Epitope mapping of anti-PR3 antibodies using chimeric human/mouse PR3 recombinant proteins

High-resolution epitope mapping of commercial antibodies to ANCA antigens by yeast surface display

Epitope mapping provides critical insight into antibody-antigen interactions. Epitope mapping of autoantibodies from patients with autoimmune diseases can help elucidate disease immunogenesis and guide the development of antigen-specific therapies. Similarly, epitope mapping of commercial antibodies targeting known autoantigens enables the use of those antibodies to test specific hypotheses. Anti-Neutrophil Cytoplasmic Autoantibody (ANCA) vasculitis results from the formation of autoantibodies to multiple autoantigens, including myeloperoxidase (MPO), proteinase-3 (PR3), plasminogen (PLG), and peroxidasin (PXDN). To perform high-resolution epitope mapping of commercial antibodies to these autoantigens, we developed a novel yeast surface display library based on a series of >5000 overlapping peptides derived from their protein sequences. Using both FACS and magnetic bead isolation of reactive yeast, we screened 19 commercially available antibodies to the ANCA autoantigens. This approach to epitope mapping resulted in highly specific, fine epitope mapping, down to single amino acid resolution in many cases. Our study also identified cross-reactivity between some commercial antibodies to MPO and PXDN, which suggests that patients with apparent autoantibodies to both proteins may be the result of cross-reactivity. Together, our data validate yeast surface display using maximally overlapping peptides as an excellent approach to linear epitope mapping.

Defensin Interactions in Relation to Monoclonal and Disease-Related Proteinase 3 Antibodies Binding at the Catalytic Site

Proteinase 3 (PR3) is a neutrophil granulocyte enzyme and an autoantigen found in several forms of vasculitis. Due to the diagnostic and clinical importance of antibodies (Abs) to PR3, it is important to characterize the protein and the nature of its epitopes. Here, we have characterized PR3 monoclonal antibodies (MAbs) and disease-associated Abs and their dependency on the PR3 structure and modifications, especially interactions with α-defensins. Three MAbs (HYB 172-01, 172-04, 172-05), which bind to PR3 in its native and denatured forms and provide the disulphide bridges, were intact. α-1-antitrypsin (AT) binds to purified human neutrophil granulocyte PR3 and inhibits its proteolytic activity, towards a small synthetic peptide substrate and a large protein substrate (casein). AT also inhibited the binding of the three MAbs to PR3, indicating that they bind in a region affected by AT binding. However, the MAbs did not inhibit PR3 proteolytic activity with a small substrate, showing that they bound at the active site without restricting access to the substrate cleft. Patient-derived Abs showed essentially the same characteristics as the MAbs, with important implications for vasculitis diagnostics and pathophysiology. Current findings illustrate that PR3 epitopes depend on the three-dimensional structure of the PR3/defensin complex, and that the epitopes depend to a smaller or larger degree on PR3/defensin associations.

Inhibitors and Antibody Fragments as Potential Anti-Inflammatory Therapeutics Targeting Neutrophil Proteinase 3 in Human Disease

Proteinase 3 (PR3) has received great scientific attention after its identification as the essential antigenic target of antineutrophil cytoplasm antibodies in Wegener's granulomatosis (now called granulomatosis with polyangiitis). Despite many structural and functional similarities between neutrophil elastase (NE) and PR3 during biosynthesis, storage, and extracellular release, unique properties and pathobiological functions have emerged from detailed studies in recent years. The development of highly sensitive substrates and inhibitors of human PR3 and the creation of PR3-selective single knockout mice led to the identification of nonredundant roles of PR3 in cell death induction via procaspase-3 activation in cell cultures and in mouse models. According to a study in knockout mice, PR3 shortens the lifespan of infiltrating neutrophils in tissues and accelerates the clearance of aged neutrophils in mice. Membrane exposure of active human PR3 on apoptotic neutrophils reprograms the response of macrophages to phagocytosed neutrophils, triggers secretion of proinflammatory cytokines, and undermines immune silencing and tissue regeneration. PR3-induced disruption of the anti-inflammatory effect of efferocytosis may be relevant for not only granulomatosis with polyangiitis but also for other autoimmune diseases with high neutrophil turnover. Inhibition of membrane-bound PR3 by endogenous inhibitors such as the α-1-protease inhibitor is comparatively weaker than that of NE, suggesting that the adverse effects of unopposed PR3 activity resurface earlier than those of NE in individuals with α-1-protease inhibitor deficiency. Effective coverage of PR3 by anti-inflammatory tools and simultaneous inhibition of both PR3 and NE should be most promising in the future.

