Serum samples and ethical approval
Ethical approval for study of vaccine elicited antibodies in sera from vaccinees was obtained from the East of England – Cambridge Central Research Ethics Committee Cambridge (REC ref: 17/EE/0025). Studies involving health care workers (including testing and sequencing of respiratory samples) were reviewed and approved by The Institutional Human Ethics Committees of NCDC and CSIR-IGIB(NCDC/2020/NERC/14 and CSIR-IGIB/IHEC/2020-21/01). Participants provided informed consent.
The virus isolation procedures in this study were approved by the Institutional Review Board of National Institute for Infectious Diseases (approval ID: 1178) and Tokyo Metropolitan Institute of Public Health (approval ID: 3KenKenKen-466) according to the Declaration of Helsinki 2013. All protocols involving specimens from human subjects recruited at Kyoto University, Kuramochi Clinic Interpark and Universidad San Francisco de Quito were reviewed and approved by the Institutional Review Boards of Kyoto University (approval ID: G0697), Kuramochi Clinic Interpark (approval ID: G2021-004) and Universidad San Francisco de Quito (approval ID: CEISH P2020-022IN), and the Ecuadorian Ministry of Health (approval IDs: MSP-CGDES-2020-0121-O and MSP-CGDES-061-2020). The export of sera from Ecuador to Japan was approved by ARCSA ID: ARCSA-ARCSA-CGTC-DTRSNSOYA-2021-1626-M. All human subjects provided written informed consent. All protocols for the use of human specimens were reviewed and approved by the Institutional Review Boards of The Institute of Medical Science, The University of Tokyo (approval ID: 2021-1-0416), Kumamoto University (approval IDs: 2066 and 2074), University of Miyazaki (approval ID: O-1021)
Sequencing
Spike genomes for the original Wuhan strain, and Omicron VOC were obtained from GISAID EpiCoV database accessed on 30th November 2021. A consensus Spike genome was created from all complete and high coverage genomes, excluding all sequences with >5% Ns using Geneious Prime v2022. The consensus genome was translated to poly-proteins and the Spike gene was aligned to the original Wuhan strain using mafft v7.49038 with the --globalpair --maxiterate 1000 flags.
Data availability:
The above structural models used in this study for Delta and Omicron variants are available at https://github.com/CSB-Thukral-Lab/Spike_structural_models_Delta_and_Omicron
3D structural models of the spike homotrimer protein complex were alternatively also generated using Alphafold v2.1.139. In its validation at the 14th edition of the Critical Assessment of protein Structure Prediction (CASP14) the predictions generated were demonstrated to be comparative to experimental structures. When a close homolog structure is available to Alphafold the predictions it generates for those positions are within typical experimental error. Required databases were downloaded on 02/12/2021. The program was run with the parameters --max_template_date=2021-12-01 --model_preset=monomer --db_preset=full_dbs --is_prokaryote_list=false. Protein structures were visualised in ChimeraX v1.340. As predicted structures for the whole spike protein include poorly resolved chains at the terminal ends, these residues were identified by overlaying structures on PDB entry 6ZP2, then selected and removed from PDB files using the delete atoms/bonds action. Two further monomers were overlayed on 6zp2 to generate a homotrimer structure. Mutated residues were then coloured in red and labelled manually with respect to the Wuhan strain.
To model interactions between the omicron spike RBD and REGN 10933 and 10987, 6XDG was downloaded from PDB and aligned to the alphafolded omicron spike using the matchermaker function in ChimeraX v1.3. Predicted hydrogen bonds were plot within ChimeraX using a relaxed distance of 0.400Å and are shown as dashed green and blue lines. Predicted contact points between the RBD and REGN 10933 and 10987chains are indicated as red spheres using the same criteria as for hydrogen bonds.
Data availability:
All protein structures shown are freely available at github.com/ojcharles/viral_alphafold
ACE2 binding affinity assays
Biolayer Interferometry
Assays were performed on an Octet Red (ForteBio) instrument at 30°C with shaking at 1,000 RPM. Streptavidin (SA) biosensors were hydrated in water for 10 min prior to a 1 min incubation in undiluted 10X kinetics buffer. SARS-CoV-2 Wuhan, Alpha, or Omicron RBDs were loaded at 2.5-7.5 μg/mL in 10X Kinetics Buffer for 100-400 s prior to baseline equilibration for 120 s in 10X kinetics buffer. Association of ACE2 to Wuhan RBD was assessed in 10X kinetics buffer at various concentrations in a three-fold dilution series from 333.3 to 4.1 nM was carried out for 150 s prior to dissociation for 150 s for. Association of ACE2 to Alpha or Omicron RBD was assessed in 10X kinetics buffer at various concentrations in a three-fold dilution series from 266.7 nM to 3.3 nM was carried out for 200 s prior to dissociation for 200 s. The data were normalized to the baseline and nonspecific binding was subtracted, prior to fitting performed using a 1:1 binding model and the ForteBio data analysis software. Kinetics values (KD, kon, koff) were determined with a global fit applied to all data.
