Structure of CD20 in complex with the therapeutic monoclonal antibody rituximab

Cluster of Differentiation 20 (CD20) is a B cell membrane protein that is targeted by monoclonal antibodies for the treatment of malignancies and auto-immune disorders, but whose structure and function are unknown. Rituximab (RTX) has been in clinical use for two decades, but how it activates complement to kill B cells remains poorly understood. We obtained a structure of CD20 in complex with RTX, revealing CD20 as a compact double-barrel dimer bound by two RTX antigen-binding fragments (Fabs), each of which engages a composite epitope and an extensive homotypic Fab:Fab interface. Our data suggest that RTX crosslinks CD20 into circular assemblies and lead to a structural model for complement recruitment. Our results further highlight the potential relevance of homotypic Fab:Fab interactions in targeting oligomeric cell-surface markers.