Aducanumab binding to Aβ _1-42 fibrils alters dynamics of the N-terminal tail while preserving the fibril core

Aducanumab, a human IgG1 antibody with plaque-clearing effects and modest clinical benefit, binds selectively to aggregated Aβ via the N-terminal region. Yet, the molecular details of how the antibody engages Aβ_1-42 fibrils remain unresolved. Using magic-angle spinning NMR, we show that binding of aducanumab preserves the overall architecture of the Aβ_1-42 fibril core while inducing significant structural and dynamic perturbations in the N-terminal region. Antibody binding markedly reduces flexibility in this domain, with the appearance of side-chain resonances from residues D1, E3, and histidine (likely H6) in dipolar-based experiments. These side chains-previously observed in scalar-coupling spectra of the unbound state-indicate rigidification of residues that were dynamic. The interaction extends to S8 and Y10, indicating broader fibril engagement than the minimal epitope (residues 3 to 7) defined in fragment-based studies. Perturbations in the C-terminal segment (G37-A42) are consistent with its spatial proximity to the antibody-bound N termini of neighboring monomers. Cryo-TEM images reveal fibrils bundling in the presence of aducanumab, consistent with lateral association via antibody cross-linking, supporting a model where surface coating and steric hindrance suppress secondary nucleation. This mode of action restricts monomer access to catalytic sites on the fibril surface, resulting in partial inhibition (~threefold reduction) of secondary nucleation. The effect depends on high avidity and relatively high stoichiometry but is ultimately limited by antibody size relative to N-terminal spacing along the fibril. These findings provide atomic-level insights into aducanumab's binding mode and supply a structural framework for understanding antibody-mediated fibril recognition and for guiding next-generation therapies targeting Aβ aggregates in Alzheimer's disease.