The SARS-CoV-2-RBD-Avi constructs were synthesized by GenScript into pcDNA3.1- with an N-terminal mu-phosphatase signal peptide and a C-terminal octa-histidine tag, flexible linker, and avi tag (GHHHHHHHHGGSSGLNDIFEAQKIEWHE). The boundaries of the construct are N-328RFPN331 and 528KKST531-C, PMID: 32155444, PMID: 33706364. The human angiotensin-converting enzyme ectodomain (hACE2) consists of residues 1–614 fused to a C-terminal octa-histidine tag PMID: 32841599. Proteins were produced in Expi293F Cells (ThermoFisher Scientific) grown in suspension using Expi293 Expression Medium (ThermoFisher Scientific) at 37°C in a humidified 8% CO2 incubator rotating at 130 rpm. Cells grown to a density of 3 million cells per mL were transfected using the ExpiFectamine 293 Transfection Kit (ThermoFisher Scientific) and cultivated for 4 days. Proteins were purified from clarified supernatants using a nickel HisTrap HP affinity column (Cytiva) and washed with ten column volumes of 20 mM imidazole, 25 mM sodium phosphate pH 8.0, and 300 mM NaCl before elution on a gradient to 500 mM imidazole. Proteins were buffer exchanged into 20 mM sodium phosphate pH 8 and 100 mM NaCl and concentrated using centrifugal filters (Amicon Ultra) before being flash frozen.
Flow Cytometry analysis of Spike proteins binding to recombinant human ACE2
ExpiCHO cells were seeded into 50 ml Mini Bioreactor Centrifuge Tube (Corning) at 6 x 106 cells/ml in 5 ml ExpiCHO Expression medium (Life technology). Plasmids encoding SARS-CoV-2 Wuhan, Alpha (B.1.1.7), Delta (B.1.617.2) or Omicron (B.1.1.529) Spike protein (5 mg) were diluted in OptiPRO SFM (Life Technology) and mixed with Expifectamine CHO Reagent (Life Technology). After 1 min incubation at room temperature, transfection mixes were added to cell suspensions. Next, cells were incubated at 37°C 8% CO2 with an orbital shaking speed of 120 rpm for the following 48 hours.
Transiently transfected cells were harvested and washed with PBS, 1% BSA, 2 mM EDTA. Cells were counted, dispensed into round-bottom 96 well plates (Corning) and incubated with human IgG Fc-conjugated ACE2 serial dilutions (concentration range: 30’000 – 0.17 ng/ml) for 45 min at 4°C. After two washes, Alexa Fluor647 Goat Anti-Human IgG secondary Ab (1.5 mg/ml) (Jackson Immunoresearch) was added to the cells, which were then incubated for 30 min at 4°C in the dark. Cells were washed twice and resuspended in wash buffer for data acquisition at ZE5 cytometer (Biorad). To assess Spike protein expression level, an aliquot of each transfectant cell line was stained with 10 mg/ml of S2P6 antibody (Pinto et al., Science 2021) for 45 min at 4°C.
Neutralisation titre analyses
Neutralisation by vaccine-elicited antibodies after two doses of the BNT162b2 and Chad-Ox-1 vaccine, in addition to a third dose with BNT162b2 was determined by PV infections in the presence of serial dilutions of sera as described10. The ID50 within groups were summarised as a geometric mean titre (GMT) and statistical comparison between groups were made with Mann-Whitney or Wilcoxon ranked sign tests. Statistical analyses were performed using Stata v13 and Prism v9.
Pseudotype virus (PV) experiments for neutralisation assays
HEK 293T CRL-3216, Hela-ACE-2 (Gift from James Voss), Vero CCL-81 were maintained in Dulbecco’s Modified Eagle Medium (DMEM) supplemented with 10% fetal calf serum (FCS), 100 U/ml penicillin, and 100mg/ml streptomycin. All cells were regularly tested and are mycoplasma free.
