False-positive myeloperoxidase binding activity due to DNA/anti-DNA antibody complexes: a source for analytical error in serologic evaluation of anti-neutrophil cytoplasmic autoantibodies

Anti-myeloperoxidase antibodies (anti-MPO) are a major type of anti-neutrop hil cytoplasmic antibody (ANCA). While evaluating anti-MPO monoclonal antib odies from SCG/Kj mice, we observed several hybridomas that appeared to rea ct with both MPO and DNA. Sera from some patients with systemic lupus eryth ematosus (SLE) also react with MPO and DNA. We hypothesized that the MPO bi nding activity is a false-positive result due to the binding of DNA, contai ned within the antigen binding site of anti-DNA antibodies, to the cationic MPO. Antibodies from tissue culture supernatants from 'dual reactive' hybr idomas were purified under high-salt conditions (3 m NaCl) to remove any an tigen bound to antibody. The MPO and DNA binding activity were measured by ELISA. The MPO binding activity was completely abrogated while the DNA bind ing activity remained. The MPO binding activity was restored, in a dose-dep endent manner, by the addition of increasing amount of calf-thymus DNA (CT- DNA) to the purified antibody. Sera from six patients with SLE that reacted with both MPO and DNA were treated with DNase and showed a decrease in MPO binding activity compared with untreated samples. MPO binding activity was observed when CT-DNA was added to sera from SLE patients that initially re acted with DNA but not with MPO. These results suggest that the DNA contain ed within the antigen binding site of anti-DNA antibodies could bind to the highly cationic MPO used as substrate antigen in immunoassays, resulting i n a false-positive test.