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Allergic diseases, such as asthma and AD, are the most common chronic diseases encountered by pediatricians in their clinical practice. Recent studies indicate that over 10% of children have AD at some point in their childhood(1). In the case of asthma alone, it is estimated that nearly 5 million children are affected. Asthma is one of the leading admitting diagnoses to children's hospitals throughout the world, accounts for nearly one-third of visits to pediatric emergency rooms, and is the most common cause of school days lost in childhood. More than 5000 patients with asthma die annually in the United States with death rates highest among blacks aged 15-24 y(2). The overall prevalence of allergic diseases and morbidity related to them have risen progressively during the past 20 y.

To meet the challenge of developing more effective strategies in the management of this common group of illnesses, researchers throughout the world have been actively investigating the pathogenesis of allergic immune responses and inflammation. I would like to thank the Awards Committee for choosing me to be a recipient of the 1997 E. Meade Johnson Award for Research in Pediatrics, and giving me the opportunity to review for the Pediatric Academic Societies our current understanding of the mechanisms that give rise to chronic allergic diseases such as asthma and AD. Although the focus of our research has primarily been on the role of T cells and cytokines in this process, I will also review some important recent advances in the mechanisms of inflammatory cell recruitment and the genetics of atopy as it is the interaction of genetic, environmental, and immunologic host factors that play a critical role in determining the development of clinical phenotypes found in this important set of diseases.

ROLE OF INFLAMMATION IN ALLERGIC DISEASES

An important distinguishing feature of atopy is the production of a sustained high level IgE response to environmental allergens(3). IgE binds to the α-chain of the high affinity IgE receptor (FcεR1) on mast cells, basophils, and dendritic cells, as well as to the low affinity IgE receptor (FcεR2; CD23) on monocytes/macrophages and lymphocytes. The cross-linking by allergen of IgE bound to these cell types results in cellular activation and leads to the release of a variety of mediators, proteases, and cytokines. IgE bearing Langerhans cells from AD skin lesions, but not Langerhans cells that lack surface IgE, are capable of presenting house dust mite allergen to T cells(4). These results suggest that cell-bound IgE on Langerhans cells facilitate binding of allergens to Langerhans cells before their processing and antigen presentation. More importantly, it indicates that IgE has a multifunctional role in the pathogenesis of allergic responses(5).

Clinically important allergen-induced reactions are generally associated with an IgE-dependent biphasic response(6). After allergen challenge, atopic patients have an immediate reaction that subsides within 90 min. Elevated plasma histamine and mast cell-derived tryptase can be detected in bronchoalveolar lavage fluid of asthmatics after challenge with allergen, as well as skin chamber fluids of atopic individuals undergoing skin allergen challenges(7). Three to 4 h later, an intense inflammatory reaction termed the “late phase response” occurs. During this period, the predominant cellular infiltrations are eosinophils, mononuclear cells, and, to a lesser extent, neutrophils. Twenty-four to 48 h after allergen challenge, the cellular infiltrate is predominantly T cells and monocytes/macrophages. These T cells primarily express mRNA for IL-4, IL-5, and GM-CSF, but no mRNA for IFN-γ(8). This IgE-mediated inflammatory late phase response plays an important role in allergic diseases. For example, in asthma, the intensity of nonspecific bronchial hypereactivity in asthmatic reactions after allergen bronchoprovocation is proportional to the intensity of the late phase response(9). Furthermore, clinical improvement in asthmatic symptoms after allergen immunotherapy correlates with an attenuation of late phase response after bronchoprovocation challenge(10).

