Abstract

Vaccines directed against SARS-CoV-2 spike are able to induce neutralising antibodies and prevent COVID-19 disease. Nonetheless, SARS-CoV-2 variants have emerged with the ability to escape antibody neutralisation in vitro, and protective immunity in vivo. Defining correlates of protection has been complicated by variability in the assays used to measure neutralising antibody titres. Whilst plaque reduction neutralisation tests (PRNT) remain the gold standard, their utility is limited by low throughput and long turnaround times. To overcome these problems, we have developed a luminescent reporter cell line for SARS-CoV-2, exploiting the expression of viral protease for the quantitation of infected cells. In these cells, cleavage of a specific oligopeptide linker by the Papain-Like Protease of SARS-CoV-2 leads to activation of a circularly permuted firefly luciferase-based biosensor. Accordingly, viral replication may be measured within 24 h in a 96- or 384-well plate format, using a simple microplate luminometer. The reporter cells may be activated by different SARS-CoV-2 variants, and are ideally suited for titration of neutralising activity in sera, from individuals infected with SARS-CoV-2 virus or inoculated with SARS-CoV-2 vaccine. Beyond SARS-CoV-2, the Papain-Like protease cleavage sequence in the biosensor is highly conserved across related betacoronaviruses. Our reporter cells therefore provide an “off the shelf” solution for the quantitation of viral replication and neutralising antibody activity during this and (potentially) future coronavirus pandemics. As well as serological surveys, they are equally appropriate for high throughput screens of candidate antiviral compounds.