**2. Pathophysiology and pathogenesis of asthma**

Asthma can be classified as atopic/allergic (extrinsic), which is the most common form, or nonatopic/nonallergic (intrinsic) asthma, which is more rare, has a later onset, and tends to be more severe than atopic asthma [11]. Atopic asthma involves inflammation mediated by specific IgE antibodies directed against common environmental allergens, whereas nonatopic asthma involves inflammation and airway constriction mediated by local production of IgE antibodies that are possibly directed at bacterial or viral antigens. The pathophysiology of nonatopic asthma is very similar to that of atopic asthma, but it is not caused by exposure to an allergen [2, 11].

The gross pathology of asthma reveals significant overinflation of the lungs [12]. Microscopi‐ cally, this overinflation of lungs is manifest as marked distension of the alveoli. Notable airway smooth muscle (ASM) hyperplasia, basement membrane thickening, mucous gland hyper‐ plasia, mucosal epithelium sloughing, and tissue edema are also seen [12]. This increase in muscle mass, mucous gland tissues, and tissue edema leads to a thickened airway wall, with a resultant decrease in airway caliber [12, 13]. These structural changes have been described as remodeling, a term used to define complex morphological changes that involve all of the structures of the bronchial wall [12, 13].

The initiation of bronchial epithelial damage by environmental agents (allergens, viruses, irritants, etc.) or their inflammatory products activates a sequence of events that amplify the inflammation and induce airway remodeling [13, 14]. Bousquet et al. suggested that asthma pathophysiology involved overlapping interactions of smooth muscle dysfunction, airway inflammation, and airway remodeling [13]. The inflammatory, physiological, and structural factors that contribute to the pathogenesis of asthma will be described below.

#### **2.1. Airway inflammation**

Asthma, a life-long condition, frequently presents in early childhood and is the leading chronic disease in children in the Western world. Although the prevalence of childhood varies widely across the world as described in the Phase III ISAAC study [3], most studies have reported that this prevalence has increased in recent decades [4–6]. This increase has been associated with a rise in atopic sensitization and other allergic disorders, such as eczema and rhinitis [6]. Approximately 25.9 million Americans (including 7.1 million children) had asthma in 2011, which equates to a rate of 84.8 per 1,000 in the population. The highest prevalence rate was seen in those in the 5–17 years of age bracket (105.5 per 1,000). Overall, the rate in those under the age of 18 years (94.9 per 1,000) was significantly greater than that in those over 18 years (81.6 per 1,000). The current asthma prevalence rate for boys under 18 years (101.7 per 1,000) was 16% higher than the rate among similarly aged girls (87.8 per 1,000) [7]. In 2008, the condition accounted for an estimated 14.4 million lost school days in children and 14.2 million lost work days in adults. Asthma is thus a leading cause of activity limitation and amounts to

Approximately 80 percent of children with asthma develop symptoms before 5 years of age, but the disease is frequently misdiagnosed or not suspected, particularly in infants [8]. Coughing and wheezing are the most common symptoms of childhood asthma. Breathless‐ ness, chest tightness or pressure, and chest pain have also been reported [1, 2]. Descriptors may vary between cultures and by age; for example, children may be described as having heavy breathing [2]. Confirmation of the diagnosis of asthma in children requires a careful review of a child's current and past medical history, family history, as well as a physical

Asthma is characterized by variable expiratory airflow limitation. Pulmonary function tests are sometimes needed to diagnose asthma and to rule out other possible causes of the symptoms. Spirometry is the most common pulmonary function test; it measures the flow and volume of air blown out after a child takes a very deep breath and then forcefully exhales. The important parameters derived from spirometry include forced expiratory volume in 1 s (FEV1), forced vital capacity (FVC), flow between 25% and 75% of the vital capacity (FEF 25– 75%), and peak expiratory flow rate (PEFR) [9]. The greater the variation in lung function, or the more times excess variation is seen in a patient with respiratory symptoms, the more likely the diagnosis is to be one of asthma. The FEV1/FVC ratio is normally >0.75–0.80 and usually exceeds 0.90 in children [10]. In asthma, at least once during diagnostic process, the FEV1 is low, confirming that the FEV1/FVC ratio is reduced. Generally, an increase in FEV1 of >12% of that predicted after inhalation of a rapid-acting bronchodilator and/or average daily diurnal peak expiratory flow (PEF) variability exceeding 13% indicates that a child has asthma [2]. In young children, in whom lung function testing is not feasible, including most preschool

children, asthma is defined by the presence of variable respiratory symptoms.