Pseudotype virus preparation for testing against vaccine elicited antibodies and cell entry
Plasmids encoding the spike protein of SARS-CoV-2 D614 with a C terminal 19 amino acid deletion with D614G were used. Omicron and Delta spikes were generated by gene synthesis. Viral vectors were prepared by transfection of 293T cells by using Fugene HD transfection reagent (Promega). 293T cells were transfected with a mixture of 11ul of Fugene HD, 1µg of pCDNAΔ19 spike, 1ug of p8.91 HIV-1 gag-pol expression vector and 1.5µg of pCSFLW (expressing the firefly luciferase reporter gene with the HIV-1 packaging signal). Viral supernatant was collected at 48 and 72h after transfection, filtered through 0.45um filter and stored at -80˚C as previously described. Infectivity was measured by luciferase detection in target cells.
Sera Collection for live virus neutralisation experiments
Vaccine sera were collected from fourteen vaccinees four weeks after the second vaccination with BNT162b2 (Pfizer-BioNTech) (average age: 46, range: 38-55, 21% male). The sera obtained from seven vaccinees 2-3 weeks after the second vaccination with ChAdOx1 (Oxford-AstraZeneca) (average age: 46, range: 35-54, 71% male) were purchased from BioIVT.
Convalescent sera were collected from ten vaccine-naive individuals who had been infected with Delta variant (AY.29) (average age: 46, range: 22-63, 70% male). To determine the SARS-CoV-2 variants infected, saliva were collected from COVID-19 patients during onset and RNA was extracted using a QIAamp viral RNA mini kit (Qiagen, Qiagen, Cat# 52906) according to the manufacturer’s protocol. The sample was subjected to whole genome sequencing based on a modified ARTIC Network protocol41, and the near full-length SARS-CoV-2 genome sequences were obtained. Alternatively, we also performed the capture-hybridization method. The RNA isolated from saliva was treated with DNase I (Takara, Cat# EN0521) and the sequencing library was prepared using Twist library preparation kit (Twist Bioscience, Cat# 101058). The capture-hybridization was conducted using xGen COVID-19 capture panel and xGen hybridization and wash kit (Integrated DNA Technologies, Cat# 1080578) according to the manufacturer’s instruction. Illumina sequencing was performed using MiSeq reagent kit v2 (300 cycles) and a MiSeq sequencer (Illumina). For the data analysis, trimmomatic-0.39 (reference42) was used to remove the adaptors and low-quality reads from the raw sequence data. The trimmed paired-end reads were aligned to the human genome hg38 using bowtie2 v2.3.4.3 (reference43) and unmapped reads were mapped to the original SARS-CoV-2 genome (strain Wuhan-Hu-1, GenBank accession no. NC_045512.2) using Bwa-mem2 (https://github.com/bwa-mem2/bwa-mem2). The PCR duplicates were removed by gencore v0.16.0 (reference 44) and a consensus sequence was obtained by IGV v2.10.2 (reference 45). The mutations detected and viral lineage were determined by using CoVsurver (https://corona.bii.a-star.edu.sg) and Pangolin COVID-19 lineage assigner (https://pangolin.cog-uk.io/). The twelve convalescent sera during early pandemic (until April 2020) (average age: 71, range: 52-92, 8% male) were purchased from RayBiotech. Sera were inactivated at 56°C for 30 min and stored at –80°C until use.
Live virus experiments in Caco-2
Vero E6 (ATCC-CRL1586) and Caco-2 cells transduced with BVDV NPro (Caco-NPro) were cultured in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10 % (v/v) FCS, non-essential amino acid (NEAA), glutamine and 100 U penicillin ml−1 and 100 µg streptomycin ml−1 at 37 °C with 5 % CO2. Propagated live infectious SARS-CoV-2 viruses, lineages B.1.1.617.2 (Delta) and B.1.1.529 (Omicron) were kindly received from Professor Wendy Barclay (Imperial College London) and Dr Jonathan Brown (Imperial College London) as part of the work conducted by G2P-UK National Virology Consortium. The virus isolate matching Omicron variant, kindly donated by Gavin Screaton at Oxford University, was propagated on Vero-ACE2-TMPRSS2 (VAT) cells for 3 days until cytopathic effect was observed. Caco-2 live virus infections were conducted in containment level 3 facility at the Cambridge Institute Therapeutic Immunology and Infectious Disease (CITIID) under approved standard operating procedures and protocols.
