Identification of C3d Complement Fragments in Lesional Tissues in ANCA-Associated Vasculitis

Antineutrophil cytoplasmic antibody-associated vasculitis (AAV) is typically described as "pauci-immune" due to the absence or weak detection of Igs and complement activation within AAV tissue lesions. However, complement fragments have been recently reported in significant amounts in damaged tissue in AAV, notably in traps formed by neutrophil extravasation. This study aimed to characterize complement activation, in particular the long-lasting fragment C3d, in affected tissue in AAV.

Biopsies from patients with AAV (kidney, n = 40; lung, n = 5; sinus, n = 2; and skin, n = 3), IgA nephropathy (IgAN; kidney, n = 29), C3 glomerulonephritis (C3G; kidney, n = 9), lupus nephritis (LN; kidney, n = 30), and cutaneous leukocytoclastic vasculitis (skin, n = 41) were stained using a novel anti-C3d monoclonal antibody (3d8b), and, in some instances, a commercially available anti-C3c polyclonal antibody.

Both semiquantitative scoring and digital quantification revealed moderate glomerular staining in AAV and IgAN and higher signal in C3G and LN. A distinct C3c/C3d glomerular staining pattern was seen in AAV and IgAN with more deposition in mesangium than capillaries. C3G and LN exhibited more homogenous deposition. Three of seven biopsies that tested negative for C3 by routine diagnostic immunostaining were positive for C3d. C3d deposition was also detected in extra-glomerular small vessels in the kidney, in superficial dermal vasculature in leukocytoclastic vasculitis, and in lung and sinus lesional tissues in AAV.

Immunostaining for complement fragments, particularly C3d, reveals evidence of complement activation in multiple tissues in AAV, including biopsies deemed negative by routine C3 staining. These data indicate that glomerulonephritis in AAV is less pauci-immune than previously appreciated.