Asthma

JAY A. NADEL and WILLIAM W. BUSSE

University of California, San Francisco, Cardiovascular Research Institute, San Francisco, California; and University of Wisconsin Medical School, University of Wisconsin Hospital and Clinics, Madison, Wisconsin

	INTRODUCTION

TOP INTRODUCTION CONCLUSION REFERENCES

In December 1971, the National Heart and Lung Institute prepared an analysis of current pulmonary research programs. Their report on asthma research stated: "[Asthma] is relatively common, accounts for a considerable amount of acute respiratory incapacity in the population and is incompletely understood from the point of view of pathogenesis." In their program, "several studies [were funded to] examine the associated physiologic changes in acute asthmatic attacks. In asthmatics with irritable airways, the possible interaction between directly inhaled antigenic stimuli [leading] to bronchospasm and the non-antigenic stimuli such as SO₂ will be studied.

"Basic physiologic studies in asthmatic patients between attacks seek to separate this group from patients with chronic bronchitis and emphysema on the basis of elastic recoil and diffusing capacity characteristics. However, other investigators have planned studies on the natural history of asthma in order to examine the possibility that patients with spasmodic asthma may ultimately develop a COPD-like disease.

"Investigations on lung innervation will contribute to a better understanding of bronchoconstriction and ultimately, rational pharmacologic therapy.

"Bronchopulmonary lavage, now applied as a therapeutic measure in some resistant asthmatic attacks, is being evaluated for more precise clinical indications for therapy and with attention to associated changes in mechanics, gas exchange and blood flow."

From this modest portfolio has emerged a comprehensive program, which includes studies on cellular and molecular mechanisms in asthma, population studies to define the features and risk factors in asthma, efforts to identify the genetic basis of asthma, programs to determine and define the influence of asthma therapy on childhood asthma, Childhood Asthma Management Program, Specialized Centers of Research on asthma, and Expert Panels to establish guidelines for the treatment of asthma. This diverse, multidisciplinary approach has provided support for improved and expanded insight into mechanisms of asthma. The diversity of these research efforts reflects the current concepts that asthma is a complex disease of the airway, and a number of different approaches will be necessary to define the mechanisms and pathogenesis of this disease.

Although a precise definition of asthma is still elusive, the major features of asthma were uniformly accepted: airway obstruction, bronchial hyperresponsiveness, and airway inflammation. The evolution of this current perception has been the result of research, development of new techniques, the discovery of important factors important in the regulation of inflammatory and immune responses, and the utilization of molecular and genetic techniques for study. To appreciate the evolution of hypotheses of asthma pathogenesis, it is important and helpful to review the history of asthma research over the past decades. The idea that asthma is an inflammatory disease of the airway is not necessarily new; however, the evidence to support this position is now based upon research with patients with asthma, isolated cells or cell lines, and animal models, including knockouts and transgenic animals. The historic events and key research observations that have moved us to view asthma as a chronic disease of the airways, in which special inflammatory events exist, are strongly linked to NIH-funded research. The following "story" identifies some of the important "events" that have directed us in the quest to elucidate the pathogenesis of asthma. Asthma has become a major health care issue, and with this marked increase in the number of patients with asthma has come a more urgent need to understand this disease.

	THE CHANGE IN ASTHMA PREVALENCE

The incidence of asthma has increased substantially in the last two decades. Although identification of who has asthma may differ now than in prior decades, current data indicate that asthma and wheezing illnesses have increased both in children and adults (1). Gergen and colleagues (2) found that the prevalence of asthma in 6- to 11-yr-olds was 4.8% in 1971-1974 in the United States. This was based upon a questionnaire response: "Has asthma ever been diagnosed?" From 1976 to 1980, the prevalence of asthma increased to 7.6% in a 3- to 17-yr-old cohort. Similar trends have emerged worldwide, with dramatic increases occurring in New Zealand and Australia. For example, the prevalence of asthma in Australia in 8- to 11-yr-olds in 1982 was 12.9% (3); but 1992 the prevalence in the same age bracket soared to 29.7% (4).

Epidemiologic studies have also given insight into the risk factors that may be associated with the increased incidence of asthma (1). The presence of allergy has emerged as the greatest risk factor for the development of asthma (see below), and data show that the frequency of asthma in a population parallels the degree of atopy(s) (e.g., positive immediate skin test responses). Moreover, the development of allergic reactions to certain environmental allergens, i.e., house dust mite, cat, Alternaria, and cockroach, has shown the highest association with asthma. These data provide convincing evidence that factors that determine allergic sensitization, and their reactions, are linked to the pathogenesis of asthma. Furthermore, these data suggest that possible increases in the prevalence of allergy in the community are an important risk factor in the pathogenesis of asthma. Thirty years ago this was not the view of researchers, and the importance of allergy in the pathogenesis of asthma represents an important change in the concept of asthma.

