Apoptosis-induced proteinase 3 membrane expression is independent from degranulation

A variety of death modes of neutrophils and their role in the etiology of autoimmune diseases

Neutrophils are important in the context of innate immunity and actively contribute to the progression of diverse autoimmune disorders. Distinct death mechanisms of neutrophils may exhibit specific and pivotal roles in autoimmune diseases and disease pathogenesis through the orchestration of immune homeostasis, the facilitation of autoantibody production, the induction of tissue and organ damage, and the incitement of pathological alterations. In recent years, more studies have provided in-depth examination of various neutrophil death modes, revealing nuances that challenge conventional understanding and underscoring their potential clinical utility in diagnosis and treatment. This review explores the multifaceted processes and characteristics of neutrophil death, with a focus on tailored investigations within various autoimmune diseases. It also highlights the potential interplay between neutrophil death and the landscape of autoimmune disorders. The review encapsulates the pertinent pathways implicated in various neutrophil death mechanisms across diverse autoimmune diseases while also charts possible avenues for future research.

β2 Integrin Regulation of Neutrophil Functional Plasticity and Fate in the Resolution of Inflammation

Neutrophils act as the first line of cellular defense against invading pathogens or tissue injury. Their rapid recruitment into inflamed tissues is critical for the elimination of invading microorganisms and tissue repair, but is also capable of inflicting damage to neighboring tissues. The β₂ integrins and Mac-1 (CD11b/CD18, α_Mβ₂ or complement receptor 3) in particular, are best known for mediating neutrophil adhesion and transmigration across the endothelium and phagocytosis of microbes. However, Mac-1 has a broad ligand recognition property that contributes to the functional versatility of the neutrophil population far beyond their antimicrobial function. Accumulating evidence over the past decade has demonstrated roles for Mac-1 ligands in regulating reverse neutrophil transmigration, lifespan, phagocytosis-induced cell death, release of neutrophil extracellular traps and efferocytosis, hence extending the traditional β₂ integrin repertoire in shaping innate and adaptive immune responses. Understanding the functions of β₂ integrins may partly explain neutrophil heterogeneity and may be instrumental to develop novel therapies specifically targeting Mac-1-mediated pro-resolution actions without compromising immunity. Thus, this review details novel insights on outside-in signaling through β₂ integrins and neutrophil functional heterogeneity pertinent to the resolution of inflammation.

Proteomic analysis of neutrophils in ANCA-associated vasculitis reveals a dysregulation in proteinase 3-associated proteins such as annexin-A1 involved in apoptotic cell clearance

Granulomatosis with polyangiitis (GPA) is an autoimmune vasculitis associated with anti-neutrophil-cytoplasmic antibodies (ANCA) against proteinase 3 leading to kidney damage. Neutrophils from those patients have increased expression of membrane proteinase 3 during apoptosis. Here we examined whether neutrophils from patients with GPA have dysregulated protein expressions associated with apoptosis. A global proteomic analysis was performed comparing neutrophils from patients with GPA, with healthy individuals under basal conditions and during apoptosis. At disease onset, the cytosolic proteome of neutrophils of patients with GPA before treatment was significantly different from healthy controls, and this dysregulation was more pronounced following ex vivo apoptosis. Proteins involved in cell death/survival were altered in neutrophils of patients with GPA. Several proteins identified were PR3-binding partners involved in the clearance of apoptotic cells, namely calreticulin, annexin-A1 and phospholipid scramblase 1. These proteins form a platform at the membrane of apoptotic neutrophils in patients with GPA but not healthy individuals and this was associated with the clinical presentation of GPA. Thus, our study shows that neutrophils from patients with GPA have an intrinsic dysregulation in proteins involved in apoptotic cell clearance, which could contribute to the unabated inflammation and autoimmunity in GPA. Hence, harnessing these dysregulated pathways could lead to novel biomarkers and targeted therapeutic opportunities to treat kidney disease.

Regulation of macrophage activation by proteins expressed on apoptotic neutrophils: Subversion towards autoimmunity by proteinase 3

Neutrophils are critically involved in host defence and they also modulate the inflammatory process. Turning the inflammatory response towards a resolutive outcome requires a dialogue between apoptotic neutrophils and proresolving macrophages through complex key molecular interactions controlling efferocytosis, anti-inflammatory reprogramming and ultimately immune regulation. In this review, we will first focus on recent molecular analyses aiming at characterizing the role of proteins expressed on apoptotic neutrophils and their cognate partners expressed on macrophages in the resolution of inflammation. These will include chemokine receptors and their ligands and annexin A1 and its receptor FPR2. We will next depict how the structural and enzymatic properties of proteinase 3 (PR3), the autoantigen in vasculitis, allow its expression on apoptotic neutrophils, which in turn affects efferocytosis and immune response associated with the clearance of apoptotic cells. This example illustrates that the fate of apoptotic neutrophils directly influences the resolution of inflammation and immune responses thereby potentially contributing to systemic and nonresolving inflammation as well as autoimmunity.