Pathologic studies of patients with ongoing symptoms of asthma, AD, and allergic rhinitis have revealed evidence of chronic inflammation accompanied by the presence and activation of eosinophils, T lymphocytes, mast cells, basophils, neutrophils, and epithelial cells(11, 12). It is highly unlikely that a single cell type, mediator, or cytokine accounts for all of the features of allergic inflammation. Indeed, as cellular and molecular techniques are applied to pathologic specimens from these different diseases, it is becoming apparent that there is considerable disease heterogeneity. For example, there is data to suggest that different cell types and their mediators may have lesser or greater importance in the various forms of asthma and at different stages in the natural history of this illness. Mast cells play an important role in the immediate response and initiation of inflammatory responses(7). Neutrophils likely play a role in more severe forms of asthma, particularly fatal asthma(13, 14). In the majority of patients, T lymphocytes are thought to play a key role in orchestrating the nature and magnitude of allergic inflammatory response, and eosinophils are critical effector cells by virtue of their capacity to secrete basic proteins, i.e. major basic protein and eosinophil cationic protein, which are cytotoxic to the respiratory epithelium of asthmatics, thereby contributing to the bronchial hyperreactivity observed in these patients(11). The importance of inflammatory responses in chronic allergic diseases is supported by the observation that treatment with antiinflammatory drugs, such as corticosteroids, are highly effective in reducing clinical symptoms due to these illnesses.

MECHANISMS OF INFLAMMATORY CELL RECRUITMENT

The presence of increased numbers of eosinophils in the circulation and at local tissue sites of inflammation is a characteristic feature of allergic diseases. The development of new treatments for allergic diseases requires a detailed understanding of the development and selective recruitment of eosinophils from the bone marrow into local tissue sites. This involves a multistep process that includes eosinophil hematopoietic development, endothelial adhesion, chemotaxis, and survival. IL-5 plays a critical role in eosinophil hematopoiesis, eosinophil maturation and activation, and prolonged eosinophil survival(15). Elevated IL-5 mRNA expression and increased numbers of activated eosinophils have been detected in bronchial biopsies of asthmatics and skin biopsies of patients with AD(16, 17). Because IL-5 knockout mice maintain low level eosinophilia, other cytokines likely contribute to eosinophil development. In addition to IL-5, IL-3, and GM-CSF also play a role in activating or priming eosinophils(15). Compared with resting eosinophils, cytokine stimulated eosinophils bind vascular endothelium with greater avidity, migrate more rapidly and produce higher levels of mediators(18).

In the circulation, eosinophils bind to endothelium at sites of tissue inflammation. Initial binding occurs as low affinity tethering or rolling on the endothelium. This is mediated by several endothelial selectins, including E-, and P-selectins(19). For subsequent extravasation to occur, rolling must be followed by eosinophil activation and firm adhesion to the endothelium. This process requires eosinophil activation by cytokines such as IL-5, and the newly described family of chemokines(20). The most important consequence of eosinophil activation is an increased affinity by which eosinophil surface integrins such as β2-integrin and VLA-4 bind to their counterreceptors, i.e. ICAM-1 and VCAM-1 on the endothelial cell(18). This results in leukocyte arrest or firm adhesion followed by transendothelial migration(21).

Immunohistochemical studies have demonstrated increased expression of E-selectin, ICAM-1, and VCAM-1 in tissue biopsies from patients with various allergic diseases(22, 23). Intradermal allergen challenge in sensitized subjects also induces significant E-selectin and ICAM-1 expression in parallel with leukocyte recruitment(24, 25). These vascular endothelial adhesion molecules are induced by several cytokines. The expression of ICAM-1, E-selectin, and VCAM-1 on endothelial cells is up-regulated by IL-1, TNF-α, and other cytokines(26). The initial phase of eosinophil and lymphocyte, but not neutrophil, recruitment during allergic responses is thought to depend on endothelial expression of VCAM-1. In addition to IL-1 and tissue necrosis factor, this adhesion molecule can also be induced after local release of IL-4 and IL-13 by resident cells, e.g. mast cells, at sites of allergic inflammation, and interacts with VLA-4 on eosinophils, basophils, and lymphocytes(27). Importantly, neutrophils do not express VLA-4(28).

