PROPERTIES OF CIRCULATING IGA MOLECULES IN HENOCH-SCHONLEIN PURPURA NEPHRITIS WITH FOCUS ON NEUTROPHIL CYTOPLASMIC ANTIGEN IGA BINDING (IGA-ANCA) - NEW INSIGHT INTO A DEBATED ISSUE

Background. The presence and the pathogenetic role of circulating IgA reacting with neutrophil cytoplasmic antigens (IgA-ANCA) in patients w ith Henoch-Schonlein purpura (HSP) is still debated. This study was ai med to investigate some characteristics of serum IgA and macromolecula r IgA in HSP patients, focusing on IgA-ANCA. Methods. Eighty-seven HSP patients with biopsy proved renal involvement (51 adults and 36 child ren) enrolled in a multicentre study of the Italian Group of Immunopat hology were investigated. Results. Significantly high levels of IgA im mune complexes were found in both adults (P<0.05) and children (P<0.01 ), while the binding of IgA to jacalin, was significantly low in child ren with HSP (P<0.01) only. Two series of ELISA were done for IgA-ANCA , in two different laboratories. Increased binding to PMN crude extrac ts (P<0.01) without any modification in IgA binding to proteinase 3 wa s found by either specific ELISA. Conversely, the binding of IgA to my eloperoxidase (MPO) was found to be significantly (P<0.05) increased w ith positive values in 25% of patients by one assay only. Three of fou r sera with positive IgA-MPO ANCA exhibited binding in Western-blot st udies with the MPO preparation used in ELISA to a 28-kDa species. D-ga lactose and N-acetyl-glucosamine decreased the binding of serum IgA to MPO more in HSP than in controls (P<0.05). Conclusions. The conflicti ng reports on IgA-ANCA may reflect some atypical characteristics of th e reaction which can be detected only by some ELISAs. We suggest that not an antigen-antibody reaction but a lectinic interaction due to abn ormal composition of IgA carbohydrate side chains may account for the IgA-ANCA reaction in patients with HSP nephritis.