STRUCTURE OF A HUMAN IgM RHEUMATOID FACTOR COMPLEXED WITH ITS AUTOANTIGEN IgG Fc. Corper, A. L.¹, Sohi, M. K.¹, Jefferis, R.², Steinitz, M.³, Feinstein, A.⁴, Beale, D.⁴, Taussig, M. J.⁴, Sutton, B. J.^1*, ¹The Randall Institute, King's College London, 26-29 Drury Lane, London WC2B SRL, UK, ²Immunology Dept., Birmingham University Medical School, Birmingham, B15 2TT, UK, ³The Hebrew University, Hadassah Medical School, Jerusalem, Israel, ⁴Lab.of Structural Studies, Immunology Dept., Babraham Institute, Cambridge CB2 4AT, UK

We have determined the crystal structure of a complex between a human IgM rheumatoid factor (RF) and IgG Fc, revealing for the first time the nature of autoantibody recognition of an autoantigen. RFs are found in the sera and synovia of patients with rheumatoid arthritis (RA), and form immune complexes which cause inflammation and tissue damage. We crystallised a complex between the Fab fragment an IgM RF from an RA patient (RF-AN), and human IgG4 Fc, with a stoichiometry of 2:1 Fab:Fc. The crystals are space group C2 (a = 160.3Å, b = 81.9Å, c = 64.2Å, [[beta]] =

98.3deg.) and diffract to 3.2Å resolution. The structure was solved by molecular replacement, and refined to a final R_cryst of 0.22 (R_free = 0.29). The epitope in IgG Fc consists principally of residues in the CH3 domain, with a smaller contribution from CH2, and overlaps with the binding sites for the bacterial proteins A and G. The interaction between the antibody and the epitope differs in topology from that of antibody-antigen or complexes seen to date, using only one edge of the potential combining site surface. The small number of contact residues accounts for the low binding affinity of the Fab-Fc interaction. The light chain contributes only a single contact residue, but this is the result of somatic mutation of the germline gene, and implicates a process of antigen driven selection in the generation of this RF. These results provide a basis for the design of agents with therapeutic applications in RA.