Epidemiology has identified the change in the "face" of asthma in society and suggested the mechanisms likely to be involved. With the discovery and understanding of immune and inflammatory events in the lungs, these observations can now be linked to the pathogenesis of asthma. This allows for studies of how "allergic" events can affect airway histology, physiology and pathology, and may ultimately elucidate the mechanisms underlying the persistent nature of this disease.

	THE DEFINITION OF ASTHMA: AN EVOLVING PROCESS

Working definitions of asthma emerged from two Ciba Foundation symposia in 1959 (6) and 1971 (7), which stated that asthma was predominantly a disease of airway smooth muscle, which was altered so that it contracted too easily and too much. In the NIH 1997 Guidelines for the Diagnosis and Management of Asthma, a working definition of asthma is given:

Asthma is a chronic inflammatory disorder of the airways in which many cells and cellular elements play a role, in particular, mast cells, eosinophils, T lymphocytes, macrophages, neutrophils, and epithelial cells. In susceptible individuals, this inflammation causes recurrent episodes of wheezing, breathlessness, chest tightness, and coughing, particularly at night or in the early morning. These episodes are usually associated with widespread but variable airflow obstruction that is often reversible either spontaneously or with treatment. The inflammation also causes an associated increase in the existing bronchial hyperresponsiveness to a variety of stimuli. (8)

Asthma research has provided the evidence to construct their much expanded, and likely more accurate, definition of asthma. Table 1 shows the expansion of the definition of asthma from the 1960s to 1997. 

	FACTORS IN THE DEVELOPMENT AND REGULATION OF AIRWAY INFLAMMATION

Bronchial Hyperresponsiveness

In the Ciba Foundation definition of asthma in 1959 (6), a principal defining feature of asthma was "widespread narrowing of the bronchial airways, which changes its severity over short periods of time, either spontaneously or under treatment." However, subjects with asthma were known for a long time to have extreme sensitivity of the airways, and the recognition of this abnormal response led the American Thoracic Society Committee on Diagnostic Standards to use bronchial hyperresponsiveness as a diagnostic criterion (9). Although many factors have been incriminated in causing bronchial hyperresponsiveness, including airway geometry and autonomic control of the smooth muscle, the principal focus is now directed toward those events that cause and regulate airway inflammation.

Allergy and Allergic Inflammation

Prior to 1966, the serum factor associated with allergic diseases and reactions was labeled "reagin." In 1966 Ishizaka and coworkers (10) reported that reaginic activity belonged to a new class of immunoglobulins: IgE. IgE molecules were found to have a strong affinity for tissue mast cells and basophils, and when specific allergens combine with IgE antibody on these target cells, a noncytotoxic release of a variety of potent chemical mediators was triggered. These observations identified IgE antibodies as the cause of immediate allergic reactions. Subsequent studies over the next 30 yr have provided information of the molecular regulation of IgE synthesis, the nature of the high- and low-affinity receptors for this immunoglobulin, the mechanisms by which allergen tissue-bound IgE signals sensitized cells to release bronchoactive mediators, and, finally, evidence as to how these events relate to asthma. Serum concentrations of IgE, independent of antigen specificity, also have been linked to the presence of asthma and are considered a risk factor for this disease (11).

The Role of Mast Cells, Mast Cell Mediators, and Asthma

With IgE antibody identified, it was possible to begin to analyze and understand the biochemistry of mediator release. Early studies used human lung fragments that were sensitized with IgE antibody and then challenged with antigen ragweed to define the regulation to mast cell activation and secretion, as well as the specific mast cell mediators and their pharmacology (12). For example, mediators identified with the mast cell included preformed compounds like histamine and eosinophil chemotactic factor of anaphylaxis (ECF-A) and others, such as slow-reacting substance of anaphylaxis (SRS-A), which was synthesized and released as a consequence of mast cell activation by antigen- and cell-bound IgE antibody (13, 14). The identification of other mediators soon followed as mast cell biology was integrated into the pathobiology of asthma. From the expanding array of mediators and their function, it was proposed that mast cell activation led to a humoral and cellular response phase (Figure 1). The humoral phase of the mast cell reaction was the consequence of acute mediator release and the action of these vasoactive factors on vascular permeability and smooth muscle tone. By contrast, putative mediators like ECF-A and neutrophil chemotactic factor were proposed to regulate the movement of cells to the site of the allergic response. Thus, the mast cell was felt to be involved in both the immediate phase response to antigen and a transition from this acute phase of the allergic reaction to a cellular inflammatory response. These observations marked the beginning of efforts to link acute phase reactions, i.e., bronchospasm, to the development of persistent changes in the airway such as inflammation.

