Immune responses of meningococcal B outer membrane vesicles in middle-aged mice

Outer membrane vesicles as a platform for the discovery of antibodies to bacterial pathogens

Bacterial outer membrane vesicles (OMVs) are nanosized spheroidal particles shed by gram-negative bacteria that contain biomolecules derived from the periplasmic space, the bacterial outer membrane, and possibly other compartments. OMVs can be purified from bacterial culture supernatants, and by genetically manipulating the bacterial cells that produce them, they can be engineered to harbor cargoes and/or display molecules of interest on their surfaces including antigens that are immunogenic in mammals. Since OMV bilayer-embedded components presumably maintain their native structures, OMVs may represent highly useful tools for generating antibodies to bacterial outer membrane targets. OMVs have historically been utilized as vaccines or vaccine constituents. Antibodies that target bacterial surfaces are increasingly being explored as antimicrobial agents either in unmodified form or as targeting moieties for bactericidal compounds. Here, we review the properties of OMVs, their use as immunogens, and their ability to elicit antibody responses against bacterial antigens. We highlight antigens from bacterial pathogens that have been successfully targeted using antibodies derived from OMV-based immunization and describe opportunities and limitations for OMVs as a platform for antimicrobial antibody development. KEY POINTS: • Outer membrane vesicles (OMVs) of gram-negative bacteria bear cell-surface molecules • OMV immunization allows rapid antibody (Ab) isolation to bacterial membrane targets • Review and analysis of OMV-based immunogens for antimicrobial Ab development.

A novel cold-chain free VCG-based subunit vaccine protects against Chlamydia abortus-induced neonatal mortality in a pregnant mouse model

Chlamydia abortus (Cab) causes spontaneous abortion and neonatal mortality in infected ruminants and pregnant women. Most Cab infections are asymptomatic, although they can be treated with antibiotics, signifying that control of these infections may require alternative strategies, including the use of effective vaccines. However, the limitations imposed by live attenuated and inactivated vaccines further suggest that employment of subunit vaccines may need to be considered. The efficacy of a newly generated Vibrio cholerae ghost (rVCG)-based subunit vaccine harboring the N-terminal portion of the Cab Pmp18D protein (rVCG-Pmp18.3) in preventing Cab-induced abortion or neonatal mortality was evaluated in pregnant mice. Mice were intranasally (IN) immunized and boosted twice, 2 weeks apart with the vaccine, and immunized and unimmunized mice were caged with males 4 weeks postimmunization. The mice were then infected either IN or transcervically (TC) 10 days after pregnancy, and the fertility rate was determined 7 days postpartum. Eight days after delivery, the mice were sacrificed, and Cab infectivity in the lungs and spleens was evaluated by culturing tissue homogenates in tissue culture. Our results demonstrated that the vaccine induced immune effectors that mediated complete clearance of infection in the lungs and significantly reduced Cab infectivity in the spleen following IN immunization. Vaccine immunization also afforded protection against Cab-induced upper genital tract pathology (uterine dilation). Furthermore, while there was no incidence of abortion in both immunized and unimmunized mice, immunized mice were completely protected against neonatal mortality compared to unimmunized infected controls, which lost a significant percentage of their litter 7 days postpartum. Our results establish the capability of the rVCG-Pmp18.3 vaccine to prevent infection in the lungs (mucosal) and spleen (systemic) and protect mice from Cab-induced tubal pathologies and neonatal mortality, a hallmark of Cab infection in ruminants. To advance the commercial potential of this vaccine, future studies will optimize the antigen dose and the number of vaccine doses required for protection of ruminants.

Intranasal and Intramuscular Immunization with Outer Membrane Vesicles from Serogroup C Meningococci Induced Functional Antibodies and Immunologic Memory

BACKGROUND

Immunization is the key to prevent invasive meningococcal disease (IMD), caused by Neisseria meningitidis. Outer membrane vesicles (OMVs) can be used as meningococcal antigens.

METHODS

Isogenic mice A/Sn (H2^a) were immunized with low antigenic doses of OMVs of an N. meningitidis C:2a:P1.5 strain, via intranasal/intramuscular route, adjuvanted by cholera toxin subunit B (CTB) or via intramuscular route only, adjuvanted by aluminium hydroxide (AH). Mice were followed until old age and humoral and cellular responses were assessed by ELISA, Immunoblotting, Dot-blot, Serum-bactericidal assay, Immunohistochemistry and ELISpot.

RESULTS

OMV+CTB and OMV+AH groups presented statistically higher antibodies titers, which persisted until middle and old ages. IgG isotypes point to a Th2 type of response. Avidity indexes were considered high, regardless of adjuvant use, but only groups immunized with OMVs and adjuvants (OMV+CTB and OMV+AH) presented bactericidal activity. The antibodies recognized antigens of molecular weights attributed to porin and cross-reactivity proteins. Although the spleen of old mice did not present differences in immunohistochemistry marking of CD68+, CD4+, CD79+ and CD25+ cells, splenocytes of immune groups secreted IL-4 and IL-17 when stimulated with OMVs and meningococcal C polysaccharide.

CONCLUSION

We concluded that both adjuvants, CTB and AH, improved the immunogenicity of low doses of OMVs and contributed to a persistent immune response. Even though AH is well established in the vaccinology area, CTB seems to be a promising adjuvant candidate for meningococcal vaccines: it is suitable for mucosal delivery and supports a Th2 type of response. Therefore, OMVs are still a relevant vaccine platform.

