Cellular Mechanisms in Airways Inflammation pp 125-146 | Cite as
Neutrophils
Abstract
Neutrophils are the rapid response cells of acute inflammation and a major component of host defence. Their ability to ingest bacteria and other microbes was recognised by Elie Metchnikoff in the 19th century, but in recent years the double-edged nature of the inflammatory response has been recognised. Despite their beneficial role in host defence, neutrophils and their pro-inflammatory products are increasingly implicated in the pathogenesis of acute and chronic inflammatory diseases. In the lung these include chronic bronchitis and emphysema, asthma, respiratory distress syndrome, bronchiectasis and a number of interstitial lung diseases. Thus, understanding the pro-inflammatory host defence functions of the neutrophil, and the mechanisms by which these are terminated, may yield insights into the pathogenesis of diseases which are a major cause of morbidity and mortality in the developed world.
Keywords
NADPH Oxidase Idiopathic Pulmonary Fibrosis Human Neutrophil Neutrophil Elastase Neutrophil ApoptosisPreview
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References
- 1.Dancey JT, Deubelbeiss KA, Harker LA, Finch CA (1976) Neutrophil kinetics in man. J Clin Invest 58: 705–715PubMedCrossRefGoogle Scholar
- 2.Cronkite EP, Fliedner T (1964) Granulocytopoiesis. New Engl J Med 270: 1347–1352PubMedCrossRefGoogle Scholar
- 3.Peters AM (1997) Just how big is the pulmonary granulocyte pool? Clin Sci 94: 7–19Google Scholar
- 4.Spitznagel JK (1990) Antibiotic proteins of human neutrophils. J Clin Invest 86: 1381–1386PubMedCrossRefGoogle Scholar
- 5.Smolen JE, Boxer LA (1990) Functions of neutrophils. In: WJ Williams et al (eds): Hematology. Mc Graw Hill, New York, USA, 780–794Google Scholar
- 6.Gallin JI (1984) Neutrophil specific granules: a fuse that ignites the inflammatory response. Clin Res 32: 320–327PubMedGoogle Scholar
- 7.Ryder MI, Weinreb RN, Niederman R (1984) The organisation of actin filaments in human polymorphonuclear leukocytes. Anat Rec 209: 7–20PubMedCrossRefGoogle Scholar
- 8.Lloyd AR, Oppenheim JJ (1992) Polys lament — the neglected role of the polymor-phonuclear neutrophil in the afferent limb of the immune-response. Immunol Today 13: 169–172PubMedCrossRefGoogle Scholar
- 9.Sklar LA, Finney DA, Oades ZG, Jesaitis RG, Painter RG, Cochrane CG (1984) The dynamics of ligand-receptor interactions. J Biol Chem 259: 5661–5669PubMedGoogle Scholar
- 10.Smith CD, Cox CC, Synderman R (1986) Receptor-coupled activation of phosphoinositide-specific phospholipase C by an N protein. Science 232: 97–99PubMedCrossRefGoogle Scholar
- 11.Stephens L, Eguinoa A, Corey S, Jackson T, Hawkins PT (1993) Receptor stimulated accumulation of phosphatidylinositol(3,4,5)-triphosphate by G-protein mediated pathways in human myeloid derived cells. EMBO J 12: 2265–2272PubMedGoogle Scholar
- 12.Erickson RW, Peveri P, Traynorkaplan AE, Curnutte JT (1993) Activation of human neutrophil NADPH oxidase by phosphatidic-acid and diacylglycerol. FASEB J 7: A1059Google Scholar
- 13.Daniels RH, Bird IN, Hill ME, Finnen MJ (1993) Differential regulation of early phase and late phase responses in human neutrophils by cAMP. Biochem Pharmacol 45: 1613–1620PubMedCrossRefGoogle Scholar
- 14.Moilanen E, Vuorinen P, Kankaanranta H, Metsa-Keteka T, Vapaatalo H (1993) Inhibition by nitric-oxide-donors of polymorphonuclear leucocyte functions. Br J Pharmacol 109: 852–858PubMedCrossRefGoogle Scholar