View larger version (20K): [in this window] [in a new window]   Figure 1.   The role of mast cell mediators in allergic reactions.

The Discovery of the Cysteinyl Leukotrienes

By the early 1980s, the structure and synthesis of SRS-A was established (15). With this discovery came clarification of how lipid molecules were produced, the cell sources of these products, the pharmacology of the mediators in relationship to control of airway smooth muscle tone, and the potential importance of these compounds to asthma. This quest is an excellent example of translational research, with the culmination being new compounds for the treatment of asthma (cysteinyl leukotriene D₄ receptor antagonists and 5-lipoxygenase inhibitors). The importance of these studies is noted in the work of two major investigators in this area, E. J. Corey and B. Samuelsson, who received Nobel prizes in medicine for their work on eicosanoids.

Recognition of Allergic Inflammation: The Discovery of the Late-phase Allergic Reaction

A number of observations have expanded and extended the revelance of mast cell biology to asthma (18). Mast cell mediators, tryptase and chymase, have been shown to alter substantially bronchial muscle function and airway histology (19). Tryptase sensitizes bronchial smooth muscle to histamine and, as a consequence, promotes bronchial responsiveness. Tryptase also stimulates fibroblasts to produce fibrosis; chymase causes basement membrane injury (20).

Mast cells, like most other immune cells, are not only affected by cytokines, which regulate their growth and development, but are also likely sources of these molecules, including interleukin (IL)-1, IL-2, IL-3, IL-4, IL-5, and IL-10, granulocyte/macrophage colony-stimulating factor (GMCSF), transforming growth factor beta (TGF-), interferon  (IFN-), tumor necrosis factor alpha (TNF-), and a variety of chemokines. The identification of mast cell cytokines and recognition that some of their constituents can produce bronchial responsiveness paralleled a realization that allergic reaction may have more than an isolated immediate effect.

Mast cell-derived mediators also influence cell trafficking, particularly neutrophils and eosinophils (21). Interest that an inflammatory response may be associated with immediate reactions to allergens came from descriptions of the late-phase cutaneous reactions by Pepys and his coworkers (22). These clinical investigators found that an intradermal injection of Aspergillis antigen in patients with allergic bronchopulmonary aspergillosis caused an initial wheal and flare, which usually resolved, only to be followed by reaction 2 to 6 h later at the same site, which was characterized by diffuse erythema and edema. Initial interpretation of skin test site histology suggested this reaction to be the result of a type II, or Arthus, reaction. Dolovich and colleagues (23), however, were able to demonstrate that late cutaneous responses also followed the administration of a variety of antigens, including ragweed and a monospecific antiserum to IgE. Such observations suggested that certain late cutaneous responses were dependent primarily on IgE and do not require other immunoglobulins or complement for their development.

Solley and coworkers (24) extended these findings and demonstrated that a wide variety of allergens, i.e., ragweed, Timothy grass, guinea pig, and Alternaria, elicited cutaneous late-phase reactions (LPRs). IgE antibodies were found responsible, or at least involved, for the LPR, and the histology of the late-phase reactions was characterized by edema and a mixed cellular infiltrate; the inflammatory response was predominantly lymphocytic but also contained eosinophils, neutrophils, and basophils. These and other studies were an impetus to identify the factor(s) that contribute to and dictate the allergic inflammatory response. The recognition of the late phase and its link to allergic sensitization marked a transition from the study of bronchospasm (the immediate response) to the inflammatory reaction (the late phase). The next phase of investigation was to link these events to asthma.

	LATE-PHASE REACTIONS AND ASTHMA: EARLY EVIDENCE FOR INFLAMMATION IN ASTHMA

The recognition and integration of this LPR information in asthma research began a movement to define the role and contribution of inflammation to asthma. Airway responsiveness to a variety of mediators, including histamine and methacholine, was known to be part of asthma. Robertson and coworkers (25) demonstrated that late-phase reactions also occurred with inhaled antigens. Moreover, the late asthmatic responses to inhaled allergens were accompanied by an increase in bronchial responsiveness, a key feature of asthma. If the late response was particularly severe, it was followed by recurrent episodes of asthma at night (26). These clinical observations suggested that the effect of inhaled allergen elicits not only an immediate and a late response, but that antigen-associated events could also change features of the airways, causing asthma to become more severe. Thus, the concept and importance of acute allergic reactions to inhaled allergens changed; allergens could not only cause acute bronchospasm, but also could initiate a series of airway events that lead to enhanced bronchial hyperresponsiveness and increased asthma severity (Figure 2).

