[Causes and mechanisms of vasculitides]

Vasculitic lesions are defined by necrosis of the vascular wall with perivascular inflammatory infiltrates. Secondary forms of vasculitis are observed in some systemic diseases, after infections, secondary to drugs or to malignancies. Usually, these secondary forms are characterised by the presence of immunoglobulin and complement deposits and thus probably caused by immune complex deposits. Conversely, necrotizing vasculitis occurring without known causal factor are named primary vasculitis, are not associated with immune deposits. Wegener's granulomatosis, microscopic polyangitis, Churg and Strauss syndrome and the isolated form of necrotizing and crescent glomerulonephritis are closely associated with the anti-neutrophil cytoplasm auto-antibodies (ANCA). The two main auto-antigen targets recognised by ANCA are lysosomal enzymes, proteinase 3 and myeloperoxidase, both contained in azurophilic granules of polymorphonuclear neutrophils and in primary lysosomes of monocytes. It has been demonstrated that ANCA antigens are expressed at the neutrophil (and monocyte) membrane after preactivation and are accessible to antibodies. Thus, ANCA may amplify neutrophils and monocyte activation. However, the causal or induced nature of anti-neutrophil auto-immunity in vasculitis remains unknown.