Why Does Airway Inflammation Persist? Is It Leukotrienes?

PAUL M. O'BYRNE

Asthma Research Group, St. Joseph's Hospital and the Department of Medicine, McMaster University, Hamilton, Ontario, Canada

	CYSTEINYL LEUKOTRIENES AS MEDIATORS OF ASTHMATIC BRONCHOCONSTRICTION

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The leukotrienes are eicosanoids, derived from a cell membrane phospholipid constituent arachidonic acid, which is selectively cleaved by phospholipase A₂ from cell membranes. Arachidonic acid is converted sequentially to 5-hydroperoxyeicosatetraenoic acid (5-HPETE) and then to leukotriene A₄ (LTA₄) by a catalytic complex consisting of 5-lipoxygenase (5-LO) and the 5-lipoxygenase-activating protein (FLAP). In the presence of leukotriene-C₄ synthase, glutathione is adducted at the C-6 position of LTA₄ to yield the molecule known as LTC₄, which is exported from the cytosol to the extracellular microenvironment, where the glutamic acid moiety is cleaved to form LTD₄. Cleavage of the glycine moiety from LTD₄ results in the formation of LTE₄, which is the stable excretory product. LTC₄, LTD₄, and LTE₄, are known as the cysteinyl leukotrienes; together these molecules constitute the material formerly known as the slow-reacting substance of anaphylaxis (SRS-A). All three cysteinyl leukotrienes have the same range of biological effects; however, LTE₄ is much less potent than its precursor molecules. Among the cells in the lung that possess the enzymatic activities to produce the cysteinyl leukotrienes are mast cells, eosinophils, and alveolar macrophages.

There is, now, compelling information to implicate the cysteinyl leukotrienes as mediators of asthmatic bronchoconstriction. Increased production of the cysteinyl leukotrienes, as measured by increases in urinary excretion of LTE₄, has been demonstrated after allergen inhalation (1, 2) in subjects with atopic asthma and during episodes of acute severe asthma (2). Also, subjects with aspirin-induced asthma have higher baseline levels of urinary LTE₄ when compared with other subjects with asthma (3); this level increases after aspirin ingestion, and is associated with an overexpression of LTC₄ synthase in airway biopsies (4).

Treatment with drugs that inhibit the synthesis of the cysteinyl leukotrienes or block the CysLT₁ receptor, which mediates the action of the cysteinyl leukotrienes in the airways, markedly attenuates allergen-induced, aspirin-induced, and exercise-induced bronchoconstriction (5) and allergen-induced airway hyperresponsiveness (8), as well as improving spontaneous bronchoconstriction (9, 10), and other indices of asthma control (11). Taken together, these studies indicate an important role for the cysteinyl leukotrienes in causing bronchoconstriction.

	CAN CYSTEINYL LEUKOTRIENES CAUSE ASTHMATIC AIRWAY INFLAMMATION?

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The most widely studied biological effects of the cysteinyl leukotrienes are smooth muscle contraction and vascular leakage. There is much less convincing evidence that the cysteinyl leukotrienes cause chemotaxis or activation of inflammatory cells in individuals with asthma. LTD₄ is a potent chemoattractant for human eosinophils in vitro (12) and inhaled LTE₄ has been shown to cause eosinophil accumulation in asthmatic airway biopsies (13). Also, inhaled LTD₄ has been demonstrated to increase eosinophils in induced sputum (14), although the magnitude of this effect was not different from that of inhaled methacholine, an agonist not known to cause airway inflammation. In one study, we examined the ability of inhaled LTD₄ to cause eosinophil and mast cell accumulation in induced sputum from asthmatic subjects with documented allergen-induced airway eosinophilia, and could not demonstrate that LTD₄ caused an increase in sputum eosinophils (15).

Two studies have used segmental allergen challenge to examine the ability of antileukotrienes to attenuate allergen- induced airway eosinophilia in subjects with asthma; one study (16) demonstrated attenuation of eosinophil influx after challenge, and the other (17) demonstrated attenuation of basophil, but not eosinophil, influx. The effect of antileukotrienes on airway inflammation in patients with persistent asthma has also been studied. These studies have shown a significant reduction in circulation blood eosinophils in patients with nocturnal asthma (18) and in adults (19) and children (20) with persisting asthma. Also, treatment with the CysLT₁ antagonist, montelukast, reduced the numbers of sputum eosinophils in adults with persisting asthma (21).

