Generation of active myeloid and lymphoid granule serine proteases requires processing by the granule thiol protease dipeptidyl peptidase I

Human Neutrophil Elastase: Characterization of Intra- vs. Extracellular Inhibition

Neutrophil elastase (HNE), like other members of the so-called GASPIDs (Granule-Associated Serine Peptidases of Immune Defense), is activated during protein biosynthesis in myeloid precursors and stored enzymatically active in cytoplasmic granules of resting neutrophils until secreted at sites of host defense and inflammation. Inhibitors thus could bind to the fully formed active site of the protease intracellularly in immature progenitors, in circulating neutrophils, or to HNE secreted into the extracellular space. Here, we have compared the ability of a panel of diverse inhibitors to inhibit HNE in the U937 progenitor cell line, in human blood-derived neutrophils, and in solution. Most synthetic inhibitors and, surprisingly, even a small naturally occurring proteinaceous inhibitor inhibit HNE intracellularly, but the extent and dynamics differ markedly from classical enzyme kinetics describing extracellular inhibition. Intracellular inhibition of HNE potentially affects neutrophil functions and has side effects, but it avoids competition of inhibitors with extracellular substrates that limit its efficacy. As both intra- and extracellular inhibition have advantages and disadvantages, the quantification of intracellular inhibition, in addition to classical enzyme kinetics, will aid the design of novel, clinically applicable HNE inhibitors with targeted sites of action.

Pharmacologic inhibition of dipeptidyl peptidase 1 (cathepsin C) does not block in vitro granzyme-mediated target cell killing by CD8 T or NK cells

Recently developed small-molecule inhibitors of the lysosomal protease dipeptidyl peptidase 1 (DPP1), also known as cathepsin C (CatC), can suppress suppurative inflammation in vivo by blocking the processing of zymogenic (pro-) forms of neutrophil serine proteases (NSPs), including neutrophil elastase, proteinase 3, and cathepsin G. DPP1 also plays an important role in activating granzyme serine proteases that are expressed by cytotoxic T lymphocytes (CTL) and natural killer (NK) cells. Therefore, it is critical to determine whether DPP1 inhibition can also cause off-target suppression of CTL/NK-cell-mediated killing of virus-infected or malignant cells. Herein, we demonstrate that the processing of human granzymes A and B, transitioning from zymogen to active proteases, is not solely dependent on DPP1. Thus, the killing of target cells by primary human CD8+ T cells, NK cells, and gene-engineered anti-CD19 CAR T cells was not blocked in vitro even after prior exposure to high concentrations of the reversible DPP1 inhibitor brensocatib. Consistent with this observation, the turnover of model granzyme A/B peptide substrates in the human CTL/NK cell lysates was not significantly reduced by brensocatib. In contrast, preincubation with brensocatib almost entirely abolished (>90%) both the cytotoxic activity of mouse CD8+ T cells and granzyme substrate turnover. Overall, our finding that the effects of DPP1 inhibition on human cytotoxic lymphocytes are attenuated in comparison to those of mice indicates that granzyme processing/activation pathways differ between mice and humans. Moreover, the in vitro data suggest that human subjects treated with reversible DPP1 inhibitors, such as brensocatib, are unlikely to experience any appreciable deficits in CTL/NK-cell-mediated immunities.

Neutrophil-Derived Proteases in Lung Inflammation: Old Players and New Prospects

Neutrophil-derived proteases are critical to the pathology of many inflammatory lung diseases, both chronic and acute. These abundant enzymes play roles in key neutrophil functions, such as neutrophil extracellular trap formation and reactive oxygen species release. They may also be released, inducing tissue damage and loss of tissue function. Historically, the neutrophil serine proteases (NSPs) have been the main subject of neutrophil protease research. Despite highly promising cell-based and animal model work, clinical trials involving the inhibition of NSPs have shown mixed results in lung disease patients. As such, the cutting edge of neutrophil-derived protease research has shifted to proteases that have had little-to-no research in neutrophils to date. These include the cysteine and serine cathepsins, the metzincins and the calpains, among others. This review aims to outline the previous work carried out on NSPs, including the shortcomings of some of the inhibitor-orientated clinical trials. Our growing understanding of other proteases involved in neutrophil function and neutrophilic lung inflammation will then be discussed. Additionally, the potential of targeting these more obscure neutrophil proteases will be highlighted, as they may represent new targets for inhibitor-based treatments of neutrophil-mediated lung inflammation.

The mast cells - Cytokines axis in Autism Spectrum Disorder

Autism Spectrum Disorder (ASD) is a neurodevelopmental disturbance, diagnosed in early childhood. It is associated with varying degrees of dysfunctional communication and social skills, repetitive and stereotypic behaviors. Regardless of the constant increase in the number of diagnosed patients, there are still no established treatment schemes in global practice. Many children with ASD have allergic symptoms, often in the absence of mast cell (MC) positive tests. Activation of MCs may release molecules related to inflammation and neurotoxicity, which contribute to the pathogenesis of ASD. The aim of the present paper is to enrich the current knowledge regarding the relationship between MCs and ASD by providing PPI network analysis-based data that reveal key molecules and immune pathways associated with MCs in the pathogenesis of autism. Network and enrichment analyzes were performed using receptor information and secreted molecules from activated MCs identified in ASD patients. Our analyses revealed cytokines and key marker molecules for MCs degranulation, molecular pathways of key mediators released during cell degranulation, as well as various receptors. Understanding the relationship between ASD and the activation of MCs, as well as the involved molecules and interactions, is important for elucidating the pathogenesis of ASD and developing effective future treatments for autistic patients by discovering new therapeutic target molecules.

Cysteine Cathepsins as Therapeutic Targets in Immune Regulation and Immune Disorders

Cysteine cathepsins, as the most abundant proteases found in the lysosomes, play a vital role in several processes-such as protein degradation, changes in cell signaling, cell morphology, migration and proliferation, and energy metabolism. In addition to their lysosomal function, they are also secreted and may remain functional in the extracellular space. Upregulation of cathepsin expression is associated with several pathological conditions including cancer, neurodegeneration, and immune-system dysregulation. In this review, we present an overview of cysteine-cathepsin involvement and possible targeting options for mitigation of aberrant function in immune disorders such as inflammation, autoimmune diseases, and immune response in cancer.

