# A SARS-CoV-2 variant-induced NTD-targeting antibody enhances viral infection via a distinctive binding mode

## TL;DR

A new antibody from a Delta variant infection enhances SARS-CoV-2 entry into cells through a unique binding pattern, similar to antibodies from earlier strains.

## Contribution

Discovery of a Delta-induced antibody that enhances viral infection via a novel binding orientation despite NTD mutations.

## Key findings

- ConD-854 enhances infection of pre-Omicron variants but not Omicron or later strains.
- ConD-854 binds to a shared epitope on NTD with a distinct heavy-light chain orientation.
- Delta variant mutations do not affect induction of NIEAs targeting Loop211–215.

## Abstract

SARS-CoV-2 infection elicits both neutralizing and non-neutralizing monoclonal antibodies (mAbs), primarily targeting to the N-terminal domain (NTD), receptor-binding domain (RBD), and S2 subunit of the spike protein. Notably, a unique subset of NTD-targeting mAbs isolated from prototype Wuhan-Hu-1 strain infected donors displayed a capacity of facilitating the viral infection independent of the fragment crystallizable (Fc) region in vitro. However, the rapid evolution of SARS-CoV-2 variants, particularly with NTD mutations, has led to widespread immune evasion. Whether SARS-CoV-2 variants could still induce NTD-targeting infection-enhancing antibodies (NIEAs) remains unclear. Here, we identified a distinctive NIEA, ConD-854, from a Delta variant primarily infected donor, with broad infection-enhancing activities against most pre-Omicron variants but not against post-Omicron variants. Structural and functional analysis revealed that ConD-854 enhanced the viral infection through an Fc-independent bivalent binding mechanism with a largely shared recognition epitope, but its heavy-light chain orientation was nearly perpendicular relative to the reported prototype strain-induced NIEAs. Collectively, our findings demonstrated that the primary infection of Delta variant could still induce the NIEAs targeting the similar epitope as those elicited by prototype strain infection. Mutations in Delta NTD were located outside the infection-enhancing epitope and did not affect the induction of NIEAs. Remarkably, we defined a distinctive structural paradigm for an NIEA to recognize the viral epitope. These results enriched our understanding of antiviral antibodies and provided insights for future vaccine design.

SARS-CoV-2 infection triggers the production of various antibodies, however, in in-vitro assays, some antibodies targeting the N-terminal domain (NTD) of the viral spike protein unexpectedly promote viral entry into cells in an Fc-independent manner. Previously, it remained unclear whether infection with SARS-CoV-2 variants such as Delta would still trigger the body to produce such NTD-targeting infection-enhancing antibodies (NIEAs). In this study, we identified one NIEA from a donor primarily infected with the Delta variant, which enhanced the infection of most pre-Omicron viruses but did not enhance the infection of Omicron and subsequent variants. Our cryo-electron microscopy (cryo-EM) data revealed that this Delta-induced NIEA, like NIEAs induced by the prototype strain, binds to a roughly similar region on the NTD containing the Loop211–215 but exhibits a distinct binding mode when compared with the previously reported prototype-induced NIEAs, as its heavy-light chain orientation is nearly perpendicular to that of the prototype-induced NIEAs even though their approaching angles are similar. Our study provides new insights into the functional mechanism of NIEAs and the design of vaccines.