Development of multivalent next-generation vaccine strategy against ancestral SARS-CoV-2 and emerging variants

By Tarun Sai LomteReviewed by Danielle Ellis, B.Sc.Feb 14 2022

A recent study published in the journal Cell demonstrated that intranasal (i.n.) delivery of coronavirus 2019 (COVID-19) vaccine in murine models elicits greater immune responses against the wildtype severe acute respiratory coronavirus-2 (SARS-CoV-2) and its variants.

Since the emergence of SARS-CoV-2 in late 2019, there have been over 412 million COVID-19 infections to date, causing more than 5.8 million deaths worldwide. Several antiviral drugs and therapeutic antibodies are being used for treatment, and SARS-CoV-2 vaccines have been approved for emergency use to raise immunity at a population level.

Different vaccines like nucleic acid-based, Adenovirus-vectored (Adv), recombinant protein subunit, and whole-virus attenuated vaccines, among others, have been developed. These first-generation vaccines are based on the wild type (WT) SARS-CoV-2 and administered conventionally through intramuscular (i.m.) injections and, according to some reports, are less efficacious against SARS-CoV-2 variants. Besides, the need to design effective vaccines against the variants and devise novel next-generation strategies to improve vaccine efficiency is growing amid speculations of developing a variant-specific vaccine.

The study

In the present study, researchers designed next-generation human/chimpanzee Adv vaccines and tested them in murine models. The human Adv (Tri:HuAd) vaccine or chimpanzee Adv (Tri:ChAd) vaccine expressing the complete S1 domain of spike (S) protein, full nucleocapsid (N) protein, and a truncated nsp12 (RNA-dependent RNA polymerase or RdRp) as SARS-CoV-2 antigens was injected intranasally or intramuscularly in mice.

The three antigens’ transgene was constructed through overlapping polymerase chain reactions (PCR). Tissue plasminogen (tPA) signal sequence was cloned upstream of spike S1 sequence of Wuhan-Hu-1 SARS-CoV-2 isolate fused to vesicular stomatitis G protein transmembrane (VSVG TM) domain to synthesize the first PCR product.

The VSVG TM domain was included to enable trimerization and exosomal targeting, and the PCR product was placed under the control of murine cytomegalovirus (CMV) promoter in a pCY1 plasmid. The porcine teschovirus-1 2A (P2A) skip sequence was included upstream of full-length N protein fused to a conserved portion of RdRp (truncated polymerase) to generate the second PCR product. The second product was cloned into a pCY1 plasmid downstream of the VSVG TM sequence to yield the final expression cassette. This expression cassette was introduced in shuttle vectors for transfection and synthesis of Tri:HuAd and Tri:ChAd vaccines.

Findings

The safety profile of Tri:HuAd and Tri:ChAd was first evaluated in murine models after a single dose. The evaluation of respiratory mucosa four weeks post-vaccination showed S-specific immunoglobulin-G (IgG) responses in mice with i.n. delivery of either vaccine. In contrast, IgG responses in the airways were absent in those with i.m. administration of either vaccine. The humoral responses were more pronounced locally and systemically with an i.n. vaccine than i.m. vaccine and particularly enriched with the Tri:ChAd vaccine.

The S1-specific CD8+ cells were multifunctional expressing interferon-γ (IFNγ) and tumor necrosis factor-α (TNFα) while N- and RdRp-specific CD8+ cells were monofunctional. Mucosal tissue-resident T (T_RM) lymphocytes were induced only with i.n. vaccination and markedly higher with the Tri:ChAd vaccine. Alveolar macrophages (AM), which represent the innate defenses, are implicated in early innate responses upon SARS-CoV-2 infection. The team assessed AM responses after i.m. or i.n. immunization with either vaccine and noted that i.n. Tri:ChAd vaccine generated robust AM responses.

The inoculation of a lethal dose of mouse-adapted virus (SARS-CoV-2 MA 10) four weeks post-vaccination resulted in a humane endpoint after 4-5 days in about 80% of mice injected with either vaccine intramuscularly. Interestingly, i.n. vaccinated mice were well protected throughout experimentation.

A highly susceptible mice strain, K18-hACE2, immunized with Tri:ChAd i.n. the vaccine was further evaluated. The K18-hACE2 mice tolerated a lethal dose of WT SARS-CoV-2, while the unvaccinated mice succumbed after five to seven days. The authors then inoculated these mice with a lethal dose of SARS-CoV-2 variants: B.1.1.7 (Alpha) and B.1.351 (Beta).

Sera from Tri:ChAd vaccinated mice demonstrated equal potential to neutralize WT virus and B.1.1.7 variant, but it had reduced potency against the Beta variant. Overall, the mice were well protected with a single dose of the Tri: ChAd vaccine against WT isolate and the two variants. Furthermore, the authors investigated the immune responses of a trivalent vaccine over bivalent (N:RdRp) and monovalent (S1) ChAd vaccines and observed that a trivalent ChAd vaccine with S1, N, and RdRp antigens offered additional protection than the bivalent or monovalent ChAd vaccine.

Conclusions

The research team observed a superior immune response against SARS-CoV-2 with an i.n. Adv vaccination than i.m. vaccination. Moreover, the ChAd vaccine was more potent than the HuAd vaccine to elicit both humoral and cellular responses. Trivalent vaccine resulted in robust immune responses than either bivalent or monovalent vaccine.

In conclusion, the findings showed that an intranasal vaccine with three SARS-CoV-2 antigens offered strong protection, mainly in the respiratory tract, against the WT isolate and the immune-evasive Beta variant. These findings were illustrated in murine models, and it must be tested whether ChAd or HuAd platform is effective in humans.