Approval from the University of Minnesota Institutional Biosafety Committee (IBC) was obtained to perform in vitro infection studies using infectious SARS-CoV-2 virus. All handling of viral samples and infected tissue was performed in a certified biosafety level 3 (BSL-3) microbiology research laboratory. Human donor corneas and whole globes were obtained from Lions Gift of Sight (Minneapolis, MN, USA) with research consent under University of Minnesota Institutional Review Board (IRB) exemption for cadaveric tissue. All tissues used in this study were obtained from donors with negative COVID-19 testing within three days prior to death or no concerns for COIVD-19 infection based on EBAA/FDA screening criteria guidelines.
Virus Isolates and Virus Propagation
Two SARS-CoV-2 isolates, isolate 2019-nCoV/USA-WA1/2020 (NR-52281), and isolate hCoV-19/Hong Kong/VM20001061/2020 (NR-52282), deposited by the Centers for Disease Control and Prevention, were obtained through BEI Resources, NIAID, NIH (Manassas, VA). The USA-WA1 isolate was isolated from an oropharyngeal swab taken on January 2020 from a patient in Washington, USA who developed COVID-19 after recent travel to China. The Hong Kong isolated was isolated from a nasopharyngeal aspirate and throat swab taken on January 2020 from an adult male patient in Hong Kong. Both isolates were assigned lineage A and GISAID clade S.29
Infection into Vero E6 cells (ATCC CRL-1586) was performed to confirm the infectivity of both viral isolates and to propagate the viruses. Briefly, Vero E6 cells were cultured and maintained in Dulbecco’s modified Eagle medium (DMEM) supplemented with 5% heat inactivated fetal bovine serum (FBS). Cells in 75cm2 cell culture flasks were inoculated with each SARS-CoV-2 isolate at a multiplicity of infection (MOI) of 0.1 in 3 mL of DMEM and incubated at 37oC/5% CO2 for 1 h. After 1 h adsorption period, 7 mL of DMEM supplemented with 5% FBS was added to the flasks and incubated at 37oC/5% CO2 for 4 to 6 days. Cells were observed every day for cytopathic effect (CPE). When CPE reached 50-60%, the culture supernatant was harvested and clarified by centrifugation at 3,000 rpm for 15 min at 4oC. Virus stocks were stored in aliquots at -80oC. Virus titer was determined by 50% tissue culture infectious dose (TCID50) assay on Vero E6 cells. The infected Vero E6 cells were then fixed and immunostained with SARS-CoV-2 nucleocapsid (N)-protein and SARS-CoV-2 spike (S)-protein to confirm infection and to establish positive controls for the primary antibodies.
In vitro Infection of Ocular Surface Epithelium
To test infection of ocular surface epithelium, cell cultures of limbal, corneal, and conjunctival epithelial cells were established from SARS-CoV2 negative donor tissue. After dissecting the limbal ring, bulbar conjunctival, and central cornea from donor corneoscleral disks, tissue samples were segmented into ~1x1 mm tissue blocks and plated on plastic culture dishes for explant cultures as previously described.18 Explants were cultured for outgrowth at 37°C/5% CO2 using EpiLife medium (Thermo Fisher Scientific, Waltham, MA, USA) with S7 supplement/penicillin/streptomycin for up to 14 days. Cells were then trypsinized and reseeded into 24-well tissue culture plates for in vitro infection studies.
Cultured cell lines were then each infected with both the US-WA1 or the Hong Kong SARS-CoV-2 isolates at an MOI of 0.5 to compare infectivity rates. At 2 days post infection (DPI), culture supernatant was removed, and the cells were washed twice with phosphate buffered saline (PBS). The cells were then fixed with 4% paraformaldehyde for 30 min at room temperature and immunostained for N-protein expression to detect viral infection.
In vitro Infection of Endothelial Cells
Infection of corneal endothelium was tested in both primary endothelial cell cultures and in flat-mounted Descemet’s Stripping Automated Endothelial Keratoplasty (DSAEK) grafts. Human primary endothelial cells were obtained from Celprogen (Torrance, CA, USA) and cultivated according to manufacturer instructions. Cells were then infected with each of the SARS-CoV-2 isolates at an MOI of 0.5. At 2 DPI, cells were fixed with 4% paraformaldehyde for 30 min at room temperature and then immunostained for N-protein expression to detect viral infection.
