Inhibitors of mast cell tryptase beta as therapeutics for the treatment of asthma and inflammatory disorders

Synovial mast cells and osteoarthritis: Current understandings and future perspectives

Osteoarthritis (OA) is a prevalent joint disease worldwide that significantly impacts the quality of life of individuals, particularly those in middle-aged and elderly populations. OA was initially considered as non-inflammatory arthritis, but recent studies have identified a substantial number of immune responses in OA, leading to the recognition of inflammation as a key factor in its pathogenesis. An increasing number of studies have found that mast cell (MC) and MC-secreted inflammatory mediators and cytokines are notably increased in the synovial fluid of OA patients, indicating a potential association between MCs and the onset and progression of synovial inflammation. The present review aims to summarize the significance and mechanism of MCs in the pathogenesis of OA. Meanwhile, we also discuss the clinical potential of using MCs as therapeutic target for OA therapy. Modulating the activities of MCs or the mediators of MCs in the synovial fluid inflammatory microenvironment will be promising new options for the treatment of OA.

Update on Mast Cell Proteases as Drug Targets

Mast cell granules are packed with proteases, which are released with other mediators by degranulating stimuli. Several of these proteases are targets of potentially therapeutic inhibitors based on hypothesized contributions to diseases, notably asthma and ulcerative colitis for β-tryptases, heart and kidney scarring for chymases, and airway infection for dipeptidyl peptidase-I. Small-molecule and antibody-based β-tryptase inhibitors showing preclinical promise were tested in early-phase human trials with some evidence of benefit. Chymase inhibitors were given safely in Phase II trials without demonstrating benefits, whereas dipeptidyl peptidase-I inhibitor improved bronchiectasis, in effects likely related to inactivation of the enzyme in neutrophils.

Thioredoxin-1: A Promising Target for the Treatment of Allergic Diseases

Thioredoxin-1 (Trx1) is an important regulator of cellular redox homeostasis that comprises a redox-active dithiol. Trx1 is induced in response to various stress conditions, such as oxidative damage, infection or inflammation, metabolic dysfunction, irradiation, and chemical exposure. It has shown excellent anti-inflammatory and immunomodulatory effects in the treatment of various human inflammatory disorders in animal models. This review focused on the protective roles and mechanisms of Trx1 in allergic diseases, such as allergic asthma, contact dermatitis, food allergies, allergic rhinitis, and drug allergies. Trx1 plays an important role in allergic diseases through processes, such as antioxidation, inhibiting macrophage migration inhibitory factor (MIF), regulating Th1/Th2 immune balance, modulating allergic inflammatory cells, and suppressing complement activation. The regulatory mechanism of Trx1 differs from that of glucocorticoids that regulates the inflammatory reactions associated with immune response suppression. Furthermore, Trx1 exerts a beneficial effect on glucocorticoid resistance of allergic inflammation by inhibiting the production and internalization of MIF. Our results suggest that Trx1 has the potential for future success in translational research.

Mast cell tryptase enhances wound healing by promoting migration in human bronchial epithelial cells

Epithelial damage and increase of intraepithelial mast cells (MC) are characteristics of asthma. The role of MC mediator tryptase and the protease-activated receptor-2 (PAR2) on epithelial wound healing is not fully investigated. Stimulation of bronchial epithelial cells (BECs) with tryptase promoted gap closure, migration and cellular speed compared to controls. Stimulated BECs had higher expression of migration marker CD151 compared to controls. Proliferation marker KI67 was upregulated in tryptase-stimulated BECs compared to controls. Treatment with PAR2 antagonist I-191 reduced gap closure, migration and cell speed compared to BECs stimulated with tryptase. We found that tryptase enhances epithelial wound healing by increased migration and proliferation, which is in part regulated via PAR2. Our data suggest that tryptase might be beneficial in tissue repair under baseline conditions. However, in a pathological context such as asthma with increased numbers of activated MCs, it might lead to epithelial remodeling and loss of function.

The Genetic Basis and Clinical Impact of Hereditary Alpha-Tryptasemia

Hereditary alpha-tryptasemia (HαT) is an autosomal dominant genetic trait found in 4% to 6% of the general population and defined by excess copies of alpha-tryptase at TPSAB1. Elevated basal serum tryptase (sBT >8 ng/mL) is a defining feature of HαT and appears to result from increased pro-alpha-tryptase synthesis and secretion rather than mast cell activation. It is estimated that approximately one-third of individuals with HαT have associated symptoms, including cutaneous, gastrointestinal, atopic, musculoskeletal, autonomic, and neuropsychiatric manifestations. HαT is found at a disproportionately high rate in systemic mastocytosis and idiopathic anaphylaxis, and is a modifying factor that independently increases the incidence and severity of anaphylaxis. The varied phenotypes associated with HαT may, in part, result from coinheritance of other genetic variants, increased expression of α-/ß-tryptase heterotetramers, and/or overexpression of pro-alpha-tryptase, although further studies are needed. There is an accurate diagnostic test available to confirm HαT in patients that can be used in combination with sBT to help risk-stratify individuals in whom bone marrow biopsy is being considered. There is no specific treatment for symptoms associated with HαT, and management is focused on controlling clinical manifestations with mast cell mediator antagonists, aspirin, inhalers, epinephrine, omalizumab, and involvement of other specialists.

Bivalent antibody pliers inhibit β-tryptase by an allosteric mechanism dependent on the IgG hinge

Human β-tryptase, a tetrameric trypsin-like serine protease, is an important mediator of allergic inflammatory responses in asthma. Antibodies generally inhibit proteases by blocking substrate access by binding to active sites or exosites or by allosteric modulation. The bivalency of IgG antibodies can increase potency via avidity, but has never been described as essential for activity. Here we report an inhibitory anti-tryptase IgG antibody with a bivalency-driven mechanism of action. Using biochemical and structural data, we determine that four Fabs simultaneously occupy four exosites on the β-tryptase tetramer, inducing allosteric changes at the small interface. In the presence of heparin, the monovalent Fab shows essentially no inhibition, whereas the bivalent IgG fully inhibits β-tryptase activity in a hinge-dependent manner. Our results suggest a model where the bivalent IgG acts akin to molecular pliers, pulling the tetramer apart into inactive β-tryptase monomers, and may provide an alternative strategy for antibody engineering.

β-tryptases are responsible for most of the proteolytic activity during mast cell activation. Here, the authors develop β-tryptase-inhibiting antibodies and provide structural and biochemical evidence that the bivalency of the antibodies is a prerequisite for their inhibitory activity.

