Cysteine cathepsins and caspases in silicosis

Lysosomal cysteine proteases are mediators of cell death in macrophages following exposure to amorphous silica nanoparticles

Increasing use of nanomaterials in everyday products such as cosmetics, medicines and food packaging is of grave concern given the lack of understanding with regards the impact such materials have on biological systems. The aim of this study is to examine cell death induced by cationic amorphous silica nanoparticles and determine the involvement of lysosomal cysteine proteases in this process. We report that multiple forms of cell death including apoptosis and pyroptosis are elicited following exposure to amorphous silica nanoparticles and that lysosomal cysteine proteases are involved in both cell death pathways in macrophages. Interestingly, lysosomal cysteine protease mRNA expression and release into the extracellular environment is induced following exposure to amorphous silica nanoparticles. Previously, the determination of nanoparticle-induced toxicity has focused on cytokine readouts, but the work presented here demonstrates that changes to normal protease biology should also be considered when evaluating the molecular mechanisms by which nanoparticulate matter causes cellular inflammation and death.

A Comparative Analysis of In Vitro Toxicity of Synthetic Zeolites on IMR-90 Human Lung Fibroblast Cells

Broad industrial application of zeolites increases the opportunity of inhalation. However, the potential impact of different types and compositions of zeolite on cytotoxicity is still unknown. Four types of synthetic zeolites have been prepared for assessing the effect on lung fibroblast: two zeolite L (LTL-R and LTL-D), ZSM-5 (MFI-S), and faujasite (FAU-S). The cytotoxicity of zeolites on human lung fibroblast (IMR-90) was assessed using WST1 cell proliferation assay, mitochondrial function, membrane leakage of lactate dehydrogenase, reduced glutathione levels, and mitochondrial membrane potential were assessed under control. Intracellular changes were examined using transmission electron microscopy (TEM). Toxicity-related gene expressions were evaluated by PCR array. The result showed significantly higher toxicity in IMR-90 cells with FAU-S than LTL-R, LTL-D and MFI-S exposure. TEM showed FAU-S, spheroidal zeolite with a low Si/Al ratio, was readily internalized forming numerous phagosomes in IMR-90 cells, while the largest and disc-shaped zeolites showed the lowest toxicity and were located in submembranous phagosomes in IMR-90 cells. Differential expression of TNF related genes was detected using PCR arrays and confirmed using qRT-PCR analysis of selected genes. Collectively, the exposure of different zeolites shows different toxicity on IMR-90 cells.

Oxidation of cathepsin S by major chemicals of cigarette smoke

Lung cysteine cathepsin S (CatS) that is a potent elastase plays a deleterious role in alveolar remodeling during smoke-induced emphysema. Despite the presence of a reactive nucleophilic cysteine (Cys25) within its active site, most of its elastinolytic activity is preserved after exposure to cigarette smoke extract (CSE), a major source of sulfhydryl oxidants. This result led us to decipher CatS resistance to major and representative CSE oxidants: hydrogen peroxide, formaldehyde, acrolein and peroxynitrite. CatS was inactivated by hydrogen peroxide, peroxynitrite and acrolein in a time- and dose-dependent manner, while formaldehyde was a weaker oxidant. Hydrogen peroxide, but not CSE, formaldehyde, and peroxynitrite impaired the autocatalytic maturation of pro-CatS, whereas acrolein prevented the formation of mature CatS without hindering the initial step of the two-step autocatalytic process. Far-UV CD spectra analysis supported that oxidation by CSE and hydrogen peroxide did not led to a structural alteration of CatS, despite a notable increase of protein carbonylation, a major hallmark of oxidative damage. Evaluation of the oxidation status of Cys25 by specific biotinylated redox sensing probes suggested the formation of sulfenic acid followed by a slower conversion to sulfinic acid after incubation with hydrogen peroxide. Addition of reducing reagents (dithiothreitol, glutathione and N-acetyl cysteine) led to a partial recovery of CatS activity following incubation with CSE, hydrogen peroxide and peroxynitrite. Current results provide some mechanistic evidence of CatS stability and activity in the presence of CSE, supporting its harmful contribution to the pathophysiology of emphysema.