d-Peptides as inhibitors of PR3-membrane interactions

Proteinase 3 (PR3) is a neutrophil serine protease present in cytoplasmic granules but also expressed at the neutrophil surface where it mediates proinflammatory effects. Studies of the underlying molecular mechanisms have been hampered by the lack of inhibitors of the PR3 membrane anchorage. Indeed while there exist inhibitors of the catalytic activity of PR3, its membrane interfacial binding site (IBS) is distinct from its catalytic site. The IBS has been characterized both by mutagenesis experiments and molecular modeling. Through docking and molecular dynamics simulations we have designed d-peptides targeting the PR3 IBS. We used surface plasmon resonance to evaluate their effect on the binding of PR3 to phospholipid bilayers. Next, we verified their ability of binding to PR3 via fluorescence spectroscopy and isothermal titration calorimetry. The designed peptides did not affect the catalytic activity of PR3. A few peptides bound to PR3 hydrophobic pockets and inhibited PR3 binding to lipids. While the (KFF)₃K d-peptide inconveniently showed a significant affinity for the lipids, another d-peptide (SAKEAFFKLLAS) did not and it inhibited the PR3-membrane binding site with IC₅₀ of about 40μM. Our work puts forward d-peptides as promising inhibitors of peripheral protein-membrane interactions, which remain high-hanging fruits in drug design.

Transgenic Mice Expressing Human Proteinase 3 Exhibit Sustained Neutrophil-Associated Peritonitis

Proteinase 3 (PR3) is a myeloid serine protease expressed in neutrophils, monocytes, and macrophages. PR3 has a number of well-characterized proinflammatory functions, including cleaving and activating chemokines and controlling cell survival and proliferation. When presented on the surface of apoptotic neutrophils, PR3 can disrupt the normal anti-inflammatory reprogramming of macrophages following the phagocytosis of apoptotic cells. To better understand the function of PR3 in vivo, we generated a human PR3 transgenic mouse (hPR3Tg). During zymosan-induced peritonitis, hPR3Tg displayed an increased accumulation of neutrophils within the peritoneal cavity compared with wild-type control mice, with no difference in the recruitment of macrophages or B or T lymphocytes. Mice were also subjected to cecum ligation and puncture, a model used to induce peritoneal inflammation through infection. hPR3Tg displayed decreased survival rates in acute sepsis, associated with increased neutrophil extravasation. The decreased survival and increased neutrophil accumulation were associated with the cleavage of annexin A1, a powerful anti-inflammatory protein known to facilitate the resolution of inflammation. Additionally, neutrophils from hPR3Tg displayed enhanced survival during apoptosis compared with controls, and this may also contribute to the increased accumulation observed during the later stages of inflammation. Taken together, our data suggest that human PR3 plays a proinflammatory role during acute inflammatory responses by affecting neutrophil accumulation, survival, and the resolution of inflammation.

Proteinase 3: the odd one out that became an autoantigen

Neutrophils are critical in the defense against bacterial and fungal pathogens, and they also modulate the inflammatory process. The areas where neutrophils are studied have expanded from the restricted field of antibacterial defense to the modulation of inflammation and finally, to fine-tuning immune responses. As a result, recent studies have shown that neutrophils are implicated in several systemic autoimmune diseases, although exactly how neutrophils contribute to these diseases and the molecular mechanisms responsible are still under investigation. In a group of autoimmune vasculitides associated with anti-neutrophil cytoplasmic antibodies (AAVs), granulomatosis with polyangiitis (GPA) illustrates the concept that autoimmunity can develop against one specific neutrophil protein, namely, proteinase 3 (PR3), one of the four serine protease homologs contained within azurophilic granules. In this review, we will focus on recent molecular analyses combined with functional studies that provide clear evidence that the pathogenic properties of PR3 are not only a result of its enzymatic activity but also mediated by a particular structural element-the hydrophobic patch-which facilitates associations with various proteins and lipids and permits anchorage into the plasma membrane. Furthermore, these unique structural and functional characteristics of PR3 might be key contributors to the systemic inflammation and to the immune dysregulation observed in GPA.

Proteinase 3 Is a Phosphatidylserine-binding Protein That Affects the Production and Function of Microvesicles

Proteinase 3 (PR3), the autoantigen in granulomatosis with polyangiitis, is expressed at the plasma membrane of resting neutrophils, and this membrane expression increases during both activation and apoptosis. Using surface plasmon resonance and protein-lipid overlay assays, this study demonstrates that PR3 is a phosphatidylserine-binding protein and this interaction is dependent on the hydrophobic patch responsible for membrane anchorage. Molecular simulations suggest that PR3 interacts with phosphatidylserine via a small number of amino acids, which engage in long lasting interactions with the lipid heads. As phosphatidylserine is a major component of microvesicles (MVs), this study also examined the consequences of this interaction on MV production and function. PR3-expressing cells produced significantly fewer MVs during both activation and apoptosis, and this reduction was dependent on the ability of PR3 to associate with the membrane as mutating the hydrophobic patch restored MV production. Functionally, activation-evoked MVs from PR3-expressing cells induced a significantly larger respiratory burst in human neutrophils compared with control MVs. Conversely, MVs generated during apoptosis inhibited the basal respiratory burst in human neutrophils, and those generated from PR3-expressing cells hampered this inhibition. Given that membrane expression of PR3 is increased in patients with granulomatosis with polyangiitis, MVs generated from neutrophils expressing membrane PR3 may potentiate oxidative damage of endothelial cells and promote the systemic inflammation observed in this disease.