Bowman-Birk Inhibitors: Insights into Family of Multifunctional Proteins and Peptides with Potential Therapeutical Applications

Bowman-Birk inhibitors (BBIs) are found primarily in seeds of legumes and in cereal grains. These canonical inhibitors share a highly conserved nine-amino acids binding loop motif CTP1SXPPXC (where P1 is the inhibitory active site, while X stands for various amino acids). They are natural controllers of plants' endogenous proteases, but they are also inhibitors of exogenous proteases present in microbials and insects. They are considered as plants' protective agents, as their elevated levels are observed during injury, presence of pathogens, or abiotic stress, i.a. Similar properties are observed for peptides isolated from amphibians' skin containing 11-amino acids disulfide-bridged loop CWTP1SXPPXPC. They are classified as Bowman-Birk like trypsin inhibitors (BBLTIs). These inhibitors are resistant to proteolysis and not toxic, and they are reported to be beneficial in the treatment of various pathological states. In this review, we summarize up-to-date research results regarding BBIs' and BBLTIs' inhibitory activity, immunomodulatory and anti-inflammatory activity, antimicrobial and insecticidal strength, as well as chemopreventive properties.

Monocyclic β-Lactam: A Review on Synthesis and Potential Biological Activities of a Multitarget Core

A monocyclic ring in their structure characterizes monobactams, a subclass of β-lactam antibiotics. Many of these compounds have a bactericidal mechanism of action and acts as penicillin and cephalosporins, interfering with bacterial cell wall biosynthesis. The synthesis of novel β-lactams is an emerging area of organic synthesis research due to the problem of increasing bacterial resistance to existing β -lactam antibiotics, and, in this way, new compounds have been presented with several structural modifications, aiming to improve biological activities. Among the biological activities studied, the most outstanding are antibacterial, antitubercular, anticholesterolemic, anticancer, antiinflammatory, antiviral, and anti-enzymatic, among others. This review explores the vast number of works related to monocyclic β-lactams, compounds of great importance in scientific research.

When alpha meets beta, mast cells get hyper

The evolutionary conservation of the catalytically inactive α-tryptase gene has remained a mystery. In this issue of JEM, Le et al. (2019. J. Exp. Med. https://doi.org/10.1084/jem.20190701) unveil the existence of a novel but natural tryptase, heteromeric α/β-tryptase, a critical mediator of α-tryptase-associated diseases.

IgE-mediated mast cell activation promotes inflammation and cartilage destruction in osteoarthritis

Osteoarthritis is characterized by articular cartilage breakdown, and emerging evidence suggests that dysregulated innate immunity is likely involved. Here, we performed proteomic, transcriptomic, and electron microscopic analyses to demonstrate that mast cells are aberrantly activated in human and murine osteoarthritic joint tissues. Using genetic models of mast cell deficiency, we demonstrate that lack of mast cells attenuates osteoarthritis in mice. Using genetic and pharmacologic approaches, we show that the IgE/FcεRI/Syk signaling axis is critical for the development of osteoarthritis. We find that mast cell-derived tryptase induces inflammation, chondrocyte apoptosis, and cartilage breakdown. Our findings demonstrate a central role for IgE-dependent mast cell activation in the pathogenesis of osteoarthritis, suggesting that targeting mast cells could provide therapeutic benefit in human osteoarthritis.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

An integrated molecular modeling approach for the tryptase monomer-curcuminoid recognition analysis: conformational and bioenergetic features

Human mast cell tryptase has been shown as an activating enzyme in matrix degradation process. The previous study suggest that tryptase either alone or in joining with activation of metalloproteinases, can associate in extra cellular matrix damage and the possible destruction of the basement membrane resulting in photoaging. Therefore the inhibition of tryptase activity is one of the most important therapeutic strategies against the photoaging. Curcumin has been shown to be a potential agent for preventing and/or treating the photoaging induced by UV radiation. However, the protective effect of curcumin against the photoaging through the tryptase inhibition is still inadequately understood. In this work, computational methods to characterize the structural framework and define the atomistic details of the determinants for the tryptase inhibition mechanism by curcuminoids were performed. By molecular docking, three putative binding models able to efficiently bind all curcuminoids were identified. Analysis of molecular dynamics simulations revealed that cyclocurcumin, curcumin glucuronide, and curcumin, the most effective inhibitors from the three models, modified significant tryptase monomer rigidity by binding in all the possible sites. The result of these binding events is the suppression of the functional enzymatic motions involving the binding of substrates to the catalytic site. On the basis of this finding may thus be beneficial for the development of new natural inhibitors for the therapeutic remedy of photoaging, targeting and modulating the activity of tryptase.

A Novel, Nonpeptidic, Orally Active Bivalent Inhibitor of Human β-Tryptase

β-Tryptase is released from mast cells upon degranulation in response to allergic and inflammatory stimuli. Human tryptase is a homotetrameric serine protease with 4 identical active sites directed toward a central pore. These active sites present an optimized scenario for the rational design of bivalent inhibitors, which bridge 2 adjacent active sites. Using (3-[1-acylpiperidin-4-yl]phenyl)methanamine as the pharmacophoric core and a disiloxane linker to span 2 active sites we have successfully produced a novel bivalent tryptase inhibitor, compound 1a, with a comparable profile to previously described inhibitors. Pharmacological properties of compound 1a were studied in a range of in vitro enzymic and cellular screening assays, and in vivo xenograft models. This non-peptide inhibitor of tryptase demonstrated superior activity (IC50 at 100 pmol/L tryptase = 1.82 nmol/L) compared to monomeric modes of inhibition. X-ray crystallography validated the dimeric mechanism of inhibition, and 1a demonstrated good oral bioavailability and efficacy in HMC-1 xenograft models. Furthermore, compound 1a demonstrated extremely slow off rates and high selectivity against-related proteases. This highly potent, orally bioavailable and selective inhibitor of human tryptase will be an invaluable tool in future studies to explore the therapeutic potential of attenuating the activity of this elusive target.

