Normal Neutrophil Function in Cathepsin G-Deficient Mice

Neutrophil proteases are protective against SARS-CoV-2 by degrading the spike protein and dampening virus-mediated inflammation

Studies on severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) have highlighted the crucial role of host proteases for viral replication and the immune response. The serine proteases furin and TMPRSS2 and lysosomal cysteine proteases facilitate viral entry by limited proteolytic processing of the spike (S) protein. While neutrophils are recruited to the lungs during COVID-19 pneumonia, little is known about the role of the neutrophil serine proteases (NSPs) cathepsin G (CatG), elastase (NE), and proteinase 3 (PR3) on SARS-CoV-2 entry and replication. Furthermore, the current paradigm is that NSPs may contribute to the pathogenesis of severe COVID-19. Here, we show that these proteases cleaved the S protein at multiple sites and abrogated viral entry and replication in vitro. In mouse models, CatG significantly inhibited viral replication in the lung. Importantly, lung inflammation and pathology were increased in mice deficient in NE and/or CatG. These results reveal that NSPs contribute to innate defenses against SARS-CoV-2 infection via proteolytic inactivation of the S protein and that NE and CatG limit lung inflammation in vivo. We conclude that therapeutic interventions aiming to reduce the activity of NSPs may interfere with viral clearance and inflammation in COVID-19 patients.

Innate mechanism of mucosal barrier erosion in the pathogenesis of acquired colitis

The colonic mucosal barrier protects against infection, inflammation, and tissue ulceration. Composed primarily of Mucin-2, proteolytic erosion of this barrier is an invariant feature of colitis; however, the molecular mechanisms are not well understood. We have applied a recurrent food poisoning model of acquired inflammatory bowel disease using Salmonella enterica Typhimurium to investigate mucosal barrier erosion. Our findings reveal an innate Toll-like receptor 4-dependent mechanism activated by previous infection that induces Neu3 neuraminidase among colonic epithelial cells concurrent with increased Cathepsin-G protease secretion by Paneth cells. These anatomically separated host responses merge with the desialylation of nascent colonic Mucin-2 by Neu3 rendering the mucosal barrier susceptible to increased proteolytic breakdown by Cathepsin-G. Depletion of Cathepsin-G or Neu3 function using pharmacological inhibitors or genetic-null alleles protected against Mucin-2 proteolysis and barrier erosion and reduced the frequency and severity of colitis, revealing approaches to preserve and potentially restore the mucosal barrier.

The formation and function of the neutrophil phagosome

Neutrophils are the most abundant circulating leukocyte and are crucial to the initial innate immune response to infection. One of their key pathogen-eliminating mechanisms is phagocytosis, the process of particle engulfment into a vacuole-like structure called the phagosome. The antimicrobial activity of the phagocytic process results from a collaboration of multiple systems and mechanisms within this organelle, where a complex interplay of ion fluxes, pH, reactive oxygen species, and antimicrobial proteins creates a dynamic antimicrobial environment. This complexity, combined with the difficulties of studying neutrophils ex vivo, has led to gaps in our knowledge of how the neutrophil phagosome optimizes pathogen killing. In particular, controversy has arisen regarding the relative contribution and integration of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-derived antimicrobial agents and granule-delivered antimicrobial proteins. Clinical syndromes arising from dysfunction in these systems in humans allow useful insight into these mechanisms, but their redundancy and synergy add to the complexity. In this article, we review the current knowledge regarding the formation and function of the neutrophil phagosome, examine new insights into the phagosomal environment that have been permitted by technological advances in recent years, and discuss aspects of the phagocytic process that are still under debate.

Granule Leakage Induces Cell-Intrinsic, Granzyme B-Mediated Apoptosis in Mast Cells

Mast cells are multifunctional immune cells scattered in tissues near blood vessels and mucosal surfaces where they mediate important reactions against parasites and contribute to the pathogenesis of allergic reactions. Serine proteases released from secretory granules upon mast cell activation contribute to these functions by modulating cytokine activity, platelet activation and proteolytic neutralization of toxins. The forced release of granule proteases into the cytosol of mast cells to induce cell suicide has recently been proposed as a therapeutic approach to reduce mast cell numbers in allergic diseases, but the molecular pathways involved in granule-mediated mast cell suicide are incompletely defined. To identify intrinsic granule proteases that can cause mast cell death, we used mice deficient in cytosolic serine protease inhibitors and their respective target proteases. We found that deficiency in Serpinb1a, Serpinb6a, and Serpinb9a or in their target proteases did not alter the kinetics of apoptosis induced by growth factor deprivation in vitro or the number of peritoneal mast cells in vivo. The serine protease cathepsin G induced marginal cell death upon mast cell granule permeabilization only when its inhibitors Serpinb1a or Serpinb6a were deleted. In contrast, the serine protease granzyme B was essential for driving apoptosis in mast cells. On granule permeabilization, granzyme B was required for caspase-3 processing and cell death. Moreover, cytosolic granzyme B inhibitor Serpinb9a prevented caspase-3 processing and mast cell death in a granzyme B-dependent manner. Together, our findings demonstrate that cytosolic serpins provide an inhibitory shield preventing granule protease-induced mast cell apoptosis, and that the granzyme B-Serpinb9a-caspase-3 axis is critical in mast cell survival and could be targeted in the context of allergic diseases.

Cathepsin G Inhibition by Serpinb1 and Serpinb6 Prevents Programmed Necrosis in Neutrophils and Monocytes and Reduces GSDMD-Driven Inflammation

Neutrophil granule serine proteases contribute to immune responses through cleavage of microbial toxins and structural proteins. They induce tissue damage and modulate inflammation if levels exceed their inhibitors. Here, we show that the intracellular protease inhibitors Serpinb1a and Serpinb6a contribute to monocyte and neutrophil survival in steady-state and inflammatory settings by inhibiting cathepsin G (CatG). Importantly, we found that CatG efficiently cleaved gasdermin D (GSDMD) to generate the signature N-terminal domain GSDMD-p30 known to induce pyroptosis. Yet GSDMD deletion did not rescue neutrophil survival in Sb1a.Sb6a^-/- mice. Furthermore, Sb1a.Sb6a^-/- mice released high levels of pro-inflammatory cytokines upon endotoxin challenge in vivo in a CatG-dependent manner. Canonical inflammasome activation in Sb1a.Sb6a^-/- macrophages showed increased IL-1β release that was dependent on CatG and GSDMD. Together, our findings demonstrate that cytosolic serpins expressed in myeloid cells prevent cell death and regulate inflammatory responses by inhibiting CatG and alternative activation of GSDMD.

Mouse Models and Tools for the in vivo Study of Neutrophils

Neutrophils are the most abundant leukocytes in human blood and critical actors of the immune system. Many neutrophil functions and facets of their activity in vivo were revealed by studying genetically modified mice or by tracking fluorescent neutrophils in animals using imaging approaches. Assessing the roles of neutrophils can be challenging, especially when exact molecular pathways are questioned or disease states are interrogated that alter normal neutrophil homeostasis. This review discusses the main in vivo models for the study of neutrophils, their advantages and limitations. The side-by-side comparison underlines the necessity to carefully choose the right model(s) to answer a given scientific question, and exhibit caveats that need to be taken into account when designing experimental procedures. Collectively, this review suggests that at least two models should be employed to legitimately conclude on neutrophil functions.