Proteinase 3 on apoptotic cells disrupts immune silencing in autoimmune vasculitis

Granulomatosis with polyangiitis (GPA) is a systemic necrotizing vasculitis that is associated with granulomatous inflammation and the presence of anti-neutrophil cytoplasmic antibodies (ANCAs) directed against proteinase 3 (PR3). We previously determined that PR3 on the surface of apoptotic neutrophils interferes with induction of antiinflammatory mechanisms following phagocytosis of these cells by macrophages. Here, we demonstrate that enzymatically active membrane-associated PR3 on apoptotic cells triggered secretion of inflammatory cytokines, including granulocyte CSF (G-CSF) and chemokines. This response required the IL-1R1/MyD88 signaling pathway and was dependent on the synthesis of NO, as macrophages from animals lacking these pathways did not exhibit a PR3-associated proinflammatory response. The PR3-induced microenvironment facilitated recruitment of inflammatory cells, such as macrophages, plasmacytoid DCs (pDCs), and neutrophils, which were observed in close proximity within granulomatous lesions in the lungs of GPA patients. In different murine models of apoptotic cell injection, the PR3-induced microenvironment instructed pDC-driven Th9/Th2 cell generation. Concomitant injection of anti-PR3 ANCAs with PR3-expressing apoptotic cells induced a Th17 response, revealing a GPA-specific mechanism of immune polarization. Accordingly, circulating CD4+ T cells from GPA patients had a skewed distribution of Th9/Th2/Th17. These results reveal that PR3 disrupts immune silencing associated with clearance of apoptotic neutrophils and provide insight into how PR3 and PR3-targeting ANCAs promote GPA pathophysiology.

Pathogenesis of antineutrophil cytoplasmic autoantibody-mediated disease

Antineutrophil cytoplasmic autoantibodies (ANCAs) are the probable cause of a distinct form of vasculitis that can be accompanied by necrotizing granulomatosis. Clinical and experimental evidence supports a pathogenesis that is driven by ANCA-induced activation of neutrophils and monocytes, producing destructive necrotizing vascular and extravascular inflammation. Pathogenic ANCAs can originate from precursor natural autoantibodies. Pathogenic transformation might be initiated by commensal or pathogenic microbes, legal or illegal drugs, exogenous or endogenous autoantigen complementary peptides, or dysregulated autoantigen expression. The ANCA autoimmune response is facilitated by insufficient T-cell and B-cell regulation. A putative pathogenic mechanism for vascular inflammation begins with ANCA-induced activation of primed neutrophils and monocytes leading to activation of the alternative complement pathway, which sets in motion an inflammatory amplification loop in the vessel wall that attracts and activates neutrophils with resultant respiratory burst, degranulation, extrusion of neutrophil extracellular traps, apoptosis and necrosis. The pathogenesis of extravascular granulomatosis is less clear, but a feasible scenario proposes that a prodromal infectious or allergic condition positions primed neutrophils in extravascular tissue in which they can be activated by ANCAs in interstitial fluid to produce extravascular necrotizing injury that would initiate an innate granulomatous inflammatory response to wall off the necrotic debris.

B cell-mediated pathogenesis of ANCA-mediated vasculitis

B cells and their progeny that produce and release anti-neutrophil cytoplasmic autoantibodies (ANCA) are the primary cause for an aggressive form of necrotizing small vessel vasculitis. Cytoplasmic ANCA antigens are released at the surface and in the microenvironment of cytokine-primed neutrophils. Binding of ANCA to ANCA antigens activates neutrophils by both Fc receptor engagement and direct Fab'2 binding to antigen on the cell surface. ANCA-activated neutrophils release factors that induce alternative complement pathway activation, which establishes a potent inflammatory amplification loop that causes severe necrotizing vascular inflammation. The origin of the ANCA autoimmune response is unknown but appears to involve genetically determined HLA specificities that allow the autoimmune response to develop. One putative immunogenic mechanism begins with an immune response to a peptide that is complementary to the autoantigen and evolves through an anti-idiotypic network to produce autoantibodies to the autoantigen. Another putative immunogenic mechanism begins with an immune response to a microbe-derived molecular mimic of the autoantigen resulting in antibodies that cross-react with the autoantigen. Release of neutrophil extracellular traps, apoptosis, and increased granule protein expression of ANCA antigens may facilitate the initiation of an ANCA autoimmune response, augment established pathogenic ANCA production, or both. The ANCA B cell autoimmune response is facilitated by quantitatively and qualitatively impaired T cell and B cell suppression and by release from activated neutrophils of B cell-activating factors that enhance B cell proliferation and retard B cell apoptosis.