Multiparameter Longitudinal Imaging of Immune Cell Activity in Chimeric Antigen Receptor T Cell and Checkpoint Blockade Therapies

Longitudinal multimodal imaging presents unique opportunities for noninvasive surveillance and prediction of treatment response to cancer immunotherapy. In this work we first designed a novel granzyme B activated self-assembly small molecule, G-SNAT, for the assessment of cytotoxic T lymphocyte mediated cancer cell killing. G-SNAT was found to specifically detect the activity of granzyme B within the cytotoxic granules of activated T cells and engaged cancer cells in vitro. In lymphoma tumor-bearing mice, the retention of cyanine 5 labeled G-SNAT-Cy5 correlated to CAR T cell mediated granzyme B exocytosis and tumor eradication. In colorectal tumor-bearing transgenic mice with hematopoietic cells expressing firefly luciferase, longitudinal bioluminescence and fluorescence imaging revealed that after combination treatment of anti-PD-1 and anti-CTLA-4, the dynamics of immune cell trafficking, tumor infiltration, and cytotoxic activity predicted the therapeutic outcome before tumor shrinkage was evident. These results support further development of G-SNAT for imaging early immune response to checkpoint blockade and CAR T-cell therapy in patients and highlight the utility of multimodality imaging for improved mechanistic insights into cancer immunotherapy.

Granzyme B in Epithelial Barrier Dysfunction and Related Skin Diseases

The predominant function of the skin is to serve as a barrier - to protect against external insults and to prevent water loss. Junctional and structural proteins in the stratum corneum, the outermost layer of the epidermis, are critical to the integrity of the epidermal barrier as it balances ongoing outward migration, differentiation, and desquamation of keratinocytes in the epidermis. As such, epidermal barrier function is highly susceptible to upsurges of proteolytic activity in the stratum corneum and epidermis. Granzyme B is a serine protease scarce in healthy tissues but present at high levels in tissues encumbered by chronic inflammation. Discovered in the 1980s, Granzyme B is currently recognized for its intracellular roles in immune cell-mediated targeted apoptosis as well as extracellular roles in inflammation, chronic injuries, tissue remodeling, and processing of cytokines, matrix proteins, and autoantigens. Increasing evidence has emerged in recent years supporting a role for Granzyme B in promoting barrier dysfunction in the epidermis by direct cleavage of barrier proteins and eliciting immunoreactivity. Likewise, Granzyme B contributes to impaired epithelial function of the airways, retina, gut and vessels. In the present review, the role of Granzyme B in cutaneous epithelial dysfunction is discussed in the context of specific conditions with an overview of underlying mechanisms as well as utility of current experimental and therapeutic inhibitors.

Role of Serine Proteases at the Tumor-Stroma Interface

During tumor development, invasion and metastasis, the intimate interaction between tumor and stroma shapes the tumor microenvironment and dictates the fate of tumor cells. Stromal cells can also influence anti-tumor immunity and response to immunotherapy. Understanding the molecular mechanisms that govern this complex and dynamic interplay, thus is important for cancer diagnosis and therapy. Proteolytic enzymes that are expressed and secreted by both cancer and stromal cells play important roles in modulating tumor-stromal interaction. Among, several serine proteases such as fibroblast activation protein, urokinase-type plasminogen activator, kallikrein-related peptidases, and granzymes have attracted great attention owing to their elevated expression and dysregulated activity in the tumor microenvironment. This review highlights the role of serine proteases that are mainly derived from stromal cells in tumor progression and associated theranostic applications.

Cathepsin C inhibitors as anti-inflammatory drug discovery: Challenges and opportunities

Cathepsin C, an important lysosomal cysteine protease, mediates the maturation process of neutrophil serine proteases, and participates in the inflammation and immune regulation process associated with polymorphonuclear neutrophils. Therefore, cathepsin C is considered to be an attractive target for treating inflammatory diseases. With INS1007 (trade name: brensocatib) being granted a breakthrough drug designation by FDA for the treatment of Adult Non-cystic Fibrosis Bronchiectasis and Coronavirus Disease 2019, the development of cathepsin C inhibitor will attract attentions from medicinal chemists in the future soon. Here, we summarized the research results of cathepsin C as a therapeutic target, focusing on the development of cathepsin C inhibitor, and provided guidance and reference opinions for the upcoming development boom of cathepsin C inhibitor.

Patients with COVID-19: in the dark-NETs of neutrophils

SARS-CoV-2 infection poses a major threat to the lungs and multiple other organs, occasionally causing death. Until effective vaccines are developed to curb the pandemic, it is paramount to define the mechanisms and develop protective therapies to prevent organ dysfunction in patients with COVID-19. Individuals that develop severe manifestations have signs of dysregulated innate and adaptive immune responses. Emerging evidence implicates neutrophils and the disbalance between neutrophil extracellular trap (NET) formation and degradation plays a central role in the pathophysiology of inflammation, coagulopathy, organ damage, and immunothrombosis that characterize severe cases of COVID-19. Here, we discuss the evidence supporting a role for NETs in COVID-19 manifestations and present putative mechanisms, by which NETs promote tissue injury and immunothrombosis. We present therapeutic strategies, which have been successful in the treatment of immunο-inflammatory disorders and which target dysregulated NET formation or degradation, as potential approaches that may benefit patients with severe COVID-19.