View larger version (11K): [in this window] [in a new window]   Figure 2.   The immediate and late-phase reactions to inhaled antigen and the consequence of these reactions on pulmonary physiology.

Initially, many factors were felt to play a role in the late-phase asthmatic reactions: inflammatory cells, complement, mediators, and the target organ, in the case of asthma-airway susceptibility (27). Although mast cells release histamine upon allergen activation, there was no evidence that this mediator, or others such as bradykinin or prostaglandin E ₁, were by themselves the cause of LPRs. Rather, cellular recruitment to the airway appeared as a key factor in this process. To identify these cellular factors, direct sampling of the airway in relationship to immediate and late-phase reactions was used and became instrumental to clarify the putative cells involved in the various phases of the allergic reaction. This marked an important technical milestone in asthma research: the use of fiberoptic bronchoscopy and lavage. Bronchoscopy allows the clinician to diagnose lung pathology by lavage and biopsy; these same tools were used by investigators to explore the airways in response to antigen by retrieving lavage fluid, cells, and tissue for analysis. This approach removed a major barrier of study and allowed direct access to the target organ of interestthe lung. No longer was it necessary to rely on surrogate markers.

The Cellular Features of Late-phase Reactions: Insight into the Inflammatory Processes in Asthma

Metzger and coworkers (28) were among the first investigators to evaluate asthma subjects with fiberoptic bronchoscopy and lavage during antigen challenge. They found an increase in airway cells beginning 4 h after antigen challenge. There was an initial increase in neutrophils, which was followed by the appearance of eosinophils. Mucosal biopsies showed mast cell degranulation, and airway eosinophils had "piecemeal" degranulation (29). T-helper lymphocytes appeared in the lungs later. These studies and others suggested that antigen interacts with cells and tissues of the airway to recruit cells to the lung. With this trafficking of cells to the lung, the airways narrow and bronchial responsiveness increases. The important cells recruited to the airway following antigen challenge included eosinophils and T cells.

The Role of Eosinophils in Asthma: Host Defense versus Inflammation

Eosinophils have always been associated with asthma. Peripheral blood and tissue eosinophilia are hallmark features of asthmaboth allergic and nonallergic asthma (30); and eosinophils are a histologic feature of severe asthma at autopsy (31). Less than 20 years ago, there was little information on the role of eosinophils in asthma, or other diseases for that matter. Initial perceptions about eosinophils in allergic diseases suggested that they inactivate mast cell mediators and downregulate ongoing secretion of mast cell mediators (32). However, a number of observations helped to define more precisely what eosinophils do in asthma.

Eosinophils kill parasites in vitro. From these findings it was speculated that the mechanisms that are deleterious to parasites could also be harmful to airway tissues (33). To explore this possibility, Gleich and coworkers (34) investigated the composition of eosinophil granules. Major basic protein (MBP), a principal granular protein, killed parasites and was toxic to respiratory tissue, e.g., MBP reduced ciliary activity and disrupted bronchial epithelium. Immunofluorescent stains of airway tissue in severe or fatal asthma found MBP localized to cells in the airway and extracellular regions of bronchial mucosa, including areas of damaged epithelium (35). Furthermore, MBP damage to epithelium occurred at concentrations found in sputum of patients with severe asthma. Thus, evidence emerged to demonstrate that eosinophils were not only an association with asthma but a likely contributor to airway injury.

Lymphocytes and Asthma: `The Captain' of the Inflammatory Process

In 1989 Mosmann and Coffman (36) described subpopulations of T helper (Th) cells in mice with distinct cytokine patterns. Th1 cells generated cytokines whose actions were important for host defense against fungus and virus, in transplantation, and as a component of immune surveillance. By contrast, cytokines derived from Th2 cells, i.e., IL-4 and IL-5, were instrumental to the development of allergic inflammation. IL-4, along with IL-13, is a key cytokine in regulating an isotype switch to IgE synthesis; IL-5 is involved in eosinophil-driven inflammation (37). In humans, there is also evidence that subpopulations of T-helper cells exist and that asthma may be a Th2-like disease (Figure 3).

