A major target for warm immunoglobulin G autoantibodies: the third external loop of Band 3.

BACKGROUND: Rh proteins and the Wr(b) antigen, which results from an interaction between Band 3 and glycophorin A, are the most common targets for warm immunoglobulin (Ig)G autoantibodies. Apart from autoanti-Di(b) , a scarce specificity, IgG warm autoantibodies specific for Band 3 have never been characterized by serologic methods. STUDY DESIGN AND METHODS: Blood samples from 120 patients with autoimmune hemolytic anemia (AIHA) and IgG-coated red blood cells (RBCs) were studied by serologic methods. Some autoantibodies were investigated by immunochemical methods. RESULTS: Autoantibodies against the third external loop of Band 3 have a distinctive pattern of reactivity in that they fail to react after RBC treatment with α-chymotrypsin and pronase, whereas papain, ficin, and trypsin have no effect. Eleven (9%) patients had pure anti-Band 3 autoantibodies. Autoanti-Band 3 antibodies were associated with other specificities in 66 (55%) patients. Immunoprecipitation and rare RBCs from a Wu+ homozygote, known to have an unusual pattern of reactivity after protease treatment, were used to confirm the Band 3 specificity. Treatment with sodium hypochlorite, believed to oxidize the Met residue at Position 559 in the third loop, showed that these autoantibodies were heterogeneous. Most antibodies reacted optimally at 37 °C, but two patients had incomplete cold IgG autoantibodies. Unlike autoantibodies to Rh proteins, warm autoanti-Band 3 activate complement and are almost totally bound to autologous RBCs. CONCLUSION: We describe the first cases of warm IgG autoantibodies specific for the third loop of Band 3. This external loop also appears as a major target in patients with warm antibody AIHA.