Protection afforded by previous Vibrio cholerae infection against subsequent disease and infection: A review

BACKGROUND

Cholera is an acute, diarrheal disease caused by Vibrio cholerae O1 or 139 that is associated with a high global burden.

METHODS

We analyzed the estimated duration of immunity following cholera infection from available published studies. We searched PubMed and Web of Science for studies of the long-term immunity following cholera infection. We identified 22 eligible studies and categorized them as either observational, challenge, or serological.

RESULTS

We found strong evidence of protection at 3 years after infection in observational and challenge studies. However, serological studies show that elevated humoral markers of potential correlates of protection returned to baseline within 1 year. Additionally, a subclinical cholera infection may confer lower protection than a clinical one, as suggested by 3 studies that found that, albeit with small sample sizes, most participants with a subclinical infection from an initial challenge with cholera had a symptomatic infection when rechallenged with a homologous biotype.

CONCLUSIONS

This review underscores the need to elucidate potential differences in the protection provided by clinical and subclinical cholera infections. Further, more studies are warranted to bridge the gap between the correlates of protection and cholera immunity. Understanding the duration of natural immunity to cholera can help guide control strategies and policy.

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.

Immunogenicity of a killed bivalent whole cell oral cholera vaccine in forcibly displaced Myanmar nationals in Cox's Bazar, Bangladesh

After the large influx of Rohingya nationals (termed Forcibly Displaced Myanmar National; FDMN) from Rakhine State of Myanmar to Cox’s Bazar in Bangladesh, it was apparent that outbreaks of cholera was very likely in this setting where people were living under adverse water and sanitation conditions. Large campaigns of oral cholera vaccine (OCV) were carried out as a preemptive measure to control cholera epidemics. The aim of the study was to evaluate the immune responses of healthy adults and children after administration of two doses of OCV at 14 days interval in FDMN population and compare with the response observed in Bangladeshi’s vaccinated earlier. A cross-sectional immunogenicity study was conducted among FDMNs of three age cohort; in adults (18+years; n = 83), in older children (6–17 years; n = 63) and in younger children (1–5 years; n = 80). Capillary blood was collected at three time points to measure vibriocidal antibodies using either plasma or dried blood spot (DBS) specimens. There was a significant increase of responder frequency of vibriocidal antibody titer at day 14 in all groups for Vibrio cholerae O1 (Ogawa/Inaba: adults-64%/64%, older children-70%/89% and younger children-51%/75%). There was no overall difference of vibriocidal antibody titer between FDMN and Bangladeshi population at baseline (p = 0.07–0.08) and at day 14, day 28 in all age groups for both serotypes. The seroconversion rate and geometric mean titer (GMT) of either serotype were comparable using both plasma and DBS specimens. These results showed that OCV is capable of inducing robust immune responses in adults and children among the FDMN population which is comparable to that seen in Bangladeshi participants in different age groups or that reported from other cholera endemic countries. Our results also suggest that the displaced population were exposed to V. cholerae prior to seeking shelter in Bangladesh.

Oral cholera vaccines (OCV) are now on the WHO stockpile and targeted for use for countries with outbreaks and epidemics but also for control of endemic cholera. In Bangladesh many studies have been carried out to assess the safety, immunogenicity as well as feasibility of vaccination in the endemic settings of the country. However, with the large recent influx of displaced Rohingya nationals from Myanmar (termed Forcibly Displace Myanmar National; FDMN), mass campaigns with OCV were conducted between October 2017-December 2018. However, no data is available of the previous exposure to cholera of this population, prior to their arrival in Bangladesh. An assessment of immunogenicity status of FDMN is needed to find out if OCV is able to elicit comparable immune response and whether the same dose regimen of OCV was immunogenic among the FDMNs. In this study, we have measured the immune responses to the OCV, Shanchol, in adults (18 years and above), older children (6–17 years), and younger children (1–5 years). The results of this study shows that the oral cholera vaccine capable of inducing an immune response in adults and children among this FDMN population and the responses were comparable to that seen in Bangladeshi participants in earlier studies.

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.

Induction of systemic, mucosal and memory antibody responses targeting Vibrio cholerae O1 O-specific polysaccharide (OSP) in adults following oral vaccination with an oral killed whole cell cholera vaccine in Bangladesh

BACKGROUND

Oral cholera vaccine (OCV) containing killed Vibrio cholerae O1 and O139 organisms (Bivalent-OCV; Biv-OCV) are playing a central role in global cholera control strategies. OCV is currently administered in a 2-dose regimen (day 0 and 14). There is a growing body of evidence that immune responses targeting the O-specific polysaccharide (OSP) of V. cholerae mediate protection against cholera. There are limited data on anti-OSP responses in recipients of Biv-OCV. We assessed serum antibody responses against O1 OSP, as well as antibody secreting cell (ASC) responses (a surrogate marker for mucosal immunity) and memory B cell responses in blood of adult recipients of Biv-OCV in Dhaka, Bangladesh.

METHODOLOGY/PRINCIPAL FINDINGS

We enrolled 30 healthy adults in this study and administered two doses of OCV (Shanchol) at days 0 and 14. Blood samples were collected before vaccination (day 0) and 7 days after each vaccination (day 7 and day 21), as well as on day 44. Serum responses were largely IgA with minimal IgG and IgM responses in this population. There was no appreciable boosting following day 14 vaccination. There were significant anti-OSP IgA ASC responses on day 7 following the first vaccination, but none after the second immunization. Anti-OSP IgA memory B cell responses were detectable 30 days after completion of the vaccination series, with no evident induction of IgG memory responses. In this population, anti-Ogawa OSP responses were more prominent than anti-Inaba responses, perhaps reflecting impact of previous exposure. Serum anti-OSP responses returned to baseline within 30 days of completing the vaccine series.

CONCLUSION

Our results call into question the utility of the 2-dose regimen separated by 14 days in adults in cholera endemic areas, and also suggest that Biv-OCV-induced immune responses targeting OSP are largely IgA in this highly endemic cholera area. Studies in children in cholera-endemic areas need to be performed. Protective efficacy that extends for more than a month after vaccination presumably is mediated by direct mucosal immune response which is not assessed in this study. Our results suggest a single dose of OCV in adults in a cholera endemic zone may be sufficient to mediate at least short-term protection.

Safety and immunogenicity of a killed bivalent (O1 and O139) whole-cell oral cholera vaccine in adults and children in Vellore, South India

This open-label study assessed the safety and immunogenicity of two doses (14 days apart) of an indigenously manufactured, killed, bivalent (Vibrio cholerae O1 and O139), whole-cell oral cholera vaccine (SHANCHOL; Shantha Biotechnics) in healthy adults (n = 100) and children (n = 100) in a cholera endemic area (Vellore, South India) to fulfill post-licensure regulatory requirements and post-World Health Organization (WHO) prequalification commitments. Safety and reactogenicity were assessed, and seroconversion rates (i.e. proportion of participants with a ≥ 4-fold rise from baseline in serum vibriocidal antibody titers against V. cholerae O1 Inaba, O1 Ogawa and O139, respectively) were determined 14 days after each vaccine dose. No serious adverse events were reported during the study. Commonly reported solicited adverse events were headache and general ill feeling. Seroconversion rates after the first and second dose in adults were 67.7% and 55.2%, respectively, against O1 Inaba; 47.9% and 45.8% against O1 Ogawa; and 19.8% and 20.8% against O139. In children, seroconversion rates after the first and second dose were 80.2% and 68.8%, respectively, against O1 Inaba; 72.9% and 67.7% against O1 Ogawa; and 26.0% and 18.8% against O139. The geometric mean titers against O1 Inaba, O1 Ogawa, and O139 in both adults and children were significantly higher after each vaccine dose compared to baseline titers (P < 0.001; for both age groups after each dose versus baseline). The seroconversion rates for O1 Inaba, O1 Ogawa, and O139 in both age groups were similar to those in previous studies with the vaccine. In conclusion, the killed, bivalent, whole-cell oral cholera vaccine has a good safety and reactogenicity profile, and is immunogenic in healthy adults and children. Trial Registration: ClinicalTrials.gov NCT00760825; CTRI/2012/01/002354.

