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

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 (Delta N-NP) of SARS-CoV-2 in anEscherichia coliexpression system and described the purification of the soluble recombinant NP and Delta N-NP in details. The identities of the NP and Delta N-NP were confirmed with mass spectrometry. We then used immunoglobulin G detection ELISAs to compare the sensitivity of NP and Delta N-NP in detecting anti-SARS-CoV-2 antibodies. Delta N-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. Delta N-NP showed a positive detection rate similar to that of the SARS-CoV-2 S protein in human sera. We conclude that Delta N-NP is a better serological marker than NP for evaluating the immunogenicity of inactivated SARS-CoV-2.

Automated summary

This study compared the sensitivity of NP and Delta N-NP in detecting anti-SARS-CoV-2 antibodies using immunoglobulin G detection ELISAs. The results showed that Delta N-NP had greater sensitivity than NP, making it a better serological marker for evaluating the immunogenicity of inactivated SARS-CoV-2.