Phage display and kinetic selection of antibodies that specifically inhibit amyloid self-replication

The aggregation of the amyloid beta peptide (A beta) into amyloid fibrils is a defining characteristic of Alzheimer's disease. Because of the complexity of this aggregation process, effective therapeutic inhibitors will need to target the specific microscopic steps that lead to the production of neurotoxic species. We introduce a strategy for generating fibril-specific antibodies that selectively suppress fibril-dependent secondary nucleation of the 42-residue form of A beta (A beta 42). We target this step because it has been shown to produce the majority of neurotoxic species during aggregation of A beta 42. Starting from large phage display libraries of single-chain antibody fragments (scFvs), the three-stage approach that we describe includes (i) selection of scFvs with high affinity for A beta 42 fibrils after removal of scFvs that bind A beta 42 in its monomeric form; (ii) ranking, by surface plasmon resonance affinity measurements, of the resulting candidate scFvs that bind to the A beta 42 fibrils; and (iii) kinetic screening and analysis to find the scFvs that inhibit selectively the fibril-catalyzed secondary nucleation process in Av42 aggregation. By applying this approach, we have identified four scFvs that inhibit specifically the fibril-dependent secondary nucleation process. Our method also makes it possible to discard antibodies that inhibit elongation, an important factor because the suppression of elongation does not target directly the production of toxic oligomers and may even lead to its increase. On the basis of our results, we suggest that the method described here could form the basis for rationally designed immunotherapy strategies to combat Alzheimer's and related neurodegenerative diseases.