Proteinase 3, the autoantigen in granulomatosis with polyangiitis, associates with calreticulin on apoptotic neutrophils, impairs macrophage phagocytosis, and promotes inflammation

Proteinase 3 (PR3) is the target of anti-neutrophil cytoplasm Abs in granulomatosis with polyangiitis, a form of systemic vasculitis. Upon neutrophil apoptosis, PR3 is coexternalized with phosphatidylserine and impaired macrophage phagocytosis. Calreticulin (CRT), a protein involved in apoptotic cell recognition, was found to be a new PR3 partner coexpressed with PR3 on the neutrophil plasma membrane during apoptosis, but not after degranulation. The association between PR3 and CRT was demonstrated in neutrophils by confocal microscopy and coimmunoprecipitation. Evidence for a direct interaction between PR3 and the globular domain of CRT, but not with its P domain, was provided by surface plasmon resonance spectroscopy. Phagocytosis of apoptotic neutrophils from healthy donors was decreased after blocking lipoprotein receptor-related protein (LRP), a CRT receptor on macrophages. In contrast, neutrophils from patients with granulomatosis with polyangiitis expressing high membrane PR3 levels showed a lower rate of phagocytosis than those from healthy controls not affected by anti-LRP, suggesting that the LRP-CRT pathway was disturbed by PR3-CRT association. Moreover, phagocytosis of apoptotic PR3-expressing cells potentiated proinflammatory cytokine in vitro by human monocyte-derived macrophages and in vivo by resident murine peritoneal macrophages, and diverted the anti-inflammatory response triggered by the phagocytosis of apoptotic cells after LPS challenge in thioglycolate-elicited murine macrophages. Therefore, membrane PR3 expressed on apoptotic neutrophils might amplify inflammation and promote autoimmunity by affecting the anti-inflammatory "reprogramming" of macrophages.

Decreased neutrophil apoptosis in quiescent ANCA-associated systemic vasculitis

Background

ANCA-Associated Systemic Vasculitis (AASV) is characterized by leukocytoclasis, accumulation of unscavenged apoptotic and necrotic neutrophils in perivascular tissues. Dysregulation of neutrophil cell death may contribute directly to the pathogenesis of AASV.

Methods

Neutrophils from Healthy Blood Donors (HBD), patients with AASV most in complete remission, Polycythemia Vera (PV), Systemic Lupus Erythematosus (SLE), Rheumatoid Arthritis (RA) and renal transplant recipients (TP) were incubated in vitro, and the rate of spontaneous apoptosis was measured by FACS. Plasma levels of cytokines and sFAS were measured with cytometric bead array and ELISA. Expression of pro/anti-apoptotic factors, transcription factors C/EBP-α, C/EBP-β and PU.1 and inhibitors of survival/JAK2-pathway were measured by real-time-PCR.

Results

AASV, PV and RA neutrophils had a significantly lower rate of apoptosis compared to HBD neutrophils (AASV 50±14% vs. HBD 64±11%, p<0.0001). In RA but not in AASV and PV, low apoptosis rate correlated with increased plasma levels of GM-CSF and high mRNA levels of anti-apoptotic factors Bcl-2A1 and Mcl-1. AASV patients had normal levels of G-CSF, GM-CSF and IL-3. Both C/EBP-α, C/EBP-β were significantly higher in neutrophils from AASV patients than HBD. Levels of sFAS were significantly higher in AASV compared to HBD.

Conclusion

Neutrophil apoptosis rates in vitro are decreased in AASV, RA and PV but mechanisms seem to differ. Increased mRNA levels of granulopoiesis-associated transcription factors and increased levels of sFAS in plasma were observed in AASV. Additional studies are required to define the mechanisms behind the decreased apoptosis rates, and possible connections with accumulation of dying neutrophils in regions of vascular lesions in AASV patients.

Profilin 1 is a potential biomarker for bladder cancer aggressiveness

Of the most important clinical needs for bladder cancer (BC) management is the identification of biomarkers for disease aggressiveness. Urine is a "gold mine" for biomarker discovery, nevertheless, with multiple proteins being in low amounts, urine proteomics becomes challenging. In the present study we applied a fractionation strategy of urinary proteins based on the use of immobilized metal affinity chromatography for the discovery of biomarkers for aggressive BC. Urine samples from patients with non invasive (two pools) and invasive (two pools) BC were subjected to immobilized metal affinity chromatography fractionation and eluted proteins analyzed by 1D-SDS-PAGE, band excision and liquid chromatography tandem MS. Among the identified proteins, multiple corresponded to proteins with affinity for metals and/or reported to be phosphorylated and included proteins with demonstrated association with BC such as MMP9, fibrinogen forms, and clusterin. In agreement to the immobilized metal affinity chromatography results, aminopeptidase N, profilin 1, and myeloblastin were further found to be differentially expressed in urine from patients with invasive compared with non invasive BC and benign controls, by Western blot or Elisa analysis, nevertheless exhibiting high interindividual variability. By tissue microarray analysis, profilin 1 was found to have a marked decrease of expression in the epithelial cells of the invasive (T2+) versus high risk non invasive (T1G3) tumors with occasional expression in stroma; importantly, this pattern strongly correlated with poor prognosis and increased mortality. The functional relevance of profilin 1 was investigated in the T24 BC cells where blockage of the protein by the use of antibodies resulted in decreased cell motility with concomitant decrease in actin polymerization. Collectively, our study involves the application of a fractionation method of urinary proteins and as one main result of this analysis reveals the association of profilin 1 with BC paving the way for its further investigation in BC stratification.

