Role of complement system in patients with biopsy-proven immunoglobulin G4-related kidney disease.

Hypocomplementemia has been frequently reported in immunoglobulin G4-related kidney disease (IgG4-RKD). However, studies on the role of complement system in IgG4-RKD are lacking. A total of 40 429 renal biopsies from January 2010 to January 2018 were reexamined in the present study, and 17 patients were confirmed to meet the criteria of IgG4-RKD. According to the serum C3 levels, they were divided into 2 groups: the low-C3 group (C3 <0.8 g/L, n = 7) and the normal-C3 group (C3 ≥0.8 g/L, n = 10). Compared with the normal-C3-level group, the patients in the low-C3-level group had lower serum C4 concentrations (P = .025), higher serum IgG4 concentrations (P = .003), higher positive rates in rheumatoid factor (P = .033), more severe storiform fibrosis (P = .007) at diagnosis, and higher blood urea nitrogen levels at the latest test (P = .04). The serum levels of C3 were in negative correlation with the serum levels of IgG4 (P = .003), the levels of rheumatoid factor (P = .002), renal deposition of C1q (P = .028), storiform fibrosis (P < .001), scores of interstitial fibrosis (P = .015), the amount of renal IgG4-positive (IgG4+) plasma cells (P = .020), the ratios of IgG4+ plasma cells/CD138+ cells (P = .018), and the blood urea nitrogen concentrations at the last test (P = .023). Our study shows that IgG4-RKD is a relatively rare entity. Complement system may participate in the development of IgG4-RKD.