Neutrophil extracellular traps in neuropathy with anti-neutrophil cytoplasmic autoantibody-associated microscopic polyangiitis

To clarify the roles of neutrophils in anti-neutrophil cytoplasmic autoantibody (ANCA)-associated vasculitic neuropathy, we studied neutrophil extracellular traps (NETs) in peripheral nerve vasculitis. Stored nerve samples from 17 patients with microscopic polyangiitis (MPA) were immunohistochemically analyzed using antibodies for citrullinated histone H3 (citH3) and various neutrophil enzymes. We defined merged citH3 and extracellularly released myeloperoxidase (MPO) as NET formation. We also compared NET formation between MPO-ANCA-positive/negative MPA and rheumatoid arthritis (RA)-associated vasculitic neuropathy. NETs were identified mostly in vasculitic small arterioles of 6 of 12 MPO-ANCA-positive MPA patients, and their frequency was higher (p < 0.05) than in ANCA-negative patients. NETs were not found in vasculitic neuropathy with RA or patients with chronic inflammatory demyelinating polyradiculoneuropathy. NETs were also observed in the peripheral nervous system of MPA patients as well as in the lung and kidney. These results suggest that NETs may be involved in the pathogenesis of MPA neuropathy.