Inactivated whole-cell vaccines, also known as ‘killed vaccines,’ are amongst the most traditionally employed varieties of vaccine, having been developed against a wide range of pathogens, including cholera and E. coli.
In a new study, released as a preprint on the bioRxiv* server, a team of researchers inactivated whole-cell E.coli with a reduced genome and utilized this to express coronavirus-specific fusion peptides, demonstrating a potent anamnestic response.
Designing the vaccine
Gram-negative bacteria bear transport proteins, autotransporters, that allow the proteins expressed on the cell surface to be exchanged, and modern genetic engineering allows a protein of interest to be expressed on the surface in this way. As many as 200,000 foreign proteins can be displayed on the surface of a single bacterium, and this technology has previously been employed to elicit an immune response in vivo by displaying selected pathogenic antigens. However, the range of antigens that can be expressed by this method is limited by size, to below around 50 kDa, and bacteria do not produce proteins with mammalian glycolysation, and thus may not elicit the production of the appropriate antibodies.
No vaccines have yet been licensed that employ this technology, largely due to the low immunogenicity displayed due to the improper glycolysation. In this paper, the group hypothesized that removal of extraneous surface proteins from the bacteria, besides the expressed antigen of interest, may elicit a stronger immune response. The whole SARS-CoV-2 spike protein utilized in the deployed vaccines is a large 180 kDa protein, and thus could not be deployed using this method. Several virus families, including coronaviruses and SARS-CoV-2, bear the HIV-1 fusion peptide, which is involved in cell membrane penetration. As this region is highly conserved, it could potentially make a good vaccine candidate, and the authors additionally point out that the use of the whole spike protein has been implicated with an increased risk of rare inflammatory syndromes.
Testing the vaccine
The group utilized killed whole-cell E. coli that were genome-reduced in order to present fewer native surface cell proteins. A plasmid was inserted into the bacteria to express the SARS-CoV-2 specific HIV-1 fusion peptide, subsequently demonstrated by antibody binding assays. Alternative vaccines were also prepared for porcine epidemic diarrhea virus (PEDV), expressing the correlating fusion peptide for this similar virus.
Pigs were vaccinated with either of the prepared vaccines and a control, administrated with a booster at day 21, then challenged with oral PEDV infection by the respective virus at day 35. Blood was collected weekly, and circulating anti-fusion protein antibodies quantified, not being significantly higher following vaccination but before the challenge. However, following the viral challenge, the anti-FP values were notably higher amongst the vaccinated, suggesting that the vaccines had primed the pigs to respond. Serum interferon-γ levels were also higher amongst the vaccinated, and fewer of these pigs expressed outward signs of PEDV-related symptoms.
As expected, the PEDV vaccine was more effective at immunizing against PEDV than the SARS-CoV-2 vaccine. However, the similarity of the fusion proteins originating from each virus was such that a statistically significant response was still detected. Indeed, the 13 amino acid sequences surrounding the 6 core residues of each sequence are identical.
The group argues that these vaccines could be easier to produce and transport than the currently deployed mRNA vaccines, particularly given the need for a strong cold-chain in the latter case, requiring constant temperatures of -20°C or colder. They do, however, concede that the possibility of eliciting an immune response in hosts towards the genome-reduced E. coli bears a low but possible risk, and requires further investigation.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.