Molecular analysis of the membrane insertion domain of proteinase 3, the Wegener's autoantigen, in RBL cells: implication for its pathogenic activity

PR3, also called myeloblastin, is a neutrophil serine protease that promotes myeloid cell proliferation by cleaving the cyclin-dependent kinase inhibitor p21(cip1/waf1). In addition, it is the target of ANCA in GPA, a necrotizing vasculitis. Anti-PR3 ANCA binding to membrane-expressed PR3 triggers neutrophil activation, potentiating vascular inflammation. This study performed in RBL cells identifies the structural motifs of PR3 membrane anchorage and examines its impact on PR3 proinflammatory and proliferative functions. With the use of MD simulations and mutagenesis, we demonstrate that the mutations of four hydrophobic (F180, F181, L228, F229) or four basic (R193, R194, K195, R227) amino acids abrogated PR3 membrane anchorage. The hydrophobic patch-deficient PR3 mutant (PR34H4A) was still able to cleave the synthetic substrate Boc-Ala-Pro-Val in cell lysates. However, in contrast to WT PR3, PR34H4A was not expressed at the plasma membrane after degranulation and failed to cleave extracellular fibronectin, was not externalized after apoptosis and did not impair macrophage phagocytosis of apoptotic cells, did not promote myeloid cell proliferation and failed to cleave p21/waf1. PR3 membrane insertion appears to be pivotal for its proinflammatory activities, such as extracellular proteolysis and impairment of apoptotic cell clearance, but also for myeloid cell proliferation. Targeting membrane-associated PR3 might constitute a novel, anti-inflammatory therapeutic strategy in inflammatory disease especially in vasculitis, but this approach has to be validated in mature neutrophils.

ANCA-associated vasculitis: is there a role for neutrophil apoptosis in autoimmunity?

The primary small vessel systemic vasculitides are disorders that target small blood vessels, inducing vessel wall inflammation, and are associated with the development of antineutrophil cytoplasmic antibodies. Multiple organs are attacked including the lungs and kidneys. Increasing knowledge of pathogenesis suggests that the antibodies activate neutrophils inappropriately, leading to endothelial and vascular damage. Cytokines, such as tumor necrosis factor, can facilitate damage by priming neutrophils and activating endothelial cells. Apoptosis of infiltrating neutrophils is also disrupted by antineutrophil cytoplasmic antibody activation. Removal of these effete cells occurs in a proinflammatory manner, promoting persistent inflammation. The autoimmune response may be promoted by aberrant phagocytosis of apoptotic neutrophils by dendritic cells. Understanding pathogenesis can help to rationalize existing therapies and indicate new approaches to therapy.

How does proteinase 3 interact with lipid bilayers?

Proteinase 3 (PR3) is a serine protease of the neutrophils whose membrane expression is relevant in a number of inflammatory pathologies. It has been shown to strongly interact with reconstituted bilayers containing dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidylglycerol (DMPG) or mixtures of both phospholipids. Here we present the results of molecular dynamics simulations of PR3 anchored at three different phospholipid bilayers: DMPC, DMPG and an equimolar mixture of DMPC/DMPG. We present for the first time a detailed model of membrane-bound PR3. A thorough inventory of the interaction between the lipids and the enzyme reveals three types of interactions contributing to the anchorage of PR3. Basic residues (R177, R186A, R186B, K187 and R222) interact via hydrogen bonds with the lipid headgroups to stabilize PR3 at the interfacial membrane region. Hydrophobic amino acids (V163, F165, F166, I217, L223, and F224) insert into the hydrophobic core below the carbonyl groups of the bilayers and six aromatic amino acids (F165, F192, F215, W218, F224, and F227) contribute electrostatic interaction via cation-pi interactions with the choline groups of DMPC. PR3 presents all the characteristics of a peripheral membrane protein with an ability to bind negative phospholipids. Although the catalytic triad remains unperturbed by the presence of the membrane, the ligand binding sites are located in close proximity to the membrane and amino acids K99 and I217 interact significantly with the lipids. We expect the binding of long ligands to be modified by the presence of the lipids.

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.

The use of small molecule high-throughput screening to identify inhibitors of the proteinase 3-NB1 interaction

Anti-neutrophil cytoplasmic antibodies (ANCA) to proteinase 3 (PR3) are found in patients with small-vessel vasculitis. PR3-ANCA bind strongly to membrane PR3 (mPR3) that is presented by the NB1 receptor. We performed high-throughput screening using a small molecule library to identify compounds that inhibit PR3-NB1 binding. We established a human embryonic kidney (HEK293) cell-based system, where approximately 95 +/- 2% of the NB1-transfected cells expressed the NB1 receptor on the cell surface. Addition of 0.1 microg/ml human PR3 to 10(4) NB1-expressing HEK293 cells resulted in PR3 binding that was detected by immunofluorescence using a fluorescence plate reader assay. We identified 13 of 20 000 molecules that inhibited PR3 binding by >70%. Seven of 13 substances showed reproducible inhibition in four additional validation experiments. Two selected compounds (27519 and 27549) demonstrated a dose-dependent inhibition over a range from 6.25 to 100 microM as measured by the plate reader assay. We used flow cytometry as a second assay, and found that both compounds reproducibly inhibited PR3 binding to NB1-transfected HEK293 cells at 50 microM (inhibition to 42 +/- 4% with compound 27519 and to 47 +/- 6% with compound 27549 compared to the dimethylsulphoxide control). Furthermore, compounds 27519 and 27549 also inhibited binding of exogenous PR3 to human neutrophils. In contrast, the compounds did not decrease mPR3 expression on resting neutrophils, but reduced the tumour necrosis factor-alpha-mediated mPR3 increase on NB1(pos) neutrophils when present continuously during the assay. The findings suggest that small inhibitory compounds provide a potential therapeutic tool to reduce mPR3 by preventing its binding to NB1.

