 or placebo. Immunogenicity endpoints included SVA levels on days 1, 11, and 29 in volunteers aged 6-17 years and also on days 91 and 181 in volunteers aged 12-17 years. Safety was assessed by comparing solicited signs and symptoms on days 1-8, unsolicited adverse events (AEs) through day 29, and serious AEs through day 181. A total of 374 participants were enrolled, comprising 321 vaccine and 53 placebo recipients. The SVA seroconversion rates 10 days after immunization were 98.6% and 2.1% in vaccine and placebo recipients, respectively, and the vaccine seroconversion rate was non-inferior to the 93.5% rate seen in adults aged 18-45 years. Most reactogenicity was mild to moderate, and there were no vaccine-related serious AEs. The complete dose was consumed in 95.3% and 98.1% of vaccine and placebo recipients, respectively. PXVX0200 appears safe, immunogenic, and well tolerated in children and adolescents aged 6-17 years.

Immune responses to O-specific polysaccharide (OSP) in North American adults infected with Vibrio cholerae O1 Inaba

Background

Antibodies targeting O-specific polysaccharide (OSP) of Vibrio cholerae may protect against cholera; however, little is known about this immune response in infected immunologically naïve humans.

Methodology

We measured serum anti-OSP antibodies in adult North American volunteers experimentally infected with V. cholerae O1 Inaba El Tor N16961. We also measured vibriocidal and anti-cholera toxin B subunit (CtxB) antibodies and compared responses to those in matched cholera patients in Dhaka, Bangladesh, an area endemic for cholera.

Principal findings

We found prominent anti-OSP antibody responses following initial cholera infection: these responses were largely IgM and IgA, and highest to infecting serotype with significant cross-serotype reactivity. The anti-OSP responses peaked 10 days after infection and remained elevated over baseline for ≥ 6 months, correlated with vibriocidal responses, and may have been blunted in blood group O individuals (IgA anti-OSP). We found significant differences in immune responses between naïve and endemic zone cohorts, presumably reflecting previous exposure in the latter.

Conclusions

Our results define immune responses to O-specific polysaccharide in immunologically naive humans with cholera, find that they are largely IgM and IgA, may be blunted in blood group O individuals, and differ in a number of significant ways from responses in previously humans. These differences may explain in part varying degrees of protective efficacy afforded by cholera vaccination between these two populations.

Trial registration number

ClinicalTrials.gov NCT01895855.

Cholera is an acute, secretory diarrheal disease caused by Vibrio cholerae O1. There is a growing body of evidence that immune responses targetting the O-specific polysaccharide (OSP) of V. cholerae are associated with protecton against cholera. Despite this, little is known about immune responses targeting OSP in immunologically naive individals. Cholera affects populations in severely resource-limited areas. To address this, we assessed anti-OSP immune responses in North American volunteers experimentally infected with wild type V. cholerae O1 El Tor Inaba strain N16961. We found that antibody responses were largely IgM and IgA, cross-reacted to both Inaba and Ogawa serotypes, and correlated with vibriocidal responses. We found no association of responses to severity of disease, but did find that blood group O individuals mounted lower IgA fold-changes to OSP than did non-blood group O individuals. Individuals with blood group O are at particular risk for severe cholera, and are less well protected against cholera following oral vaccination. We also compared anti-OSP responses in previously unexposed individuals to responses in matched endemic zone patients, and found a number of significant differences. Such differences may explain in part the varying degrees of protective efficacy afforded by cholera vaccination between these two populations.

An overview of VaxchoraTM, a live attenuated oral cholera vaccine

Cholera remains a public health threat among the least privileged populations and regions affected by conflicts and natural disasters. Together with Water, Sanitation and Hygiene practices, use of oral cholera vaccines (OCVs) is a key tool to prevent cholera. Bivalent whole-cell killed OCVs have been extensively used worldwide and found effective in protecting populations against cholera in endemic and outbreak settings. No cholera vaccine had been available for United States (US) travelers at risk for decades until 2016 when CVD 103-HgR (Vaxchora™), an oral live attenuated vaccine, was licensed by the US FDA. A single dose of Vaxchora™ protected US volunteers against experimental challenge 10 days and 3 months after vaccination. However, use of Vaxchora™ poses several challenges in resource poor settings as it requires reconstitution, is age-restricted to 18 to 64 years, has no data in populations endemic for cholera, and faces challenges related to cold chain and cost.