Epitope Mapping of Neutralizing Monoclonal Antibodies to Human Interferon-γ Using Human-Bovine Interferon-γ Chimeras

Our aim was to identify conformational epitopes, recognized by monoclonal antibodies (mAbs) made against human (h) interferon (IFN)-γ. Based on the mAbs' (n = 12) ability to simultaneously bind hIFN-γ in ELISA, 2 epitope clusters with 5 mAbs in each were defined; 2 mAbs recognized unique epitopes. Utilizing the mAbs' lack of reactivity with bovine (b) IFN-γ, epitopes were identified using 7 h/bIFN-γ chimeras where the helical regions (A-F) or the C terminus were substituted with bIFN-γ residues. Chimeras had a N-terminal peptide tag enabling the analysis of mAb recognition of chimeras in ELISA. The 2 mAb clusters mapped to region A and E, respectively; the epitopes of several mAbs also involved additional regions. MAbs in cluster A neutralized, to various degrees, IFN-γ-mediated activation of human cells, in line with the involvement of region A in the IFN-γ receptor interaction. MAbs mapping to region E displayed a stronger neutralizing capacity although this region has not been directly implicated in the receptor interaction. The results corroborate earlier studies and provide a detailed picture of the link between the epitope specificity and neutralizing capacity of mAbs. They further demonstrate the general use of peptide-tagged chimeric proteins as a powerful and straightforward method for efficient mapping of conformational epitopes.

Neutrophils from vasculitis patients exhibit an increased propensity for activation by anti-neutrophil cytoplasmic antibodies

Anti-neutrophil cytoplasmic antibodies (ANCA) are thought to be pathogenic in ANCA-associated vasculitis (AAV) by stimulating polymorphonuclear leucocytes (PMNs) to degranulate and produce reactive oxygen species (ROS). The aim of this study was to investigate if PMNs from AAV patients are stimulated more readily by ANCA compared with PMNs from healthy controls (HCs). Differences in ANCA characteristics that can account for different stimulation potential were also studied. PMNs from five AAV patients and five HCs were stimulated with 10 different immunoglobulins (Ig)Gs, purified from PR3-ANCA-positive patients, and ROS production, degranulation and neutrophil extracellular trap (NET) formation was measured. ANCA levels, affinity and clinical data of the AAV donors were recorded. The results show that PMNs from AAV patients produce more intracellular ROS (P = 0·019), but degranulate to a similar extent as PMNs from HCs. ROS production correlated with NET formation. Factors that may influence the ability of ANCA to activate PMNs include affinity and specificity for N-terminal epitopes. In conclusion, our results indicate that PMNs from AAV patients in remission behave quite similarly to HC PMNs, with the exception of a greater intracellular ROS production. This could contribute to more extensive NET formation and thus an increased exposure of the ANCA autoantigens to the immune system.

Discrimination and variable impact of ANCA binding to different surface epitopes on proteinase 3, the Wegener's autoantigen

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.

Longitudinal studies of patients with ANCA vasculitis demonstrate concurrent reactivity to complementary PR3 protein segments cPR3m and cPR3C and with no reactivity to cPR3N

