Editorials The Eosinophil and Acute Lung Injury
T
he eosinophil has been recognized as a discrete cell type by microscopic examination of the blood, bone marrow, and other tissues for about 150 years. As detailed by Spyr (1), the eosinophil was probably first observed in the peripheral blood of humans in 1846 by WhartonJ ones, an anatomist at Charing Cross Hospital, London, England. It was Paul Ehrlich who discovered the best known characteristic of the cell. In 1879, Ehrlich described a leukocyte that avidly bound acidic dyes (1). He termed this cell the eosinophil because of the intense avidity of its granules for eosin, a brominated fluorescein derivative. Ehrlich's staining procedure has been widely used for examination of peripheral blood. Despite a vast accumulation of reports of the association of eosinophilia and specific clinical conditions, progress in developing knowledge of the eosinophil function has been slow. The presence of high numbers of eosinophils in diseases associated with parasite infection led to the widespread assumption that the cell plays a unique and beneficial role in host defense against such organisms. Eosinophils accumulate about parasites in vivo and deposit toxic granule contents on them (2). Both eosinophil granule proteins and oxygen metabolites have been shown to kill parasite worms (3). Similarly, the presence of high numbers of eosinophils in allergic diseases such as asthma coupled with the knowledge that eosinophil-associated enzymes can neutralize mediators of anaphylaxis, including LTC 4 , histamine, and plateletactivating factor, led to the suggestion that one function of the eosinophil is downregulation of the inflammation after immediate-type hypersensitivity reactions (4). However, as knowledge of the toxicity of the eosinophil to human tissues developed over the past decade, the view of their role in asthma changed. The eosinophil is now regarded by many as a potent proinflammatory cell with considerable tissue-injuring potential and a AM REV RESPIR DIS 1990; 142:1245-1246
prime mediator of epithelial injury and bronchial hyperreactivity (5). Eosinophils are rarely found in the normal human lower respiratory tract (6); however, many inflammatory disorders of the lower respiratory tract are associated with an accumulation of eosinophils in the parenchyma. The possibility that constituents of the eosinophil might be involved in injury to the lung parenchyma has gained clinical and experimental support. Eosinophil activation appears to be part of the inflammatory processes in idiopathic pulmonary fibrosis (lPF), sarcoidosis, hypersensitivity pneumonitis, histiocytosis X, eosinophilic pneumonia, and the interstitial diseases associated with collagen-vascular or druginduced disorders (6, 7). Eosinophilia in bronchoalveolar lavage may be a marker of progressive lung disease in patients with IPF (8, 9). Eosinophil activation in the lung has also been related to the lung damage in adult respiratory distress syndrome (ARDS) (10). A role for the eosinophil in mediating injury to the lung parenchyma in ARDS is supported by experiments demonstrating that eosinophils are cytotoxic to several types of lung parenchymal cells in vitro and that they can degrade matrix components of the human lung parenchyma (6, 11). The recent August 1990 issue of the AMERICAN REVIEW OF RESPIRAlORY DISEASE contained an article by Rowen and colleagues (12) that professed that activated eosinophils caused acute edematous injury in isolated perfused rat lungs. In the current issue of the REVIEW, Fujimoto and coworkers (13) report on studies in a similar model that also implicate the eosinophil's participation in production of microvascular injury. Although the eosinophil preparations in both studies contained> 10070 contaminating cells, the assertions that products of activated eosinophils were primary contributors to the injury are warranted. The two studies offer complimentary information. The study by Fujimoto and coworkers provides sound physiologic