Repression of CTSG, ELANE and PRTN3-mediated histone H3 proteolytic cleavage promotes monocyte-to-macrophage differentiation

Chromatin undergoes extensive reprogramming during immune cell differentiation. Here we report the repression of controlled histone H3 amino terminus proteolytic cleavage (H3ΔN) during monocyte-to-macrophage development. This abundant histone mark in human peripheral blood monocytes is catalyzed by neutrophil serine proteases (NSPs) cathepsin G, neutrophil elastase and proteinase 3. NSPs are repressed as monocytes mature into macrophages. Integrative epigenomic analysis reveals widespread H3ΔN distribution across the genome in a monocytic cell line and primary monocytes, which becomes largely undetectable in fully differentiated macrophages. H3ΔN is enriched at permissive chromatin and actively transcribed genes. Simultaneous NSP depletion in monocytic cells results in H3ΔN loss and further increase in chromatin accessibility, which likely primes the chromatin for gene expression reprogramming. Importantly, H3ΔN is reduced in monocytes from patients with systemic juvenile idiopathic arthritis, an autoinflammatory disease with prominent macrophage involvement. Overall, we uncover an epigenetic mechanism that primes the chromatin to facilitate macrophage development.

Transcriptomic Profiling Identifies Neutrophil-Specific Upregulation of Cystatin F as a Marker of Acute Inflammation in Humans

Cystatin F encoded by CST7 is a cysteine peptidase inhibitor known to be expressed in natural killer (NK) and CD8⁺ T cells during steady-state conditions. However, little is known about its expression during inflammatory disease states in humans. We have developed an analytic approach capable of not only identifying previously poorly characterized disease-associated genes but also defining regulatory mechanisms controlling their expression. By exploring multiple cohorts of public transcriptome data comprising 43 individual datasets, we showed that CST7 is upregulated in the blood during a diverse set of infectious and non-infectious inflammatory conditions. Interestingly, this upregulation of CST7 was neutrophil-specific, as its expression was unchanged in NK and CD8⁺ T cells during sepsis. Further analysis demonstrated that known microbial products or cytokines commonly associated with inflammation failed to increase CST7 expression, suggesting that its expression in neutrophils is induced by an endogenous serum factor commonly present in human inflammatory conditions. Overall, through the identification of CST7 upregulation as a marker of acute inflammation in humans, our study demonstrates the value of publicly available transcriptome data in knowledge generation and potential biomarker discovery.

Extracellular vesicles and female reproduction

Extracellular vesicles (EVs) are nano-sized membrane bound complexes that have been identified as a mean for intercellular communication between cells and tissues both in physiological and pathological conditions. These vesicles contain numerous molecules involved in signal transduction including microRNAs, mRNAs, DNA, proteins, lipids, and cytokines and can affect the behavior of recipient cells. Female reproduction is dependent on extremely fine-tuned endocrine regulation, and EVs may represent an added layer that contributes to this regulation. This narrative review article provides an update on the research of the role of EVs in female reproduction including folliculogenesis, fertilization, embryo quality, and implantation. We also highlight potential pitfalls in typical EV studies and discuss gaps in the current literature.

Perforin and granzymes in neurological infections: From humans to cattle

Perforin and granzymes are essential components of the cytotoxic granules present in cytotoxic T lymphocytes and natural killer cells. These proteins play a crucial role in a variety of conditions, including viral infections, tumor immune surveillance, and tissue rejection. Besides their beneficial effect in most of these situations, perforin and granzymes have also been associated with tissue damage and immune diseases. Moreover, it has been reported that perforin and granzymes released during viral infections could contribute to the pathogenesis of diseases. In this review, we summarize the information available on human perforin and granzymes and their relationship with neurological infections and immune disorders. Furthermore, we compare this information with that available for bovine and present data on perforin and granzymes expression in cattle infected with bovine alphaherpesvirus types1 and -5. To our knowledge, this is the first review analyzing the impact of perforin and granzymes on neurological infections caused by bovine herpesviruses.

Identification of Plasmodium dipeptidyl aminopeptidase allosteric inhibitors by high throughput screening

Dipeptidyl aminopeptidases (DPAPs) are cysteine proteases that cleave dipeptides from the N-terminus of protein substrates and have been shown to play important roles in many pathologies including parasitic diseases such as malaria, toxoplasmosis and Chagas's disease. Inhibitors of the mammalian homologue cathepsin C have been used in clinical trials as potential drugs to treat chronic inflammatory disorders, thus proving that these enzymes are druggable. In Plasmodium species, DPAPs play important functions at different stages of parasite development, thus making them potential antimalarial targets. Most DPAP inhibitors developed to date are peptide-based or peptidomimetic competitive inhibitors. Here, we used a high throughput screening approach to identify novel inhibitor scaffolds that block the activity of Plasmodium falciparum DPAP1. Most of the hits identified in this screen also inhibit Plasmodium falciparum DPAP3, cathepsin C, and to a lesser extent other malarial clan CA proteases, indicating that these might be general DPAP inhibitors. Interestingly, our mechanism of inhibition studies indicate that most hits are allosteric inhibitors, which opens a completely new strategy to inhibit these enzymes, study their biological function, and potentially develop new inhibitors as starting points for drug development.

Primary immunodeficiencies reveal the essential role of tissue neutrophils in periodontitis

Periodontitis is a common human inflammatory disease. In this condition, microbiota trigger excessive inflammation in oral mucosal tissues surrounding the dentition, resulting in destruction of tooth-supporting structures (connective tissue and bone). While susceptibility factors for common forms of periodontitis are not clearly understood, studies in patients with single genetic defects reveal a critical role for tissue neutrophils in disease susceptibility. Indeed, various genetic defects in the development, egress from the bone marrow, chemotaxis, and extravasation are clearly linked to aggressive/severe periodontitis at an early age. Here, we provide an overview of genetic defects in neutrophil biology that are linked to periodontitis. In particular, we focus on the mechanisms underlying Leukocyte Adhesion Deficiency-I, the prototypic Mendelian defect of impaired neutrophil extravasation and severe periodontitis.