Target-Directed Self-Assembly of Homodimeric Drugs Against β-Tryptase

Tryptase, a serine protease released from mast cells, is implicated in many allergic and inflammatory disorders. Human tryptase is a donut-shaped tetramer with the active sites facing inward forming a central pore. Bivalent ligands spanning two active sites potently inhibit this configuration, but these large compounds have poor drug-like properties. To overcome some of these challenges, we developed self-assembling molecules, called coferons, which deliver a larger compound in two parts. Using a pharmacophoric core and reversibly binding linkers to span two active sites, we have successfully produced three novel homodimeric tryptase inhibitors. Upon binding to tryptase, compounds reassembled into flexible homodimers, with significant improvements in IC₅₀ (0.19 ± 0.08 μM) over controls (5.50 ± 0.09 μM), and demonstrate good activity in mast cell lines. These studies provide validation for this innovative technology that is especially well-suited for the delivery of dimeric drugs to modulate intracellular macromolecular targets.

Functional Proteomic Profiling of Secreted Serine Proteases in Health and Inflammatory Bowel Disease

While proteases are essential in gastrointestinal physiology, accumulating evidence indicates that dysregulated proteolysis plays a pivotal role in the pathophysiology of inflammatory bowel disease (IBD). Nonetheless, the identity of overactive proteases released by human colonic mucosa remains largely unknown. Studies of protease abundance have primarily investigated expression profiles, not taking into account their enzymatic activity. Herein we have used serine protease-targeted activity-based probes (ABPs) coupled with mass spectral analysis to identify active forms of proteases secreted by the colonic mucosa of healthy controls and IBD patients. Profiling of (Pro-Lys)-ABP bound proteases revealed that most of hyperactive proteases from IBD secretome are clustered at 28-kDa. We identified seven active proteases: the serine proteases cathepsin G, plasma kallikrein, plasmin, tryptase, chymotrypsin-like elastase 3 A, and thrombin and the aminopeptidase B. Only cathepsin G and thrombin were overactive in supernatants from IBD patient tissues compared to healthy controls. Gene expression analysis highlighted the transcription of genes encoding these proteases into intestinal mucosae. The functional ABP-targeted proteomic approach that we have used to identify active proteases in human colonic samples bears directly on the understanding of the role these enzymes may play in the pathophysiology of IBD.

Controlling Mast Cell Activation and Homeostasis: Work Influenced by Bill Paul That Continues Today

Mast cells are tissue resident, innate immune cells with heterogenous phenotypes tuned by cytokines and other microenvironmental stimuli. Playing a protective role in parasitic, bacterial, and viral infections, mast cells are also known for their role in the pathogenesis of allergy, asthma, and autoimmune diseases. Here, we review factors controlling mast cell activation, with a focus on receptor signaling and potential therapies for allergic disease. Specifically, we will discuss our work with FcεRI and FγR signaling, IL-4, IL-10, and TGF-β1 treatment, and Stat5. We conclude with potential therapeutics for allergic disease. Much of these efforts have been influenced by the work of Bill Paul. With many mechanistic targets for mast cell activation and different classes of therapeutics being studied, there is reason to be hopeful for continued clinical progress in this area.

Tryptase as a polyfunctional component of mast cells

Mast cells are haematopoietic cells that arise from pluripotent precursors of the bone marrow. They play immunomodulatory roles in both health and disease. When appropriately activated, mast cells undergo degranulation, and preformed granule compounds are rapidly released into the surroundings. In many cases, the effects that mast cells have on various inflammatory settings are closely associated with the enzymatic characteristics of tryptase, the main granule compound of mast cells. Tryptase degranulation is often linked with the development of an immune response, allergy, inflammation, and remodelling of tissue architecture. Tryptase also represents an informative diagnostic marker of certain diseases and a prospective target for pharmacotherapy. In this review, we discuss the current knowledge about mast cell tryptase as one of the mast cell secretome proteases. The main points of the reviewed publications are highlighted with our microscopic images of mast cell tryptases visualized using immunohistochemical staining.

Effect of tryptase inhibition on joint inflammation: a pharmacological and lentivirus-mediated gene transfer study

Background

Increasing evidences indicate that an unbalance between tryptases and their endogenous inhibitors, leading to an increased proteolytic activity, is implicated in the pathophysiology of rheumatoid arthritis. The aim of the present study was to evaluate the impact of tryptase inhibition on experimental arthritis.

Methods

Analysis of gene expression and regulation in the mouse knee joint was performed by RT-qPCR and in situ hybridization. Arthritis was induced in male C57BL/6 mice with mBSA/IL-1β. Tryptase was inhibited by two approaches: a lentivirus-mediated heterologous expression of the human endogenous tryptase inhibitor, sperm-associated antigen 11B isoform C (hSPAG11B/C), or a chronic treatment with the synthetic tryptase inhibitor APC366. Several inflammatory parameters were evaluated, such as oedema formation, histopathology, production of IL-1β, -6, -17A and CXCL1/KC, myeloperoxidase and tryptase-like activities.

Results

Spag11c was constitutively expressed in chondrocytes and cells from the synovial membrane in mice, but its expression did not change 7 days after the induction of arthritis, while tryptase expression and activity were upregulated. The intra-articular transduction of animals with the lentivirus phSPAG11B/C or the treatment with APC366 inhibited the increase of tryptase-like activity, the late phase of oedema formation, the production of IL-6 and CXCL1/KC. In contrast, neutrophil infiltration, degeneration of hyaline cartilage and erosion of subchondral bone were not affected.

Conclusions

Tryptase inhibition was effective in inhibiting some inflammatory parameters associated to mBSA/IL-1β-induced arthritis, notably late phase oedema formation and IL-6 production, but not neutrophil infiltration and joint degeneration. These results suggest that the therapeutic application of tryptase inhibitors to rheumatoid arthritis would be restrained to palliative care, but not as disease-modifying drugs. Finally, this study highlighted lentivirus-based gene delivery as an instrumental tool to study the relevance of target genes in synovial joint physiology and disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-017-1326-9) contains supplementary material, which is available to authorized users.

The role of tryptase and anti-type II collagen antibodies in the pathogenesis of idiopathic epiretinal membranes

Purpose

To investigate the pathogenesis of idiopathic epiretinal membrane (ERM) from a biochemical perspective, the relationships between ERM and tryptase activity, a serine protease, and the levels of anti-type II collagen (anti-IIC) antibodies in the serum.