Oral immunization with a probiotic cholera vaccine induces broad protective immunity against Vibrio cholerae colonization and disease in mice

Oral cholera vaccines (OCVs) are being increasingly employed, but current killed formulations generally require multiple doses and lack efficacy in young children. We recently developed a new live-attenuated OCV candidate (HaitiV) derived from a Vibrio cholerae strain isolated during the 2010 Haiti cholera epidemic. HaitiV exhibited an unexpected probiotic-like activity in infant rabbits, preventing intestinal colonization and disease by wild-type V. cholerae before the onset of adaptive immunity. However, it remained unknown whether HaitiV would behave similarly to other OCVs to stimulate adaptive immunity against V. cholerae. Here, we orally immunized adult germ-free female mice to test HaitiV’s immunogenicity. HaitiV safely and stably colonized vaccinated mice and induced known adaptive immune correlates of cholera protection within 14 days of administration. Pups born to immunized mice were protected against lethal challenges of both homologous and heterologous V. cholerae strains. Cross-fostering experiments revealed that protection was not dependent on vaccine colonization in or transmission to the pups. These findings demonstrate the protective immunogenicity of HaitiV and support its development as a new tool for limiting cholera.

Oral cholera vaccines are increasingly used as public health tools for prevention of cholera and curtailing the spread of outbreaks. However, current killed vaccines provide minimal protection in young children, who are especially susceptible to this diarrheal disease, and require ~7–14 days between vaccination and development of protective immunity. We recently created HaitiV, a live-attenuated oral cholera vaccine candidate derived from a clinical isolate from the Haiti cholera outbreak. Unexpectedly, HaitiV protected against cholera-like illness in infant rabbits within 24 hours of administration, before the onset of adaptive immunity. However, HaitiV’s capacity to stimulate adaptive immune responses against the cholera pathogen were not investigated. Here, we report that HaitiV induces immunological correlates of protection against cholera in adult germ-free mice and leads to protection against disease in their offspring. Protection against disease was transferable through the milk of the immunized mice and was not due to transmission or colonization of HaitiV in this model. Coupling the immunogenicity data presented here with our previous observation that HaitiV can protect from cholera prior to the induction of adaptive immunity, we propose that HaitiV may provide both rapid-onset short-term protection from disease while eliciting stable and long-lasting immunity against cholera.

Comparison of the immunogenicity of Dukoral® oral cholera vaccine between renal transplant recipients on either a calcineurin inhibitor or mycophenolate - A controlled trial

BACKGROUND

The evidence for recommendations regarding vaccination in solid organ transplant recipients is sparse. There is little data comparing vaccine responses between groups on different immunosuppressive drugs. This study was conducted to evaluate the antibody response to Dukoral® oral cholera vaccine in renal transplant recipients (RTR).

METHODS

In a single-center non-randomized controlled clinical trial, healthy volunteers (n = 21) and renal transplant recipients (n = 30) were vaccinated with the oral whole cell/recombinant B subunit cholera vaccine Dukoral® (Valneva Inc., Vienna, Austria). The RTR were stratified according to their maintenance immunosuppressive therapy: either prednisone and a calcineurin inhibitor (cyclosporine A or tacrolimus; P/CNI group; n = 15) or prednisone and mycophenolate (P/MMF group; n = 15). All volunteers ingested Dukoral® at baseline and at day 14. Serum samples were drawn at day 0 and day 21. The primary outcome was seroconversion, defined as either a 3-fold IgA serum titer increase in anti-cholera toxin B antibodies and/or a 4-fold rise in the serum vibriocidal titer.

RESULTS

Follow-up was complete. Seroconversion after vaccination was 57% (standard error, SE 9%) in RTR and 81% (SE 9%) in healthy controls (Relative Risk, RR 0.70; 95% CI 0.48-1.02). When stratified according to maintenance immunosuppression, the seroconversion rate was 67% (SE 12%) in the P/CNI group (RR compared with controls 0.82; 95% CI 0.55-1.25) and 47% (SE 13%) in the P/MMF group (RR compared with controls 0.58; 95% CI 0.32-1.03).

CONCLUSION

Adverse events were mild to moderate and transient. The response to Dukoral was weaker and the seroconversion rate was lower in renal transplant recipients than in healthy controls. In particular, those using mycophenolate had a poor response. Nevertheless, more than half of the transplant recipients seroconverted. Therefore oral vaccines should not be discarded as a potential tool for protection of solid organ transplant recipients. This trial is registered in clinicaltrials.gov under NCT01109914.

