IMMUNE-BRAIN CROSSTALK IN CASPR2 ENCEPHALITIS: THE ROLE OF BISPECIFIC IGG4

Anti-CASPR2 disease is a severe form of encephalitis belonging to the emerging group of IgG4-neurological autoimmune disorders (IgG4-NAID). These disorders are characterized by pathogenic IgG4 antibodies, which have the unique feature of the Fab-arm exchange (FAE) to form bispecific antibodies. Anti-CASPR2 patients present with three distinct clinical phenotypes: limbic encephalitis, neuromyotonia and Morvan’s syndrome which highlight the importance of characterizing these antibodies and what role the FAE plays in disease pathophysiology.
For this study, antibodies were cloned from patients and produced for functional characterization in different backbones: IgG1, monospecific IgG4 and bispecific IgG4. To create stable bispecific antibodies, controlled FAE was induced by combining the IgG4 antibodies with another negative IgG4.
Pathogenicity was first assessed in vitro using CBA, TBA in mouse sciatic nerve and brain and in hippocampal neurons cultures with several immunofluorescence stainings. Then we used a passive transfer mouse model to check pathogenicity in vivo, where antibodies were administered into the right lateral ventricle via osmotic minipumps. Behavioral assays, infrared monitoring of circadian rhythms and sleep disturbances, and ex vivo analyses of brain tissue were conducted to evaluate antibody binding and functional effects. This methodology enabled the direct comparison of IgG1 and IgG4 pathogenic antibodies, as well as differences between monospecific and bispecific IgG4 antibodies.
This approach enables the investigation of isotype-specific pathogenicity and the role of FAE in disease mechanisms. These findings also provide a more precise characterization of antibody–neuron interactions in autoimmune encephalitis and improving our understanding of emerging IgG4-NAID