Abstract
Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitides (AAV) are autoimmune inflammatory small-vessel disorders with potentially life-threatening organ manifestations. Recent disease definitions and classification criteria allow distinction between granulomatosis with polyangiitis (GPA), eosinophilic granulomatosis with polyangiitis (EGPA), and non-granulomatous microscopic polyangiitis (MPA). The discovery of ANCA-autoantibodies directed against proteolytic enzymes of neutrophil granules-has enabled earlier diagnosis of AAV and paved the way to stage-adapted treatments. ANCA testing initially relied on different immunofluorescence patterns, i.e., cytoplasmic ANCA (C-ANCA) vs. perinuclear ANCA (P-ANCA), in ethanol-fixed neutrophils. This is nowadays outperformed by well-standardized immunoassays against the ANCA target antigens proteinase 3 (PR3) and myeloperoxidase (MPO) for the diagnosis of small-vessel vasculitides. The discovery of ANCA has contributed substantially to unravelling the pathogenesis of AAV, which comprises neutrophil degranulation, NETosis with concurrently amplified ANCA antigen presentation, and intra- and transmural vascular inflammation involving the alternative complement system in susceptible individuals. There is a genetic disposition concerning certain HLA alleles and polymorphisms of the proteinase 3 gene. Furthermore, epigenetic modifications impact on disease activity and relapse. During follow-up, the ANCA titer is not a reliable mirror of disease activity; however, PR3-ANCA positivity is associated with a greater likelihood of relapse and a better treatment response to rituximab as compared to cyclophosphamide/azathioprine. Within the past 60 years, the discovery of ANCA has revolutionized the ability to diagnose, understand, classify, and treat AAV in a targeted manner.