Complement in ANCA-associated glomerulonephritis

Anti-neutrophil cytoplasmic antibodies (ANCA) are found in pauci-immune necrotizing crescentic glomerulonephritis. In the past, the role of complement in ANCA-associated vasculitis (AAV) was assumed to be minimal. More recently, however, it was found that blocking the complement cascade in a mouse model of AAV reduces glomerular damage. Immune complex deposits have been found in biopsies from AAV patients. In this study, we questioned whether immune complex formation or deposition may result in complement activation in ANCA-associated glomerulonephritis.ANCA-positive patients from the Limburg Renal Registry were included between 1979 and 2011. Renal histology was documented together with immunoglobulin and complement immunofluorescence. In addition, C3d, properdin, C4d and mannose-binding lectin (MBL) were stained. Electron microscopy was performed. Circulating immune complexes were determined in a subset of patients, as well as C3 allotypes.C3c was found in 78 of 187 renal biopsies (41.7%) divided over 32.3% of proteinase-3 (PR3)-AAV patients and 52.3% of myeloperoxidase (MPO)-AAV patients (P = 0.006), whereas C3d was found positive in 51.1% of PR3-AAV patients and 70.4% of MPO-AAV patients (P = 0.105). C4d was found positive in 70.8%, properdin in 38.7% and MBL in 30.4% of patients. Whereas C4d and MBL positivity was similar between the AGN groups, properdin was more common in biopsies classified as crescentic compared with biopsies classified as focal or mixed. Renal biopsies positive for C3d and/or properdin showed more cellular crescents and less normal glomeruli compared with biopsies negative for C3d and/or properdin (P < 0.05). In 3 out of 43 renal biopsies analysed by electron microscopy, small electron dense deposits were found. In 14 of 46 patients analysed, circulating immune complexes were detectable. No association between histological findings and C3 allotypes was found.In the majority of AAV patients, no immune complex deposits were found in their renal biopsies. C3d, C4d and C5b-9 staining, however, was found to be positive in a majority of analysed renal biopsies. Importantly, C3d and properdin staining was associated with cellular crescents. We hypothesize that local immune complexes are quickly degraded in AAV and therefore not visible by electron microscopy. Our findings are compatible with the hypothesis that complement activation in AAV occurs predominantly via alternative pathway activation.