Pathogenesis of Wegener's granulomatosis.

Although a single disease entity, Wegener's granulomatosis (WG) displays a set of clinical manifestations, each with a different immunopathogenesis. Granuloma formation, "pauci-immune" vasculitis and glomerulonephritis (= renal vasculitis) are the histologic hallmarks of WG which can occur together (WG triad) in full-blown disease, or separately in "initial phase" disease or the formes frustes. The different clinical manifestations are characterised by multiple immune abnormalities that culminate in the over-production of autoantibodies directed mainly against proteinase 3 (PR3-ANCA). A number of in vitro observations point to the potential mechanisms by which ANCA could induce neutrophil-mediated vascular injury, i.e. vasculitis. The most commonly postulated scenario for ANCA-mediated vasculitis involves the interaction of polymorphonuclear neutrophils (PMN) and endothelial cells (EC) via cell adhesion molecule interactions. The initiating event is ANCA-induced leukocyte activation, in which PMN-derived mediators (i.e. cytokines, lipid metabolites, etc.) are intimately involved. The result is necrotizing inflammation of blood vessel walls. However, the clinical and pathological hallmark of WG is the coexistence of vasculitis and granuloma. The causative agent(s) leading to granuloma formation, predominantly in the respiratory tract, is still unknown, but the presence of T cells in the granulomatous inflammation indicates T-cell hyperactivity. Immunohistochemical studies have shown that the cellular infiltrations in renal and pulmonary lesions of WG primarily contain CD4+ T-cells and macrophages. Recent investigations have demonstrated that CD4+ T-cells from granulomatous lesions in the nose and from bronchoalveolar lavage (BAL) mainly express the Th1 cytokine profile, which stimulates predominantly cell-mediated immune responses. This result supports the hypothesis that due to the two-phase course of WG there occurs a polarisation of the T-cell sub-population (Th1 versus Th2 type) which may explain the transition from the initial (granulomatous) phase of WG (so-called localized or locoregional restricted WG) to the generalized (vasculitic) phase. In conclusion, although the initiating events in the development of WG are still unknown, remarkable advances have been made in characterizing the infiltrating inflammatory cells and their products, which is of major importance in understanding the pathogenesis of this disease. In this regard, the use of specific mediators (i.e. cytokines, adhesions molecule antagonists, anti-Id ANCA, etc.) to modulate the inflammatory response may prove beneficial in the therapy of WG.