*Purpose: To determine if IgM has a direct effect in preventing SARS-CoV-2 replication in Vero E6 cells, and delaying or preventing disease in infected K18-hACE2 mice.

*Methods: 1) Vero E6 cells, grown to confluence in 12 well plates, were used to test the effect of IgM in reducing the number of plaque-forming units (PFU).There were 4 groups: a) 25PFU WA-1 SARS-CoV-2 was combined with 20, 5 or 0.8μg IgM in growth medium, and incubated for 1hr in a final volume of 500ul. 100μl was added to Vero E6 cells in replicate wells and incubated for 1hr; b) 100μl of 20, 5 or 0.8μg IgM was added to Vero E6 cells and incubated. Media was aspirated and the cells were then inoculated with 25PFU WA-1 and incubated for 1hr; c) Virus control – as above, but with no IgM; d) No virus or IgM. FBS growth medium containing Avicel was overlain in the wells and incubated for 48 hours. Virus replication was stopped by incubating with 10% buffered formalin. Following removal of formalin, plates were stained with Giemsa violet, dried, and photographed. 2) A COVID -19 Spike-ACE2 binding assay kit was used to determine if IgM (2ug, 4.5ug, 20ug, 45ug IgM) inhibits the interaction between the Spike-receptor binding domain (S-RBD) and Angiotensin I ConvertingEnzyme 2 (ACE2) receptor. 3) K18-hACE2 mice were divided into 3 groups based on treatment regimen; Group 1: with IgM, No virus; 2: with Saline, with virus; 3: with IgM, with virus. 35ug IgM was injected intraperitoneal in a single dose, 2 days prior to infection. Mice were innoculated intranasally with 1250 pfu of HK SARS-CoV-2.

*Results: 1) Exposure of 25PFU SARS-CoV-2 to IgM (at all concentrations) prior to incubation with Vero E6 cells, inhibited its replication in Vero E6 cells. When Vero E6 cells were incubated with IgM prior to infection, no plaques were seen in wells with 20ug and 5ug IgM but were observed in wells with 0.8ug IgM. Plaques were also observed in the Virus alone group, but none were seen in the ‘No IgM-No virus’ group. 2) 45ug IgM/100uls inhibited the binding of S-RBD to ACE2 by ~94-100%, 20ug IgM/100uls inhibited it by ~80%, and 2 or 4.5ug/100ul by ~70-75%. Control without IgM did not inhibit the S-RBD─ACE-2 binding. 3) Pretreatment with a single low dose IgM injection delayed weight loss and mortality.

*Conclusions: IgM inhibits the replication of SARS-CoV-2 in Vero cells in vitro. It also inhibits the interaction between S-RBD that is present on the viral surface and the ACE2 receptor, by binding to S-RBD. A single low dose of IgM given prechallenge delayed disease in infected mice. The discovery that IgM interferes with the formation of the S-RBD─ACE2 complex, and that a single low dose can delay disease, indicates its translational potential as a vaccine/therapeutic to prevent or treat COVID-19.

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