Eosinophils extravasating into the tissue respond to chemotactic gradients and migrate toward the epithelium of both airways and the skin. Production of chemoattractants such as chemokines by epithelial cells likely attracts critical effector cells such as eosinophils(20). Three groups of chemokines have been classified according to the primary sequence of their first two cysteines: C-X-C, C-C, and C families. The C-X-C and C families act primarily on neutrophils and lymphocytes, whereas the C-C family members act on eosinophils, basophils, lymphocytes, and macrophages.

The chemokines that cause eosinophil chemotaxis and that have been implicated in the pathogenesis of allergic disease include RANTES, eotaxin, MCP-2, MCP-3, and MCP-4(29). Chemokines bind and signal through G protein-coupled, seven-membrane spanning receptors. Eosinophils express chemokine receptor 3 (CCR3). Interesting, a recent study demonstrated that the response of eosinophils to eotaxin, RANTES, MCP-2, MCP-3, and MCP-4 can all be blocked by a MAb directed to CCR3(30). These results have important therapeutic implications as the blockage of eosinophil chemotaxis may not require the development of individual chemokine inhibitors but the blockade of a common receptor for these various chemokines.

ROLE OF T CELLS IN ALLERGIC RESPONSES

Studies of T cell clones support the concept that polarized T cell responses leads to the release of cytokines important in the pathogenesis of allergic diseases (Fig. 1). CD4+ TH cells can be divided into three major subsets termed TH1, TH2, and TH0, based on the pattern of cytokines they secrete(31). TH1 produce IFN-γ but not IL-4 or IL-5, and predominantly promote cell-mediated immune respones. In contrast, TH2 cells elaborate IL-4, IL-5, and IL-13 but not IFN-γ, and provide help for humoral immune responses. IL-4 and IL-13 induce germ line transcription of Igγ4 and Igε heavy chain constant region genes(32). In T cell-dependent responses, switch recombination to IgG4 and IgE synthesis requires engagement of CD40 on the B cell by its ligand on activated T cells [reviewed in de Vries(33). IL-5 promotes differentiation, vascular endothelial adhesion, and cell survival of eosinophils as well as enhances basophil histamine release(15). In contrast, IFN-γ inhibits IgE synthesis, expression of the IL-4 receptor on T cells, as well as the proliferation of TH2 cells(34–36).

Figure 1
figure 1

Interactions between cells and cytokines in allergy. Allergen capture and processing involves dendritic cells, particularly those expressing high affinity IgE receptors present at mucosal or epithelial surfaces. After presentation of appropriate peptides via major histocompatibility complex class II molecules on dendritic cells to the T cell receptor and the engagement of key costimulatory molecules, e.g. between B7 and CD28, naive TH0 cells can differentiate into two different pathways, depending on the cytokine microenvironment. IL-4 is a key determinant of TH2 cell differentiation, whereas IL-12, IFN-α, and IFN-γ are important determinants for TH1 cell differentiation. TH2 cells produce the major cytokines in allergic inflammation, i.e. IL-4 and IL-13, which in combination with a second signal, i.e. CD40 ligand and CD40 interactions, results in IgE synthesis; IL-4, IL-9, and IL-10 play a role in mast cell growth and differentiation; and IL-3, IL-5 and GMCSF are important cytokines involved in eosinophil differentiation, survival, and chemotaxis. The dashed lines represent inhibitory pathways, whereas solid lines represent stimulatory pathways.

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The expansion of TH2- or TH2-like cells is therefore thought to play a critical role in the pathogenesis of allergic diseases(37). In humans, TH2-like allergen-specific clones have been grown from the bronchial mucosa of allergic asthma patients and skin biopsies of patients with AD(38, 39). Other reports have demonstrated that infiltrating T cells expressing mRNA for IL-4, IL-5, and IL-13, but not for IFN-γ, are found in bronchial biopsy specimens and in bronchoalveolar lavage cells of patients with allergic asthma by in situ hybridization(16, 40). T cells expressing TH2-like cytokines have also been found in the acute skin lesions of AD(17, 41) (Fig. 2).

