Fig 1.

A, SARS-CoV-2 RBD antibody binding from commercially available IVIG at dilution 1:50 by ELISA. Each dot represents a different lot number of immunoglobulin product. The positive control is anti-spike antibody (CR3022, Creative Biolabs) at dilution 1:1000.¹ The negative control is human whole serum obtained from Sigma. Concentration indicates the protein percentage (wt/vol) in the commercial lot, before dilution for the ELISA. The 5% and 10% IVIG samples were first diluted to 0.66% (wt/vol) to achieve a common concentration of immunoglobulin, then diluted 1:50 for the ELISA. The dilution solution was PBS (Fisher Scientific, item no. BP3994) with 0.05% (vol/vol) Tween-20 (Sigma Aldrich, item no. P1379) and 1% (wt/vol) BSA (Rockland Antibodies & Assays, item no. BSA-50). The final concentration of immunoglobulin is approximately 0.0133% (wt/vol) or roughly 133 μg/mL of immunoglobulin. B, SARS-CoV-2 (Isolate USA-WA1/2020) was obtained from the Centers for Disease Control and Prevention and expanded in Vero E6 cells (ATCC). The virus titer used was 1 × 10⁵ plaque-forming units (PFU)/mL. Vero cells were seeded at 7500/well in 96-well plates and cultured overnight. IVIG diluted 1:4 (20 μL) was mixed with 100 PFU of SARS-CoV-2 in 100 μL DMEM, incubated at 37 °C for 1 hour, and added to monolayers of Vero E6 cells in duplicate. Cytopathic effect was assessed after 3 days. Images of Vero E6 cells by microscopy (10×) after 72 hours in culture. Cytopathic effect (CPE) is visible in SARS-CoV-2–infected Vero E6 cells mixed with IVIG. Images show results of 2 of the 4 highest RBD-binding samples; the 3 lowest RBD-binding samples showed similar results (not shown). The insets show co-culture of Vero E6 cells with immunoglobulin but without virus, demonstrating that IVIG does not kill Vero E6 cells. C, Reduction of CPE in SARS-CoV-2–infected Vero E6 cells mixed with convalescent patient serum at dilutions of 1:4, 1:16, and 1:64; however, CPE was seen when serum was diluted to 1:256. DMEM, Dulbecco modified Eagle medium.