Patients and methods

Vitreous samples for measurement of tryptase activity were obtained from 54 eyes of 54 patients who underwent a vitrectomy for vitreoretinal disease, ie, 14 eyes of 14 patients with idiopathic macular hole, 14 eyes of 14 patients with proliferative diabetic retinopathy (PDR), 13 eyes of 13 patients with ERM, and 13 eyes of 13 patients with rhegmatogenous retinal detachment (RRD). Tryptase activity was measured by spectrophotometry. Anti-IIC antibodies were measured in the serum obtained from 17 patients with ERM, eight patients who underwent cataract surgery, 12 patients with PDR, and nine patients with RRD. In these 46 patients, the anti-IIC antibodies were measured using a Human/Monkey Anti-Type I and Type II Collagen IgG Assay Kit.

Results

Vitreal tryptase activity (mean ± standard deviation [SD]) in macular hole, PDR, ERM, and RRD was 0.0146±0.0053, 0.0018±0.0018, 0.0166±0.0046, and 0.0117±0.0029 mU/mg protein, respectively. Vitreal tryptase activity was significantly higher in macular hole and ERM than in PDR and RRD (P<0.05, Fisher’s protected least significant difference). The serum levels of anti-IIC immunoglobulin G (IgG) antibody (mean ± SD) in ERM, cataract surgery, PDR, and RRD were 58.222±30.986, 34.890±18.165, 55.760±26.008, and 35.453±12.769 units/mL, respectively. The serum levels of anti-IIC IgG antibody were significantly higher in ERM and PDR than in cataract surgery and RRD (P<0.05, Fisher’s protected least significant difference, two-sided).

Conclusion

In the pathogenesis of ERM, increased vitreal tryptase activity may be involved in tissue fibrosis, and elevated serum anti-IIC antibodies may lead to an immune response at the vitreoretinal interface, thus resulting in membrane formation.

Mast cell proteases as pharmacological targets

Mast cells are rich in proteases, which are the major proteins of intracellular granules and are released with histamine and heparin by activated cells. Most of these proteases are active in the granule as well as outside of the mast cell when secreted, and can cleave targets near degranulating mast cells and in adjoining tissue compartments. Some proteases released from mast cells reach the bloodstream and may have far-reaching actions. In terms of relative amounts, the major mast cell proteases include the tryptases, chymases, cathepsin G, carboxypeptidase A3, dipeptidylpeptidase I/cathepsin C, and cathepsins L and S. Some mast cells also produce granzyme B, plasminogen activators, and matrix metalloproteinases. Tryptases and chymases are almost entirely mast cell-specific, whereas other proteases, such as cathepsins G, C, and L are expressed by a variety of inflammatory cells. Carboxypeptidase A3 expression is a property shared by basophils and mast cells. Other proteases, such as mastins, are largely basophil-specific, although human basophils are protease-deficient compared with their murine counterparts. The major classes of mast cell proteases have been targeted for development of therapeutic inhibitors. Also, a human β-tryptase has been proposed as a potential drug itself, to inactivate of snake venins. Diseases linked to mast cell proteases include allergic diseases, such as asthma, eczema, and anaphylaxis, but also include non-allergic diseases such as inflammatory bowel disease, autoimmune arthritis, atherosclerosis, aortic aneurysms, hypertension, myocardial infarction, heart failure, pulmonary hypertension and scarring diseases of lungs and other organs. In some cases, studies performed in mouse models suggest protective or homeostatic roles for specific proteases (or groups of proteases) in infections by bacteria, worms and other parasites, and even in allergic inflammation. At the same time, a clearer picture has emerged of differences in the properties and patterns of expression of proteases expressed in human mast cell subsets, and in humans versus other mammals. These considerations are influencing prioritization of specific protease targets for therapeutic inhibition, as well as options of pre-clinical models, disease indications, and choice of topical versus systemic routes of inhibitor administration.

A brief exposure to tryptase or thrombin potentiates fibrocyte differentiation in the presence of serum or serum amyloid p

A key question in both wound healing and fibrosis is the trigger for the initial formation of scar tissue. To help form scar tissue, circulating monocytes enter the tissue and differentiate into fibroblast-like cells called fibrocytes, but fibrocyte differentiation is strongly inhibited by the plasma protein serum amyloid P (SAP), and healthy tissues contain very few fibrocytes. In wounds and fibrotic lesions, mast cells degranulate to release tryptase, and thrombin mediates blood clotting in early wounds. Tryptase and thrombin are upregulated in wound healing and fibrotic lesions, and inhibition of these proteases attenuates fibrosis. We report that tryptase and thrombin potentiate human fibrocyte differentiation at biologically relevant concentrations and exposure times, even in the presence of concentrations of serum and SAP that normally completely inhibit fibrocyte differentiation. Fibrocyte potentiation by thrombin and tryptase is mediated by protease-activated receptors 1 and 2, respectively. Together, these results suggest that tryptase and thrombin may be an initial trigger to override SAP inhibition of fibrocyte differentiation to initiate scar tissue formation.

Allosteric control of βII-tryptase by a redox active disulfide bond

The S1A serine proteases function in many key biological processes such as development, immunity, and blood coagulation. S1A proteases contain a highly conserved disulfide bond (Cys(191)-Cys(220) in chymotrypsin numbering) that links two β-loop structures that define the rim of the active site pocket. Mast cell βII-tryptase is a S1A protease that is associated with pathological inflammation. In this study, we have found that the conserved disulfide bond (Cys(220)-Cys(248) in βII-tryptase) exists in oxidized and reduced states in the enzyme stored and secreted by mast cells. The disulfide bond has a standard redox potential of -301 mV and is stoichiometrically reduced by the inflammatory mediator, thioredoxin, with a rate constant of 350 m(-1) s(-1). The oxidized and reduced enzymes have different substrate specificity and catalytic efficiency for hydrolysis of both small and macromolecular substrates. These observations indicate that βII-tryptase activity is post-translationally regulated by an allosteric disulfide bond. It is likely that other S1A serine proteases are similarly regulated.

Therapeutic potential of the peptide leucine arginine as a new nonplant bowman-birk-like serine protease inhibitor

The peptide leucine arginine (pLR) belongs to a new class of cyclic peptides isolated from frog skin. Its primary sequence is similar to the reactive loop of plant Bowman-Birk inhibitors (BBI), and the recently discovered circular sunflower trypsin inhibitor-1 (SFTI-1). The conformational properties of pLR in solution were determined by NMR spectroscopy and revealed excellent structural similarity to BBI and SFTI-1. Moreover, pLR is a highly potent trypsin inhibitor, with Ki values in the nanomolar range, and, due to its small size, a potential inhibitor of the serine protease tryptase. Since tryptase plays a crucial role in the development of allergic airway inflammation, the therapeutic potential of pLR in a murine asthma model was investigated. Treatment of ovalbumin-sensitized mice with pLR during allergen challenge reduced the acute asthma phenotype. Most importantly, application even at the end of a long-lasting chronic asthma model decreased the development of chronic airway inflammation and tissue remodeling.

