Chimeric 3C-RBD Recombinant COVID-19 Vaccines to Enhance Cellular Immunity

Background

SARS-CoV-2 is the newly emerging member of the Coronaviridae (CoV) family, responsible for the Coronavirus disease-2019 (COVID-19) pandemic. Person-to-person transmission of the virus resulted in rapid spreading of SARS-CoV-2 worldwide. it is reported that SARS-CoV-2 has resulted in more than 86 million infections and almost 4 million deaths already around the world.

SARS-CoV-2 is an enveloped virus that contains a single-stranded positive-sense RNA. Infection begins when the virus attaches to pulmonary cells via the angiotensin converting enzyme 2 (ACE-2) receptor mediated by a glycoprotein expressed on its surface, the Spike protein (Spro) (Letko et al., 2020). Fusion of the viral membrane with the lumen of the endosomal membrane leads to endocytosis, facilitating infection via entry of the viral RNA into the cytosol. During the intracellular viral life cycle, two large polyproteins, pp1a and pp1ab, are translated. Sixteen non-structural proteins (nsp) are co-translationally and post-translationally released from pp1a and pp1ab upon proteolytic activity of two virus proteases, the papain-like protease (PLpro) and the 3C-like protease/main protease. These proteins are responsible for allowing the establishment of the viral replication and transcription complex (RTC) which is crucial for virus replication inside the cells (V’kovski et al., 2020).

Control of COVID-19, and emerging more transmissible variants, depends on the development of an efficacious vaccine(s) that can prevent the virus infecting pulmonary cells or surviving within these cells. Several vaccines are currently on the market, most notably those developed by BioNtech/Pfizer, Moderna and Oxford University/AstraZeneca. The former two vaccines are mRNA vaccines that enter human cells after injection and induce the cell to produce a viral Spro protein that stimulates the immune system. The efficacy of these vaccines is reported as >90% but because mRNA is highly instable they require storage and transport at -80°C, a costly and laborious procedure. The latter is a live recombinant Adenovirus-based vaccine that survives in cells for a short time but long enough to induce the production of viral Spro protein that stimulates the immune system. These vaccines reportedly have efficacy of ~60 – 70% but, in their favour, are stable and, therefore, can be stored at 4°C.

Regardless of the means by which these vaccines are delivered the final outcome is the induction of immune responses (mainly antibodies) that bind to the SARS-CoV-2 virus and prevents infection of the pulmonary cells. The target on the virus, to which these antibodies bind, is the Spro (Spike protein) that is responsible for binding to ACE on these cells. More specifically, the vaccine-induced antibodies bind to the section of the Spike protein that directly interacts with ACE, referred to as the Receptor Binding Domain (RBD). The antibodies are often referred to as ‘neutralising antibodies’ because they stop the virus entry into the cell.

Technology Overview

NUI Galway have developed a vaccine construct that is novel and not amongst any of the vaccines currently being used or under development.

The vaccine is a ‘chimeric’ because it is made of two parts of the SARS-CoV-2 virus, one of which is the RBD. The other part is made of a major protein that is produced by the virus after it enters the pulmonary cell, the 3C-like protease.

These two proteins are linked by a bridge which the researchers refer to as GP-linker, allowing the production of a stable and bioactive chimeric protein.

This GP linker, as far as the researchers know, is unique. The design is to allow space between the two proteins so that the can function separately on the immune system while still attached.

It is anticipated that linking the two proteins would induce more potent responses in humans than using them as a non-linked mixture.

The vaccine is produced using a ‘traditional method’ by which many vaccines currently on the market are made. This involves inserting the gene that encodes the COVID-19 vaccine construct into a surrogate host – in this case a bacterium. The bacterium is then grown in standard fermentation media after which they are removed and lysed. The vaccine protein is then isolated from the bacterial lysate.

Benefits

1. It uses two parts of the virus – the unique pairing of these molecules could potentially induce more potent immune responses than each alone, which could reach efficacy of >90%
2. Combining the two parts has made the recombinant chimer more soluble, stable and easier to produce.
3. The parts are linked with a novel bridge allowing each part to function separately.
4. The system allows the two parts to be co-expressed as a single molecule which can be purified as a single vaccine rather than isolating two parts separately and then mixing (the system allows other molecules to be added to this chimer to make trimers, tetramers etc).
5. The vaccine would have the capacity to induce neutralising antibodies to the RBD that would prevent virus entry into the cell. Additionally, it can trigger ‘cellular’ immune responses to the 3C-like protease that could potentially kill virus-infected cells and thus prevent spread of the virus within the body and reduce transmission to other cells. This could also prevent or reduce person-to-person transmission.
6. The vaccine is a protein and therefore it does not use the cells of the body to make the vaccine. NUI Galway’s researchers immunize directly with the protein.
7. The above makes it easier to determine the correct dose of vaccine to administer.
8. The researchers Galway have a clearer idea of how protein vaccines work compared to the new technologies.
9. The vaccine protein is stable and can be stored in a regular refrigerator at 4°C. (often proteins produced in this way are stable even at room temperature). This makes storage, transportation and administration of the vaccine easier. Furthermore, its stability means it can readily be transported and used in developing countries with lack the resources to store vaccines at very low temperatures.
10. Production is cheap – vaccines could be made at costs as low as €1 to 5 each.
11. This chimeric vaccine not only targets the RBD, which is the main point of attack for many vaccines, but also includes another target (3C-like protease) and thus would give a broader immunity – this immunity would include both antibodies and cellular components of the immune system.
12. Additional trimeric and bigger vaccines could be made to broaden the immune response. Immune potentiating proteins or peptides could be added to the chimer using the same linker system.
13. A broader immune response has the potential to fight new variants that may emerge.
14. New chimeric vaccines can be designed and made within days to fight any emerging SARS-CoV-2 variants.
15. Despite the fact that vaccines are currently being used there is a massive shortage of vaccines. The chimeric vaccine uses different and fast production methods and therefore could fill the gap.
16. Manufacture could be carried out in various countries including low- to middle-income countries.
17. The 3C-RBD chimeric vaccine should be safe to administer to young children and pregnant women.

Applications

Vaccines against COVID-19 and future variants

Opportunity

NUI Galway are interested in investment to continue the development of several chimeric vaccines