proof that preactivated cell preparations increase lung microvascular permeability and resistance. Activated eosinophils caused a biphasic pulmonary vascular response; an initial intense vasoconstriction was followed by increased pulmonary microvascular permeability that resulted in lung edema. In the study by Rowen and colleagues, the physiologic methods employed make it difficult to determine whether the edema was caused by increased hydrostatic pressure, microvascular injury, or both. However, the study furnishes excellent histologic evidence of epithelial and capillary endothelial injury in the model. Meaningful quantitative comparisons of the two studies are difficult because they differ in the source and methods used to obtain the eosinophils, the numbers of cells perfused in the isolated lungs, and the methods used for eosinophil activation. What are the specific processes used by the eosinophil to mediate lung injury? Initial attention should logically focus upon the constituents that account for the striking morphologic appearance of the eosinophil. The cationic proteins contained within the eosin-straining granules include eosinophil peroxidase (EPO), major basic protein (MBP), eosinophil catipnic protein (ECP), and eosinophilderived neurotoxin (EDN). These distinct proteins make up about 90% of the granule proteins. Cationic proteins can directly injure pulmonary endothelial cells, increase the transvascular flux of proteins across endothelial monolayers, and cause lung edema in isolated perfused rat lungs (14, 15). Charge interactions between cationic proteins and the negatively charged endothelial cell surface appear to be the common mechanism for endothelial injury. Because the isoelectric point of some of the granule proteins of the eosinophil is above pH 11, the probability of charge-related endothelial injury by these granule components is high. The eosinophil cationic proteins may mediate lung injury by mechanisms not directly related to charge interactions. 1245
EDITORIAL
1246
The protective effect of catalase observed in the study by Rowen and colleagues (12) suggests that oxidant-mediated processes are likely involved in the lung injury and make the involvement of EPO of potential importance. Normal eosinophils have a respiratory burst that is two or three times more intense than that of normal neutrophils in vitro (16). A majority of the H 2 0 2 produced by the respiratory burst of the eosiriophil is used as a substrate by EPO to generate halogenated oxidants. The eosinophil preferentially uses bromide instead of chloride to make the unusual but potent oxidant hypobromous acid. Hypobromous acid is capable of destroying a wide range of mammalian targets. Studies on the effects of supplemental bromide on the injury produced in the eosinophil-mediated, isolated perfused lung models would be informative, perhaps, in further defining the role of oxidants. Fujimoto and coworkers (13) did not determine whether oxidant processes were involved in the microvascular inju- . ry that they observed. In view of the preactivation protocol they used, the phorbol myristate acetate-induced respiratory burst of the eosinophils would likely have subsided before their infusion into the lungs. Thus, it is likely that nonoxidative processes were largely responsible for injury in their model. MBP, ECP, and EDN are all toxic to mammalian cells in vitro. Because the cDNA encoding these proteins have been cloned, forthcoming information on specific functional domains as well as methods to produce large quantities of
the intact proteins will facilitate a dissection of their roles in acute lung injury. We are now beginning to understand more fully and appreciate how the unique properties of the eosinophil may be important in tissue injury. In addition to injury caused by the cationic proteins, the eosinophil could mediate lung injury by elaborating matrix-degrading proteases or by generating proinflammatory lipid autocoids. Much remains to be determined regarding the contribution of this cell to various forms of acute lung injury. The notion that the eosinophil may cause serious damage to host tissues in the setting of acute lung injury is gaining support. JOHN R. HOIDAL, M.D. Division of Respiratory, Critical Care and Occupational Pulmonary Medicine University of Utah Medical Center Salt Lake City, UT
References 1. Spyr CJF. Eosinophils: a comprehensive review and guide to the scientific and medical literature. New York: Oxford University Press, 1988; 3-9. 2. Kephart GM, Gleich GJ, Connor DH, Gibson OW, Ackerman SJ. Deposition of eosinophil granule major basic proteins into microfilariae of Onchocerca volvulus in the skin of patients treated with diethylcarbamazine. Lab Invest 1984; 50:51-61. 3. Yazdanbakhsh M, Tai PC, Spyr CJF, Gleich GJ, Roos D. Synergism between eosinophil cationic protein and oxygen metabolites in killing of schistosomula of Schistosoma mansoni. J Immunol 1987; 138:3443-7. 4. Goetz! EJ, Wasserman SI, Austen KF. Eosinophil polymorphonuclear leukocyte function in immediate hypersensitivity. Arch Patho11975; 99:1-4. 5. Gleich GJ. The eosinophil and bronchial asthma: current understanding. J Allergy Clin Immunol
