Crystal structure of a conformation-dependent rabbit IgG Fab specific for amyloid prefibrillar oligomers

Although rabbit antibodies are widely used in research, no structures of rabbit antigen-binding fragments (Fab) have been reported. M204 is a rabbit monoclonal antibody that recognizes a generic epitope that is common to prefibrillar amyloid oligomers formed from many different amyloidogenic sequences. Amyloid oligomers are widely suspected to be a primary causative agent of pathogenesis in several age-related neurodegenerative diseases, such as Alzheimer's disease. The detailed structure of these amyloid oligomers is not known nor is the mechanism for the recognition of the generic epitope by conformation-dependent monoclonal antibodies. As a first approach to understanding the mechanism of conformation-dependent antibody recognition, we have crystallized the Fab of M204. We have determined the structure of the Fab of M204 at 1.54 Å resolution. The crystal structure reveals details of the M204 antigen combining site and features unique to rabbit Fabs such as an interdomain disulfide bond on its light chain. Based on the structural features of the antigen-combining site of the M204, we rule out a "steric zipper" formation, as found in numerous amyloid fibril structures, as a mechanism of antibody-antigen recognition. The details of the first rabbit immunoglobulin Fab structure might also be useful for exploiting the potential of rabbit monoclonal antibodies for the development of humanized rabbit antibodies as therapeutic agents.