Age-related immunogenicity and reactogenicity of live oral cholera vaccine CVD 103-HgR in a randomized, controlled clinical trial

Aging is accompanied by a decline in immune function which can lead to decreased responses to vaccines. Attenuated recombinant Vibrio cholerae O1 vaccine strain CVD 103-HgR elicits a rapid serum vibriocidal antibody (SVA) response and protects against cholera diarrhea in volunteer challenge studies but has not been studied in older adults. We evaluated CVD 103-HgR (PXVX0200) in adults age 46-64, compared them to previously studied adults age 18-45, and studied age-related immunogenicity across adults 18-64 years of age. Volunteers were randomized to receive a single dose of 1 × 10⁹ CFU of PXVX0200 or placebo. Immunogenicity endpoints included SVA and anti-cholera toxin (CT) antibody levels on days 1, 11, 29, 91 and 181 and lipopolysaccharide (LPS) and CT-specific IgA and IgG memory B cells on days 1, 91 and 181. Safety was assessed by comparing solicited signs and symptoms on days 1-8 and other adverse events through day 181. 2979 volunteers received vaccine, including 291 age 45-64. Day 11 seroconversion occurred in 90.4% of older adults vs 93.5%% of younger adults and met the endpoint of demonstrating non-inferiority between the two groups. Significant increases in LPS-specific IgG and IgA and CT-specific memory IgG memory B cells were seen at days 91 and 181. There appeared to be a continuous age-related decline in SVA seroconversion and geometric mean titers, but not memory B cell responses, across the 18-64 year age range. Most reactogenicity was mild and was more common in the placebo group. PXVX0200 appears safe and immunogenic in older adults. Clinical Trials Registration: clinicaltrials.gov NCT02100631.

Immunogenicity and Protection From a Single Dose of Internationally Available Killed Oral Cholera Vaccine: A Systematic Review and Metaanalysis

In addition to improved water supply and sanitation, the 2-dose killed oral cholera vaccine (OCV) is an important tool for the prevention and control of cholera. We aimed to document the immunogenicity and protection (efficacy and effectiveness) conferred by a single OCV dose against cholera. The metaanalysis showed that an estimated 73% and 77% of individuals seroconverted to the Ogawa and Inaba serotypes, respectively, after an OCV first dose. The estimates of single-dose vaccine protection from available studies are 87% at 2 months decreasing to 33% at 2 years. Current immunologic and clinical data suggest that protection conferred by a single dose of killed OCV may be sufficient to reduce short-term risk in outbreaks or other high-risk settings, which may be especially useful when vaccine supply is limited. However, until more data suggest otherwise, a second dose should be given as soon as circumstances allow to ensure robust protection.

Lipopolysaccharide-specific memory B cell responses to an attenuated live cholera vaccine are associated with protection against Vibrio cholerae infection

BACKGROUND

The single-dose live attenuated vaccine CVD 103-HgR protects against experimental Vibrio cholerae infection in cholera-naïve adults for at least 6 months after vaccination. While vaccine-induced vibriocidal seroconversion is associated with protection, vibriocidal titers decline rapidly from their peak 1-2 weeks after vaccination. Although vaccine-induced memory B cells (MBCs) might mediate sustained protection in individuals without detectable circulating antibodies, it is unknown whether oral cholera vaccination induces a MBC response.

METHODS

In a study that enrolled North American adults, we measured lipopolysaccharide (LPS)- and cholera toxin (CtxB)-specific MBC responses to PXVX0200 (derived from the CVD 103-HgR strain) and assessed stool volumes following experimental Vibrio cholerae infection. We then evaluated the association between vaccine-induced MBC responses and protection against cholera.

RESULTS

There was a significant increase in % CT-specific IgG, % LPS-specific IgG, and % LPS-specific IgA MBCs which persisted 180 days after vaccination as well as a significant association between vaccine-induced increase in % LPS-specific IgA MBCs and lower post-challenge stool volume (r = -0.56, p < 0.001).

DISCUSSION

Oral cholera vaccination induces antigen-specific MBC responses, and the anamnestic LPS-specific responses may contribute to long-term protection and provide correlates of the duration of vaccine-induced protection.

CLINICAL TRIALS REGISTRATION

NCT01895855.

Anti-O-specific polysaccharide (OSP) immune responses following vaccination with oral cholera vaccine CVD 103-HgR correlate with protection against cholera after infection with wild-type Vibrio cholerae O1 El Tor Inaba in North American volunteers

Background

Cholera is an acute voluminous dehydrating diarrheal disease caused by toxigenic strains of Vibrio cholerae O1 and occasionally O139. A growing body of evidence indicates that immune responses targeting the O-specific polysaccharide (OSP) of V. cholerae are involved in mediating protection against cholera. We therefore assessed whether antibody responses against OSP occur after vaccination with live attenuated oral cholera vaccine CVD 103-HgR, and whether such responses correlate with protection against cholera.

Methodology

We assessed adult North American volunteers (n = 46) who were vaccinated with 5 × 10⁸ colony-forming units (CFU) of oral cholera vaccine CVD 103-HgR and then orally challenged with approximately 1 × 10⁵ CFU of wild-type V. cholerae O1 El Tor Inaba strain N16961, either 10 or 90 days post-vaccination.

