Editorial Inflammation and Airway Function: The Asthma Syndrome T
he hypothesis that inflammatory cells versus guinea pigs) and different models activated and sequestered within the (aerosol endotoxin versus allergen challungs could modulate airway function lenge) and addressed completely different has been the subject of numerous publi- aspects of airway function (nonspecific cations and symposia. Contained with- airway responsiveness versus allergenin the lungs are inflammatory cells, in- induced late changes in lung mechanics). cluding neutrophils, the various subtypes This editorial will not rigorously critique of lymphocytes, eosinophils, mast cells, the two articles but rather, it will discuss macrophages (alveolar, interstitial, and the apparent disassembling of the arguintravascular), plasma cells, and mono- ment that neutrophils control airway cytes, as well as a variety of cells (such function. It will also attempt to develop as platelets) not generally regarded as in- a conceptual understanding of asthma flammatory that can be intimately in- that may allow us to glean meaningful volved in the inflammatory response. An information from apparently dissimilar apriori assumption has been that the in- animal experiments and to apply these flammatory response was a required step data with the hope of increasing our unin the repair of tissue damage and that derstanding of asthma. the process was inherently beneficial. It Pauwels and coworkers (1)take a most is only recently that we have begun to in- interesting approach to addressing the terpret the process itself as potentially role of neutrophils in altered airway reharmful, with the inflammatory cells sponsiveness. Their data argue against a themselves, either directly or indirectly, role for inflammatory cells in the alteracapable of causing lung injury. Inflam- tions in airway responsivenessnoticed afmatory cells such as neutrophils and eo- ter exposure to aerosol endotoxin in rats sinophils are veritable factories capable in two ways: (1) there was no temporal of producing a wide range of mediators, relationship between endotoxin-induced cytokines, proteinases, and toxic oxidants increases in airway responsivenessand the and interacting with the host of other cell type or severity of inflammation as astypes contained within the lungs. Inflam- sessed by measuring the cellular compomatory cellsmay contribute to functional sition of bronchoalveolar lavage (BAL) abnormalities such as altered airway re- fluid, and (2) although aerosol endotoxsponsiveness and lung mechanics. Al- in caused similar neutrophil influx into though we lack a complete understand- BAL fluid in all three inbred rat strains ing of the specific pharmacologic actions studied, it only increased airway responof steroids, the success of these agents sivenessin two of the three strains of rats. Hutson and coworkers(2)examinedthe in the treatment of asthma (including the increased use of aerosolized steroids in role of neutrophils in inhaled, allergenoutpatient maintenance therapy) has induced, early and late changes in lung been used to support a link between in- mechanics in ovalbumin-sensitized guinea pigs. The investigators had previously flammation and asthma. Two articles appear in the current is- noted a temporal relationship between sue of the AMERICAN REVIEW OF RESPI- neutrophil accumulation in BAL fluid and the late changes in lung mechanics RAlORY DISEASE that address the role of inflammatory cells in alterations in air- caused by inhaled ovalbumin. Neutrophil way function. These articles are "The Ef- depletion with rabbit anti-guinea pig fect of Endotoxin Inhalation on Airway neutrophil serum did not affect either the Responsiveness and Cellular Influx in early or late changes in lung mechanics Rats" by Pauwels and colleagues (1)and caused by allergen. The investigators used "Evidence That Neutrophils Do Not Par- these data to arguethat pulmonary neutroticipate in the Late-Phase Airways Re- phils are not, despite their coincident apsponse Provoked by Ovalbumin Inhala- pearance in BAL fluid, causally involved tion in Conscious, Sensitized Guinea in the late changes in lung mechanics. Pigs" by Hutson and associates (2). AlBoth reports summarize relevant literthough both articles address the role of ature arguing that leukocytes can moduinflammation in altered airway function, late, in the case of the study by Pauwels they were done in different species (rats and colleagues, airway responsiveness AM REV RESPIR DIS 1990; 141:531-533
