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

Preparation and Evaluation of a New Lipopolysaccharide-based Conjugate as a Vaccine Candidate for Brucellosis

Objectives

Development of an efficacious vaccine against brucellosis has been a challenge for scientists for many years. At present, there is no licensed vaccine against human brucellosis. To overcome this problem, currently, antigenic determinants of Brucella cell wall such as Lipopolysaccharide (LPS) are considered as potential candidates to develop subunit vaccines.

Methods

In this study, Brucella abortus LPS was used for conjugation to Neisseria meningitidis serogroup B outer membrane vesicle (OMV) as carrier protein using carbodiimide and adipic acid–mediated coupling and linking, respectively. Groups of eight BALB/c mice were injected subcutaneously with 10 μg LPS alone, combined LPS + OMV and conjugated LPS–OMV on 0 days, 14 days, 28 days and 42 days. Anti-LPS IgG was measured in serum.

Results

The yield of LPS to OMV in LPS–OMV conjugate was 46.55%, on the basis of carbohydrate content. The ratio for LPS to OMV was 4.07. The LPS–OMV conjugate was the most immunogenic compound that stimulated following the first injection with increased IgG titer of ∼5-fold and ∼1.3-fold higher than that produced against LPS and LPS in noncovalent complex to OMV (LPS + OMV), respectively. The highest anti-LPS IgG titer was detected 2 weeks after the third injection (Day 42) of LPS–OMV conjugate. The conjugated compound elicited higher titers of IgG than LPS + OMV, that showed a 100–120-fold rise of anti-LPS IgG in mice.

Conclusion

These results indicate that our conjugated LPS–OMV can be used as a brucellosis vaccine, but further investigation is required.

Lipopolysaccharide modifications of a cholera vaccine candidate based on outer membrane vesicles reduce endotoxicity and reveal the major protective antigen

The causative agent of the life-threatening gastrointestinal infectious disease cholera is the Gram-negative, facultative human pathogen Vibrio cholerae. We recently started to investigate the potential of outer membrane vesicles (OMVs) derived from V. cholerae as an alternative approach for a vaccine candidate against cholera and successfully demonstrated the induction of a long-lasting, high-titer, protective immune response upon immunization with OMVs using the mouse model. In this study, we present immunization data using lipopolysaccharide (LPS)-modified OMVs derived from V. cholerae, which allowed us to improve and identify the major protective antigen of the vaccine candidate. Our results indicate that reduction of endotoxicity can be achieved without diminishing the immunogenic potential of the vaccine candidate by genetic modification of lipid A. Although the protective potential of anti-LPS antibodies has been suggested many times, this is the first comprehensive study that uses defined LPS mutants to characterize the LPS-directed immune response of a cholera vaccine candidate in more detail. Our results pinpoint the O antigen to be the essential immunogenic structure and provide a protective mechanism based on inhibition of motility, which prevents a successful colonization. In a detailed analysis using defined antisera, we can demonstrate that only anti-O antigen antibodies, but not antibodies directed against the major flagellar subunit FlaA or the most abundant outer membrane protein, OmpU, are capable of effectively blocking the motility by binding to the sheathed flagellum and provide protection in a passive immunization assay.

Vaccines for preventing cholera: killed whole cell or other subunit vaccines (injected)

BACKGROUND

Injected cholera vaccines are rarely used today, although they may have some benefit. It is valuable to summarize the evidence for effectiveness of injected cholera vaccines for comparison with newer oral vaccines (subject of a separate Cochrane Review).

OBJECTIVES

To evaluate killed whole cell (KWC) cholera vaccines and other inactive subunit vaccines (administered by injection) for preventing cholera and death, and to evaluate the adverse effects.

SEARCH STRATEGY

In September 2008, we searched the Cochrane Infectious Diseases Group Specialized Register, CENTRAL (The Cochrane Library 2008, Issue 3), EMBASE, and LILACS. We also searched reference lists and handsearched the journal Vaccine up to 1997.

