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Anti-neutrophil cytoplasmic antibodies (ANCA) with proteinase 3 (PR3) specificity are a useful laboratory biomarker for the diagnosis of Granulomatosis with Polyangiitis (GPA) and are believed to be implicated in the pathogenesis. It has been repeatedly suggested that disease activity of GPA is more closely related to the appearance and rise of PR3-inhibiting ANCA than to an increase of total ANCA. Previous studies on a limited number of patient samples, however, have yielded inconclusive results. To overcome the previous methodological limitations, we established a new ultrasensitive method to quantify the inhibitory capacity of PR3-ANCA using small volumes of plasma from patients with GPA. A large collection of longitudinally-collected samples from the Wegener Granulomatosis Etanercept Trial (WGET) became available to us to determine the functional effects of ANCA on PR3 in comparison to clinical disease manifestations. In these patient samples we not only detected PR3-ANCA with inhibitory capacity, but also PR3-ANCA with enhancing effects on PR3 activity. However no correlation of these activity-modulating PR3-ANCA with disease activity at either the time of enrollment or over the course of disease was found. Only patients with pulmonary involvement, especially patients with nodule formation in the respiratory tract, showed a slight, but not significant, decrease of inhibitory capacity. Epitope mapping of the activity-modulating PR3-ANCA revealed a binding on the active site surface of PR3. Yet these ANCA were able to bind to PR3 with an occupied active site cleft, indicating an allosteric mechanism of inhibition. The recently described signal ratio between the MCPR3-3 and MCPR3-2 capture ELISA was consistent with the binding of activity-modulating ANCA to the active site surface. Evidence for a shared epitope between activity-modulating PR3-ANCA and MCPR3-7, however, was very limited, suggesting that a majority of PR3-ANCA species do not inhibit PR3 by the same mechanism as previously reported for MCPR3-7. [ABSTRACT FROM AUTHOR]

Abstract: Proteinase 3 (PR3)-specific antineutrophil cytoplasmic antibodies (ANCA) are highly specific for the autoimmune small vessel vasculitis, Wegener’s granulomatosis (WG). PR3-ANCA have proven diagnostic value but their pathogenic potential and utility as a biomarker for disease activity remain unclear. PR3-ANCA recognize conformational epitopes, and epitope-specific PR3-ANCA subsets with variable impact on biological functions of PR3 have been postulated. The aims of this study were to identify specific PR3 surface epitopes recognized by monoclonal antibodies (moAbs) and to determine whether the findings can be used to measure the functional impact of epitope-specific PR3-ANCA and their potential relationship to disease activity. We used a novel flow cytometry assay based on TALON-beads coated with recombinant human (H) and murine (M) PR3 and 10 custom-designed chimeric human/mouse rPR3-variants (Hm1-5/Mh1-5) identifying 5 separate non-conserved PR3 surface epitopes. Anti-PR3 moAbs recognize 4 major surface epitopes, and we identified the specific surface location of 3 of these with the chimeric rPR3-variants. The ability of PR3-ANCA to inhibit the enzymatic activity of PR3 was measured indirectly using a capture-ELISA system based on the different epitopes recognized by capturing moAbs. Epitope-specific PR3-ANCA capture-ELISA results obtained from patient plasma (n =27) correlated with the inhibition of enzymatic activity of PR3 by paired IgG preparations (r =0.7, P <0.01). The capture-ELISA results also seem to reflect disease activity. In conclusion, insights about epitopes recognized by anti-PR3 moAbs can be applied to separate PR3-ANCA subsets with predictable functional qualities. The ability of PR3-ANCA to inhibit the enzymatic activity of PR3, a property linked to disease activity, can now be gauged using a simple epitope-based capture-ELISA system. [Copyright &y& Elsevier]