Figure 2
figure 2

Darkfield illumination of autoradiographs of a skin biopsy from an acute atopic dermatitis skin lesion hybridized with IL-4 cRNA probe (A) and IFN-γ-cRNA probe (B). The magnification of these photographs is ×300. Reproduced from The Journal of Clinical Investigation, 1994, 94:870-876(17) by copyright permission of The American Society for Clinical Investigation.

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In the absence of distinct polarizing signals, TH0 cells develop(31). These cells produce both TH1 and TH2 cytokines and have intermediate effects depending upon the ratio of cytokines produced and the nature of the responding cells. The further development of TH0 cells into the TH1 or TH2 pathway is dependent upon a number of determinants, including the subject's particular genetic background, the nature of the antigenic stimulus, and the costimulatory signals used during T cell activation(31). Cytokines present at the time of antigen exposure, however, are one of the major determinants directing TH cells toward the TH1 or TH2 phenotype. IL-4 promotes TH2 development, whereas IL-12 produced by macrophages or dendritic cells is a potent inducer of TH1 cells(42, 43). Recently, it has also been shown that the IL-12 receptor (IL-12R) β2 subunit which is the binding and signal transducing component of the IL-12R, is expressed on TH1 but not TH2 clones(44). Interestingly, IL-4 inhibited the expression of IL-12R β2. In contrast, IL-12 and IFN-α induces expression of the IL-12R β2 chain after antigen triggering, thereby providing a basis by which these two cytokines induce the differentiation of TH1 cells.

Taken together, these observations suggest that exposure of the atopic host to specific allergens or stimuli that modulate the balance of TH1/TH2 cells can promote allergic responses. Indeed, a dose-response relation between exposure to house dust mite allergens and asthma has been found(45). Even more intriguing is the recent report from Japan of a reciprocal relationship between the prevalence of atopy and immunity to tuberculosis as measured by delayed cutaneous hypersensitivity to tuberculin(46). It is known that tuberculosis infections trigger a TH1 response. Based on this study it has been postulated that the rising prevalence of atopy in certain Westernized countries may relate to reduced TH1 responses that accompany persistent infection. Of note, the incidence of other infections may also be declining with the use of vaccination and these could account as well for alterations in the TH2/TH1 balance. Overall, these results emphasize the complexity of the environmental contribution to asthma and atopy, and the importance of reducing the exposure of allergic children to potentially harmful environmental stimuli.

ORGAN-SPECIFIC HOMING OF T H 2 CELLS

Because organ-specific infiltration of TH2-like memory T cells play a critical role in the induction of local allergic inflammatory responses, the mechanisms which control recruitment of T lymphocytes to different tissue sites are of great interest. Studies in animal models have demonstrated clear heterogeneity in the ability of memory T cells to migrate to mucosal as opposed to nonmucosal tissues(47, 48). This tissue-selective homing is regulated primarily at the level of T lymphocyte recognition of vascular endothelial cell surface antigens through the interaction of differentially expressed T lymphocyte homing receptors and their endothelial cell ligands. Several lymphocyte/endothelial cell adhesion molecule pairs participate in “tissue-selective” lymphocyte homing. These include 1) the CLA and its counterreceptor, E-selectin, which direct lymphocyte homing to skin; 2) L-selectin and its ligand, peripheral lymph node addressin, which plays a role in lymphocyte homing to peripheral lymph nodes, and 3) the α4β7 integrin and its ligand, MAdCAM-1 (mucosal addressin), which directs T cell homing to Peyer's patch and intestinal lamina propria(48). In humans, the CLA antigen has been the best studied of the putative tissue-selective homing receptors.

