The COVID-19 pandemic spread like wildfire in the first six months of 2020 over the entire globe, spurring intensive efforts to develop a vaccine that would be both effective and inexpensive so that mass immunization could be carried out.
Now, a new Israeli study published on the preprint server bioRxiv* in June 2020 reports a new recombinant vaccine expressing the S or spike protein of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) to be highly protective with a single dose in an animal model. This could be a gamechanger in the fight to contain the outbreak.
Novel Coronavirus SARS-CoV-2 Transmission electron micrograph of SARS-CoV-2 virus particles, isolated from a patient. Image captured and color-enhanced at the NIAID Integrated Research Facility (IRF) in Fort Detrick, Maryland. Credit: NIAID
The Spike Protein
The SARS-CoV-2 is a single-stranded RNA virus bearing the spike (S) surface glycoprotein on the viral membrane. This is a trimeric protein that binds with the angiotensin-converting enzyme (ACE) 2 on the human host cell. The post-binding cleavage of the S protein into its S1 and S2 subunits induces membrane fusion, with the entry of the virus into the host cell.
The S protein binding is thus a significant vaccine target because inhibition of this binding blocks subsequent viral entry and infection. The S1/S2 interface is the site of activity of host cell proteases like the transmembrane serine protease (TMPRSS2), or furin. The latter acts at a novel furin cleavage site on the boundary between S1 and S2 and its presence makes a huge impact on the ability of the virus to enter Vero E6 and BHK culture cells, which do not express TMPRSS2.
Vesicular Stomatitis Virus
The viral vector used in the current study is the vesicular stomatitis virus (VSV), which is a rhabdovirus, a single-stranded RNA virus that causes mostly animal disease but rarely affects humans. It was developed as a recombinant VSV (rVSV) platform and used to create vaccines against Ebola, HIV, and other zoonotic infections.
The reasons for this choice include the ease of viral propagation, the rapidity with which it reaches high titers in the immunized individual; the strong immune responses, both cellular and humoral, in vivo; and the ability to attenuate the virus by merely knocking out G protein, which is its major virulence factor. Additionally, most people have not been infected by VSV.
Development of rVSV-ΔG-spike
At present, over 100 vaccines are being raced through the development process on a variety of platforms, but none are close to being approved. The current attempt is based on rVSV, termed the VSV-ΔG-spike. The VSV-G is replaced by the S protein of the SARS-CoV-2.
The passaging of the rVSV-ΔG-spike vector caused a couple of critical mutations, which appears to be required for successful adaptation to Vero E6 culture since their expression rapidly becomes dominant and correlates with a fall in the VSV-G titer.
The researchers found that the rVSV-ΔG-spike vaccine candidate was expressed on the infected cells, while the newly generated viruses strongly express the spike protein on the membrane.
Recognition by COVID-19 Convalescent Serum
The rVSV-ΔG-spike is neutralized by convalescent serum from COVID-19 patients, as well as spike protein in SARS-CoV-2-infected cells, at 24 hours from infection, which shows that the spike antigen is similar to that of the wildtype SARS-CoV-2. A plaque reduction neutralization test (PRNT) demonstrates that both the recombinant and native versions could be neutralized by several convalescent sera in an efficient manner. Thus, the rVSV-ΔG-spike can induce effective immunity against SARS-CoV-2.
Successful Animal Model
The use of the right animal model is important to ensure generalizable results. The researchers here established the golden Syrian hamster model for COVID-19 under experimental conditions. Infected animals showed clinical features like weight loss, and changes in the lungs at lung tissue, which correlated with COVID-19 infection. They also stained positive for SARS-CoV-2.
The use of this model ensures that the rVSV-ΔG spike vaccine could be appropriately evaluated. They used single doses of the vaccine at increasing dosage to find the right dose, monitored them daily, and looked for adverse effects. They found that not a single animal showed post-immunization weight loss or signs of illness. Thus, a single dose of this vaccine was safe at all doses, eliciting neutralizing antibodies against SARS-CoV-2.
One dose of 106 plaque-forming units (PFU) per hamster was selected and used subcutaneously rather than intramuscularly as with the first round. Both the dosage and route were proved to be effective in inducing neutralizing and binding antibodies to the virus. Here again, all animals remained healthy and did not lose weight, indicating the protective effect of the vaccine.
Future Directions for Safe Effective Vaccines
The vaccine was thus proved to be both safe and effective in protecting against infection after exposure to an intranasal dose of the SARS-CoV-2, whereas non-immunized animals showed weight loss, and their lungs showed signs of pneumonia
The study showed that “A single dose of the rVSV-ΔG-spike vaccine candidate was found to be safe, efficacious, and provides protection against a deleterious SARS-CoV-2 challenge.” The researchers say, “These results pave the way for further examination of rVSV-ΔG-spike in clinical trials as a vaccine against SARS-CoV-2.”
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.