Background The vast majority of SARS-CoV-2 vaccines which are
licensed or under development focus on the spike (S) protein and its
receptor binding domain (RBD). However, S and RBD from SARS-CoV-2
variants of concerns show considerable sequence variations and repeated
injections for boosting specific immunity are necessary. Aim of this
study was to develop and characterize a SARS-CoV-2 vaccine targeting the
highly conserved nucleocapsid (N) protein. Methods Recombinant
N protein was expressed in Escherichia coli, purified to
homogeneity by chromatography and characterized by SDS-PAGE,
immunoblotting, mass spectrometry, dynamic light scattering and
differential scanning calorimetry. The N protein vaccine was obtained by
formulation of recombinant N as squalane-based emulsion and used to
immunize Balb/c mice, NOD scid gamma (NSG) mice engrafted with human
PBMC, rabbits and marmoset monkeys to study safety as well as antibody
and cellular immunity using ELISA for antibodies, measurement of
N-specific Th1 and Th2 cytokine secretion and carboxyfluorescein
succinimidyl ester (CFSE) dilution assays for CD4 +
and CD8 + T cell responses. The protective effect of
the vaccine was studied in SARS-CoV-2-infected Syrian hamsters.
Results Immunization of mice, rabbits and Syrian hamsters with
the recombinant N protein-based vaccine formulated as squalane-based
emulsion (Convacell®) induced sustainable N-specific IgG responses and a
N-specific mixed Th1/Th2 cytokine response. In marmoset monkeys a
N-specific CD4 + as well as CD8 + T
cell response was observed. Vaccinated and then infected Syrian hamsters
showed reduced lung histopathology, reduced virus was detected in lung
tissue, lung weight relative to the body was not increased after
challenge and body weight was regained faster than in non-vaccinated
animals. Repeated dose toxicity studies in mice and rabbits showed that
Convacell® was well tolerated and safe. Conclusions Convacell®
induced a SARS-CoV-2-specific protective immune response in Syrian
hamsters. It is a new vaccine targeting the nucleocapsid protein of
SARS-CoV-2 and thus may augment the armamentarium of vaccines for
COVID-19.