A high-affinity RBD-targeting nanobody improves fusion partner's potency against SARS-CoV-2 (preprint)

A key step to the SARS-CoV-2 infection is the attachment of its Spike receptor-binding domain (S RBD) to the host receptor ACE2. Considerable research have been devoted to the development of neutralizing antibodies, including llama-derived single-chain nanobodies, to target the receptor-binding motif (RBM) and to block ACE2-RBD binding. Simple and effective strategies to increase potency are desirable for such studies when antibodies are only modestly effective. Here, we identify and characterize a high-affinity synthetic nanobody (sybody, SR31) as a fusion partner to improve the potency of RBM-antibodies. Crystallographic studies reveal that SR31 binds to RBD at a conserved and greasy site distal to RBM. Although SR31 distorts RBD at the interface, it does not perturb the RBM conformation, hence displaying no neutralizing activities itself. However, fusing SR31 to two modestly neutralizing sybodies dramatically increases their affinity for RBD and neutralization activity against SARS-CoV-2 pseudovirus. Our work presents a tool protein and an efficient strategy to improve nanobody potency.

A potent synthetic nanobody targets RBD and protects mice from SARS-CoV-2 infection (preprint)

SARS-CoV-2, the causative agent of COVID-191, recognizes host cells by attaching its receptor-binding domain (RBD) to the host receptor ACE22-7. Neutralizing antibodies that block RBD-ACE2 interaction have been a major focus for therapeutic development8-18. Llama-derived single-domain antibodies (nanobodies, ~15 kDa) offer advantages including ease of production and possibility for direct delivery to the lungs by nebulization19, which are attractive features for bio-drugs against the global respiratory disease. Here, we generated 99 synthetic nanobodies (sybodies) by in vitro selection using three libraries. The best sybody, MR3 bound to RBD with high affinity (KD = 1.0 nM) and showed high neutralization activity against SARS-CoV-2 pseudoviruses (IC50 = 0.40 μg mL-1). Structural, biochemical, and biological characterization of sybodies suggest a common neutralizing mechanism, in which the RBD-ACE2 interaction is competitively inhibited by sybodies. Various forms of sybodies with improved potency were generated by structure-based design, biparatopic construction, and divalent engineering. Among these, a divalent MR3 conjugated with the albumin-binding domain for prolonged half-life displayed highest potency (IC50 = 12 ng mL-1) and protected mice from live SARS-CoV-2 challenge. Our results pave the way to the development of therapeutic nanobodies against COVID-19 and present a strategy for rapid responses for future outbreaks.

Potent synthetic nanobodies against SARS-CoV-2 and molecular basis for neutralization (preprint)

SARS-CoV-2, the Covid-19 causative virus, adheres to human cells through binding of its envelope Spike protein to the receptor ACE2. The Spike receptor-binding domain (S-RBD) mediates this key event and thus is a primary target for therapeutic neutralizing antibodies to mask the ACE2-interacting interface. Here, we generated 99 synthetic nanobodies (sybodies) using ribosome and phage display. The best sybody MR3 binds the RBD with KD of 1.0 nM and neutralizes SARS-CoV-2 pseudovirus with IC50 of 0.40 g mL-1. Crystal structures of two sybody-RBD complexes reveal a common neutralizing mechanism through which the RBD-ACE2 interaction is competitively inhibited by sybodies. The structures allowed the rational design of a mutant with higher affinity and improved neutralization efficiency by [~]24-folds, lowering the IC50 from 12.32 to 0.50 g mL-1. Further, the structures explain the selectivity of sybodies between SARS-CoV strains. Our work presents an alternative approach to generate neutralizers against newly emerged viruses. One sentence summaryStructural and biochemical studies revealed the molecular basis for the neutralization mechanism of in vitro-selected and rationally designed nanobody neutralizers for SARS-CoV-2 pseudovirus.