Premedication with a cathepsin C inhibitor alleviates early primary graft dysfunction in mouse recipients after lung transplantation

Neutrophil serine proteases (NSPs), like proteinase 3 (PR3) and neutrophil elastase (NE) are implicated in ischemia-reperfusion responses after lung transplantation (LTx). Cathepsin C (CatC) acts as the key regulator of NSP maturation during biosynthesis. We hypothesized that CatC inhibitors would reduce vascular breakdown and inflammation during reperfusion in pretreated lung transplant recipients by blocking NSP maturation in the bone marrow. An orthotopic LTx model in mice was used to mimic the induction of an ischemia-reperfusion response after 18 h cold storage of the graft and LTx. Recipient mice were treated subcutaneously with a chemical CatC inhibitor (ICatC) for 10 days prior to LTx. We examined the effect of the ICatC treatment by measuring the gas exchange function of the left lung graft, protein content, neutrophil numbers and NSP activities in the bone marrow 4 h after reperfusion. Pre-operative ICatC treatment of the recipient mice improved early graft function and lead to the disappearance of active NSP protein in the transplanted lung. NSP activities were also substantially reduced in bone marrow neutrophils. Preemptive NSP reduction by CatC inhibition may prove to be a viable and effective approach to reduce immediate ischemia reperfusion responses after LTx.

Characterization of P. falciparum dipeptidyl aminopeptidase 3 specificity identifies differences in amino acid preferences between peptide-based substrates and covalent inhibitors

Malarial dipeptidyl aminopeptidases (DPAPs) are cysteine proteases important for parasite development thus making them attractive drug targets. In order to develop inhibitors specific to the parasite enzymes, it is necessary to map the determinants of substrate specificity of the parasite enzymes and its mammalian homologue cathepsin C (CatC). Here, we screened peptide-based libraries of substrates and covalent inhibitors to characterize the differences in specificity between parasite DPAPs and CatC, and used this information to develop highly selective DPAP1 and DPAP3 inhibitors. Interestingly, while the primary amino acid specificity of a protease is often used to develop potent inhibitors, we show that equally potent and highly specific inhibitors can be developed based on the sequences of nonoptimal peptide substrates. Finally, our homology modelling and docking studies provide potential structural explanations of the differences in specificity between DPAP1, DPAP3, and CatC, and between substrates and inhibitors in the case of DPAP3. Overall, this study illustrates that focusing the development of protease inhibitors solely on substrate specificity might overlook important structural features that can be exploited to develop highly potent and selective compounds.

Up-regulated cathepsin C induces macrophage M1 polarization through FAK-triggered p38 MAPK/NF-κB pathway

Increasing evidence indicates that in response to environmental changes, macrophages can dynamically change into two main functional phenotypes, namely M1 and M2. Depending on these different phenotypes, macrophages can produce either pro-inflammatory or anti-inflammatory factors which may affect the outcome of inflammation. Mastering the switching of M1/M2 phenotypes may provide therapeutic approaches to chronic inflammatory disease, such as atherosclerosis, rheumatoid arthritis, even the metabolic disorders. Cathepsin C (CTSC), as a member of the papain family of cysteine proteases, is a key enzyme in the activation of granule serine proteases thereby involved in modulating the inflammatory responses. Moreover, abundant expression of CTSC has been found in M1 macrophages in plaques of atherosclerosis and related to the progression of disease. However, whether CTSC can regulate macrophage activation status in inflammatory responses has not been fully investigated. In the present study, using peritoneal macrophages (PMs) and mouse macrophage cell line RAW264.7 treated with LPS and active monomer of CTSC, we found that CTSC was not only expressed in macrophages in M1 activation status, but also facilitated macrophages towards M1 phenotype, suggesting a self-activation mechanism involved in this process which may lead to a vicious circle in chronic inflammation. Then we attempted to explore the underlying molecular mechanisms of CTSC resulting in M1 activation. Focal adhesion kinase (FAK) is one of the non-receptor cytoplasmic protein tyrosine kinases, serving as an upstream mediator that leads to transcription of many pro-inflammatory factors. We found FAK expression was up-regulated at both mRNA and protein levels following CTSC stimulation, and FAK phosphorylation level was also significantly increased. The p38MAPK/NF-κB pathway, as the downstream of FAK, were also found activated in CTSC-treated macrophages, suggesting that CTSC may promote macrophage towards M1 activation status through FAK-induced p38MAPK/NF-κB signaling pathway activation. Our study provides a new therapeutic target in the treatment of chronic inflammatory diseases.

Structure-based design and in vivo anti-arthritic activity evaluation of a potent dipeptidyl cyclopropyl nitrile inhibitor of cathepsin C

Cathepsin C (CatC) is a dipeptidyl-exopeptidase which activates neutrophil serine protease precursors (elastase, proteinase 3, cathepsin G and NSP4) by removing their N-terminal propeptide in bone marrow cells at the promyelocytic stage of neutrophil differentiation. The resulting active proteases are implicated in chronic inflammatory and autoimmune diseases. Hence, inhibition of CatC represents a therapeutic strategy to suppress excessive protease activities in various neutrophil mediated diseases. We designed and synthesized a series of dipeptidyl cyclopropyl nitrile compounds as putative CatC inhibitors. One compound, IcatC_XPZ-01 ((S)-2-amino-N-((1R,2R)-1-cyano-2-(4'-(4-methylpiperazin-1-ylsulfonyl)biphenyl-4-yl)cyclopropyl)butanamide)) was identified as a potent inhibitor of both human and rodent CatC. In mice, pharmacokinetic studies revealed that IcatC_XPZ-01 accumulated in the bone marrow reaching levels suitable for CatC inhibition. Subcutaneous administration of IcatC_XPZ-01 in a monoclonal anti-collagen antibody induced mouse model of rheumatoid arthritis resulted in statistically significant anti-arthritic activity with persistent decrease in arthritis scores and paw thickness.

Cathepsins: Potent regulators in carcinogenesis

Cathepsins (CTS) are mainly lysosomal acid hydrolases extensively involved in the prognosis of different diseases, and having a distinct role in tumor progression by regulating cell proliferation, autophagy, angiogenesis, invasion, and metastasis. As all these processes conjunctively lead to cancer progression, their site-specific regulation might be beneficial for cancer treatment. CTS regulate activation of the proteolytic cascade and protein turnover, while extracellular CTS is involved in promoting extracellular matrix degradation and angiogenesis, thereby stimulating invasion and metastasis. Despite cancer regulation, the involvement of CTS in cellular adaptation toward chemotherapy and radiotherapy augments their therapeutic potential. However, lysosomal permeabilization mediated cytosolic translocation of CTS induces programmed cell death. This complex behavior of CTS generates the need to discuss the different aspects of CTS associated with cancer regulation. In this review, we mainly focused on the significance of each cathepsin in cancer signaling and their targeting which would provide noteworthy information in the context of cancer biology and therapeutics.

