The current coronavirus disease 2019 (COVID-19) pandemic has resulted in unprecedented social and economic disruption, with a projected global “optimistic loss” of almost $3.3 trillion USD. The severe acute respiratory syndrome coronavirus (SARS-CoV-2), which is the virus responsible for COVID-19, continues to spread at an exponential rate throughout Europe, Latin America, and Asia. Unfortunately, no specific treatment has been shown to prevent or cure COVID-19.
Study: A live measles-vectored COVID-19 vaccine induces strong immunity and protection from SARS-CoV-2 challenge in mice and hamsters. Image Credit: Blue Planet Studio / Shutterstock.com
Several vaccines have been successfully developed to combat COVID-19. In particular, those based on messenger ribonucleic acid (mRNA) and adenovirus vector platforms have demonstrated high levels of protection from COVID-19.
Despite their utility, numerous limitations are associated with these vaccines, some of which include a lack of knowledge on the duration of protection, their capacity to help control new variants, cold chain logistics, manufacturing issues, and approval of vaccines for the younger population, adolescents and children. Therefore, exploring other vaccine strategies is essential.
Among the vaccine platforms being implemented, live attenuated viral vectors are of particular interest, as this method of immunization induces lasting protective immunity and is inexpensive to manufacture at a large scale. For example, the live attenuated measles vaccine (MV) is among the safest and most efficacious. This vaccine elicits neutralizing antibodies and robust, long-lasting Th1 cellular responses.
Therefore, the approach of live attenuated viral vectors for COVID-19 immunization is an attractive candidate with minimal risk of vaccine-associated enhanced respiratory disease (VAERD). To this end, an MV-based vaccine targeting the spike (S) protein of SARS-CoV-2 shows promise.
Several recombinant MV (rMV)-based vaccines against viral pathogens are currently in preclinical and clinical trials, such as vaccines against chikungunya, Zika, and Lassa viruses.
About the study
A new Nature Communications study explores the potential of MV-based vaccines targeting the S protein of SARS-CoV-2 through the generation of a series of rMVs expressing either full-length S or the S2 subunit protein of SARS-CoV-2. In the current study, the researchers tested the capacity of these rMVs to elicit neutralizing antibodies and T-cell responses in a mouse model of measles vaccination, as well as their ability to protect immunized mice from intranasal challenge with mouse-adapted SARS-CoV-2.
Additionally, the immunogenicity and protective efficacy of the lead rMV were tested in the relevant golden Syrian hamster model of SARS-CoV-2 challenge.
Since SARS-CoV and SARS-CoV-2 S proteins share a high degree of similarity, the full-length S protein of SARS-CoV-2 with a transmembrane domain was chosen as the main antigen to be expressed by the MV vector.
To improve its expression, a number of modifications were introduced in the native S sequence, including human codon-optimization and mutation of two prolines, K986P and V987P, in the S2 region, thus increasing its expression and immunogenicity. To increase the surface expression of the S protein in MV-infected cells, 11 C-terminal amino acids (aa 1263-1273) were deleted from the S cytoplasmic tail (CT) to generate dER constructs. Antigens based on the S2 subunit were designed to investigate the possibility of generating a broad-spectrum vaccine targeting SARS-CoV-2 and its variants.
Here, the functionality of S proteins was analyzed by transfecting the same plasma cloning deoxyribonucleic acid (pcDNA) mammalian expression vectors in Vero cells, which express the angiotensin-converting enzyme 2 (ACE2) receptor. It was observed that functional S proteins were expressed on the cell surface.
The expression of the S2 subunit alone resulted in a hyper-fusion phenotype in Vero cells, thus suggesting the triggering of non-receptor-mediated membrane fusion by proteases cleaving at the S2’ site and freeing the fusion peptide. On the other hand, both the 2P-stabilized SF-2P-dER and S2-2P-dER did not induce syncytium formation, thereby indicating that their fusion activity was abrogated by the 2P mutation.