Imaging of extracellular cathepsin S activity by a selective near infrared fluorescence substrate-based probe

We designed a near-infrared fluorescent substrate-based probe (SBP), termed MG101, for monitoring extracellular cathepsin S (CatS) activity. We conceived a fused peptide hairpin loop-structure, combining a CatS recognition domain, an electrostatic zipper (with complementary charges of a polyanionic (D-Glu)₅ segment and a polycationic (D-Arg)₅ motif, as well as a N and C terminal Förster resonance energy transfer pair (donor: AlexaFluor680; quencher: BHQ3) to facilitate activity-dependent imaging. MG101 showed excellent stability since no fluorescence release corresponding to a self-dequenching was observed in the presence of either 2 M NaCl or after incubation at a broad range of pH (2.2-8.2). Cathepsins B, D, G, H, and K, neutrophil elastase and proteinase 3 did not cleave MG101, while CatS, and to a lesser extent CatL, hydrolysed MG101 at pH 5.5. However MG101 was fully selective for CatS at pH 7.4 (k_cat/K_m = 140,000 M^-1 s^-1) and sensitive to low concentration of CatS (<1 nM). The selectivity of MG101 was successfully endorsed ex vivo, as it was hydrolysed in cell lysates derived from wild-type but not knockout CatS murine spleen. Furthermore, application of the SBP probe with confocal microscopy confirmed the secretion of active CatS from THP-1 macrophages, which could be abrogated by pharmacological CatS inhibitors. Taken together, present data highlight MG101 as a novel near-infrared fluorescent SBP for the visualization of extracellular active CatS from macrophages and other cell types.

Curcumin inhibits the TGF-β1-dependent differentiation of lung fibroblasts via PPARγ-driven upregulation of cathepsins B and L

Pulmonary fibrosis is a progressive disease characterized by a widespread accumulation of myofibroblasts and extracellular matrix components. Growing evidences support that cysteine cathepsins, embracing cathepsin B (CatB) that affects TGF-β1-driven Smad pathway, along with their extracellular inhibitor cystatin C, participate in myofibrogenesis. Here we established that curcumin, a potent antifibrotic drug used in traditional Asian medicine, impaired the expression of both α-smooth muscle actin and mature TGF-β1 and inhibited the differentiation of human lung fibroblasts (CCD-19Lu cells). Curcumin induced a compelling upregulation of CatB and CatL. Conversely cystatin C was downregulated, which allowed the recovery of the peptidase activity of secreted cathepsins and the restoration of the proteolytic balance. Consistently, the amount of both insoluble and soluble type I collagen decreased, reaching levels similar to those observed for undifferentiated fibroblasts. The signaling pathways activated by curcumin were further examined. Curcumin triggered the expression of nuclear peroxisome proliferator-activated receptor γ (PPARγ). Contrariwise PPARγ inhibition, either by an antagonist (2-chloro-5-nitro-N-4-pyridinyl-benzamide) or by RNA silencing, restored TGF-β1-driven differentiation of curcumin-treated CCD-19Lu cells. PPARγ response element (PPRE)-like sequences were identified in the promoter regions of both CatB and CatL. Finally, we established that the transcriptional induction of CatB and CatL depends on the binding of PPARγ to PPRE sequences as a PPARγ/Retinoid X Receptor-α heterodimer.