View larger version (16K): [in this window] [in a new window]   Figure 3.   T-helper lymphocyte subpopulations and their contribution to lung immune responses.

With bronchoalveolar lavage (BAL) and endobronchial biopsies, the immunohistologic features of asthma have begun to be described as well as the search for the causative cells in this process. Mucosal biopsies showed airway epithelial damage even when asthma was mild (38, 39). Moreover, some subjects with asthma had subepithelial fibrosis early in the course of the disease (40); these findings suggested that airway injury in asthma may not be fully reversible. Principal cellular infiltrates in asthma included eosinophils and activated lymphocytes (41). Finally, the process of airway inflammation in asthma was variable and could be reduced by anti-inflammatory therapy (i.e., corticosteroids) or accentuated by allergen exposure (42). Thus, the patterns of inflammation initially found at postmortem examinations of subjects with severe asthma were also noted in milder versions of the disease and felt to represent the disease process, not simply an extreme event. Because T cells are long-lived and have "immune memory," their involvement in asthma was felt to be instrumental in the persistence of inflammation in asthma.

Another major step in understanding the regulation of inflammation in asthma has been the identification of cytokines, their cell source, and the immune responses they cause. Robinson and colleagues (43) obtained airway cells from normal subjects and patients with mild asthma by BAL; immunohistochemistry was used to identify gene products of various cytokines and their cell source. In asthma, more BAL cells stained positive for mRNA for IL-3, IL-4, IL-5, and GMCSF than in normal subjects, by contrast, IFN- expression was similar in normal and asthma populations. From these findings, the investigators concluded that atopic asthma is associated with predominant activation of Th2-like T-cell populations (44). This same approach has been applied to various asthma phenotypes, i.e., allergic versus intrinsic asthma, with less than definitive results. Moreover, intervention with anti-inflammatory therapy, principally inhaled corticosteroids, has reduced the airway eosinophilic infiltration and cells expressing Th2-like cytokines. Thus, it appears that regulation of lymphocytes by corticosteroids can repress genetic expression of pro-inflammatory cytokines and consequently the generation and development of airway injury.

Mucus Hypersecretion

As discussed above, the use of bronchoscopy, BAL and bronchial biopsy have enabled the study of cellular components involved in asthmatic airway responses. One important component of these responses has been an appreciated, but neglected, component of asthma until recently: mucous hypersecretion.

In 1960, Dunhill (45) reported that mucus obstruction of peripheral airways is an important pathologic finding in fatal asthma. Postmortem studies of patients who died of severe, acute attacks of asthma showed a marked increase in goblet cells and luminal mucus in peripheral airways. Studies such as these suggested that an acute trigger for goblet cell degranulation might be involved in acute asthma and asthmatic deaths (46). In a mouse model of allergic asthma, local delivery of antigen results in occlusion of the airways with mucus and leukocyte infiltration (47). The mechanisms underlying mucus secretion in airways are not completely understood, but recent efforts provided needed information (48). For example, many mediators with receptors in the airway epithelium have secretagogue activity, e.g., histamine, leukotrienes, and serotonin, but they are not very potent. In the past decade, Sommerhoff and colleagues (49) showed that purified neutrophil elastase is an extremely potent secretagogue in airway submucosal glands and goblet cells. These observations link the presence of neutrophils in the airway and one of their products to an important component of asthmamucus hypersecretion (50).

Cytokines, Chemokines, and Adhesion Proteins: Mechanisms for Directed Cell Migration

The discovery of cytokines has been a major advance in understanding the intricacies of the allergic inflammatory response in asthma. Cytokines are small secreted proteins or, more often, glycoproteins that have the capacity to influence growth, differentiation, and activation functions. These, in turn, regulate and determine the nature of immune responses, including inflammation and repair (51). However, it has also been recognized that the regulation by cytokines occurs not only by direct actions on cells, but also by feedback or inhibitory loops. Thus, the outcome of an immune reaction will be determined by many factors, including the particular cytokine, the environment by which the reaction takes place, the cells involved, and the ultimate network that emerges.