- 15.Wallerath T, Gath I, Aulitizky WE, Pollock JS, Kleinert H, Forstermann U (1997) Identification of the NO synthase isoforms expressed in human neutrophil granulocytes, megakaryocytes and platelets. Thrombosis Haemostasis 77: 163–167Google Scholar
- 16.Hallet MB, Lloyds D (1995) Neutrophil priming: the cellular signals that say “amber” but not “green”. Immunol Today 16: 264–268CrossRefGoogle Scholar
- 17.Brummel JH, Chan CK, Butler J, Borregaard N, Siminovitch KA, Grinstein S, Downey GP (1997) Regulation of Src homology 2-containing tyrosine phosphatase 1 during activation of human neutrophils — Role of protein kinase C. J Biol Chem 272: 875–882CrossRefGoogle Scholar
- 18.Pozzan T, Lew CB, Wollheim RY, Tsien RY (1983) Is cystolic ionized calcium regulating neutrophil activation? Science 221: 1413–1415PubMedCrossRefGoogle Scholar
- 19.Whyte MKB, Hardwick SJ, Meagher LC, Savill JS, Haslett C (1993) Transient elevations of cytosolic free calcium retard subsequent apoptosis in neutrophils in vitro.J Clin Invest 92: 446–455PubMedCrossRefGoogle Scholar
- 20.Segal AW, Abo A (1993) The biochemical basis of the NADPH oxidase of phagocytes. Trends Biochem Sci 18: 43–47PubMedCrossRefGoogle Scholar
- 21.DeLeo FR, Quinn MT (1996) Assembly of the phagocyte NADPH oxidase: molecular interaction of oxidase proteins. J Leukoc Biol 60: 677–691PubMedGoogle Scholar
- 22.Hallet MB, Davies EV, Campbell AK (1993) Oxidase activation in individual neutrophils is dependent on the onset and magnitude of the Ca2+ signal. Cell Calcium 11: 655–663CrossRefGoogle Scholar
- 23.Warren JS, Mandel DM, Johnson KJ, Ward PA (1989) Evidence for the role of platelet-activating factor in immune complex vasculitis in the rat. J Clin Invest 83: 669–678PubMedCrossRefGoogle Scholar
- 24.Forehand JR, Pabst MJ, Phillips WA, Johnston Jr. RB (1989) Lipopolysaccheride priming of human neutrophils for an enhanced respiratory burst. J Clin Invest 83: 74–83PubMedCrossRefGoogle Scholar
- 25.O’Flaherty JT, Redman JF, Jacobson DP, Rossi AG (1990) Stimulation and priming of protein kinase C translocation by a Ca2+ transient-independent mechanism. J Biol Chem 265: 21619–21623PubMedGoogle Scholar
- 26.Kitchen E, Rossi A, Condcliffe AM, Haslett C, Chilvers ER (1996) Demonstration of reversible priming of human neutrophils using platelet-activating factor. Blood 88: 4330–4337PubMedGoogle Scholar
- 27.Carre PC, Mortenson RL, King TE, Noble PW, Sable CL, Riches DWH (1991) Increased expression of the Interleukin-8 gene by alveolar macrophages in idiopathic pulmonary fibrosis — a potential mechanism for the recruitment and activation of neutrophils in lung fibrosis. J Clin Invest 88: 1802–1810PubMedCrossRefGoogle Scholar
- 28.Kunkel SL, Lukas M, Streiter RM (1995) Expression and biology of neutrophil and endothelial cell-derived cytokines. Sem Cell Biol 6: 327–336CrossRefGoogle Scholar
- 29.Worthen GS, Schwab B, Elson EL, Downey GP (1989) Mechanisms of stimulated neutrophils: cell stiffening induces retention in capillaries. Science 249: 183–186CrossRefGoogle Scholar
- 30.Tedder TF, Steeber DA, Chen A, Engel P (1995) The selectins: vascular adhesion molecules. FASEB J 9: 866–873PubMedGoogle Scholar
- 31.Springer TA (1990) Adhesion receptors of the immune system. Nature 346: 425–434PubMedCrossRefGoogle Scholar
- 32.Springer TA (1994) Traffic signals for lymphocyte recirculation and leukocyte emigration: The multistep paradigm. Cell 76: 301–314PubMedCrossRefGoogle Scholar
