Ongoing threats from infectious diseases such as pandemic influenza, as well as the promise of emerging immunotherapies, are driving the demand for new vaccine platforms. Although traditional protein subunit-based vaccines are safe, they are often poorly immunogenic and expensive to manufacture. Now, Chen and colleagues present a potential new vaccine platform, which is based on engineered bacterial outer membrane vesicles (OMVs). These comprise vaccine, adjuvant and particulate carrier all in one, and can easily and economically be purified by ultracentrifugation.
OMVs are proteoliposomes about 100 nm in size, which constitutively bud from the outer membrane of Gram-negative bacteria. There are two vaccines on the market that consist of bacterial surface proteins naturally incorporated into OMVs, purified from the pathogens the vaccine is directed at. Although such particulate nanoscale antigen delivery systems are of great interest, a more general applicability of OMVs as vaccine delivery vectors hinged on the challenge of how to incorporate a desired antigen into the vesicles.
The authors had previously shown that genetic coupling of protein subunit antigens to cytolysin A (ClyA), a bacterial haemolysin that is enriched in OMVs, results in efficient secretion of the fusion protein into the vesicles. Now, the authors tested the applicability of OMVs as vaccine carriers by using the poorly immunogenic green fluorescent protein (GFP) as a model antigen. To test the immunogenicity of the ClyA–GFP fusion protein, mice were inoculated with either GFP alone, ClyA alone, the ClyA–GFP fusion protein or a mixture of GFP and ClyA. Mice immunized with the ClyA–GFP fusion protein showed much higher GFP-specific antibody titres compared with all other groups.
They then tested whether the fusion protein retained its antigenic and adjuvant properties when incorporated into OMVs. The OMVs were prepared from a hyper-producing Escherichia coli strain, with or without the vector coding for the fusion protein. Mice were injected with either blank OMVs and purified ClyA–GFP, or engineered OMVs containing the ClyA–GFP protein. Both groups developed comparable GFP-specific antibody titres to mice immunized with the fusion protein alone. In a head-to-head comparison of engineered OMVs with the gold standard adjuvant, alum — the only adjuvant that is approved for human use in North America — ClyA–GFP OMVs provoked comparable GFP-specific antibody titres to GFP adsorbed to alum, ClyA–GFP adsorbed to alum, and ClyA–GFP with empty OMVs.
The authors point out that although the vaccine seemed to be well tolerated in mice, further studies are needed to investigate the immune-enhancing activity of ClyA. As ClyA a cytolytic protein, it might need to be detoxified by mutation or truncation or by chemical methods. It also remains to be established whether OMVs might need to be depleted of lipopolysaccharide for human application. Nevertheless, as a diverse range of proteins can be fused to ClyA, this system could potentially be turned into a robust and tunable platform, which sidesteps the complex purification process of traditional subunit vaccines.
ORIGINAL RESEARCH PAPER
Chen, D. J. et al. Delivery of foreign antigens by engineered outer membrane vesicle vaccines. Proc. Natl Acad. Sci. USA 27 Jan 2010 (doi/10.1073/pnas.0805532107)
Kim, J.-Y. et al. Engineered bacterial outer membrane vesicles with enhanced functionality. J. Mol. Biol. 380, 51–66 (2008).
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Proceedings of the National Academy of Sciences (2016)