Mechanisms of vasculitis: how pauci-immune is ANCA-associated renal vasculitis?

Both the innate and the acquired immune system are involved in the pathophysiology of renal vasculitis. However, anti-neutrophil cytoplasmic antibody (ANCA)-associated renal vasculitis is characterized by a 'pauci-immune' pattern of immunofluorescence during kidney biopsy, indicating the relative lack of immunoglobulin and complement deposition within the kidney. On the other hand, evidence is accumulating that ANCA, autoantibodies against constituents of primary granules of neutrophils and the lysosomes of monocytes, play a pathogenic role in renal vasculitis. In this review we will discuss both in vitro and in vivo experimental data providing compelling evidence that ANCA are a primary pathogenic factor in renal vasculitis, mainly by augmenting leukocyte-endothelial interactions. We will also address novel data, pointing at the role of, in addition to ANCA, non-specific proinflammatory signals. Finally, we propose a working hypothesis of the pathogenesis of ANCA-associated renal vasculitis.