Distinct gut metagenomics and metaproteomics signatures in prediabetics and treatment-naïve type 2 diabetics

Background

The gut microbiota plays important roles in modulating host metabolism. Previous studies have demonstrated differences in the gut microbiome of T2D and prediabetic individuals compared to healthy individuals, with distinct disease-related microbial profiles being reported in groups of different age and ethnicity. However, confounding factors such as anti-diabetic medication hamper identification of the gut microbial changes in disease development.

Method

We used a combination of in-depth metagenomics and metaproteomics analyses of faecal samples from treatment-naïve type 2 diabetic (TN-T2D, n = 77), pre-diabetic (Pre-DM, n = 80), and normal glucose tolerant (NGT, n = 97) individuals to investigate compositional and functional changes of the gut microbiota and the faecal content of microbial and host proteins in Pre-DM and treatment-naïve T2D individuals to elucidate possible host-microbial interplays characterizing different disease stages.

Findings

We observed distinct differences characterizing the gut microbiota of these three groups and validated several key features in an independent TN-T2D cohort. We also demonstrated that the content of several human antimicrobial peptides and pancreatic enzymes differed in faecal samples between three groups.

Interpretation

Our findings suggest a complex, disease stage-dependent interplay between the gut microbiota and the host and point to the value of metaproteomics to gain further insight into interplays between the gut microbiota and the host.

Fund

The study was supported by the National Natural Science Foundation of China (No. 31601073), the National Key Research and Development Program of China (No. 2017YFC0909703) and the Shenzhen Municipal Government of China (No. JCYJ20170817145809215). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

An elastase activity reporter for Electronic Paramagnetic Resonance (EPR) and Overhauser-enhanced Magnetic Resonance Imaging (OMRI) as a line-shifting nitroxide

Pulmonary inflammatory diseases are a major burden worldwide. They have in common an influx of neutrophils. Neutrophils secrete unchecked proteases at inflammation sites consequently leading to a protease/inhibitor imbalance. Among these proteases, neutrophil elastase is responsible for the degradation of the lung structure via elastin fragmentation. Therefore, monitoring the protease/inhibitor status in lungs non-invasively would be an important diagnostic tool. Herein we present the synthesis of a MeO-Suc-(Ala)₂-Pro-Val-nitroxide, a line-shifting elastase activity probe suitable for Electron Paramagnetic Resonance spectroscopy (EPR) and Overhauser-enhanced Magnetic Resonance Imaging (OMRI). It is a fast and sensitive neutrophil elastase substrate with K_m = 15 ± 2.9 µM, k_cat/K_m = 930,000 s^-1 M^-1 and K_m = 25 ± 5.4 µM, k_cat/K_m = 640,000 s^-1 M^-1 for the R and S isomers, respectively. These properties are suitable to detect accurately concentrations of neutrophil elastase as low as 1 nM. The substrate was assessed with broncho-alveolar lavages samples derived from a mouse model of Pseudomonas pneumonia. Using EPR spectroscopy we observed a clear-cut difference between wild type animals and animals deficient in neutrophil elastase or deprived of neutrophil Elastase, Cathepsin G and Proteinase 3 or non-infected animals. These results provide new preclinical ex vivo and in vivo diagnostic methods. They can lead to clinical methods to promote in time lung protection.

The NADPH Oxidase and Microbial Killing by Neutrophils, With a Particular Emphasis on the Proposed Antimicrobial Role of Myeloperoxidase within the Phagocytic Vacuole

This review is devoted to a consideration of the way in which the NADPH oxidase of neutrophils, NOX2, functions to enable the efficient killing of bacteria and fungi. It includes a critical examination of the current dogma that its primary purpose is the generation of hydrogen peroxide as substrate for myeloperoxidase-catalyzed generation of hypochlorite. Instead, it is demonstrated that NADPH oxidase functions to optimize the ionic and pH conditions within the vacuole for the solubilization and optimal activity of the proteins released into this compartment from the cytoplasmic granules, which kill and digest the microbes. The general role of other NOX systems as electrochemical generators to alter the pH and ionic composition in compartments on either side of a membrane in plants and animals will also be examined.

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.

Cathepsin G Controls Arterial But Not Venular Myeloid Cell Recruitment

BACKGROUND

Therapeutic targeting of arterial leukocyte recruitment in the context of atherosclerosis has been disappointing in clinical studies. Reasons for such failures include the lack of knowledge of arterial-specific recruitment patterns. Here we establish the importance of the cathepsin G (CatG) in the context of arterial myeloid cell recruitment.

METHODS

Intravital microscopy of the carotid artery, the jugular vein, and cremasteric arterioles and venules in Apoe^-/-and CatG-deficient mice (Apoe^-/-Ctsg^-/-) was used to study site-specific myeloid cell behavior after high-fat diet feeding or tumor necrosis factor stimulation. Atherosclerosis development was assessed in aortic root sections after 4 weeks of high-fat diet, whereas lung inflammation was assessed after inhalation of lipopolysaccharide. Endothelial deposition of CatG and CCL5 was quantified in whole-mount preparations using 2-photon and confocal microscopy.

RESULTS

Our observations elucidated a crucial role for CatG during arterial leukocyte adhesion, an effect not found during venular adhesion. Consequently, CatG deficiency attenuates atherosclerosis but not acute lung inflammation. Mechanistically, CatG is immobilized on arterial endothelium where it activates leukocytes to firmly adhere engaging integrin clustering, a process of crucial importance to achieve effective adherence under high-shear flow. Therapeutic neutralization of CatG specifically abrogated arterial leukocyte adhesion without affecting myeloid cell adhesion in the microcirculation. Repetitive application of CatG-neutralizing antibodies permitted inhibition of atherogenesis in mice.

CONCLUSIONS

Taken together, these findings present evidence of an arterial-specific recruitment pattern centered on CatG-instructed adhesion strengthening. The inhibition of this process could provide a novel strategy for treatment of arterial inflammation with limited side effects.

Cathepsin B modulates lysosomal biogenesis and host defense against Francisella novicida infection

Kanneganti and collaborators propose that the lysosomal protease cathepsin B provides a checkpoint for activation of the transcription factor TFEB and lysosomal biogenesis and explore the impact of this pathway on host defense against bacterial infection.

Lysosomal cathepsins regulate an exquisite range of biological functions, and their deregulation is associated with inflammatory, metabolic, and degenerative diseases in humans. In this study, we identified a key cell-intrinsic role for cathepsin B as a negative feedback regulator of lysosomal biogenesis and autophagy. Mice and macrophages lacking cathepsin B activity had increased resistance to the cytosolic bacterial pathogen Francisella novicida. Genetic deletion or pharmacological inhibition of cathepsin B down-regulated mechanistic target of rapamycin activity and prevented cleavage of the lysosomal calcium channel TRPML1. These events drove transcription of lysosomal and autophagy genes via transcription factor EB, which increased lysosomal biogenesis and activation of autophagy initiation kinase ULK1 for clearance of the bacteria. Our results identified a fundamental biological function of cathepsin B in providing a checkpoint for homeostatic maintenance of lysosome populations and basic recycling functions in the cell.