1990; 85:422-36. 6. Davis WB, Fels GA, Sun XH, Gadek JE, Venet A, Crystal RG. Eosinophil mediated injury in lung parenchymal cells and interstitial matrix. A possible role for eosinophils in chronic inflammatory disorders of the lower respiratory tract. J Clin Invest 1984; 74:269-78. 7. Hallgren R, Bjernier L, Lundgren R, Venge P. The eosinophil component of the alveolitis in idiopathic pulmonary fibrosis. Signs of eosinophil activation in the lung are related to impaired lung function. Am Rev Respir Dis 1989; 139:373-7. 8. Watters Le, Schwarz MI, Cherniack RM et al. Idiopathic pulmonary fibrosis: pretreatment bronchoalveolar lavage cellular constituents and their relationship with lung histopathology and clinical response to therapy. Am Rev Respir Dis 1987; 135:6%-704. 9. Peterson MW, Monick M, Hunninghake CWo Prognostic role of eosinophils in pulmonary fibrosis. Chest 1987; 92:51-6. 10. Hallgren R, Samuelson T, Venge P, Modig J. Eosinophil activation in the lung is related to lung damage in adult respiratory distress syndrome. Am Rev Respir Dis 1987; 135:639-42. 11. Ayers GH, Altman Le, Gleich GJ, Loegering DA, Baker CB. Eosinophil and eosinophil granule mediated pneumocyte injury. J Allergy Clin Immunol 1985; 76:595-604. 12. Rowen JL, Hyde DM, McDonald RJ. Eosinophils cause acute edematous injury in isolated perfused rat lungs. Am Rev Respir Dis 1990; 142: 215-20. 13. Fujimoto K, Parker JC, Kayes SG. Activated eosinophils increase vascular permeability and resistance in isolated perfused rat lungs. Am Rev Respir Dis 1990; 142:000-000. 14. Peterson MW, Stone P, Shasby DM. Cationic neutrophil proteins increase transendothelial albumin movement. J Appl Physiol 1987; 62:1521-30. 15. Chang S, Westcott J, Henson JE, Voekel NF. Pulmonary vascular injury by polycations in perfused rat lung. J Appl Physiol 1987; 62:1932-43. 16. Weiss SJ, Test ST, Eckman CM, Ross D, Regiani S. Brominating oxidants generated by human eosinophils. Science 1984; 236:200-3.
T
he eosinophil has been recognized as a discrete cell type by microscopic examination of the blood, bone marrow, and other tissues for about 150 years. As detailed by Spyr (1), the eosinophil was probably first observed in the peripheral blood of humans in 1846 by WhartonJ ones, an anatomist at Charing Cross Hospital, London, England. It was Paul Ehrlich who discovered the best known characteristic of the cell. In 1879, Ehrlich described a leukocyte that avidly bound acidic dyes (1). He termed this cell the eosinophil because of the intense avidity of its granules for eosin, a brominated fluorescein derivative. Ehrlich's staining procedure has been widely used for examination of peripheral blood. Despite a vast accumulation of reports of the association of eosinophilia and specific clinical conditions, progress in developing knowledge of the eosinophil function has been slow. The presence of high numbers of eosinophils in diseases associated with parasite infection led to the widespread assumption that the cell plays a unique and beneficial role in host defense against such organisms. Eosinophils accumulate about parasites in vivo and deposit toxic granule contents on them (2). Both eosinophil granule proteins and oxygen metabolites have been shown to kill parasite worms (3). Similarly, the presence of high numbers of eosinophils in allergic diseases such as asthma coupled with the knowledge that eosinophil-associated enzymes can neutralize mediators of anaphylaxis, including LTC 4 , histamine, and plateletactivating factor, led to the suggestion that one function of the eosinophil is downregulation of the inflammation after immediate-type hypersensitivity reactions (4). However, as knowledge of the toxicity of the eosinophil to human tissues developed over the past decade, the view of their role in asthma changed. The eosinophil is now regarded by many as a potent proinflammatory cell with considerable tissue-injuring potential and a AM REV RESPIR DIS 1990; 142:1245-1246