- 33.Hellewell PG, Young S, Henson P, Worthen G (1994) Disparate role of the beta 2-integrin CD18 in the local accumulation of neutrophils in pulmonary and cutaneous inflammation in the rabbit. Am J Respir Cell Mol Biol 10: 391–398PubMedGoogle Scholar
- 34.Holtzman MJ, Look DC (1992) Cell adhesion molecules as targets for unravelling the genetic regulation of airway inflammation. Am J Resp Cell Mol Biol 7: 246–247Google Scholar
- 35.Weiss SJ (1989) Tissue destruction by neutropils. N Engl J Med 320: 365–376PubMedCrossRefGoogle Scholar
- 36.Jones HA, Sriskandan AM, Peters NB, Krausz T, Boobis AR, Haslett C (1997) Dissociation of neutrophil emigration and metabolic activity in lobar pneumonia and bronchiectasis. Eur Respir J 10: 795–803PubMedGoogle Scholar
- 37.Savill JS, Wyllie AH, Henson JE, Henson PM, Haslett C (1989) Macrophage phagocytosis of aging neutrophils in inflammation. Programmed cell death in the neutrophil leads to its recognition by macrophages. J Clin Invest 83: 865–875PubMedCrossRefGoogle Scholar
- 38.Haslett C (1992) Resolution of acute inflammation and the role of apoptosis in the tissue fate of granulocytes. Clin Sci 83: 639–648PubMedGoogle Scholar
- 39.Grigg JM, Savill JS, Sarraf C, Haslett C, Silverman M (1991) Neutrophil apoptosis and clearance by macrophages in the lungs of neonates with pulmonary inflammation. Lancet 338: 720–722PubMedCrossRefGoogle Scholar
- 40.Cox G, Crossley J, Xing Z (1995) Macrophage engulfment of apoptotic neutrophils contributes to the resolution of acute pulmonary inflammation in vivo. Am J Resp Cell Mol Biol 12: 232–237Google Scholar
- 41.Whyte MKB, Meagher LC, MacDermot J, Haslett C (1993) Impairment of function in aging neutrophils is associated with apoptosis. J Immunol 150: 5124–5134PubMedGoogle Scholar
- 42.Meagher LC, Savill JS, Baker A, Fuller RW, Haslett C (1992) Phagocytosis of apoptotic neutrophils does not induce macrophage release of thromboxane B2. J Leukoc Biol 52: 269–273PubMedGoogle Scholar
- 43.Lee A, Whyte MKB, Haslett C (1993) Inhibition of apoptosis and prolongation of neutrophil functional longevity by inflammatory mediators. J Leukoc Biol 54: 283–288PubMedGoogle Scholar
- 44.Rossi AG, Cousin JM, Dransfield I, Lawson MF, Chilvers ER, Haslett C (1995) Agents that elevate cAMP inhibit human neutrophil apoptosis. Biochem Biophys Res Comm 217: 892–899PubMedCrossRefGoogle Scholar
- 45.Stockley RA (1995) The pathogenesis of chronic obstructive lung disease — Implications for therapy. Q J Med 88: 141–146Google Scholar
- 46.Thompson AB, Daughton D, Robbins RA, Ghafouri MA, Oehlerking M, Rennard SI (1989) Intraluminal airway inflammation in chronic bronchitis. Characterisation and correlation with clinical parameters. Am Rev Resp Dis 140: 1527–1537PubMedCrossRefGoogle Scholar
- 47.Keatings VM, Collins PD, Scott DM, Barnes PJ (1996) Differences in interleukin-8 and tumour necrosis factor in induced sputum from patients with chronic obstructive pulmonary disease or asthma. Am J Respir Crit Care Med 153: 530–534PubMedGoogle Scholar
- 48.Martin TR, Pistorese BP, Chi EY, Goodman RB, Matthay MA (1989) Effects of leukotriene B4 in the human lung. Recruitment of neutrophils into the alveolar spaces without a change in protein permeability. J Clin Invest 84: 1609–1619PubMedCrossRefGoogle Scholar
- 49.Bingle L, Tetley TD (1996) Secretory leukoprotease inhibitor: partnering alpha 1-proteinase inhibitor to combat pulmonary inflammation. Thorax 51: 1273–1274PubMedCrossRefGoogle Scholar