The application of cytokine biology to events that contribute to, or participate in, asthma has provided unique insights into a wide variety of processes, identified mechanisms involved in the development and regulation of inflammatory events, and led to potential new therapeutics. For example, IgE synthesis is influenced by a number of key cytokines: IL-4, IL-13, and IFN-. IL-4 was originally identified as a B-cell growth factor and, although previously derived from T-helper lymphocytes, can also be synthesized by mast cells, eosinophils, and basophils (52). IL-4 induces the immunoglobulin isotype switch from IgM to IgE by initiating the transcription of  germ line heavy-chain transcripts (53). In addition to its ability to regulate synthesis of IgE, IL-4 induces endothelial cell expression of vascular cell adhesion molecule (VCAM)-1, a primary adhesion protein for eosinophils, basophils, and mononuclear cells. By contrast, IFN- functions as a regulator of allergic expression through a variety of means, including its capacity to inhibit the IL-4 isotype switch to IgE.

A second family of inflammatory proteins includes the chemokines, which are chemotactic for eosinophils and found in the C-C chemokine family, including RANTES, MIP-1, MCP-3, and eotaxin. In contrast to other chemoattractants of granulocytic cells, these factors are selective for eosinophils. RANTES and eotaxin, when acting in synergy with IL-5, are apparently the most important eosinophil chemoattractants in the development of allergic inflammation.

Asthma is characterized by a localized accumulation of inflammatory cells in the airway, particularly eosinophils and activated lymphocytes (54). The processes that direct cell recruitment are critical to the orchestration of the inflammatory response and the selectivity of the cells in these reactions. Adhesion proteins are molecules that contribute to these selective recruitment responses and permit cell-cell and cell-substratum attachments. The discovery of adhesion molecules has been important to understand the steps in cell recruitment, leukocyte-endothelial interactions (margination), diapedesis (transendothelial migration), and in the lung, transepithelial migration (55).

Some of these adhesion molecules have already been shown to have potential importance to asthma. Intercellular adhesion molecule (ICAM)-1 is found on vascular endothelium and epithelium, and was the first of the adhesion molecules to demonstrate a direct relevance to the inflammatory process in asthma. To determine the contribution of ICAM-1 adhesion molecules to airway eosinophilia and bronchial hyperresponsiveness to antigen, Wegener and associates (56) treated Ascaris-sensitized monkeys with a monoclonal antibody to ICAM-1. In sensitized monkeys, an inhalation antigen challenge increased airway eosinophil and caused an enhancement of bronchial responsiveness. When sensitized monkeys were treated with a monoclonal antibody to ICAM-1, eosinophil recruitment to the airway and the enhanced bronchial responsiveness to antigen was diminished. These observations suggest that ICAM-1 adhesion with eosinophils was important to cell recruitment to the airway and the eventual development of bronchial inflammation and, hence, hyperresponsiveness. The discovery of the adhesion molecules has helped in understanding how cells get to sites of inflammation, how this process can be selective for certain cells, and how these molecules can be future targets of treatment.

The Role of Allergens in the Pathogenesis of Asthma

House dust mite allergen has proven to be an important factor in the prevalence of asthma, and the host response to this allergen serves as a model to identify the role and contribution of other airborne allergens to asthma. The evidence to support a role for, and importance of, allergens, particularly house dust mite, to asthma is convincing, strong, and universal. In a prospective study, Sporik and colleagues (57) showed that sensitization to house dust mite increased the odds ratio for subjects to develop asthma; moreover, where house dust mites do not exist, other allergens, i.e., animal dander and cockroach are associated with allergic sensitization and the development of asthma. There is also evidence that a dose-response relationship exists between exposure to mite allergens and sensitization, and eventually asthma (58). If dust mite-sensitized subjects with asthma live in an environment lacking this particular allergen, asthma severity decreases, as does the level of bronchial responsiveness. With reintroduction of allergen to the environment, there is a rapid return in symptoms and intensity of bronchial responsiveness. Therefore, allergens are risk factors for asthma development and expression and important provokers of the inflammatory process. Observations on the importance of environmental allergens as factors in the pathogenesis of asthma have led to strong recommendations that allergen sensitivity be identified and causative allergens be avoided as much as possible.

Respiratory Infections

Respiratory infections are the most common cause of asthma exacerbations. Years ago, bacterial infections were considered the cause of asthma exacerbations with respiratory tract illnesses. However, studies in the late 1960s and early 1970s used culture techniques to show that viral respiratory infections, particularly rhinovirus, influenza, and respiratory syncytial virus, were the causes of respiratory infections provoking asthma, not bacteria (59). With the use of polymerized chain reaction (PCR) technology, Johnston and coworkers (60) detected respiratory viruses in over 80% of asthma exacerbations; rhinovirus was the most frequent infectious cause of asthma exacerbation.

