Rapid, reliable, and reproducible cell fusion assay to quantify SARS-Cov-2 spike interaction with hACE2

COVID-19 is a global crisis of unimagined dimensions. Currently, Remedesivir is only fully licensed FDA therapeutic. A major target of the vaccine effort is the SARS-CoV-2 spike-hACE2 interaction, and assessment of efficacy relies on time consuming neutralization assay. Here, we developed a cell fusion assay based upon spike-hACE2 interaction. The system was tested by transient co-transfection of 293T cells, which demonstrated good correlation with standard spike pseudotyping for inhibition by sera and biologics. Then established stable cell lines were very well behaved and gave even better correlation with pseudotyping results, after a short, overnight co-incubation. Results with the stable cell fusion assay also correlated well with those of a live virus assay. In summary we have established a rapid, reliable, and reproducible cell fusion assay that will serve to complement the other neutralization assays currently in use, is easy to implement in most laboratories, and may serve as the basis for high throughput screens to identify inhibitors of SARS-CoV-2 virus-cell binding and entry.
Author summary COVID-19 continues to be a global public health concern. Currently, the in vitro assessment of efficacy of any vaccine or therapeutic is based on time-consuming neutralization assays using live or pseudotyped virus at BSL2/BSL3 biocontainment. There are many factors which will affect the neutralization assay, including quality of plasmid, transfection efficiency, virus titer, etc. Here, we developed a novel cell fusion assay based upon spike-hACE2 interaction, which demonstrated excellent correlation with standard spike pseudotyping for inhibition by convalescent sera, cloned antibodies, and biologics, after 16–24 hours co-incubation. It also correlated well with a live virus assay. Other than two stable cell lines, our cell fusion assay requires no specialized research reagents or laboratory equipment and should be easy to adapt for use in most investigative and clinical settings. It will allow for the testing of sera after vaccination or infection, to assess for level of immune protection, and it could be used for high throughput screening for monoclonal antibodies, compounds, and biologics that interfere with virus-cell binding and entry.