Evidence of mast-cell activation in a subset of patients with eosinophilic chronic obstructive pulmonary disease

Does Chronic Obstructive Pulmonary Disease Originate from Different Cell Types?

The onset of chronic obstructive pulmonary disease (COPD) is heterogeneous, and current approaches to define distinct disease phenotypes are lacking. In addition to clinical methodologies, subtyping COPD has also been challenged by the reliance on human lung samples from late-stage diseases. Different COPD phenotypes may be initiated from the susceptibility of different cell types to cigarette smoke, environmental pollution, and infections at early stages that ultimately converge at later stages in airway remodeling and destruction of the alveoli when the disease is diagnosed. This perspective provides discussion points on how studies to date define different cell types of the lung that can initiate COPD pathogenesis, focusing on the susceptibility of macrophages, T and B cells, mast cells, dendritic cells, endothelial cells, and airway epithelial cells. Additional cell types, including fibroblasts, smooth muscle cells, neuronal cells, and other rare cell types not covered here, may also play a role in orchestrating COPD. Here, we discuss current knowledge gaps, such as which cell types drive distinct disease phenotypes and/or stages of the disease and which cells are primarily affected by the genetic variants identified by whole genome-wide association studies. Applying new technologies that interrogate the functional role of a specific cell type or a combination of cell types as well as single-cell transcriptomics and proteomic approaches are creating new opportunities to understand and clarify the pathophysiology and thereby the clinical heterogeneity of COPD.

Increased mast cell activation in eosinophilic chronic obstructive pulmonary disease

Objectives

A subset of chronic obstructive pulmonary disease (COPD) patients have increased numbers of airway eosinophils associated with elevated markers of T2 inflammation. This analysis focussed on mast cell counts and mast cell‐related gene expression in COPD patients with higher vs lower eosinophil counts.

Methods

We investigated gene expression of tryptase (TPSAB1), carboxypeptidase A3 (CPA3), chymase (CMA1) and two mast cell specific gene signatures; a bronchial biopsy signature (MC_bb) and an IgE signature (MC_IgE) using sputum cells and bronchial epithelial brushings. Gene expression analysis was conducted by RNA‐sequencing. We also examined bronchial biopsy mast cell numbers by immunohistochemistry.

Results

There was increased expression of TPSAB1, CPA3 and MC_bb in eosinophil^high than in eosinophil^low COPD patients in sputum cells and bronchial epithelial brushings (fold change differences 1.21 and 1.28, respectively, P < 0.01). Mast cell gene expression was associated with markers of T2 and eosinophilic inflammation (IL13, CLCA1, CST1, CCL26, eosinophil counts in sputum and bronchial mucosa; rho = 0.4–0.8; P < 0.05). There was no difference in MC_IgE gene expression between groups. There was no difference in the total number of bronchial biopsy mast cells between groups.

Conclusion

These results demonstrate that eosinophilic inflammation is associated with altered mast cell characteristics in COPD patients, implicating mast cells as a component of T2 inflammation present in a subset of COPD patients.

Novel Strategies to Target Mast Cells in Disease

Mast cells (MCs) are versatile effector cells of the immune system, characterized by a large content of secretory granules containing a variety of inflammatory mediators. They are implicated in the host protection toward various external insults, but are mostly well known for their detrimental impact on a variety of pathological conditions, including allergic disorders such as asthma and a range of additional disease settings. Based on this, there is currently a large demand for therapeutic regimens that can dampen the detrimental impact of MCs in these respective pathological conditions. This can be accomplished by several strategies, including targeting of individual mediators released by MCs, blockade of receptors for MC-released compounds, inhibition of MC activation, limiting mast cell growth or by inducing mast cell apoptosis. Here, we review the currently available and emerging regimens to interfere with harmful mast cell activities in asthma and other pathological settings and discuss the advantages and limitations of such strategies.

The Role of Electronic Noses in Phenotyping Patients with Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is a common progressive disorder of the respiratory system which is currently the third leading cause of death worldwide. Exhaled breath analysis is a non-invasive method to study lung diseases, and electronic noses have been extensively used in breath research. Studies with electronic noses have proved that the pattern of exhaled volatile organic compounds is different in COPD. More recent investigations have reported that electronic noses could potentially distinguish different endotypes (i.e., neutrophilic vs. eosinophilic) and are able to detect microorganisms in the airways responsible for exacerbations. This article will review the published literature on electronic noses and COPD and help in identifying methodological, physiological, and disease-related factors which could affect the results.