and, for the study by Hutson and associates, allergen-induced changes in lung mechanics. As pointed out by Hutson and coworkers (2), much of these data are circumstantial and do not necessarily establish a cause-and-effect relationship between specific inflammatory cell types and altered airway function. Most of the literature is either descriptive or involvesdepletion (with or without repletion) of specific inflammatory cell types. The reports found in this issue of the journal fall into these categories. The report by Pauwels and colleagues, though essentially descriptive, is able to address directly the role of inflammatory cells in altered airway responsiveness byexploiting differences between rat strains in their response to aerosol endotoxin. Characterizing the cellular constituents of BAL fluid does not necessarily yield useful information about the location, type, or severity of inflammatory cell infiltration within the lungs. Pauwels and coworkers (1) have attempted, by measuring elastase activity in the lavage supernatant, to differentiate between the mere presence of neutrophils within BAL fluid and whether or not the cells are activated. The investigators did not observe a temporal relationship between either BAL fluid neutrophil counts or supernatant elastase activity and increased airway responsiveness to aerosol or intravenous S-hydroxytryptamine (SHT) after aerosol endotoxin exposure. The inclusion of data on both aerosol and intravenous SHT also allows the investigators to conclude (1) that increased airway responsiveness does not result solely from increased airway epithelial permeability and (2) that the increased mucus secretion associated with aerosol endotoxin does not prevent the bronchoconstriction caused by SHT. If it were possible to deplete only one inflammatory cell type or to inhibit specifically the actions of only one cell line in vivo, it would be relatively easy to address the importance of the cells in altered airway function. Such experiments are more easily conceived than done. Hutson and coworkers (2)supply detailed information on the effects of their rabbit anti-guinea pig neutrophil serum on a variety of inflammatory cell types both 531
532
at baselineand after ovalbumin challenge in their sensitized guinea pigs. In studies in which effects are noted with depletion, one can ask if the method of cell depletion, rather than depletion itself, caused the observed changes. Such issues have been raised with studies using cytotoxic drugs such as hydroxyurea, nitrogen mustard, or cyclophosphamide (3). In the case of the negative results reported in the study by Hutson and coworkers (2), a different potential pitfall exists. Were the guinea pigs sufficiently depleted of neutrophils to observe an effect? What is the nature of the neutrophils that are resistant to the rabbit anti-guinea pig neutrophil serum and are these cells responsible for the late changes in lung mechanics caused by allergen? The results reported by Hutson and associates (2) in guinea pigs differ from those reported by other investigators in rabbits and rats. The investigators appropriately discuss possible explanations for these differences, including interspecies differences and differences in the experimental models and methods. Neutrophils may not be important in the control of airway function in guinea pigs. Data similar to those reported by Hutson and coworkers have been published for guinea pigs in models using Ascaris suum, cigarette smoke, ozone, and toluene diisocyanate. Methodologic differences may also be important. For example, the late changes in lung mechanics observed in the study by Hutson and colleagues are very late (17 h) compared to those reported in other models and humans. The plethysmographic methods used by the investigators to measure lung mechanics do not clearly differentiate between alterations in the upper and lower airways. The resultsof the study by Pauwelsand colleagues and the study by Hutson and associates argue against neutrophils being important in the control of airway function. Although no one would want to base the care of asthmatic patients solely upon experimental findings in small animals, it is the job, in part, of an editorial to try to determine the relevance of animal research to humans. What kind of conclusions can be drawn from these two studies that may prove relevant to the human disease of asthma? Should one, based on these observations, assume that inflammatory cells are not important in the control of airway function and in the disease of asthma? This may be a valid conclusion. Still, considering the complexity of the inflam-
EDITORIAL
