Anti-amyloid nanobody-Fc fusion protein drives potent amyloid clearance through microglial recruitment

Anti-amyloid beta (Aβ) monoclonal antibodies are effective at lowering amyloid in Alzheimer's disease (AD). However, whether Fc-mediated effector function is absolutely required for efficacy is not completely understood. This is important for optimizing therapeutic efficacy and mitigating side effects such as amyloid-related imaging abnormalities (ARIA). Antibodies lacking Fc effector function, like single-domain antibodies (nanobodies), offer a unique tool to dissect these mechanisms, as their small size facilitates blood-brain barrier (BBB) penetration and allows Fc-mediated functions to be studied independently.

We immunized a llama with Aβ aggregates and constructed a phage display library to screen for aggregate-specific nanobodies. Lead candidates were characterized by epitope mapping and binding affinity to amyloid plaques in both murine and human AD brain tissues. We further assessed their BBB permeability and evaluated their efficacy in clearing pre-existing plaques in amyloid precursor protein (APP)/presenilin 1 (PS1) mice.

We identified two lead nanobodies, 3A11 and 2D10, that bind distinct epitopes and specifically bind Aβ plaques in murine and human AD brain tissues. Following systemic administration, the monovalent, unmodified (Fc-less) 2D10 nanobody, but not 3A11, successfully crossed the BBB and engaged amyloid plaques in APP/PS1 mice. However, despite robust target engagement, the Fc-less 2D10 failed to recruit microglia or reduce plaque burden. In contrast, an engineered 2D10-Fc fusion antibody potently cleared amyloid plaques, achieving a reduction in pathology comparable to aducanumab treatment. This efficacy was directly correlated with Fc-mediated microglial recruitment and activation, demonstrating that the Fc domain is essential for phagocytic plaque removal.

Our findings demonstrate that Fc effector function is indispensable for microglial-mediated amyloid clearance in vivo. By clarifying this fundamental mechanism, this study provides a framework for the rational design of next-generation immunotherapies. Furthermore, 2D10-Fc represents a promising therapeutic candidate, combining the high-affinity targeting of nanobodies with the effector power necessary for robust plaque clearance.