title: A major target for warm immunoglobulin G autoantibodies: the third external loop of Band 3.
abstract: 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.
  
  
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Etablissement Français du Sang Ile-de-France, Paris, France.
authors.forename: Daniel
authors.initials: D
authors.lastname: Janvier
authors.forename: Yin
authors.initials: Y
authors.lastname: Lam
authors.forename: Isabelle
authors.initials: I
authors.lastname: Lopez
authors.forename: Lalia
authors.initials: L
authors.lastname: Elakredar
authors.forename: Philippe
authors.initials: P
authors.lastname: Bierling
mesh_terms: D000744:Anemia, Hemolytic, Autoimmune / Q000276:immunology; D001323:Autoantibodies / Q000276:immunology*; D002918:Chymotrypsin / Q000378:metabolism; D005260:Female; D005361:Ficain / Q000378:metabolism; D006801:Humans; D007074:Immunoglobulin G / Q000276:immunology*; D047468:Immunoprecipitation; D008297:Male; D011402:Pronase / Q000378:metabolism
chemical_list: D001323:Autoantibodies; D007074:Immunoglobulin G; D002918:Chymotrypsin; C117608:alpha-chymotrypsin; D005361:Ficain; D011402:Pronase