Pathogenesis of ANCA-associated vasculitides

Antineutrophil cytoplasmic autoantibody (ANCA)associated vasculitides are characterised by necrotising inflammation of small vessels in conjunction with ANCA directed to either proteinase 3 (PR3) or myeloperoxidase (MPO). The aetiopathogenesis of these disorders is still not fully elucidated but clinical as well as in vitro and in vivo experimental data strongly suggest a role for the autoimmune responses to PR3 and MPO in disease development. Clinically, PR3-ANCA are strongly associated with granulomatous vasculitis as in Wegener's granulomatosis, and MPO-ANCA with necrotising small vessel vasculitis as in microscopic polyangiitis. Levels of PR3-ANCA and MPO-ANCA do, however, not fully reflect disease activity. In vitro, ANCA activate primed neutrophils to release lytic enzymes and reactive oxygen species, a process reinforced by the alternative pathway of complement. In the context of endothelial cells, this process leads to endothelial detachment and lysis. In vivo experimental studies have clearly demonstrated the pathogenic potential of MPO-ANCA for necrotising glomerulonephritis and pulmonary capillaritis. For PR3-ANCA-associated granulomatous vasculitis, an animal model is lacking. Here, effector T cells, in particular Th17 cells, appear to have a major pathogenic role in addition to ANCA. Finally, microbial factors, derived in particular from S aureus and Gram-negative bacteria, could play a part in disease induction and expression. These new insights into the pathogenesis of ANCA-associated vasculitides have opened new ways for targeted treatment.