SELECTION CRITERIA

Randomized and quasi-randomized controlled trials comparing injected cholera vaccines (KWC or other inactive subunit) with placebo, control vaccines, or no intervention in adults and children irrespective of immune status or special risk category.

DATA COLLECTION AND ANALYSIS

Two authors extracted data and assessed trial methodological quality independently. Dichotomous data were reported using the risk ratio (RR) with 95% confidence intervals (CI). Vaccine efficacies were also calculated (% vaccine efficacy = (1-RR) x 100%).

MAIN RESULTS

Sixteen trials, involving over one million adults, children and infants, fulfilled the inclusion criteria. Twenty-four comparisons reported on vaccine efficacy (cholera cases and/or deaths) and 11 comparisons considered adverse effects (nine reported on both). Compared to placebo, vaccinees had a reduced risk of death from cholera (RR 0.49, 95% CI 0.25 to 0.93; 837,442 participants) and a reduced risk of contracting cholera at 12 months (RR 0.52, 95% CI 0.42 to 0.65, random-effects model; 1,512,573 participants). This translates to an efficacy of 48%, 95% confidence interval 35% to 58%. Significant protection lasted for two years, even after only a single dose, and for three years with an annual booster. Children over five years and adults were protected for up to three years, while children under five years were protected for up to a year. Injected cholera vaccines were associated with more systemic and local adverse effects compared to placebo, but these were not severe or life-threatening.

AUTHORS' CONCLUSIONS

Injected cholera vaccines appear to be safe and relatively more effective than usually realized. Protection against cholera persists for up to two years following a single dose of vaccine, and for three years with an annual booster. However, they have been superseded by oral vaccines.

Immunization with Vibrio cholerae outer membrane vesicles induces protective immunity in mice

The gram-negative bacterium Vibrio cholerae releases outer membrane vesicles (OMVs) during growth. In this study, we immunized female mice by the intranasal, intragastric, or intraperitoneal route with purified OMVs derived from V. cholerae. Independent of the route of immunization, mice induced specific, high-titer immune responses of similar levels against a variety of antigens present in the OMVs. After the last immunization, the half-maximum total immunoglobulin titer was stable over a 3-month period, indicating that the immune response was long lasting. The induction of specific isotypes, however, was dependent on the immunization route. Immunoglobulin A, for example, was induced to a significant level only by mucosal immunization, with the intranasal route generating the highest titers. We challenged the offspring of immunized female mice with V. cholerae via the oral route in two consecutive periods, approximately 30 and 95 days after the last immunization. Regardless of the route of immunization, the offspring was protected against colonization with V. cholerae in both challenge periods. Our results show that mucosal immunizations via both routes with OMVs derived from V. cholerae induce long-term protective immune responses against this gastrointestinal pathogen. These findings may contribute to the development of "nonliving," OMV-based vaccines against V. cholerae and other enteric pathogens, using the oral or intranasal route of immunization.

Investigation towards bivalent chemically defined glycoconjugate immunogens prepared from acid-detoxified lipopolysaccharide of Vibrio cholerae O1, serotype Inaba

A free amino group present on the acid-detoxified lipopolysaccharide (pmLPS) of V. cholerae O1 serotype Inaba was investigated for site-specific conjugation. Chemoselective pmLPS biotinylation afforded the corresponding mono-functionalized derivative, which retained antigenicity. Thus, pmLPS was bound to carrier proteins using thioether conjugation chemistry. Induction of an anti-LPS antibody (Ab) response in BALB/c mice was observed for all conjugates. Interestingly, the sera had vibriocidal activity against both Ogawa and Inaba strains opening the way to a possible bivalent vaccine. However, the level of this Ab response was strongly affected by both the nature of the linker and of the carrier. Furthermore, no switch from IgM to IgG, i.e. from a T cell-independent to a T cell-dependent immune response was detected, a result tentatively explained by the possible presence of free polysaccharide in the formulation. Taken together, these results encourage further investigation towards the development of potent pmLPS-based neoglycoconjugate immunogens, fully aware of the challenge faced in the development of a cholera vaccine that will provide efficient serogroup coverage.