Myeloperoxidase (MPO) is one of the major target antigens of antineutrophil cytoplasmic autoantibodies (ANCA) found in patients with small-vessel vasculitis and pauci-immune necrotizing glomerulonephritis. To date, the target epitopes of MPO-ANCA remain poorly defined. Human MPO-ANCA do not typically bind mouse MPO. We utilized the differences between human and mouse MPO to identify the target regions of MPO-ANCA. We generated five chimeric MPO molecules in which we replaced different segments of the human or mouse molecules with their homologous counterpart from the other species. Of serum samples from 28 patients screened for this study, 43 samples from 14 patients with MPO-ANCA-associated vasculitis were tested against recombinant human and mouse MPO and the panel of chimeric molecules. Sera from 64 and 71% of patients bound to the carboxy-terminus of the heavy chain, in the regions of amino acids 517-667 or 668-745, respectively. No patient serum bound the MPO light chain or the amino-terminus of the heavy chain. All sera bound to only one or two regions of MPO. Although the pattern of MPO-ANCA binding changed over time (4-27 months) in 6 of 10 patients with several serum samples, such changes were infrequent. Other target regions of MPO-ANCA may not have been detected due to conformational differences between the native and recombinant forms of MPO. MPO-ANCA do not target a single epitope, but rather a small number of regions of MPO, primarily in the carboxy-terminus of the heavy chain.
Bibliographical noteFunding Information:
This work was supported by National Institute for Diabetes and Digestive and Kidney Diseases grant PO1DK58335, the Swedish Research Council and the Swedish Association for Medical research. This work was presented in part in abstract form at the 2002 (Erdbrüegger U, Hellmark T, Majure MCM et al. Comparative binding of human anti-MPO antibodies to human and mouse MPO. J Am Soc Nephrol 2002; 13 ) and 2004 (Erdbrüegger U, Hellmark T, Alcorta DA et al. Most myeloperoxidase (MPO)-ANCA show stable binding to a restricted number of epitopes. J Am Soc Nephrol 2004; 15 : 680A) meetings of the American Society of Nephrology.