Use of oral cholera vaccine as a vaccine probe to determine the burden of culture-negative cholera

Analyses of stool from patients with acute watery diarrhea (AWD) using sensitive molecular diagnostics have challenged whether fecal microbiological cultures have acceptably high sensitivity for cholera diagnosis. If true, these findings imply that current estimates of the global burden of cholera, which rely largely on culture-confirmation, may be underestimates. We conducted a vaccine probe study to evaluate this possibility, assessing whether an effective killed oral cholera vaccine (OCV) tested in a field trial in a cholera-endemic population conferred protection against cholera culture-negative AWD, with the assumption that if cultures are indeed insensitive, OCV protection in such cases should be detectable. We re-analysed the data of a Phase III individually-randomized placebo-controlled efficacy trial of killed OCVs conducted in Matlab, Bangladesh in 1985. We calculated the protective efficacy (PE) of a killed whole cell-only (WC-only) OCV against first-episodes of cholera culture-negative AWD during two years of post-dosing follow-up. In secondary analyses, we evaluated PE against cholera culture-negative AWD by age at vaccination, season of onset, and disease severity. In this trial 50,770 people received at least 2 complete doses of either WC-only OCV or placebo, and 791 first episodes of AWD were reported during the follow-up period, of which 365 were culture-positive for Vibrio cholerae O1. Of the 426 culture-negative AWD episodes, 215 occurred in the WC group and 211 occurred in the placebo group (adjusted PE = -1.7%; 95%CI -23.0 to 13.9%, p = 0.859). No measurable PE of OCV was observed against all or severe cholera culture-negative AWD when measured overall or by age and season subgroups. In this OCV probe study we detected no vaccine protection against AWD episodes for which fecal cultures were negative for Vibrio cholera O1. Results from this setting suggest that fecal cultures from patients with AWD were highly sensitive for cholera episodes that were etiologically attributable to this pathogen. Similar analyses of other OCV randomized controlled trials are recommended to corroborate these findings.

Conventional microbiological culture has remained a relatively uncontested ‘gold standard’ for the diagnosis of cholera; however, emerging methods, including sensitive molecular tests, challenge the current paradigm. One pivotal article demonstrated that culture failed to detect cholera in one-third of the cholera-positive stool specimens confirmed by other methods. This finding underscored the absence of a reliable reference test, further complicated by newer tests outperforming the gold standard, leaving no suitable comparator. In this study, we used oral cholera vaccine as a probe to investigate the reliability of conventional culture as a diagnostic for cholera by measuring the effectiveness of the vaccine against cholera culture-negative acute watery diarrhea. We did not find any evidence of protection, implying that the culture diagnostics used were reliable. The dynamics of cholera transmission require a rapid response, and ascertaining the best rapid diagnostic test for early detection of outbreaks will maximize the effectiveness of chronically limited resources in high risk regions. As techniques advance, well-designed studies should be implemented to systematically evaluate their merit against established methods, and improved diagnostics, including rapid diagnostics and microbiological culture, should be implemented into cholera control programs to reduce cholera transmission by creating a better trigger for outbreak response.

Development of a new dipstick (Cholkit) for rapid detection of Vibrio cholerae O1 in acute watery diarrheal stools

Recognizing cholera cases early, especially in the initial phase of an outbreak and in areas where cholera has not previously circulated, is a high public health priority. Laboratory capacity in such settings is often limited. To address this, we have developed a rapid diagnostic test (RDT) termed Cholkit that is based on an immunochromatographic lateral flow assay for the diagnosis of cholera cases using stool. Cholkit contains a monoclonal antibody (ICL-33) to the O-specific polysaccharide (OSP) component of V. cholerae O1 lipopolysaccharide, and recognizes both Inaba and Ogawa serotypes. We tested the Cholkit dipstick using fresh stool specimens of 76 adults and children presenting with acute watery diarrhea at the icddr,b hospital in Dhaka, Bangladesh. We compared Cholkit’s performance with those of microbial culture, PCR (targeting the rfb and ctxA genes of V. cholerae) and the commercially available RDT, Crystal VC (Span Diagnostics; Surat, India). We found that all stool specimens with a positive culture for V. cholerae O1 (n = 19) were positive by Cholkit as well as Crystal VC. We then used Bayesian latent class modeling to estimate the sensitivity and specificity of each diagnostic assay. The sensitivity of Cholkit, microbiological culture, PCR and Crystal VC was 98% (95% CI: 88–100), 71% (95% CI: 59–81), 74% (95% CI: 59–86) and 98% (95% CI: 88–100), respectively. The specificity for V. cholerae O1 was 97% (95% CI: 89–100), 100%, 97% (95% CI: 93–99) and 98% (95% CI: 92–100), respectively. Of note, two Crystal VC dipsticks were positive for V. cholerae O139 but negative by culture and PCR in this area without known circulating epidemic V. cholerae O139. In conclusion, the Cholkit dipstick is simple to use, requires no dedicated laboratory capacity, and has a sensitivity and specificity for V. cholerae O1 of 98% and 97%, respectively. Cholkit warrants further evaluation in other settings.

Cholera is a severely dehydrating diarrheal disease that can lead to death if remains untreated. The incidence of case fatality is higher at the beginning of the outbreak. Diagnosis of cholera in the early stage of outbreak is a high public health priority. Although countries facing complex emergencies are more vulnerable to cholera outbreak, laboratory capacity in such settings is usually limited. To address this, here we report the development of a rapid diagnostic test (RDT) termed Cholkit for the diagnosis of cholera cases using stool and the assessment of its performance with those of microbial culture, PCR and Crystal VC assay, a commercially available dipstick using a latent class modeling approach.

Vibrio cholerae O1 secretes an extracellular matrix in response to antibody-mediated agglutination

Vibrio cholerae O1 is one of two serogroups responsible for epidemic cholera, a severe watery diarrhea that occurs after the bacterium colonizes the human small intestine and secretes a potent ADP-ribosylating toxin. Immunity to cholera is associated with intestinal anti-lipopolysaccharide (LPS) antibodies, which are known to inhibit V. cholerae motility and promote bacterial cell-cell crosslinking and aggregation. Here we report that V. cholerae O1 classical and El Tor biotypes produce an extracellular matrix (ECM) when forcibly immobilized and agglutinated by ZAC-3 IgG, an intestinally-derived monoclonal antibody (MAb) against the core/lipid A region of LPS. ECM secretion, as demonstrated by crystal violet staining and scanning electron microscopy, occurred within 30 minutes of antibody exposure and peaked by 3 hours. Non-motile mutants of V. cholerae did not secrete ECM following ZAC-3 IgG exposure, even though they were susceptible to agglutination. The ECM was enriched in O-specific polysaccharide (OSP) but not Vibrio polysaccharide (VPS). Finally, we demonstrate that ECM production by V. cholerae in response to ZAC-3 IgG was associated with bacterial resistant to a secondary complement-mediated attack. In summary, we propose that V. cholerae O1, upon encountering anti-LPS antibodies in the intestinal lumen, secretes an ECM (or O-antigen capsule) possibly as a strategy to shield itself from additional host immune factors and to exit an otherwise inhospitable host environment.

Safety and immunogenicity of the killed bivalent (O1 and O139) whole-cell cholera vaccine in the Philippines

The killed bivalent (O1 and O139) whole cell oral cholera vaccine (OCV) (Shanchol™) was first licensed in India in 2009 and World Health Organization pre-qualified in 2011. We assessed the safety and immunogenicity of this OCV in the Philippines. This was a phase IV, single-arm, descriptive, open-label study. We recruited 336 participants from 2 centers: 112 participants in each age group (1-4, 5-14 and ≥ 15 years). Participants received 2 OCV doses 14 d apart. Safety was monitored throughout the trial. Blood samples were collected at baseline (pre-vaccination) and 14 d after each dose. Serum vibriocidal antibody titers to V. cholerae O1 (El Tor Inaba and El Tor Ogawa) and O139 strains were assessed, with seroconversion defined as ≥ 4-fold increase from baseline in titers. No immediate unsolicited systemic adverse events/reactions were observed. Unsolicited systemic adverse events were mostly grade 1 intensity. One serious adverse event occurred after the first dose, but was unrelated to vaccination. High seroconversion rates (range 69-92%) were achieved against the O1 serotypes with a trend toward higher rates in the 1-4 y (86-92%) and 5-14 y (86-88%) age groups than the ≥ 15 y age group (69-83%). Lower seroconversion rates were achieved against the O139 serotype (35-70%), particularly in those aged ≥ 15 y (35-42%). The 2-dose regimen of the killed bivalent whole cell OCV was well-tolerated in this study conducted in the Philippines, a cholera-endemic country. Robust immune responses were observed even after a single-dose.