Principal findings

Vaccination was associated with induction of significant serum IgM and IgA anti-OSP and vibriocidal antibody responses within 10 days of vaccination. There was significant correlation between anti-OSP and vibriocidal antibody responses. IgM and IgA anti-OSP responses on day 10 following vaccination were associated with lower post-challenge stool volume (r = −0.44, P = 0.002; r = −0.36, P = 0.01; respectively), and none of 27 vaccinees who developed a ≥1.5 fold increase in any antibody isotype targeting OSP on day 10 following vaccination compared to baseline developed moderate or severe cholera following experimental challenge, while 5 of 19 who did not develop such anti-OSP responses did (P = 0.01).

Conclusion

Oral vaccination with live attenuated cholera vaccine CVD 103-HgR induces antibodies that target V. cholerae OSP, and these anti-OSP responses correlate with protection against diarrhea following experimental challenge with V. cholerae O1.

Trial registration

ClinicalTrials.gov NCT01895855

Cholera is a severe watery diarrheal disease, caused by pathogenic strains of V. cholerae. Protective immunity against cholera is serogroup specific, and serogroup specificity is determined by the O-specific polysaccharide (OSP) of V. cholerae lipopolysaccharide (LPS). Despite this, no previous work has directly assessed correlation of OSP-immune responses and protection against cholera. In this study, we assessed adult North American volunteer’s antibody responses targeting OSP after vaccination with live attenuated oral cholera vaccine CVD 103-HgR, and we assessed correlation of protection against cholera with such antibody responses. Oral vaccination was associated with the induction of significant IgM and IgA responses against OSP, and these responses correlated with vibriocidal responses. There was significant negative correlation of OSP responses and post-challenge stool volume, and none of the volunteers who developed an anti-OSP antibody responses of any isotype of ≥1.5 fold developed moderate or severe cholera following experimental challenge. In summary, vaccination with live attenuated oral cholera vaccine CVD 103-HgR induces antibodies that target V. cholerae OSP, and these responses highly correlate with protection against cholera.

Randomized, Placebo-Controlled, Double-Blind Phase 2 Trial Comparing the Reactogenicity and Immunogenicity of a Single Standard Dose to Those of a High Dose of CVD 103-HgR Live Attenuated Oral Cholera Vaccine, with Shanchol Inactivated Oral Vaccine as an Open-Label Immunologic Comparator

Reactive immunization with a single-dose cholera vaccine that could rapidly (within days) protect immunologically naive individuals during virgin soil epidemics, when cholera reaches immunologically naive populations that have not experienced cholera for decades, would facilitate cholera control. One dose of attenuated Vibrio cholerae O1 classical Inaba vaccine CVD 103-HgR (Vaxchora) containing ≥2 × 10⁸ CFU induces vibriocidal antibody seroconversion (a correlate of protection) in >90% of U.S. adults. A previous CVD 103-HgR commercial formulation required ≥2 × 10⁹ CFU to elicit high levels of seroconversion in populations in developing countries. We compared the vibriocidal responses of Malians (individuals 18 to 45 years old) randomized to ingest a single ≥2 × 10⁸-CFU standard dose (n = 50) or a ≥2 × 10⁹-CFU high dose (n = 50) of PaxVax CVD 103-HgR with buffer or two doses (n = 50) of Shanchol inactivated cholera vaccine (the immunologic comparator). To maintain blinding, participants were dosed twice 2 weeks apart; CVD 103-HgR recipients ingested placebo 2 weeks before or after ingesting vaccine. Seroconversion (a ≥4-fold vibriocidal titer rise) between the baseline and 14 days after CVD 103-HgR ingestion and following the first and second doses of Shanchol were the main outcomes measured. By day 14 postvaccination, the rates of seroconversion after ingestion of a single standard dose and a high dose of CVD 103-HgR were 71.7% (33/46 participants) and 83.3% (40/48 participants), respectively. The rate of seroconversion following the first dose of Shanchol, 56.0% (28/50 participants), was significantly lower than that following the high dose of CVD 103-HgR (P = 0.003). The vibriocidal geometric mean titer (GMT) of the high dose of CVD 103-HgR exceeded the GMT of the standard dose at day 14 (214 versus 95, P = 0.045) and was ∼2-fold higher than the GMT on day 7 and day 14 following the first Shanchol dose (P > 0.05). High-dose CVD 103-HgR is recommended for accelerated evaluation in developing countries to assess its efficacy and practicality in field situations. (This study has been registered at ClinicalTrials.gov under registration no. NCT02145377.)