Proteinase 3, the Wegener autoantigen, is externalized during neutrophil apoptosis: evidence for a functional association with phospholipid scramblase 1 and interference with macrophage phagocytosis

Proteinase 3 (PR3), a serine proteinase contained in neutrophil azurophilic granules, is considered a risk factor for vasculitides and rheumatoid arthritis when expressed on the outer leaflet of neutrophil plasma membrane and is the preferred target of antineutrophil cytoplasm autoantibodies (ANCA) in Wegener granulomatosis. ANCA binding to PR3 expressed at the surface of neutrophils activates them. Evidence is provided that neutrophil apoptosis induced significantly more membrane PR3 expression without degranulation (but no enhanced membrane CD35, CD66b, CD63, myeloperoxidase, or elastase expression). This observation was confirmed on cytoplasts, a model of granule-free neutrophils. We hypothesized that PR3 could interact with proteins involved in membrane flip-flop (eg, phospholipid scramblase 1 [PLSCR1]). PR3-PLSCR1 interaction in neutrophils was demonstrated by confocal microscopy and coimmunoprecipitation. In the RBL-2H3 rat mast-cell line stably transfected with PR3 or its inactive mutant (PR3S203A), PR3 externalization depended on PLSCR1, as shown by less PR3 externalization in the presence of rPLSCR1 siRNA, but independently of its serine-proteinase activity. Finally, apoptosis-externalized PR3 decreased the human macrophage-phagocytosis rate of apoptotic PR3 transfectants. Therefore, in addition to ANCA binding in vasculitis, the proinflammatory role of membrane PR3 expression may involve interference with macrophage clearance of apoptotic neutrophils.

Apoptosis of human neutrophils induced by protein phosphatase 1/2A inhibition is caspase-independent and serine protease-dependent

Protein phosphatase (PP) activity is associated with the regulation of apoptosis in neutrophils. However, the underlying regulatory mechanism(s) in apoptosis remain unclear. The type of cell death induced by okadaic acid (OA), the inhibitor of PP1 and PP2A, is characterized by apoptotic morphological changes of the cells and annexin V-positive staining without DNA fragmentation. The apoptotic effects of OA and calyculin A on neutrophils were observed at concentrations ranging from 50 to 200 nM, or 10 to 50 nM, respectively. Cyclosporine A (a PP2B specific inhibitor), however, did not exhibit any pro-apoptotic effects. OA and calyculin A, but not cyclosporine A, exhibited significant effects on protein levels and on the electrophoretic mobility of Mcl-1. zVAD-fmk, a pancaspase inhibitor, failed to inhibit the effect of OA on the caspase-3 activity, procaspase-3 processing, and the apoptotic rate of neutrophils. However, 4-(2-aminoethyl) benzenesulfonylfluoride (AEBSF), a general serine protease inhibitor, significantly abrogated the OA-induced mobility shift in procaspase-3, caspase-3 activation, and the apoptotic morphological changes in neutrophils. Moreover, OA enhanced the serine protease activity of the neutrophils. The addition of the proteinase-3 protein increased the rate of neutrophil apoptosis, which was also blocked by AEBSF but not by zVAD-fmk. These results suggest that OA induces procaspase-3 processing but that OA-induced apoptosis is caspase-independent and serine protease-dependent.

Increased membrane expression of proteinase 3 during neutrophil adhesion in the presence of anti proteinase 3 antibodies

We investigated membrane proteinase 3 (mPR3) expression during TNF-alpha-induced adhesion of neutrophils in the presence of anti-PR3 antibodies, a situation occurring during anti-neutrophil cytoplasmic autoantibodies (ANCA)-associated vasculitis. Three increasing levels of mPR3 expression were observed on the mPR3(+) neutrophil subset after stepwise cell activation. TNF-alpha activation without adhesion, TNF-alpha-induced adhesion, and adhesion in the presence of anti-PR3 mAb or human anti-PR3 ANCA resulted, respectively, in a two-, seven-, and 24-fold increase of mPR3 levels. In plasma, anti-PR3 antibodies poorly recognized suspended neutrophils, whereas they bound to mPR3 on adherent cells. mPR3 upregulation was also triggered by IL-8, formyl-methionyl-leucyl-phenylalanine (fMLP), and neutrophil adhesion to activated human umbilical vein endothelial cells. It involved beta2 integrins and Fcgamma receptor, because it was prevented by anti-CD18 antibodies and was not observed with anti-PR3 F(ab')(2). Furthermore, it was specific to anti-PR3 mAb, and no mPR3 upregulation was observed with anti-myeloperoxidase or anti-HLA-ABC mAb. Newly expressed mPR3 molecules, after TNF-induced adhesion, were mobilized from secretory vesicles (CD35(+)) and secondary granules (CD11b(+)). The adhesion- and antibody-dependent upregulations of mPR3 expression occurred with little azurophilic granule degranulation, no sign of apoptosis, and no further CD177 upregulation. In conclusion, this study describes an amplifying loop in polymorphonuclear neutrophil activation process, whereby ANCA are involved in the membrane expression of their own antigen during cell adhesion. This could explain the restriction of ANCA-associated vasculitis to small vessels, the main site of neutrophil adhesion.

