N-terminally truncated nucleocapsid protein of SARS-CoV-2 as a better serological marker than whole nucleocapsid protein in evaluating the immunogenicity of inactivated SARS-CoV-2.

Abstract

The coronavirus disease 2019 pandemic caused by severe acute respiratory syndrome‐coronavirus 2 (SARS‐CoV‐2) had led to a serious public health crisis, and no specific treatments or vaccines are available yet. A nucleocapsid protein (NP)‐based enzyme‐linked immunosorbent assay (ELISA) detection method is not only important in disease diagnosis, but is required for the evaluation of vaccine efficacy during the development of an inactivated SARS‐CoV‐2 vaccine. In this study, we expressed both the NP and N‐terminally truncated NP (ΔN‐NP) of SARS‐CoV‐2 in an Escherichia coli expression system and described the purification of the soluble recombinant NP and ΔN‐NP in details. The identities of the NP and ΔN‐NP were confirmed with mass spectrometry. We then used immunoglobulin G detection ELISAs to compare the sensitivity of NP and ΔN‐NP in detecting anti‐SARS‐CoV‐2 antibodies. ΔN‐NP showed greater sensitivity than NP in the analysis of serially diluted sera from mice and rabbits vaccinated with inactive SARS‐CoV‐2 and in human sera diluted 1:400. ΔN‐NP showed a positive detection rate similar to that of the SARS‐CoV‐2 S protein in human sera. We conclude that ΔN‐NP is a better serological marker than NP for evaluating the immunogenicity of inactivated SARS‐CoV‐2.