- 50.Busse WW, Calhoun WF, Sedgwick J (1993) Mechanism of airway inflammation in Asthma. Am Rev Resp Dis 147: S20–24PubMedGoogle Scholar
- 51.Laitinen LA, Laitinen A, Haahtela T (1993) Airway mucosal inflammation even in patients with newly diagnosed asthma. Am Rev Resp Dis 147: 697–704PubMedGoogle Scholar
- 52.Frangova V, Sacco O, Silvestri M, Oddera S, Balbo A, Crimi E, Rossi GA (1996) BAL neutrophilia in asthmatic patients. A by-product of eosinophil recruitment? Chest 110: 1236–1242PubMedCrossRefGoogle Scholar
- 53.Montefort S, Gratziou C, Goulding D, Polosa R, Haskard DO, Howarth PH, Holgate ST, Carroll MP (1994) Bronchial biopsy evidence for leukocyte infiltration and upregulation of leukocyte-endothelial cell adhesion molecules for 6 hours after local allergen challenge in sensitized asthmatic airways. J Clin Invest 94: 1411–1421CrossRefGoogle Scholar
- 54.Sur S, Crotty TB, Kephart GM (1993) Sudden-onset fatal asthma. A distant entity with few eosinophils and relatively more neutrophils in the airway submucosa? Am Rev Resp Dis 148: 713–719PubMedCrossRefGoogle Scholar
- 55.Lamblin C, Gosset P, TillieLeblond I, Saulnier F, Marquette CH, Wallaert B, Tonnel AB (1998) Bronchial neutrophilia in patients with noninfectious status asthmaticus. Am J Resp Crit Care Med 157: 394–402PubMedGoogle Scholar
- 56.Fabbri LM, Boschetto P, Zocco E, Milani G, Pivirotto F, Plebani M, Burlina A, Licata B, Mapp CE (1987) Bronchoalveolar neutrophilia during late asthmatic reactions induced by toluene diisocyanate. Am Rev Resp Dis 136: 36–42PubMedCrossRefGoogle Scholar
- 57.Reid PT, Donnelly SC, Haslett C (1995) Inflammatory predictors for the development of the adult respiratory distress syndrome. Thorax 50: 1023–1026PubMedCrossRefGoogle Scholar
- 58.Brus F, van Oeveren W, Okken A, Oettomo SB (1997) Number and activation of circulating polymorphonuclear leukocytes and platelets are associated with neonatal respiratory distress syndrome severity. Pediatrics 99: 672–680PubMedCrossRefGoogle Scholar
- 59.Donnelly SC, MacGregor I, Zamani A, Gordon MW, Robertson CE, Steedman DJ, Little K, Haslett C (1995) Plasma elastase levels and the development of the adult respiratory distress syndrome. Am J Respir Crit Care Med 151: 1428–1433PubMedGoogle Scholar
- 60.Elborn JS, McCloskey M (1997) Cystic fibrosis. Curr Opin Inf Dis 10: 128–131CrossRefGoogle Scholar
- 61.Mulligan MS, Jones ML, Bolanowski MA, Baganoff MP, Deppeler CL, Meyers DM, Ryan US, Ward PA (1993) Inhibition of lung inflammatory reactions in rats by an antihuman IL-8 antibody. J Immunol 150: 5585–5595PubMedGoogle Scholar
- 62.Matsumoto T, Yokoi K, Mukaida N, Harada A, Yamashita J, Watanabe Y, Matsushima K (1997) Pivotal role of interleukin-8 in the acute respiratory distress syndrome and cerebral reperfusion injury. J Leukoc Biol 62: 581–587PubMedGoogle Scholar
- 63.Villard J, Dayerpastore F, Hamacher J, Aubert JD, Schlegelhaueter S, Nicod LP (1995) Gro-alpha and interleukin-8 in pneumocytosis-carini or bacterial pneumonia and adultrespiratory-distress-syndrome. Am J Resp Crit Care Med 152: 1549–1554Google Scholar
- 64.Elliott MJ, Maini RN, Feldmann M, Longfox A, Charles P, Bijl H, Woody JN (1994) Repeated therapy with monoclonal-antibody to tumour-necrosis-factor-alpha (CA2) in patients with rheumatiod arthritis. Lancet 8930: 1125–1127CrossRefGoogle Scholar