matory response, more discrete information on a cellular and molecular levelmay yet yield positive results. The two studies do not specifically address the possible role of inflammatory cells other than neutrophils in the control of airway function. Also, nonspecific airwayresponsiveness and allergen-induced alterations in lung mechanics are different airway responses. There is no reason to assume that inflammatory cells would have the same or similar effects on all aspects of airway function. The studies raise even more basic questions related to the application of results in experimental animals to the disease of asthma. Asthma is not a disease of either rats or guinea pigs. Asthma is also probably not a specificdisease, but rather a syndrome involving a heterogeneous group of precipitating mechanisms that result in the common clinical presentation of reversible airway obstruction. Most drugs in current use treat the endpoint airway obstruction rather than block the action or production of specific mediators that may be involved in its pathogenesis. Asthma even lacks a completely acceptable definition. The original definition of the American Thoracic Society included increased, nonspecific airway responsivenesss and the reversible nature of the observed airway obstruction. It did not include any mention of endotoxin, allergic mechanisms, or late asthmatic responses. Are alterations in nonspecific airway responsivenessessential to any definition of asthma? Increased airway responsiveness is observed in subjects who have no history or symptoms of asthma, and some subjects with the clinical diagnosis of and/or symptoms compatible with asthma do not demonstrate increased responsiveness to nonallergic bronchial provocation tests. Animal studies, such as that by Pauwels and coworkers, examining mechanisms responsible for alteration in airway responsiveness probably are important. If one accepts that asthma is a syndrome rather than a disease, then altered airway responsiveness may help to define a subgroup of asthmatics sharing a common mechanism or mechanisms. Common mechanisms may be found that are responsible for increased airwayresponsivenessin both animal models and humans. The stimulus used to increase airway responsiveness in the study by Pauwels and colleagues was aerosolized endotoxin. Endotoxin can increase airwayresponsiveness in a variety of animals, includ-
ing asthmatic humans (4). Although it has been argued that endotoxin may be important in the pathogenesis of lung diseases such as byssinosis,it is unlikely that endotoxin is central to the pathogenesis of all types of airway dysfunction observed in humans; rather endotoxin serves as a useful model to study the airways' response to injury. How important are allergic mechanisms and, in particular, the late asthmatic response? Recent data from humans, measuring serum IgE levels, argue for allergic mechanisms being more centrally involvedin asthma than waspreviously thought (5). Only a subgroup of the total asthmatic population, on the other hand, have late asthmatic responses. Still, an understanding of the mechanisms responsible for the late asthmatic response may yield information generally applicable to asthma. A variety of pharmacologic agents, such as 5-lipoxygenaseinhibitors and sulfidopeptide leukotriene and platelet-activating factor receptor antagonists, are now in early clinical trials on humans. Experimental evidencesuggeststhat these agents may affect the late changes in lung mechanics observed after allergen challenge in animal models. Research such as that of Hutson and coworkers may ultimately help guide the intelligent use of new agents if and when they ultimately become available for general clinical use in humans. In summary, the studies by Pauwels and colleagues (1) and Hutson and coworkers (2) argue against a simple relationship between the accumulation of neutrophils within the lungs and altered airway function. They also highlight the problems inherent in trying to draw general conclusions from animals that do not develop clinical asthma and applying these results to humans. More detailed information is needed before wecan fully understand the role of inflammatory cells in the control of airway function. These studies also underscore the problem inherent in treating asthma as a distinct disease tather than as a syndrome. If we treat asthma as a syndrome, wecan then include potentially diverse studies such as these in one conceptual package. The mechanisms responsible for endotoxin-induced alterations in airway responsiveness and for late changes in lung mechanics observed after allergen challange may all prove important if asthma is, in fact, a syndrome in which a number of mechanisms cause a common response.
I
533
EDITORIAL
JAMES R. SNAPPER M.D. The Center for Lung Research Department of Medicine Vanderbilt University School of Medicine Nashville, TN
References 1. Pauwels RA, Kips JC, Peleman RA, Van Der Straeten ME. The effect of endotoxin inhalation on airway responsiveness and cellular influx in rats. Am Rev Respir Dis 1990; 141:540-5. 2. Hutson PA, VarleyJO, Sanjar S, Kings M, Holgate ST, Church MK. Evidence that neutrophils do not participate in the late-phase airways response provoked by ovalbumin inhalation in conscious, sensitized guinea pigs, Am Rev Respir Dis 1990; 141:535-9.