DSAEK grafts were prepared from donor research corneas with transplant-quality endothelium. Grafts were cut to ≤150 µm thickness using a microkeratome (Moria SA, Antony, France) and trephined using a 7.75 mm corneal vacuum trephine (Katena, Parsippany, NJ) per standard protocols.30 DSAEK grafts were placed into 6 well tissue culture plates and incubated with 8 x 105 TCID50 of SARS-CoV-2 Hong Kong isolate in 1 mL of DMEM without FBS for 1 h at 37oC/5% CO2. After 1 h adsorption period, grafts were transferred to 100 mm dishes containing 20 mL of DMEM/F12 supplemented with 10% FBS and incubated at 37oC/5% CO2 for 4 days. Grafts were then fixed and immunostained for SARS-CoV-2 N-protein expression to detect viral infection.
Immunostaining of Cultured Epithelial and Endothelial cells
Fixed cells were washed with PBS containing 0.3% Triton X-100 (PBST), blocked with 1% bovine serum albumin (BSA) and 2% normal goat serum in PBST for 1 h at room temperature and then incubated with rabbit anti-SARS-CoV-2 nucleocapsid antibody (SB40588-T62, Sino Biological; 1:2000 dilution) or anti-SARS-CoV2 spike antibody (SB40592-T62, Sino Biological, 1:500 dilution) overnight at 4oC. After washing twice with PBST, cells were incubated with Alexa Fluor 594-conjugated goat anti-rabbit IgG (Life Technologies; 1:1000 dilution) at room temperature for 1 h. Cells were subsequently washed twice with PBST and treated with bis-Benzimide Hoechst 33258 (Sigma; 2µg/mL) for 20 min at room temperature to counterstain the cell nuclei. After washing twice with PBST, cells were imaged using Nikon TE2000-E inverted fluorescent microscope.
Immunostaining of DSAEK Grafts
Virus-infected DSAEK grafts were fixed in 4% paraformaldehyde for 24 h and then permeated with 0.1% PBST. After extensive washing, non-specific immunoglobulin binding was blocked with 10% normal goat serum/1xPBS for 1 h at room temperature. The tissues were incubated overnight with the anti-SARS-CoV2 nucleocapsid antibody (1:500) followed by Alexa Fluor 594-conjugated goat anti-rabbit IgG (1:800). Cell nuclei were counterstained with Hoechst 33258 for visualization. All images were taken with a Leica DM 4000B fluorescence microscope using Leica Application Suite version 3.8 (Leica Microsystems, Buffalo Grove, IL).
Virus viability in Life4C medium
Corneoscleral discs were dissected from whole globes and placed in cornea viewing chambers (CVC) with 20 ml of Life4C (Numedis Inc., Isanti, MN, USA) preservation medium for viral stability testing. Viability of infectious SARS-CoV-2 in Life4C cornea storage medium was assessed by inoculating SARS-CoV-2 Hong Kong isolate into CVCs containing Life4C with or without donor corneoscleral disks to a final concentration of 1 x 105 TCID50/mL. The medium was stored at 4oC for 14 days. An aliquot of preservation medium was collected at 0, 1, 2, 4, 7, and 14 DPI and infectious virus titer were determined by TCID50 assay using Vero E6 cells.
TCID50 assay
Vero E6 cells were seeded into 48-well cell culture plate 24 h prior to infection. Each aliquot sample of preservation medium were serially diluted eight times in DMEM using a 10-fold dilution each time. 100 µL of each dilution was then added to 48 well plates containing Vero E6 cells (6 wells for each dilution). Plates were incubated for 1 h at 37oC/5% CO2. After 1 h incubation, 1 mL of DMEM supplemented with 2% FBS was added to each well and incubated for 48 h. Culture medium was then removed from the wells, cells were washed twice with PBS, and fixed with 4% paraformaldehyde for 30 min at room temperature. Wells were scored for presence or absence of viral protein immunostaining as described above. TCID50 was then calculated using Reed and Muench method based on the loss of positive immunostaining with serial dilution.