Fine specificity of antibodies to SARS-CoV-2 nucleoprotein in patients with COVID-19.

The production of antibodies against viral structural proteins such as the spike (S) and the nucleocapsid (N) is a hallmark of SARS-CoV-2 infection. The N protein contains several immunogenic regions. In this work we analysed epitope specificity and avidity of anti-N antibodies in COVID-19 patients. Eighty-nine COVID-19 patients were recruited. IgG, IgA, IgM antibodies to recombinant N protein and to 6 different peptides containing predicted B epitopes were measured by ELISA. Antibody avidity was evaluated by chaotropic ELISA. Anti-N IgG, IgA, and IgM were detected in 59%, 41% and 30% respectively; in 11% cases anti-N IgG are the only antibodies present in COVID patients. IgG and IgA anti-N antibodies levels correlate with levels of IL-6 and inversely with PaO2/FiO2 ratio (p < 0,005). Anti-N IgG displayed medium avidity in 51% and high avidity in 49% COVID patients; anti-N avidity was negatively correlated with PaO2/FiO2 (p < 0,05). Median anti-N antibody levels were similar in discharged or deceased patients and antibody avidity was not correlated with outcome. Among peptides, N366-388 is the most frequently recognized by IgG, IgA and IgM antibodies followed by N380-400, bound by patients IgG and IgA but anti-N380-400 IgG display higher avidity than anti-N366-388 IgG. These results indicate that anti-N antibodies are characterized by medium-high avidity and preferentially bind the COOH-terminus of the nucleoprotein. Anti-N antibodies, especially directed towards specific epitopes, might be relevant for the course of the infection, and their induction might improve current vaccination strategies.