Epitope Mapping Using the X-Ray Crystallographic Structure of Complement Receptor Type 2 (CR2)/CD21: Identification of a Highly Inhibitory Monoclonal Antibody That Directly Recognizes the CR2-C3d Interface

Abstract

Complement receptor type 2 (CR2)/CD21 is a B lymphocyte cell membrane C3d/iC3b receptor that plays a central role in the immune response. Human CR2 is also the receptor for the EBV viral membrane glycoprotein gp350/220. Both C3d and gp350/220 bind CR2 within the first two of 15–16 repetitive domains that have been designated short consensus/complement repeats. Many mAbs react with human CR2; however, only one currently available mAb is known to block both C3d/iC3b and gp350/220 binding. We have used a recombinant form of human CR2 containing the short consensus/complement repeat 1-2 ligand-binding fragment to immunize Cr2−/− mice. Following fusion, we identified and further characterized four new anti-CR2 mAbs that recognize this fragment. Three of these inhibited binding of CR2 to C3d and gp350/220 in different forms. We have determined the relative inhibitory ability of the four mAbs to block ligand binding, and we have used overlapping peptide-based approaches to identify linear epitopes recognized by the inhibitory mAbs. Placement of these epitopes on the recently solved crystal structure of the CR2-C3d complex reveals that each inhibitory mAb recognizes a site either within or adjacent to the CR2-C3d contact site. One new mAb, designated 171, blocks CR2 receptor-ligand interactions with the greatest efficiency and recognizes a portion of the C3d contact site on CR2. Thus, we have created an anti-human CR2 mAb that blocks the C3d ligand by direct contact with its interaction site, and we have provided confirmatory evidence that the C3d binding site seen in its crystal structure exists in solution.

Mapping of the C3d Ligand Binding Site on Complement Receptor 2 (CR2/CD21) Using Nuclear Magnetic Resonance and Chemical Shift Analysis

“…Subsequent biochemical studies were consistent with a requirement for both SCR1 and SCR2 in the interaction with C3d (37,49). However, contrary to suggestions in earlier studies, the x-ray co-crystal structure showed only SCR2 to be in contact with C3d (33).…”

Anti-Sm Autoantibodies in Systemic Lupus Target Highly Basic Surface Structures of Complexed Spliceosomal Autoantigens

“…A relatively new approach, presented herein, has been to identify the fine specificity of polyclonal Abs and then to apply those data to the crystal structure of the autoantigen. In fact, we know of only two such cases where a similar study has even been attempted (19,40). In all likelihood, this is due to the relative paucity of autoantigens that have had their crystal structures solved.…”

B Cells from Mice Prematurely Expressing Human Complement Receptor Type 2 Are Unresponsive to T-Dependent Antigens

“…Purified and biotin conjugated mAb 171 (anti-hCR2, IgG1 isotype) (25), IgG1 isotype control, and the polyclonal anti-hCR2 Ab were produced in the laboratory following standard methods. 2.4G2 (anti-mCD16/mCD32, Fc block), PE-conjugated B-Ly-4 (anti-hCR2) and CD19, allophycocyanin-or FITC-conjugated RA3-6B2 (anti-mCD45R, B220), FITC-conjugated anti-B7.1 (CD80), FITC-conjugated anti-B7.2 (CD86), FITC-conjugated Gl-7, biotin-conjugated anti-CD95 (jo-1), biotin-conjugated anti-I-A b (MHC II), biotin-conjugated anti-CD44, biotin-conjugated anti-CD25 (Il-2R␣-chain), biotin-conjugated anti-CD69, and streptavidin (SA)-allophycocyanin were all obtained from BD Pharmingen.…”