Antibodies recognizing the complement of the middle of PR3 (cPR3m) occur in ∼30% of PR3-anti-neutrophil cytoplasmic autoantibodies (ANCA)-vasculitis patients and immunization of animals with a peptide complementary to the middle of PR3 (cPR3m) induces not only anti-complementary PR3 antibodies, but also anti-PR3 antibodies derived through an anti-idiotypic response. PR3 epitopes recognized by patient ANCA, however, are not restricted to the middle of PR3. This prompted us to test for antibodies that react with proteins complementary to the terminal regions of PR3 (cPR3C and cPR3N) in PR3-ANCA patients. Anti-cPR3C reactivity was detected in 28% of patients but anti-cPR3N reactivity in only 15%. Ranked anti-cPR3C and anti-cPR3m reactivity correlated in the cohort, whereas there was no significant relationship between cPR3C and cPR3N reactivity. Serial samples from 3 patients' revealed that anti-cPR3C and anti-cPR3m reactivity followed a similar pattern over time. Serial samples from a fourth patient demonstrated an anti-cPR3N response without concurrent cPR3m or cPR3C reactivity. Epitope determination by mass spectrometry identified a 13-amino acid sequence on cPR3C that contained a common binding site recognized by antibodies from three patients. This peptide sequence contains a "PHQ" motif which was reported to be the basis for cross-reactivity of anti-cPR3m antibodies with plasminogen. Why these antibodies are detected in only ∼30% of the patients remains unclear. The data reveal that it is not due to lack of inclusion of flanking regions of complementary PR3 during screening. Instead, quite unexpectedly, the data demonstrate that patients' antibodies react with a restricted epitope that exists in both cPR3m and cPR3C.

Mapping of conformational epitopes on human proteinase 3, the autoantigen of Wegener's granulomatosis

Anti-neutrophil cytoplasmic Abs (cANCAs) against conformational epitopes of proteinase 3 (PR3) are regarded as an important pathogenic marker in Wegener's granulomatosis (WG). Although the three-dimensional structure of PR3 is known, binding sites of mAbs and cANCAs have not been mapped to date. Competitive binding and biosensor experiments suggested the existence of four nonoverlapping areas on the PR3 surface. In this paper, we present an approach to identify these discontinuous surface regions that cannot be mimicked by linear peptides. The very few surface substitutions found in closely related PR3 homologs from primates, which were further varied by the construction of functional human-gibbon hybrids, resulted in the differential loss of three Ab binding sites, two of which were mapped to the N-terminal beta-barrel and one to the linker segment connecting the N- and C-terminal barrels of PR3. The sera from WG patients differed in their binding to gibbon PR3 and the gibbon-human PR3 hybrid, and could be divided into two groups with similar or significantly reduced binding to gibbon PR3. Binding of almost all sera to PR3-alpha1-protease inhibitor (alpha1-PI) complexes was even more reduced and often absent, indicating that major antigenic determinants overlap with the active site surface on PR3 that associates with alpha1-PI. Similarly, the mouse mAbs CLB12.8 and 6A6 also did not react with gibbon PR3 and PR3-alpha1-PI complexes. Our data strongly suggest that cANCAs from WG patients at least in part recognize similar surface structures as do mouse mAbs and compete with the binding of alpha1-PI to PR3.

Structures of human proteinase 3 and neutrophil elastase--so similar yet so different

Proteinase 3 and neutrophil elastase are serine proteinases of the polymorphonuclear neutrophils, which are considered to have both similar localization and ligand specificity because of their high sequence similarity. However, recent studies indicate that they might have different and yet complementary physiologic roles. Specifically, proteinase 3 has intracellular specific protein substrates resulting in its involvement in the regulation of intracellular functions such as proliferation or apoptosis. It behaves as a peripheral membrane protein and its membrane expression is a risk factor in chronic inflammatory diseases. Moreover, in contrast to human neutrophil elastase, proteinase 3 is the preferred target antigen in Wegener's granulomatosis, a particular type of vasculitis. We review the structural basis for the different ligand specificities and membrane binding mechanisms of both enzymes, as well as the putative anti-neutrophil cytoplasm autoantibody epitopes on human neutrophil elastase 3. We also address the differences existing between murine and human enzymes, and their consequences with respect to the development of animal models for the study of human proteinase 3-related pathologies. By integrating the functional and the structural data, we assemble many pieces of a complicated puzzle to provide a new perspective on the structure-function relationship of human proteinase 3 and its interaction with membrane, partner proteins or cleavable substrates. Hence, precise and meticulous structural studies are essential tools for the rational design of specific proteinase 3 substrates or competitive ligands that modulate its activities.