Targeting of cathepsin C induces autophagic dysregulation that directs ER stress mediated cellular cytotoxicity in colorectal cancer cells

As Autophagy is a pivotal mechanism of cancer cell survival and the development of chemotherapeutic resistance; therefore, new approaches are warranted for its targeting which may be fulfilled by cathepsins regulation. Amongst cathepsins, cathepsin C (CTSC) is highly expressed in various cancers and possesses significant therapeutic potential in autoimmune disorders; however, its role in colorectal cancer has not been explored. Herein, we aimed to investigate the role of CTSC in autophagy regulation mediated colorectal carcinoma cell proliferation. Cathepsin C targeting through inhibitors/siRNA leads to the accumulation of light chain 3 II and p62 without affecting the lysosomal integrity, revealed dysfunctional autolysosomal degradation which is also substantiated by proteolytic studies. Cathepsin C inhibition showed comparable autophagy blockade with E64d and augmented the autophagy blockade mediated by bafilomycin. Loss of CTSC function also induced ER stress-mediated JNK phosphorylation accompanied by the translocation of mitochondrial cyt c followed by apoptotic cell death in colorectal carcinoma cells. Taken together, the study reveals that CTSC targeting plays a key role in the regulation of autophagy mediated colorectal cancer cell proliferation. Further investigations are required to determine the functional role of CTSC in other tumors also which may have implications for the therapeutic prevention of cancer in the future.

Involvement of cystatin C in immunity and apoptosis

As an abundantly expressed cysteine protease inhibitor widely distributed in the organisms, cystatin C is involved in various physiological processes. Due to its relatively small molecular weight and easy detection, cystatin C is commonly used as a measure for glomerular filtration rate. In pathological conditions, however, growing evidences suggest that cystatin C is associated with various immune responses against either exogenous or endogenous antigens, which ultimately result in inflammatory autoimmune diseases or tumor development if not properly controlled. Thus the fluctuation of cystatin C levels might have more clinical implications than a reflection of kidney functions. Here, we summarize the latest development of studies on the pathophysiological functions of cystatin C, with focus on its immune regulatory roles at both cellular and molecular levels including antigen presentation, secretion of cytokines, synthesis of nitric oxide, as well as apoptosis. Finally, we discuss the clinical implications and therapeutic potentials of what this predominantly expressed protease inhibitor can bring to us.

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

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

Experimental Arthritis Is Dependent on Mouse Mast Cell Protease-5

The constitutive heparin⁺ (HP) mast cells (MCs) in mice express mouse MC protease (mMCP)-5 and carboxypeptidase A (mMC-CPA). The amino acid sequence of mMCP-5 is most similar to that of human chymase-1, as are the nucleotide sequences of their genes and transcripts. Using a homologous recombination approach, a C57BL/6 mouse line was created that possessed a disrupted mMCP-5 gene. The resulting mice were fertile and had no obvious developmental abnormality. Lack of mMCP-5 protein did not alter the granulation of the IL-3/IL-9-dependent mMCP-2⁺ MCs in the jejunal mucosa of Trichinella spiralis-infected mice. In contrast, the constitutive HP⁺ MCs in the tongues of mMCP-5-null mice were poorly granulated and lacked mMC-CPA protein. Bone marrow-derived MCs were readily developed from the transgenic mice using IL-3. Although these MCs contained high levels of mMC-CPA mRNA, they also lacked the latter exopeptidase. mMCP-5 protein is therefore needed to target translated mMC-CPA to the secretory granule along with HP-containing serglycin proteoglycans. Alternately, mMCP-5 is needed to protect mMC-CPA from autolysis in the cell's granules. Fibronectin was identified as a target of mMCP-5, and the exocytosis of mMCP-5 from the MCs in the mouse's peritoneal cavity resulted in the expression of metalloproteinase protease-9, which has been implicated in arthritis. In support of the latter finding, experimental arthritis was markedly reduced in mMCP-5-null mice relative to wild-type mice in two disease models.

Prolonged pharmacological inhibition of cathepsin C results in elimination of neutrophil serine proteases

Cathepsin C (CatC) is a tetrameric cysteine dipeptidyl aminopeptidase that plays a key role in activation of pro-inflammatory serine protease zymogens by removal of a N-terminal pro-dipeptide sequence. Loss of function mutations in the CatC gene is associated with lack of immune cell serine protease activities and cause Papillon-Lefèvre syndrome (PLS). Also, only very low levels of elastase-like protease zymogens are detected by proteome analysis of neutrophils from PLS patients. Thus, CatC inhibitors represent new alternatives for the treatment of neutrophil protease-driven inflammatory or autoimmune diseases. We aimed to experimentally inactivate and lower neutrophil elastase-like proteases by pharmacological blocking of CatC-dependent maturation in cell-based assays and in vivo. Isolated, immature bone marrow cells from healthy donors pulse-chased in the presence of a new cell permeable cyclopropyl nitrile CatC inhibitor almost totally lack elastase. We confirmed the elimination of neutrophil elastase-like proteases by prolonged inhibition of CatC in a non-human primate. We also showed that neutrophils lacking elastase-like protease activities were still recruited to inflammatory sites. These preclinical results demonstrate that the disappearance of neutrophil elastase-like proteases as observed in PLS patients can be achieved by pharmacological inhibition of bone marrow CatC. Such a transitory inhibition of CatC might thus help to rebalance the protease load during chronic inflammatory diseases, which opens new perspectives for therapeutic applications in humans.

