Rahul Tharoor

Introduction. COVID-19 is a respiratory illness that as of June 4, 2021, has infected over 170 million people and led to the death of over 3.7 million individuals globally.¹ The SARS-CoV-2 virus may cause injury by binding to and downregulating ACE-2 receptors, as well as activating mass inflammatory “Cytokine Storms” that induce damage in the lungs and other organs.^2,3. Monoclonal antibodies (mAbs) serve a unique role amongst COVID-19 therapeutics, since they can reduce the risk of developing severe symptoms in individuals who have tested positive for SARS-CoV-2. ⁴ While most SARS-CoV-2 mAbs target the receptor binding domain (RBD) of the S1 spike subunit to prevent ACE-2 binding, other mAbs have been noted to target epitopes such as the S1 N-terminal domain (NTD). ⁴ LY-CoV555 and LY-CoV016 remain the mainstay prophylactic mAb cocktail; however, SARS-CoV-2 variants could pose threat to their efficacy.^5,7 Methods. Researchers isolated mAbs from previously infected COVID-19 patients and ran ELISA and neutralization assays to identify mAbs specifically targeting the RBD. ⁴ Cry-electron microscopy of neutralizing mAbs with S1 protein and ACE-2 was visualized to pinpoint binding patterns. ⁴ Another experiment analyzed the mAb LY-CoV555 via x-ray crystallography.⁵ Two NTD mAbs, COV2-2676 and COV2-2489, were examined alongside SARS-CoV-2 in electron microscopy imaging, Vero cell model, and a mice experiment. ⁶ Deep mutational scanning was performed on LY-CoV555 and LY-CoV016 with SARS-CoV-2 variants, noting interactions under fluorescence-activated cell sorting.⁷ Results. mAbs targeting the top of the RBD competed strongly with ACE2 binding, while those binding at the sides did not.⁴ Some mAbs, such as Fab2-4, bound the RBDs in an “all-down” conformation, locking the RBDs into place. ⁴ LY-CoV555 bound 7 of the 25 RBD sidechains as well as the ACE-2 receptor. ⁵ Multiple mAbs targeting the NTD in Vero cells neutralize SARS-CoV-2 at a post-attachment step of the viral life cycle. ⁶ COV2-2676 and COV2-2489 recognize the closed conformational state of the S1 protein, binding the N3 and N5 loops of the NTD.⁶ LY-CoV555 and LY-CoV016 display binding to opposite sides of the RBD binding ridge.⁷ SARS-CoV-2 variants of note that evaded both LY-CoV555 and LY-CoV016 include E484K and K417N/T.⁷ Conclusions. These results help explain how both RBD and NTD targeting mAbs function to neutralize SARS-CoV-2. Additionally, such data helps identify the limitations of current mAb therapy against existing variants. By understanding the underlying mechanisms, new mAb cocktails can be created to overcome evasion by SARS-CoV-2 variants.

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4. Liu L, Wang P, Nair MS, et al. Potent neutralizing antibodies against multiple epitopes on SARS-CoV-2 spike. Nature. 2020. doi:10.1038/s41586-020-2571-7
5. Jones BE, Brown-Augsburger PL, Corbett KS, et al. LY-CoV555, a rapidly isolated potent neutralizing antibody, provides protection in a non-human primate model of SARS-CoV-2 infection. bioRxiv. 2020. doi:10.1101/2020.09.30.318972
6. Suryadevara N, Shrihari S, Gilchuk P, et al. Neutralizing and protective human monoclonal antibodies recognizing the N-terminal domain of the SARS-CoV-2 spike. Cell. 2021. doi:10.1016/j.cell.2021.03.029
7. Starr TN, Greaney AJ, Dingens AS, Bloom JD. Complete map of SARS-CoV-2 RBD mutations that escape the monoclonal antibody LY-CoV555 and its cocktail with LY-CoV016. bioRxiv. 2021. doi:10.1101/2021.02.17.431683