3. Winn R, Maunder R, Chi E, Harlan J. Neutrophil depletion does not prevent lung edema after endotoxin in goats. J Appl Physiol1987; 62:116-21. 4. Michel 0, Duchateau J, Sergysels R. Effect of inhaled endotoxin on bronchial reactivity in asthmatic and normal subjects. J Appl Physiol1989; 66:1059-64. 5. Burrows B, Martinez FD, Halonen M Barbee RA, Clines MG. Association of asthma 'with serum IgE levels and skin-test reactivity to allergens. N Engl J Med 1989; 320:271-7.
he hypothesis that inflammatory cells versus guinea pigs) and different models activated and sequestered within the (aerosol endotoxin versus allergen challungs could modulate airway function lenge) and addressed completely different has been the subject of numerous publi- aspects of airway function (nonspecific cations and symposia. Contained with- airway responsiveness versus allergenin the lungs are inflammatory cells, in- induced late changes in lung mechanics). cluding neutrophils, the various subtypes This editorial will not rigorously critique of lymphocytes, eosinophils, mast cells, the two articles but rather, it will discuss macrophages (alveolar, interstitial, and the apparent disassembling of the arguintravascular), plasma cells, and mono- ment that neutrophils control airway cytes, as well as a variety of cells (such function. It will also attempt to develop as platelets) not generally regarded as in- a conceptual understanding of asthma flammatory that can be intimately in- that may allow us to glean meaningful volved in the inflammatory response. An information from apparently dissimilar apriori assumption has been that the in- animal experiments and to apply these flammatory response was a required step data with the hope of increasing our unin the repair of tissue damage and that derstanding of asthma. the process was inherently beneficial. It Pauwels and coworkers (1)take a most is only recently that we have begun to in- interesting approach to addressing the terpret the process itself as potentially role of neutrophils in altered airway reharmful, with the inflammatory cells sponsiveness. Their data argue against a themselves, either directly or indirectly, role for inflammatory cells in the alteracapable of causing lung injury. Inflam- tions in airway responsivenessnoticed afmatory cells such as neutrophils and eo- ter exposure to aerosol endotoxin in rats sinophils are veritable factories capable in two ways: (1) there was no temporal of producing a wide range of mediators, relationship between endotoxin-induced cytokines, proteinases, and toxic oxidants increases in airway responsivenessand the and interacting with the host of other cell type or severity of inflammation as astypes contained within the lungs. Inflam- sessed by measuring the cellular compomatory cellsmay contribute to functional sition of bronchoalveolar lavage (BAL) abnormalities such as altered airway re- fluid, and (2) although aerosol endotoxsponsiveness and lung mechanics. Al- in caused similar neutrophil influx into though we lack a complete understand- BAL fluid in all three inbred rat strains ing of the specific pharmacologic actions studied, it only increased airway responof steroids, the success of these agents sivenessin two of the three strains of rats. Hutson and coworkers(2)examinedthe in the treatment of asthma (including the increased use of aerosolized steroids in role of neutrophils in inhaled, allergenoutpatient maintenance therapy) has induced, early and late changes in lung been used to support a link between in- mechanics in ovalbumin-sensitized guinea pigs. The investigators had previously flammation and asthma. Two articles appear in the current is- noted a temporal relationship between sue of the AMERICAN REVIEW OF RESPI- neutrophil accumulation in BAL fluid and the late changes in lung mechanics RAlORY DISEASE that address the role of inflammatory cells in alterations in air- caused by inhaled ovalbumin. Neutrophil way function. These articles are "The Ef- depletion with rabbit anti-guinea pig fect of Endotoxin Inhalation on Airway neutrophil serum did not affect either the Responsiveness and Cellular Influx in early or late changes in lung mechanics Rats" by Pauwels and colleagues (1)and caused by allergen. The investigators used "Evidence That Neutrophils Do Not Par- these data to arguethat pulmonary neutroticipate in the Late-Phase Airways Re- phils are not, despite their coincident apsponse Provoked by Ovalbumin Inhala- pearance in BAL fluid, causally involved tion in Conscious, Sensitized Guinea in the late changes in lung mechanics. Pigs" by Hutson and associates (2). AlBoth reports summarize relevant literthough both articles address the role of ature arguing that leukocytes can moduinflammation in altered airway function, late, in the case of the study by Pauwels they were done in different species (rats and colleagues, airway responsiveness AM REV RESPIR DIS 1990; 141:531-533