Development and evaluation of an up-converting phosphor technology-based lateral flow assay for the rapid, simultaneous detection of Vibrio cholerae serogroups O1 and O139

Vibrio cholerae serogroups O1 and O139 are etiological agents of cholera, a serious and acute diarrheal disease, and rapid detection of V. cholerae is a key method for preventing and controlling cholera epidemics. Here, a point of care testing (POCT) method called Vch-UPT-LF, which is an up-converting phosphor technology-based lateral flow (UPT-LF) assay with a dual-target detection mode, was developed to detect V. cholerae O1 and O139 simultaneously from one sample loading. Although applying an independent reaction pair made both detection results for the two Vch-UPT-LF detection channels more stable, the sensitivity slightly declined from 104 to 105 colony-forming units (CFU) mL-1 compared with that of the single-target assay, while the quantification ranges covering four orders of magnitude were maintained. The strip showed excellent specificity for seven Vibrio species that are highly related genetically, and nine food-borne species whose transmission routes are similar to those of V. cholerae. The legitimate arrangement of the two adjacent test lines lessened the mutual impact of the quantitation results between the two targets, and the quantification values did not differ by more than one order of magnitude when the samples contained high concentrations of both V. cholerae O1 and O139. Under pre-incubation conditions, 1×101 CFU mL-1 of V. cholerae O1 or O139 could be detected in fewer than 7 h, while the Vch-UPT-LF assay exhibited sensitivity as high as a real-time fluorescent polymerase chain reaction with fewer false-positive results. Therefore, successful development of Vch-UPT-LF as a dual-target assay for quantitative detection makes this assay a good candidate POCT method for the detection and surveillance of epidemic cholera.

Biomarkers of Environmental Enteropathy are Positively Associated with Immune Responses to an Oral Cholera Vaccine in Bangladeshi Children

Environmental enteropathy (EE) is a poorly understood condition that refers to chronic alterations in intestinal permeability, absorption, and inflammation, which mainly affects young children in resource-limited settings. Recently, EE has been linked to suboptimal oral vaccine responses in children, although immunological mechanisms are poorly defined. The objective of this study was to determine host factors associated with immune responses to an oral cholera vaccine (OCV). We measured antibody and memory T cell immune responses to cholera antigens, micronutrient markers in blood, and EE markers in blood and stool from 40 Bangladeshi children aged 3-14 years who received two doses of OCV given 14 days apart. EE markers included stool myeloperoxidase (MPO) and alpha anti-trypsin (AAT), and plasma endotoxin core antibody (EndoCab), intestinal fatty acid binding protein (i-FABP), and soluble CD14 (sCD14). We used multiple linear regression analysis with LASSO regularization to identify host factors, including EE markers, micronutrient (nutritional) status, age, and HAZ score, predictive for each response of interest. We found stool MPO to be positively associated with IgG antibody responses to the B subunit of cholera toxin (P = 0.03) and IgA responses to LPS (P = 0.02); plasma sCD14 to be positively associated with LPS IgG responses (P = 0.07); plasma i-FABP to be positively associated with LPS IgG responses (P = 0.01) and with memory T cell responses specific to cholera toxin (P = 0.01); stool AAT to be negatively associated with IL-10 (regulatory) T cell responses specific to cholera toxin (P = 0.02), and plasma EndoCab to be negatively associated with cholera toxin-specific memory T cell responses (P = 0.02). In summary, in a cohort of children 3-14 years old, we demonstrated that the majority of biomarkers of environmental enteropathy were positively associated with immune responses after vaccination with an OCV.

Retrospective Analysis of Serotype Switching of Vibrio cholerae O1 in a Cholera Endemic Region Shows It Is a Non-random Process

Genomic data generated from clinical Vibrio cholerae O1 isolates collected over a five year period in an area of Kolkata, India with seasonal cholera outbreaks allowed a detailed genetic analysis of serotype switching that occurred from Ogawa to Inaba and back to Ogawa. The change from Ogawa to Inaba resulted from mutational disruption of the methyltransferase encoded by the wbeT gene. Re-emergence of the Ogawa serotype was found to result either from expansion of an already existing Ogawa clade or reversion of the mutation in an Inaba clade. Our data suggests that such transitions are not random events but rather driven by as yet unidentified selection mechanisms based on differences in the structure of the O1 antigen or in the serotype-determining wbeT gene.

Cholera is a major health problem in many parts of the world causing seasonal outbreaks in endemic areas. Essentially only the O1 serogroup of Vibrio cholerae causes epidemic cholera. This serogroup has two immunologically distinguishable serotype variants called Ogawa and Inaba. The Inaba serotype is a consequence of a mutation in a single gene, wbeT, that in its intact form encodes for an enzyme that methylates the terminal perosamine sugar of the lipopolysaccharide side chain thus resulting in the Ogawa serotype. By careful examination over a five-year period of the genetic lineages of bacteria causing cholera in an endemic area we show data indicating that serotype switching is not a random process but is driven by selection pressures that have yet to be identified.

A Randomized, Placebo-Controlled Trial Evaluating Safety and Immunogenicity of the Killed, Bivalent, Whole-Cell Oral Cholera Vaccine in Ethiopia

Killed whole-cell oral cholera vaccine (OCV) has been a key component of a comprehensive package including water and sanitation measures for recent cholera epidemics. The vaccine, given in a two-dose regimen, has been evaluated in a large number of human volunteers in India, Vietnam, and Bangladesh, where it has demonstrated safety, immunogenicity, and clinical efficacy. We conducted a double-blind randomized placebo-controlled trial in Ethiopia, where we evaluated the safety and immunogenicity of the vaccine in 216 healthy adults and children. OCV was found to be safe and elicited a robust immunological response against Vibrio cholerae O1, with 81% adults and 77% children demonstrating seroconversion 14 days after the second dose of vaccine. This is the first study to evaluate safety and immunogenicity of the vaccine in a population outside Asia using a placebo-controlled, double-blind, randomized study design.

Development of stable Vibrio cholerae O1 Hikojima type vaccine strains co-expressing the Inaba and Ogawa lipopolysaccharide antigens

We describe here the development of stable classical and El Tor V. cholerae O1 strains of the Hikojima serotype that co–express the Inaba and Ogawa antigens of O1 lipopolysaccharide (LPS). Mutation of the wbeT gene reduced LPS perosamine methylation and thereby gave only partial transformation into Ogawa LPS on the cell surface. The strains express approximately equal amounts of Inaba– and Ogawa–LPS antigens which are preserved after formalin–inactivation of the bacteria. Oral immunizations of both inbred and outbred mice with formalin–inactivated whole–cell vaccine preparations of these strains elicited strong intestinal IgA anti–LPS as well as serum vibriocidal antibody responses against both Inaba and Ogawa that were fully comparable to the responses induced by the licensed Dukoral vaccine. Passive protection studies in infant mice showed that immune sera raised against either of the novel Hikojima vaccine strains protected baby mice against infection with virulent strains of both serotypes. This study illustrates the power of using genetic manipulation to improve the properties of bacteria strains for use in killed whole–cell vaccines.

Circulating mucosal associated invariant T cells are activated in Vibrio cholerae O1 infection and associated with lipopolysaccharide antibody responses

Background

Mucosal Associated Invariant T (MAIT) cells are innate-like T cells found in abundance in the intestinal mucosa, and are thought to play a role in bridging the innate-adaptive interface.

Methods

We measured MAIT cell frequencies and antibody responses in blood from patients presenting with culture-confirmed severe cholera to a hospital in Dhaka, Bangladesh at days 2, 7, 30, and 90 of illness.

Results

We found that MAIT (CD3⁺CD4⁻CD161^hiVα7.2⁺) cells were maximally activated at day 7 after onset of cholera. In adult patients, MAIT frequencies did not change over time, whereas in child patients, MAITs were significantly decreased at day 7, and this decrease persisted to day 90. Fold changes in MAIT frequency correlated with increases in LPS IgA and IgG, but not LPS IgM nor antibody responses to cholera toxin B subunit.

