The Roles of Inflammatory Cells in the Pathogenesis of Asthma and of Chronic Obstructive Pulmonary Disease':' C. J. CORRIGAN and A. B. KAY Definition of Asthma and COPO The definition of both asthma and chronic obstructive pulmonary disease (COPD) remain entirely functional, and this has hindered studies of pathogenesis, particularly because many of the abnormalities in the two diseases demonstrate a degree of overlap. Asthma is generally defined as reversible airwayobstruction (1) with nonspecific bronchial hyperresponsiveness (BHR) as an additional important feature. BHR appears to correlate with disease severity (2) and the need for therapy (3). COPD, on the other hand, can be defined in terms of irreversible obstruction of the small airways, although irreversible airway obstruction also can complicate chronic asthma (4). Mucus hypersecretion is a feature of COPD in some patients. Nonspecific BHR has, in addition, been demonstrated in patients with COPD (5, 6) although this may simply reflect the mechanical effect of reduced baseline airway caliber (6). The Site of the Lesion in Asthma and in COPO In order to relate inflammation to functional changes, it is first necessary to define precisely the site of the lesion responsible for the changes observed. In asthma many, if not all, airways may be involved, ranging from the glottis (7) to the peripheral airways (8). In severe asthma, complete closure of some airways may occur, resulting in an increase in residual volume. In addition, there may be a marked degree of hyperinflation caused by loss of lung elastic recoil. How these changes are related to bronchial inflammation is unclear. In COPD, the precise relationship between functional abnormalities of excessive sputum production, obstruction of the small airways, and emphysema is ill defined. It would seem that the pathologic changes observed in the large airways in chronic bronchitis are not related to significant airflow obstruction (9). In patients with COPD dying with moderate to severe airway obstruction, inflammation of the peripheral airways appeared to be less important than emphysema as a cause of the airflow obstruction (10, 11). The effects of small airways inflammation were more prominent in milder disease where the elastic recoil properties of the lungs were relatively preserved (12-15). Nevertheless, some patients with fatal COPD were found to have little or no emphysema at autopsy (16). The uncertainty of the relationship between these two pathologic processes makes investigation of the role of inflammation in COPD pathogenesis very difficult.
AM REV RESPIR DIS 1991; 143:1165-1168
Pathologic Features of Asthma and COPD The pathologic features of asthma and COPD show many similarities, including desquamation of the epithelium, goblet and squamous cell metaplasia, an increase in the number of mucous glands, and hypertrophy of the bronchial smooth muscle (17), suggestingthat these changes are a nonspecific accompaniment of the inflammatory process. In addition, both diseases are characterized by exudation of fluid and inflammatory cells into the airway lumen. The principal difference between the inflammatory responses in asthma and those in COPD is not only the site of the inflammation (as discussed previously) but also the nature of the inflammatory cells. The inflammatory cell infiltrate in asthma is characterized by a predominance of eosinophils and mononuclear cells (17). In COPD, neutrophils and tissue macrophages predominate (10, 18). In asthma, most of this information has been derived from autopsy studies of asthma deaths. These observations have not shed light on the temporal course of the inflammatory changes in asthma and do not allow a comparison of the extent of inflammation with disease severity. Furthermore, no information is yet available as to whether inflammatory changes are uniform both within and between the various "forms" of asthma. In COPD, much information has been obtained from resected surgical specimens where anatomic changes can be related accurately to lung function. It has been possible, therefore, to obtain evidence that the extent of the inflammatory cell infiltrate in the small airways increases with the degree of functional impairment, at least in mild to moderate disease (12-15). The relationship between inflammatory cells and the genesis of emphysema remains unclear. The Roles of Inflammatory Cells in Asthma and in COPO In considering the possible roles of inflammatory cells in the pathogenesis of asthma and COPD, a firm distinction must be made between cellular changes associated with the true pathologic disorders of the disease and those that are simply a consequence of the effects of exacerbating factors. In the case of asthma, many "trigger factors" have been identified (table 1). Some, such as allergens, occupational agents, and aspirin, are specific in the sense that they exacerbate symptoms only in susceptible asthmatic persons, whereas others such as dust and cold air are nonspecific. Although all these factors clearly exacerbate asthma, it is less clear whether any