We have found that T cells migrating into the skin of cell-mediated reactions express significantly higher levels of CLA than T cells isolated from the airways of asthmatics(49). Thus, we have hypothesized that the propensity of patients to develop AD as opposed to asthma depend on differences in the skin- versus the lung-seeking behavior of their memory/effector T cells. Children with food-induced AD provide a unique opportunity to analyze the relationship between tissue specificity of a clinical reaction to an allergen and the expression of homing receptors on T cells activated in vitro by the relevant allergen. In this regard, we assessed the expression of CLA and L-selectin on peripheral blood T cells from patients with milk-induced AD, and compared their homing receptor expression after stimulation with casein to T cells collected from patients with milk-induced enterocolitis or nonatopic healthy controls(50). We found that the casein-reactive T cells from patients with milk-induced eczema displayed significantly higher levels of CLA than Candida albicans-reactive T cells from the same patients, and either casein- or C. albicans-reactive T cells from nonatopic controls or noneczematous atopic patients. More recently, we have also found that children with milk-induced asthma, compared with milk-induced AD, have significantly lower expression of CLA+ T cells after stimulation with casein (Fig. 3) (D. Y. M. Leung and H. A. Sampson, unpublished observations).

Figure 3
figure 3

Casein induces CLA+ T cell expression in patients with milk-induced AD. CLA+ T cell expression was analyzed at baseline (d 0) and after 6 d of in vitro stimulation with casein in peripheral blood mononuclear cells from patients with milk-induced AD, patients with gastroenteropathy, and normal subjects. At baseline, patients with milk-induced AD had significantly greater CLA+ T cell expression than children with milk-induced gastroenteropathy. After 6 d of casein stimulation, children with milk-induced AD had significantly greater CLA+ T cell expression than the other two control groups. Reproduced from The Journal of Clinical Investigation, 1995, 95:913-918(50) by copyright permission of The American Society for Clinical Investigation.

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The relationship between CLA and cutaneous T cell responses in atopic disease has also recently been reported by Santamaria Babi et al.(51). These investigators analyzed the expression of CLA on circulating memory T cells in AD patients versus asthmatics who were sensitized with house dust mite. When peripheral blood CLA+CD3+CD45RO+ T cells were separated from CLA-CD3+CD45RO+ T cells, the mite-specific T cell proliferation response in patients with AD sensitized to dust mite was localized to CLA+ T cells. In contrast, mite-sensitive patients with asthma had a strong mite-dependent proliferation response in their CLA- T cells. A further link between CLA expression and skin disease-associated T cell function in AD was demonstrated by the observation that freshly isolated CLA+ T cells in AD patients, but not normal control subjects, selectively demonstrated both evidence of activation (HLA-DR expression) and spontaneous production of IL-4 but not IFN-γ.

These observations strongly support the concept that, in human allergic diseases, mechanisms exist to target memory TH2 cells with a particular allergen reactivity to specific organs. In the case of skin-seeking T cells compared with lung-seeking T cells, there is selective expression of the CLA homing receptor. Future studies are needed to determine whether this preferential CLA induction on allergen-specific T cells is due to allergen preferentially entering the body via the skin, or to other regulatory influences promoting CLA induction in non-skin-associated microenvironments, such as the gut-associated lymphoid tissue, which could potentially tie the increased prevalence of abnormal gut permeability with AD in food allergic patients.

CLA expression can be differentially regulated on virgin and memory T cells in vitro by microenvironmental signals(47, 48). Thus, three cytokines, transforming growth factor-β1, IL-12, and to a lesser extent IL-6, are able to up-regulate CLA expression. In contrast, other cytokines such as IL-1, IL-2, IL-3, IL-4, IL-5, IL-7, and IFN-γ lack CLA up-regulatory activity. Furthermore, we have observed that, when bacterial superantigens, e.g. staphylococcal enterotoxins or toxic shock syndrome toxin-1, are used to stimulate T cells (with accessory cells present), there is a profound up-regulation of CLA, in an IL-12-dependent manner(52). In contrast, the mitogen, phytohemagglutinin, had no such effect. Thus, the nature of the antigen, the cytokine milieu in which the immune responses occurs, and the location of the response may all contribute to the regulation of homing receptors such as CLA. This observation may be particularly relevant to patients with AD who are heavily colonized with staphylococci-secreting superantigens(53).