Traditional Chinese medicine (TCM), particularly herbal medicine, has been used for the treatment of asthma for hundreds of years, as documented in the *Yellow Emperor's Inner Canon* (*Huangdi Neijing*) and the *Essential Prescriptions from the Golden Cabinet* (*Jin Gui Yao Lue*). In Taiwan, Chinese herbal medicine is commonly used as complementary and alternative therapy for the treatment of atopic diseases such as asthma, allergic rhinitis, and atopic

\$56.0 billion in health care costs annually in the United States [7].

218 Asthma - From Childhood Asthma to ACOS Phenotypes

examination.

Inflammation plays a central role in the pathophysiology of asthma [15]. Airway inflammation remains a consistent pattern throughout the distinct phenotypes of asthma (e.g., intermittent, persistent, exercise-associated, aspirin-sensitive, or severe asthma) [16]. Airway inflammation involves an interaction of many cell types and multiple mediators with the airway, which eventually results in the characteristic pathophysiological features of asthma. The principal cells involved in airway inflammation are mast cells, eosinophils, epithelial cells, macrophages, and activated T lymphocytes [12, 13].

T lymphocytes play an important role in the regulation of airway inflammation through the release of numerous cytokines. Airway inflammation in asthma may indeed represent a loss of the normal balance between Th1 and Th2 lymphocytes [12, 16]. Th1 cells produce interleukin (IL)-2 and interferon gamma (IFN-γ), which are critical in the defense mechanisms of cells in response to infection. Th2 cells, in contrast, generate a family of cytokines (IL-4, IL-5, IL-6, IL-9, and IL-13) that can stimulate the growth, differentiation, and recruitment of mast cells, basophils, eosinophils, and B-cells, all of which are involved in humoral immunity and in the allergic response [13, 14, 16].

IgE plays an essential role in type I hypersensitivity, which results in various allergic diseases, such as allergic asthma, most types of sinusitis, allergic rhinitis, food allergies, and specific types of chronic urticaria and atopic dermatitis [17]. Antigen-specific IgE is partly responsible for the initiation of an allergic response in asthma. IgE primes the IgE-mediated allergic response by binding to Fc receptors expressed on the surface of mast cells, basophils, eosino‐ phils, monocytes, macrophages, or platelets in humans [18]. Antigens cross-link to the IgE on mast cells, which then release bronchoconstricting mediators (histamine, cysteinyl-leuko‐ trienes, prostaglandin D2) and further amplify the inflammatory response by damaging local tissue and attracting other lymphocytes [17]. IL-4 produced by Th2 cells stimulates IgE production in B-lymphocytes and expression of vascular cell adhesion molecule 1 (VCAM-1) on endothelial cells, whereas IL-5 stimulates eosinophil differentiation and mobilization to inflammatory sites [13, 16]. Circulating eosinophils enter the area of allergic inflammation and begin migrating to the lung by rolling, through interactions with selectins, and eventually adhere to the endothelium by means of binding between integrins and members of the immunoglobulin superfamily of adhesion proteins, namely VCAM-1 and intercellular adhesion molecule 1 (ICAM-1) [13, 16]. As the eosinophils enter the matrix of the airway through the influence of various chemokines, such as monocyte chemotactic protein (MCP-1), macrophage inflammatory protein (MIP-1α), eotaxin or RANTES, and cytokines, their survival is prolonged by IL-4 and granulocyte-macrophage colony–stimulating factor (GM-CSF) [13, 16]. Upon activation, the eosinophils release inflammatory mediators, such as leukotrienes and granule proteins, which injure airway tissues [19]. In addition, eosinophils can generate GM-CSF to prolong and potentiate their survival and thereby contribute to persistent airway inflammation [16]. Eosinophils are the most characteristic cells accumulated in asthma and allergic inflammation; their presence is often related to disease severity. Eosinophils are recruited or activated by IL-5, the eotaxin family of chemokines, via the eosinophil-selective chemokine receptor CCR3, and by Toll-like receptors (TLRs). Activated eosinophils produce lipid mediators, such as leukotrienes and platelet-activating factor, which mediate smooth muscle contraction; toxic granule products (e.g., major basic protein, eosinophil-derived neurotoxin, eosinophil peroxidase, or eosinophil cationic protein) that can damage airway epithelium and nerves; and cytokines, such as GM-CSF, transforming growth factors (TGF) α and β, and interleukins, which may be involved in airway remodeling and fibrosis [13]. Recently, Th regulatory cells that exclusively produce IL-17 cytokines (TH17 cells) have been identified in patients with severe asthma [19]. The involvement of TH17 responses in the pathogenesis of asthma has been shown by the overexpression of IL-17 mRNA in the airways of asthma model mice [19]. It is now suggested that TH17-related cytokines play a critical role in airway remodeling and may be involved in interactions with structural cells [13, 19].