Lentivirus-transduced Caco-2 Npro cells were prepared in 48-well plates and infected in biological triplicates with either SARS-CoV-2 Delta or Omicron variant at m.o.i. 1 or 0.1 TCID50 per cell in 250 µl DMEM containing 2% FBS for 2 h at 37 °C with 5% CO2. Unabsorbed inoculum was then removed, cells were washed and the media was replaced with DMEM containing 2% FBS. Virus infections were maintained up to 72 h. Cell culture supernatants were harvested at 0, 24, 48 and 72 h post infection (p.i.) where viral RNAs were quantitated by RT-qPCR and/or infectious virus units were titrated by TCID50.
RNA extractions. Viral RNA was extracted using guanidinium-based reagent containing 2% TX-100. SARS-CoV-2 gene specific primer probe against E gene was used to evaluate levels of viral RNA by RT-qPCR, with an addition of no-template control analysed routinely as negative control. The levels of SARS-CoV-2 mRNA were determined based on absolute quantitation against a standard curve generated using in vitro-transcribed RNA from fragment 11 clone kindly provided by Volker Thiel (University of Bern). Data were collected using a ViiA 7 Real-Time PCR System (Applied Biosystems).
TCID50 assay. Ten-fold serial dilutions of collected virus supernatants were prepared in DMEM culture media. Of these dilutions, 50 µl was inoculated onto monolayers of Vero E6 cells grown on 96-well plates and incubated at 37 °C in a 5 % CO2 incubator. Virus titres were collected at 72 hours p.i. and expressed as TCID50 ml−1 values by the Reed–Muench method (Reed & Muench, 1938).
Standardisation of virus input by SYBR Green-based product-enhanced PCR assay (SG-PERT)
The reverse transcriptase activity of virus preparations was determined by qPCR using a SYBR Green-based product-enhanced PCR assay (SG-PERT) as previously described46. Briefly, 10-fold dilutions of virus supernatant were lysed in a 1:1 ratio in a 2x lysis solution (made up of 40% glycerol v/v 0.25% Triton X-100 v/v 100mM KCl, RNase inhibitor 0.8 U/ml, TrisHCL 100mM, buffered to pH7.4) for 10 minutes at room temperature.
Sample lysates (12 µl) were added to 13 µl of SYBR Green master mix (containing 0.5µM of MS2-RNA Fwd and Rev primers, 3.5pmol/ml of MS2-RNA, and 0.125U/µl of Ribolock RNAse inhibitor and cycled in a QuantStudio. Relative amounts of reverse transcriptase activity were determined as the rate of transcription of bacteriophage MS2 RNA, with absolute RT activity calculated by comparing the relative amounts of RT to an RT standard of known activity.
Cells for replication experiments and entry assays
H1299 cells were a kind gift from Simon Cook. Calu-3 cells were a kind gift from Paul Lehner, A549 A2T247 cells were a kind gift from Massimo Palmerini. Vero E6 Ace2/TMPRSS2 cells were a kind gift from Emma Thomson.
Nasal epithelial tissue infection
Primary human nasal epithelial cells (Cat# EP02, Batch# MP0010) were purchased from Epithelix and maintained according to the manufacturer's procedure. The infection experiment using primary human nasal epithelial cells was performed as previously described (Saito et al, Nature, 2021)48. Briefly, the working viruses were diluted with Opti-MEM (Thermo Fisher Scientific, Cat# 11058021). The diluted viruses (1,000 TCID50 in 100 μl) were inoculated onto the apical side of the culture and incubated at 37°C for 1 h. The inoculated viruses were removed and washed twice with Opti-MEM. To harvest the viruses on the apical side of the culture, 100 μl Opti-MEM was applied onto the apical side of the culture and incubated at 37°C for 10 min. The Opti-MEM applied was harvested and used for RT–qPCR to quantify the viral RNA copy number (see below).
Western blot on infected cells and virions
VeroE6-ACE2/TMPRSS2 cells were infected with SARS-CoV-2 at MOI 1. At 24 and 48 h.p.i., cells and culture media were harvested. The culture media were centrifuged with supernatants collected . To pellet down SARS-CoV-2 in the supernatants, virus were centrifuged at 20,000 g for 2 h at 4 °C through a 20% sucrose cushion or by PEG6000 (Papa et al plos pathogen 2021), and virus pellets were resuspended in 1 x SDS sample buffer.
