Outer membrane vesicle

Abstract

Nowadays adjuvants are extensively used as immuno-stimulatory and immuno-modulatory compounds as components of subunit and combination vaccine formulations. The adjuvants of microbial origin are more frequently used among currently used licensed or experimental adjuvants. The outer membrane vesicle (OMV) of Neisseria meningitidis is among the newly studied components of microbial origin, which could be applied as an adjuvant. Although the potency of OMV as a carrier (conjugated to a hapten) is now proven, the adjuvant properties of OMV have particular significance as a potential target for protective immunity. Since it has immune-stimulatory activity, OMV has been utilized in vaccine development. This commentary reviews the different applications of OMV as potential adjuvant in the field of vaccine development.

Keywords: :

• neisseria meningitidis
• adjuvant
• outer membrane vesicle
• vaccine development

Objective

The use of adjuvants in a vaccine originated approximately 70 y ago with the incorporation of bacterial vaccines into aluminum hydroxide gels. Their efficacy along with minimal side effects dampened for many years any impetus to substitute other adjuvant compounds. Nevertheless, substances that amplified humoral and cell-mediated immunity continued to be developed and characterized in animals, and these contributed greatly to the magnification and subsequent understanding of heretofore complex, subliminal, immunological events.

Despite great success in the development of adjuvants in the last few decades, many adjuvants were never accepted for routine vaccination because of safety concerns, e.g., acute toxicity and the possibility of delayed side effects. Therefore, there is a continuing need for powerful adjuvants that are relatively safe and without any significant adverse reaction inherent to their use. Thus, the adjuvants of microbial origins are more common among the other currently used adjuvants.¹

Universal Adjuvant Properties of Outer Membrane Vesicle

The outer membrane vesicle (OMV) of Neisseria meningitidis serogroup B (GBOMV) is among the more studied components with microbial origin, which could be applied as an adjuvant.¹ While the adjuvant properties of meningococcal OMV were expected due to their known propensity to activate dendritic cells and macrophages, the potency of OMV as a carrier (conjugated to a hapten) is now proven. As mentioned above, most adjuvants cause local and systemic reactions and are not licensed for human use; According to these drawbacks of currently applied adjuvants, OMV would be a relatively safe adjuvant with high potency to induce a typical secondary response; the OMV used in our vaccine formulations has been used previously in human trials and was found to be safe. Several reports have described that polysaccharide antigens, normally T-independent in animal models, stimulate immunologic memory when combined to GBOMV because the carrier has been shown to have T-helper-cell mitogenic activity.² OMVs contain the native configuration of different classes of outer membrane proteins (OMPs) and other antigens such as lipo-oligosaccharide. The immunodominant antigen of these vaccines is the sero-subtype outer membrane protein PorA, a class 1 outer membrane protein. The current hypothesis for the adjuvant activity of OMV is related to PorA, a major protein component of OMV, acting as an adjuvant or mitogen inducing the expression of costimulatory molecules like CD80, CD86 or CD40, which are essential for cognate T-cell-dependent antibody production. Meningococci group B OMP is also known to increase expression of costimulatory molecules on murine B cells and to enhance antibody responses to polysaccharide antigens by a CD40-CD40L-mediated mechanisim.³ Conformational structure and material components of the OMV are very critical in antigen delivery to immune system cells. In addition, OMV isolated from commensal Neisseria can be a delivery vehicle for heterologous vaccine antigens. Overall, the previous studies have shown that the predominant OMPs (PorA, PorB and RmpM) from N. meningitidis present in the Meningococci B Cuban vaccine had differing capacities to prime the immune system.

Among the described adjuvant properties of OMV is that the mucosal and systemic antibody responses to an influenza virus vaccine are greatly augmented when co-administered with OMV.¹ In addition, the adjuvant properties of OMV-derived particles have been demonstrated for potential cancer vaccines.¹

Outer Membrane Vesicle Based Vaccines

N. meningitidis serogroup B vaccines, based on OMVs, have been developed and used in many countries, and have shown efficacy in children and young adults. OMVs have adjuvant properties, and the cross-reactive antibodies generated by one or more OMV antigens might also provide further protection against serogroup B meningococcal disease caused by heterologous strains.^2,4

Oliver et al.⁵ have demonstrated that the application of OMV from commensal Neisseria lactamica is an alternative approach for a meningococcal vaccine. Immunization with N. lactamica OMV has provided protection against the challenge with a range of meningococcal strains in a mouse model of meningococcal disease. It has also been shown that immunization of mice with N. lactamica OMV induces antibodies cross-reactive with N. meningitidis and primes for an enhanced response to N. meningitidis immunization.⁶ In addition, OMV isolated from commensal Neisseria can be a delivery vehicle for heterologous vaccine antigens. Also, OMVs of Vibrio cholera, Bordetella pertussis, Francisella tularensis and Bacillus anthracis are being evaluated as vaccine candidates.⁴

OMV Act as Effective Intranasal Adjuvants Eliciting Humoral Responses Against HBsAg

Sardinas et al.⁷ have employed a hepatitis B surface antigen (HBsAg) vaccine as a model antigen to assess the mucosal adjuvant properties of N. lactamica OMV. It was encouraging that OMV from both N. lactamica and N. meningitidis, intranasally co-administered with HBsAg, modified the subclass pattern and increased IgG2a responses. This suggests that the use of these bacterial OMV as mucosal adjuvants could favor the induction of antibodies with enhanced functional activity against the heterologous antigen by exerting an immunomodulatory effect.

