Antibody-mediated spike activation promotes cell-cell transmission of SARS-CoV-2

The COVID pandemic fueled by emerging SARS-CoV-2 new variants of concern remains a major global health concern, and the constantly emerging mutations present challenges to current therapeutics. The spike glycoprotein is not only essential for the initial viral entry, but is also responsible for the transmission of SARS-CoV-2 components via syncytia formation. Spike-mediated cell-cell transmission is strongly resistant to extracellular therapeutic and convalescent antibodies via an unknown mechanism. Here, we describe the antibody-mediated spike activation and syncytia formation on cells displaying the viral spike. We found that soluble antibodies against receptor binding motif (RBM) are capable of inducing the proteolytic processing of spike at both the S1/S2 and S2' cleavage sites, hence triggering ACE2-independent cell-cell fusion. Mechanistically, antibody-induced cell-cell fusion requires the shedding of S1 and exposure of the fusion peptide at the cell surface. By inhibiting S1/S2 proteolysis, we demonstrated that cell-cell fusion mediated by spike can be re-sensitized towards antibody neutralization in vitro. Lastly, we showed that cytopathic effect mediated by authentic SARS-CoV-2 infection remain unaffected by the addition of extracellular neutralization antibodies. Hence, these results unveil a novel mode of antibody evasion and provide insights for antibody selection and drug design strategies targeting the SARS-CoV-2 infected cells. SARS-CoV-2 has been found to mediate breakthrough infections in vaccinated individuals and re-infections of convalescent patients, but the molecular mechanism of its immune escaping strategies remains elusive. Unlike virus-to-cell transmission, spike promotes SARS-CoV-2 cell-to-cell transmission, which is strongly resistant to extracellular antibodies and patient plasma. Here we show that receptor binding-motif antibodies mediate the ACE2-independent activation of spike at the cell plasma membrane. This mode of spike activation is enabled by the protease-mediated S1/S2 cleavage event and can be genetically and pharmacologically prevented. Through targeting the S1/S2 site, antibody neutralization against spike-mediated cell-cell fusion can be restored in various SARS-CoV-2 variants. Hence, these data highlight a role for S1/S2-cleaved spike and inform therapeutic strategies to restore antibody neutralization against cell-cell transmission of SARS-CoV-2.