Role of eosinophils in airway inflammation of chronic obstructive pulmonary disease

COPD is a significant cause of morbidity and mortality. In some patients with COPD, eosinophils contribute to inflammation that promotes airway obstruction; approximately a third of stable COPD patients have evidence of eosinophilic inflammation. Although the eosinophil threshold associated with clinical relevance in patients with COPD is currently subject to debate, eosinophil counts hold potential as biomarkers to guide therapy. In particular, eosinophil counts may be useful in assessing which patients may benefit from inhaled corticosteroid therapy, particularly regarding exacerbation prevention. In addition, several therapies targeting eosinophilic inflammation are available or in development, including monoclonal antibodies targeting the IL5 ligand, the IL5 receptor, IL4, and IL13. The goal of this review was to describe the biologic characteristics of eosinophils, their role in COPD during exacerbations and stable disease, and their use as biomarkers to aid treatment decisions. We also propose an algorithm for inhaled corticosteroid use, taking into consideration eosinophil counts and pneumonia history, and emerging eosinophil-targeted therapies in COPD.

Eosinophilia and clinical outcome of chronic obstructive pulmonary disease: a meta-analysis

Numerous studies have investigated the association between eosinophilia and clinical outcome of patients with chronic obstructive pulmonary disease (COPD) but the evidence is conflicting. We conducted a pooled analysis of outcome measures comparing eosinophilic and non-eosinophilic COPD patients. We searched articles indexed in four databases using Medical Subject Heading or Title and Abstract words including COAD, COPD, eosinophil, eosinophilia, eosinopenia from inception to December 2016. Observational studies and randomized controlled trials with parallel groups comparing COPD patients with and without eosinophilia were included. Comparing to the non-eosinophilic group, those with eosinophilic COPD had a similar risk for exacerbation in 12 months [Odds ratio = 1.07, 95% confidence interval (CI) 0.86–1.32, P = 0.55] and in-hospital mortality [OR = 0.52, 95% CI 0.25–1.07]. Eosinophilia was associated with reduced length of hospital stay (P = 0.04). Subsequent to therapeutic interventions, eosinophilic outpatients performed better in pulmonary function tests [Mean Difference = 1.64, 95% CI 0.05–3.23, P < 0.001]. Inclusion of hospitalized patients nullified the effect. Improvement of quality of life was observed in eosinophilic subjects [Standardized Mean Difference = 1.83, 95% CI 0.02–3.64, P = 0.05], independent of hospitalization status. In conclusion, blood eosinophilia may be predictive of favorable response to steroidal and bronchodilator therapies in patients with stable COPD.

Mast cells and their activation in lung disease

Mast cells and their activation contribute to lung health via innate and adaptive immune responses to respiratory pathogens. They are also involved in the normal response to tissue injury. However, mast cells are involved in disease processes characterized by inflammation and remodeling of tissue structure. In these diseases mast cells are often inappropriately and chronically activated. There is evidence for activation of mast cells contributing to the pathophysiology of asthma, pulmonary fibrosis, and pulmonary hypertension. They may also play a role in chronic obstructive pulmonary disease, acute respiratory distress syndrome, and lung cancer. The diverse mechanisms through which mast cells sense and interact with the external and internal microenvironment account for their role in these diseases. Newly discovered mechanisms of redistribution and interaction between mast cells, airway structural cells, and other inflammatory cells may offer novel therapeutic targets in these disease processes.

New Developments in Mast Cell Biology: Clinical Implications

Mast cells (MCs) are present in connective tissue and at mucosal surfaces in all classes of vertebrates. In health, they contribute to tissue homeostasis, host defense, and tissue repair via multiple receptors regulating the release of a vast stockpile of proinflammatory mediators, proteases, and cytokines. However, these potentially protective cells are a double-edged sword. When there is a repeated or long-term stimulus, MC activation leads to tissue damage and dysfunction. Accordingly, MCs are implicated in the pathophysiologic aspects of numerous diseases covering all organs. Understanding the biology of MCs, their heterogeneity, mechanisms of activation, and signaling cascades may lead to the development of novel therapies for many diseases for which current treatments are lacking or are of poor efficacy. This review will focus on updates and developments in MC biology and their clinical implications, with a particular focus on their role in respiratory diseases.

