The coronavirus disease 2019 (COVID‐19) pandemic is transmitted by severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) and has affected millions of people all around the world, leading to more than 6.5 million deaths. The nucleocapsid (N) phosphoprotein plays important roles in modulating viral replication and transcription, virus‐infected cell cycle progression, apoptosis, and regulation of host innate immunity. As an immunodominant protein, N protein induces strong humoral and cellular immune responses in COVID‐19 patients, making it a key marker for studying N‐specific B cell and T cell responses and the development of diagnostic serological assays and efficient vaccines. In this review, we focus on the structural and functional features and the kinetic and epitope mapping of B cell and T cell responses against SARS‐CoV‐2 N protein to extend our understanding on the development of sensitive and specific diagnostic immunological tests and effective vaccines.

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Introduction. Neutralizing antibodies have been widely used for the prophylaxis and treatment of COVID-19. Hypothesis. The major target for these neutralizing antibodies is the receptor-binding domain (RBD) of the viral spike protein. Aim. In the present study, we developed and characterized three neutralizing chimeric mouse-human mAbs for potential therapeutic purposes. Methodology. Light and heavy chain variable region genes of three mouse mAbs (m4E8, m3B6, and m1D1) were amplified and ligated to human Cγ1 and Cκ constant region genes by PCR. After cloning into a dual promoter mammalian expression vector, the final constructs were transiently expressed in DG-44 cells and the purified chimeric antibodies were characterized by ELISA and Western blotting. The neutralizing potency of the chimeric mAbs was determined by three different virus neutralization tests including sVNT, pVNT, and cVNT. Results. All three recombinant chimeric mAbs display human constant regions and are able to specifically bind to the RBD of SARS-CoV-2 with affinities comparable to the parental mAbs. Western blot analysis showed similar epitope specificity profiles for both the chimeric and the parental mouse mAbs. The results of virus neutralization tests (sVNT, pVNT, and cVNT) indicate that c4E8 had the most potent neutralizing activity with IC50 values of 1.772, 0.009, and 0.01 µg ml−1, respectively. All chimeric and mouse mAbs displayed a similar pattern of reactivity with the spike protein of the SARS-CoV-2 variants of concern (VOC) tested, including alpha, delta, and wild-type. Conclusion. The chimeric mAbs displayed neutralizing potency similar to the parental mouse mAbs and are potentially valuable tools for disease control.