Western blotting on virions was performed as previously described13. For cell lysates, the harvested cells were washed and lysed in lysis buffer [25 mM HEPES (pH 7.2), 20% glycerol, 125 mM NaCl, 1% Nonidet P40 substitute (Nacalai Tesque, Cat# 18558-54), protease inhibitor cocktail (Nacalai Tesque, Cat# 03969-21)] and lysates were diluted with 2 × sample buffer [100 mM Tris-HCl (pH 6.8), 4% SDS, 12% β-mercaptoethanol, 20% glycerol, 0.05% bromophenol blue] and boiled for 10 m. Then, 10 μl samples were subjected to Western blotting. For protein detection, the following antibodies were used: mouse anti-SARS-CoV-2 S monoclonal antibody (clone 1A9, GeneTex, Cat# GTX632604, 1:10,000), rabbit anti-SARS-CoV-2 N monoclonal antibody (clone HL344, GeneTex, Cat# GTX635679, 1:5,000), mouse anti-alpha tubulin (TUBA) monoclonal antibody (clone DM1A, Sigma-Aldrich, Cat# T9026, 1:10,000), horseradish peroxidase (HRP)-conjugated donkey anti-rabbit IgG polyclonal antibody (Jackson ImmunoResearch, Cat# 711-035-152, 1:10,000) and HRP-conjugated donkey anti-mouse IgG polyclonal antibody (Jackson ImmunoResearch, Cat# 715-035-150, 1:10,000). Chemiluminescence was detected using ChemiDoc Touch Imaging System (Bio-Rad, Cat #170-6545).
Analysis of single-nuclei and single-cell RNA sequencing datasets.
Normalised and log-transformed expression values per cell were obtained from human lung single-nuclei and epithelial non-small cell lung carcinoma cell lines. scanpy 1.7.1 was used to process the data. For the tissue data24, only single-nuclei RNA-seq were selected. From the cancer cell lines study, only mock samples were selected. ACE2 and TMPRSS2 expression was plotted with violin plot function in scanpy.
Plasmids for split GFP system to measure cell-cell fusion
pQCXIP‐BSR‐GFP11 and pQCXIP‐GFP1‐10 were from Yutaka Hata 49 Addgene plasmid #68716; http://n2t.net/addgene:68716; RRID:Addgene_68716 and Addgene plasmid #68715; http://n2t.net/addgene:68715; RRID:Addgene_68715)
Generation of GFP1‐10 or GFP11 lentiviral particles
Lentiviral particles were generated by co-transfection of 293T or Vero cells with pQCXIP‐BSR‐GFP11 or pQCXIP‐GFP1‐10 as previously described 50. Supernatant containing virus particles was harvested after 48 and 72 hours, 0.45 µm filtered, and used to infect 293T or Vero cells to generate stable cell lines. 293T and Vero cells were transduced to stably express GFP1‐10 or GFP11 respectively and were selected with 2 μg/ml puromycin.
Cell-cell fusion assay
Cell-cell fusion assays were carried out as previously described 13,50,51. Briefly, 293T GFP1-10 and Vero-GFP11 cells were seeded at 80% confluence in a 1:1 ration in 48 multiwell plate the day before. Cells. were co-transfected with 0.5 µg of spike expression plasmids in pCDNA3 using Fugene 6 and following the manufacturer’s instructions (Promega). Cell-cell fusion was measured using an Incucyte and determined as the proportion of green area to total phase area. Data were then analysed using Incucyte software analysis. Graphs were generated using Prism 8 software. To measure cell surface spike expression HEK293 cells were transfected with S expression plasmids and stained with rabbit anti-SARS-CoV-2 S S1/S2 polyclonal antibody (Thermo Fisher Scientific, Cat# PA5-112048, 1:100). Normal rabbit IgG (SouthernBiotech, Cat# 0111-01, 1:100) was used as a negative control, and APC-conjugated goat anti-rabbit IgG polyclonal antibody (Jackson ImmunoResearch, Cat# 111-136-144, 1:50) was used as a secondary antibody. Surface expression level of S proteins was analysed using FACS Canto II (BD Biosciences) and FlowJo software v10.7.1 (BD Biosciences). Gating strategy for flow cytometry is shown in Supplementary Figure 8.
Cell Culture for live virus experiments
VeroE6/TMPRSS2 and Vero E6 ACE2/TMPRSS2 cells [an African green monkey (Chlorocebus sabaeus) kidney cell line; JCRB1819]52 were maintained in Dulbecco’s modified Eagle's medium (low glucose) (Wako, Cat# 041-29775) containing 10% FBS, G418 (1 mg/ml; Nacalai Tesque, Cat# G8168-10ML) and 1% PS. Calu-3 cells (a human lung epithelial cell line; ATCC HTB-55) were maintained in Eagle’s minimum essential medium (Sigma-Aldrich, Cat# M4655-500ML) containing 10% FBS and 1% PS.