An important early observation by Empey and colleagues (61) showed that respiratory infections increased airway hyperresponsiveness; this observation raised the possibility that respiratory viruses affect airway responsiveness and accomplish this by causing or enhancing bronchial inflammation. Efforts have now focused on how respiratory viruses influence lower airway function and inflammation.

To investigate how rhinoviruses induce asthma, experimental infections with this virus have shown that they: (1) increase airway responsiveness; (2) promote the development of the late asthmatic response to inhaled antigen; (3) enhance the recruitment of eosinophils to the airway; and (4) promote mucosal airway inflammation (increased lymphocytes and eosinophils) during an acute infection (62). Although the mechanisms by which respiratory viruses provoke asthma have yet to be fully established, current evidence suggests that viruses stimulate cells to secrete a variety of cytokines that upregulate existing airway inflammation.

Another issue of importance and interest has been the influence of early life respiratory infections on the development of asthma. Respiratory syncytial viruses are the usual cause of wheezing in children less than 2 yr of age. In a prospective study, Martinez and associates (63) followed nearly 826 children from birth to 6 yr of age. When evaluated at 6 yr of age, a segment of these children were found to have wheezed early in life. In many, these episodes were transient and likely related to small airways and maternal smoke exposure. Others had either persistent wheezing or late-onset wheezing. At present, it is not clear what role respiratory viral infections play in either persistent or late-onset wheezing in children.

It has also been observed that some respiratory infections may be protective for the development of asthma. In families with a large number of children, the younger siblings are less likely to have wheezing. Furthermore, natural measles infections appear protective for asthma (64). Finally, Shirakawa and colleagues (65) found that the presence of a positive PPD response appeared to reduce the risk for allergic diseases and asthma. Collectively, these studies suggest that some respiratory infections may stimulate a Th1 cytokine profile, i.e., IFN-, and the increased presence of this cytokine shifts the balance away from allergic inflammation. This is an area that needs further exploration.

Late-Onset Asthma: A Continued Mystery

Allergy is an important factor for asthma in patients with the onset of asthma in their first few decades of life. However, adult-onset asthma also occurs in individuals with no allergic background. These observations have led to the theory that airway inflammation itself could reproduce the manifestations of asthma (66). Nearly 20 years ago, it was proposed that epithelial damage could play an important role in the pathogenesis of asthma, "possibly by causing effects on mast cells, leukocytes, macrophages or platelets" (67). Thus, the epithelium has moved from being considered a protective "coating" on the airways to a cell instrumental in causing and regulating airway inflammation. It is clear from the evolution of studies on airway inflammation that multiple cells and mediators, and thus multiple mechanisms, are involved in asthma. It is no longer possible to consider a single cell or a single mediator as the cause of the various aspects of asthma (Figure 4). Like inflammation in other diseases, e.g., arthritis or colitis, the process in asthma is complex, multicellular, and diverse. Techniques to study this complex process have also emerged in the past 20 years.

View larger version (14K): [in this window] [in a new window]   Figure 4.   Current concepts of the multicellular, multimediator aspects of allergic inflammation in asthma.

	THE ROLE OF ANIMAL MODELS IN THE STUDY OF ASTHMA

There has been considerable debate over the potential roles of animal models in asthma. It has been argued that no animal develops spontaneous, chronic asthma, and therefore studies of asthma should be restricted to humans. As has been noted, the use of bronchoscopy, lavage, and biopsy in humans with asthma has seen major advances in the understanding of asthma and particularly the presence and role of inflammation. On the other hand, studies in patients with asthma have limitations. Over the past several decades, it has become clear that innovative animal studies can contribute greatly to defining mechanisms of asthma.

Many of the initial studies involved dogs and provided advantages for measurement of pulmonary physiology. Small animals, in contrast, were useful in studies of experimental asthma (68). Studies in guinea pigs were especially useful to study airway smooth muscle contraction and bronchial hyperresponsiveness. Major advances have followed the development of over-expression and knockout models in mice, and promise to provide unique insight into the role of specific genes and gene-related products. The utilization of molecular genetics has given various animal models a key role in understanding the regulation of airway inflammation and its contributions to alter lung function.

	TRANSLATION OF BASIC SCIENCE TO THERAPEUTIC RECOMMENDATIONS

In 1991, under the leadership of Claude Lenfant, Director of the NHLBI, the "Expert Panel Report: Guidelines for the Diagnosis and Management of Asthma" was issued (69). The recommendations for the treatment of asthma focused on four components of management: (1) use of objective measurements of lung function to assess asthma severity, direct treatment, and monitor response to therapy; (2) establish environmental control measures to avoid or eliminate factors that provoke asthma; (3) use of pharmacologic therapy for long-term management to control airway inflammation; and (4) patient education is important in the successful management of asthma.