Conclusions

In the acute phase of cholera, MAIT cells are activated, depleted from the periphery, and as part of the innate response against V. cholerae infection, are possibly involved in mechanisms underlying class switching of antibody responses to T cell-independent antigens.

Vibrio cholerae is the bacterium that causes cholera, which can be a potentially fatal diarrheal disease that affects millions of people worldwide each year. How our immune system provides protection against cholera is poorly understood. Mucosal Associated Invariant T (MAIT) cells are recently discovered immune cells found in the blood and intestinal tract of humans. In this study of cholera patients in Dhaka, Bangladesh, we found that blood MAIT cells are activated during cholera, and that in children, blood MAIT cells are decreased in number during the course of disease. We also found that the MAIT cell response correlates with the antibody response to V. cholerae O1 lipopolysaccharide, which in the past has been shown to be an important determinant of protection. These findings suggest that MAIT cells may play an important role in the body's defense against cholera.

Immunogenicity of a killed bivalent (O1 and O139) whole cell oral cholera vaccine, Shanchol, in Haiti

Background

Studies of the immunogenicity of the killed bivalent whole cell oral cholera vaccine, Shanchol, have been performed in historically cholera-endemic areas of Asia. There is a need to assess the immunogenicity of the vaccine in Haiti and other populations without historical exposure to Vibrio cholerae.

Methodology/Principal Findings

We measured immune responses after administration of Shanchol, in 25 adults, 51 older children (6–17 years), and 47 younger children (1–5 years) in Haiti, where cholera was introduced in 2010. A≥4-fold increase in vibriocidal antibody titer against V. cholerae O1 Ogawa was observed in 91% of adults, 74% of older children, and 73% of younger children after two doses of Shanchol; similar responses were observed against the Inaba serotype. A≥2-fold increase in serum O-antigen specific polysaccharide IgA antibody levels against V. cholerae O1 Ogawa was observed in 59% of adults, 45% of older children, and 61% of younger children; similar responses were observed against the Inaba serotype. We compared immune responses in Haitian individuals with age- and blood group-matched individuals from Bangladesh, a historically cholera-endemic area. The geometric mean vibriocidal titers after the first dose of vaccine were lower in Haitian than in Bangladeshi vaccinees. However, the mean vibriocidal titers did not differ between the two groups after the second dose of the vaccine.

Conclusions/Significance

A killed bivalent whole cell oral cholera vaccine, Shanchol, is highly immunogenic in Haitian adults and children. A two-dose regimen may be important in Haiti, and other populations lacking previous repeated exposures to V. cholerae.

Studies evaluating the ability of the killed bivalent whole cell oral cholera vaccine, Shanchol, to elicit an immune response have been performed in historically cholera-endemic areas of Asia. There is a need to assess whether the vaccine is able to elicit an immune response in Haiti and other populations without historical exposure to cholera. In this study, we measure immune responses after administration of Shanchol, in 25 adults, 51 older children (6–17 years), and 47 younger children (1–5 years) in Haiti, where cholera was introduced in 2010. A killed bivalent whole cell oral cholera vaccine (Shanchol) is capable of inducing an immune response in adults and children living in Haiti. However, a two-dose regimen may be important in Haiti and other populations lacking historical exposure to cholera.

Safety and immunogenicity assessment of an oral cholera vaccine through phase I clinical trial in Korea

The safety, tolerability and immunogenicity of an oral cholera vaccine (OCV) was assessed in adult Korean male through an open-label, non-comparative clinical study. Two doses of vaccine with an interval of 2 weeks were given to 20 healthy subjects. A total of 7 adverse events occurred in 6 subjects. However, no clinically significant change was observed in electrocardiograms, vital signs, physical examinations, and clinical laboratory tests. The immunogenicity of OCV was evaluated by serum vibriocidal assay where anti-Vibrio cholerae O1 and O139 antibodies were measured at day 0, 14, and 28 of vaccine administration. The antibody titers ranged from < 2.5-5,120 for V. cholerae O1 Inaba, < 2.5-10,240 for V. cholerae O1 Ogawa and < 2.5-480 for V. cholerae O139. In addition, the fold increase in antibody titers ranged from 1-4,096 for O1 Inaba, 1-8,192 for O1 Ogawa, and 1-384 for O139. The seroconversion rate was 95% and 45% for O1 and O139 antibodies, respectively. Our study clearly shows that administration of two doses of OCV at a 2 week-interval increases an appropriate level of antibody titer in the serum of healthy Korean adult males (Clinical Trial Number, NCT01707537).

Postgenomic approaches to cholera vaccine development

Cholera remains an important public health threat. A cholera vaccine that provides durable protection at the mucosal surface, especially among children in endemic settings, is urgently needed. The availability of the complete genome sequence of a clinical isolate of Vibrio cholerae O1 El Tor has allowed for comparative and functional genomic approaches in the study of cholera. This work holds promise for the identification of bacterial targets of protective human immune responses and may contribute to the development of a new generation of cholera vaccines.

Immune adjuvant effect of V. cholerae O1 derived Proteoliposome coadministered by intranasal route with Vi polysaccharide from Salmonella Typhi

The proteoliposome derived from Vibrio cholerae O1 (PLc) is a nanoscaled structure obtained by a detergent extraction process. Intranasal (i.n) administration of PLc was immunogenic at mucosal and systemic level vs. V. cholerae; however the adjuvant potential of this structure for non-cholera antigens has not been proven yet. The aim of this work was to evaluate the effect of coadministering PLc with the Vi polysaccharide antigen (Poli Vi) of S. Typhi by the i.n route. The results showed that Poli Vi coadministered with PLc (PLc+Poli Vi) induce a higher IgA response in saliva (p<0.01) and faeces (p<0.01) than Poli Vi administered alone. Likewise, the IgG response in sera was higher in animals immunised with PLc+Poli Vi (p<0.01). Furthermore, IgG induced in sera of mice immunised with PLc+Poli Vi was similar (p>0.05) to that induced in a group of mice immunised by the parenteral route with the Cuban anti-typhoid vaccine vax-TyVi, although this vaccine did not induce a mucosal response. In conclusion, this work demonstrates that PLc can be used as a mucosal adjuvant to potentiate the immune response against a polysaccharide antigen like Poli Vi.

[The optimization of conditions of diagnostic fluorescent monoclonal immunoglobulins production to identify comma bacillus of serogroups O1 and O139]

The article considers, the issue of producing the species-specific fluorescent monoclonal immunoglobulins to detect comma bacillus of serogrups O1 and O139 in the reaction of direct immunfluorescence. It is established that not only ascitic but culture fluid too can be the source of monoclonal antibodies for producing fluorescent conjugates. The optimal conditions are selected to produce the fluorescent monoclonal immunoglobulins-monoclonal antibodies. The corresponding producing techology can be reproduced at any time in view of availability of hybrid-producers of monoclonal antibodies O1 and monoclonal antibodies O139 in the institute cryodepositoty. The results of testing the fluorescent preparations on homologous and heterologous strains demonstrated their strict specificiy and high sensibility regarding comma bacillus of serogroups O1 and O139. The new preparations favor significant increase of effectiveness of diagnostics of V. cholerae O1 and O139.

Phase variable O antigen biosynthetic genes control expression of the major protective antigen and bacteriophage receptor in Vibrio cholerae O1

The Vibrio cholerae lipopolysaccharide O1 antigen is a major target of bacteriophages and the human immune system and is of critical importance for vaccine design. We used an O1-specific lytic bacteriophage as a tool to probe the capacity of V. cholerae to alter its O1 antigen and identified a novel mechanism by which this organism can modulate O antigen expression and exhibit intra-strain heterogeneity. We identified two phase variable genes required for O1 antigen biosynthesis, manA and wbeL. manA resides outside of the previously recognized O1 antigen biosynthetic locus, and encodes for a phosphomannose isomerase critical for the initial step in O1 antigen biosynthesis. We determined that manA and wbeL phase variants are attenuated for virulence, providing functional evidence to further support the critical role of the O1 antigen for infectivity. We provide the first report of phase variation modulating O1 antigen expression in V. cholerae, and show that the maintenance of these phase variable loci is an important means by which this facultative pathogen can generate the diverse subpopulations of cells needed for infecting the host intestinal tract and for escaping predation by an O1-specific phage.

