A protease-activatable luminescent biosensor and reporter cell line for authentic SARS-CoV-2 infection

Efforts to define serological correlates of protection against COVID-19 have been hampered by the lack of a simple, scalable, standardised assay for SARS-CoV-2 infection and antibody neutralisation. Plaque assays remain the gold standard, but are impractical for high-throughput screening. In this study, we show that expression of viral proteases may be used to quantitate infected cells. Our assays exploit the cleavage of specific oligopeptide linkers, leading to the activation of cell-based optical biosensors. First, we characterise these biosensors using recombinant SARS-CoV-2 proteases. Next, we confirm their ability to detect viral protease expression during replication of authentic virus. Finally, we generate reporter cells stably expressing an optimised luciferase-based biosensor, enabling viral infection to be measured within 24 h in a 96- or 384-well plate format, including variants of concern. We have therefore developed a luminescent SARS-CoV-2 reporter cell line, and demonstrated its utility for the relative quantitation of infectious virus and titration of neutralising antibodies.

to quantitate reporter activation (S2A-B Fig). All reporters showed an increase in FlipGFP 97 fluorescence, but the magnitude of effect was variable (Fig 1B). We therefore selected the 98 Opt3c-FlipGFP (Fig 1C, MPro) and PLP2-FlipGFP (Fig 1D, PLPro) biosensors for further 99 evaluation. Similar results were obtained using epifluorescence microscopy (Fig 1E). 100 To confirm the specificity of these biosensors for their cognate protease, the same co-101 transfection assay was used to measure Opt3c-FlipGFP, PLP2-FlipGFP and TEV-FlipGFP 102 activation in pair-wise combination with MPro, PLPro or TEV protease. In each case, an 103 increase in FlipGFP fluorescence was observed only in the presence of cognate protease (Fig 104 These fluorescent biosensors therefore provide proof-of-concept data confirming that viral 124 protease activity may be used to signal SARS-CoV-2 infection. Nonetheless, the magnitude 125 of effect is markedly reduced compared with over-expression of recombinant viral proteases 126 (compare Fig 2B with Fig 1G). This likely reflects lower levels of protease expression during 127 viral infection (Fig 2C). In addition, it is possible that the localisation of proteases and/or 128 presence of other viral proteins and endogenous polyprotein substrates during authentic viral 129 infection reduces their likelihood of encountering reporter molecules. 130

Luciferase-based biosensors of authentic SARS-CoV-2 infection 131
To enzymatically amplify the signal from viral protease activity, we next sought to generate 132 equivalent luciferase-based biosensors. The 30F-GloSensor reporter comprises an inactive, 133 circularly permuted firefly luciferase (FFluc) molecule, in which cleavage of a specific 134 oligopeptide linker is able to restore luminescence [12]. Co-expression with Renilla luciferase 135 (Rluc) from the same vector allows reporter levels and cell viability to be normalised between 136 conditions. We therefore generated 30F-GloSensor constructs with the same sequence-137 specific oligopeptide linkers as our FlipGFP-based reporters, together with the equivalent non-138 cleavable controls. These constructs were co-transfected with/without cognate SARS-CoV-2 139 protease into HEK293T cells, and luminescence analysed after 24 h (Fig 3A). Normalising for 140 observed a modest yet statistically significant reduction in neutralising activity against the 204 B.1.617.2 (delta) variant of concern (Fig 5D). virological assays, and screens of candidate antiviral compounds. Luminescent assays may 212 be readily adapted to high-throughput platforms and, whilst the mechanism of reporter 213 activation is different, the range of potential applications is similar to TZM-bl reporter cells for 214

FlipGFP-based reporters 268
To generate FlipGFP-based reporters, the TEV FlipGFP plasmid PCDNA3-FlipGFP(TEV 269 cleavage seq) T2A mCherry (Addgene, #124429, a gift from Xiaokun Shu [9]) was used as a 270 template for pairs of PCR reactions including primers designed to generate overlapping 271 products replacing the TEV cleavage site with the indicated cleavage sequence (S9 Table). 272 For example, to replace the TEV cleavage site with the PLP2 cleavage sequence, products 273 generated by PCR reactions including 'Near AfeI'/'PLP2 Rv' and 'PLP2 Fw'/'Near AflII' primer 274 pairs were used. The same plasmid was then digested with AfeI and AflII and assembled with 275 the gel purified PCR products using the HiFi Assembly Master Mix (NEB, E5520). 276 For MPro, we selected a wildtype self-cleavage sequence (SAVLQ/SGF, herein termed WT3c) 277 present between nsp4 and nsp5, and an "optimal" cleavage sequence ( To generate a non-cleavable Opt3c-FlipGFP (MPro) reporter, the critical glutamine residue in 282 the cleavage site was changed to isoleucine (TVRLI/SGF). For the PLP2-FlipGFP (PLPro) 283 reporter, the critical LKGG sequence was scrambled to GLGK (TGLGK/APT). 284

Viral proteases 300
To generate an expression vector for SARS-  In brief, images were acquired using a Cellomics ArrayScan XTI high-throughput imaging 409 platform (Thermo Fisher) using a 386 nm excitation/emission filter to detect DAPI-stained 410 nuclei and a 560 nm excitation/emission filter to detect AF594. Images were then analysed 411 with built-in high content HCS Studio software (by Thermo Fisher) using the Target Activation 412 application. For this, cellular objects were identified by applying overlays (masks) based on 413 DAPI intensity. Necessary steps to exclude non-cellular artefacts (large or small objects) were 414 activated based on average nuclei size. Additionally, background correction was performed 415 on both channels. The generated nuclei masks were then applied to the AF594 channel and 416 the threshold for AF594 staining was determined using stained mock-infected cells. Finally, 417 cells were considered infected if their AF594 signal was above this threshold. 42 fields were 418 scanned for each sample/condition to ensure the analysis of a sufficient number of cells. 419

Confocal microscopic analysis of FlipGFP-based reporters and spike protein 420 expression in SARS-CoV-2-infected cells 421
HEK293T-ACE2 cells were seeded at a density 9 x 10 4 cells/well of an 8-well µ-Slide (Ibidi, 422 80826) in 250 µL complete media. After 1 h, cells were transfected in duplicate with the 423 indicated FlipGFP-based reporter constructs as above. The following morning, cells were 424 infected with SARS-CoV-2 at the indicated MOI and incubated for 24 h. 425 To measure reporter activation in infected cells using confocal microscopy, cells were first 426 Diagnostic cutoffs were determined by receiver operating characteristic (ROC) curve analysis. 518

Ethics statement 519
Ethical approval for this study was granted by the East of England -Cambridge Central 520 Research Ethics Committee (08/H0308/176). Written informed consent was obtained from all 521 volunteers prior to providing serum samples. 522