Cytopathological Aspects in the Bronchoalveolar Lavage Fluid in Chronic Obstructive Pulmonary Disease

Chronic Obstructive Pulmonary Disease (COPD) is a clinical syndrome characterised by a slow progressive decline in expiratory airflow [1], a process that has gradually developed over the years. Studies of patients with COPD show an inflammatory process in the small airways [2]. The aim of this paper is to identify the cytopathological aspects of the liquids present in the bronchoalveolar lavage in patients with COPD. We were performed a descriptive analytical case-control and prospective study on forty patients with COPD and ten asymptomatic smokers (healthy smokers or patients at risk). The percentage of marcophage, the type of the dominant inflamatory cell, in the bronchoalveolar lavage (BAL) liquid was significantly higher at patients with mild and moderate COPD as compared to patients with severe and very severe COPD. In the present work, the percentage of the neutrophil in the BAL liquid was significantly higher at patients with severe and very severe COPD, as compared to the patients with mild and moderate COPD and to aparently healthy smokers. In conclusion, we can say that COPD is characterized by an inflammatory process located in the small airways with predominant participation of macrophages, the procentage of macrophages in BAL fluid variyng inversely proportional to the severity of the disease.

Primary lysis of eosinophils in severe desquamative asthma

Primary lysis of eosinophils liberates free eosinophil granules (FEGs) releasing toxic proteins in association with bronchial epithelial injury repair. Eosinophil lysis may be significantly pathogenic. Bronchial mucosal FEGs are associated with uncontrolled asthma, severe asthma, aspirin-sensitive asthma, and lethal asthma. FEGs in the bronchial wall may characterize severe asthma without sputum eosinophilia. Excessive numbers of sputum FEGs occur in severe exacerbations of asthma and are reduced along with clinical improvement. Occurrence of FEGs affects interpretation of other sputum biomarkers including numbers of eosinophils, ECP, and eosinophil-stained macrophages. Thus, eosinophil lysis produces FEGs as bronchial biomarkers of severe asthma. Blood eosinophils in severe asthma seem primed exhibiting a propensity to lyse that is greater the more severe the asthma. Proclivity of blood eosinophils to lyse also distinguished three levels of severity among children with exacerbations of asthma. Numerous FEGs releasing toxic proteins occur in association with grave derangement and shedding of epithelium in severe asthma. Subepithelial FEGs correlate negatively with intact bronchial epithelium in clinically uncontrolled asthma. Significant correlations between sputum ECP, Creola bodies, and severity of asthma exacerbations have also been demonstrated. Hence, eosinophil lysis apparently causes epithelial desquamation in severe asthma. Exaggerated epithelial repair in turn would contribute to inflammatory and remodelling features of severe asthma. Perseverance of FEGs together with maintained disease activity, despite treatment with 'eosinophil-depleting' steroids and anti-IL5 biologicals, agrees with the possibility that eosinophil lysis is worthy target for novel anti-asthma drugs. Priming and lysis of eosinophils, and protein release from FEGs, are regulated and can be targeted. Eosinophil lysis and FEGs belong to the disease picture of severe asthma and need consideration in asthma studies concerned with phenotypes, biomarkers, roles of epithelial injury/repair, and targeting novel drugs.

Distribution of sputum cellular phenotype in a large asthma cohort: predicting factors for eosinophilic vs neutrophilic inflammation

Background

Phenotyping asthma according to airway inflammation allows identification of responders to targeted therapy. Induced sputum is technically demanding. We aimed to identify predictors of sputum inflammatory phenotypes according to easily available clinical characteristics.

Methods

This retrospective study was conducted in 508 asthmatics with successful sputum induction recruited from the University Asthma Clinic of Liege. Receiver-operating characteristic (ROC) curve and multiple logistic regression analysis were used to assess the relationship between sputum eosinophil or neutrophil count and a set of covariates. Equations predicting sputum eosinophils and neutrophils were then validated in an independent group of asthmatics.

Results

Eosinophilic (≥3%) and neutrophilic (≥76%) airway inflammation were observed in 46% and 18% of patients respectively. Predictors of sputum eosinophilia ≥3% were high blood eosinophils, FE_NO and IgE level and low FEV₁/FVC. The derived equation was validated with a Cohen’s kappa coefficient of 0.59 (p < 0.0001). ROC curves showed a cut-off value of 220/mm³ (AUC = 0.79, p < 0.0001) or 3% (AUC = 0.81, p < 0.0001) for blood eosinophils to identify sputum eosinophilia ≥3%. Independent predictors of sputum neutrophilia were advanced age and high FRC but not blood neutrophil count.

Conclusion

Eosinophilic and paucigranulocytic asthma are the dominant inflammatory phenotypes. Blood eosinophils provide a practical alternative to predict sputum eosinophilia but sputum neutrophil count is poorly related to blood neutrophils.