Taken together, these studies suggest that the cysteinyl leukotrienes can cause eosinophilic airway inflammation and may be, in part, responsible for the persisting eosinophilic airway inflammation present in some individuals with asthma. However, the antileukotrienes are not known to improve the persisting airway hyperresponsiveness present in individuals with asthma, as inhaled corticosteroids (which usually eliminate eosinophilic airway inflammation) do. This suggests that either reducing airway eosinophilia does not influence airway hyperresponsiveness, or more likely, the magnitude of the effect of the antileukotrienes on the airway eosinophilia is insufficient.

	CAN LEUKOTRIENE B4 CAUSE ASTHMATIC AIRWAY INFLAMMATION?

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LTB₄ is another eicosanoid produced by the action of 5-LO on arachidonic acid. Cells that contain LTB₄ synthetase, mainly alveolar macrophages and neutrophils, convert LTA₄ to LTB₄, which has a different range of biological activities than the cysteinyl leukotrienes. LTB₄ acts on a distinct G protein-coupled receptor, different from the CysLT₁ receptor, and has no direct action on airway smooth muscle to cause bronchoconstriction. LTB₄ is a potent neutrophil chemoattractant (12) and can also cause eosinophil chemotaxis. Increases in plasma LTB₄ levels have been described in children (22) and adults (23) during acute asthmatic episodes, which persisted for up to 1 mo after the episodes. Also, LTB₄ levels in bronchoalveolar lavage (BAL) fluid are increased in patients with nocturnal asthma (18), as are BAL neutrophil numbers in patients with severe persisting asthma (24). Both BAL LTB₄ levels and neutrophils in nocturnal asthma are reduced by treatment with oral prednisone, in association with an increase in FEV₁ (25). Inhaled LTB₄ causes a rapid and transient neutropenia followed by a neutrophilia in both healthy subjects and subjects with asthma which lasts < 6 h (26). These changes in circulating neutrophils are not associated with any changes in lung function. In addition, treatment with an LTB₄ receptor antagonist does not influence allergen-induced asthmatic responses (27). There are, however, no published studies of an effect of an LTB₄ antagonist on clinical indices of airway inflammation in patients with persisting asthma.

	SUMMARY

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Both LTB₄ and the cysteinyl leukotrienes cause inflammatory cell chemotaxis in vitro as well as in vivo, in animal models of airway inflammation. These eicosanoids are produced during both acute and experimental asthma and the cysteinyl leukotrienes are likely responsible for part of the persisting airway eosinophilia in asthma. The importance of LTB₄ in causing the airway neutrophilia associated with some exacerbations of asthma (28), and with severe persisting asthma (24), is not known.

	IMPORTANT QUESTIONS

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• Do the cysteinyl leukotrienes act alone, or in combination with other inflammatory mediators, to cause airway eosinophilia in asthma?
• How important is the cysteinyl leukotriene-induced airway eosinophilia in causing persisting asthma?
• Is LTB₄ involved in causing the airway neutrophilia that occurs in some exacerbations of asthma and in patients with severe asthma?
• Will an LTB₄ antagonist be useful in treating patients with severe, noneosinophilic asthma?

	Footnotes

Correspondence and requests for reprints should be addressed to P. M. O'Byrne, M.D., Firestone Regional Chest and Allergy Unit, St. Joseph's Hospital, 50 Charlton Ave. East, Hamilton, ON, L8N 4A6 Canada. E-mail: obyrnep{at}fhs.mcmaster.ca.

	References

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1. Manning, P. J., J. Rokach, J. L. Malo, D. Ethier, A. Cartier, Y. Girard, S. Charleson, and P. M. O'Byrne. 1990. Urinary leukotriene E4 levels during early and late asthmatic responses. J. Allergy Clin. Immunol. 86: 211-220 [Medline].

2. Taylor, G. W., P. Black, N. Turner, I. Taylor, N. H. Maltby, R. W. Fuller, and C. T. Dollery. 1989. Urinary leukotriene E4 after antigen challenge and in acute asthma and allergic rhinitis. Lancet i: 585-587 .

