Combination of High-Resolution Structures for the B Cell Receptor and Co-Receptors Provides an Understanding of Their Interactions with Therapeutic Antibodies

SIMPLE SUMMARY: The treatment of B cell malignancies was transformed by the development of a monoclonal antibody—rituximab—that targets CD20, a protein expressed on the surface of B cells. However, some types of B cell malignancies do not express CD20 or can reduce the expression of the protein to escape therapy. Consequently, there is a need to develop tailored therapies against other targets expressed by B cells. The structures of key B cell proteins, the B cell receptor, and its co-receptors CD22, CD19 and CD81, have recently been solved and provide the opportunity to guide development of new antibodies and other therapies targeted at malignant B cells. Here, we review high-resolution protein structures of the BCR, CD22, CD19 and CD81 molecules, treatments that have been developed against these targets and discuss structural features that will enable the design of novel antibodies. ABSTRACT: B cells are central to the adaptive immune response, providing long lasting immunity after infection. B cell activation is mediated by a cell surface B cell receptor (BCR) following recognition of an antigen. BCR signaling is modulated by several co-receptors including CD22 and a complex that contains CD19 and CD81. Aberrant signaling through the BCR and co-receptors promotes the pathogenesis of several B cell malignancies and autoimmune diseases. Treatment of these diseases has been revolutionized by the development of monoclonal antibodies that bind to B cell surface antigens, including the BCR and its co-receptors. However, malignant B cells can escape targeting by several mechanisms and until recently, rational design of antibodies has been limited by the lack of high-resolution structures of the BCR and its co-receptors. Herein we review recently determined cryo-electron microscopy (cryo-EM) and crystal structures of the BCR, CD22, CD19 and CD81 molecules. These structures provide further understanding of the mechanisms of current antibody therapies and provide scaffolds for development of engineered antibodies for treatment of B cell malignancies and autoimmune diseases.