NLRP3 inflammasome plays a redundant role with caspase 8 to promote IL-1β-mediated osteomyelitis

Missense mutation in the proline-serine-threonine phosphatase-interacting protein 2 (Pstpip2) gene results in the development of spontaneous chronic bone disease characterized by bone deformity and inflammation that is reminiscent of patients with chronic multifocal osteomyelitis (cmo). Interestingly, this disease is specifically mediated by IL-1β but not IL-1α. The precise molecular pathways that promote pathogenic IL-1β production inPstpip2(cmo)mice remain unidentified. Furthermore, how IL-1β provokes inflammatory bone disease inPstpip2(cmo)mice is not known. Here, we demonstrate that double deficiency of Nod like receptor family, pyrin domain containing 3 (NLRP3) and caspase 8 inPstpip2(cmo)mice provides similar protection as observed in caspase-1 and caspase-8-deficientPstpip2(cmo)mice, demonstrating redundant roles for the NLRP3 inflammasome and caspase 8 in provoking osteomyelitic disease inPstpip2(cmo)mice. Consistently, immunofluorescence studies exhibited distinct caspase-1 and caspase-8 puncta in diseasedPtpn6(spin)neutrophils. Data from our chimera studies demonstrated that IL-1β produced by hematopoietic cells is sensed by the radioresistant compartment to promote bone disease. Furthermore, our results showed that the IL-1β signaling is unidirectional and feedback signaling of IL-1β onto the hematopoietic compartment is not important for disease induction. In conclusion, our studies have uncovered the combined actions of the NLRP3 inflammasome and caspase 8 leading to IL-1β maturation and the directionality of IL-1β in driving disease inPstpip2(cmo)mice.

Lactoferrin Is an Allosteric Enhancer of the Proteolytic Activity of Cathepsin G

Protease-mediated degradation of proteins is critical in a plethora of physiological processes. Neutrophils secrete serine proteases including cathepsin G (CatG), neutrophile elastase (NE), and proteinase 3 (PR3) together with lactoferrin (LF) as a first cellular immune response against pathogens. Here, we demonstrate that LF increases the catalytic activity of CatG at physiological concentration, with its highest enhancing capacity under acidic (pH 5.0) conditions, and broadens the substrate selectivity of CatG. On a functional level, the enzymatic activity of CatG was increased in the presence of LF in granulocyte-derived supernatant. Furthermore, LF enhanced CatG-induced activation of platelets as determined by cell surface expression of CD62P. Consequently, LF-mediated enhancement of CatG activity might promote innate immunity during acute inflammation.

Staphylococcus aureus protects its immune-evasion proteins against degradation by neutrophil serine proteases

Neutrophils store large quantities of neutrophil serine proteases (NSPs) that contribute, via multiple mechanisms, to antibacterial immune defences. Even though neutrophils are indispensable in fighting Staphylococcus aureus infections, the importance of NSPs in anti-staphylococcal defence is yet unknown. However, the fact that S. aureus produces three highly specific inhibitors for NSPs [the extracellular adherence proteins (EAPs) Eap, EapH1 and EapH2], suggests that these proteases are important for host defences against this bacterium. In this study we demonstrate that NSPs can inactivate secreted virulence factors of S. aureus and that EAP proteins function to prevent this degradation. Specifically, we find that a large group of S. aureus immune-evasion proteins is vulnerable to proteolytic inactivation by NSPs. In most cases, NSP cleavage leads to functional inactivation of virulence proteins. Interestingly, proteins with similar immune-escape functions appeared to have differential cleavage sensitivity towards NSPs. Using targeted mutagenesis and complementation analyses in S. aureus, we demonstrate that all EAP proteins can protect other virulence factors from NSP degradation in complex bacterial supernatants. These findings show that NSPs inactivate S. aureus virulence factors. Moreover, the protection by EAP proteins can explain why this antibacterial function of NSPs was masked in previous studies. Furthermore, our results indicate that therapeutic inactivation of EAP proteins can help to restore the natural host immune defences against S. aureus.

Neutrophil serine proteases in antibacterial defense

Neutrophil serine proteases (NSPs) are critical for the effective functioning of neutrophils and greatly contribute to immune protection against bacterial infections. Thanks to their broad substrate specificity, these chymotrypsin-like proteases trigger multiple reactions that are detrimental to bacterial survival such as direct bacterial killing, generation of antimicrobial peptides, inactivation of bacterial virulence factors and formation of neutrophil extracellular traps. Recently, the importance of NSPs in antibacterial defenses has been further underscored by discoveries of unique bacterial evasion strategies to combat these proteases. Bacteria can indirectly disarm NSPs by protecting bacterial substrates against NSP cleavage, but also produce inhibitory molecules that potently block NSPs. Here we review recent insights in the functional contribution of NSPs in host protection against bacterial infections and the elegant strategies that bacteria use to counteract these responses.

Staphylococcus aureus secretes a unique class of neutrophil serine protease inhibitors

Neutrophils are indispensable for clearing infections with the prominent human pathogen Staphylococcus aureus. Here, we report that S. aureus secretes a family of proteins that potently inhibits the activity of neutrophil serine proteases (NSPs): neutrophil elastase (NE), proteinase 3, and cathepsin G. The NSPs, but not related serine proteases, are specifically blocked by the extracellular adherence protein (Eap) and the functionally orphan Eap homologs EapH1 and EapH2, with inhibitory-constant values in the low-nanomolar range. Eap proteins are together essential for NSP inhibition by S. aureus in vitro and promote staphylococcal infection in vivo. The crystal structure of the EapH1/NE complex showed that Eap molecules constitute a unique class of noncovalent protease inhibitors that occlude the catalytic cleft of NSPs. These findings increase our insights into the complex pathogenesis of S. aureus infections and create opportunities to design novel treatment strategies for inflammatory conditions related to excessive NSP activity.

Cathepsin G activity lowers plasma LDL and reduces atherosclerosis

Cathepsin G (CatG), a serine protease present in mast cells and neutrophils, can produce angiotensin-II (Ang-II) and degrade elastin. Here we demonstrate increased CatG expression in smooth muscle cells (SMCs), endothelial cells (ECs), macrophages, and T cells from human atherosclerotic lesions. In low-density lipoprotein (LDL) receptor-deficient (Ldlr(-/-)) mice, the absence of CatG reduces arterial wall elastin degradation and attenuates early atherosclerosis when mice consume a Western diet for 3months. When mice consume this diet for 6months, however, CatG deficiency exacerbates atherosclerosis in aortic arch without affecting lesion inflammatory cell content or extracellular matrix accumulation, but raises plasma total cholesterol and LDL levels without affecting high-density lipoprotein (HDL) or triglyceride levels. Patients with atherosclerosis also have significantly reduced plasma CatG levels that correlate inversely with total cholesterol (r=-0.535, P<0.0001) and LDL cholesterol (r=-0.559, P<0.0001), but not with HDL cholesterol (P=0.901) or triglycerides (P=0.186). Such inverse correlations with total cholesterol (r=-0.504, P<0.0001) and LDL cholesterol (r=-0.502, P<0.0001) remain significant after adjusting for lipid lowering treatments among this patient population. Human CatG degrades purified human LDL, but not HDL. This study suggests that CatG promotes early atherogenesis through its elastinolytic activity, but suppresses late progression of atherosclerosis by degrading LDL without affecting HDL or triglycerides.