Marine macro- and microalgae as potential agents for the prevention of asthma: hyperresponsiveness and inflammatory subjects

Asthma is a variable disease and various factors are affected to increase the asthmatic symptoms and level of asthma control. It is believed that the cause for this disease is a combination of genetic and environmental factors. Numerous medications are available at present to treat this disease but it has been failed to control number of incidences successfully. Hence, recently many researchers have paid their interest to identify potential drugs from marine-based resources such as marine algae. In vitro and in vivo experiments have been conducted with extracts or compounds from algae and found that they showed significant activities against asthma. Accordingly, many marine macro- and microalgae have been reported to have potential to ameliorate the effect of asthma. However, detailed studies are needed in relation to identify the molecular mechanism of this disease to apply those marine resources against asthma effectively. In this chapter, an attempt has been taken to discuss the potential antiasthmatic activity of marine macro- and microalgae.

Anaphylaxis on reperfusion during liver transplantation with coagulopathy

We present a case in which anaphylaxis on hepatic reperfusion during liver transplantation presented only with hypotension and coagulopathy. There were no cutaneous manifestations or clinical features distinguishing anaphylaxis from postreperfusion syndrome. The recipient regularly consumed seafood, and the organ donor died of anaphylaxis to shellfish. The trigger for anaphylaxis was postulated to be passive transfer of immunoglobulin to the recipient. Anesthesiologists should be notified of donor factors to anticipate anaphylaxis. In this report, we discuss coagulopathy of anaphylaxis and contrast it with disseminated intravascular coagulation.

Dimerization of β-tryptase inhibitors, does it work for both basic and neutral P1 groups?

The tetrameric folding of β-tryptase and the pair-wise distribution of its substrate binding sites offer a unique opportunity for development of inhibitors that span two adjacent binding sites. A series of dimeric inhibitors with two basic P1 moieties was discovered using this design strategy and exhibited tight-binder characteristics. Using the same strategy, an attempt was made to design and synthesize dimeric inhibitors with two neutral-P1 groups in hope to exploit the dimeric binding mode to achieve a starting point for further optimization. The unsuccessful attempt, however, demonstrated the important role played by Ala190 in neutral-P1 binding and casted further doubt on the possibility of developing neutral-P1 inhibitors for β-tryptase.

Oxidative folding and structural analyses of a Kunitz-related inhibitor and its disulfide intermediates: functional implications

Tick-derived protease inhibitor (TdPI) is a tight-binding Kunitz-related inhibitor of human tryptase β with a unique structure and disulfide-bond pattern. Here we analyzed its oxidative folding and reductive unfolding by chromatographic and disulfide analyses of acid-trapped intermediates. TdPI folds through a stepwise generation of heterogeneous populations of one-disulfide, two-disulfide, and three-disulfide intermediates, with a major accumulation of the nonnative three-disulfide species IIIa. The rate-limiting step of the process is disulfide reshuffling within the three-disulfide population towards a productive intermediate that oxidizes directly into the native four-disulfide protein. TdPI unfolds through a major accumulation of the native three-disulfide species IIIb and the subsequent formation of two-disulfide and one-disulfide intermediates. NMR characterization of the acid-trapped and further isolated IIIa intermediate revealed a highly disordered conformation that is maintained by the presence of the disulfide bonds. Conversely, the NMR structure of IIIb showed a native-like conformation, with three native disulfide bonds and increased flexibility only around the two free cysteines, thus providing a molecular basis for its role as a productive intermediate. Comparison of TdPI with a shortened variant lacking the flexible prehead and posthead segments revealed that these regions do not contribute to the protein conformational stability or the inhibition of trypsin but are important for both the initial steps of the folding reaction and the inhibition of tryptase β. Taken together, the results provide insights into the mechanism of oxidative folding of Kunitz inhibitors and pave the way for the design of TdPI variants with improved properties for biomedical applications.

Intestinal epithelial cells express galectin-9 in patients with food allergy that plays a critical role in sustaining allergic status in mouse intestine

BACKGROUND AND AIMS

Mechanisms in sustaining the allergic hypersensitivity status in the body are unclear. Galectin-9 (Gal-9) has strong immune regulatory capacity. The present study aims to elucidate the role of Gal-9 in sustaining allergic status in the intestine.

METHODS

Duodenal biopsies were obtained from 20 patients with peptic ulcer and food allergy (FA). The expression of Gal-9 in intestinal tissue was examined at both protein level and mRNA level. Two coculture systems with intestinal epithelial cells (IEC) and mast cells, or dendritic cells (DC) and T cells were established to investigate the source of Gal-9 in the intestine and the mechanism by which Gal-9 modulated DC's phenotyping and sustained the T helper 2 polarization.

RESULTS

Normal IEC showed mild expression of Gal-9 that was markedly enhanced in patients with FA. Mast cells had the capability to induce IEC to produce Gal-9 via releasing tryptase that activated the proteinase-activated receptor 2 on IEC. Gal-9 activated DC to produce TIM4 (T-cell immunoglobulin mucin domain) via ligating TIM3 on DC via activating the cyclic guanosine monophosphate (cGMP) pathway. In a mouse FA model, blocking Gal-9 inhibited the allergic hypersensitivity status and the antigen-specific Th2 response in the intestine.

CONCLUSIONS

IEC-derived Gal-9 contributes to sustaining the allergic status in the intestine.