of them can be implicated in its cause. Thus, only a proportion of sensitized atopic subjects develop symptoms and objective features of asthma on exposure to specific allergens, suggesting that innate factors specific to the host rather than to the effects of allergen exposure per se are more relevant to the development of this disease. This distinction is an important one because many studies implicating inflammatory cells in the pathogenesis of asthma and COPD are based on the effects of such trigger factors on these cells. In COPD, cigarette smoke is a clear trigger factor (19), whereas the roles of occupational exposure to dust and air pollution are more controversial (20). Nevertheless, only a proportion of heavy smokers develop mucous hypersecretion, and a minority, perhaps only 10 to 15070, develop chronic airflow obstruction (19). Again, innate host susceptibility is clearly relevant. The evidence for implicating inflammatory cellsin the pathogenesis of asthma and COPD has been derived from studies of total cell numbers, their activation status, and the release of putative proinflammatory mediators. Patients with disease have been compared with normal control subjects, and measurements also have been related to disease severity. Cells are often obtained by bronchoalveolar lavage (BAL), although it is not entirely certain that this procedure samples the relevant inflammatory sites. The numbers of eosinophils and the concentrations of the eosinophil major basic protein wereincreased in the BAL fluid of asthmatics when compared with those in normal control subjects (21,22). Elevated numbers of eosinophils were observed in the BAL fluid of atopic asthmatic subjects who developed a late-phase fall in FEV 1 after allergen bronchial challenge (23, 24). Eosinophils generate appreciable quantities of leukotriene C 4/D 4 and plateletactivating factor, and these together with granule-derived proteins and superoxide ions may, at least in part, cause many of the pathophysiologic changes observed in asthma (25). Increased numbers of neutrophils in BAL fluid wereobserved during the late-phase fall in FEV 1 after specific challenge of both
I From the Department of Allergy and Clinical Immunology, National Heart & Lung Institute, London, United Kingdom. 2 Correspondence and requests for reprints should be addressed to A. B. Kay, Professor and Director, Department of Allergy and Clinical Immunology, National Heart & Lung Institute, Dovehouse Street, London SW3 6LY, UK.
CORRIGAN AND KAY
TABLE 1 COMPARISON OF THE PRINCIPAL PATHOLOGIC FEATURES OF ASTHMA AND COPO Feature
Site of inflammation
Entire bronchial tree but not alveoli; relative importance of various sites uncertain
Small airways and alveoli
Infiltrating inflammatory cells
Predominance of eosinophils, monocytes, T lymphocytes; mast cells and neutrophils also implicated
Predominance of neutrophils and monocytes
Immunologically specific: allergens, occupational agents, aspirin. Nonspecific: dust, fog, cold air
Immunologically nonspecific: cigarette smoke, (?) dust, and air pollution
Proinflammatory substances implicated in disease pathogenesis
Leukotrienes, PAF-acether, prostaglandins, histamine, eosinophil basic proteins, neutrophil and monocyte granule proteases Lymphokines
Leukotrienes Superoxide ions Neutrophil and monocyte granule proteases
Evidence for involvement of specific immunologic mechanisms
atopic and occupational asthmatics (26, 27). Activation of peripheral blood neutrophils also was found in ongoing asthma and after allergen bronchial challenge and exerciseinduced asthma (28-30). Neutrophils also have the propensity to elaborate lipid mediators, particularly leukotriene B4 , as well as superoxide ions and a number of granuleassociated proteases, all of which could, at least in theory, contribute to the pathophysiologic features of asthma. Mast cell numbers were increased in the BAL fluid of asthmatics when compared with those in normal control subjects (21, 22), and the numbers could be correlated with the degree of BHR (21). These cells release histamine and elaborate PGD 2 and LTB4 after activation by crosslinking of surface-bound 19B.Mast cells from asthmatics showed elevated releasability when compared with those from normal control subjects (22). Alveolar macrophages isolated from asthmatic subjects showed enhanced expression of the low affinity IgE receptors, enhanced lipid mediator release, and enhanced exocytosis of granule-derived proteases in response to specific allergen and anti-IgE challenge as compared with those from normal control subjects (31-33). In addition to their role in antigen presentation, these cells have the capacity to release lipid mediators, including PAF-acether and leukotrienes, and also may be activated because of binding of antigen-specific IgE through low-affinity receptors (34). There therefore exists circumstantial evidence that a wide range of granulocytic leukocytes as well as monocytes may be implicated in the pathogenesis of asthma, although the precise roles of these cells are ill defined. Evidence for a Role for Sensitized T Lymphocytes in Asthma Sensitized T lymphocytes are unique among