Cathepsin G deficiency reduces periaortic calcium chloride injury-induced abdominal aortic aneurysms in mice

OBJECTIVE

Cathepsin G (CatG) is a serine protease that mediates angiotensin I to angiotensin II (Ang-II) conversion and is highly expressed in human abdominal aortic aneurysms (AAAs). However, it remains untested whether this protease participates in the pathogenesis of AAA.

METHODS AND RESULTS

Immunofluorescent double staining demonstrated the expression of CatG in smooth muscle cells (SMCs), macrophages, and endothelial cells in human AAA lesions (n = 12) but not in AAA-free aortas (n = 10). Whereas inflammatory cytokines induced CatG expression, high glucose concentration increased CatG activity in producing Ang-II and angiotensin-converting enzyme in SMCs, which could be fully blocked by a CatG-selective inhibitor or its small interfering RNA. To test whether CatG contributes to AAA development, we generated CatG and low-density lipoprotein receptor double deficient (Ldlr(-/-)Ctsg(-/-)) mice and their littermate controls (Ldlr(-/-)Ctsg(+/+)). Absence of CatG did not affect Ang-II infusion-induced AAAs. In contrast, in Ang-II-independent AAAs induced by periaortic CaCl2 injury (n = 12 per group), CatG deficiency significantly reduced aortic diameter increase (58.33% ± 6.83% vs 31.67% ± 5.75%; P = .007), aortic lesion area (0.35 ± 0.04 mm(2) vs 0.21 ± 0.02 mm(2); P = .005), and aortic wall elastin fragmentation grade (2.75 ± 0.18 vs 1.58 ± 0.17; P = .002) along with reduced lesion collagen content grade (2.80 ± 0.17 vs 2.12 ± 0.17; P = .009) without affecting indices of lesion inflammation, angiogenesis, cell proliferation, or apoptosis. In vitro elastin degradation assays demonstrated that CaCl2-induced AAA lesions from Ldlr(-/-)Ctsg(-/-) mice contained much lower elastinolytic activity than in those from littermate control mice. Gelatin gel zymogram assay suggested that absence of CatG in CaCl2-induced AAA lesions also reduced the activity of elastinolytic matrix metalloproteinases 2 and 9.

CONCLUSIONS

CatG may contribute to CaCl2-induced experimental AAAs directly through its elastinolytic activity and indirectly by regulating lesion matrix metalloproteinases 2 and 9 activities. Increased expression of CatG in vascular and inflammatory cells of human AAAs and its increased activity in producing Ang-II and angiotensin-converting enzyme by SMCs suggest an additional mechanism by which CatG contributes to AAA lesion progression.

Cathepsin G-dependent modulation of platelet thrombus formation in vivo by blood neutrophils

Neutrophils are consistently associated with arterial thrombotic morbidity in human clinical studies but the causal basis for this association is unclear. We tested the hypothesis that neutrophils modulate platelet activation and thrombus formation in vivo in a cathepsin G-dependent manner. Neutrophils enhanced aggregation of human platelets in vitro in dose-dependent fashion and this effect was diminished by pharmacologic inhibition of cathepsin G activity and knockdown of cathepsin G expression. Tail bleeding time in the mouse was prolonged by a cathepsin G inhibitor and in cathepsin G knockout mice, and formation of neutrophil-platelet conjugates in blood that was shed from transected tails was reduced in the absence of cathepsin G. Bleeding time was highly correlated with blood neutrophil count in wildtype but not cathepsin G deficient mice. In the presence of elevated blood neutrophil counts, the anti-thrombotic effect of cathepsin G inhibition was greater than that of aspirin and additive to it when administered in combination. Both pharmacologic inhibition of cathepsin G and its congenital absence prolonged the time for platelet thrombus to form in ferric chloride-injured mouse mesenteric arterioles. In a vaso-occlusive model of ischemic stroke, inhibition of cathepsin G and its congenital absence improved cerebral blood flow, reduced histologic brain injury, and improved neurobehavioral outcome. These experiments demonstrate that neutrophil cathepsin G is a physiologic modulator of platelet thrombus formation in vivo and has potential as a target for novel anti-thrombotic therapies.

SerpinB1 is critical for neutrophil survival through cell-autonomous inhibition of cathepsin G

Bone marrow (BM) holds a large reserve of polymorphonuclear neutrophils (PMNs) that are rapidly mobilized to the circulation and tissues in response to danger signals. SerpinB1 is a potent inhibitor of neutrophil serine proteases neutrophil elastase (NE) and cathepsin G (CG). SerpinB1 deficiency (sB1(-/-)) results in a severe reduction of the BM PMN reserve and failure to clear bacterial infection. Using BM chimera, we found that serpinB1 deficiency in BM cells was necessary and sufficient to reproduce the BM neutropenia of sB1(-/-) mice. Moreover, we showed that genetic deletion of CG, but not NE, fully rescued the BM neutropenia in sB1(-/-) mice. In mixed BM chimera and in vitro survival studies, we showed that CG modulates sB1(-/-) PMN survival through a cell-intrinsic pathway. In addition, membrane permeabilization by lysosomotropic agent l-leucyl-l-leucine methyl ester that allows cytosolic release of granule contents was sufficient to induce rapid PMN death through a CG-dependent pathway. CG-mediated PMN cytotoxicity was only partly blocked by caspase inhibition, suggesting that CG cleaves a distinct set of targets during apoptosis. In conclusion, we have unveiled a new cytotoxic function for the serine protease CG and showed that serpinB1 is critical for maintaining PMN survival by antagonizing intracellular CG activity.

Neutrophil-derived cathelicidin promotes adhesion of classical monocytes

RATIONALE

The leukocyte response in acute inflammation is characterized by an initial recruitment of neutrophils preceding a second wave of monocytes. Neutrophil-derived granule proteins were suggested to hold an important role in this cellular switch. The exact mechanisms by which neutrophils mediate these processes are only partially understood.

OBJECTIVE

To investigate the role of neutrophils and their granule contents in the adhesion of monocyte subpopulations in acute inflammation.

METHODS AND RESULTS

Here, we show that neutrophil-derived cathelicidins (human: LL37, mouse: CRAMP) induce adhesion of classical monocytes but not of nonclassical monocytes in the mouse cremaster muscle and in in vitro flow chamber assays. CRAMP is released from emigrated neutrophils and then transported across the endothelium, where it is presented to rolling leukocytes. Endothelial-bound cathelicidin activates formyl-peptide receptor 2 on classical monocytes, resulting in monocytic β1- and β2-integrin conformational change toward an extended, active conformation that allows for adhesion to their respective ligands, vascular cell adhesion molecule 1 and intercellular adhesion molecule 1.

CONCLUSIONS

These data elucidate a novel mechanism of neutrophil-mediated monocyte recruitment, which could be targeted in conditions where recruitment of classical monocytes plays an unfavorable role.

