Complement activation in pauci-immune necrotizing and crescentic glomerulonephritis: results of a proteomic analysis

Background. Complement activation plays an important role in the pathophysiology of anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV), although it remains unclear which pathway is activated. Whether pauci-immune necrotizing crescentic glomerulonephritis (pauci-immune GN) with negative ANCA serology is part of the spectrum of AAV or a different disease entity is essentially unknown. Methods. We used proteomic analysis to delineate the complement profile in a series of 13 kidney biopsies of patients with pauci-immune GN, with either proteinase 3 (PR3) (five patients) or myeloperoxidase (MPO) antibodies (four patients) or with consistently negative ANCA serology (four patients). Immunofluorescence staining of glomeruli was essentially negative in the PR3-ANCA and MPO-ANCA groups, while a mild staining for C3 was seen in the ANCA-negative cases. No electron-dense deposits were found in the PR3-ANCA and MPO-ANCA groups, but mesangial and few subepithelial deposits were clearly present in the ANCA-negative specimens. Results. Mass spectrometry revealed low spectra numbers for C3 and immunoglobulins in both PR3-positive and MPO-positive patients with minimal or no C4 and C9. In contrast, larger spectra numbers for C3, moderate spectra numbers for C9, complement factor H-related protein-1 and low spectra numbers for C4, C5 and immunoglobulins were found in the ANCA-negative cases. Conclusion. While complement activation is noted in AAV, the complement activation appears to be more prominent in the ANCA-negative glomerulonephritis. The larger amount of C3 and moderate amount of C9 in the ANCA-negative glomerulonephritis implies activation of the alternate and terminal pathway of complement, suggesting that this entity may be caused or promoted by a genetic or acquired defect in the alternative pathway.