Characterization of Circulating Immune Complexes in Patients with Myeloperoxidase–Antineutrophil Cytoplasmic Antibody-Associated Glomerulonephritis

Circulating immune complexes (CICs) are frequently detected in the sera of patients with myeloperoxidase (MPO)–antineutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis (MPO-AAGN) and their role in activating the classical complement pathway has been suggested. However, the precise composition and functional characteristics of CICs in AAGN remain poorly understood. We analyzed serum samples from four patients with MPO-AAGN and confirmed CICs. An immunoadsorbent column was prepared using a monoclonal rheumatoid factor antibody that binds to CICs, enabling extraction via immunoprecipitation. CICs were analyzed by Western blotting to detect MPO and IgG. Their capacity to activate complement was assessed in vitro by measuring complement activation products using the enzyme-linked immunosorbent assay following incubation with normal human serum. Western blotting revealed distinct bands for both MPO and IgG in all samples. Incubation of extracted CICs with normal human serum resulted in elevated levels of complement activation products, including C5a and C5b-9. This increase was completely inhibited by the addition of EDTA or EGTA. In conclusion, CICs in the serum of patients with MPO-AAGN contain MPO and IgG (presumably MPO-ANCA). These CICs can activate the complement system, likely through the classical pathway. Our findings support the hypothesis that CICs composed of MPO and MPO-ANCA contribute to complement activation via the classical pathway and play a significant role in AAGN pathogenesis.