Specific Application of OMV in Brucellosis Vaccines Design

Bhattacharjee et al.⁸ have evaluated the N. meningitidis serogroup B OMP (GBOMP) complex - Brucella melitensis strain 16M LPS non-covalent complex to elicit the an effective and long-lasting immune response against brucellosis in mice. In parallel with the adjuvant properties of OMV, Bhattacharjee et al.⁸ have developed a vaccine composed of purified LPS of Brucella melitensis 16M noncovalent complex with the GBOMP and studied its immunological properties. In a recent paper they have reported that the purified B. melitensis LPS+GBOMP non-covalent complex vaccine could be a safe candidate vaccine against brucellosis.²

We previously reported the efficacy of B. abortus LPS+ GBOMV after subcutaneous injections with two boosters (vs. one booster) administrated two weeks (vs. one week) after the previous immunizations.^1,9 In fact, we designed a subunit brucellosis vaccine composed of B. abortus S99 LPS with GBOMV as a non-covalent complex and then evaluated specific serum antibody responses against the LPS of B. abortus S99 as well as production of T-helper-1 cytokine (IFN-γ) and T-helper-2 cytokines (IL-4 and IL-10) in spleen cell suspension supernatants assayed by ELISA. After both booster doses, the induced anti-B. abortus S99 LPS IgG titers increased significantly, but as could be predicted, the booster injections did not lead to any increase in anti-B. abortus S99 LPS IgM titer, which is a sign of specificity of our designed ELISA kit to detect anti-B. abortus S99 LPS IgM. We also achieved higher titers of anti-LPS IgG in comparison with studies reported by Bhattacharjee et al.^1,8

General and Specific Adjuvant Properties of GBOMV in Meningococcal Vaccines Development

We previously synthesized a conjugate vaccine candidate with group A meningococcal capsular polysaccharide (GAMP) conjugated to OMV, which was immunogenic in rabbits, and the respective antisera showed bactericidal and opsonophagocytic activity against N. meningitidis serogroup A,^10-12 but the method of conjugation is very complex and needs to be controlled by multiple parameters in light of the risk of conformational changes of dominant and essential epitopes as a result of the covalent reactions between GAMP and OMV. In this respect, protective antibodies against native polysaccharide or OMV proteins would not be induced following this conformational change. The covalent attachment of the GAMP to the GBOMV may have induced some modifications in the GBOMV protein epitopes through directly covalent linkage or steric hindrance. Furthermore, a reactive reagent is used for coupling of polysaccharide to the protein, and the control of conjugate vaccines may be complicated for ensuring the absence of the coupling reagent and its reaction products. An alternative to these conjugates could be polysaccharide-OMV complexes, in which OMV can be act as a microbial adjuvant. Thus in a recent study,⁴ GBOMV was used as an adjuvant with GAMP and tested in New Zealand White (NZW) rabbits to evaluate bactericidal antibody response and opsonophagocytic activity against serogroup A meningococci. Non-covalent combination of GBOMV and GAMP and three controls including GAMP, GBOMV and normal saline were injected intramuscularly into groups of four female NZW rabbits with two booster doses.⁴ The serum samples were collected on different days at intervals of two weeks and tested by complement-mediated bactericidal assay and opsonophagocytic activity against serogroup A meningococci according to the World Health Organization protocol. The results indicate that the combination of GBOMV with GAMP, in noncovalent form, was able to induce a high level of bactericidal antibody and opsonophagocytic activity in comparison with GAMP alone. The GBOMV was shown to be a potent carrier protein in the induction of an immune response, but in this study GBOMV in non-covalent form and without any conjugation process is considered an adjuvant for inducing an immune response against the three prevalent serogroups of N. meningitidis. The vesicular nature of the complex, demonstrated by electron microscopy (data not shown), may increase the uptake and processing of the GAMP by antigen–presenting cells.^3,13

GBOMV Could Be Applied as an Adjuvant Co-administered with mzNL4–3 VLPs

In a recent report, we further demonstrated the potency of non-infectious virus-like particles (mzNL4–3 VLPs) mixed with GBOMV to induce humoral and cellular responses against HIV-1.¹⁴ Analysis of anti-HIV-1 responses elicited in immunized BALB/c mice following different immunization regimens indicated that GBOMV+VLP was an immunopotent combination which significantly induced anti-HIV-1 IgG with IgG2a dominance. Results of cytokine and ELISpot assays also showed the capability of the VLP+OMV immunogen for effective induction of IFNγ and IL4-secreting cells and further suggested the promotion of a Th1-oriented response that was evidenced by the increased IFNγ /IL4 secretion ratio. According to our study, HIV-1 VLPs combined with N. meningitidis B OMVs seem to be a promising approach in vaccine development against HIV-1.

Conclusion

A number of scientific reports have demonstrated the adjuvant properties of N. meningitidis serogroup B OMVs. Our findings confirm this subject found by others.

In this commentary, we explore the potential properties of OMV delivered with a vaccine antigen and used as a hapten carrier, and we discuss its capacity to strongly induce an immune response when co-administered as a general adjuvant. In addition, it is well known that meningococcal OMV are effective and relatively safe vaccines, as demonstrated in several clinical trials. Thus, the availability of such an OMV component with adjuvant properties will be of great importance for the development of improved and combined vaccines for a wide variety of diseases.