#### **2.2. Airway remodeling and ASM dysfunction**

cells involved in airway inflammation are mast cells, eosinophils, epithelial cells, macrophages,

T lymphocytes play an important role in the regulation of airway inflammation through the release of numerous cytokines. Airway inflammation in asthma may indeed represent a loss of the normal balance between Th1 and Th2 lymphocytes [12, 16]. Th1 cells produce interleukin (IL)-2 and interferon gamma (IFN-γ), which are critical in the defense mechanisms of cells in response to infection. Th2 cells, in contrast, generate a family of cytokines (IL-4, IL-5, IL-6, IL-9, and IL-13) that can stimulate the growth, differentiation, and recruitment of mast cells, basophils, eosinophils, and B-cells, all of which are involved in humoral immunity and in the

IgE plays an essential role in type I hypersensitivity, which results in various allergic diseases, such as allergic asthma, most types of sinusitis, allergic rhinitis, food allergies, and specific types of chronic urticaria and atopic dermatitis [17]. Antigen-specific IgE is partly responsible for the initiation of an allergic response in asthma. IgE primes the IgE-mediated allergic response by binding to Fc receptors expressed on the surface of mast cells, basophils, eosino‐ phils, monocytes, macrophages, or platelets in humans [18]. Antigens cross-link to the IgE on mast cells, which then release bronchoconstricting mediators (histamine, cysteinyl-leuko‐ trienes, prostaglandin D2) and further amplify the inflammatory response by damaging local tissue and attracting other lymphocytes [17]. IL-4 produced by Th2 cells stimulates IgE production in B-lymphocytes and expression of vascular cell adhesion molecule 1 (VCAM-1) on endothelial cells, whereas IL-5 stimulates eosinophil differentiation and mobilization to inflammatory sites [13, 16]. Circulating eosinophils enter the area of allergic inflammation and begin migrating to the lung by rolling, through interactions with selectins, and eventually adhere to the endothelium by means of binding between integrins and members of the immunoglobulin superfamily of adhesion proteins, namely VCAM-1 and intercellular adhesion molecule 1 (ICAM-1) [13, 16]. As the eosinophils enter the matrix of the airway through the influence of various chemokines, such as monocyte chemotactic protein (MCP-1), macrophage inflammatory protein (MIP-1α), eotaxin or RANTES, and cytokines, their survival is prolonged by IL-4 and granulocyte-macrophage colony–stimulating factor (GM-CSF) [13, 16]. Upon activation, the eosinophils release inflammatory mediators, such as leukotrienes and granule proteins, which injure airway tissues [19]. In addition, eosinophils can generate GM-CSF to prolong and potentiate their survival and thereby contribute to persistent airway inflammation [16]. Eosinophils are the most characteristic cells accumulated in asthma and allergic inflammation; their presence is often related to disease severity. Eosinophils are recruited or activated by IL-5, the eotaxin family of chemokines, via the eosinophil-selective chemokine receptor CCR3, and by Toll-like receptors (TLRs). Activated eosinophils produce lipid mediators, such as leukotrienes and platelet-activating factor, which mediate smooth muscle contraction; toxic granule products (e.g., major basic protein, eosinophil-derived neurotoxin, eosinophil peroxidase, or eosinophil cationic protein) that can damage airway epithelium and nerves; and cytokines, such as GM-CSF, transforming growth factors (TGF) α and β, and interleukins, which may be involved in airway remodeling and fibrosis [13]. Recently, Th regulatory cells that exclusively produce IL-17 cytokines (TH17 cells) have been

and activated T lymphocytes [12, 13].