inflammatory cells in the sense that they are the only cells that can directly recognize antigens through specific receptors and thereby initiate an inflammatory response with subsequent release of cytokines. In the case of asthma, this process could occur irrespective of the presence or absence of antigen-specific IgE, thus providing a basis for unifying the pathogenesis of atopic and nonatopic asthma. Activated lymphocytes were indeed observed within the bronchial epithelium in asthmatic patients (35). These cells also have the capacity to bring about selective accumulation of inflammatory cells. This might be achieved in at least four ways: (1) by selective stimulation of granulocyte precursors; (2) by prolongation of the survival of cells (e.g., eosinophils) in tissues; (3) by direct chemotaxis of neutrophils and eosinophils; and (4) by priming of inflammatory cells in the tissues so that their response to activating agents (such as IgE-dependent activation) is enhanced. For example, in the case of the eosinophil, activated T lymphocytes are a major source of the lymphokine interleukin-5, which was shown specifically to enhance eosinophil survival and activate mature eosinophils (36, 37). Interleukin-5 was reported to be chemotactic for eosinophils although this effect was weak (38). Activated T lymphocytes were demonstrated in the peripheral blood of patients with status asthmaticus but not in those with milder asthma or in patients with COPD (39). The activated cells were solely of the CD4 phenotype. The percentages of activated cells decreased after therapy and clinical improvement. Similarly, elevated concentrations of interferon-gamma and soluble interleukin-2 receptor were found in the serum of patients with status asthmaticus but not in those with mild asthma or COPD or in normal control subjects (40). Changes in concentrations of
interleukin-2 receptor and numbers of activated CD4 + T cells could be correlated with clinical improvement after therapy. These studies emphasize the fact that different profiles of inflammatory mediators in asthma may be associated with disease of varying severity. In studies employing allergen bronchial challenge, a selective increase in CD4+ T cells in BAL fluid was observed 48 h after challenge in subjects who had previously been shown to develop a late-phase fall in FEV 1 (41). These findings complement those of a fall in the numbers of peripheral blood CD4 + T cells after allergen inhalation (42), and together they suggest that a process of selective recruitment of CD4 + T cells in the lung occurs in association with the late-phase asthmatic reaction. In contrast, an expansion of the CD8 + T cell population in BAL fluid was observed in patients with an isolated early response to allergen bronchial challenge (43). In COPD, most observations relating to inflammatory cells have concerned neutrophils and macrophages. As discussed previously, the relevance of effects observed in cigarette smokers who do not have COPD must remain open to question. Increased numbers ofneutrophils were observed in the BAL fluid of asymptomatic smokers and those with chronic cough and sputum production (44). The amounts of superoxide ion generated in response to a phorbol myristate acetate stimulus by peripheral blood neutrophils were shown to correlate with the degree of nonspecific BHR in patients with COPD (45). Putative neutrophil membrane-derived mediators such as LTB4 were shown to be present in COPD sputum (46). Numbers of macrophages in BAL fluid were elevated in smokers compared with those in nonsmokers (47). BAL macrophagesfrom smokers and patients with COPD showed significantly elevated elastase release compared with those in normal control subjects (48). Increased concentrations of macrophage-derived elastase in BAL fluid were documented in smokers (49). Cathepsin-Belike cysteine proteinase activity was described in COPD sputum and localized histochemically to alveolar macrophages (50). A large body of data, based in part on the study of homozygous alpha-l-protease inhibitor deficiency and in part on in vitro and other experimental studies, has established the protease-antiprotease hypothesis for the pathogenesis of emphysema (51), so that release of elastase and collagenases by inflammatory cells may be relevant to the pathogenesis of this disease. Furthermore, collagen and elastin breakdown products were shown to be chemotactic for neutrophils and monocytes (52, 53). Despite these observations, there is no direct evidence implicating elastases in the pathogenesis of emphysema in cigarette smokers who develop this disease despite adequate concentrations of alpha-lantiproteinases. By analogy with asthma, inflammatory cell numbers and activation status might change in acute exacerbations of COPD, but little experimental information is available concerning this problem (54).