Mast cell proteases as protective and inflammatory mediators

Proteases are the most abundant class of proteins produced by mast cells. Many of these are stored in membrane-enclosed intracellular granules until liberated by degranulating stimuli, which include cross-linking of high affinity IgE receptor F(c)εRI by IgE bound to multivalent allergen. Understanding and separating the functions of the proteases is important because expression differs among mast cells in different tissue locations. Differences between laboratory animals and humans in protease expression also influence the degree of confidence with which results obtained in animal models of mast cell function can be extrapolated to humans. The inflammatory potential of mast cell proteases was the first aspect of their biology to be explored and has received the most attention, in part because some of them, notably tryptases and chymases, are biomarkers of local and systemic mast cell degranulation and anaphylaxis. Although some of the proteases indeed augment allergic inflammation and are potential targets for inhibition to treat asthma and related allergic disorders, they are protective and even anti-inflammatory in some settings. For example, mast cell tryptases may protect from serious bacterial lung infections and may limit the "rubor" component of inflammation caused by vasodilating neuropeptides in the skin. Chymases help to maintain intestinal barrier function and to expel parasitic worms and may support blood pressure during anaphylaxis by generating angiotensin II. In other life-or-death examples, carboxypeptidase A3 and other mast cell peptidases limit systemic toxicity of endogenous peptideslike endothelin and neurotensin during septic peritonitis and inactivate venom-associated peptides. On the other hand, mast cell peptidase-mediated destruction of protective cytokines, like IL-6, can enhance mortality from sepsis. Peptidases released from mast cells also influence nonmast cell proteases, such as by activating matrix metalloproteinase cascades, which are important in responses to infection and resolution of tissue injury. Overall, mast cell proteases have a variety of roles, inflammatory and anti-inflammatory, protective and deleterious, in keeping with the increasingly well-appreciated contributions of mast cells in allergy, tissue homeostasis and innate immunity.

Mast cells squeeze the heart and stretch the gird: their role in atherosclerosis and obesity

Mast cells are crucial for the development of allergic and anaphylactic reactions, but they are also involved in acquired and innate immunity. Increasing evidence now implicates mast cells in inflammatory diseases through activation by non-allergic triggers such as neuropeptides and cytokines. This review discusses how mast cells contribute to the inflammatory processes associated with coronary artery disease and obesity. Animal models indicate that mast cells, through the secretion of various vasoactive mediators, cytokines and proteinases, contribute to coronary plaque progression and destabilization, as well as to diet-induced obesity and diabetes. Understanding how mast cells participate in these inflammatory processes could help in the development of unique inhibitors with novel therapeutic applications for these diseases, which constitute the greatest current threat to global human health and welfare.

Airway remodeling in asthma: new mechanisms and potential for pharmacological intervention

The chronic inflammatory response within the airways of asthmatics is associated with structural changes termed airway remodeling. This remodeling process is a key feature of severe asthma. The 5-10% of patients with a severe form of the disease account for the higher morbidity and health costs related to asthma. Among the histopathological characteristics of airway remodeling, recent reports indicate that the increased mass of airway smooth muscle (ASM) plays a critical role. ASM cell proliferation in severe asthma implicates a gallopamil-sensitive calcium influx and the activation of calcium-calmodulin kinase IV leading to enhanced mitochondrial biogenesis through the activation of various transcription factors (PGC-1α, NRF-1 and mt-TFA). The altered expression and function of sarco/endoplasmic reticulum Ca(2+) pump could play a role in ASM remodeling in moderate to severe asthma. Additionally, aberrant communication between an injured airway epithelium and ASM could also contribute to disease severity. Airway remodeling is insensitive to corticosteroids and anti-leukotrienes whereas the effect of monoclonal antibodies (the anti-IgE omalizumab, the anti-interleukin-5 mepolizumab or anti-tumor necrosis factor-alpha) remains to be investigated. This review focuses on potential new therapeutic strategies targeting ASM cells, especially Ca(2+) and mitochondria-dependent pathways.

Protease inhibitors in respiratory disease: focus on asthma and chronic obstructive pulmonary disease

Respiratory diseases, such as asthma and chronic obstructive pulmonary disease (COPD), are a major health burden on society and current treatment modalities for these diseases have not significantly changed over the past 40 years. The only major pharmacological advancement for the treatment of these diseases has been to increase the duration of action of bronchodilators (asthma: salmeterol; COPD: tiotropium bromide) and glucocorticosteroids (asthma: fluticasone propionate) and, increasingly, to formulate these agents in the same delivery device. Despite our increasing understanding of the cell and molecular biology of these diseases, the development of novel treatments remains beyond the reach of the scientific community. Proteases are a family of proteins with diverse biological activity, which are found in abundance within the airways of asthma and COPD, and have been implicated in the pathogenesis of these diseases. The targeting of proteases, including mast cell tryptase, neutrophil elastase and matrix metalloprotease with low-molecular-weight inhibitors, has highlighted the potential role of these enzymes in mediating certain aspects of the disease process in preclinical studies. Several challenges remain regarding the development of protease inhibitors, including the synthesis of highly potent and specific inhibitors, and target validation in man.

Engineered cystine knot miniproteins as potent inhibitors of human mast cell tryptase beta

Here we report the design, chemical and recombinant synthesis, and functional properties of a series of novel inhibitors of human mast cell tryptase beta, a protease of considerable interest as a therapeutic target for the treatment of allergic asthma and inflammatory disorders. These inhibitors are derived from a linear variant of the cyclic cystine knot miniprotein MCoTI-II, originally isolated from the seeds of Momordica cochinchinensis. A synthetic cyclic miniprotein that bears additional positive charge in the loop connecting the N- and C-termini inhibits all monomers of the tryptase beta tetramer with an overall equilibrium dissociation constant K(i) of 1 nM and thus is one of the most potent proteinaceous inhibitors of tryptase beta described to date. These cystine knot miniproteins may therefore become valuable scaffolds for the design of a new generation of tryptase inhibitors.

Physiological and pathophysiological functions of intestinal mast cells

The normal gastrointestinal (GI) mucosa is equipped with mast cells that account for 2-3% of lamina propria cells under normal conditions. Mast cells are generally associated with allergic disease, and indeed, food allergy that manifests in the GI tract is usually mast cell dependent. On the other hand, mast cells have a number of physiological functions in the GI tract, namely regulatory functions such as control of blood flow and coagulation, smooth muscle contraction and peristalsis, and secretion of acid, electrolytes, and mucus by epithelial cells. One of the most intriguing functions of intestinal mast cells is their role in host defense against microbes like bacteria, viruses, or parasites. Mast cells recognize microbes by antibody-dependent mechanisms and through pattern-recognition receptors. They direct the subsequent immune response by attracting both granulocytes and lymphocytes to the site of challenge via paracrine cytokine release. Moreover, mast cells initiate, by releasing proinflammatory mediators, innate defense mechanisms such as enhanced epithelial secretion, peristalsis, and alarm programs of the enteric nervous This initiation can occur in response to a primary contact to the microbe or other danger signals, but becomes much more effective if the triggering antigen reappears and antibodies of the IgE or IgG type have been generated in the meantime by the specific immune system. Thus, mast cells operate at the interface between innate and adaptive immune responses to enhance the defense against pathogens and, most likely, the commensal flora. In this respect, it is important to note that mast cells are directly involved in controlling the function of the intestinal barrier that turned out to be a crucial site for the development of infectious and immune-mediated diseases. Hence, intestinal mast cells perform regulatory functions to maintain tissue homeostasis, they are involved in host defense mechanisms against pathogens, and they can induce allergy once they are sensitized against foreign antigens. The broad spectrum of functions makes mast cells a fascinating target for future pharmacological or nutritional interventions.