[COPD and inflammation: statement from a French expert group. Phenotypes related to inflammation]

INTRODUCTION

The objective of the present article is to review available data on possible links between phenotypes and inflammatory profiles in patients with chronic obstructive pulmonary disease (COPD).

BACKGROUND

Chronic bronchitis is associated with proximal bronchial inflammation and small airway inflammation with remodeling at the site of obstruction. CT scanning enables patients to be phenotyped according to the predominantly bronchial or emphysematous nature of the morphological abnormality. Exacerbations, in a context of persistently elevated baseline inflammation, are associated with increased inflammation and a poor prognosis. Long-term studies have correlated inflammatory markers (and anti-inflammatory drug effects) with dynamic hyperinflation, possibly confirming that inflammation promotes hyperinflation. The inflammatory cell count in the pulmonary arterial walls correlates with the severity of endothelial dysfunction. The risk of developing pulmonary hypertension would seem to increase with low-grade systemic inflammation. The role of low-grade systemic inflammation in COPD co-morbidities, and in nutritional and muscular involvement in particular, remains a matter of debate. Regular physical exercise may help reduce this inflammation.

CONCLUSIONS

In COPD, many aspects of the clinical phenotype are related to inflammation. Better knowledge of these relationships could help optimize current and future treatments.

Airway inflammation and mannitol challenge test in COPD

Background

Eosinophilic airway inflammation has successfully been used to tailor anti-inflammatory therapy in chronic obstructive pulmonary disease (COPD). Airway hyperresponsiveness (AHR) by indirect challenges is associated with airway inflammation. We hypothesized that AHR to inhaled mannitol captures eosinophilia in induced sputum in COPD.

Methods

Twenty-eight patients (age 58 ± 7.8 yr, packyears 40 ± 15.5, post-bronchodilator FEV₁ 77 ± 14.0%predicted, no inhaled steroids ≥4 wks) with mild-moderate COPD (GOLD I-II) completed two randomized visits with hypertonic saline-induced sputum and mannitol challenge (including sputum collection). AHR to mannitol was expressed as response-dose-ratio (RDR) and related to cell counts, ECP, MPO and IL-8 levels in sputum.

Results

There was a positive correlation between RDR to mannitol and eosinophil numbers (r = 0.47, p = 0.03) and level of IL-8 (r = 0.46, p = 0.04) in hypertonic saline-induced sputum. Furthermore, significant correlations were found between RDR and eosinophil numbers (r = 0.71, p = 0.001), level of ECP (r = 0.72, p = 0.001), IL-8 (r = 0.57, p = 0.015) and MPO (r = 0.64, p = 0.007) in sputum collected after mannitol challenge. ROC-curves showed 60% sensitivity and 100% specificity of RDR for >2.5% eosinophils in mannitol-induced sputum.

Conclusions

In mild-moderate COPD mannitol hyperresponsiveness is associated with biomarkers of airway inflammation. The high specificity of mannitol challenge suggests that the test is particularly suitable to exclude eosinophilic airways inflammation, which may facilitate individualized treatment in COPD.

Trial registration

Netherlands Trial Register (NTR): NTR1283

Pathologic similarities and differences between asthma and chronic obstructive pulmonary disease

PURPOSE OF REVIEW

Classically, asthma and chronic obstructive pulmonary disease present distinct clinical, physiologic and pathologic features. However, not infrequently, patients may present with overlapping clinical symptoms and physiological abnormalities: patients with severe asthma may present with fixed airway obstruction and patients with chronic obstructive pulmonary disease may have hyperresponsiveness and eosinophilia. At pathological level, inflammatory and structural similarities also occur and may be related to the phenotypic overlaps.

RECENT FINDINGS

In patients with asthma overlaps at inflammatory level exist with chronic obstructive pulmonary disease, such as increased neutrophilia in patients with severe asthma or an association of CD8+ T cells and lung-function decline. In chronic obstructive pulmonary disease, minimizing eosinophilia may be important to reduce exacerbations. Structural alterations occur in both diseases, but involving airway compartments differently. Airway epithelial changes, extracellular matrix deposition and mucus gland hypertrophy occur in both diseases. Asthmatics have thicker reticular basement membrane and more prominent smooth-muscle abnormalities, whereas emphysema is a distinct feature of chronic obstructive pulmonary disease.

SUMMARY

Recognizing the differences and similarities at pathological level in both diseases may lead to a better understanding of the overlapping clinical and physiological phenotypes, thereby helping to better plan specific treatment and long-term management.

