An overlap of antineutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis and IgG4-related kidney disease

We investigated the characteristics of antineutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis (ANCA-GN) and immunoglobulin G4 (IgG4)-related kidney disease (IgG4-RKD) overlap syndrome. This is a 2-center study with 19 patients. Fifteen patients were classified as microscopic polyangiitis (MPA). The initial serum creatinine levels were 320.9 ± 191.4 μmol/l and the BVAS was 19.7 ± 7.4 at diagnosis. Hematuria was absent or slight in 13 (68.4%) cases. Renal histology of these patients revealed concurrent ANCA-GN and IgG4-RKD. Regarding the histological classification of ANCA-GN, 9 (47.4%), 8 (42.1%), 1 (5.3%) and 1 (5.3%) patients were classified as focal, crescentic, mixed and sclerotic ANCA-GN, respectively. MPO-ANCA could be detected in 17/19 (89.5%) patients. IgG subclasses of MPO-ANCA were tested in 10 patients, and all were positive for IgG4-MPO-ANCA. In patients with combined ANCA-GN and IgG4-RKD, the percentage of positive IgG1-MPO-ANCA was significantly lower than the control group of 20 AAV patients without IgG4-RKD (P = 0.002), and the percentage of positive IgG4-MPO-ANCA was higher than the control group, but this difference was not statistically significant (P = 0.14). ANCA-GN and IgG4-RKD overlap syndrome concerned mainly MPO-ANCA positive patients. The IgG subclass analysis of MPO-ANCA showed lower percentage of IgG1 subclass. The association between ANCA-GN and IgG4-RKD is possible and represents a special entity.