Activation, apoptosis, and clearance of neutrophils in Wegener's granulomatosis

Wegener's granulomatosis (WG) is strongly associated with the presence of antineutrophil cytoplasmic autoantibodies (ANCAs). Within WG these ANCAs are usually (80-90%) directed against the azurophilic enzyme proteinase 3, the so called PR3-ANCA. A pathophysiological role for these autoantibodies, supported by numerous in vitro and in vivo studies, is specifically based on their capacity to bind and activate neutrophils and potentially may damage vessels. In this review, the pathogenic potential of different developmental stages of the neutrophil in the pathogenesis of WG is discussed. After release from the bone marrow into the circulation, neutrophils can be primed by TNFalpha and become attached to locally activated endothelium. Once attached to the endothelium, ANCAs can fully activate these primed neutrophils. In this activation process, the degree of activation after stimulation with PR3-ANCAs associates with the level of PR3 expression on the membrane of the neutrophil. Following activation, infiltrated neutrophils become apoptotic with further membrane expression of PR3. In WG patients, clearance of apoptotic neutrophils can be disturbed due to the opsonization of PR3-expressing apoptotic neutrophils with PR3-ANCAs, thereby perpetuating inflammation by the release of proinflammatory cytokines during clearance; or it may favor autoimmunity by PR3 presentation in an inflammatory environment. Furthermore, the presence of ANCAs and the release of the vessel-related pentraxin PTX3 may lead to the persistence of late apoptotic neutrophils in tissues, thereby inducing leukocytoclastic lesions that are characteristic in patients with WG. All together, alive neutrophils as well as apoptotic neutrophils play a key role in different inflammatory phenomena seen in patients suffering from WG.

Wegener autoantigen induces maturation of dendritic cells and licenses them for Th1 priming via the protease-activated receptor-2 pathway

Autoantibodies to proteinase 3 (PR3) are involved in the pathogenesis of autoimmune-mediated vasculitis in Wegener granulomatosis (WG). To address the question how the autoantigen PR3 becomes a target of adaptive immunity, we investigated the effect of PR3 on immature dendritic cells (iDCs) in patients with WG, healthy blood donors, and patients with Crohn disease (CD), another granulomatous disease. PR3 induces phenotypic and functional maturation of a fraction of blood monocyte-derived iDCs. PR3-treated DCs express high levels of CD83, a DC-restricted marker of maturation, CD80 and CD86, and HLA-DR. Furthermore, the DCs become fully competent antigen-presenting cells and can induce stimulation of PR3-specific CD4+ T cells, which produce IFN-γ. PR3-maturated DCs derived from WG patients induce a higher IFN-γ response of PR3-specific CD4+ T cells compared with patients with CD and healthy controls. The maturation of DCs mediated through PR3 was inhibited by a serine protease inhibitor, by antibodies directed against the protease-activated receptor-2 (PAR-2), and by inhibition of phospholipase C, suggesting that the interactions of PR3 with PAR-2 are involved in the induction of DC maturation. Wegener autoantigen interacts with a “gateway” receptor (PAR-2) on iDCs in vitro triggering their maturation and licenses them for a T helper 1 (Th1)–type response potentially favoring granuloma formation in WG.

Fever-like temperatures affect neutrophil NF-kappaB signaling, apoptosis, and ANCA-antigen expression

The neutrophil is pivotal to ANCA vasculitis pathogenesis. Fever frequently complicates ANCA diseases. This study investigated the effects of short-term heat exposure on apoptosis in neutrophils that were treated with LPS, GM-CSF, IL-8, and dexamethasone. All compounds delayed apoptosis. Heat abrogated the apoptosis-delaying effect of LPS without affecting constitutive apoptosis or delayed apoptosis by GM-CSF, IL-8, or dexamethasone. The heat effect was dose dependent over the 39 to 42 degrees C range. NF-kappaB but not extracellular signal-regulated kinase, p38 mitogen-activated protein kinase (MAPK), or phosphatidylinositol 3-kinase/Akt controlled LPS-delayed apoptosis. Furthermore, LPS-induced IkappaBalpha degradation, DNA binding, and NF-kappaB-dependent gene transcription activation were abrogated by short-term heat. When core temperatures were raised to 40.5 degrees C for 30 min in mice, LPS-induced neutrophil NF-kappaB activation also was prevented. Short-term heat removed heat-shock protein 90 from the IkappaB kinase complex, resulting in failure of LPS-induced IkappaB kinase activation. Despite delayed apoptosis, ANCA antigen expression was increased in LPS-treated neutrophils. ANCA antigen increase was prevented by p38 MAPK inhibition and by heat exposure. Heat exposure did not inhibit LPS-induced p38 MAPK phosphorylation. Instead, apoptosis-mediated p38 MAPK degradation was accelerated, thereby decreasing the p38 MAPK that was available for LPS-mediated ANCA antigen upregulation. These data suggest that fever-like temperatures modulate neutrophil behavior in this disease.