Epitope shift of proteinase-3 anti-neutrophil cytoplasmic antibodies in patients with small vessel vasculitis

Anti-neutrophil cytoplasmic antibodies against proteinase 3 (PR3-ANCA) are used as diagnostic tools for patients with small vessel vasculitis (AASV). We have produced chimeric mouse/human PR3 molecules and investigate changes in reactivity over time and the possible relationship between epitope specificity and clinical course. Thirty-eight PR3-ANCA-positive patients diagnosed between 1990 and 2003 were followed until December 2005. Plasma was collected at each out-patient visit and older samples were retrieved retrospectively. Patients reacted with multiple epitopes at the time of diagnosis. At subsequent relapses 12 patients shifted reactivity, in 11 cases from epitopes located in the C-terminal towards epitopes in the N-terminal. Patients with reactivity against N-terminal parts of PR3 at diagnosis had a significantly lower relapse rate, 30% compared to 78% in the group with predominantly C-terminal reactivity (P = 0.04). The reactivity pattern did not correlate to outcome measured as death, end-stage renal disease or vasculitis activity index score (VDI) at 5 years. Further research is necessary to conclude if this is a general phenomenon.

Epitope-specific anti-neutrophil cytoplasmic antibodies: do they matter? Can they be detected?

Proteinase 3 (PR3)-anti-neutrophil cytoplasmic antibodies (ANCA) and myeloperoxidase (MPO)-ANCA are suggested to play a pathogenic role as they are closely related to the small-vessel vasculitis syndromes, Wegener's granulomatosis and microscopic polyangiitis. A large body of in vitro and animal experiments supports this concept. The mechanisms of action involve a direct interaction between ANCA and its antigen. The epitope specificity of ANCA may therefore influence the functional effects of ANCA and/or may reflect the mechanisms behind different disease manifestations or disease courses.

Rats and mice immunised with chimeric human/mouse proteinase 3 produce autoantibodies to mouse Pr3 and rat granulocytes

AIM

In this study, we employed chimeric human/mouse Proteinase 3 (PR3) proteins as tools to induce an autoantibody response to PR3 in rats and mice.

METHOD

Rats and mice were immunised with recombinant human PR3 (HPR3), recombinant murine PR3 (mPR3), single chimeric human/mouse PR3 (HHm, HmH, mHH, mmH, mHm, Hmm) or pools of chimeric proteins. Antibodies to mPR3 and HPR3 were measured by ELISA. Antibodies to rat PR3 were determined by indirect immunofluorescence (IIF) on rat white blood cells. Urinalysis was performed by dipstick analysis. Kidney and lung tissue was obtained for pathological examination.

RESULTS

In mice, immunisation with the chimeric human/mouse PR3 Hmm led to an autoantibody response to mPR3. Rats immunised with the chimeric human/mouse PR3 Hmm, HmH and mmH, or a pool of the chimeric human/mouse PR3 proteins, produced antibodies selectively binding to rat granulocytes as detected by IIF. No gross pathological abnormalities could be detected in kidney or lungs of mice or rats immunised with chimeric human/mouse PR3.

CONCLUSION

Immunisation with chimeric human/mouse proteins induces autoantibodies to PR3 in rats and mice. Chimeric proteins can be instrumental in developing experimental models for autoimmune diseases.

Mapping of myeloperoxidase epitopes recognized by MPO-ANCA using human-mouse MPO chimers

Myeloperoxidase (MPO) is one of the major target antigens of antineutrophil cytoplasmic autoantibodies (ANCA) found in patients with small-vessel vasculitis and pauci-immune necrotizing glomerulonephritis. To date, the target epitopes of MPO-ANCA remain poorly defined. Human MPO-ANCA do not typically bind mouse MPO. We utilized the differences between human and mouse MPO to identify the target regions of MPO-ANCA. We generated five chimeric MPO molecules in which we replaced different segments of the human or mouse molecules with their homologous counterpart from the other species. Of serum samples from 28 patients screened for this study, 43 samples from 14 patients with MPO-ANCA-associated vasculitis were tested against recombinant human and mouse MPO and the panel of chimeric molecules. Sera from 64 and 71% of patients bound to the carboxy-terminus of the heavy chain, in the regions of amino acids 517-667 or 668-745, respectively. No patient serum bound the MPO light chain or the amino-terminus of the heavy chain. All sera bound to only one or two regions of MPO. Although the pattern of MPO-ANCA binding changed over time (4-27 months) in 6 of 10 patients with several serum samples, such changes were infrequent. Other target regions of MPO-ANCA may not have been detected due to conformational differences between the native and recombinant forms of MPO. MPO-ANCA do not target a single epitope, but rather a small number of regions of MPO, primarily in the carboxy-terminus of the heavy chain.

