A functional assay for serum detection of antibodies against SARS‐CoV‐2 nucleoprotein

The humoral immune response to SARS‐CoV‐2 results in antibodies against spike (S) and nucleoprotein (N). However, whilst there are widely available neutralization assays for S antibodies, there is no assay for N‐antibody activity. Here, we present a simple in vitro method called EDNA (electroporated‐antibody‐dependent neutralization assay) that provides a quantitative measure of N‐antibody activity in unpurified serum from SARS‐CoV‐2 convalescents. We show that N antibodies neutralize SARS‐CoV‐2 intracellularly and cell‐autonomously but require the cytosolic Fc receptor TRIM21. Using EDNA, we show that low N‐antibody titres can be neutralizing, whilst some convalescents possess serum with high titres but weak activity. N‐antibody and N‐specific T‐cell activity correlates within individuals, suggesting N antibodies may protect against SARS‐CoV‐2 by promoting antigen presentation. This work highlights the potential benefits of N‐based vaccines and provides an in vitro assay to allow the antibodies they induce to be tested.