ANCA-associated pauci-immune glomerulonephritis: always pauci-immune?

Anti-neutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis (GN) is considered "pauci-immune" with absent or mild glomerular tuft staining for immunoglobulin (Ig) and/or complement. However, it is not unusual to see some immune deposits (ID) within glomeruli on immunofluorescence (IF). We determined to evaluate the prevalence and clinical significance of immune deposits in ANCA-associated GN.

We included all patients with ANCA associated vasculitis with renal biopsies between January 2002 and May 2014: granulomatosis with polyangiitis, eosinophilic granulomatosis with polyangiitis, microscopic polyangiitis and renal limited vasculitis. Patients were divided into Group A: biopsy without ID (≤2+ intensity of immunostaining) and Group B: biopsy with ID (>2+ intensity of immunostaining). Serum creatinine, estimated glomerular filtration rate (eGFR) at time of the biopsy, amount of proteinuria and hematuria, requirement of dialysis and extra renal involvement were recorded.

Fifty-three patients (75.4% females) were included. Mean age at biopsy was 66.3 years. Typical pauci-immune GN was found in 39 patients (73.5%, group A). In 14 patients (26.4%, group B) examination revealed substantial deposition of Ig or complement in the mesangium and/or along the glomerular capillary wall. The only difference comparing both groups was significantly higher proteinuria in group B (mean 1.6/24 h (SD: 10.7) vs. 0.8/24 h (SD: 7.6), p=0.0036).

In ANCA GN at least a quarter of patients were not "pauci-immune" (26.4%). In this subgroup, immune deposits were only associated with a significantly higher proteinuria. Further basic and clinical research is needed to elucidate the significance of immune deposition in ANCA GN.