- 65.Evans DJ, Barnes PJ, Spaethe SM, Vanalstyne EL, Mitchell MI, O’Connor BJ (1996) Effect of a Leukotriene B-4 receptor antagonist, LY293111, on allergen-induced responses in asthma. Thorax 51:1178–1184PubMedCrossRefGoogle Scholar
- 66.Guidot DM, Repine MJ, Westcott JY, Repine JE (1994) Intrinsic 5-lipoxygenase activi-ty is required for neutrophil responsivity. Proc Natl Acad Sci USA 91: 8156–8159PubMedCrossRefGoogle Scholar
- 67.Coughlan AF, Hau H, Dunlop LC, Berndt MC, Hancock WW (1994) P-selectin and platelet-activating-factor mediate initial endotoxin-induced neutropenia. J Exp Med 179: 329–334PubMedCrossRefGoogle Scholar
- 68.Albelda SM, Smith CW, Ward PA (1994) Adhesion molecules and inflammatory injury. FASEB J 8: 504–512PubMedGoogle Scholar
- 69.Mulligan MS, Miyasaka M, Tamatani T, Jones ML, Ward PA (1994) Requirements for L-selectin in neutrophil-mediated lung injury in rats. J Immunol 152: 832–840PubMedGoogle Scholar
- 70.Bullard DC, Qin L, Lorenzo I, Quinlim WM, Doyle NA, Bosse R, Vestweber D, Doershuk CM, Beaudet AL (1995) P-selectin ICAM-1 double mutant mice — acute emigration of neutrophils into the peritoneum is completely absent but is normal into pulmonary alveoli. J Clin Invest 95: 1782–1788PubMedCrossRefGoogle Scholar
- 71.Muller T, Engels P, Fozard J (1996) Subtypes of the type 4 cAMP phosphodiesterase: structure, regulation and selective inhibition. Trends Pharm Sci 17: 294–298PubMedCrossRefGoogle Scholar
- 72.Bloemem PGM, VandenTweel MC, Henricks PAJ, Engels F, Kester MHA, VandeLoo PGF, Blomjous FJ, Nijkamp FP (1997) Increased cAMP levels in stimulated neutrophils inhibit their adhesion to human bronchial epithelial cells. Am J Phys-Lung Cell Mol Phys 16: L580–L587Google Scholar
- 73.Sullivan GW, Carper HT, Mandell GL (1995) The specific type-IV phosphodiesterase inhibitor rolipram combined with adenosine reduces tumour necrosis factor-alphaprimed neutrophils oxidative activity. Int J Immunopharm 17: 793–803CrossRefGoogle Scholar
- 74.Llewellyn Jones CG, Stockley RA (1994) The effects of 132-agonists and methylxanthines on neutrophil function in vitro.Eur Respir J 7: 1460–1466PubMedCrossRefGoogle Scholar
- 75.Downy GP, Butler JR, Tapper H, Fialkow L, Satiel AR, Rubin BB, Grinstein S (1998) Importance of MEK in neutrophil microbicidal responsiveness. J Immunol 160: 434–443Google Scholar
- 76.Niggli V, Keller H (1997) The phosphatidylinositol 3-kinase inhibitor wortmannin markedly reduces chemotactic peptide-induced locomotion and increases in cytoskeletal actin in human neutrophils. Eur J Pharm 335: 43–52CrossRefGoogle Scholar
- 77.SueAQuan AK, Fialkow L, Vlahos CJ, Schlem JA, Grinstein S, Butler J, Downey GP (1997) Inhibition of neutrophil oxidative burst and granule secretion by Wortmannin: Potential role of MAP kinase and renaturable kinases. J Cell Physiol 172: 94–108CrossRefGoogle Scholar
- 78.Talpain E, Armstrong RA, Coleman RA, Vardey CJ (1995) Characterization of the PGE receptor subtype mediating inhibition of superoxide production in human neutrophils. Br J Pharm 114: 1459–1465CrossRefGoogle Scholar
- 79.Armstrong RA (1995) Investigation of the inhibitory effects of PGE(2) and selective EP agonists on chemotaxis of human neutrophils. Br J Pharm 116: 2903–2908CrossRefGoogle Scholar
- 80.Filep JG, Delalandre A, Payette Y, FoldesFilep E (1997) Glucocorticoid receptor regulates expression of L-selectin and CD11/CD18 on human neutrophils. Circulation 96: 295–301PubMedCrossRefGoogle Scholar