Individuals with Le(a+b-) blood group have increased susceptibility to symptomatic vibrio cholerae O1 infection

BACKGROUND

Human genetic factors such as blood group antigens may affect the severity of infectious diseases. Presence of specific ABO and Lewis blood group antigens has been shown previously to be associated with the risk of different enteric infections. The aim of this study was to determine the relationship of the Lewis blood group antigens with susceptibility to cholera, as well as severity of disease and immune responses to infection.

METHODOLOGY

We determined Lewis and ABO blood groups of a cohort of patients infected by Vibrio cholerae O1, their household contacts, and healthy controls, and analyzed the risk of symptomatic infection, severity of disease if infected and immune response following infection.

PRINCIPAL FINDINGS

We found that more individuals with cholera expressed the Le(a+b-) phenotype than the asymptomatic household contacts (OR 1.91, 95% CI 1.03-3.56) or healthy controls (OR 1.90, 95% CI 1.13-3.21), as has been seen previously for the risk of symptomatic ETEC infection. Le(a-b+) individuals were less susceptible to cholera and if infected, required less intravenous fluid replacement in hospital, suggesting that this blood group may be associated with protection against V. cholerae O1. Individuals with Le(a-b-) blood group phenotype who had symptomatic cholera had a longer duration of diarrhea and required higher volumes of intravenous fluid replacement. In addition, individuals with Le(a-b-) phenotype also had lessened plasma IgA responses to V. cholerae O1 lipopolysaccharide on day 7 after infection compared to individuals in the other two Lewis blood group phenotypes.

CONCLUSION

Individuals with Lewis blood type Le(a+b-) are more susceptible and Le(a-b+) are less susceptible to V. cholerae O1 associated symptomatic disease. Presence of this histo-blood group antigen may be included in evaluating the risk for cholera in a population, as well as in vaccine efficacy studies, as is currently being done for the ABO blood group antigens.

Simple, direct conjugation of bacterial O-SP-core antigens to proteins: development of cholera conjugate vaccines

Bacterial O-SP-core antigens can be conjugated to proteins in the same, simple way as synthetic, linker-equipped carbohydrates by applying squaric acid chemistry. Introduction of spacers (linkers) to either O-SP-core antigens or protein carriers, which is involved in commonly applied protocols, is not required. The newly developed method described here consists of preparation of a squaric acid monoester derivative of O-SP-core antigen, utilizing the amino group inherent in the core, and reaction of the monoester with the carrier protein. The intermediate monoester can be easily purified; its conjugation can be monitored by SELDI-TOF mass spectrometry and, thus, readily controlled, since the conjugation can be terminated when the desired carbohydrate-protein ratio is reached. Here, we describe production of conjugates containing the O-SP-core antigen of Vibrio cholerae O1, the major cause of cholera, a severe dehydrating diarrheal disease of humans. The resultant products are recognized by convalescent phase sera from patients recovering from cholera in Bangladesh, and anti-O-SP-core-protein responses correlate with plasma antilipopolysaccharide and vibriocidal responses, which are the primary markers of protection from cholera. The results suggest that such conjugates have potential as vaccines for cholera and other bacterial diseases.

Safety and immunogenicity study of a killed bivalent (O1 and O139) whole-cell oral cholera vaccine Shanchol, in Bangladeshi adults and children as young as 1 year of age

BACKGROUND

Safety and immunogenicity study of an oral, killed, bivalent whole-cell, cholera vaccine, Shanchol was carried out in Bangladeshi participants. This study was conducted prior to initiating a feasibility study in Bangladesh.

STUDY PARTICIPANTS

The double-blind, randomized placebo controlled study was carried out in adults (18-45 years), toddlers (2-5 years) and younger children (12-23 months). Two doses of the vaccine/placebo were given 14 days apart.

RESULTS

Shanchol did not elicit major adverse events in any age group. Vibriocidal antibody responses in adults were 60% against Vibrio cholerae O1 Inaba, 72% against V. cholerae O1 Ogawa and 21% against V. cholerae O139. In toddlers, responses were 84%, 75% and 64% and in younger children it was 74%, 78% and 54% against Inaba, Ogawa and O139 serotypes. The responses in all ages were higher in vaccinees compared to pre-immune titers or to responses in placebo recipients (P<0.001). Plasma IgA antibody response to O1 Inaba LPS was seen in 61%, 73% and 45% of adults, toddlers and younger children, respectively.

CONCLUSIONS

The safety and immunogenicity data for Shanchol is promising and warrants future use in large scale trial in cholera endemic areas, high risk Bangladeshi population and in other countries in the region.

[Evaluation of toxin producing abilities of non-O1/non-O139 serogroup Vibrio cholerae isolated from humans]

AIM

Determination of non-O1/non-O139 Vibrio cholerae toxin (CT) gene expression by using EIA, and biological effect of non-O1/non-O139 V. cholerae supernatant on cell cultures evaluation.

MATERIALS AND METHODS

39 V. cholerae strains from various serological groups were studied. Hemolytic activity of strains was determined by using Greig test, and cholera toxin production--in GM1-EIA and in continuous cell lines by registering cytotonic, cytotoxic and proteolitic effect.

RESULTS

GM1-EIA method does not detect CT production in 29 museum strains of non-O1/non-O139 V. cholerae in vitro. CT was detected only in 1 non-O1/non-O139 V. cholerae strain supernatant with OD = 0.577 that is substantially lower than in O1 V. cholerae strains (OD = 2.176). In cell cultures non-O1/non-O139 V. cholerae supernatants diluted to 1:100 caused elongation only in single cells.

CONCLUSION

Cytological model is a more sensitive technique to evaluate toxin producing abilities of non-O1/non-O139 V. cholerae strains and is appropriate for use.

[Comparative assessment of dot-immunoassay and immunochromatography methods for detection of 01 serogroup of vibrio cholerae]

AIM

Comparative study of sensitivity and specificity of immunochromatographic (IC) assay kit and dot-immunoanalysis for assessment of feasibility of their use for laboratory diagnostics of cholera.

MATERIALS AND METHODS

Experimental lots of IC assay kit and dot-immunoassay (DIA) for detection of Vibrio cholerae serogroup 01 serovars Ogava and Inaba were constructed on the basis of species-specific monoclonal antibodies (MCA) conjugated with colloid gold (IC) and peroxidase (DIA). Hybridoma-producer of MCAwas obtained and stored in liquid nitrogen in Rostov-on-Don Research Institute for Plague Control. It was deposited in specialized collection of cell cultures of vertebrates in Institute of Cytology (Saint Petersburg).

RESULTS

Strong specificity of IC assay kit and DIA relative to cholera vibrios 01 and absence of crossreactivity with closely related and heterologous microorganisms were shown. Minimal quantity of vibrios, which could be detected using IC assay kit and DIA, was 107 and 105-106 microbial cells respectively.

CONCLUSION

Performance of IC assay takes 5-15 min, DIA--1.5 hour, they allow to visually assess the reaction, do not require instrumentation and in perspective both methods could be used on defined stages of scheme for laboratory analysis of cholera.

Diagnostic limitations to accurate diagnosis of cholera

The treatment regimen for diarrhea depends greatly on correct diagnosis of its etiology. Recent diarrhea outbreaks in Bangladesh showed Vibrio cholerae to be the predominant cause, although more than 40% of the suspected cases failed to show cholera etiology by conventional culture methods (CMs). In the present study, suspected cholera stools collected from every 50th patient during an acute diarrheal outbreak were analyzed extensively using different microbiological and molecular tools to determine their etiology. Of 135 stools tested, 86 (64%) produced V. cholerae O1 by CMs, while 119 (88%) tested positive for V. cholerae O1 by rapid cholera dipstick (DS) assay; all but three samples positive for V. cholerae O1 by CMs were also positive for V. cholerae O1 by DS assay. Of 49 stools that lacked CM-based cholera etiology despite most being positive for V. cholerae O1 by DS assay, 25 (51%) had coccoid V. cholerae O1 cells as confirmed by direct fluorescent antibody (DFA) assay, 36 (73%) amplified primers for the genes wbe O1 and ctxA by multiplex-PCR (M-PCR), and 31 (63%) showed El Tor-specific lytic phage on plaque assay (PA). Each of these methods allowed the cholera etiology to be confirmed for 97% of the stool samples. The results suggest that suspected cholera stools that fail to show etiology by CMs during acute diarrhea outbreaks may be due to the inactivation of V. cholerae by in vivo vibriolytic action of the phage and/or nonculturability induced as a host response.

