Antineutrophil cytoplasmic antibodies reacting with the pro form of proteinase 3 and disease activity in patients with Wegener's granulomatosis and microscopic polyangiitis

Objective Antineutrophil cytoplasmic antibodies (ANCA) directed against proteinase 3 (PR3) are diagnostic markers for the small vessel vasculitides Wegener's granulomatosis (WG) and microscopic polyangiitis (MPA). Correlation of disease activity with PR3 ANCA levels, as determined by standard methods, is not apparent in every patient. PR3 ANCA react with yet to be identified conformational epitopes. We have identified PR3 ANCA subsets that react differentially with mature recombinant PR3 (rPR3; lacking the N-terminal activation dipeptide) and the pro form of this enzyme (pro-rPR3). The present study was performed to determine the association of these PR3 ANCA subsets with disease activity. Methods Sera from 61 PR3 ANCA–positive patients with WG or MPA were assayed by capture enzyme-linked immunosorbent assay using pro-rPR3 and rPR3 as target antigens, and were correlated with disease activity as determined by the Birmingham Vasculitis Activity Score (BVAS). Results Median levels of PR3 ANCA reacting with pro-rPR3 were higher during active (n = 32) than during inactive (n = 29) disease (P = 0.016). Reactivity with mature rPR3 was not significantly different (P = 0.71). Serial followup in individual patients also indicated better correlation of PR3 ANCA reactivity with pro-rPR3 than with mature rPR3. Conclusion PR3 ANCA subsets reactive with epitopes accessible on pro-PR3 correlate better with disease activity than do subsets reactive with epitopes accessible only on mature PR3. This observation may explain why ANCA levels determined with current standard methods are suboptimal for monitoring disease activity. It raises new questions about the primary target of the PR3 ANCA immune response in patients with small vessel vasculitis.