The chymase mouse mast cell protease 4 degrades TNF, limits inflammation, and promotes survival in a model of sepsis

Mouse mast cell protease 4 (mMCP-4), the mouse counterpart of human mast cell chymase, is thought to have proinflammatory effects in innate or adaptive immune responses associated with mast cell activation. However, human chymase can degrade the proinflammatory cytokine TNF, a mediator that can be produced by mast cells and many other cell types. We found that mMCP-4 can reduce levels of mouse mast cell-derived TNF in vitro through degradation of transmembrane and soluble TNF. We assessed the effects of interactions between mMCP-4 and TNF in vivo by analyzing the features of a classic model of polymicrobial sepsis, cecal ligation and puncture (CLP), in C57BL/6J-mMCP-4-deficient mice versus C57BL/6J wild-type mice, and in C57BL/6J-Kit(W-sh/W-sh) mice containing adoptively transferred mast cells that were either wild type or lacked mMCP-4, TNF, or both mediators. The mMCP-4-deficient mice exhibited increased levels of intraperitoneal TNF, higher numbers of peritoneal neutrophils, and increased acute kidney injury after CLP, and also had significantly higher mortality after this procedure. Our findings support the conclusion that mMCP-4 can enhance survival after CLP at least in part by limiting detrimental effects of TNF, and suggest that mast cell chymase may represent an important negative regulator of TNF in vivo.

Cathepsin G-regulated release of formyl peptide receptor agonists modulate neutrophil effector functions

Neutrophil serine proteases play an important role in inflammation by modulating neutrophil effector functions. We have previously shown that neutrophils deficient in the serine proteases cathepsin G and neutrophil elastase (CG/NE neutrophils) exhibit severe defects in chemokine CXCL2 release and reactive oxygen species (ROS) production when activated on immobilized immune complex. Exogenously added active CG rescues these defects, but the mechanism remains undefined. Using a protease-based proteomic approach, we found that, in vitro, the addition of exogenous CG to immune complex-stimulated CG/NE neutrophils led to a decrease in the level of cell-associated annexin A1 (AnxA1) and cathelin-related antimicrobial peptide (CRAMP), both known inflammatory mediators. We further confirmed that, in vivo, CG was required for the extracellular release of AnxA1 and CRAMP in a subcutaneous air pouch model. In vitro, CG efficiently cleaved AnxA1, releasing the active N-terminal peptide Ac2-26, and processed CRAMP in limited fashion. Ac2-26 and CRAMP peptides enhanced the release of CXCL2 by CG/NE neutrophils in a dose-dependent manner via formyl peptide receptor (FPR) stimulation. Blockade of FPRs by an antagonist, Boc2 (t-Boc-Phe-d-Leu-Phe-d-Leu-Phe), abrogates CXCL2 release, whereas addition of FPR agonists, fMLF and F2L, relieves Boc2 inhibition. Furthermore, the addition of active CG, but not inactive CG, also relieves Boc2 inhibition. These findings suggest that CG modulates neutrophil effector functions partly by controlling the release (and proteolysis) of FPR agonists. Unexpectedly, we found that mature CRAMP, but not Ac2-26, induced ROS production through an FPR-independent pathway.

Neutrophil serine proteases promote IL-1β generation and injury in necrotizing crescentic glomerulonephritis

The pathogenesis of anti-neutrophil cytoplasmic antibody (ANCA)-associated necrotizing crescentic GN (NCGN) is incompletely understood. Dipeptidyl peptidase I (DPPI) is a cysteine protease required for the activation of neutrophil serine proteases (NSPs) cathepsin G, neutrophil elastase, and proteinase 3, which are enzymes that modulate inflammation. We used a mouse model of anti-myeloperoxidase (MPO) antibody-induced NCGN to determine whether active NSPs contribute to its pathogenesis. MPO-deficient animals immunized with murine MPO, irradiated, and transplanted with wild-type bone marrow developed NCGN. In contrast, transplantation with bone marrow that lacked DPPI or lacked both neutrophil elastase and proteinase 3 protected mice from NCGN induced by anti-MPO antibody. The kidneys of mice reconstituted with DPPI-deficient bone marrow generated significantly less IL-1β than did those of mice reconstituted with wild-type bone marrow; similarly, in vitro, DPPI-deficient monocytes produced significantly less IL-1β in response to anti-MPO antibody than did wild-type monocytes. This reduction in IL-1β was NSP dependent; exogenous addition of PR3 restored IL-β production in DPPI-deficient monocytes. Last, the IL-1 receptor antagonist anakinra protected animals against anti-MPO antibody-induced NCGN (16.7%±6.0% versus 2.4%±1.7% crescents), suggesting that IL-1β is a critical inflammatory mediator in this model. These data suggest that the development of anti-MPO antibody-induced NCGN requires NSP-dependent IL-1β generation and that these processes may provide therapeutic targets for ANCA-mediated diseases in humans.

Towards a four-dimensional view of neutrophils

Neutrophils are constitutively produced throughout adult life and are essential for host responses to many types of pathogen. Neutropenia has long been associated with poor prognosis in the clinic, yet we have an incomplete understanding of their life cycle, not only during homeostasis but also during infection and chronic inflammation. Here, we review recent advances that provide insight into the genetic and biochemical regulators of neutrophil production, function, and survival.

Role of NADPH oxidase versus neutrophil proteases in antimicrobial host defense

NADPH oxidase is a crucial enzyme in mediating antimicrobial host defense and in regulating inflammation. Patients with chronic granulomatous disease, an inherited disorder of NADPH oxidase in which phagocytes are defective in generation of reactive oxidant intermediates (ROIs), suffer from life-threatening bacterial and fungal infections. The mechanisms by which NADPH oxidase mediate host defense are unclear. In addition to ROI generation, neutrophil NADPH oxidase activation is linked to the release of sequestered proteases that are posited to be critical effectors of host defense. To definitively determine the contribution of NADPH oxidase versus neutrophil serine proteases, we evaluated susceptibility to fungal and bacterial infection in mice with engineered disruptions of these pathways. NADPH oxidase-deficient mice (p47^phox−/−) were highly susceptible to pulmonary infection with Aspergillus fumigatus. In contrast, double knockout neutrophil elastase (NE)^−/−×cathepsin G (CG)^−/− mice and lysosomal cysteine protease cathepsin C/dipeptidyl peptidase I (DPPI)-deficient mice that are defective in neutrophil serine protease activation demonstrated no impairment in antifungal host defense. In separate studies of systemic Burkholderia cepacia infection, uniform fatality occurred in p47^phox−/− mice, whereas NE^−/−×CG^−/− mice cleared infection. Together, these results show a critical role for NADPH oxidase in antimicrobial host defense against A. fumigatus and B. cepacia, whereas the proteases we evaluated were dispensable. Our results indicate that NADPH oxidase dependent pathways separate from neutrophil serine protease activation are required for host defense against specific pathogens.