Effects of enteropathogenic bacteria & lactobacilli on chemokine secretion & Toll like receptor gene expression in two human colonic epithelial cell lines

BACKGROUND & OBJECTIVE

The intestinal epithelium is part of the innate immune system responding to contact with pathogenic or commensal bacteria. The objective of this study was to compare innate responses of intestinal epithelial cell lines to pathogenic bacteria and to lactobacilli.

METHODS

Two human intestinal epithelial cell lines, HT29 (enterocyte-like) and T84 (crypt-like), were exposed to pathogenic bacteria representative of non invasive (Vibrio cholerae O1 and O139), adherent (enterohaemorrhagic Escherichia coli, EHEC) or invasive (Salmonella Typhimurium and Shigella flexneri) phenotypes and to non pathogenic Lactobacillus rhamnosus GG or Lactobacillus plantarum. Interleukin-8 (IL-8) was measured in culture supernatant by ELISA, while mRNA from cells was subjected to quantitative reverse transcriptase PCR for several other chemokines (CXCL1, CCL5 and CXCL5) and for Toll-like receptors (TLR) 2, 4, 5 and 9.

RESULTS

V. cholerae, S. Typhimurium, S. flexneri and EHEC induced IL-8 secretion from epithelial cells into the medium. Salmonella, Shigella and EHEC, but not V. cholerae, significantly increased mRNA expression of CXCL1. None of the pathogens induced CCL5 or CXCL5. Salmonella and Vibrio significantly increased TLR4 expression, while Vibrio and EHEC decreased TLR5 expression. EHEC also decreased TLR9 expression. Lactobacilli attenuated the IL-8 response of the cell lines to V. cholerae, Salmonella, and EHEC but did not significantly change the IL-8 response to Shigella.

INTERPRETATION & CONCLUSION

Distinct patterns of epithelial cell chemokine responses were induced by the bacterial pathogens studied and these were modulated by commensal lactobacilli. Alterations in TLR expression by these pathogens are likely to be important in pathogenesis.

Characterization of a novel protective monoclonal antibody that recognizes an epitope common to Vibrio cholerae Ogawa and Inaba serotypes

A novel protective monoclonal antibody (mAb) that recognizes a lipopolysaccharide (LPS) epitope common between serotypes Ogawa and Inaba of the O1 serogroup of Vibrio cholerae was characterized and the potential to develop peptide mimics of this protective LPS epitope was investigated. mAb 72.1 recognizes both Ogawa and Inaba LPS and it is vibriocidal and protective in passive immunization against infection by strains of both serotypes. The cDNA-derived amino acid sequence of mAb 72.1 is closely related to the previously characterized mAb ZAC-3, which is thought to recognize an epitope in the lipid A core region of O1 LPS. In an attempt to develop a peptide mimic-based vaccine against V. cholerae, phage display libraries were screened with mAb 72.1 and 11 peptide mimics were identified. Remarkably, all of the peptide sequences identified from linear phage display libraries contained two cysteine residues, suggesting that mAb 72.1 preferentially binds to peptides constrained with a disulphide bond. One of the peptide mimics was immunologically characterized. Although immunization of mice with this peptide mimic conjugated to KLH elicited antibodies against the peptide itself, these antibodies did not cross-react with Ogawa or Inaba LPS. Effectiveness of a peptide mimic as a vaccine may depend on how well the peptide can mimic the carbohydrate interactions when binding to the anti-carbohydrate antibody. Thus, investigating how peptides and LPS bind to mAb 72.1 may be useful in improving current peptide mimics or designing more effective peptide mimics. Identification and characterization of novel protective anti-LPS antibodies may be useful in studying protective epitopes of LPS, which may help develop LPS-based therapeutics against V. cholerae.

A comparison of clinical and immunologic features in children and older patients hospitalized with severe cholera in Bangladesh

BACKGROUND

: Infection with Vibrio cholerae induces protection from subsequent severe disease, suggesting that an effective vaccine could be an important preventive strategy. Available vaccines provide less protection against cholera than natural infection, particularly in children.

METHODS

: We examined a cohort of 121 children (2 years-12 years of age) and 276 older patients (>12 years of age) hospitalized with cholera in Dhaka, Bangladesh over a 4-year period, to compare clinical features in older patients and children and immune responses to key antigens.

RESULTS

: Older patients had more severe disease. Children with cholera were more commonly retinol deficient, while zinc deficiency was equally prevalent in both groups. Children developed higher vibriocidal and serum immune responses to the B subunit of cholera toxin (CTB). In contrast, older patients mounted higher immune responses to 2 other key V. cholerae antigens, the lipopolysaccharide (LPS) and toxin coregulated pilus antigens (TcpA). We compared immune responses following infection with those occurring after receipt of a live, oral vaccine in both children and older patients in Bangladesh, during a similar time period. The response rates for vibriocidal and LPS antibodies were higher after infection than after vaccination. Both vaccinated older patients and children responded poorly to CTB and TcpA.

CONCLUSIONS

: Although children developed vigorous vibriocidal and CTB-specific responses following infection, they had lessened responses to LPS and TcpA compared with older patients, as well as lessened responses to vaccination. More studies need to be carried out to determine factors, including micronutrient interventions that can improve responses in children to both natural infection and vaccination.

Biofilm acts as a microenvironment for plankton-associated Vibrio cholerae in the aquatic environment of Bangladesh

The role of biofilm as a microenvironment of plankton-associated Vibrio cholerae was investigated using plexiglass as a bait. A total of 72 biofilm samples were tested using culture, direct fluorescent antibody (DFA) and molecular techniques following standard procedures. Culturable V. cholerae (smooth and rugose variants) were isolated from 33% of the samples. V. cholerae O1 were detected by FA technique throughout the year except April and June. All V. cholerae O1 isolates were positive for tcpA, ctxA and ace genes while V. cholerae non-O1, non-O139 isolates lacked these genes. V. cholerae O1 (both Inaba and Ogawa) strains had identical ribotype pattern (R1), but V. cholerae non-O1, non-O139 had different ribotype patterns. All V. cholerae O1 strains were resistant to vibrio-static compound (O/129). All V. cholerae O1 except one were resistant to trimethoprime-sulphamethoxazole, streptomycin, nalidixic acid and furazolidone but sensitive to ciprofloxacin, and tetracycline. This study indicates that plexiglass can act as a bait to form biofilm, a microenvironment that provides shelter for plankton containing V. cholerae in the aquatic environment of Bangladesh.

[Complex morphological approach to assessment of protective properties of preparations for cholera specific prophylaxis]

Using developed scheme, complex study of protective properties of avirulent recombinant strain Vibrio cholerae El Tor Inaba KM 184 was performed. Necessity for broadening of standard procedure of testing of cholera vaccines protective properties by using of quantitative methods of assessment of morphological changes and state of biomodel's functional systems, which could increase the information value of assessment of studied vaccines, was experimentally substantiated.

Hemolysin and the multifunctional autoprocessing RTX toxin are virulence factors during intestinal infection of mice with Vibrio cholerae El Tor O1 strains

The seventh cholera pandemic that started in 1961 was caused by Vibrio cholerae O1 strains of the El Tor biotype. These strains produce the pore-forming toxin hemolysin, a characteristic used clinically to distinguish classical and El Tor biotypes. Even though extensive in vitro data on the cytolytic activities of hemolysin exist, the connection of hemolysin to virulence in vivo is not well characterized. To study the contribution of hemolysin and other accessory toxins to pathogenesis, we utilized the model of intestinal infection in adult mice sensitive to the actions of accessory toxins. In this study, we showed that 4- to 6-week-old streptomycin-fed C57BL/6 mice were susceptible to intestinal infection with El Tor strains, which caused rapid death at high doses. Hemolysin had the predominant role in lethality, with a secondary contribution by the multifunctional autoprocessing RTX (MARTX) toxin. Cholera toxin and hemagglutinin/protease did not contribute to lethality in this model. Rapid death was not caused by increased dissemination due to a damaged epithelium since the numbers of CFU recovered from spleens and livers 6 h after infection did not differ between mice inoculated with hemolysin-expressing strains and those infected with non-hemolysin-expressing strains. Although accessory toxins were linked to virulence, a strain defective in the production of accessory toxins was still immunogenic since mice immunized with a multitoxin-deficient strain were protected from a subsequent lethal challenge with the wild type. These data suggest that hemolysin and MARTX toxin contribute to vaccine reactogenicity but that the genes for these toxins can be deleted from vaccine strains without affecting vaccine efficacy.