Cysteinyl-leukotriene levels in sputum differentiate asthma from rhinitis patients with or without bronchial hyperresponsiveness

BACKGROUND

We have previously reported that asthma differs from rhinitis with or without bronchial hyperresponsiveness in the perception and degree of lower airway inflammation.

OBJECTIVE

The aim of the present study was to investigate whether sputum levels of inflammatory markers could further distinguish these patient groups.

METHODS

Patients with seasonal allergic rhinitis with or without asthma or bronchial hyperresponsiveness to methacholine were investigated. Induced sputum was performed during as well as off season, and analysed for cysteinyl-leukotrienes, hyaluronan, eosinophilic cationic protein (ECP) and other inflammatory markers.

RESULTS

Asthmatic patients differentiated from those with rhinitis with or without bronchial hyperresponsiveness in levels of cysteinyl-leukotrienes [geometric mean: 3.3 (lower 95%-upper 95% confidence interval (CI) of geometric mean: 1.9-5.1) vs. 1.4 (0.9-2.2) and 0.7 (0.3-1.6) pg/microg total protein] and hyaluronan [0.30 (0.22-0.43) vs. 0.15 (0.10-0.20) and 0.20 (0.12-0.35) ng/microg total protein] in sputum. The levels of cysteinyl-leukotrienes decreased in sputum from the asthmatic patients, while the levels of hyaluronan remained elevated off-season. Furthermore, elevated levels of ECP were noticed among both the asthmatic and rhinitis patients with hyperresponsiveness compared with controls [0.022 (0.014-0.033) and 0.015 (0.011-0.021) compared with 0.010 (0.007-0.014) ng/microg total protein]. The level of ECP remained elevated off season.

CONCLUSION

Cysteinyl-leukotrienes are possibly more related to mast cell-mediated inflammation and remodelling, also indicated by increased levels of hyaluronan during and off season. This inflammation may be partly different from the eosinophil-driven inflammation.

Aspects on pathophysiological mechanisms in COPD

Chronic obstructive pulmonary disease (COPD) is a condition which is characterized by irreversible airway obstruction due to narrowing of small airways, bronchiolitis, and destruction of the lung parenchyma, emphysema. It is the fourth most common cause of mortality in the world and is expected to be the third most common cause of death by 2020. The main cause of COPD is smoking but other exposures may be of importance. Exposure leads to airway inflammation in which a variety of cells are involved. Besides neutrophil granulocytes, macrophages and lymphocytes, airway epithelial cells are also of particular importance in the inflammatory process and in the development of emphysema. Cell trafficking orchestrated by chemokines and other chamoattractants, the proteinase-antiproteinase system, oxidative stress and airway remodelling are central processes associated with the development of COPD. Recently systemic effects of COPD have attracted attention and the importance of systemic inflammation has been recognized. This seems to have direct therapeutic implications as treatment with inhaled glucocorticosteroids has been shown to influence mortality. The increasing body of knowledge regarding the inflammatory mechanism in COPD will most likely have implications for future therapy and new drugs, specifically aimed at interaction with the inflammatory processes, are currently being developed.

Patterns of inflammation and the use of reversibility testing in smokers with airway complaints

BACKGROUND

Although both smoking and respiratory complaints are very common, tools to improve diagnostic accuracy are scarce in primary care. This study aimed to reveal what inflammatory patterns prevail in clinically established diagnosis groups, and what factors are associated with eosinophilia.

METHOD

Induced sputum and blood plasma of 59 primary care patients with COPD (n = 17), asthma (n = 11), chronic bronchitis (CB, n = 14) and smokers with no respiratory complaints ('healthy smokers', n = 17) were collected, as well as lung function, smoking history and clinical work-up. Patterns of inflammatory markers per clinical diagnosis and factors associated with eosinophilia were analyzed by multiple regression analyses, the differences expressed in odds ratios (OR) with 95% confidence intervals.

RESULTS

Multivariately, COPD was significantly associated with raised plasma-LBP (OR 1.2 [1.04-1.37]) and sTNF-R55 in sputum (OR 1.01 [1.001-1.01]), while HS expressed significantly lowered plasma-LBP (OR 0.8 [0.72-0.95]). Asthma was characterized by higher sputum eosinophilic counts (OR 1.3 [1.05-1.54]), while CB showed a significantly higher proportion of sputum lymphocytic counts (OR 1.5 [1.12-1.9]). Sputum eosinophilia was significantly associated with reversibility after adjusting for smoking, lung function, age, gender and allergy.