Abstract: Direct comparisons of human (h) and murine (m) neutrophil elastase (NE) and proteinase 3 (PR3) are important for the understanding and interpretation of inflammatory and PR3-related autoimmune processes investigated in wild-type-, mNE- and mPR3/mNE knockout mice. To this end, we purified recombinant mPR3 and mNE expressed in HMC1 and 293 cells and compared their biophysical properties, proteolytic activities and susceptibility to inhibitors with those of their human homologues, hPR3 and hNE. Significant species differences in physico-chemical properties, substrate specificities and enzyme kinetics towards synthetic peptide substrates, oxidized insulin B chain, and fibrinogen were detected. MeOSuc-AAPV-pNA and Suc-AAPV-pNA were hydrolyzed more efficiently by mPR3 than hPR3, but enzymatic activities of mNE and hNE were very similar. Fibrinogen was cleaved much more efficiently by mPR3 than by hPR3. All four proteases were inhibited by α1-antitrypsin and elafin. Eglin C inihibited mNE, hNE, mPR3, but not hPR3. SLPI inhibited both NEs, but neither PR3. The custom-designed hNE inhibitor, Val15-aprotinin, is a poor inhibitor for mNE. In conclusion, appropriate interpretation of experiments in murine models requires individual species-specific assessment of neutrophil protease function and inhibition. [Copyright &y& Elsevier]

Effects of carboxy-terminal modifications of proteinase 3 (PR3) on the recognition by PR3-ANCA. Background. Autoantibodies directed against neutrophil proteinase 3 (PR3-ANCA) from patients with Wegener's granulomatosis and microscopic polyangiitis recognize conformational epitopes of PR3. During maturation of neutrophils, PR3 undergoes amino-terminal and carboxy-terminal processing. In contrast to amino-terminal processing, the effects of carboxy-terminal processing on recognition of PR3 by PR3-ANCA remain unknown. Carboxy-terminally modified or tagged recombinant PR3 (rPR3) molecules may be useful for the refinement of diagnostic assays and for the study of biological processes. Methods. This study was designed to determine whether 293 cells can be used to express specifically designed carboxy-terminal variants of rPR3, and to evaluate the effects of different carboxy-terminal modifications on the recognition by PR3-ANCA in the capture ELISA. Results. The rPR3-variants secreted into the media supernatants of transfected 293 cells escaped proteolytic processing. Furthermore, in contrast to the effects of amino-terminal pro-peptide deletion on PR3-ANCA binding, carboxy-terminal modifications (deletion and additions) did not significantly affect recognition by PR3-ANCA. Conclusions. This expression system is ideally suited for the expression of custom-designed carboxy-terminal rPR3 variants, and major conformational effects of carboxy-terminal modifications seem unlikely. [ABSTRACT FROM AUTHOR]

Abstract: Testing for antineutrophil cytoplasmic antibodies (ANCA) reacting with proteinase 3 (PR3) is part of the routine diagnostic evaluation of patients with small vessel vasculitis. For PR3-ANCA detection, capture ELISAs are reported to be superior to direct ELISAs. Standard capture ELISAs, in which PR3 is anchored by anti-PR3 monoclonal antibodies (moAB), have two potential disadvantages. First, the capturing moAB may compete for epitopes recognized by some PR3-ANCA, causing occasional false-negative results. Second, the capture of recombinant PR3 mutant molecules becomes unpredictable as modifications of specific conformational epitopes may not only affect the binding of PR3-ANCA, but also the affinity of the capturing anti-PR3 moAB. Here, we describe a new capture ELISA, and its application for PR3-ANCA detection. This new assay is based on the standardized capture of a variety of different carboxy-terminally c-myc tagged recombinant ANCA target antigens using anti-c-myc coated ELISA plates. Antigen used include c-myc tagged human rPR3 variants (mature and pro-form conformations), mouse mature rPR3 and human recombinant neutrophil elastase. This new anti-c-myc-capture ELISA for PR3-ANCA detection has an intra- and inter-assay coefficient of variation of 3.6% to 7.7%, and 15.8% to 18.4%, respectively. The analytical sensitivity and specificity for PR3-ANCA positive serum samples were 93% and 100%, respectively when rPR3 with mature conformation was used as target antigen, and 83% and 100% when the pro-enzyme conformation was employed. In conclusion, this new anti-c-myc capture ELISA compares favorably to our standard capture ELISA for PR3-ANCA detection, enables the unified capture of different ANCA target antigens through binding to a c-myc tag, and allows capture of rPR3 mutants necessary for PR3-ANCA epitope mapping studies. [Copyright &y& Elsevier]