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

Keywords: B cell; B cell receptor; CD19; CD22; CD81; cryo-electron microscopy; crystallography; monoclonal antibody.

Figure 1

Model of BCR dynamics during B cell activation. Antigen binding to the BCR promotes the interaction between BCRs and lipid rafts. The BCR moves away from the negative co-receptor CD22 and the CD19-CD81 complex dissociates, allowing CD19 to diffuse in the membrane and interact with the BCR. Interaction with CD19 amplifies signaling through the BCR and activation of the B cell. CD22 is not shown here as part of the CD19-CD81 complex. BCR, B cell receptor; ITAM, immunoreceptor tyrosine-based activation motifs; ITIM, immunoreceptor tyrosine-based inhibition motif; Lyn, Lck/Yes-related novel (Lyn) tyrosine kinase; P, phosphate; Syk, spleen tyrosine kinase. Modified from Susa et al. [5].

Figure 2

Ribbon structure of the IgM-BCR. Cryo-EM reconstruction of the IgM-BCR showing CD79a (green), CD79b (blue), IgM heavy chains (orange and cyan) and light chains (magenta and yellow). Using the cryo-EM structure of IgM-BCR, three surface (1–3) epitopes that are free of glycosylation sites are identified for recognition by an antibody. The glycan-free surface epitopes are indicated by red circles and magnified views are shown in the right-side panels. ECDα structural model produced using PyMOL software version 2.5.2, Schrödinger, Inc., New York, NY, USA [35].

Figure 3

Ribbon representation of the human CD19-CD81 complex within a cell membrane bound to coltuximab. CD19 is colored blue, CD81 tan, coltuximab pink, and phospholipids in grey cartoon representation. The model shows the heavy and light chains of coltuximab (pink), the complete ECD and TM of CD19 (blue), and the full-length of CD81 (tan). The structural model was produced using PyMOL software version 2.5.2., Schrödinger, Inc., New York, NY, USA [35].

Figure 4

Surface epitope of CD22 recognized by epratuzumab. X-ray crystallographic structure of epratuzumab (blue and cyan) bound to the CD22 d2 (pale green)/d3 (lilac) interface. The CD22 d1 Ig domain is shown in sand. The structural model was produced using PyMOL software version 2.5.2, Schrödinger, Inc., New York, NY, USA [35].