Immunological Effects of Dimethyldioctadecylammonium Bromide and Saponin as Adjuvants for Outer Membrane Vesicles from Neisseria meningitidis

The meningococcal disease is a global health threat, but is preventable through vaccination. Adjuvants improve meningococcal vaccines and are able to trigger different aspects of the immune response. The present work evaluated the immune response of mice against Neisseria meningitidis outer membrane vesicles (OMV) complexed with the adjuvants aluminium hydroxide (AH), via subcutaneous route; and dimethyldioctadecylammonium bromide (DDA) or Saponin (Sap), via intranasal/subcutaneous routes. ELISA demonstrated that all adjuvants increased IgG titers after the booster dose, remaining elevated for 18 months. Additionally, adjuvants increased the avidity of the antibodies and the bactericidal titer: OMVs alone were bactericidal until 1:4 dilution but, when adjuvanted by Alum, DDA or Sap, it increased to 1/32. DDA and Sap increased all IgG isotypes, while AH improved IgG1 and IgG2a levels. Thus, Sap led to the recognition of more proteins in Immunoblot, followed by DDA and AH. Sap and AH induced higher IL-4 and IL-17 release, respectively. The use of adjuvants improved both cellular and humoral immune response, however, each adjuvant contributed to particular parameters. This demonstrates the importance of studying different adjuvant options and their suitability to stimulate different immune mechanisms, modulating the immune response.

Experimental studies using OMV in a new platform of SARS-CoV-2 vaccines

Although COVID-19 vaccines have recently been approved for emergency use, search for new vaccines are still urgent, since the access of the countries, especially the poorest, to the vaccines, has shown to be slower than the necessary to rapidly control the pandemic. We proposed a novel platform for vaccine using recombinant receptor binding domain (rRBD) from Sars-Cov-2 spike protein and Neisseria meningitidis outer membrane vesicles (OMVs). The antigen preparation produced a humoral and cellular immune response. Taken together our findings suggest a good immunostimulatory patter in response to immunization with rRBD plus N. meningitidis OMV.

Immunogenicity of antigens from outer membrane vesicles of Neisseria meningitidis associated with bilayer fragment of dioctadecyldimethylammonium in Swiss adult mice

Purpose

In the present study, meningococcal serogroup B outer membrane vesicles (OMVs) were associated with bilayer fragments of a cationic lipid, dioctadecyldimethylammonium (DDA-BF), used as adjuvant, in an antigenic preparation tested in adult female outbred mice. This adjuvant was compared to the traditional adjuvant aluminum hydroxide.

Materials and Methods

The potential in generating humoral response was evaluated by enzyme-linked immunosorbent assay (ELISA). Individual serum was collected and immunoglobulin G (IgG), IgG1, IgG2a, and IgG2b were quantified. Analyses were carried out 15 and 60 days after immunization. Antibodies avidity index were also analyzed by ELISA. Immunoblot and dot-ELISA were carried out to evaluate specific reaction for homologous strains and cross-reactive antigens present in other meningococcal strains isolated in 2011–2012 year, in Brazil. Delayed type hypersensitivity was used as indicative of cellular immunity and compared between two experimental groups, 24 hours after homologous strain challenge.

Results

The OMVs of Neisseria meningitidis, and N. lactamica (related species) were characterized by electrophoretic separation of proteins in 13% polyacrylamide gel. The strains presented antigens in the range of 8 to 130 kDa, showing a heterogeneous protein migration pattern. In the group immunized with OMVs/DDA-BF, we found no significant production of total IgG 15 days after the first immunization. On the other hand, 60 days after first immunization both adjuvants act benefiting total IgG production similarly. The antibodies of the IgG isotype produced by animals immunized after one or two doses after first immunization, showed intermediate and high avidity, independent on the adjuvant used. In both experimental groups the swelling of the footpads was significantly higher than those of the controls, suggesting that only one dose was enough to stimulate the generation of cellular immunity.

Conclusion

The use of this cationic adjuvant for N. meningitidis OMVs preparation revealed good potential for future new antigen preparation for N. meningitidis vaccine.

Gut Microbe-Derived Outer Membrane Vesicles: A Potential Platform to Control Cecal Load of Campylobacter jejuni

Acute diarrheal illness and gastroenteritis caused by Campylobacter jejuni infection remain significant public health risks in developing countries with substantial mortality and morbidity in humans, particularly in children under the age of five. Genetic diversities among Campylobacter jejuni and limited understanding of immunological correlations of host protection remain primary impediments for developing an effective measure to controlCampylobacter infection. Moreover, the lack of a reliable in vivo model to mimic natural infection against Campylobacter jejuni has substantially delayed the vaccine-development process. Given the role of bacterial outer membrane associated proteins in intestinal adherence and invasion as well as modulating dynamic interplay between host and pathogens, bacterial outer-membrane vesicles have emerged as a potential vaccine target against a number of gut pathogens, including Campylobacter jejuni. Here, we describe a mucosal vaccine strategy using chitosan-coated outer-membrane vesicles to induce specific immune responses against Campylobacter jejuni in mice. To overcome the challenges of mucosal delivery of outer membrane vesicles in terms of exposure to variable pH and risk of enzymatic degradation, we preferentially used chitosan as a nontoxic, mucoadhesive polymer. We show that intragastric delivery of chitosan-coated outer-membrane vesicles imparts significant immune protection against Campylobacter jejuni with high level local and systemic antibody production. Further, immunization with the outer membrane vesicles resulted in potent cellular responses with an increased CD4⁺ and CD8⁺ T cell population. Moreover, significant upregulation of IFN-γ and IL-6 gene expression suggests that mucosal delivery of outer membrane vesicles promotes a Th1/Th2 mixed-type immune response. Together, as an acellular and nonreplicating canonical end product of bacterial secretion, mucosal delivery of outer membrane vesicles may represent a promising platform for developing an effective vaccine againstCampylobacter jejuni.