Hypocomplementemia is related to elevated serum levels of IgG subclasses other than IgG4 in IgG4-related kidney disease

Objectives: This study investigated the clinical features of IgG4-RKD patients with hypocomplementemia compared with those without it, so as to clarify the factors related to hypocomplementemia. Methods: In this single-center retrospective study, we analyzed the clinical features of 25 patients with IgG4-RKD according to the presence/absence of hypocomplementemia. Additionally, we validated the results of a single-center study in a separate large multicenter cohort of 328 patients with IgG4-RD, and searched for factors related to hypocomplementemia. Results: Serum IgG levels (p < .001), non-IgG4 IgG levels, calculated as the total IgG minus IgG4 (p < .001), serum IgG1 levels (p = .017), and the number of involved organs (p = .018) were significantly higher in the hypocomplementemia group. At relapse of renal lesions in four patients, all had serum IgG4 re-elevation, with the three with hypocomplementemia presenting worsening of hypocomplementemia and re-elevation of non-IgG4 IgG levels. In a validation cohort of 328 patients with IgG4-RD, multivariate logistic regression analysis indicated elevation of non-IgG4 IgG levels to be an independent factor related to hypocomplementemia in the patients with IgG4-RKD. Conclusion: The present study suggests that hypocomplementemia is associated with elevation of IgG subclasses other than IgG4 including IgG1 in IgG4-RKD.