Evolutionary and structural pathways of antibody maturation reveal principles for engineering potent SARS-CoV-2 neutralizers.

The continual emergence of SARS-CoV-2 variants underscores the need to elucidate antibody maturation and apply rational design principles to optimize therapeutics. This study focused on vaccine-elicited 9C lineage, encoded by IGHV4-34, as a model for affinity maturation and structural refinement. Sequence analyses revealed strong framework conservation alongside focused diversification within CDRH3, where the invariant RWFD anchor and a recurrent SYTV block were consistently associated with potent neutralization. Phylogenetic reconstruction traced a trajectory from weakly neutralizing precursors, such as 9A91, to ultrapotent clones, like 9C2, showing nearly a 3000-fold improvement in IC₅₀ values. Computational modeling, molecular dynamics, and docking analyses demonstrated that potency arises from progressive stabilization of the CDRH3 apex, with the SYT motif at VH106-108 serving as a critical structural driver with improved IC50 = 5.52 μg/mL. Residues at positions 98, 102, and 105 further fine-tuned stability and binding geometry, while rigid versus flexible apex conformations revealed convergent solutions for high-affinity binding. Engineered variants of 9A91 recapitulated these principles, with SYT installation and flanking substituents improving stability, energetics, and antigen engagement. ELISA validation confirmed up to 64% enhanced binding and neutralization with IC50 = 0.76 μg/mL compared with the germline precursor. Collectively, these findings define a hierarchical mechanism of antibody maturation, where conserved anchoring motifs, apex stabilization, and secondary refinements synergize to achieve potency, establishing a framework for rational antibody design against SARS-CoV-2 and emerging viral threats.