CONCLUSION

Patterns of inflammatory markers in a panel of blood plasma and sputum cells and mediators were discernable in clinical diagnosis groups of respiratory disease. COPD and so-called healthy smokers showed consistent opposite associations with plasma LBP, while chronic bronchitics showed relatively predominant lymphocytic inflammation compared to other diagnosis groups. Only sputum eosinophilia remained significantly associated with reversibility across the spectrum of respiratory disease in smokers with airway complaints.

Asthma and COPD: differences and similarities. With special reference to the usefulness of budesonide/formoterol in a single inhaler (Symbicort) in both diseases

Asthma and chronic obstructive pulmonary disease (COPD) both have a high prevalence worldwide and yet each condition remains underdiagnosed. Despite a number of common features, these inflammatory respiratory syndromes have distinct clinical outcomes. COPD represents a greater economic burden than asthma because it has a less favourable prognosis and is associated with greater morbidity and mortality. Therefore, it is important to distinguish between these two diseases at an early stage, so that appropriate therapy can be prescribed to prevent deterioration. However, effective treatments that may be used in both conditions can minimise the effects of misdiagnosis and maximise the impact of treatment without the associated complexity when both conditions occur together. The current review summarises the differences and similarities of asthma and COPD, in terms of risk factors, pathophysiology, symptoms and diagnosis, to provide greater understanding of the role of budesonide/formoterol in a single inhaler in both diseases.

Sputum eosinophilia and the short term response to inhaled mometasone in chronic obstructive pulmonary disease

BACKGROUND

An association between the sputum eosinophil count and the response to a 2 week course of prednisolone has previously been reported in patients with chronic obstructive pulmonary disease (COPD). Whether the response to inhaled corticosteroids is related to the presence of eosinophilic inflammation is unclear.

METHODS

A randomised, double blind, crossover trial of placebo and mometasone furoate (800 microg/day), each given for 6 weeks with a 4 week washout period, was performed in subjects with COPD treated with bronchodilator therapy only. Spirometric tests, symptom scores, chronic respiratory disease questionnaire (CRQ), and induced sputum were performed before and after each treatment phase.

RESULTS

Ninety five patients were recruited of which 60 were randomised. Overall there were no treatment associated changes in forced expiratory volume in 1 second (FEV(1)), total CRQ, or sputum characteristics. After stratification into tertiles by baseline eosinophil count, the net improvement in post-bronchodilator FEV(1) increased with mometasone compared with placebo progressively from the least to the most eosinophilic tertile. The mean change in post-bronchodilator FEV(1) with mometasone compared with placebo in the highest tertile was 0.11 l (95% CI 0.03 to 0.19). This improvement was not associated with a fall in the sputum eosinophil count.

CONCLUSIONS

An increased sputum eosinophil count is related to an improvement in post-bronchodilator FEV(1) following treatment with inhaled mometasone in COPD, but the improvement is not associated with a reduction in the sputum eosinophil count.

A novel serine protease predominately expressed in macrophages

We have identified a novel serine protease designated EOS by sequence identity searches. The deduced protein contains 284 amino acids with an active form containing 248 amino acids starting from an Ile-Val-Gly-Gly motif. The active form comprises a catalytic triad of conserved amino acids: His77, Asp126 and Ser231. It shares 44% identity with beta-tryptase and belongs to the S1 trypsin-like serine-protease family. Interestingly, this gene also maps to human chromosome 16p13.3. The purified protease showed amidolytic activity, cleaving its substrates before arginine residues. Tissue distribution by immunohistochemistry analysis demonstrated that EOS is highly expressed in spleen and moderately expressed in intestine, colon, lung and brain. We confirmed this expression pattern at the mRNA level by performing in situ hybridization. The results from both immunohistochemistry and in situ hybridization indicate that EOS is associated with macrophages. We corroborated this observation by double immunofluorescence using the anti-EOS antibody and an anti-CD68 antibody, a macrophage specific marker. Furthermore, we have detected a dramatic increase in immune staining of EOS in cultured U937 cells treated with PMA, which represent activated macrophages. This up-regulation is also reflected by elevated EOS mRNA in the PMA-treated U937 cells detected by Northern blotting. Since macrophages have important roles in various pathological conditions, such as wound healing, atherosclerosis and numerous inflammatory diseases, the localization of this novel serine protease to active macrophages may help to further the elucidation of the roles of this gene product in modulating these disorders.