Development of an SPR-based binding assay for characterization of anti-CD20 antibodies to CD20 expressed on extracellular vesicles

Characterization of anti-CD20 antibody binding to CD20 is critical to development of anti-CD20 therapeutics. While SPR is widely used to characterize binding of therapeutics to their targets, its application to the characterization of anti-CD20 therapeutics has been limited by the challenges of obtaining recombinant or native fulllength CD20 suitable for ligand binding assays. Extracellular vesicles (EVs) are nanoparticles naturally released from cells that provide a favorable microenvironment for membrane proteins such as CD20 to maintain proper conformation and activity. Here, we report a novel SPR-based assay that enables elucidation of binding kinetics and affinity measurements for anti-CD20 antibody binding to EV-expressed CD20. Our SPR assay is label-free, easy to perform, and demonstrates specific interaction of rituximab and obinutuzumab to CD20 expressed on EVs. The SPR assay revealed that rituximab and obinutuzumab have different binding kinetics and mechanisms to CD20 although both bind to CD20 with high affinity. Our results are consistent with existing literature and verified the validity of this method. The detailed binding kinetics information may also contribute to a better understanding of the interaction between these two antibodies and CD20. Moreover, our method provides a platform with which to characterize other therapeutic antibodies binding to EV-expressed membrane proteins.