ROLES OF INFLAMMATORY CELLS IN PATHOGENESIS OF ASTHMA AND COPO
Despite these many demonstrations of the potential role of inflammatory cellsin asthma and COPD, it remains to be shown how far inflammatory cells and their products are involved in the genesis of the phenomenon of BHR in asthma, and how in COPD inflammatory cells can cause small airways obstruction and emphysema. The relationship between inflammatory infiltration and cell activation and disease severity in asthma and in COPD is ill defined. An increased understanding of the role of inflammatory cells in asthma and in COPD willrequire a better definition of the temporal course of the inflammatory process and an investigation of the properties of inflammatory cells directly at the relevant sitesof inflammation in these diseases. Only then will it be possible to assess the true contribution of these cells to the pathogenesis of asthma and COPD.
Summary and Conclusions Studies of the inflammatory processes in asthma and in COPD have been hindered by the imprecise definitions of these diseases, the uncertainty as to the location of the relevant inflammatory sites for these diseases within the lungs, and the difficulties in relating observed histopathologic changes to the effects of particular proinflammatory leukocytes or their products. Circumstantial evidence exists implicating neutrophils, eosinophils, mast cells, and monocytes in asthma pathogenesis based on increased numbers and activation of these cells in blood, BAL fluid, or biopsy material, and the propensity of these cells to releaseproducts causing tissue damage. There is increasing evidence that the accumulation and activation of these cells is orchestrated by lymphokines elaborated by sensitized T lymphocytes. In COPD, attention has been focused on the secretion of proteases by infiltrating neutrophils and macrophages, although there is no incontrovertible evidence that these are relevant to the disease pathogenesis in the majority of patients with normal serum concentrations of protease inhibitors. In the future, studies of the functions of inflammatory cells actually at the sites of inflammation using the techniques of immunocytochemistry and molecular biology may provide further information as to the role of these cells in the inflammatory processes associated with asthma and COPD. References 1. American Thoracic Society. Standards for the diagnosis and care of patients with chronic obstructive pulmonary disease and asthma. Am Rev Respir Dis 1987; 136:225-44. 2. Hargreave FE, Ryan G, Thomson NC, et al. Bronchial responsivenessto histamine or methacholine in asthma: measurement and clinical significance. J Allergy Clin Immunoll981; 68:347-55. 3. Juniper EF, Frith PA, Hargreave FE. Airway responsivenessto histamine and methacholine: relationship to minimum treatment to control symptoms of asthma. Thorax 1981; 36:575-9. 4. Brown PJ, Greville HW, Finucane KE. Asthma and irreversible airflow obstruction. Thorax 1984; 39:131-6.
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Comments Dr. Anderson: What challenge was used to measure BHR in the asthmatics when comparing with number of mast cells and responsiveness? Dr. Corrigan: This was methacholine challenge. The asthmatics had relatively mild symptoms and were not challenged with antigen. Dr. Lee: There are recent data indicating that mast cells can release cytokines. Clearly, this provides a potentially important mechanism for amplification of cellular recruitment and priming. The role of the mast cell in the mechanisms of asthma and bronchial hyperreactivity willneed to be reassessedin viewof these new findings.