Induction of neutrophil responsiveness to myeloperoxidase antibodies by their exposure to supernatant of degranulated autologous neutrophils

Antibodies against myeloperoxidase (MPO) and proteinase 3 (PR3) are the predominant autoantibodies present in antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis. Their binding to the corresponding antigen on the surface of polymorphonuclear neutrophils (PMNs) is believed to trigger the disease process. Cytokines released during an inflammatory reaction are thought to prime resting PMNs, making them responsive to autoantibodies. In the present study we found that MPO but not PR3 could be detected on the cell surface of unstimulated PMNs after incubation with the supernatants of activated autologous PMNs. MPO was shown to be acquired from these supernatants, because PMNs did not express MPO when the supernatants were specifically MPO-depleted. In addition, purified soluble MPO bound to unstimulated PMNs. Unstimulated PMNs that had passively acquired MPO released oxygen radicals when incubated with monoclonal antibody anti-MPO or the immunoglobulin G fraction of a patient with MPO-ANCA. The data presented here suggest that, in ANCA-associated vasculitis, soluble MPO released by activated PMNs may bind to unstimulated PMNs, thereby making them reactive to anti-MPO antibodies. This mechanism of dispersing PMN activation would be specific for MPO-ANCA and may explain differences in the pathologic and clinical expression of MPO-ANCA versus PR3-ANCA vasculitis.