SARS-CoV-2 live virus isolation, preparation and titration for hNEC experiments
To isolate an Omicron variant (BA.1 lineage, strain TY38-873; GISAID ID: EPI_ISL_7418017), saliva was collected from a traveller arrived at Japan, and quantitative RT-PCR testing for SARS-CoV-2 was performed in an airport quarantine station, Japan. The sample was subjected to whole genome sequencing based on a modified ARTIC Network protocol41, and the near full-length SARS-CoV-2 genome sequence (GISAID ID: EPI_ISL_6913953) was deposited in GISAID. Virus isolation was performed as previously described52. In brief, the saliva was inoculated into VeroE6/TMPRSS2 cells and cytopathic effect (CPE) was observed 4 days after inoculation. The supernatant was then harvested and stored at –80°C as an original virus (GISAID ID: EPI_ISL_7418017). After one more passage in VeroE6/TMPRSS2 cells, the virus was obtained from National Institute of Infectious Diseases, Japan. A D614G-bearing early pandemic isolate (B.1.1lineage, strain TKYE610670; GISAID ID: EPI_ISL_479681) and a Delta isolate (B.1.617.2 lineage, strain TKYTK1734; GISAID ID: EPI_ISL_2378732) were used in the previous study3.
Virus preparation and titration was performed as previously described3,53. To prepare the working virus stock, 100 μl of the seed virus was inoculated into VeroE6/TMPRSS2 cells (5 × 106 cells in a T-75 flask). One hour after infection, the culture medium was replaced with Dulbecco’s modified Eagle's medium (low glucose) (Wako, Cat# 041-29775) containing 2% FBS and 1% PS. At 3 days postinfection, the culture medium was harvested and centrifuged, and the supernatants were collected as the working virus stock.
The titre of the prepared working virus was measured as the 50% tissue culture infectious dose (TCID50). Briefly, one day prior to infection, VeroE6/TMPRSS2 cells (10,000 cells/well) were seeded into a 96-well plate. Serially diluted virus stocks were inoculated into the cells and incubated at 37°C for 4 days. The cells were observed under microscopy to judge the CPE appearance. The value of TCID50/ml was calculated with the Reed–Muench method as previously described13.
SARS-CoV-2 Infection with live virus
One day prior to infection, cells (10,000 cells) were seeded into a 96-well plate. SARS-CoV-2 (1,000 TCID50 as measured with plaque assay on Vero cells) was inoculated and incubated at 37°C for 1 h. The infected cells were washed, and 180 µl of culture medium was added. The culture supernatant (10 µl) was harvested at the indicated time points and used for real-time RT-PCR to quantify viral RNA copy number (see below).
Real-Time RT-PCR
Real-time RT-PCR was performed as previously described3,53. Briefly, 5 μl of culture supernatant was mixed with 5 μl of 2 × RNA lysis buffer [2% Triton X-100, 50 mM KCl, 100 mM Tris-HCl (pH 7.4), 40% glycerol, 0.8 U/μl recombinant RNase inhibitor (Takara, Cat# 2313B)] and incubated at room temperature for 10 min. RNase-free water (90 μl) was added, and the diluted sample (2.5 μl) was used as the template for real-time RT-PCR performed according to the manufacturer’s protocol using the One Step TB Green PrimeScript PLUS RT-PCR kit (Takara, Cat# RR096A) and the following primers: Forward N, 5'-AGC CTC TTC TCG TTC CTC ATC AC-3'; and Reverse N, 5'-CCG CCA TTG CCA GCC ATT C-3'. The viral RNA copy number was standardized with a SARS-CoV-2 direct detection RT-qPCR kit (Takara, Cat# RC300A). Fluorescent signals were acquired using a QuantStudio 3 Real-Time PCR system (Thermo Fisher Scientific), a CFX Connect Real-Time PCR Detection system (Bio-Rad), an Eco Real-Time PCR System (Illumina) or a 7500 Real Time PCR System (Applied Biosystems).
Preparation of Monoclonal Antibodies
Casirivimab and Imdevimab were prepared as previously described54. To construct the plasmids expressing anti-SARS-CoV-2 monoclonal antibodies (Casirivimab and Imdevimab), the sequences of the variable regions of Casirivimab and Imdevimab were obtained from KEGG Drug Database (https://www.genome.jp/kegg/drug/) and were artificially synthesized (Fasmac). The obtained coding sequences of the variable regions of the heavy and light chains were cloned into the pCAGGS vector containing the sequences of the human immunoglobulin 1 and kappa constant region [kindly provided by Dr. Hisashi Arase (Osaka University, Japan)].