Selective inhibition of human cathepsin S by 2,4,6-trisubstituted 1,3,5-triazine analogs

We report herein the synthesis and biological evaluation of a new series of 2,4,6-trisubstituted 1,3,5-triazines as reversible inhibitors of human cysteine cathepsins. The desired products bearing morpholine and N-Boc piperidine, respectively, were obtained in three to four steps from commercially available trichlorotriazine. Seventeen hitherto unknown compounds were evaluated in vitro against various cathepsins for their inhibitory properties. Among them, compound 7c (4-(morpholin-4-yl)-6-[4-(trifluoromethoxy)anilino]-1,3,5-triazine-2-carbonitrile) was identified as the most potent and selective inhibitor of cathepsin S (Ki  =  2  ±  0.3 nM). Also 7c impaired the autocatalytic maturation of procathepsin S. Molecular docking studies support that 7c bound within the active site of cathepsin S, by interacting with Gly23, Cys25 and Trp26 (S1 subsite), with Asn67, Gly69 and Phe70 (S2 subsite) and with Gln19 (S1' pocket).

Biological chemistry and functionality of protein sulfenic acids and related thiol modifications

Selective modification of proteins at cysteine residues by reactive oxygen, nitrogen or sulfur species formed under physiological and pathological states is emerging as a critical regulator of protein activity impacting cellular function. This review focuses primarily on protein sulfenylation (-SOH), a metastable reversible modification connecting reduced cysteine thiols to many products of cysteine oxidation. An overview is first provided on the chemistry principles underlining synthesis, stability and reactivity of sulfenic acids in model compounds and proteins, followed by a brief description of analytical methods currently employed to characterize these oxidative species. The following chapters present a selection of redox-regulated proteins for which the -SOH formation was experimentally confirmed and linked to protein function. These chapters are organized based on the participation of these proteins in the regulation of signaling, metabolism and epigenetics. The last chapter discusses the therapeutic implications of altered redox microenvironment and protein oxidation in disease.

Antimicrobial proteins and peptides in human lung diseases: A friend and foe partnership with host proteases

Lung antimicrobial proteins and peptides (AMPs) are major sentinels of innate immunity by preventing microbial colonization and infection. Nevertheless bactericidal activity of AMPs against Gram-positive and Gram-negative bacteria is compromised in patients with chronic obstructive pulmonary disease (COPD), cystic fibrosis (CF) and asthma. Evidence is accumulating that expression of harmful human serine proteases, matrix metalloproteases and cysteine cathepsins is markedely increased in these chronic lung diseases. The local imbalance between proteases and protease inhibitors compromises lung tissue integrity and function, by not only degrading extracellular matrix components, but also non-matrix proteins. Despite the fact that AMPs are somewhat resistant to proteolytic degradation, some human proteases cleave them efficiently and impair their antimicrobial potency. By contrast, certain AMPs may be effective as antiproteases. Host proteases participate in concert with bacterial proteases in the degradation of key innate immunity peptides/proteins and thus may play immunomodulatory activities during chronic lung diseases. In this context, the present review highlights the current knowledge and recent discoveries on the ability of host enzymes to interact with AMPs, providing a better understanding of the role of human proteases in innate host defense.

Regulation of TGF-β1-driven differentiation of human lung fibroblasts: emerging roles of cathepsin B and cystatin C

Lung matrix homeostasis partly depends on the fine regulation of proteolytic activities. We examined the expression of human cysteine cathepsins (Cats) and their relative contribution to TGF-β1-induced fibroblast differentiation into myofibroblasts. Assays were conducted using both primary fibroblasts obtained from patients with idiopathic pulmonary fibrosis and human lung CCD-19Lu fibroblasts. Pharmacological inhibition and genetic silencing of Cat B diminished α-smooth muscle actin expression, delayed fibroblast differentiation, and led to an accumulation of intracellular 50-kDa TGF-β1. Moreover, the addition of Cat B generated a 25-kDa mature form of TGF-β1 in Cat B siRNA-pretreated lysates. Inhibition of Cat B decreased Smad 2/3 phosphorylation but had no effect on p38 MAPK and JNK phosphorylation, indicating that Cat B mostly disturbs TGF-β1-driven canonical Smad signaling pathway. Although mRNA expression of cystatin C was stable, its secretion, which was inhibited by brefeldin A, increased during TGF-β1-induced differentiation of idiopathic pulmonary fibrosis and CCD-19Lu fibroblasts. In addition, cystatin C participated in the control of extracellular Cats, because its gene silencing restored their proteolytic activities. These data support the notion that Cat B participates in lung myofibrogenesis as suggested for stellate cells during liver fibrosis. Moreover, we propose that TGF-β1 promotes fibrosis by driving the effective cystatin C-dependent inhibition of extracellular matrix-degrading Cats.