220 Asthma - From Childhood Asthma to ACOS Phenotypes

allergic response [13, 14, 16].

The histopathologic changes of airway remodeling include damage or loss of the normal pseudostratified structure of airway epithelium, an increase in the proportion of mucousproducing goblet cells, fibrotic thickening of the subepithelial reticular basement membrane or "lamina reticularis," increased numbers of myofibroblasts, increased vascularity, increased ASM mass, and increased extracellular matrix [20]. These structural changes contribute to bronchial wall thickening, alterations in the physiological consequences of smooth muscle contraction, or loss of airway-parenchymal interdependence [13, 20].

Epithelial alterations in asthma include epithelial shedding, destruction of ciliated cells, goblet cell hyperplasia, upregulation of growth factor release, and overexpression of receptors, such as the epidermal growth factor receptors [21]. Loss of epithelial surface and the resultant denudation of the basement membrane may decrease this protective effect, thereby increasing the propensity for allergic insult to the airway [21]. A second important feature of airway remodeling is subepithelial fibrosis, which has been consistently reported in asthma of all levels of severity, in patients with atopic rhinitis, and even in children with treatment-resistant asthma [21]. Subepithelial fibrosis occurs in the lamina reticularis, immediately below the basement membrane, resulting in thickening of the basement membrane just below the epithelium [22]. In the asthmatic airway, fibroblasts are activated and differentiate into myofibroblasts, which secrete proinflammatory mediators and extracellular matrix proteins, including collagens I, III, and V; fibronectin; tenascin; lumican; and biglycan [21, 22]. Asthmatic airway fibroblasts promote fibrosis though expression of a higher ratio of tissue inhibitor of metalloproteinase (TIMP)-2 to matrix metalloproteinase (MMP)-2, resulting in increased matrix deposition [21]. MMPs are a family of proteases implicated in collagen degradation. MMP-2, MMP-3, MMP-8, and MMP-9 have been associated with asthma [20]. Among these, MMP-9 levels have been reported to be significantly higher in the sputum of patients with asthma than in that of control subjects [20–23].

Respiratory ASM cells are the critical effector cells that modulate airway tone [22]. In asthmatic airways, smooth muscle mass is increased due to a coordinated increase in the size (hyper‐ trophy) and number (hyperplasia) of ASM cells [21, 22]. ASM remodeling is considered to be the primary cause of airway obstruction [21]. ASM cells participate in the inflammatory and remodeling process through the expression of cellular adhesion molecules, receptors for cytokines (e.g., TNF-α), chemokines (RANTES, eotaxin, MIP-1α, and IL-8), and TLRs [21]. Additionally, the migration of ASM cells toward the epithelium contributes to remodeling. A wide range of inflammatory mediators, such as TNF-α, IL-1b, and IFN-γ, have been shown to induce the expression of ICAM-1 and VCAM-1 on cultured ASM cells [21]. The surface expression of cellular adhesion molecules by ASM cells might be pivotal in regulating interactions with a variety of inflammatory cells, including eosinophils and T cells [21]. Additionally, accumulating evidence has indicated an abnormal increase in the number and size of microvessels within bronchial tissue in remodeled airways [21]. This occurs mainly below the basal lamina, in the space between the muscle layer and the surrounding paren‐ chyma [21]. An imbalance between vascular endothelial growth factor (VEGF) and angiopoie‐ tin-1 has been shown to be involved in these abnormalities [21]. In fact, VEGF acts by increasing the permeability of these abnormal blood vessels, resulting in vessel dilation and edema, which contribute to airway narrowing [21, 22]. In addition to providing nutrition to the airways, these vessels are the source of inflammatory cells and plasma-derived mediators and cytokines [21].