To prepare these monoclonal antibodies, the pCAGGS vectors containing the sequences encoding the immunoglobulin heavy and light chains were cotransfected into HEK293T cells using PEI Max (Polysciences, Cat# 24765-1). At 48 h posttransfection, the cell culture supernatants were harvested, and the antibodies were purified using NAb protein A plus spin kit (Thermo Fisher Scientific, Cat# 89948) according to the manufacturer’s protocol.
Neutralisation Assay for monoclonal antibodies with live virus
One day prior to infection, VeroE6/TMPRSS2 (10,000 cells) were seeded into a 96-well plate. The monoclonal antibodies (Casirivimab, Imdevimab, or Casirivimab/Imdevimab) and the heat-inactivated human sera were serially diluted with DMEM supplemented with 10% FCS and 1% PS. The diluted antibodies and sera were incubated with SARS-CoV-2 (120 TCID50) at 37°C for 1 h. The viruses without antibodies or sera were included as controls. The mixture (containing the virus at 100 TCID50) was inoculated onto a monolayer of VeroE6/TMPRSS2 cells and incubated at 37°C for 1 h. Then, the cells were washed with DMEM and cultured in DMEM supplemented with 10% FCS and 1% PS. At 24 h postinfection, the culture supernatants were harvested and viral RNA was quantified by real-time RT-PCR (see above). The assay of each antibody or serum was performed in triplicate or quadruplicate, and the 50% neutralization titre was calculated using Prism 9 (GraphPad Software).
Antiviral drug assay with live virus
One day prior to infection, HOS-ACE2-TMPRSS2 cells (10,000 cells) were seeded into a 96-well plate. The cells were infected with SARS-CoV-2 (100 TCID50) at 37°C for 1 h. Then, the cells were washed with DMEM and cultured in DMEM supplemented with 10% FCS and 1% PS and the serially diluted Remdesivir (Selleck, Cat# S8932) or beta-d-N4-hydroxycytidine (a derivative of Molnupiravir; Cell Signaling Technology, Cat# 81178S). At 24 h postinfection, the culture supernatants were harvested and viral RNA was quantified by real-time RT-PCR (see above). The assay of each compound was performed in quadruplicate, and the 50% neutralization titre was calculated using Prism 9 (GraphPad Software).
Cytotoxicity Assay
The CPE of the Remdesivir and beta-d-N4-hydroxycytidine were tested using a cell counting kit-8 (Dojindo, Cat# CK04-11) according to the manufacturer’s instructions. One day prior to the assay, HOS-ACE2-TMPRSS2 cells (10,000 cells) were seeded into a 96-well plate. The cells were cultured with the serially diluted compound for 24 hours. The cell counting kit-8 solution (10 µl) was added to each well, and the cells were incubated at 37°C for 90 m. Absorbance was measured at 450 nm by GloMax explorer microplate reader (Promega).
Entry Inhibitors in A549-ACE2-TMPRSS2-based luminescent reporter cells using live virus
Quantification of viral replication in A549-ACE2-TMPRSS2-based luminescent reporter cells was performed as previously described55 and Gerber et al., manuscript in preparation). In brief, A549-ACE2-TMPRSS2 (A54-9A2T2) reporter cells (clone E8) over-expressing Renilla luciferase (Rluc) and SARS-CoV-2 Papain-like protease-activatable circularly permuted firefly luciferase (FFluc) were seeded in flat-bottomed 96-well plates. The following morning, cells were treated with the indicated doses of Camostat or E64d for 1 h, then infected with Delta or Omicron variants of SARS-CoV-2 at MOI = 0.01. After 24 h, cells were lysed in Dual-Glo Luciferase Buffer (Promega) diluted 1:1 with PBS and 1% NP-40. Lysates were then transferred to opaque 96-well plates, and viral replication quantitated as the ratio of FFluc/Rluc luminescence measured using the Dual-Glo kit (Promega) according to the manufacturer’s instructions. % inhibition was calculated relative to the maximum luminescent signal for each condition, then analysed using the Sigmoidal, 4PL, X is log(concentration) function in GraphPad Prism.
