he COVID-19 pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is now causing over 160,000 cases globally every day, with above 504,000 deaths having been recorded so far as a direct effect of the outbreak. With no specific antiviral or vaccine yet available, scientists are working day and night to produce a solution.
Now, a new study by researchers at Emory University, the University of Texas Medical Branch and the Ragon Institute and published on the preprint server bioRxiv* in June 2020 reports the development of a promising new vaccine candidate that may rapidly and effectively induce the production of neutralizing antibodies against the spike protein of the virus.
The Study: Modified Vaccinia Ankara Vector
The current study uses the modified vaccinia Ankara (MVA), which is a profoundly weakened strain of the vaccinia virus, known to be safe, immunogenic, and protective when used to produce vaccines against many infectious diseases and even cancer. It has been developed in preclinical and in human research.
In addition to the above features, MVA-vaccines induce long-term immunity, and can thus produce mucosal as well as systemic immunity. The magnitude of the response can be multiplied tenfold with two doses. The virus can accommodate inserts larger than 10 kb, so allowing more than one antigen to be expressed at the same time. The recombinant vaccines are also stable and their production scalable to high titers, allowing for the rapid manufacture of vaccines. Both CD4 and CD8 T cell responses are also generated, which may be equally important in neutralizing the infection. Finally, there is already evidence of the efficacy of these vaccines against SARS-CoV, MERS, Zika, and Ebola virus.
Two MVA-Based Vaccines Tested
The researchers used two methods to develop MVA-based vaccines – one expressing the full-length spike protein anchored to the cell membrane but before membrane fusion (MVA/S), and the other expressing only the S1 region of the secreted spike protein in its trimeric state (MVA/S1). The MVA/S is stabilized in the prefusion state by the presence of two mutations.
The two vaccines contain the receptor-binding domain (RBD) that is known to be a primary target of neutralizing antibodies. Both vaccines were tested in a mouse model.
Strong Binding Antibody Response
Both vaccines were found to induce antibodies with strong binding to different parts of the S protein. However, the MVA/S induced antibodies against the RBD, but the MVA/S1 induced binding to S1 subunit. This exciting finding indicates that antibodies generated in response to the second vaccine could be directed against non-RBD domains on the S1 subunit.
Selective Neutralizing Antibody Response
After the booster dose, at two weeks, the repeat analysis showed that MVA/S mice induced antibodies that bound to the RBD, S1, and S2 proteins. On the other hand, the MVA/S1 vaccine induced a response against the non-RBD region of the S1 subunit. Thus, the vaccines targeted the spike proteins differentially.
Secondly, the vaccines induced robust lung immune responses in the bronchus-associated lymphoid tissue (BALT) and antibody responses as well. The BALT could enable the faster expansion of lung immune responses after exposure to SARS-CoV-2, along with IgG antibodies targeting the spike protein. These were higher in the MVA/S than in the MVA/S1 group.
Thirdly, they found that whereas MVA/S was capable of inducing the formation of strong neutralizing antibody response, MVA/S1 was not, even though antibody binding was comparable with both vaccines. The neutralizing antibodies produced by the former was directly proportional to the RBD binding titer, and negatively correlated with the S1 binding titer. In other words, MVA/S could be an effective potential vaccine for SARS-CoV-2, but not MVA/S1.
S1-RBD Becomes Unstable on Incubation
To explore the failure of MVA/S1 as a vaccine, the trimeric S1 protein expressed by the virus was assayed for its binding capacity to human ACE2. They found that compared to the RBD protein, the virus S1 bound firmly to the receptor, but with half the affinity, and one-tenth of the association rate, of the former.
When S1 was incubated at 25oC for 60 minutes, its affinity was still further reduced, while RBD remained just as stable. Thus, the S1 protein does present the RBD in the proper conformation for strong binding but becomes unstable on incubation at room temperature for a more extended period. This causes neutralizing antibodies to be induced to other non-RBD regions in S1 after exposure to the vaccine.
The MVA/S vaccine thus induces strong specific anti-RBD neutralizing antibodies. Again, MVA vaccines also caused the generation of T cell responses as well as humoral responses in the lung, both of which are essential for antiviral protection when it comes to respiratory viruses. Earlier work with HIV sequences using MVA vectors demonstrates the durable and robust immunogenicity of these recombinants in mice, non-human primates, and humans, provided the dose is raised tenfold in the latter two classes compared to mice. This requirement is feasible if the vaccine is manufactured on a large scale. Thus, the current study shows the potential for the use of MVA/S as a mass vaccine for SARS-CoV-2.