Role of granule proteases in the life and death of neutrophils

Neutrophils constitute a crucial component of the innate immune defenses against microbes. Produced in the bone marrow and patrolling in blood vessels, neutrophils are recruited to injured tissues and are immediately active to contain pathogen invasion. Neutrophils undergo programmed cell death by multiple, context-specific pathways, which have consequences on immunopathology and disease outcome. Studies in the last decade indicate additional functions for neutrophils - or a subset of neutrophils - in modulating adaptive responses and tumor progression. Neutrophil granules contain abundant amounts of various proteases, which are directly implicated in protective and pathogenic functions of neutrophils. It now emerges that neutral serine proteases such as cathepsin G and proteinase-3 also contribute to the neutrophil life cycle, but do so via different pathways than that of the aspartate protease cathepsin D and that of mutants of the serine protease elastase. The aim of this review is to appraise the present knowledge of the function of neutrophil granule proteases and their inhibitors in neutrophil cell death, and to integrate these findings in the current understandings of neutrophil life cycle and programmed cell death pathways.

Inherited diseases caused by mutations in cathepsin protease genes

Lysosomal cathepsins are proteolytic enzymes increasingly recognized as prognostic markers and potential therapeutic targets in a variety of diseases. In those conditions, the cathepsins are mostly overexpressed, thereby driving the respective pathogenic processes. Although less known, there are also diseases with a genetic deficiency of cathepsins. In fact, nowadays 6 of the 15 human proteases called 'cathepsins' have been linked to inherited syndromes. However, only three of these syndromes are typical lysosomal storage diseases, while the others are apparently caused by defective cleavage of specific protein substrates. Here, we will provide an introduction on lysosomal cathepsins, followed by a brief description of the clinical symptoms of the various genetic diseases. For each disease, we focus on the known mutations of which many have been only recently identified by modern genome sequencing approaches. We further discuss the effect of the respective mutation on protease structure and activity, the resulting pathogenesis, and possible therapeutic strategies.

Neutrophil maturation rate determines the effects of dipeptidyl peptidase 1 inhibition on neutrophil serine protease activity

Background and Purpose

Neutrophil serine proteases (NSPs) are activated by dipeptidyl peptidase 1 (DPP1) during neutrophil maturation. The effects of neutrophil turnover rate on NSP activity following DPP1 inhibition was studied in a rat pharmacokinetic/pharmacodynamic model.

Experimental Approach

Rats were treated with a DPP1 inhibitor twice daily for up to 14 days; NSP activity was measured in onset or recovery studies, and an indirect response model was fitted to the data to estimate the turnover rate of the response.

Key Results

Maximum NSP inhibition was achieved after 8 days of treatment and a reduction of around 75% NSP activity was achieved at 75% in vitro DPP1 inhibition. Both the rate of inhibition and recovery of NSP activity were consistent with a neutrophil turnover rate of between 4–6 days. Using human neutrophil turnover rate, it is predicted that maximum NSP inhibition following DPP1 inhibition takes around 20 days in human.

Conclusions and Implications

Following inhibition of DPP1 in the rat, the NSP activity was determined by the amount of DPP1 inhibition and the turnover of neutrophils and is thus supportive of the role of neutrophil maturation in the activation of NSPs. Clinical trials to monitor the effect of a DPP1 inhibitor on NSPs should take into account the delay in maximal response on the one hand as well as the potential delay in a return to baseline NSP levels following cessation of treatment.

Protease signaling in animal and plant-regulated cell death

This review aims to highlight the proteases required for regulated cell death mechanisms in animals and plants. The aim is to be incisive, and not inclusive of all the animal proteases that have been implicated in various publications. The review also aims to focus on instances when several publications from disparate groups have demonstrated the involvement of an animal protease, and also when there is substantial biochemical, mechanistic and genetic evidence. In doing so, the literature can be culled to a handful of proteases, covering most of the known regulated cell death mechanisms: apoptosis, regulated necrosis, necroptosis, pyroptosis and NETosis in animals. In plants, the literature is younger and not as extensive as for mammals, although the molecular drivers of vacuolar death, necrosis and the hypersensitive response in plants are becoming clearer. Each of these death mechanisms has at least one proteolytic component that plays a major role in controlling the pathway, and sometimes they combine in networks to regulate cell death/survival decision nodes. Some similarities are found among animal and plant cell death proteases but, overall, the pathways that they govern are kingdom-specific with very little overlap.

Inhibitors of Serine Proteases in Regulating the Production and Function of Neutrophil Extracellular Traps

Neutrophil extracellular traps (NETs), DNA webs released into the extracellular environment by activated neutrophils, are thought to play a key role in the entrapment and eradication of microbes. However, NETs are highly cytotoxic and a likely source of autoantigens, suggesting that NET release is tightly regulated. NET formation involves the activity of neutrophil elastase (NE), which cleaves histones, leading to chromatin decondensation. We and others have recently demonstrated that inhibitors of NE, such as secretory leukocyte protease inhibitor (SLPI) and SerpinB1, restrict NET production in vitro and in vivo. SLPI was also identified as a NET component in the lesional skin of patients suffering from the autoinflammatory skin disease psoriasis. SLPI-competent NET-like structures (a mixture of SLPI with neutrophil DNA and NE) stimulated the synthesis of interferon type I (IFNI) in plasmacytoid dendritic cells (pDCs) in vitro. pDCs uniquely respond to viral or microbial DNA/RNA but also to nucleic acids of “self” origin with the production of IFNI. Although IFNIs are critical in activating the antiviral/antimicrobial functions of many cells, IFNIs also play a role in inducing autoimmunity. Thus, NETs decorated by SLPI may regulate skin immunity through enhancing IFNI production in pDCs. Here, we review key aspects of how SLPI and SerpinB1 can control NET production and immunogenic function.