prime mediator of epithelial injury and bronchial hyperreactivity (5). Eosinophils are rarely found in the normal human lower respiratory tract (6); however, many inflammatory disorders of the lower respiratory tract are associated with an accumulation of eosinophils in the parenchyma. The possibility that constituents of the eosinophil might be involved in injury to the lung parenchyma has gained clinical and experimental support. Eosinophil activation appears to be part of the inflammatory processes in idiopathic pulmonary fibrosis (lPF), sarcoidosis, hypersensitivity pneumonitis, histiocytosis X, eosinophilic pneumonia, and the interstitial diseases associated with collagen-vascular or druginduced disorders (6, 7). Eosinophilia in bronchoalveolar lavage may be a marker of progressive lung disease in patients with IPF (8, 9). Eosinophil activation in the lung has also been related to the lung damage in adult respiratory distress syndrome (ARDS) (10). A role for the eosinophil in mediating injury to the lung parenchyma in ARDS is supported by experiments demonstrating that eosinophils are cytotoxic to several types of lung parenchymal cells in vitro and that they can degrade matrix components of the human lung parenchyma (6, 11). The recent August 1990 issue of the AMERICAN REVIEW OF RESPIRAlORY DISEASE contained an article by Rowen and colleagues (12) that professed that activated eosinophils caused acute edematous injury in isolated perfused rat lungs. In the current issue of the REVIEW, Fujimoto and coworkers (13) report on studies in a similar model that also implicate the eosinophil's participation in production of microvascular injury. Although the eosinophil preparations in both studies contained> 10070 contaminating cells, the assertions that products of activated eosinophils were primary contributors to the injury are warranted. The two studies offer complimentary information. The study by Fujimoto and coworkers provides sound physiologic
proof that preactivated cell preparations increase lung microvascular permeability and resistance. Activated eosinophils caused a biphasic pulmonary vascular response; an initial intense vasoconstriction was followed by increased pulmonary microvascular permeability that resulted in lung edema. In the study by Rowen and colleagues, the physiologic methods employed make it difficult to determine whether the edema was caused by increased hydrostatic pressure, microvascular injury, or both. However, the study furnishes excellent histologic evidence of epithelial and capillary endothelial injury in the model. Meaningful quantitative comparisons of the two studies are difficult because they differ in the source and methods used to obtain the eosinophils, the numbers of cells perfused in the isolated lungs, and the methods used for eosinophil activation. What are the specific processes used by the eosinophil to mediate lung injury? Initial attention should logically focus upon the constituents that account for the striking morphologic appearance of the eosinophil. The cationic proteins contained within the eosin-straining granules include eosinophil peroxidase (EPO), major basic protein (MBP), eosinophil catipnic protein (ECP), and eosinophilderived neurotoxin (EDN). These distinct proteins make up about 90% of the granule proteins. Cationic proteins can directly injure pulmonary endothelial cells, increase the transvascular flux of proteins across endothelial monolayers, and cause lung edema in isolated perfused rat lungs (14, 15). Charge interactions between cationic proteins and the negatively charged endothelial cell surface appear to be the common mechanism for endothelial injury. Because the isoelectric point of some of the granule proteins of the eosinophil is above pH 11, the probability of charge-related endothelial injury by these granule components is high. The eosinophil cationic proteins may mediate lung injury by mechanisms not directly related to charge interactions. 1245
EDITORIAL
1246
The protective effect of catalase observed in the study by Rowen and colleagues (12) suggests that oxidant-mediated processes are likely involved in the lung injury and make the involvement of EPO of potential importance. Normal eosinophils have a respiratory burst that is two or three times more intense than that of normal neutrophils in vitro (16). A majority of the H 2 0 2 produced by the respiratory burst of the eosiriophil is used as a substrate by EPO to generate halogenated oxidants. The eosinophil preferentially uses bromide instead of chloride to make the unusual but potent oxidant hypobromous acid. Hypobromous acid is capable of destroying a wide range of mammalian