3. Dahlén, B., M. Kumlin, H. Johannson, C. Larsson, O. Zetterström, E. Granström, and S.-E. Dahlén. 1991. Aspirin-sensitive asthmatics have elevated basal levels of leukotriene E4 in urine and bronchial provocation with lysine-aspirin results in further release. Am. Rev. Respir. Dis. 143: 599-604 .

4. Cowburn, A. S., K. Sladek, J. Soja, L. Adamek, E. Nizankowska, A. Szczeklik, B. K. Lam, J. F. Penrose, K. F. Austen, S. T. Holgate, and A. P. Sampson. 1999. Overexpression of leukotriene C4 synthetase in bronchial biopsies from patients with aspirin-intolerant asthma. J. Clin. Invest. 101: 834-846 [Medline].

5. Taylor, I. K., K. M. O'Shaughnessy, R. W. Fuller, and C. T. Dollery. 1991. Effect of a cysteinylleukotriene receptor antagonist, ICI 204-209, on allergen-induced bronchoconstriction and airway hyperactivity in atopic subjects. Lancet 337: 690-694 [Medline].

6. Israel, E., A. R. Fischer, M. A. Rosenburg, C. M. Lilly, J. C. Callery, J. Shapiro, J. Cohn, P. Rubin, and J. Drazen. 1993. The pivotal role of 5-lipoxygenase products in the reaction of aspirin-sensitive asthmatics to aspirin. Am. Rev. Respir. Dis. 148: 1447-1451 [Medline].

7. Manning, P. J., R. M. Watson, D. J. Margolskee, V. C. Williams, J. I. Schwartz, and P. M. O'Byrne. 1990. Inhibition of exercise-induced bronchoconstriction by MK-571, a potent leukotriene D4-receptor antagonist. N. Engl. J. Med. 323: 1736-1739 [Abstract].

8. Hamilton, A. L., I. Faiferman, P. Stober, R. M. Watson, and P. M. O'Byrne. 1998. Pranlukast, a leukotriene receptor antagonist, attenuates allergen-induced early and late phase bronchoconstriction and airway hyperresponsiveness in asthmatic subjects. J. Allergy Clin. Immunol. 102: 177-183 [Medline].

9. Hui, K. P., and N. C. Barnes. 1991. Lung function improvement in asthma with a cysteinyl-leukotriene receptor antagonist. Lancet 337: 1062-1063 [Medline].

10. Gaddy, J., R. K. Bush, D. Margolskee, V. C. Williams, and W. Busse. 1992. The effects of leukotriene D4 (LTD4) antagonist (MK-571) in mild to moderate asthma. Am. Rev. Respir. Dis. 146: 358-363 [Medline].

11. Israel, E., P. Rubin, J. P. Kemp, J. Grossman, W. Pierson, S. C. Siegel, D. Tinkelman, J. J. Murray, W. W. Busse, and A. T. Segal. 1993. The effect of inhibition of 5-lipoxygenase by zileuton in mild-to-moderate asthma. Ann. Intern. Med. 119: 1059-1066 [Abstract/Free Full Text].

12. Spada, C. S., A. L. Nieves, A. H. Krauss, and D. F. Woodward. 1994. Comparison of leukotriene B4 and D4 effects on human eosinophil and neutrophil motility in vitro. J. Leukocyte Biol. 55: 183-191 [Abstract].

13. Laitinen, L. A., A. Laitinen, T. Haahtela, V. Vikka, B. W. Spur, and T. H. Lee. 1993. Leukotriene E4 and granulocytic infiltration into asthmatic airways. Lancet 341: 989-990 [Medline].

14. Diamant, Z., J. T. Hiltermann, E. L. J. van Rensen, P. M. C. Callenbach, M. A. Veselic-Charvat, H. van der Veen, and P. Sterk. 1997. The effect of inhaled leukotriene D4 and methacholine on sputum cell differentials in asthma. Am. J. Respir. Crit. Care Med. 155: 1247-1253 [Abstract].

15. Mulder, A., G. M. Gauvreau, R. M. Watson, and P. M. O'Byrne. 1999. The effect of inhaled leukotriene D₄ on airway eosinophilia and airway hyperresponsiveness in asthmatic subjects. Am. J. Respir. Crit. Care Med. 159: 1562-1567 [Abstract/Free Full Text].

