Epitope specificity determines pathogenicity and detectability in ANCA-associated vasculitis

TLDR

ANCA‑associated vasculitis is an immune‑mediated small‑vessel necrotizing disease often diagnosed by detecting myeloperoxidase‑specific antibodies. The study aimed to determine whether ANCA epitope specificity explains assay discordance, the presence of anti‑MPO in healthy individuals, and undetected ANCA in ANCA‑negative patients. Researchers mapped ANCA epitopes using epitope excision/mass spectrometry and removed a ceruloplasmin fragment from purified IgG to enhance serologic detection. MPO autoantibodies from healthy people target distinct epitopes, whereas pathogenic antibodies in disease recognize a single linear sequence that activates neutrophils and induces nephritis, showing that immunodominant epitopes drive ANCA‑vasculitis and that autoantibody diversity may be common to other autoimmune diseases.

Abstract

Anti-neutrophil cytoplasmic antibody–associated (ANCA-associated) small vessel necrotizing vasculitis is caused by immune-mediated inflammation of the vessel wall and is diagnosed in some cases by the presence of myeloperoxidase-specific antibodies (MPO-ANCA). This multicenter study sought to determine whether differences in ANCA epitope specificity explain why, in some cases, conventional serologic assays do not correlate with disease activity, why naturally occurring anti-MPO autoantibodies can exist in disease-free individuals, and why ANCA are undetected in patients with ANCA-negative disease. Autoantibodies from human and murine samples were epitope mapped using a highly sensitive epitope excision/mass spectrometry approach. Data indicated that MPO autoantibodies from healthy individuals had epitope specificities different from those present in ANCA disease. Importantly, this methodology led to the discovery of MPO-ANCA in ANCA-negative disease that reacted against a sole linear sequence. Autoantibodies against this epitope had pathogenic properties, as demonstrated by their capacity to activate neutrophils in vitro and to induce nephritis in mice. The confounder for serological detection of these autoantibodies was the presence of a fragment of ceruloplasmin in serum, which was eliminated in purified IgG, allowing detection. These findings implicate immunodominant epitopes in the pathology of ANCA-associated vasculitis and suggest that autoantibody diversity may be common to other autoimmune diseases.