targets. Studies on the effects of supplemental bromide on the injury produced in the eosinophil-mediated, isolated perfused lung models would be informative, perhaps, in further defining the role of oxidants. Fujimoto and coworkers (13) did not determine whether oxidant processes were involved in the microvascular inju- . ry that they observed. In view of the preactivation protocol they used, the phorbol myristate acetate-induced respiratory burst of the eosinophils would likely have subsided before their infusion into the lungs. Thus, it is likely that nonoxidative processes were largely responsible for injury in their model. MBP, ECP, and EDN are all toxic to mammalian cells in vitro. Because the cDNA encoding these proteins have been cloned, forthcoming information on specific functional domains as well as methods to produce large quantities of
the intact proteins will facilitate a dissection of their roles in acute lung injury. We are now beginning to understand more fully and appreciate how the unique properties of the eosinophil may be important in tissue injury. In addition to injury caused by the cationic proteins, the eosinophil could mediate lung injury by elaborating matrix-degrading proteases or by generating proinflammatory lipid autocoids. Much remains to be determined regarding the contribution of this cell to various forms of acute lung injury. The notion that the eosinophil may cause serious damage to host tissues in the setting of acute lung injury is gaining support. JOHN R. HOIDAL, M.D. Division of Respiratory, Critical Care and Occupational Pulmonary Medicine University of Utah Medical Center Salt Lake City, UT
References 1. Spyr CJF. Eosinophils: a comprehensive review and guide to the scientific and medical literature. New York: Oxford University Press, 1988; 3-9. 2. Kephart GM, Gleich GJ, Connor DH, Gibson OW, Ackerman SJ. Deposition of eosinophil granule major basic proteins into microfilariae of Onchocerca volvulus in the skin of patients treated with diethylcarbamazine. Lab Invest 1984; 50:51-61. 3. Yazdanbakhsh M, Tai PC, Spyr CJF, Gleich GJ, Roos D. Synergism between eosinophil cationic protein and oxygen metabolites in killing of schistosomula of Schistosoma mansoni. J Immunol 1987; 138:3443-7. 4. Goetz! EJ, Wasserman SI, Austen KF. Eosinophil polymorphonuclear leukocyte function in immediate hypersensitivity. Arch Patho11975; 99:1-4. 5. Gleich GJ. The eosinophil and bronchial asthma: current understanding. J Allergy Clin Immunol
1990; 85:422-36. 6. Davis WB, Fels GA, Sun XH, Gadek JE, Venet A, Crystal RG. Eosinophil mediated injury in lung parenchymal cells and interstitial matrix. A possible role for eosinophils in chronic inflammatory disorders of the lower respiratory tract. J Clin Invest 1984; 74:269-78. 7. Hallgren R, Bjernier L, Lundgren R, Venge P. The eosinophil component of the alveolitis in idiopathic pulmonary fibrosis. Signs of eosinophil activation in the lung are related to impaired lung function. Am Rev Respir Dis 1989; 139:373-7. 8. Watters Le, Schwarz MI, Cherniack RM et al. Idiopathic pulmonary fibrosis: pretreatment bronchoalveolar lavage cellular constituents and their relationship with lung histopathology and clinical response to therapy. Am Rev Respir Dis 1987; 135:6%-704. 9. Peterson MW, Monick M, Hunninghake CWo Prognostic role of eosinophils in pulmonary fibrosis. Chest 1987; 92:51-6. 10. Hallgren R, Samuelson T, Venge P, Modig J. Eosinophil activation in the lung is related to lung damage in adult respiratory distress syndrome. Am Rev Respir Dis 1987; 135:639-42. 11. Ayers GH, Altman Le, Gleich GJ, Loegering DA, Baker CB. Eosinophil and eosinophil granule mediated pneumocyte injury. J Allergy Clin Immunol 1985; 76:595-604. 12. Rowen JL, Hyde DM, McDonald RJ. Eosinophils cause acute edematous injury in isolated perfused rat lungs. Am Rev Respir Dis 1990; 142: 215-20. 13. Fujimoto K, Parker JC, Kayes SG. Activated eosinophils increase vascular permeability and resistance in isolated perfused rat lungs. Am Rev Respir Dis 1990; 142:000-000. 14. Peterson MW, Stone P, Shasby DM. Cationic neutrophil proteins increase transendothelial albumin movement. J Appl Physiol 1987; 62:1521-30. 15. Chang S, Westcott J, Henson JE, Voekel NF. Pulmonary vascular injury by polycations in perfused rat lung. J Appl Physiol 1987; 62:1932-43. 16. Weiss SJ, Test ST, Eckman CM, Ross D, Regiani S. Brominating oxidants generated by human eosinophils. Science 1984; 236:200-3.