Mast cells promote airway smooth muscle cell differentiation via autocrine up-regulation of TGF-beta 1

Asthma is a major cause of morbidity and mortality worldwide. It is characterized by airway dysfunction and inflammation. A key determinant of the asthma phenotype is infiltration of airway smooth muscle bundles by activated mast cells. We hypothesized that interactions between these cells promotes airway smooth muscle differentiation into a more contractile phenotype. In vitro coculture of human airway smooth muscle cells with beta-tryptase, or mast cells with or without IgE/anti-IgE activation, increased airway smooth muscle-derived TGF-beta1 secretion, alpha-smooth muscle actin expression and agonist-provoked contraction. This promotion to a more contractile phenotype was inhibited by both the serine protease inhibitor leupeptin and TGF-beta1 neutralization, suggesting that the observed airway smooth muscle differentiation was driven by the autocrine release of TGF-beta1 in response to activation by mast cell beta-tryptase. Importantly, in vivo we found that in bronchial mucosal biopsies from asthmatics the intensity of alpha-smooth muscle actin expression was strongly related to the number of mast cells within or adjacent to an airway smooth muscle bundle. These findings suggest that mast cell localization in the airway smooth muscle bundle promotes airway smooth muscle cell differentiation into a more contractile phenotype, thus contributing to the disordered airway physiology that characterizes asthma.

Potent, nonpeptide inhibitors of human mast cell tryptase. Synthesis and biological evaluation of novel spirocyclic piperidine amide derivatives

We have explored a series of spirocyclic piperidine amide derivatives (5) as tryptase inhibitors. Thus, 4 (JNJ-27390467) was identified as a potent, selective tryptase inhibitor with oral efficacy in two animal models of airway inflammation (sheep and guinea pig asthma models). An X-ray co-crystal structure of 4 x tryptase revealed a hydrophobic pocket in the enzyme's active site, which is induced by the phenylethynyl group and is comprised of amino acid residues from two different monomers of the tetrameric protein.

The mast cell and allergic diseases: role in pathogenesis and implications for therapy

Mast cells have long been recognized for their role in the genesis of allergic inflammation; and more recently for their participation in innate and acquired immune responses. Mast cells reside within tissues including the skin and mucosal membranes, which interface with the external environment; as well as being found within vascularized tissues next to nerves, blood vessels and glandular structures. Mast cells have the capability of reacting both within minutes and over hours to specific stimuli, with local and systemic effects. Mast cells express the high affinity IgE receptor (FcepsilonRI) and upon aggregation of FcepsilonRI by allergen-specific IgE, mast cells release and generate biologically active preformed and newly synthesized mediators which are involved in many aspects of allergic inflammation. While mast cells have been well documented to be essential for acute allergic reactions, more recently the importance of mast cells in reacting through pattern recognition receptors in innate immune responses has become recognized. Moreover, as our molecular understanding of the mast cell has evolved, novel targets for modulation have been identified with promising therapeutic potential.

Inhibition of the Activation of Multiple Serine Proteases with a Cathepsin C Inhibitor Requires Sustained Exposure to Prevent Pro-enzyme Processing

Cathepsin C is a cysteine protease required for the activation of several pro-inflammatory serine proteases and, as such, is of interest as a therapeutic target. In cathepsin C-deficient mice and humans, the N-terminal processing and activation of neutrophil elastase, cathepsin G, and proteinase-3 is abolished and is accompanied by a reduction of protein levels. Pharmacologically, the consequence of cathepsin C inhibition on the activation of these serine proteases has not been described, due to the lack of stable and non-toxic inhibitors and the absence of appropriate experimental cell systems. Using novel reversible peptide nitrile inhibitors of cathepsin C, and cell-based assays with U937 and EcoM-G cells, we determined the effects of pharmacological inhibition of cathepsin C on serine protease activity. We show that indirect and complete inhibition of neutrophil elastase, cathepsin G, and proteinase-3 is achievable in intact cells with selective and non-cytotoxic cathepsin C inhibitors, at concentrations approximately 10-fold higher than those required to inhibit purified cathepsin C. The concentration of inhibitor needed to block processing of these three serine proteases was similar, regardless of the cell system used. Importantly, cathepsin C inhibition must be sustained to maintain serine protease inhibition, because removal of the reversible inhibitors resulted in the activation of pro-enzymes in intact cells. These findings demonstrate that near complete inhibition of multiple serine proteases can be achieved with cathepsin C inhibitors and that cathepsin C inhibition represents a viable but challenging approach for the treatment of neutrophil-based inflammatory diseases.

A tick protein with a modified Kunitz fold inhibits human tryptase

TdPI, a tick salivary gland product related to Kunitz/BPTI proteins is a potent inhibitor of human beta-tryptase. Kinetic assays suggest that three of the four catalytic sites of tryptase are blocked by TdPI, and that the inhibition of one of these involves a peptide flanking the Kunitz head. In the course of the inhibition, tryptase cleaves TdPI at several positions. Crystal structures of the TdPI head, on its own and in complex with trypsin, reveal features that are not found in classical Kunitz/BPTI proteins and suggest the mode of interaction with tryptase. The loop of TdPI connecting the beta-sheet with the C-terminal alpha-helix is shortened, the disulphide-bridge pattern altered and N and C termini separated to produce a highly pointed molecule capable of penetrating the cramped active sites of tryptase. TdPI accumulates in the cytosolic granules of mast cells, presumably suppressing inflammation in the host animal's skin by tryptase inhibition.