and, for the study by Hutson and associates, allergen-induced changes in lung mechanics. As pointed out by Hutson and coworkers (2), much of these data are circumstantial and do not necessarily establish a cause-and-effect relationship between specific inflammatory cell types and altered airway function. Most of the literature is either descriptive or involvesdepletion (with or without repletion) of specific inflammatory cell types. The reports found in this issue of the journal fall into these categories. The report by Pauwels and colleagues, though essentially descriptive, is able to address directly the role of inflammatory cells in altered airway responsiveness byexploiting differences between rat strains in their response to aerosol endotoxin. Characterizing the cellular constituents of BAL fluid does not necessarily yield useful information about the location, type, or severity of inflammatory cell infiltration within the lungs. Pauwels and coworkers (1) have attempted, by measuring elastase activity in the lavage supernatant, to differentiate between the mere presence of neutrophils within BAL fluid and whether or not the cells are activated. The investigators did not observe a temporal relationship between either BAL fluid neutrophil counts or supernatant elastase activity and increased airway responsiveness to aerosol or intravenous S-hydroxytryptamine (SHT) after aerosol endotoxin exposure. The inclusion of data on both aerosol and intravenous SHT also allows the investigators to conclude (1) that increased airway responsiveness does not result solely from increased airway epithelial permeability and (2) that the increased mucus secretion associated with aerosol endotoxin does not prevent the bronchoconstriction caused by SHT. If it were possible to deplete only one inflammatory cell type or to inhibit specifically the actions of only one cell line in vivo, it would be relatively easy to address the importance of the cells in altered airway function. Such experiments are more easily conceived than done. Hutson and coworkers (2)supply detailed information on the effects of their rabbit anti-guinea pig neutrophil serum on a variety of inflammatory cell types both 531
532
at baselineand after ovalbumin challenge in their sensitized guinea pigs. In studies in which effects are noted with depletion, one can ask if the method of cell depletion, rather than depletion itself, caused the observed changes. Such issues have been raised with studies using cytotoxic drugs such as hydroxyurea, nitrogen mustard, or cyclophosphamide (3). In the case of the negative results reported in the study by Hutson and coworkers (2), a different potential pitfall exists. Were the guinea pigs sufficiently depleted of neutrophils to observe an effect? What is the nature of the neutrophils that are resistant to the rabbit anti-guinea pig neutrophil serum and are these cells responsible for the late changes in lung mechanics caused by allergen? The results reported by Hutson and associates (2) in guinea pigs differ from those reported by other investigators in rabbits and rats. The investigators appropriately discuss possible explanations for these differences, including interspecies differences and differences in the experimental models and methods. Neutrophils may not be important in the control of airway function in guinea pigs. Data similar to those reported by Hutson and coworkers have been published for guinea pigs in models using Ascaris suum, cigarette smoke, ozone, and toluene diisocyanate. Methodologic differences may also be important. For example, the late changes in lung mechanics observed in the study by Hutson and colleagues are very late (17 h) compared to those reported in other models and humans. The plethysmographic methods used by the investigators to measure lung mechanics do not clearly differentiate between alterations in the upper and lower airways. The resultsof the study by Pauwelsand colleagues and the study by Hutson and associates argue against neutrophils being important in the control of airway function. Although no one would want to base the care of asthmatic patients solely upon experimental findings in small animals, it is the job, in part, of an editorial to try to determine the relevance of animal research to humans. What kind of conclusions can be drawn from these two studies that may prove relevant to the human disease of asthma? Should one, based on these observations, assume that inflammatory cells are not important in the control of airway function and in the disease of asthma? This may be a valid conclusion. Still, considering the complexity of the inflam-
EDITORIAL
