In a paper published in Frontiers in Microbiology, scientists have described the utility of outer membrane vesicles derived from Gram-negative bacteria as vaccines and methods to expand their applications.
Study: Outer Membrane Vesicles: A Bacterial Derived Vaccination System. Image credit: Maxx-Studio/Shutterstock
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Outer membrane vesicles (OMVs) are spherical lipid nanoparticles with a diameter of 20–300 nm. These vesicles are derived from the cell membrane of Gram-negative bacteria and are composed of bacterial proteins, lipids, nucleic acids and other components.
OMVs derived from pathogenic or non-pathogenic bacteria play essential roles in bacterial pathogenesis, cell-to-cell communication, horizontal gene transfer, quorum sensing and maintenance of bacterial fitness. However, as a non-replicative component, OMVs cannot independently induce disease pathogenesis.
Bacterial proteins and glycans make OMVs a potent immunogenic component that can be used as an adjuvant to induce the host immune response. Because of this property, OMVs are considered potential candidates for vaccine development.
Isolation of OMV
Gram-negative bacteria release OMVs during growth or under stressful conditions. However, these spontaneous OMVs are released in low amounts and therefore cannot be used for large-scale vaccine production.
Several strategies have been developed to increase OMV production. Sonication, vortexing, or EDTA-mediated extraction have been applied to mechanically disrupt the bacterial membrane, resulting in the release of OMVs.
EDTA-extracted OMVs are closely related to the native bacterial membrane and induce comparable immune responses. In contrast, sonication and vortexing increase the amount of non-membrane components in the final product, resulting in increased antigenicity and reduced safety.
Detergent-based extraction is another well-documented method that produces OMVs with reduced levels of lipopolysaccharides (LPS), which are bacterial toxins. Despite reducing the risk of toxicity, this process leads to the loss of many bacterial proteins and lipoproteins, which in turn results in the suppression of OMV-stimulated immune responses.
Manipulation of certain bacterial genes can increase vesiculation and thus can produce high levels of genetically modified OMVs. The genes encoding the bacterial lipoproteins Lpp and NlpI and the outer membrane protein OmpA are the main targets of genetic manipulation.
Heterologous OMVs
Non-pathogenic bacterial strains can express heterologous proteins to reduce toxicity and improve immunogenicity of OMVs.
A protein of interest can be fused to a bacterial transmembrane protein and the resulting plasmid can be introduced into the bacterial strain, which will subsequently produce recombinant OMVs that express the desired protein on the surface.
Another potential strategy for expressing heterologous proteins is glycoengineering of the LPS O antigen. Glycosylated OMVs can be produced by expressing the O antigen gene of a pathogen in a nonpathogenic O antigen mutant strain of bacteria.
OMV-induced immune response
Pathogen-associated molecular patterns present on the OMV outer membrane activate pattern recognition receptors on host cells, leading to the activation of innate immune signaling and the release of pro-inflammatory cytokines. The engulfment of OMVs by innate immune cells induces adaptive immune responses.
LPS acts as an adjuvant to induce an effective host immune response to the bacterial antigen expressed on the surface of the OMV. However, overexpression of LPS can lead to overstimulation of immune responses and the induction of systemic toxic shock. Detergent-based preparations or genetic manipulations can be used to reduce the level of highly reactive LPS on the OMV surface.
OMV-based vaccines
OMVs expressing the desired antigens can be administered into the body via various routes, including oral/intranasal, intramuscular, subcutaneous, intraperitoneal, and intradermal. OMV expressing the spike protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has recently been shown to induce robust immune responses in hamsters when administered intranasally.
Currently, there are two clinically approved OMV vaccines, VA-MENGOC-BC™ and Bexsero™, against the invasive serogroup B strain of N. meningitidis. The PorA protein expressed by this bacterium is highly variable between strains. OMVs derived from the strain causing meningitis have been successfully used to develop vaccines against this particular bacterial strain.
Many OMV vaccines are currently in development. These vaccine candidates have been designed to target N. gonorrhoeae, Shigella spp., Salmonella spp., extraintestinal pathogenic E. coli (EXPEC), V. cholerae, M. tuberculosis and non-typeable H. influenzae.
In addition to antibacterial vaccines, OMVs have been used to produce vaccines against viruses, including the influenza virus and the coronavirus. Tumor-targeted OMVs containing therapeutic siRNA or tumor antigens have also been developed as therapeutic cancer vaccines.