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ORIGINAL ARTICLE
Platelet-activating factor receptor (PAFr) is upregulated in small airways and alveoli of smokers and COPD patients SHAKTI DHAR SHUKLA,1 HANS KONRAD MULLER,1 ROGER LATHAM,1 SUKHWINDER SINGH SOHAL1,2* AND EUGENE HAYDN WALTERS1* 1
NHMRC Centre of Research Excellence for Chronic Respiratory Disease, School of Medicine, University of Tasmania, Hobart and 2School of Health Sciences, University of Tasmania, Launceston, Tasmania, Australia
ABSTRACT Background and objective: PAFr is a cell adhesion site for specific bacteria, notably non-typeable Haemophilus influenzae (NTHi) and Streptococcus pneumoniae. We previously published that PAFr expression is significantly upregulated in the large airways of smokers, especially in COPD. We have now investigated PAFr expression in the epithelium and Rbm of small airways and in the alveolar compartment in smokers and patients with both COPD and small airway disease. Methods: We evaluated PAFr expression crosssectionally in resected lung tissue from: eight smokers with normal lung function (NLFS); 10 with smokingrelated small airway narrowing only; eight COPD smokers; 10 COPD ex-smokers, and compared these with nine control tissues. Anti-PAFr immunostaining was quantified using computer-aided image analysis. Results: Significantly increased PAFr expression in small airway epithelium of all clinical groups was found compared with controls (P < 0.01). Moreover, epithelial PAFr expression was upregulated in COPD smokers compared with NLFS (P < 0.05), but not when compared with COPD ex-smokers or patients with only small airways disease. Smoking history (pack-year) correlated significantly with PAFr expression in the currently smoking individuals, especially in NLFS (r = 0.9; P < 0.002). An increase above normal in PAFr-expressing cells in the airway epithelial Rbm was only significant in COPD smokers (P < 0.007). An upregulation of PAFr-expressing cell in alveolar epithelium was uniformly found in all clinical groups compared with normal control (P < 0.01).
SUMMARY AT A GLANCE PAFr aids in specific binding of two key bacterial pathogens (Streptococcus pneumoniae and Haemophilus influenzae) involved in chronic colonization and acute exacerbations of COPD. High PAFr expression in the epithelium of both small airways and alveoli therefore could be crucial in facilitating infections in smokers and especially in COPD.
Conclusion: Epithelial PAFr expression is upregulated in small airways and alveoli in smokers and COPD. Increased expression of PAFr could be crucial in facilitating acute and chronic respiratory infection with specific respiratory pathogens. Key words: airway epithelium, chronic obstructive pulmonary disease, inhaled corticosteroid, platelet-activating factor receptor, ‘respiratory’ bacteria adhesion. Abbreviations: ChoP, phosphorylcholine; COPD, chronic obstructive pulmonary disease; FEV1, forced expiratory volume in 1 s; FVC, forced vital capacity; HE, haematoxylin and eosin; ICS, inhaled corticosteroids; mRNA, messenger RNA; NLFS, normal lung function smoker; NTHi, non-typeable Haemophilus influenza; PAFr, platelet-activating factor receptor; Rbm, reticular basement membrane; SAD, small airways disease.
INTRODUCTION Correspondence: Eugene Haydn Walters, NHMRC Centre of Research Excellence for Chronic Respiratory Disease, School of Medicine, University of Tasmania, MS1, 17 Liverpool Street, Private Bag 23, Hobart, Tas. 7000, Australia. Email:
[email protected] *EHW and SSS contributed equally to this study. Disclosure Statement: The abstract of this study was previously presented at the TSANZSRS congress 2015. Received 16 July 2015; invited to revise 8 August and 28 September 2015; revised 27 August and 1 October 2015; accepted 1 October 2015 (Associate Editor: Chi Chiu Leung)
Article first published online: 10 December 2015 © 2015 Asian Pacific Society of Respirology
Chronic obstructive pulmonary disease (COPD) is characterized by persistent and progressive airflow limitation as a result of pathological changes induced by exposure to noxious particles and gases.1 The major risk factor for COPD is cigarette smoking (worldwide), and exposure to biomass fuel smoke (low and middle-income countries). In addition to the significant suffering of patients, COPD is a huge health and finance-related burden and is also projected to become the world’s third leading cause of mortality by 2030.2–4 Respirology (2016) 21, 504–510 doi: 10.1111/resp.12709
505
Small airway PAFr expression in COPD
The role of respiratory infections and chronic bacterial colonization as a possible disease driver has been emphasized as COPD patients whose respiratory tracts are ‘colonized’ with bacteria experience more respiratory symptoms, even in the absence of an acute exacerbation.5–7 The most common ‘colonizers’ in the stable state of COPD are Streptococcus pneumonia and non-typeable Haemophilus influenzae (NTHi).8 In addition, the risk of mortality and morbidity escalates when patients encounter frequent exacerbations,9–11 which are mostly microbial in origin. These same bacteria are frequently implicated in these exacerbations, either primarily or secondary to initial viral infection. Acute exacerbations are directly linked with an accelerated decline in lung function, increased local and systemic inflammation and ongoing decreased quality of life.12–15 Bacterial adhesion to host epithelium is essentially the first step for colonization of mucous membranes.16 The two major respiratory pathogens in COPD, S. pneumoniae and NTHi, adhere to epithelial surfaces through phosphorylcholine [(CH3)3N+CH2CH2PO4-] or ChoP, which is a small zwitterionic/bipolar molecule and molecular mimic of platelet-activating factor (PAF), on the bacterial cell wall surface. Through physico-chemical interaction, ChoP specifically adheres to platelet-activating factor receptor (PAFr) which may be expressed on the surface of respiratory tract epithelial cells.17,18 PAFr expression can be upregulated by various factors such as exposure to acid, urban particulate matter, viral infections, cytokines and cigarette smoke.19 We hypothesize an upregulation in PAFr expression on the epithelial cell surface may well be the cause of chronic colonization and invasion of the respiratory tract by S. pneumoniae and NTHi and may play a central role in disease progression. Various studies have reported the importance of PAFr in the adhesion of both S. pneumoniae and NTHi in in vitro cell culture and animal models. Grigg et al.20 concluded that increased Pneumococcus adhesion to cells exposed to cigarette smoke extract was a result of increased PAFr expression and was attenuated by a PAFr antagonist. Mice exposed to cigarette smoke extract in vivo also showed significant upregulation in lung PAFr messenger (m)RNA transcripts. They also demonstrated an increased percentage of human bronchial epithelium with PAFr positivity in active smokers compared with normal non-smoking individuals.20 Suri et al.21 reported a significant increase in PAFr mRNA expression in airway biopsies from smokers and smokers with COPD. More recently, we have found significantly increased expression of PAFr in central airway bronchial biopsies of both COPD smokers and COPD ex-smokers compared with smokers with normal lung function, which was found to be independent of age or forced expiratory volume in 1 s (FEV1).22 There was a strong relationship for pack-years of smoking in the COPD ex-smokers suggesting that smoking cessation is ineffective in downregulation of PAFr once COPD has developed. Alternatively, it is possible that high expression of PAFr in some smokers may be a risk factor for development of COPD in the first place.22 © 2015 Asian Pacific Society of Respirology
Although PAFr expression patterns in large airways of smokers and COPD have been reported, PAFrprotein expression in small airways has not been explored. Published evidence has established that the small conducting airways (
