An anti-Aβ (amyloid β) single-chain variable fragment prevents amyloid fibril formation and cytotoxicity by withdrawing Aβ oligomers from the amyloid pathway

A beta (amyloid beta) immunotherapy has been revealed as a possible tool in Alzheimer's disease treatment. In contrast with complete antibodies, the administration of scFvs (single-chain variable fragments) produces neither meningoencephalitis nor cerebral haemorrhage. In the present study, the recombinant expression of scFv-h3D6, a derivative of an antibody specific for A beta oligomers, is presented, as well as the subsequent proof of its capability to recover the toxicity induced by the A beta(1-42) peptide in the SH-SY5Y neuroblastoma cell line. To gain insight into the conformational changes underlying the prevention of A beta toxicity by this antibody fragment, the conformational landscape of scFv-h3D6 upon temperature perturbation is also described. Heating the native state does not lead to any extent of unfolding, but rather directly to a beta-rich intermediate state which initiates an aggregation pathway. This aggregation pathway is not an amyloid fibril pathway, as is that followed by the A beta peptide, but rather a worm-like fibril pathway which, noticeably, turns out to be nontoxic. On the other hand, this pathway is thermodynamically and kinetically favoured when the scFv-h3D6 and A beta(1-42) oligomers form a complex in native conditions, explaining how the scFv-h3D6 withdraws A beta(1-42) oligomers from the amyloid pathway. To our knowledge, this is the first description of a conformational mechanism by which a scFv prevents A beta-oligomer cytotoxicity.