Table 1.
Study cohort information.
Fig 1.
Immunofluorescence assay (IFA) against SARS-CoV-2 proteins.
Representative pictures of IFA against either the mock, SARS-CoV-2 spike or nucleocapsid expressing HEK-293T cells. The upper row shows IFA with negative control plasma collected before the COVID-19 pandemic. The lower row shows IFA with COVID-19 plasma, where strong green color positive cells (indicated by white arrows) were only observed in cells expressing either SARS-CoV-2 spike or nucleocapsid proteins. The lack of green color positive cells with negative control plasma and mock cells demonstrated the specificity of the IFA. All pictures were taken at 20X magnification with Nikon Eclipse 50i fluorescence microscope. The white scale bars indicate 50 μm.
Fig 2.
SARS-CoV-2 spike pseudotyped virus neutralization assay.
SARS-CoV-2 spike pseudotyped lentivirus virus encoding EGFP were tested against negative control pre-pandemic plasmas (N1, N2 and N3) and COVID-19 plasmas (US1 to US17 and SSA1 to SSA3) at 1:40 plasma dilution. At 72-hours post-infection, percentage of GFP positive cells were quantified with BD Accuri C6 Plus flow cytometer. The mean of triplicate wells was shown with error bars representing SEM. P-values were calculated via one-way ANOVA and “*” denotes p < 0.05 relative to negative control plasmas.
Fig 3.
Antibody-dependent cellular cytotoxicity (ADCC) assay.
The ADCC activity of COVID-19 plasmas were tested against HEK-293T cells expressing SARS-CoV-2 spike protein, which served as the target cells. After incubation with plasma, Calcein-AM labeled target cells were incubated with NK cells (NK92.05-CD16-176V) which served as the effector cells. The amount of fluorescent calcein released into the medium was then measured with Victor3V plate reader. A) changes in ADCC (ΔADCC) activity against the spike protein, relative to their respective activity against mock cells. N denotes pre-pandemic negative control plasmas. US and SSA denotes COVID-19 samples from USA and sub-Saharan Africa, respectively. B) Comparison of ΔADCC against the spike protein between COVID-19 and negative control plasmas. P-values were calculated via Mann Whitney test.