16. Kane, G. C., M. Polloce, C. J. Kim, R. T. Dworski, J. J. Murray, J. R Sheller, J. E. Fish, and S. P. Peters. 1996. A controlled trial of the effect of the 5-lipoxygenase inhibitor, zileuton, on lung inflammation produced by segmental antigen challenge in human beings. J. Allergy Clin. Immunol. 97: 646-654 [Medline].

17. Calhoun, W. J., B. J. Lavins, M. C. Minkwitz, R. Evans, G. J. Gleich, and J. Cohn. 1998. Effect of Zafirlukast (Accolate) on cellular mediators of inflammation: bronchoalveolar lavage fluid findings after segmental allergen challenge. Am. J. Respir. Crit. Care Med. 157: 1381-1389 [Abstract/Free Full Text].

18. Wenzel, S. E., J. B. Trudeau, D. A. Kaminsky, J. Cohn, R. J. Martin, and J. Y. Westcott. 1995. Effect of 5-lipoxygenase inhibition on bronchoconstriction and airway inflammation in nocturnal asthma. Am. J. Respir. Crit. Care Med. 152: 897-905 [Abstract].

19. Malstrom, K., G. Rodriguez, Gomez, J. Guerra, C. Villaran, A. Piñeiro, L. X. Wei, B. C. Seidenberg, and T. F. Reiss. 1999. Oral montelukast, inhaled beclomethasone, and placebo for chronic asthma: a randomized, controlled trial. Montelukast/Beclomethasone Study Group. Ann. Intern. Med. 130: 487-495 [Abstract/Free Full Text].

20. Knorr, B., J. Matz, J. A. Bernstein, H. Nguyen, B. C. Seidenberg, T. Reiss, and A. Becker. 1998. Montelukast for chronic asthma in 60 to 14-year-old children: a randomized, double-blind trial. Pediatric Montelukast Study Group. J.A.M.A. 279: 1181-1186 [Abstract/Free Full Text].

21. Pizzichini, E., J. A. Leff, T. Reiss, L. Hendeles, L. P. Boulet, L. X. Wei, A. Efthimiadis, J. Zhang, and F. E. Hargreave. 1999. Montelukast reduces airway eosinophilic inflammation in asthma: a randomized, controlled trial. Eur. Respir. J. (In press)

22. Sampson, A. P., D. P. Castling, C. P. Green, and J. F. Price. 1995. Persistent increase in plasma and urinary leukotrienes after acute asthma. Arch. Dis. Child. 73: 221-225 [Abstract].

23. Shindo, K., M. Fukumura, and K. Miyakawa. 1995. Leukotriene B4 levels in the arterial blood of asthmatic patients and the effects of prednisolone. Eur. Respir. J. 8: 605-610 [Abstract].

24. Wenzel, S. E., S. J. Szefler, D. Y. M. Leung, S. I. Sloan, M. D. Rex, and R. J. Martin. 1997. Bronchoscopic evaluation of severe asthma: persistent inflammation despite high dose glucocorticosteroids. Am. J. Respir. Crit. Care Med. 156: 737-743 [Abstract/Free Full Text].

25. Wenzel, S. E., J. B. Trudeau, J. Y. Westcott, W. R. Beam, and R. J. Martin. 1994. A single oral dose of prednisone decreases leukotriene B4 production by alveolar macrophages from patients with nocturnal asthma but not control subjects: relationship to changes in cellular influx and FEV1. J. Allergy Clin. Immunol. 94: 870-881 [Medline].

26. Sampson, S. E., J. F. Costello, and A. P. Sampson. 1997. The effect of inhaled leukotriene B4 in normal and asthmatic subjects. Am. J. Respir. Crit. Care Med. 155: 1789-1792 [Abstract].

27. Evans, D. J., P. J. Barnes, S. M. Spaethe, E. L. van Alstyne, M. I. Mitchell, and B. J. O'Connor. 1996. Effect of a leukotriene B₄ receptor antagonist LY293111, on allergen-induced responses in asthma. Thorax 51: 1178-1184 [Abstract].

28. Turner, M. O., P. Hussack, M. R. Sears, J. Dolovich, and F. E. Hargreave. 1995. Exacerbations of asthma without sputum eosinophilia. Thorax 50: 1057-1061 [Abstract].