Human cystatin C: a new biomarker of idiopathic pulmonary fibrosis?

PURPOSE

Human idiopathic pulmonary fibrosis (IPF) is a progressive interstitial lung disorder with a poor prognosis. The identification of a new and specific biomarker in bronchoalveolar lavage fluids (BALFs) may assist in the diagnosis of the disease.

EXPERIMENTAL DESIGN

Characterization of cysteine Cats and their endogenous inhibitor, cystatin C, was conducted by immunochemical analysis and measurement of endopeptidase activity of control (n = 11) and IPF (n = 25) BALFs (normalized conditions, 20 μg protein/assay).

RESULTS

Cathepsin (Cat) B was detected as proform and mature enzyme for both control and IPF samples, while Cats K, L, and S were found as zymogens with a strengthened staining in IPF BALFs. The overall endopeptidase activity related mainly to Cat B and did not vary significantly between control and IPF samples. Conversely a significant increase of immunoreactive cystatin C was measured in BALFs for each of three IPF grades.

CONCLUSIONS AND CLINICAL RELEVANCE

An excessive deposition of extracellular matrix proteins is the hallmark of fibrotic disorders. Cats are potent collagenases and might be essential for lung homeostasis. Taken together, increase of cystatin C in IPF BALFs may reflect abnormal regulation of proteolytic activity of Cats in lung, which in turn can promote the development of fibrosis.

Human cysteine cathepsins are not reliable markers of infection by Pseudomonas aeruginosa in cystic fibrosis

Cysteine cathepsins have emerged as new players in inflammatory lung disorders. Their activities are dramatically increased in the sputum of cystic fibrosis (CF) patients, suggesting that they are involved in the pathophysiology of CF. We have characterized the cathepsins in CF expectorations and evaluated their use as markers of colonization by Pseudomonas aeruginosa. The concentrations of active cathepsins B, H, K, L and S were the same in P. aeruginosa-positive (19 Ps+) and P. aeruginosa-negative (6 Ps−) samples, unlike those of human neutrophil elastase. Also the cathepsin inhibitory potential and the cathepsins/cathepsin inhibitors imbalance remained unchanged and similar (∼2-fold) in the Ps+ and Ps− groups (p<0.001), which correlated with the breakdown of their circulating cystatin-like inhibitors (kininogens). Procathepsins, which may be activated autocatalytically, are a potential proteolytic reservoir. Immunoblotting and active-site labeling identified the double-chain cathepsin B, the major cathepsin in CF sputum, as the main molecular form in both Ps+ and Ps− samples, despite the possible release of the ∼31 kDa single-chain form from procathepsin B by sputum elastase. Thus, the hydrolytic activity of cysteine cathepsins was not correlated with bacterial colonization, indicating that cathepsins, unlike human neutrophil elastase, are not suitable markers of P. aeruginosa infection.

Lack of cathepsin activities alter or prevent the development of lung granulomas in a mouse model of sarcoidosis

BACKGROUND

Remodeling of lung tissues during the process of granuloma formation requires significant restructuring of the extra-cellular matrix and cathepsins K, L and S are among the strongest extra-cellular matrix degrading enzymes. Cathepsin K is highly expressed in various pathological granulomatous infiltrates and all three enzymes in their active form are detected in bronchoalveolar lavage fluids from patients with sarcoidosis. Granulomatous inflammation is driven by T-cell response and cathepsins S and L are actively involved in the regulation of antigen presentation and T-cell selection. Here, we show that the disruption of the activities of cathepsins K, L, or S affects the development of lung granulomas in a mouse model of sarcoidosis.