- 81.Lomas DA, Ip M, Chamba A, Stockley RA (1991) The effect of in vitro and in vivo dexamethasone on human neutrophil function. Agents Actions 33: 279–285PubMedCrossRefGoogle Scholar
- 82.Meagher LC, Cousin JM, Seckl JR, Haslett C (1996) Opposing effects of glucocorticoids on the rate of apoptosis in neutrophilic and eosinophil granulocytes. J Immunol 156: 4422–4428PubMedGoogle Scholar
- 83.Claesson R, Karlsson M, Zhang YY, Carlsson J (1996) Relative role of chloramines, hypochlorous acid and proteases in the activation of human polymorphonuclear leukocyte collagenase. J Leukoc Biol 60: 598–602PubMedGoogle Scholar
- 84.Ottonello L, Dapino P, Scirocco M, Dallegri F, Sacchetti C (1994) Proteolytic inactivation of alpha-1-antitrypsin by human neutrophils — involvement of multiple and inter-linked cell responses to phagocytosable targets. Eur J Clin Invest 24: 42–49PubMedCrossRefGoogle Scholar
- 85.Barnes PJ, Karin M (1997) Nuclear factor kB: a pivitol transcription factor in chronic inflammatory disease. N Engl J Med 336: 1066–1071PubMedCrossRefGoogle Scholar
- 86.Boman G, Backer U, Larsson S, Melander B, Wahlander L (1983) Oral acetylcysteine reduces exacerbation rate in chronic bronchitis: a report of a trial organized by the Swedish Society for Pulmonary Diseases. Eur J Resp Dis 64: 405–415Google Scholar
- 87.Miyajima T, Kotake Y (1995) Spin trapping agent, phenyl-N-tert-butyl nitrone, inhibits induction of nitric oxide synthase in endotoxin-induced shock in mice. Biochem Biophys Res Commun 215: 114–121PubMedCrossRefGoogle Scholar
- 88.Sommerhoff CP, Nadel JA, Basbaum CB, Caughey GH (1990) Neutrophil elastase and cathepsin G stimulate secretion from cultured bovine airway gland serous cells. J Clin Invest 85: 682–689PubMedCrossRefGoogle Scholar
- 89.Nakamura H, Yoshimura K, McElvaney NG, Crystal RG (1992) Neutrophil elastase in repiratory epithelial lining fluid of individuals with cystic fibrosis induces interleukin-8 gene expression in a human bronchial epithelial cell line. J Clin Invest 89: 1478–1484PubMedCrossRefGoogle Scholar
- 90.Chertov O, Ueda H, Xu LL, Tani K, Murphy WJ, Wang JM, Howard OMZ, Sayers TJ, Oppenheim JJ (1997) Identification of human neutrophil-derived cathepsin G and azurocidin/CAP37 as chemoattractants for mononuclear cells and neutrophils. J Exp Med 186: 739–747PubMedCrossRefGoogle Scholar
- 91.Williams JC, Falcone RC, Knee C et al (1991) Biological characterization of ICE 200, 355, novel inhibitors of human neutrophil elastase. Am Rev Respir Dis 144: 875–883PubMedCrossRefGoogle Scholar
- 92.McElvaney DG, Hubbard RC, Birper P et al (1991) Aerosol α1-antitrypsin treatment for cystic fibrosis. Lancet 337: 392–394PubMedCrossRefGoogle Scholar
- 93.Cadene M, Duranton J, North A, SiTahar M, Chignard M, Bieth JG (1997) Inhibition of neutrophil serine proteinases by suramin. J Biol Chem 272: 9950–9955PubMedCrossRefGoogle Scholar
- 94.Llewellyn Jones CG, Lomas DA, Stockley RA (1994) Potential role of recombinant secretory leucoprotease inhibitor in the prevention of neutrophil mediated matrix degradation. Thorax 49: 567–572PubMedCrossRefGoogle Scholar
- 95.Tomee JFC, Koeter GH, Hiemstra PS, Kauffman HF (1998) Secretory leukoprotease inhibitor: a native antimicrobial protein presenting a new therapeutic option? Thorax 53: 114–116PubMedCrossRefGoogle Scholar
- 96.Sallenave JM, Porteous DJ, Haslett C (1997) Gene therapy for inflammatory lung diseases: not so far away? Thorax 52: 742–744PubMedCrossRefGoogle Scholar