A Vibrio cholerae serogroup O1 vaccine candidate against CTX ET Phi infection

Cholera is a severe diarrheal disease that may spread rapidly. Vaccination is considered a valid measure against it. We developed a new vaccine candidate, IEM109, against Vibrio cholerae. To generate this candidate, a chromosomal fragment containing the TLC element, attB of the CTX Phi integration site, and RTX cluster responsible for the cytotoxic activity for mammalian cells was deleted through homologous recombination from the previously described El Tor biotype, IEM101. The protective genes ctxB and rstR, which establish resistance to CTX Phi infections, were inserted into that same location on the chromosome of IEM109 to enhance the safety and genetic stability of the vaccine candidate and to prevent horizontal gene transfer. In in vivo tests, cell cultures showed that the cytotoxic effect of IEM109 on Hep-2 was negative. Furthermore, the infection rate of El Tor biotype CTX Phi to that of IEM109 in the rabbit intestine is 3000-fold lower than that of IEM101. Intraintestinal vaccination of rabbits with a single dose of IEM109 elicits high titers of anti-CTB IgG and vibriocidal antibodies. When challenged with 0.5-2 microg CT and 10(5) to 10(8)CFU of four wild toxigenic strains of different biotypes and serogroups, IEM109 conferred full protection. Thus, IEM109 is a stable vaccine candidate that evokes not only antitoxic and vibriocidal immunities, but also resistance to the El Tor biotype CTX Phi infection.

Transcutaneous immunization with toxin-coregulated pilin A induces protective immunity against Vibrio cholerae O1 El Tor challenge in mice

Toxin-coregulated pilin A (TcpA) is the main structural subunit of a type IV bundle-forming pilus of Vibrio cholerae, the cause of cholera. Toxin-coregulated pilus is involved in formation of microcolonies of V. cholerae at the intestinal surface, and strains of V. cholerae deficient in TcpA are attenuated and unable to colonize intestinal surfaces. Anti-TcpA immunity is common in humans recovering from cholera in Bangladesh, and immunization against TcpA is protective in murine V. cholerae models. To evaluate whether transcutaneously applied TcpA is immunogenic, we transcutaneously immunized mice with 100 mug of TcpA or TcpA with an immunoadjuvant (cholera toxin [CT], 50 mug) on days 0, 19, and 40. Mice immunized with TcpA alone did not develop anti-TcpA responses. Mice that received transcutaneously applied TcpA and CT developed prominent anti-TcpA immunoglobulin G (IgG) serum responses but minimal anti-TcpA IgA. Transcutaneous immunization with CT induced prominent IgG and IgA anti-CT serum responses. In an infant mouse model, offspring born to dams transcutaneously immunized either with TcpA and CT or with CT alone were challenged with 10(6) CFU (one 50% lethal dose) wild-type V. cholerae O1 El Tor strain N16961. At 48 h, mice born to females transcutaneously immunized with CT alone had 36% +/- 10% (mean +/- standard error of the mean) survival, while mice born to females transcutaneously immunized with TcpA and CT had 69% +/- 6% survival (P < 0.001). Our results suggest that transcutaneous immunization with TcpA and an immunoadjuvant induces protective anti-TcpA immune responses. Anti-TcpA responses may contribute to an optimal cholera vaccine.

Immunogenicity of synthetic saccharide fragments of Vibrio cholerae O1 (Ogawa and Inaba) bound to Exotoxin A

Recombinant exotoxin A (rEPA) from Pseudomonas aeruginosa conjugated to Vibrio cholerae O1 serotype-specific polysaccharides (mono-, di- and hexasaccharide) were immunogenic in mice. Monosaccharide conjugates boosted the humoral responses to the hexasaccharide conjugates. Prior exposure to purified Ogawa lipopolysaccharide (LPS) enabled contra-serotype hexasaccharide conjugates to boost the vibriocidal response, but Inaba LPS did not prime for an enhanced vibriocidal response by a contra-serotype conjugate. Prior exposure to the carrier, and priming B cells with the LPS of either serotype, resulted in enhanced vibriocidal titers if the Ogawa hexasaccharides were used, but a diminished response to the Inaba LPS. These studies demonstrate that the 'functional' B cell epitopes on the LPS differ from those of the neoglycoconjugates and that the order of immunization and the serotype of the boosting conjugate can influence the epitope specificity and function of the antisera.

Preparation and evaluation of vibrio cholerae O1 EL Tor Ogawa lipopolysaccharide-tetanus toxoid conjugates

The lipopolysaccharide (LPS) of Vibrio cholerae is considered one of the most important antigens from the point of view of immunogenicity in these bacteria. We have undertaken detoxification of this LPS by basic hydrolysis and the resultant amine groups were used for their conjugation to tetanus toxoid as carrier protein using carbodiimide-mediated coupling. The resulting conjugates were inoculated in Balb/c mice for immunogenicity studies. The anti-LPS IgG and vibriocidal antibodies were measured in serum. The antigenicity of this conjugated was evaluated by ELISA, with serums of humans vaccinated with a strain genetically modified. The conjugated elicited: high titers of IgG anti-LPS, high titers of vibriocidal antibodies and there was recognition of LPS by antibodies from cholerae immunised human serum. These results show that the conjugated LPS obtained by us, could be evaluated like a potential vaccine for human use.

Safety and immunogenicity of a reformulated Vietnamese bivalent killed, whole-cell, oral cholera vaccine in adults

Vietnam currently produces an orally administered, bivalent (O1 and O139) killed whole-cell vaccine and is the only country in the world with endemic cholera to use an oral cholera vaccine in public health practice. In order to allow international use, the vaccine had to be reformulated to meet World Health Organization (WHO) requirements. We performed a randomized, placebo controlled, safety and immunogenicity studies of this reformulated vaccine among Vietnamese adults. One hundred and forty-four subjects received the two-dose regimen and 143 had two blood samples obtained for analysis. We found that this reformulated oral killed whole-cell cholera vaccine was safe, well tolerated and highly immunogenic.

Identification of proinflammatory flagellin proteins in supernatants of Vibrio cholerae O1 by proteomics analysis

The genome of Vibrio cholerae contains five flagellin genes that encode proteins (FlaA-E) of 39-41 kDa with 61-82% identity among them. Although the existing live oral attenuated vaccine strains against cholera are protective in humans, there is an intrinsic residual cytotoxic and inflammatory component associated with these candidate vaccine strains. Bacterial flagellins are known to be potent inducers of proinflammatory molecules via activation of Toll-like receptor 5. Here we found that purified flagella from wild type V. cholerae 395 induced significant release of interleukin (IL)-8 from cultured HT-29 human colonic epithelial cells. Furthermore we found that filtered supernatants of KKV90, a DeltaflaA isogenic strain unable to produce flagella, were still able to activate production of IL-8 albeit to significantly lower levels than the wild type, suggesting that other activators of proinflammatory molecules were still present in these supernatants. A comparative proteomics analysis of secreted proteins of V. cholerae 395 and KKV90 identified additional proteins with potential to induce IL-8 release in HT-29 cells. Secreted proteins in the range of 30-45 kDa identified by two-dimensional electrophoresis and mass spectrometry revealed the presence of two additional flagellins, FlaC and FlaD, that appeared to be secreted 3- and 6-fold more, respectively, in the mutant compared with the wild type. Double isogenic mutants flaAC and flaAD were unable to trigger IL-8 release from HT-29 cells. In sum, we have shown that purified flagella and secreted flagellin proteins (FlaC and FlaD) are inducers of IL-8 release from epithelial cells via Toll-like receptor 5. This observation may explain, in part, the observed reactogenicity of cholera vaccine strains in humans.