MECHANISMS OF CHRONIC ALLERGIC INFLAMMATION

Most studies on allergic responses have focused on the mechanisms controlling cellular infiltration into allergen-induced acute and late phase response. Thus, there is relatively little information about the processes that regulate persistence of local tissue immune activation and inflammation in chronic allergic diseases. Several factors are likely to play a role in this process. First, these patients frequently have ongoing exposure to environmental allergens that are repeatedly triggering allergic responses and TH2 cell expansion. Exposure to allergens can contribute to the chronicity of illness, and their elimination can result in reduced symptoms(54).

Second, once a TH2 cell response is established, it antagonizes the activation of TH1 cells. TH2 cells produce IL-4 and IL-10. Both these cytokines reduce cytokine production, e.g. IFN-γ secretion by TH1 cells and enhance the development of TH2 cells, thus polarizing the T cell response. Monocyte/macrophages in the chronic AD lesion also have increased phosphodiesterase activity leading to increased secretion of IL-10 and prostaglandin E2. Both of these molecules inhibit IFN-γ and further amplify TH2-like responses(55, 56). IFN-γ inhibits IgE synthesis and the differentiation of IL-4-producing TH2 cells(34–36). The inability to produce IFN-γ may thereby contribute to increased IgE synthesis and sustained TH2 cell activation.

Third, apoptosis or programmed cell death of effector cells contributes to the resolution of tissue inflammation. External signals that stimulate apoptosis can be generated through the cell surface receptor Fas (CD95). Recent studies indicate that anti-Fas antibody induces apoptosis in human eosinophils(57). Aside from Fas, there are a number of other genes involved in the regulation of apoptosis, including genes that inhibit apoptosis, e.g. the Bcl-2 family, and IL-1b-converting enzyme or p53 which promote apoptosis(58, 59). Enhanced survival of inflammatory cells as the result of reduced apoptosis in inflamed tissues may therefore be a factor in the establishment of chronic inflammation. Increased production of GM-CSF and IL-5 likely contribute to reduced apoptosis of monocytes and eosinophils, respectively(60, 61). In the case of chronic AD, monocyte-macrophages exhibit enhanced survival and increased GM-CSF expression(61). Eosinophil apoptosis likely plays an important role in the resolution of airway inflammation in asthma(62).

Finally, recent studies on mononuclear cells from patients with atopic asthma indicate that allergen-induced immune activation can alter T cell response to GCs by inducing cytokine-dependent abnormalities in GCR binding affinity(63). Of interest, we have found that peripheral blood mononuclear cells from patients with chronic AD also have reduced GCR binding affinity, which can be sustained with the combination of IL-2 and IL-4(64). Endogenous cortisol levels have been found to control the magnitude of late phase allergic inflammatory responses, suggesting that impaired response to GCs could contribute to chronic allergic responses(65).

GENETICS OF ATOPY

It is well established that allergic diseases, such as asthma and AD, cluster within families, suggesting a strong genetic component to these illnesses. In this regard, twin studies have revealed that monozygotic twins are more concordant for atopic allergy of any type than are dizygotic twins(66). Nevertheless, the task of unraveling the genetics of asthma and other allergic diseases has been challenging for several reasons. First, the clinical phenotype for these diseases is heterogeneous, e.g. the bronchial hyperreactivity and wheezing of asthma is the final common pathway of several mechanisms of inflammation and multiple triggers. Second, the expression of allergic diseases is strongly dependent on environmental influences. To become atopic to a particular allergen requires appropriate exposure and subsequent development of an IgE response in a genetically susceptible host. Third, unlike cystic fibrosis, there will be multiple major and minor genes involved in the development of allergic diseases(67). Several types of genetic heterogeneity are expected involving differential expression of IgE, e.g. high IgE versus low IgE responders, and inflammatory responsiveness, e.g. a predominance of mast cell versus eosinophil- or neutrophil-driven disease, and with different types of disease manifestation, e.g. asthma, allergic rhinitis versus AD, involving different combinations of these conditions. Within the different major genes involved in the expression of allergic disease, a variety of mutations are likely to exist that modulate gene function, and therefore disease expression. Furthermore, their expression will also depend on environmental influences. Nevertheless, recent advances in our understanding of the key molecules and cytokines involved in the pathogenesis of allergic diseases have led to the identification of a number of candidate genes in asthma and atopy (Table 1).

