Protective potential of N-based vaccines against SARS-CoV-2 in a Syrian hamster model

More than 700 million confirmed cases of the novel coronavirus infection COVID-19 have been registered during the 2019-2023 pandemic, and this potentially fatal pathology continues to be identified worldwide. An optimal target for creation of cross-protective vaccines against COVID-19 seems to be the nucleocapsid (N) protein – one of the most conserved and actively produced by infected cells of SARS-CoV-2 antigens. However, the patterns of immune responses induced by N-containing vaccines remain poorly understood. So, the purpose of our study was the comparative investigation of humoral and T-cell immunogenicity of recombinant N protein (rN), N-expressing live attenuated influenza vaccine (LAIV-N), and formalin-inactivated SARS-CoV-2 (SARS-FI) and assessment of the protective effects of these vaccine candidates against homologous SARS-CoV-2 (B.1, Wuhan) strain.

Syrian hamsters received two injections of experimental vaccines three weeks apart. The effectiveness of the immune responses was measured after 42 days, and protection was tested by exposing the vaccinated hamsters to 10⁵ TCID50 of the challenge virus. To assess intergroup differences, a one-factor ANOVA with Tukey's post-hoc test was used.

A pronounced production of N-specific antibodies and T cells was found following immunization with SARS-FI and rN, whereas only the formation of IFN-γ -synthesizing splenocytes in response to N antigen stimulation was shown for LAIV-N vaccinated animals. Interestingly, LAIV-N and SARS-FI administration significantly prevented virus replication in respiratory organs and progression of infection, while rN vaccination led to better lung tissue performance, but was ineffective for the inhibition of viral airway propagation.

Although intraperitoneal injection of recombinant N protein induced robust antibody responses, these effects were insufficient to reduce viral loads in the respiratory tissues of immunized hamsters. At the same time, the N protein delivered by the attenuated influenza vector via the intranasal route did not provoke anti-N serum antibodies but stimulated N-specific cellular immune responses and protected animals, partially reducing the viral replication in the upper respiratory tract. This may be explained by the need to engage mucosal or innate immune factors which may be provoked by the LAIV as a carrying vector.

Based on the high immunogenicity and significant protective potential, both inactivated SARS-CoV-2 and modified LAIV encoding the N antigen of SARS-CoV-2 (B.1) demonstrate potential as effective means for protection against COVID-19, in contrast to the recombinant N protein.