matory response, more discrete information on a cellular and molecular levelmay yet yield positive results. The two studies do not specifically address the possible role of inflammatory cells other than neutrophils in the control of airway function. Also, nonspecific airwayresponsiveness and allergen-induced alterations in lung mechanics are different airway responses. There is no reason to assume that inflammatory cells would have the same or similar effects on all aspects of airway function. The studies raise even more basic questions related to the application of results in experimental animals to the disease of asthma. Asthma is not a disease of either rats or guinea pigs. Asthma is also probably not a specificdisease, but rather a syndrome involving a heterogeneous group of precipitating mechanisms that result in the common clinical presentation of reversible airway obstruction. Most drugs in current use treat the endpoint airway obstruction rather than block the action or production of specific mediators that may be involved in its pathogenesis. Asthma even lacks a completely acceptable definition. The original definition of the American Thoracic Society included increased, nonspecific airway responsivenesss and the reversible nature of the observed airway obstruction. It did not include any mention of endotoxin, allergic mechanisms, or late asthmatic responses. Are alterations in nonspecific airway responsivenessessential to any definition of asthma? Increased airway responsiveness is observed in subjects who have no history or symptoms of asthma, and some subjects with the clinical diagnosis of and/or symptoms compatible with asthma do not demonstrate increased responsiveness to nonallergic bronchial provocation tests. Animal studies, such as that by Pauwels and coworkers, examining mechanisms responsible for alteration in airway responsiveness probably are important. If one accepts that asthma is a syndrome rather than a disease, then altered airway responsiveness may help to define a subgroup of asthmatics sharing a common mechanism or mechanisms. Common mechanisms may be found that are responsible for increased airwayresponsivenessin both animal models and humans. The stimulus used to increase airway responsiveness in the study by Pauwels and colleagues was aerosolized endotoxin. Endotoxin can increase airwayresponsiveness in a variety of animals, includ-
ing asthmatic humans (4). Although it has been argued that endotoxin may be important in the pathogenesis of lung diseases such as byssinosis,it is unlikely that endotoxin is central to the pathogenesis of all types of airway dysfunction observed in humans; rather endotoxin serves as a useful model to study the airways' response to injury. How important are allergic mechanisms and, in particular, the late asthmatic response? Recent data from humans, measuring serum IgE levels, argue for allergic mechanisms being more centrally involvedin asthma than waspreviously thought (5). Only a subgroup of the total asthmatic population, on the other hand, have late asthmatic responses. Still, an understanding of the mechanisms responsible for the late asthmatic response may yield information generally applicable to asthma. A variety of pharmacologic agents, such as 5-lipoxygenaseinhibitors and sulfidopeptide leukotriene and platelet-activating factor receptor antagonists, are now in early clinical trials on humans. Experimental evidencesuggeststhat these agents may affect the late changes in lung mechanics observed after allergen challenge in animal models. Research such as that of Hutson and coworkers may ultimately help guide the intelligent use of new agents if and when they ultimately become available for general clinical use in humans. In summary, the studies by Pauwels and colleagues (1) and Hutson and coworkers (2) argue against a simple relationship between the accumulation of neutrophils within the lungs and altered airway function. They also highlight the problems inherent in trying to draw general conclusions from animals that do not develop clinical asthma and applying these results to humans. More detailed information is needed before wecan fully understand the role of inflammatory cells in the control of airway function. These studies also underscore the problem inherent in treating asthma as a distinct disease tather than as a syndrome. If we treat asthma as a syndrome, wecan then include potentially diverse studies such as these in one conceptual package. The mechanisms responsible for endotoxin-induced alterations in airway responsiveness and for late changes in lung mechanics observed after allergen challange may all prove important if asthma is, in fact, a syndrome in which a number of mechanisms cause a common response.
I
533
EDITORIAL
JAMES R. SNAPPER M.D. The Center for Lung Research Department of Medicine Vanderbilt University School of Medicine Nashville, TN
References 1. Pauwels RA, Kips JC, Peleman RA, Van Der Straeten ME. The effect of endotoxin inhalation on airway responsiveness and cellular influx in rats. Am Rev Respir Dis 1990; 141:540-5. 2. Hutson PA, VarleyJO, Sanjar S, Kings M, Holgate ST, Church MK. Evidence that neutrophils do not participate in the late-phase airways response provoked by ovalbumin inhalation in conscious, sensitized guinea pigs, Am Rev Respir Dis 1990; 141:535-9.
3. Winn R, Maunder R, Chi E, Harlan J. Neutrophil depletion does not prevent lung edema after endotoxin in goats. J Appl Physiol1987; 62:116-21. 4. Michel 0, Duchateau J, Sergysels R. Effect of inhaled endotoxin on bronchial reactivity in asthmatic and normal subjects. J Appl Physiol1989; 66:1059-64. 5. Burrows B, Martinez FD, Halonen M Barbee RA, Clines MG. Association of asthma 'with serum IgE levels and skin-test reactivity to allergens. N Engl J Med 1989; 320:271-7.