METHODS

Apolipoprotein E-deficient mice lacking cathepsin K or L were fed Paigen diet for 16 weeks and lungs were analyzed and compared with their cathepsin-expressing littermates. The role of cathepsin S in the development of granulomas was evaluated using mice treated for 8 weeks with a potent and selective cathepsin S inhibitor.

RESULTS

When compared to wild-type litters, more cathepsin K-deficient mice had lung granulomas, but individually affected mice developed smaller granulomas that were present in lower numbers. The absence of cathepsin K increased the number of multinucleated giant cells and the collagen content in granulomas. Cathepsin L deficiency resulted in decreased size and number of lung granulomas. Apoe-/- mice treated with a selective cathepsin S inhibitor did not develop lung granulomas and only individual epithelioid cells were observed.

CONCLUSIONS

Cathepsin K deficiency affected mostly the occurrence and composition of lung granulomas, whereas cathepsin L deficiency significantly reduced their number and cathepsin S inhibition prevented the formation of granulomas.

The occluding loop of cathepsin B prevents its effective inhibition by human kininogens

Kininogens, the major plasma cystatin-like inhibitors of cysteine cathepsins, are degraded at sites of inflammation, and cathepsin B has been identified as a prominent mediator of this process. Cathepsin B, in contrast to cathepsins L and S, is poorly inhibited by kininogens. This led us to delineate the molecular interactions between this protease and kininogens (high molecular weight kininogen and low molecular weight kininogen) and to elucidate the dual role of the occluding loop in this weak inhibition. Cathepsin B cleaves high molecular weight kininogen within the N-terminal region of the D2 and D3 cystatin-like domains and close to the consensus QVVAG inhibitory pentapeptide of the D3 domain. The His110Ala mutant, unlike His111Ala cathepsin B, fails to hydrolyze kininogens, but rather forms a tight-binding complex as observed by gel-filtration analysis. K(i) values (picomolar range) as well as association rate constants for the His110Ala cathepsin B variant compare to those reported for cathepsin L for both kininogens. Homology modeling of isolated inhibitory (D2 and D3) domains and molecular dynamics simulations of the D2 domain complexed with wild-type cathepsin B and its mutants indicate that additional weak interactions, due to the lack of the salt bridge (Asp22-His110) and the subsequent open position of the occluding loop, increase the inhibitory potential of kininogens on His110Ala cathepsin B.

Regulation of cathepsin K activity by hydrogen peroxide

Although cysteine cathepsins, including cathepsin K, are sensitive to oxidation, proteolytically active forms are found at inflammatory sites. Regulation of cathepsin K activity was analyzed in the presence of H2O2 to gain an insight into these puzzling observations. H2O2 impaired processing of procathepsin K and inactivated its mature form in a time- and dose-dependent mode. However, as a result of the formation of a sulfenic acid, as confirmed by trapping in the presence of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazol, approximately one-third of its initial activity was restored by dithiothreitol. This incomplete inactivation may partially explain why active cysteine cathepsins are still found during acute lung inflammation.