Inhibition of neutrophil elastase by alpha1-protease inhibitor at the surface of human polymorphonuclear neutrophils

The uncontrolled proteolytic activity in lung secretions during lung inflammatory diseases might be due to the resistance of membrane-bound proteases to inhibition. We have used a new fluorogenic neutrophil elastase substrate to measure the activity of free and membrane-bound human neutrophil elastase (HNE) in the presence of alpha1-protease inhibitor (alpha1-Pi), the main physiological inhibitor of neutrophil serine proteases in lung secretions. Fixed and unfixed neutrophils bore the same amounts of active HNE at their surface. However, the HNE bound to the surface of unfixed neutrophils was fully inhibited by stoichiometric amounts of alpha1-Pi, unlike that of fixed neutrophils. The rate of inhibition of HNE bound to the surface of unfixed neutrophils was the same as that of free HNE. In the presence of alpha1-Pi, membrane-bound elastase is almost entirely removed from the unfixed neutrophil membrane to form soluble irreversible complexes. This was confirmed by flow cytometry using an anti-HNE mAb. HNE activity rapidly reappeared at the surface of HNE-depleted cells when they were triggered with the calcium ionophore A23187, and this activity was fully inhibited by stoichiometric amounts of alpha1-Pi. HNE was not released from the cell surface by oxidized, inactive alpha1-Pi, showing that active inhibitor is required to interact with active protease from the cell surface. We conclude that HNE activity at the surface of human neutrophils is fully controlled by alpha1-Pi when the cells are in suspension. Pericellular proteolysis could be limited to zones of contact between neutrophils and subjacent protease substrates where natural inhibitors cannot penetrate.

Cleavage of p21/WAF1/CIP1 by Proteinase 3 Modulates Differentiation of a Monocytic Cell Line

Proteinase 3 (PR3), also called myeloblastin, is involved in the control of myeloid cell growth, but the underlying molecular mechanisms have not been elucidated. In U937/PR3, stably transfected with PRCRSV/PR3 to overexpress PR3, PMA-induced p21 expression was significantly decreased as compared with control U937, and this phenomenon was reversed in the presence of the serine proteinase inhibitor, pefabloc. Conversely, when PR3 was inactivated by small interfering RNA, p21 protein was increased, and PMA-induced monocytic differentiation was potentiated. Mass spectrometry analysis identified Ala45 as the primary cleavage site on p21, and the recombinant mutated p21A45R, generated by site-directed mutagenesis and expressed in Escherichia coli, was resistant to in vitro PR3 cleavage. The U937 cells were then stably transfected with either PRCRSV/p21 or PRCRSV/p21A45R, to ectopically express wild type p21 or PR3-resistant p21, respectively. In U937/p21A45R treated with PS-341, a selective proteasome inhibitor, a significant decrease in the S phase and a blockade in the G0-G1 phase of cell cycle were observed when compared with U937/p21 or control U937. This suggested that both PR3 and the proteasome are efficiently involved in the proteolytic regulation of p21 expression in myeloid cells. Moreover, PMA-induced p21 expression was more pronounced in U937/p21A45R compared with U937/p21 and was concomitant with the morphological features of early differentiation. Our data demonstrated that p21 is one specific target of PR3 and that PR3-mediated p21 cleavage prevents monocytic differentiation.

Proteinase-3 induces procaspase-3 activation in the absence of apoptosis: potential role of this compartmentalized activation of membrane-associated procaspase-3 in neutrophils

In the present study, we provide evidence that procaspase-3 is a novel target of proteinase 3 (PR3) but not of human neutrophil elastase (HNE). Human mast cell clone 1 (HMC1) and rat basophilic leukemia (RBL) mast cell lines were transfected with PR3 or the inactive mutated PR3 (PR3S203A) or HNE cDNA. In both RBL/PR3 and HMC1/PR3, a constitutive activity of caspase-3 was measured with DEVD substrate, due to the direct processing of procaspase-3 by PR3. No caspase-3 activation was observed in cells transfected with the inactive PR3 mutant or HNE. Despite the high caspase-3 activity in RBL/PR3, no apoptosis was detected as demonstrated by an absence of 1) phosphatidylserine externalization, 2) mitochondria cytochrome c release, 3) upstream caspase-8 or caspase-9 activation, or 4) DNA fragmentation. In vitro, purified PR3 cleaved procaspase-3 into an active 22-kDa fragment. In neutrophils, the 22-kDa caspase-3 activation fragment was present only in resting neutrophils but was absent after apoptosis. The 22 kDa fragment was specific of myeloid cells because it was absent from resting lymphocytes. This 22-kDa fragment was not present when neutrophils were treated with pefabloc, an inhibitor of serine proteinase. Like in HMC1/PR3, the 22-kDa caspase-3 fragment was restricted to the plasma membrane compartment. Double immunofluorescence labeling after streptolysin-O permeabilization further showed that PR3 and procaspase-3 could colocalize in an extragranular compartment. In conclusion, our results strongly suggest that compartmentalized PR3-induced caspase-3 activation might play specific functions in neutrophil survival.