Immunofluorescence (IF) assays
HeLa-ACE2 cells were grown on 19 mm glass coverslips in 24-well plates and incubated for 24 h. Cells were fixed in 4% paraformaldehyde (Applichem GmbH, Germany), and permeabilised in PBS-T (0.1% v/v Triton X-100 in PBS) for 5 min. Cells were blocked in PBS-T containing 5% w/v BSA for 30 min and labelled with primary antibodies against SARS-CoV-2 Spike (GTX632604, 1:1000) or anti-GM130[EP892Y]-cis-Golgi (ab52649, 1:250) in PBS-T, 1% w/v BSA for 1 h at room temperature. After washing, cells were stained with fluorochrome-conjugated secondary antibodies in PBS-T (1:1000), 1% w/v BSA for 1 h at room temperature in the dark. Nuclei were counterstained with 4’,6’-diamidino-2-phenylindole dihydrochloride (DAPI) and phalloidin 647 (A22287,1:1000) (Invitrogen, Molecular Probes, Germany). Confocal images were acquired on a Leica TCS SP8 microscope. Post-acquisition analysis was performed using Leica LAS X or Fiji (v1.49) software. For distance analysis, the two-dimensional coordinates of the centroids of Spike proteins were calculated using the Analyze Particles module of Fiji (ImageJ). The distance of each particle to the edge of the nucleus, visualised using DAPI stain, was looked up using a Euclidean distance map computed with the Distance Transform module of Fiji and exported as a list of distance measurements via the Analyze Particle function. Colocalization of Spike with the Golgi were estimated through Pearson’s correlation coefficients (R) using the PSC colocalization plug-in (ImageJ, NIH) (2). R values between −1 (perfect negative correlation) and +1 (perfect positive correlation), with 0 corresponding to no correlation were assigned56. Colocalization analyses were performed on >20 cells.
Analysis of size of infection foci
Vero E6 cells (ATCC CRL-1586, obtained from Cellonex in South Africa) were propagated in complete growth medium consisting of Dulbecco's Modified Eagle Medium (DMEM) with 10% fetal bovine serum (Hyclone) containing 10mM of HEPES, 1mM sodium pyruvate, 2mM L-glutamine and 0.1mM nonessential amino acids (Sigma-Aldrich). Vero E6 cells were passaged every 3–4 days. H1299 cell lines were propagated in growth medium consisting of complete Roswell Park Memorial Institute (RPMI) 1640 medium with 10% fetal bovine serum containing 10mM of HEPES, 1mM sodium pyruvate, 2mM L-glutamine and 0.1mM nonessential amino acids. H1299 cells were passaged every second day. The H1299-E3 (H1299-ACE2, clone E3) cell line was derived from H1299 (CRL-5803) as described in our previous work57,58. To Isolate virus, ACE2-expressing H1299 cells were seeded at 4.5 × 105 cells in a 6 well plate well and incubated for 18–20 h. After one DPBS wash, the sub-confluent cell monolayer was inoculated with 500 μL universal transport medium diluted 1:1 with growth medium filtered through a 0.45-μm filter. Cells were incubated for 1 h. Wells were then filled with 3 mL complete growth medium. After 4 days of infection (completion of passage 1 (P1)), cells were trypsinized, centrifuged at 300 rcf for 3 min and resuspended in 4 mL growth medium. Then all infected cells were added to Vero E6 cells that had been seeded at 2 × 105 cells per mL, 20mL total, 18–20 h earlier in a T75 flask for cell-to-cell infection. The coculture of ACE2-expressing H1299-E3 and Vero E6 cells was incubated for 1 h and the flask was then filled with 20 mL of complete growth medium and incubated for 4 days. The viral supernatant (passage 2 (P2) stock) was used for experiments.P2 stock was sequenced and confirmed Omicron with the following substitutions: E:T9I,M:D3G,M:Q19E,M:A63T,N:P13L,N:R203K,N:G204R,ORF1a:K856R,ORF1a:L2084I,ORF1a:A2710T,ORF1a:T3255I,ORF1a:P3395H,
ORF1a:I3758V,ORF1b:P314L,ORF1b:I1566V,ORF9b:P10S,S:A67V,S:T95I,S:Y145D,S:L212I,S:G339D,S:S371L,S:S373P,S:S375F,S:K417N,
S:N440K,S:G446S,S:S477N,S:T478K,S:E484A,S:Q493R,S:G496S,S:Q498R,S:N501Y,S:Y505H,S:T547K,S:D614G,S:H655Y,S:N679K,S:P681H,
S:N764K,S:D796Y,S:N856K,S:Q954H,S:N969K,S:L981F. Sequence was deposited in GISAID, accession: EPI_ISL_7886688.