Identification of novel bovine cumulus cell molecular markers predictive of oocyte competence: functional and diagnostic implications

The present study was undertaken to discover molecular markers in bovine cumulus cells predictive of oocyte competence and to elucidate their functional significance. Differences in RNA transcript abundance in cumulus cells harvested from oocytes of adult versus prepubertal animals (a model of poor oocyte quality) were identified by microarray analysis. Four genes of interest encoding for the lysosomal cysteine proteinases cathepsins B, S, K, and Z and displaying greater transcript abundance in cumulus cells surrounding oocytes harvested from prepubertal animals were chosen for further investigation. Greater mRNA abundance for such genes in cumulus cells of prepubertal oocytes was confirmed by real-time RT-PCR. Elevated transcript abundance for cathepsins B, S, and Z also was observed in cumulus cells surrounding adult metaphase II oocytes that developed to the blastocyst stage at a low percentage following parthenogenetic activation versus those that developed at a high percentage. Functional significance of cumulus cell cathepsin expression to oocyte competence was confirmed by treatment of cumulus-oocyte complexes during in vitro oocyte maturation with a cell-permeable cysteine proteinase (cathepsin) inhibitor. Inhibitor treatment decreased apoptotic nuclei in the cumulus layer and enhanced development of parthenogenetically activated and in vitro-fertilized adult oocytes to the blastocyst stage. Stimulatory effects of inhibitor treatment during meiotic maturation on subsequent embryonic development were not observed when oocytes were matured in the absence of cumulus cells. The present results support a functional role for cumulus cell cathepsins in compromised oocyte competence and suggest that cumulus cell cathepsin mRNA abundance may be predictive of oocyte quality.

Extracellular catalase activity protects cysteine cathepsins from inactivation by hydrogen peroxide

The resistance of secreted cysteine cathepsins to peroxide inactivation was evaluated using as model THP-1 cells. Differentiated cells released mostly cathepsin B, but also cathepsins H, K, and L, with a maximum of endopeptidase activity at day 6. Addition of non-cytotoxic concentrations of H(2)O(2) did not affect mRNA expression levels and activity of cathepsins, while the catalase activity remained also unchanged, consistently with RT-PCR analysis. Conversely inhibition of extracellular catalase led to a striking inactivation of secreted cysteine cathepsins by H(2)O(2). This report suggests that catalase may participate in the protection of extracellular cysteine proteases against peroxidation.

Management of atherosclerosis with antiallergic medicine: a lesson from the mouse model

Evaluation of: Sun J, Sukhova GK, Wolters PJ et al.: Mast cells promote atherosclerosis by releasing pro-inflammatory cytokines. Nat. Med. 13, 719-724 (2007). Mast cells are important components in human allergic response and innate immunity. These cells have been implicated in the pathogenesis of atherosclerosis since the 1950s, and a series of studies have proposed their roles in the pathologic events critical to atherogenesis. Despite these studies and hypotheses, there is no evidence to suggest a direct participation of these allergic cells in atherosclerosis. Using mast cell-deficient mice and intravenous mast cell reconstitution technology with a conventional mouse atherosclerosis model, Dr Sun et al. revealed that mast cells contribute to atherogenesis by releasing proinflammatory cytokines, which are utilized to stimulate vascular cell-protease expression and further tissue remodeling. Mice that lack these cells are resistant to diet-induced atherosclerosis. These data suggest that stabilization of mast cells with antiallergic medicine may be utilized in controlling or preventing the initiation and progression of atherosclerosis in humans.

Biochemical properties and regulation of cathepsin K activity

Cysteine cathepsins (11 in humans) are mostly located in the acidic compartments of cells. They have been known for decades to be involved in intracellular protein degradation as housekeeping proteases. However, the discovery of new cathepsins, including cathepsins K, V and F, has provided strong evidence that they also participate in specific biological events. This review focuses on the current knowledge of cathepsin K, the major bone cysteine protease, which is a drug target of clinical interest. Nevertheless, we will not discuss recent developments in cathepsin K inhibitor design since they have been extensively detailed elsewhere. We will cover features of cathepsin K structure, cellular and tissue distribution, substrate specificity, and regulation (pH, propeptide, glycosaminoglycans, oxidants), and its putative roles in physiological or pathophysiological processes. Finally, we will review the kinetic data of its inhibition by natural endogenous inhibitors (stefin B, cystatin C, H- and L-kininogens).