Updated guidelines were released in 1997 (70). There are a number of features of these guidelines that are important in the "history of asthma" over the past decades. First, the guidelines are based upon the scientific observations in the study of asthma and identify airway inflammation as: (1) a central feature of disease, (2) a driving force in airway responsiveness and disease chronicity, and (3) the principal objective of therapy. The guidelines serve as an "official" shift in the therapeutic focus from the control of airway smooth muscle contraction by bronchodilators alone to a control of those factors that cause and lead to airway inflammation. The United States guidelines and those of international panels have relied on available scientific information to define asthma and to direct the basis of treatment. The guidelines represent the transition of earlier investigational emphasis on bronchial smooth muscle dysregulation to airway inflammation, and translate this information to form the cornerstones of therapy.

With the knowledge that airway inflammation is an essential component of persistent asthma, treatment recommendations include the use of anti-inflammatory medications, principally inhaled corticosteroids. Studies with corticosteroids show a reduction in airway markers of inflammation, particularly eosinophils, and associated with these improvements in airway inflammation has come better asthma control and restitution of lung function. The change in therapeutic philosophy is the result of asthma research.

It is also understood that asthma is a persistent, chronic disease of the airways. Improved treatment requires close observations of lung function. Consequently, there is a strong recommendation that optimal therapy requires close monitoring of lung function. Measurement of lung function is no longer a research tool, but rather an essential determination of lung function and marker of disease severity.

Environmental allergens are important risk factors in the pathogenesis of asthma, contribute to existing airway inflammation, and cause exacerbations of asthma. The mechanisms by which allergens propagate asthma is now better understood and appreciated. As a consequence of these research advances, optimal asthma care requires an identification of the offending allergens and steps to avoid them.

Current therapeutics and their use are also the result of asthma research. Beta₂-adrenergic agonists are the "rescue" drug of choice. They are effective for quick relief of bronchospasm; however, we know that they do not modify inflammation in asthma and need to be used in conjunction with anti-inflammatory therapy. The molecular mechanisms of corticosteroid effects in inflammation are incomplete, but better understood (42, 71). These advances are also the consequences of asthma research.

Many cells, tissues, and mediators are involved in the pathogenesis of asthma. Techniques now exist to more precisely identify these molecules, understand their function, and modify their action. Examples are currently available drugs that modify the action of leukotrienes. The future will see new therapeutics, which target very specific molecules involved in the generation of allergic inflammation.

	WHAT ARE THE FUTURE DIRECTIONS OF ASTHMA RESEARCH?

In the past 25 years, the prevalence of asthma has increased worldwide. The reasons for this increase have not been established. However, it is also recognized that asthma is a disease of westernized society. Does this mean that increased levels of hygiene are beneficial to individual members of the community, but increase the chance for disease processes such as asthma to flourish? This area of research will be important and possibly may hold clues to the pathogenesis of asthma.

Techniques to identify genes in association with diseases have been a tremendous advance (72). Similar efforts are applied to features of asthma and asthma itself. The discovery of the multiple genes associated with asthma will allow understanding of the various asthma phenotypes. Moreover, recognition of various genes will lead to research to establish what these genes actually do and how these gene-regulated events lead to asthma. Finally, the identification of genes important to asthma can improve diagnostic capabilities and possibly lead to early therapeutic intervention.

A major focus of the next decade will be primary prevention. Current therapeutics are effective for asthma control but do not cure the disease. Furthermore, there is evidence that irreversible changes in airway function may occur in asthma with delays in treatment (73). A critical next step will be efforts to identify asthma early and intervene with therapeutics or immune modulators to control or prevent the disease. As asthma is a common disease with high morbidity and costs, this approach will be most critical.

	SUMMARY

TOP INTRODUCTION CONCLUSION REFERENCES

The understanding and perception of asthma have undergone dramatic changes in the last decades. Although the mechanisms of asthma and allergic inflammation are better understood, the number of patients with asthma has increased. Based upon the advances and progress from the past decades, it is likely that future research will serve the public well and improve diagnostics, therapeutics, and eventually discover a cure for asthma. The continued support from the NIH is critical to achieving these goals. Without this support, past achievements could not have occurred and future progress will not be possible.

	Footnotes

Correspondence and requests for reprints should be addressed to Jay A. Nadel, M.D., University of California, San Francisco, Cardiovascular Research Institute, San Francisco, CA 94143-0130.

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