Therapeutic potential of sulfamides as enzyme inhibitors

Sulfamide, a quite simple molecule incorporating the sulfonamide functionality, widely used by medicinal chemists for the design of a host of biologically active derivatives with pharmacological applications, may give rise to at least five types of derivatives, by substituting one to four hydrogen atoms present in it, which show specific biological activities. Recently, some of these compounds started to be exploited for the design of many types of therapeutic agents. Among the enzymes for which sulfamide-based inhibitors were designed, are the carbonic anhydrases (CAs), a large number of proteases belonging to the aspartic protease (HIV-1 protease, gamma-secretase), serine protease (elastase, chymase, tryptase, and thrombin among others), and metalloprotease (carboxypeptidase A (CPA) and matrix metalloproteinases (MMP)) families. Some steroid sulfatase (STS) and protein tyrosine phosphatase inhibitors belonging to the sulfamide class of derivatives have also been reported. In all these compounds, many of which show low nanomolar affinity for the target enzymes for which they have been designed, the free or substituted sulfamide moiety plays important roles for the binding of the inhibitor to the active site cavity, either by directly coordinating to a metal ion found in some metalloenzymes (CAs, CPA, STS), usually by means of one of the nitrogen atoms present in the sulfamide motif, or as in the case of the cyclic sulfamides acting as HIV protease inhibitors, interacting with the catalytically critical aspartic acid residues of the active site by means of an oxygen atom belonging to the HN-SO2-NH motif, which substitutes a catalytically essential water molecule. In other cases, the sulfamide moiety is important for inducing desired physico-chemical properties to the drug-like compounds incorporating it, such as enhanced water solubility, better bioavailability, etc., because of the intrinsic properties of this highly polarized moiety when attached to an organic scaffold. This interesting motif is thus of great value for the design of pharmacological agents with a lot of applications.

Inflammatory conditions increase expression of protease-activated receptor-2 by human airway smooth muscle cells in culture

The protease-activated receptor-2 (PAR-2) has been implicated in airway inflammation and bronchial hyperresponsiveness. We wondered whether inflammatory conditions may upregulate PAR-2 expression by the human airway smooth muscle. To do so, we treated human airway smooth muscle cells (HASMC) in primary culture with interleukin-1beta (IL-1beta), a pro-inflammatory and asthma-associated cytokine. Cells were starved for 24 h and incubated with or without IL-1beta. Online fluorescent polymerase chain reaction after reverse transcription quantified PAR-2 mRNA, and Western blotting measured PAR-2 protein expression. PAR-2 was constitutively expressed by HASMC in primary culture, and IL-1beta (10 U/mL) time dependently elevated PAR-2 mRNA with a maximum of 4.7-fold after 1.5 h (P < 0.01), and PAR-2 protein expression with a maximum of 1.5-fold after 24 h (P < 0.01). The concentration dependence of the IL-1beta effect (0.1-30 U/mL) confirmed a maximal increase of PAR-2 expression at 10 U/mL. Our study clearly shows that IL-1beta upregulates PAR-2 mRNA and protein expression by HASMC in culture. This increased expression of PAR-2 in inflammatory conditions may have functional consequences in the bronchial dysfunction of asthmatic airways.

New targets for modifying mast cell activation in asthma

Mast cells play a central role in innate immunity and in orchestrating the asthmatic response. Current medication relies on beta-agonists to relieve bronchoconstriction and steroids to reduce inflammation. However, recently drugs such as leukotriene-receptor antagonists and anti-immunoglobulin E have come on to the market. In this paper, a number of potential targets for modifying mast cell activation in asthma are reviewed. Some are already under study, including clinical trials (eg, tryptase inhibitors); others are more speculative (eg, inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity). In each case, where data are available, the action of the agents on human lung mast cells is described.

Novel, potent, selective, and orally bioavailable human βII-tryptase inhibitors

The synthesis of novel [1,2,4]oxadiazoles and their structure-activity relationship (SAR) for the inhibition of tryptase and related serine proteases is presented. Elaboration of the P'-side afforded potent, selective, and orally bioavailable tryptase inhibitors.

Design and synthesis of selective keto-1,2,4-oxadiazole-based tryptase inhibitors

Using a scaleable, directed library approach based on orthogonally protected advanced intermediates, we have prepared a series of potent keto-1,2,4-oxadiazoles designed to explore the P(2) binding pocket of human mast cell tryptase, while building in a high degree of selectivity over human trypsin and other serine proteases.

Airway remodelling in asthma: current understanding and implications for future therapies

Airway remodelling refers to the structural changes that occur in the airway wall in asthma. These include epithelial hyperplasia and metaplasia, subepithelial fibrosis, muscle cell hyperplasia and angiogenesis. These structural changes result in thickening of the airway wall, airway hyperresponsiveness (AHR), and a progressive irreversible loss of lung function. The precise sequence of events that take place during the remodelling process and the mechanisms regulating these changes remain poorly understood. It is thought that airway remodelling is initiated and promoted by repeated episodes of allergic inflammation that damage the surface epithelium of the airway. However, other mechanisms are also likely to contribute to this process. Moreover, the interrelationship between airway remodelling, inflammation and AHR has not been clearly defined. Currently, there are no effective treatments that halt or reverse the changes of airway remodelling and its effects on lung function. Glucocorticoids have been unable to eliminate the progression of remodelling changes and there is limited evidence of a beneficial effect from other available therapies. The search for novel therapies that can directly target individual components of the remodelling process should be made a priority. In this review, we describe the current understanding of the airway remodelling process and the mechanisms regulating its development. The impact of currently available asthma therapies on airway remodelling is also discussed.

X-ray structures of free and leupeptin-complexed human alphaI-tryptase mutants: indication for an alpha-->beta-tryptase transition

Tryptases alpha and beta are trypsin-like serine proteinases expressed in large amounts by mast cells. Beta-tryptase is a tetramer that has enzymatic activity, but requires heparin binding to maintain functional and structural stability, whereas alpha-tryptase has little, if any, enzymatic activity but is a stable tetramer in the absence of heparin. As shown previously, these differences can be mainly attributed to the different conformations of the 214-220 segment. Interestingly, the replacement of Asp216 by Gly, which is present in beta-tryptase, results in enzymatically active but less stable alpha-tryptase mutants. We have solved the crystal structures of both the single (D216G) and the double (K192Q/D216G) mutant forms of recombinant human alphaI-tryptase in complex with the peptide inhibitor leupeptin, as well as the structure of the non-inhibited single mutant. The inhibited mutants exhibited an open functional substrate binding site, while in the absence of an inhibitor, the open (beta-tryptase-like) and the closed (alpha-tryptase-like) conformations were present simultaneously. This shows that both forms are in a two-state equilibrium, which is influenced by the residues in the vicinity of the active site and by inhibitor/substrate binding. Novel insights regarding the observed stability differences as well as a potential proteolytic activity of wild-type alpha-tryptase, which may possess a cryptic active site, are discussed.