Neutrophilic Cathepsin C Is Maturated by a Multistep Proteolytic Process and Secreted by Activated Cells during Inflammatory Lung Diseases

The cysteine protease cathepsin C (CatC) activates granule-associated proinflammatory serine proteases in hematopoietic precursor cells. Its early inhibition in the bone marrow is regarded as a new therapeutic strategy for treating proteolysis-driven chronic inflammatory diseases, but its complete inhibition is elusive in vivo Controlling the activity of CatC may be achieved by directly inhibiting its activity with a specific inhibitor or/and by preventing its maturation. We have investigated immunochemically and kinetically the occurrence of CatC and its proform in human hematopoietic precursor cells and in differentiated mature immune cells in lung secretions. The maturation of proCatC obeys a multistep mechanism that can be entirely managed by CatS in neutrophilic precursor cells. CatS inhibition by a cell-permeable inhibitor abrogated the release of the heavy and light chains from proCatC and blocked ∼80% of CatC activity. Under these conditions the activity of neutrophil serine proteases, however, was not abolished in precursor cell cultures. In patients with neutrophilic lung inflammation, mature CatC is found in large amounts in sputa. It is secreted by activated neutrophils as confirmed through lipopolysaccharide administration in a nonhuman primate model. CatS inhibitors currently in clinical trials are expected to decrease the activity of neutrophilic CatC without affecting those of elastase-like serine proteases.

Cysteine cathepsin proteases: regulators of cancer progression and therapeutic response

Cysteine cathepsin protease activity is frequently dysregulated in the context of neoplastic transformation. Increased activity and aberrant localization of proteases within the tumour microenvironment have a potent role in driving cancer progression, proliferation, invasion and metastasis. Recent studies have also uncovered functions for cathepsins in the suppression of the response to therapeutic intervention in various malignancies. However, cathepsins can be either tumour promoting or tumour suppressive depending on the context, which emphasizes the importance of rigorous in vivo analyses to ascertain function. Here, we review the basic research and clinical findings that underlie the roles of cathepsins in cancer, and provide a roadmap for the rational integration of cathepsin-targeting agents into clinical treatment.

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.

Papillon-Lefèvre syndrome patient reveals species-dependent requirements for neutrophil defenses

Papillon-Lefèvre syndrome (PLS) results from mutations that inactivate cysteine protease cathepsin C (CTSC), which processes a variety of serine proteases considered essential for antimicrobial defense. Despite serine protease-deficient immune cell populations, PLS patients do not exhibit marked immunodeficiency. Here, we characterized a 24-year-old woman who had suffered from severe juvenile periodontal disease, but was otherwise healthy, and identified a homozygous missense mutation in CTSC indicative of PLS. Proteome analysis of patient neutrophil granules revealed that several proteins that normally localize to azurophil granules, including the major serine proteases, elastase, cathepsin G, and proteinase 3, were absent. Accordingly, neutrophils from this patient were incapable of producing neutrophil extracellular traps (NETs) in response to ROS and were unable to process endogenous cathelicidin hCAP-18 into the antibacterial peptide LL-37 in response to ionomycin. In immature myeloid cells from patient bone marrow, biosynthesis of CTSC and neutrophil serine proteases appeared normal along with initial processing and sorting to cellular storage. In contrast, these proteins were completely absent in mature neutrophils, indicating that CTSC mutation promotes protease degradation in more mature hematopoietic subsets, but does not affect protease production in progenitor cells. Together, these data indicate CTSC protects serine proteases from degradation in mature immune cells and suggest that neutrophil serine proteases are dispensable for human immunoprotection.

Beyond canonical inflammasomes: emerging pathways in IL-1-mediated autoinflammatory disease

In recent years, non-communicable chronic diseases that are potentiated by sterile inflammation have replaced infectious diseases as the major threat to human health. Sterile inflammation that results from aberrant tissue damage plays pivotal roles in the pathogenesis of numerous acute and chronic inflammatory diseases including atherosclerosis, type 2 diabetes, cancer, obesity, and multiple neurodegenerative diseases. The cellular events and molecular signaling pathways that govern sterile inflammation currently remain poorly defined; however, emerging data suggest central roles for IL-1 in driving autoimmune and inflammatory disease pathogenesis. Improved characterization of the immunological pathways that contribute to sterile inflammation are desperately needed to develop effective therapeutics to treat these devastating diseases. In this review, we discuss recent advances in our understanding of how IL-1 is regulated in response to tissue damage. In particular, we highlight recent studies that describe novel roles for conventional cell death molecules in the regulation of IL-1β production.

Congenital defects in neutrophil dynamics

Neutrophil granulocytes are key effector cells of the vertebrate immune system. They represent 50-70% of the leukocytes in the human blood and their loss by disease or drug side effect causes devastating bacterial infections. Their high turnover rate, their fine-tuned killing machinery, and their arsenal of toxic vesicles leave them particularly vulnerable to various genetic deficiencies. The aim of this review is to highlight those congenital immunodeficiencies which impede the dynamics of neutrophils, such as migration, cytoskeletal rearrangements, vesicular trafficking, and secretion.

Granzyme M: behind enemy lines

The granule-exocytosis pathway is the major mechanism via which cytotoxic lymphocytes eliminate virus-infected and tumor cells. In this pathway, cytotoxic lymphocytes release granules containing the pore-forming protein perforin and a family of serine proteases known as granzymes into the immunological synapse. Pore-formation by perforin facilitates entry of granzymes into the target cell, where they can activate various (death) pathways. Humans express five different granzymes, of which granzymes A and B have been most extensively characterized. However, much less is known about granzyme M (GrM). Recently, structural analysis and advanced proteomics approaches have determined the primary and extended specificity of GrM. GrM functions have expanded over the past few years: not only can GrM efficiently induce cell death in tumor cells, it can also inhibit cytomegalovirus replication in a noncytotoxic manner. Finally, a role for GrM in lipopolysaccharide-induced inflammatory responses has been proposed. In this review, we recapitulate the current status of GrM expression, substrate specificity, functions, and inhibitors.

Unnatural amino acids increase activity and specificity of synthetic substrates for human and malarial cathepsin C

Mammalian cathepsin C is primarily responsible for the removal of N-terminal dipeptides and activation of several serine proteases in inflammatory or immune cells, while its malarial parasite ortholog dipeptidyl aminopeptidase 1 plays a crucial role in catabolizing the hemoglobin of its host erythrocyte. In this report, we describe the systematic substrate specificity analysis of three cathepsin C orthologs from Homo sapiens (human), Bos taurus (bovine) and Plasmodium falciparum (malaria parasite). Here, we present a new approach with a tailored fluorogenic substrate library designed and synthesized to probe the S1 and S2 pocket preferences of these enzymes with both natural and a broad range of unnatural amino acids. Our approach identified very efficiently hydrolyzed substrates containing unnatural amino acids, which resulted in the design of significantly better substrates than those previously known. Additionally, in this study significant differences in terms of the structures of optimal substrates for human and malarial orthologs are important from the therapeutic point of view. These data can be also used for the design of specific inhibitors or activity-based probes.