Carcinoma mucins trigger reciprocal activation of platelets and neutrophils in a murine model of Trousseau syndrome

Trousseau syndrome is classically defined as migratory, heparin-sensitive but warfarin-resistant microthrombi in patients with occult, mucinous adenocarcinomas. Injecting carcinoma mucins into mice generates platelet-rich microthrombi dependent on P- and L-selectin but not thrombin. Heparin prevents mucin binding to P- and L-selectin and mucin-induced microthrombi. This model of Trousseau syndrome explains resistance to warfarin, which inhibits fluid-phase coagulation but not selectins. Here we found that carcinoma mucins do not generate microthrombi in mice lacking P-selectin glycoprotein ligand-1 (PSGL-1), the leukocyte ligand for P- and L-selectin. Furthermore, mucins did not activate platelets in blood from PSGL-1-deficient mice. Mucins induced microthrombi in radiation chimeras lacking endothelial P-selectin but not in chimeras lacking platelet P-selectin. Mucins caused leukocytes to release cathepsin G, but only if platelets were present. Mucins failed to generate microthrombi in cathepsin G-deficient mice. Mucins did not activate platelets in blood from mice lacking cathepsin G or protease-activated receptor-4 (PAR4), indicating that cathepsin G activates platelets through PAR4. Using knockout mice and blocking antibodies, we found that mucin-triggered cathepsin G release requires L-selectin and PSGL-1 on neutrophils, P-selectin on platelets, and Src family kinases in both cell types. Thus, carcinoma mucins promote thrombosis through adhesion-dependent, bidirectional signaling in neutrophils and platelets.

Deletion of Serpina1a, a murine α1-antitrypsin ortholog, results in embryonic lethality

Chronic obstructive pulmonary disease (COPD) is the fourth leading cause of death in the United States Approximately 1% to 2% of COPD patients suffer from α(1)-antitrypsin (A1AT) deficiency, the major inheritable predisposition to COPD/emphysema. To further study the role of A1AT deficiency in the pathogenesis of COPD/emphysema, the authors attempted to generate null-mutant mice for Serpina1a, 1 of 2 A1AT orthologs in mice. Here the authors show that targeted deletion of Serpina1a results in embryonic lethality prior to 8.5 days post conception (dpc). The results are surprising given that A1AT-null humans exist and therefore do not require this gene product for normal development. The Serpina1 gene cluster is substantially different between mouse and man. Through gene duplication, mice have 3 to 5 (depending on the strain) highly homologous proteinase inhibiting (Pi) genes, 2 of which inhibit neutrophil elastase. Despite the abundance of Pi genes in mice, Serpina1a serves a critical, nonredundant function during early mouse development. A1AT-deficient mice have been highly sought after to study emphysema, cancer, and liver disease, and as a model to perfect gene replacement therapy. These results highlight important differences between human and murine serpins and point to the difficulty inherent to using gene-targeted mice to study this common human genetic disease.

Neutrophil elastase, proteinase 3, and cathepsin G as therapeutic targets in human diseases

Polymorphonuclear neutrophils are the first cells recruited to inflammatory sites and form the earliest line of defense against invading microorganisms. Neutrophil elastase, proteinase 3, and cathepsin G are three hematopoietic serine proteases stored in large quantities in neutrophil cytoplasmic azurophilic granules. They act in combination with reactive oxygen species to help degrade engulfed microorganisms inside phagolysosomes. These proteases are also externalized in an active form during neutrophil activation at inflammatory sites, thus contributing to the regulation of inflammatory and immune responses. As multifunctional proteases, they also play a regulatory role in noninfectious inflammatory diseases. Mutations in the ELA2/ELANE gene, encoding neutrophil elastase, are the cause of human congenital neutropenia. Neutrophil membrane-bound proteinase 3 serves as an autoantigen in Wegener granulomatosis, a systemic autoimmune vasculitis. All three proteases are affected by mutations of the gene (CTSC) encoding dipeptidyl peptidase I, a protease required for activation of their proform before storage in cytoplasmic granules. Mutations of CTSC cause Papillon-Lefèvre syndrome. Because of their roles in host defense and disease, elastase, proteinase 3, and cathepsin G are of interest as potential therapeutic targets. In this review, we describe the physicochemical functions of these proteases, toward a goal of better delineating their role in human diseases and identifying new therapeutic strategies based on the modulation of their bioavailability and activity. We also describe how nonhuman primate experimental models could assist with testing the efficacy of proposed therapeutic strategies.

Luminal cathepsin g and protease-activated receptor 4: a duet involved in alterations of the colonic epithelial barrier in ulcerative colitis

Impairment of the colonic epithelial barrier and neutrophil infiltration are common features of inflammatory bowel disease. Luminal proteases affect colonic permeability through protease-activated receptors (PARs). We evaluated: (i) whether fecal supernatants from patients with ulcerative colitis (UC) trigger alterations of colonic paracellular permeability and inflammation, and (ii) the roles of cathepsin G (Cat-G), a neutrophil serine protease, and its selective receptor, PAR(4), in these processes. Expression levels of both PAR(4) and Cat-G were determined in colonic biopsies from UC and healthy subjects. The effects of UC fecal supernatants on colonic paracellular permeability were measured in murine colonic strips. Involvement of Cat-G and PAR(4) was evaluated using pepducin P4pal-10 and specific Cat-G inhibitor (SCGI), respectively. In addition, the effect of PAR(4)-activating peptide was assessed. UC fecal supernatants, either untreated or pretreated with SCGI, were infused into mice, and myeloperoxidase activity was determined. PAR(4) was found to be overexpressed in UC colonic biopsies. Increased colonic paracellular permeability that was triggered by UC fecal supernatants was blocked by both SCGI (77%) and P4pal-10 (85%). Intracolonic infusion of UC fecal supernatants into mice increased myeloperoxidase activity. This effect was abolished by SCGI. These observations support that both Cat-G and PAR(4) play key roles in generating and/or amplifying relapses in UC and provide a rationale for the development of new therapeutic agents in the treatment of this disease.

Role of bone marrow renin-angiotensin system in the pathogenesis of atherosclerosis

The renin-angiotensin system (RAS) has been considered to be a circulating hormonal system that regulates blood pressure, blood flow, fluid volume and electrolyte balance. A growing body of evidence indicates local effects of an activated RAS, particularly in the cardiac, vascular, and renal systems. It is now well established that RAS, especially angiotensin II (Ang II) and Ang II type 1 receptor (AT1R) pathway, has significant pro-inflammatory actions on the vessel wall, leading to progression of atherosclerosis. Recent reports suggest that an activated RAS has local effects in bone marrow (BM), which contributes to the regulation of normal and malignant hematologic processes. We reported that AT1aR in BM cells participate in the pathogenesis of atherosclerosis by analyzing several BM chimeric mice whose BM cells were positive or negative for AT1aR. These results suggest that blockade of AT1R not only in vascular cells but also in BM could be an important strategy to prevent atherosclerosis. In this review, we overview recent findings on a role of RAS in the pathogenesis of atherosclerosis, and discuss functional contribution of a local RAS in BM to progression and destabilization of atherosclerotic plaque.

Moraxella-dependent alpha 1-antichymotrypsin neutralization: a unique virulence mechanism

The acute phase reactant and protease inhibitor alpha(1)-antichymotrypsin is considered to play a protective role in the airways, but whether it interacts with respiratory bacteria is not known. We analyzed whether the common respiratory pathogens Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, and other bacterial species interact with antichymotrypsin. M. catarrhalis was the only species that bound antichymotrypsin among 25 bacterial species tested by flow cytometry and direct binding assay. We compared a series of clinical isolates in addition to wild-type and ubiquitous surface protein-deficient Moraxella to study the nature of antichymotrypsin binding by the bacteria. Experiments with Moraxella mutants revealed that ubiquitous surface proteins A1 and A2 were responsible for the interaction, and using recombinant fragments, a consensus sequence within ubiquitous surface proteins A1 and A2 was defined. Binding of iodine-labeled antichymotrypsin was dose dependent and strong (dissociation constant [K(d)] 24.9-44.8 nM). Moreover, a chymotrypsin activity assay showed that antichymotrypsin, when bound to the bacterial surface, was neutralized. Moraxella antichymotrypsin neutralization is a novel microbial virulence mechanism that may induce excessive inflammation resulting in more exposed extracellular matrix that is beneficial for bacterial colonization.

