Three-dimensional structure of the Fab from a human IgM cold agglutinin

Cold agglutinins (CAs) are IgM autoantibodies characterized by their abilit y to agglutinate in vitro RBC at low temperatures, These autoantibodies cau se hemolytic anemia in patients with CA disease. Many diverse Ags are recog nized by CAs, most frequently those belonging to the Ui system. These are o ligosaccharides composed of repeated units of N-acetyllactosamine, expresse d on RBC, The three-dimensional structure of the Fab of KAU, a human monocl onal IgM CA with anti-I activity, was determined. The KAU combining site sh ows an extended cavity and a neighboring pocket. Residues from the hypervar iable loops V(H)CDR3, V(L)CDR1, and V(L)CDR3 form the cavity, whereas the s mall pocket is defined essentially by residues from the hypervariable loops V(H)CDR1 and V(H)CDR2, This fact could explain the V(H)4-34 germline gene restriction among CA. The KAU combining site topography is consistent with one that binds a polysaccharide. The combining site overall dimensions are 15 ij wide and 24 Angstrom long. Conservation of key binding site residues among anti-I/i CAs indicates that this is a common feature of this family o f autoantibodies, We also describe the first high resolution structure of t he human IgM C(H)1:C-L domain. The structural analysis shows that the C(H)1 -C-L interface is mainly conserved during the isotype snitch process from I gM to IgG1.