Immunological properties of complex conjugates based on Vibrio cholerae O1 Ogawa lipopolysaccharide antigen

Host protection by humoral immunity against Vibrio cholerae O1 confers lipopolysaccharide (LPS)-specific vibriocidal antibodies. Levels of relevant specific antibodies are closely related to complement-mediated inactivation of the vibrios inoculum, especially on the mucosal surface of intestine. We have tested complex V. cholerae O1 Ogawa-detoxified lipopolysaccharide (dLPS) conjugates. The first conjugate contained glucan both as the immunomodulator and the matrix; the second conjugate contained immunologically inert amylose as matrix. Both d-LPS conjugates contain multiply attached dLPS antigen. These conjugates elicited a statistically significant increase of antigen-specific IgG levels in mice (P<0.001 and P<0.05, respectively). The specific anti-conjugate IgG and IgA response after the second (booster) dose were significantly higher compared to pre-immune and whole-cell response. The most effective vibriocidal activity was observed in the case of conjugate, with glucan as the matrix. The highest correlation was found between vibriocidal activity and specific IgG2b (r=0.765) and IgA (r=0.887) sera levels. The determination of specific IgG subclasses and IgG2a + 2b/IgG1 ratio revealed a dominant T(H)1 cell response crucial for effective vaccine candidate.

Length of the linker and the interval between immunizations influences the efficacy ofVibrio choleraeO1, Ogawa hexasaccharide neoglycoconjugates

Ogawa hexasaccharide neoglycoconjugates induce protective antibodies in mice. Similar Ogawa conjugates but with a longer linker that connects the carrier to shorter saccharides are immunogenic, but generally ineffective at inducing vibriocidal or protective antibodies. The efficacy of Ogawa hexasaccharide neoglycoconjugates of different linker lengths were tested. The majority of mice given immunizations separated by a 14-day gap did not produce vibriocidal or protective antibodies. Mice immunized 28 days apart with immunogens containing the shortest or medium length linker, but not the longest, produced vibriocidal and protective antibodies. A nonprotective, priming dose of purified Ogawa LPS followed 5 days later with a booster of the Ogawa neoglycoconjugates (di-, tetra-, or hexasaccharide) resulted in vibriocidal antibodies at day 10.

Immunosensor for the detection of Vibrio cholerae O1 using surface plasmon resonance

An immunosensor for the detection of Vibrio cholerae O1 was developed on the basis of surface plasmon resonance (SPR). A protein G layer was fabricated by means of the chemical coupling between the free amine (-NH2) groups of protein G and the activated carboxyl groups present on a self-assembled monolayer (SAM) consisting of a mixture of 11-mercaptoundecanoic acid (MUA) and hexanethiol (molar ratio of 1:2). A monoclonal antibody, which was confirmed to be specific to V. cholera O1 by the Western blotting technique, was immobilized on the protein G layer. The formation of the SAM, the protein G layer and the sequential binding of the antibody against V. cholera O1 were investigated with SPR spectroscopy. As the number of fabricated layers increased, the minimum angle of plasmon resonance was increased accordingly. The target bacteria, V. cholera O1, was measured with the fabricated immunosensor, whose detection range was between 10(5) and 10(9) cells/mL.

Toxigenic Vibrio cholerae in the aquatic environment of Mathbaria, Bangladesh

Toxigenic Vibrio cholerae, rarely isolated from the aquatic environment between cholera epidemics, can be detected in what is now understood to be a dormant stage, i.e., viable but nonculturable when standard bacteriological methods are used. In the research reported here, biofilms have proved to be a source of culturable V. cholerae, even in nonepidemic periods. Biweekly environmental surveillance for V. cholerae was carried out in Mathbaria, an area of cholera endemicity adjacent to the Bay of Bengal, with the focus on V. cholerae O1 and O139 Bengal. A total of 297 samples of water, phytoplankton, and zooplankton were collected between March and December 2004, yielding eight V. cholerae O1 and four O139 Bengal isolates. A combination of culture methods, multiplex-PCR, and direct fluorescent antibody (DFA) counting revealed the Mathbaria aquatic environment to be a reservoir for V. cholerae O1 and O139 Bengal. DFA results showed significant clumping of the bacteria during the interepidemic period for cholera, and the fluorescent micrographs revealed large numbers of V. cholerae O1 in thin films of exopolysaccharides (biofilm). A similar clumping of V. cholerae O1 was also observed in samples collected from Matlab, Bangladesh, where cholera also is endemic. Thus, the results of the study provided in situ evidence for V. cholerae O1 and O139 in the aquatic environment, predominantly as viable but nonculturable cells and culturable cells in biofilm consortia. The biofilm community is concluded to be an additional reservoir of cholera bacteria in the aquatic environment between seasonal epidemics of cholera in Bangladesh.

Direct immunofluorescence assay for rapid environmental detection of Vibrio cholerae O1

An immunofluorescence assay for direct detection of V. cholerae O1 was developed using polyclonal antibodies raised against outer membrane proteins (OMPs) of V. cholerae O1. Production of OMPs varied with growth media used; maximum production was found in tryptic soy broth. The detection system was specific because no cross-reactivity was observed with other bacteria including V. cholerae O139, E. coli, S. dysenteriae and Salmonella enterica subsp. enterica serovar Typhi. The technique was able to detect 240 CFU/mL of V. cholerae O1 suspended in phosphate-buffered saline. The assay coupled with bacterial enrichment in APW for 6 h detected as few as 5 CFU of V. cholerae in spiked samples. Moreover, a 2-h incubation of enriched bacterial cells in 0.1% yeast extract with 10 ppm nalidixic acid enhanced the bacterial size and helped in morphological identification of V. cholerae. Among 32 potable water samples from afflicted hand pumps and wells collected from a cholera-plagued area 12 were found to be contaminated with V. cholerae by immunofluorescence assay as well as by conventional culture methods. The proposed method could thus be employed in environmental surveillance of V. cholerae O1.

[Development of antilipase immunoglobulin polymer diagnosticum for the detection of Vibrio cholerae eltor, possessing hemolytic and lipase activity]

The possibility of using a heterogeneous, but structurally similar antigen--the commercial preparation of Pseudomonas sp. lipase (Sigma, USA)--for the development of polymer diagnosticum aimed at determination of lipase production in cholera vibrios was shown. The new diagnosticum (antilipase antibodies) on a polymer carrier was used in the serological volume agglomeration test for the detection of hemolytic atoxigenic V. eltor, obtained from environmental, objects, which produced lipase in 80% of cases. The differentiating capacity of the diagnosticum was confirmed on 120 V. eltor cultures isolated from environmental objects. The newly developed diagnosticum makes it possible to determine the lipase activity in cholera vibrios of different biovars and serovars.

Effect of saccharide length on the immunogenicity of neoglycoconjugates from synthetic fragments of the O-SP of Vibrio cholerae O1, serotype Ogawa

A synthetic hexasaccharide, identical to the terminal hexasaccharide of Ogawa LPS, coupled to bovine serum albumin induced protective antibodies in mice. To determine if there was a minimum saccharide length required for immunogenicity and efficacy, shorter (mono- to pentasaccharide) neoglycoconjugates (CHO-BSA) were tested in mice. The Ogawa CHO-BSA was inoculated at either a constant mass but differing moles, or equal moles but differing masses. Humoral responses were essentially the same when mice received 9 microg of the carbohydrate (0.007 mM with the pentasaccharide) in each of the neoglycoconjugates prepared from mono- through the pentasaccharide, or the same molar amount (0.007 mM), proportionally less by weight when going from the penta- to the monosaccharide. These data show that, within this dose range, the responses occurred virtually independently of the amount of immunogen. Humoral antibodies induced by these immunogens were generally not vibriocidal. Selected antisera induced by CHO-BSA immunogens were protective, but the ELISA titers of the sera were not predictive of the protective capacity. Purified, Ogawa LPS induced anti-Ogawa LPS IgM antibody titers similar to those induced by the Ogawa CHO-BSA conjugates. The anti-whole LPS sera were strongly vibriocidal, as were the previously reported sera induced by hexasaccharide conjugates. This suggests either that the shorter oligosaccharides lack a conformational epitope provided by the hexasaccharide or that the LPS has additional B cell epitopes or selects different B cells in the primary response.