The molecular reach of antibodies determines their SARS-CoV-2 neutralisation potency (preprint)

Antibodies play crucial roles in health and disease and are invaluable tools for diagnostics, research, and therapy. Although antibodies bind bivalently, we lack methods to analyse bivalent binding. Here, we introduce a particle-based model and use it to analyse bivalent binding of SARS-CoV-2 RBD-specific antibodies in surface plasmon resonance assays. The method reproduces the monovalent on/off-rates and enables measurements of new parameters, including the molecular reach, which is the maximum antigen separation that supports bivalent binding. We show that the molecular reach (22-46 nm) exceeds the physical size of an antibody (15 nm) and that the variation in reach across 45 patient-isolated antibodies is the best correlate of viral neutralisation. Using the complete set of fitted parameters, the model predicts an emergent antibody binding potency that equals the neutralisation potency. This novel analytical method should improve our understanding and exploitation of antibodies and other bivalent molecules.

Effects of common mutations in the SARS-CoV-2 Spike RBD domain and its ligand the human ACE2 receptor on binding affinity and kinetics (preprint)

The interaction between the SARS-CoV-2 virus Spike protein receptor binding domain (RBD) and the ACE2 cell surface protein is required for viral infection of cells. Mutations in the RBD domain are present in SARS-CoV-2 variants of concern that have emerged independently worldwide. For example, the more transmissible B.1.1.7 lineage has a mutation (N501Y) in its Spike RBD domain that enhances binding to ACE2. There are also ACE2 alleles in humans with mutations in the RBD binding site. Here we perform a detailed affinity and kinetics analysis of the effect of five common RBD mutations (K417N, K417T, N501Y, E484K and S477N) and two common ACE2 mutations (S19P and K26R) on the RBD/ACE2 interaction. We analysed the effects of individual RBD mutations, and combinations found in new SARS-CoV-2 variants first identified in the UK (B.1.1.7), South Africa (B.1.351) and Brazil (P1). Most of these mutations increased the affinity of the RBD/ACE2 interaction. The exceptions were mutations K417N/T, which decreased the affinity. Taken together with other studies, our results suggest that the N501Y and S477N mutations primarily enhance transmission, the K417N/T mutations facilitate immune escape, and the E484K mutation facilitates both transmission and immune escape.