Neutralization or enhancement of SARS-CoV-2 infection by a monoclonal antibody targeting a specific epitope in the spike receptor-binding domain.

Neutralizing antibodies (NAbs) are believed to be promising prophylactic and therapeutic treatment against the coronavirus disease 2019 (COVID-19), which is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Here, we reported two mouse monoclonal antibodies 7 Eb-4G and 1Ba-3H that specifically recognized the receptor-binding domain (RBD) of SARS-CoV-2 spike (S) protein without exhibiting cross-reactivity with the S proteins of SARS-CoV and MERS-CoV. The binding epitopes of 7 Eb-4G and 1Ba-3H were respectively located in the regions of residues 457-476 and 477-496 in the S protein. Only 1Ba-3H exhibited the neutralizing activity for preventing the pseudotyped lentivirus from binding to the angiotensin-converting enzyme 2 (ACE2)-transfected HEK293T cells. The competitive ELISA further showed that 1Ba-3H interfered with the binding between RBD and ACE2. Epitope mapping experiments demonstrated that a single alanine replacement at residues 480, 482, 484, 485, and 488-491 in the RBD abrogated 1Ba-3H binding. 1Ba-3H exhibited the neutralizing activity against the wild-type, Alpha, Delta, and Epsilon variants of SARS-CoV-2, but lost the neutralizing activity against Gamma variant in the plaque reduction assay. On the contrary, 1Ba-3H enhanced the cellular infection of Gamma variant in a dose-dependent manner. Our findings suggest that the antibody-dependent enhancement of infection mediated by the RBD-specific antibody for different SARS-CoV-2 variants must be considered while developing the NAb.

Neutralizing Monoclonal Antibodies Inhibit SARS-CoV-2 Infection through Blocking Membrane Fusion.

Most of SARS-CoV-2 neutralizing antibodies (nAbs) targeted the receptor binding domain (RBD) of the SARS-CoV-2 spike (S) protein. However, mutations at RBD sequences found in the emerging SARS-CoV-2 variants greatly reduced the effectiveness of nAbs. Here we showed that four nAbs, S2-4D, S2-5D, S2-8D, and S2-4A, which recognized a conserved epitope in the S2 subunit of the S protein, can inhibit SARS-CoV-2 infection through blocking the S protein-mediated membrane fusion. Notably, these four nAbs exhibited broadly neutralizing activity against SARS-CoV-2 Alpha, Gamma, Delta, and Epsilon variants. Antisera collected from mice immunized with the identified epitope peptides of these four nAbs also exhibited potent virus neutralizing activity. Discovery of the S2-specific nAbs and their unique antigenic epitopes paves a new path for development of COVID-19 therapeutics and vaccines. IMPORTANCE The spike (S) protein on the surface of SARS-CoV-2 mediates receptor binding and virus-host cell membrane fusion during virus entry. Many neutralizing antibodies (nAbs), which targeted the receptor binding domain (RBD) of S protein, lost the neutralizing activity against the newly emerging SARS-CoV-2 variants with sequence mutations at the RBD. In contrast, the nAb against the highly conserved S2 subunit, which plays the key role in virus-host cell membrane fusion, was poorly discovered. We showed that four S2-specific nAbs, S2-4D, S2-5D, S2-8D, and S2-4A, inhibited SARS-CoV-2 infection through blocking the S protein-mediated membrane fusion. These nAbs exhibited broadly neutralizing activity against Alpha, Gamma, Delta, and Epsilon variants. Antisera induced by the identified epitope peptides also possessed potent neutralizing activity. This work not only unveiled the S2-specific nAbs but also discovered an immunodominant epitope in the S2 subunit that can be rationally designed as the broad-spectrum vaccine against the SARS-like coronaviruses.