How human neutrophils kill and degrade microbes: an integrated view

Neutrophils constitute the dominant cell in the circulation that mediates the earliest innate immune human responses to infection. The morbidity and mortality from infection rise dramatically in patients with quantitative or qualitative neutrophil defects, providing clinical confirmation of the important role of normal neutrophils for human health. Neutrophil-dependent anti-microbial activity against ingested microbes represents the collaboration of multiple agents, including those prefabricated during granulocyte development in the bone marrow and those generated de novo following neutrophil activation. Furthermore, neutrophils cooperate with extracellular agents as well as other immune cells to optimally kill and degrade invading microbes. This brief review focuses attention on two examples of the integrated nature of neutrophil-mediated anti-microbial action within the phagosome. The importance and complexity of myeloperoxidase-mediated events illustrate a collaboration of anti-microbial responses that are endogenous to the neutrophil, whereas the synergy between the phagocyte NADPH (nicotinamide adenine dinucleotide phosphate) oxidase and plasma-derived group IIA phospholipase A(2) exemplifies the collective effects of the neutrophil with an exogenous factor to achieve degradation of ingested staphylococci.

The neutrophil serine protease inhibitor serpinb1 preserves lung defense functions in Pseudomonas aeruginosa infection

Neutrophil serine proteases (NSPs; elastase, cathepsin G, and proteinase-3) directly kill invading microbes. However, excess NSPs in the lungs play a central role in the pathology of inflammatory pulmonary disease. We show that serpinb1, an efficient inhibitor of the three NSPs, preserves cell and molecular components responsible for host defense against Pseudomonas aeruginosa. On infection, wild-type (WT) and serpinb1-deficient mice mount similar early responses, including robust production of cytokines and chemokines, recruitment of neutrophils, and initial containment of bacteria. However, serpinb1^−/− mice have considerably increased mortality relative to WT mice in association with late-onset failed bacterial clearance. We found that serpinb1-deficient neutrophils recruited to the lungs have an intrinsic defect in survival accompanied by release of neutrophil protease activity, sustained inflammatory cytokine production, and proteolysis of the collectin surfactant protein–D (SP-D). Coadministration of recombinant SERPINB1 with the P. aeruginosa inoculum normalized bacterial clearance in serpinb1^−/− mice. Thus, regulation of pulmonary innate immunity by serpinb1 is nonredundant and is required to protect two key components, the neutrophil and SP-D, from NSP damage during the host response to infection.

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.

Cathepsin g is required for sustained inflammation and tissue injury after reperfusion of ischemic kidneys

Neutrophil activation to release granules containing proteases and other enzymes is a primary cause of tissue damage during ischemia/reperfusion injury. Because the contribution of specific granule enzymes to this injury remains poorly defined, the role of cathepsin G in renal ischemia/reperfusion injury was tested. Bilateral renal ischemia led to the expiration of 64% of wild-type mice within 4 days of reperfusion, whereas all cathepsin G-deficient mice survived. Serum creatinine increased to similar levels at 24 hours after reperfusion and then decreased to background in both groups of mice. Ischemic kidneys from both groups had similar levels of neutrophil infiltration and of CXCL1, CXCL2, and myeloperoxidase protein 9 hours after reperfusion, but at 24 hours, these acute inflammatory response components were decreased more than 50% in kidneys from cathepsin G-deficient versus wild-type mice. Ischemic kidneys from surviving wild-type mice had severe tubular necrosis and tubular cell apoptosis 24 hours after reperfusion with subsequent development of fibrosis 30 days later. In contrast, ischemic kidneys from cathepsin G-deficient mice had a 70% decrease in tubular cell apoptosis with little detectable collagen deposition. These data identify cathepsin G as a critical component sustaining neutrophil-mediated acute tissue pathology and subsequent fibrosis after renal ischemia/reperfusion injury.

LPS responsiveness and neutrophil chemotaxis in vivo require PMN MMP-8 activity

We identify matrix metalloproteinase (MMP)-8, the polymorphonuclear (PMN) leukocyte collagenase, as a critical mediator initiating lipopolysaccharide (LPS)-responsiveness in vivo. PMN infiltration towards LPS is abrogated in Mmp8-null mice. MMP-8 cleaves LPS-induced CXC chemokine (LIX) at Ser(4)-Val(5) and Lys(79)-Arg(80). LIX bioactivity is increased upon N-terminal cleavage, enhancing intracellular calcium mobilization and chemotaxis upon binding its cognate receptor, CXCR2. As there is no difference in PMN chemotaxis in Mmp8-null mice compared with wild-type mice towards synthetic analogues of MMP-8-cleaved LIX, MMP-8 is not essential for extravasation or cell migration in collagenous matrices in vivo. However, with biochemical redundancy between MMPs 1, 2, 9, and 13, which also cleave LIX at position 4 approximately 5, it was surprising to observe such a markedly reduced PMN infiltration towards LPS and LIX in Mmp8-/- mice. This lack of physiological redundancy in vivo identifies MMP-8 as a key mediator in the regulation of innate immunity. Comparable results were found with CXCL8/IL-8 and CXCL5/ENA-78, the human orthologues of LIX. MMP-8 cleaves CXCL8 at Arg(5)-Ser(6) and at Val(7)-Leu(8) in CXCL5 to activate respective chemokines. Hence, rather than collagen, these PMN chemoattractants are important MMP-8 substrates in vivo; PMN-derived MMP-8 cleaves and activates LIX to execute an in cis PMN-controlled feed-forward mechanism to orchestrate the initial inflammatory response and promote LPS responsiveness in tissue.

Residual active granzyme B in cathepsin C-null lymphocytes is sufficient for perforin-dependent target cell apoptosis

Cathepsin C activates serine proteases expressed in hematopoietic cells by cleaving an N-terminal dipeptide from the proenzyme upon granule packaging. The lymphocytes of cathepsin C–null mice are therefore proposed to totally lack granzyme B activity and perforin-dependent cytotoxicity. Surprisingly, we show, using live cell microscopy and other methodologies, that cells targeted by allogenic CD8⁺ cytotoxic T lymphocyte (CTL) raised in cathepsin C–null mice die through perforin-dependent apoptosis indistinguishable from that induced by wild-type CTL. The cathepsin C–null CTL expressed reduced but still appreciable granzyme B activity, but minimal granzyme A activity. Also, in contrast to mice with inactivation of both their granzyme A/B genes, cathepsin C deficiency did not confer susceptibility to ectromelia virus infection in vivo. Overall, our results indicate that although cathepsin C clearly generates the majority of granzyme B activity, some is still generated in its absence, pointing to alternative mechanisms for granzyme B processing and activation. Cathepsin C deficiency also results in considerably milder immune deficiency than perforin or granzyme A/B deficiency.