A novel capture-ELISA for detection of anti-neutrophil cytoplasmic antibodies (ANCA) based on c-myc peptide recognition in carboxy-terminally tagged recombinant neutrophil serine proteases

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.

Anti-endothelial cell antibodies (AECA) in patients with propylthiouracil (PTU)-induced ANCA positive vasculitis are associated with disease activity

Increasing evidence has demonstrated that propylthiouracil (PTU) could induce ANCA positive vasculitis. However, our previous work has suggested that only one-fifth of the PTU-induced ANCA positive patients had clinical vasculitis and so the mechanism is not clear. Anti-endothelial cell antibodies (AECA) have been implicated in the pathogenesis of various vasculitides, including primary ANCA positive systemic vasculitis. The purpose of this study is to investigate the prevalence of AECA and their possible role in the pathogenesis of patients with PTU-induced ANCA positive vasculitis. Sera from 11 patients with PTU-induced ANCA positive vasculitis at both active and quiescent phases, and sera from 10 patients with PTU-induced ANCA but without clinical vasculitis, were studied. Sera from 30 healthy blood donors were collected as normal controls. Soluble proteins from 1% Triton-100 extracted in vitro cultured human umbilical vein endothelial cells were used as antigens and an immunoblotting technique was performed to determine the presence of AECA, and their specific target antigens were identified. In patients with PTU-induced ANCA positive vasculitis, 10 of the 11 patients in an active phase of disease were serum IgG-AECA positive and six protein bands of endothelial antigens could be blotted (61 kD, 69 kD, 77 kD, 85 kD, 91 kD and 97 kD). However, in the quiescent phase, seven of the 10 positive sera turned negative. None of the ANCA positive but vasculitis negative patients or normal controls were AECA positive. In conclusion, AECA could be found in sera from patients with PTU-induced ANCA positive vasculitis and were associated more closely with vasculitic disease activity.

B cell epitope specificity in ANCA-associated vasculitis: does it matter?

Pauci-immune idiopathic small-vessel vasculitis is strongly associated with the presence of antineutrophil cytoplasm autoantibodies (ANCA). Antibodies to PR3 predominate in patients with Wegener's granulomatosis; antibodies to myeloperoxidase (MPO) are found more frequently in patients with microscopic polyangiitis. There is increasing in vivo and in vitro evidence for a pathogenic role of ANCA in systemic vasculitis based on associations of ANCA with disease activity. If ANCA are pathogenic, why is the course of disease different from one patient to another? Antibodies can recognize different binding sites (epitopes) on their corresponding antigens. Differences in binding specificity may influence the pathogenic potential of the antibodies. Differences between epitope specificity of ANCA between patients or changes in epitope specificity of ANCA in time in an individual patient may, accordingly, result in differences in disease expression. This review will focus on epitope specificity of autoantibodies in systemic autoimmune diseases and especially on the epitope specificity of PR3- and MPO-ANCA. We will discuss whether PR3-ANCA or MPO-ANCA recognize different epitopes on PR3 and MPO, respectively, and whether the epitopes recognized by ANCA change in parallel with the disease activity of ANCA-associated vasculitis. Finally, we will speculate if the direct pathogenic role of ANCA can be ascribed to one relapse- or disease-inducing epitope. Characterization of relapse- or disease-inducing epitopes bound by PR3-ANCA and MPO-ANCA is significant for understanding initiation and reactivation of ANCA-associated vasculitis. Elucidating a disease-inducing epitope bound by ANCA may lead to the development of epitope-specific therapeutic strategies.