Relevance of the mouse model as a therapeutic approach for neutrophil proteinase 3-associated human diseases

Proteinase 3 (PR3) is one of the four elastase-related serine proteinases stored in the azurophilic granules of neutrophils. Although it participates in the pro- and anti-inflammatory responses to infection and inflammation it also retains specific functions that make it different from neutrophil elastase in spite of their close structural resemblance. PR3 is involved in the immune response to infection and is the major autoantigen in granulomatosis with polyangiitis (GPA, formerly Wegener disease), an autoimmune systemic vasculitis with granulomas. Thus, PR3 appears to be a relevant therapeutic target in a variety of inflammatory human diseases. Animal models are required for the testing of new drugs that target PR3 specifically but differences between human and rodent neutrophil PR3 expression and substrate specificity have greatly impaired progress in this direction. This may explain that, to date, there is no spontaneous model of vasculitis associated with anti-PR3 antibodies. In this review, we will focus on the structural and functional differences between human and murine PR3, and how these differences may be by-passed in order to develop a relevant animal model.

Toward very potent, non-covalent organophosphonate inhibitors of cathepsin C and related enzymes by 2-amino-1-hydroxy-alkanephosphonates dipeptides

Cathepsins play an important role in several human disorders and therefore the design and synthesis of their inhibitors attracts considerable interest in current medicinal chemistry approaches. Due to the presence of a strong sulphydryl nucleophile in the active center of the cysteine type cathepsins, most strategies to date have yielded covalent inhibitors. Here we present a series of non-covalent β-amino-α-hydroxyalkanephosphonate dipeptidic inhibitors of cathepsin C, ranking amongst the best low-molecular weight inhibitors of this enzyme. Their binding modes determined by molecular modelling indicate that the hydroxymethyl fragment of the molecule, not the phosphonate moiety, acts as a transition state analogue of peptide bond hydrolysis. These dipeptide mimetics appear also to be potent inhibitors of other cysteine proteases such as papain, cathepsin B and cathepsin K, thus providing new leading structures for these medicinally important enzymes.

Involvement of Fenneropenaeus chinensis Cathepsin C in antiviral immunity

Cathepsin C (Cath C) is a lysosomal cysteine protease that belongs to the papain superfamily. Cath C is capable of activating many chymotrypsin-like serine proteases and is reported to be a central coordinator for the activation of many serine proteinases in immune and inflammatory cells. In this study, Cath C cDNA was cloned from Fenneropenaeus chinensis (Fc). The complete cDNA of Fc-Cath C in Chinese white shrimp was found to be 1445-base pairs (bp) long. It contained an open reading frame (ORF) 1356-bp long and encoded a 451-amino acid residue protein, including a 17-amino acid residue signal peptide. Real-time PCR analysis results indicated that Fc-Cath C was present in all the tissues detected and exhibited high level of transcription in the hepatopancreas. In hemocytes, hepatopancreas, gills and intestine, Fc-Cath C was upregulated after stimulation by the Vibrio anguillarum and the white spot syndrome viruses (WSSVs). Replication of the WSSV increased after the injection of Fc-Cath C antiserum or knockdown Cath C by RNA interference. These results implied that Cath C might play a crucial role in the antiviral immune response of shrimp.

Uric acid as a danger signal in gout and its comorbidities

Uric acid is a waste product of purine catabolism. This molecule comes to clinical attention when it nucleates to form crystals of monosodium urate (MSU) in joints or other tissues, and thereby causes the inflammatory disease of gout. Patients with gout frequently suffer from a number of comorbid conditions including hypertension, diabetes mellitus and cardiovascular disease. Why MSU crystals trigger inflammation and are associated with comorbidities of gout has been unclear, but recent studies provide new insights into these issues. Rather than simply being a waste product, uric acid could serve a pathophysiological role as a local alarm signal that alerts the immune system to cell injury and helps to trigger both innate and adaptive immune responses. The inflammatory component of these immune responses is caused when urate crystals trigger both inflammasome-dependent and independent pathways to generate the proinflammatory cytokine IL-1. The resulting bioactive IL-1 stimulates the inflammation of gout and might contribute to the development of other comorbidities. Surprisingly, the same mechanisms underlie the inflammatory response to a number of irritant particles, many of which also cause disease. These new insights help to explain the pathogenesis of gout and point to potential new therapeutic targets for this and other sterile inflammatory diseases.

The IL-1-dependent sterile inflammatory response has a substantial caspase-1-independent component that requires cathepsin C

The sterile inflammatory response to cell death and irritant crystals is medically important because it causes disease. Although these stimuli are structurally distinct, they cause inflammation through a common pathway that requires the cytokine IL-1. In vitro, the inflammasome, and in particular its generation of active caspase-1, is absolutely required to produce bioactive IL-1β. However, in this study, we report that caspase-1 is not required in vivo for much of the IL-1β-dependent sterile inflammatory response. Furthermore, we find that cathepsin C, which controls the activity of a number of leukocyte serine proteases capable of processing IL-1β, plays a major role in this caspase-1-independent pathway. Mice that are deficient in cathepsin C have reduced inflammatory responses to dying cells and silica crystals. In the absence of cathepsin C, caspase-1 becomes rate limiting such that mice doubly deficient in both of these proteases make little IL-1β in vivo and have markedly attenuated inflammatory responses to the sterile stimuli. In contrast, these mutant mice generate normal inflammation in response to exogenous IL-1β, indicating that cathepsin C and caspase-1 function upstream of IL-1β, and, in their absence, all components of the pathway downstream of mature IL-1β are intact.

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.