Cathepsin-G interferes with clearance of Pseudomonas aeruginosa from mouse lungs

The cystic fibrosis airway is susceptible to Pseudomonas aeruginosa infection, which stimulates an intense inflammatory response leading to airway obstruction and bronchiectasis. Neutrophils migrate into the airway, and once there, release high concentrations of neutral serine proteases during phagocytosis and in death. In particular, neutrophil elastase is central to progression of bronchiectasis by interfering with bacterial clearance and directly perpetuating the inflammatory response in the airway. Using a murine model of endobronchial inflammation, we found that a different neutrophil-derived serine protease, cathepsin G, inhibited the host's ability to clear Pseudomonas from the lung, based on a 1-log reduction in bacteria recovered from cathepsin G-deficient mice. Higher antibody concentrations were found in respiratory epithelial lining fluid from mice lacking cathepsin G, but there was no difference in other opsonins, such as surfactant proteins A and D. Chemokine levels measured in the lung correlated with bacterial burden and not the animal's genotype, indicating that airway inflammation was not affected by the presence (or absence) of specific serine proteases. These findings suggest that cathepsin G interferes with airway defenses, showing that proteases other than neutrophil elastase have roles in the pathogenesis of suppurative airway diseases.

Role of polymorphonuclear leukocyte-derived serine proteinases in defense against Actinobacillus actinomycetemcomitans

Periodontitis is a chronic destructive infection of the tooth-supportive tissues, which is caused by pathogenic bacteria such as Actinobacillus actinomycetemcomitans. A severe form of periodontitis is found in Papillon-Lefèvre syndrome (PLS), an inheritable disease caused by loss-of-function mutations in the cathepsin C gene. Recently, we demonstrated that these patients lack the activity of the polymorphonuclear leukocyte (PMN)-derived serine proteinases elastase, cathepsin G, and proteinase 3. In the present study we identified possible pathways along which serine proteinases may be involved in the defense against A. actinomycetemcomitans. Serine proteinases are capable to convert the PMN-derived hCAP-18 into LL-37, an antimicrobial peptide with activity against A. actinomycetemcomitans. We found that the PMNs of PLS patients released lower levels of LL-37. Furthermore, because of their deficiency in serine proteases, the PMNs of PLS patients were incapable of neutralizing the leukotoxin produced by this pathogen, which resulted in increased cell damage. Finally, the capacity of PMNs from PLS patients to kill A. actinomycetemcomitans in an anaerobic environment, such as that found in the periodontal pocket, seemed to be reduced. Our report demonstrates a mechanism that suggests a direct link between an inheritable defect in PMN functioning and difficulty in coping with a periodontitis-associated pathogen.

Serine protease cathepsin G regulates adhesion-dependent neutrophil effector functions by modulating integrin clustering

The polymorphonuclear leukocyte (PMN)-derived serine proteases play a key role in immune complex (IC)-mediated inflammation. However, the mechanisms by which these proteases regulate inflammatory response remain largely undefined. Here, we show that IC-activated cathepsin G- and neutrophil elastase-deficient (CG/NE) PMNs adhered normally to IC-coated surfaces but did not undergo CD11b clustering and failed to initiate cytoskeletal reorganization and cell spreading. As a result, CG/NE-deficient PMNs exhibited severe defects in MIP-2 secretion and reactive oxygen intermediates production. Exogenously added CG, but not proteolytically inactive CG, was sufficient to restore these defects. These findings identify an important role for CG in integrin-dependent PMN effector functions that are separate from and downstream of integrin-dependent adhesion.

Quantitative Proteomic Analysis Using Isobaric Protein Tags Enables Rapid Comparison of Changes in Transcript and Protein Levels in Transformed Cells

Isobaric tags for relative and absolute quantitation, an approach to concurrent, relative quantification of proteins present in four cell preparations, have recently been described. To validate this approach using complex mammalian cell samples that show subtle differences in protein levels, a model stem cell-like cell line (FDCP-mix) in the presence or absence of the leukemogenic oncogene TEL/PDGFRbeta has been studied. Cell lysates were proteolytically digested, and peptides within each sample were labeled with one of four isobaric, isotope-coded tags via their N-terminal and/or lysine side chains. The four labeled samples are mixed and peptides separated by two-dimensional liquid chromatography online to a mass spectrometer (LC-MS). Upon peptide fragmentation, each tag releases a distinct mass reporter ion; the ratio of the four reporters therefore gives relative abundances of the given peptide. Relative quantification of proteins is derived using summed data from a number of peptides. TEL/PDGFRbeta leukemic oncogene-mediated changes in protein levels were compared with those seen in microarray analysis of control and transfected FDCP-mix cells. Changes at the protein level in most cases reflected those seen at the transcriptome level. Nonetheless, novel differences in protein expression were found that indicate potential mechanisms for effects of this oncogene.

How neutrophils kill microbes

Neutrophils provide the first line of defense of the innate immune system by phagocytosing, killing, and digesting bacteria and fungi. Killing was previously believed to be accomplished by oxygen free radicals and other reactive oxygen species generated by the NADPH oxidase, and by oxidized halides produced by myeloperoxidase. We now know this is incorrect. The oxidase pumps electrons into the phagocytic vacuole, thereby inducing a charge across the membrane that must be compensated. The movement of compensating ions produces conditions in the vacuole conducive to microbial killing and digestion by enzymes released into the vacuole from the cytoplasmic granules.

Methionine sulfoxide and proteolytic cleavage contribute to the inactivation of cathepsin G by hypochlorous acid: an oxidative mechanism for regulation of serine proteinases by myeloperoxidase

Using myeloperoxidase and hydrogen peroxide, activated neutrophils produce high local concentrations of hypochlorous acid (HOCl). They also secrete cathepsin G, a serine protease implicated in cytokine release, receptor activation, and degradation of tissue proteins. Isolated cathepsin G was inactivated by HOCl but not by hydrogen peroxide in vitro. We found that activated neutrophils lost cathepsin G activity by a pathway requiring myeloperoxidase, suggesting that oxidants generated by myeloperoxidase might regulate cathepsin G activity in vivo. Tandem mass spectrometric analysis of oxidized cathepsin G revealed that loss of a peptide containing Asp108, which lies in the active site, associated quantitatively with loss of enzymatic activity. Catalytic domain peptides containing Asp108 were lost from the oxidized protein in concert with the conversion of Met110 to the sulfoxide. Release of this peptide was blocked by pretreating cathepsin G with phenylmethylsulfonyl fluoride, strongly implying that oxidation introduced proteolytic cleavage sites into cathepsin G. Model system studies demonstrated that methionine oxidation can direct the regiospecific proteolysis of peptides by cathepsin G. Thus, oxidation of Met110 may contribute to cathepsin G inactivation by at least two distinct mechanisms. One involves direct oxidation of the thioether residue adjacent to the aspartic acid in the catalytic domain. The other involves the generation of new sites that are susceptible to proteolysis by cathepsin G. These observations raise the possibility that oxidants derived from neutrophils restrain pericellular proteolysis by inactivating cathepsin G. They also suggest that methionine oxidation could render cathepsin G susceptible to autolytic cleavage. Myeloperoxidase may thus play a previously unsuspected role in regulating tissue injury by serine proteases during inflammation.