Due to the decreasing expression gradient of MV genes, cloning in the additional transcription unit 2 (ATU2) allows high-level expression of the antigen, while cloning in ATU3 results in lower levels of expression. The stronger the antigen is expressed, the greater the immune response. The lower expression from ATU3 facilitates the rescue of rMV encoding antigens that are toxic or difficult to express.
Upon investigating the immunogenicity of selected rMV vaccine candidates in IFNAR−/− mice, which are susceptible to MV infection, specific immunoglobulin (Ig)G antibodies against SARS-CoV-2 S were detected in 100% of immunized mice. It was found that rMVs expressing SF-2P-dER or S2-2P-dER antigens from ATU2 elicited higher levels of anti-S antibodies than the ATU3 vectors. Furthermore, reducing the immunization dose of the ATU2 candidate still induced higher neutralizing antibody (Nab) titers than the ATU3 vaccine, correlating with its higher expression level.
Notably, no Nabs were detected in animals immunized with the S2 candidates, despite the high levels of anti-S antibodies. In fact, rMV candidates elicited significantly higher IgG2a antibody titers than IgG1, thus indicating a predominant Th1-type immune response.
Immunization with the S2 protein subunit alone was not sufficient to induce strong cellular immune responses. MV-ATU3-S2-2P-dER and empty MV were unable to induce S-specific IFN-γ responses.
The results demonstrated that MV-ATU2-SF-2P-dER induces a robust Th1-driven T-cell immune response to SARS-CoV-2 S antigens at significantly higher levels than MV-ATU3-SF-2P-dER. Comparatively, the S2 candidates elicit much lower cellular responses and no Nabs.
No infectious virus was detected in the lungs of the ATU2 group; however, one of the ATU3 group was found to exhibit infectious virus in its lungs. This observation suggests that, although viral replication may have occurred at low levels, the infectivity of the inoculated and progeny virus was efficiently neutralized.
Half of the animals immunized with MV-ATU2-SF-2P-dER were negative for infectious virus in the lungs, thus depicting partial protection. However, animals immunized with MV-ATU3-SF-2P-dER were not protected.
Animals that received a prime-boost immunization with MV-ATU2-SF-2P-dER were declared clinically healthy during the post-challenge period, with a low clinical score compared to the placebo-immunized (empty MV) animals. Conversely, animals that received a single immunization with MV-ATU2-SF-2P-dER presented an intermediate response.
Hamsters immunized with a prime-boost of MV-ATU2-SF-2P-dER presented undetectable or low infectious viral titers in the lungs. These animals presented a reduction of viral load and viral titers in the nasal turbinates. Animals that received a single immunization exhibited reduced viral RNA loads and viral titers in both lungs and nasal turbinates; however, these results were less significant.
All hamsters immunized with two doses of MV-ATU2-SF-2P-dER had significantly higher NAb titers than human convalescent sera. Animals immunized with a single dose had high NAbs in their serum; however, these Nab levels were slightly lower as compared to animals immunized with two doses.
The Geometric mean titers (GMT) of NAbs elicited by vaccination was rapidly increased by day-4 post-challenge. The antibodies induced by immunization with MV-ATU2-SF-2P-dER also neutralized three of the most prevalent SARS-CoV-2 variants, including the B.1.1.7, P.1, and B.1.351 strains.
Prime-boost immunization with MV-ATU2-SF-2P-dER protected the challenged animals from lung pathology. No or mild macroscopic changes were observed in the lungs of these animals.
Lung histological sections of vaccinated animals appeared healthy with no sign of pathological changes. Contrastingly, substantial pulmonary lesions were observed in the placebo group vaccinated with empty MV.
Immunohistochemistry to detect SARS-CoV-2 antigens in lung tissues was negative in hamsters vaccinated with prime-boost, while control animals exhibited large numbers of SARS-CoV-2 positive cells. Moreover, a single dose of MV-ATU2-SF-2P-dER did not abrogate lung pathology, whereas pulmonary lesions were less severe than in placebo-vaccinated animals.