Cysteine cathepsins: cellular roadmap to different functions

Cysteine cathepsins belong to the papain-like family C1 of clan CA cysteine peptidases. These enzymes are ubiquitously expressed and exert their proteolytic activity mainly, but not exclusively within the compartments along the endocytic pathway. Moreover, cysteine cathepsins are active in pericellular environments as soluble enzymes or bound to cell surface receptors at the plasma membrane, and possibly even within secretory vesicles, the cytosol, mitochondria, and within the nuclei of eukaryotic cells. Proteolytic actions performed by cysteine cathepsins are essential in the maintenance of homeostasis and depend heavily upon their correct sorting and trafficking within cells. As a consequence, the numerous and diverse approaches to identification, qualitative and quantitative determination, and visualization of cysteine cathepsin functions in vitro, in situ, and in vivo cover the entire spectrum of biochemistry, molecular and cell biology. This review focuses upon the transport pathways directing cysteine cathepsins to their points of action and thus emphasizes the broader role and functionality of cysteine cathepsins in a number of specific cellular locales. Such understanding will provide a foundation for future research investigating the involvement of these peptidases with their substrates, inhibitors, and the intertwined proteolytic networks at the hubs of complex biological systems.

Antigen-presenting cell population dynamics during murine silicosis

Silicosis is an occupational lung disease resulting from the inhalation of silica particles over prolonged periods of time, which causes chronic inflammation and progressive pulmonary fibrosis. Alveolar macrophages (AM) are critical effector cells, while less is known about the role and function of pulmonary dendritic cells (DC) in silicosis. We hypothesize that a balance exists between the suppressive nature of AM and the stimulatory capacity of DC to regulate lung immunity, and that this equilibrium may be overcome by silica exposure in vivo. Our results demonstrate that in response to silica exposure, both the percent and absolute number of AM significantly decreased over time, with a concomitant significant increase in DC. Both AM and DC exhibited cellular activation in response to silica, indicated by increased expression of cell surface markers. In the absence of silica-induced AM apoptosis (TNFR 1/2-null and Gld mice), no change was observed in the percent or absolute number of either cell type. Furthermore, bone marrow-derived DC, but not bone marrow-derived macrophages, migrated from the alveoli into the lung parenchyma in response to silica, resulting in significantly increased numbers of activated T lymphocytes. Collectively, the results demonstrate that AM and DC are distinct antigen-presenting cells within the respiratory tract that respond to silica exposure in vivo in unique ways, with significant implications for immune reactivity of the lung in response to environmental pathogens.

New developments in the understanding of immunology in silicosis

PURPOSE OF REVIEW

There is compelling evidence that the immune responses induced by crystalline silica particles are implicated in the development of silicosis. This article reviews recent observations which further delineate how innate and adaptive immunity are involved in this lung disease.

RECENT FINDINGS

First, silica particles are recognized to have pathogen-associated molecular patterns by the innate immune system. The MARCO receptor expressed on the surface of macrophages appears crucial for the recognition and the uptake of silica as well as the activation of these immune cells in silicosis. Additional data support a major role of inflammation (mast cells, B lymphocytes and TNFalpha) in the development of lung fibrosis but also cancer. Silica-induced acute inflammation is accompanied by thrombosis; strongly suggesting that inhaled silica particles may also induce extrapulmonary lesions. Surprisingly, a pronounced anti-inflammatory reaction may also contribute to silica-induced lung fibrosis in mice and represent an additional etiopathogenic pathway of silicosis. Interestingly, it has been proposed that the pulmonary expression of IL-9 (a T lymphocyte-related interleukin) or Heme oxygenase-1 (an anti-inflammatory molecule) attenuated silicotic disease progression in animals.

SUMMARY

New pathogenic routes involving innate receptors and antiinflammation as well as new antifibrotic immune mediators have been recently described in experimental silicosis, highlighting new potential therapeutic targets and strategies.