Table 1 Candidate genes associated with asthma and atopy
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Immune response genes are important candidates. HLA linkage has been reported for several antigens both in allergic asthma such as Amb aV of ragweed (DRB1*1501) and dust mite allergens (DQB1*0101), as well as in occupational asthma caused by toluene diisocyanate exposure (DQB1*0503)(67). An association between variants of the α-chain of the T cell receptor and responsiveness to the dust mite allergen Der P II has also been reported(68).

Relevant to the current review, chromosome 5q31-33 contains multiple candidate genes for asthma and atopy(69). These include a clustered family of cytokine genes i.e. IL-3, IL-4, IL-5, IL-13, and GM-CSF, which are expressed by TH2 cells involved in allergic inflammatory responses as well as the β2-adrenergic receptor and GCR genes. Studies on the Amish population in Pennsylvania by Marsh et al.(70) and Dutch families by Postma et al.(71) have reported linkage between total IgE, bronchial hyperreactivity, and asthma with markers around the IL-4 gene cluster and the β2-adrenergic receptor. In addition, Rosenwasser et al.(72) have found in families with asthma that a polymorphism in the IL-4 promoter is associated with elevated total serum IgE levels. Taken together, these data support the concept that IL-4 gene expression plays a critical role in the pathogenesis of atopy.

Genotyping of the β2-adrenergic receptor has also resulted in several intriguing observations. Polymorphisms of this gene have been reported in normal subjects and asthmatics by Liggett and his colleagues(73). However, the overall frequencies of distribution of these common polymorphisms were similar in normal subjects and asthmatics. Based on their observation that polymorphic forms of the receptor had different pharmacologic properties, they considered the possibility that although these variants are not a primary cause of asthma they may act as disease modifiers. In this regard, functional studies revealed that an arginine-to-glycine polymorphism at position 16 increased β-agonist-related receptor desensitization. Based on a report that asthmatics with nocturnal exacerbation undergo receptor down-regulation overnight, Turki et al.(74) examined the potential association of the Gly16 polymorphism with noctrunal asthma. Their results indicated a strong association between patients with nocturnal asthma and homozygosity for this polymorphism. Other potential relationships between bronchial hyperreactivity and β2-adrenergic receptor polymorphisms have also been reported(75).

Overall, these observations suggest that although there are likely to be multiple genes primarily involved in the pathogenesis of atopy, e.g. the IL-4 gene, there will also be genes, e.g. the β2-adrenergic receptor gene, which modify disease severity. Interestingly, approximately 50% of normal subjects are homozygous for the Gly16 polymorphism in the β2-adrenergic receptor(73). However, normal subjects with this β2-adrenergic receptor polymorphism do not have any evidence of bronchial hyperreactivity after methacholine challenge consistent with the concept that other gene products are required for disease expression of asthma.

Just as β-adrenergic agents are frequently used for treatment of acute asthma, GCs are commonly used as first line therapy for control of chronic inflammation associated with asthma. Interestingly, in a study of lymphocyte activation in vitro in the presence and absence of prednisolone, it was found that the blood cells from nearly 25% of normal subjects failed to respond optimally to prednisolone(76). These observations suggest a significant proportion of the normal population may be steroid or GC resistant, raising the possibility that alterations of the GCR gene may also be disease-modifying in chronic allergic diseases(77).

SR ASTHMA: A MODEL FOR SEVERE DISEASE PROGRESSION

Current guidelines of asthma therapy have focused on the importance of antiinflammatory therapy, particularly inhaled GCs. Asthmatics, however, vary in their responses to GC. Whereas the majority of patients respond to regular inhaled GC therapy, a subset of patients have poorly controlled asthma, even when treated with high doses of oral prednisone [reviewed in Lee et al.(78). Although accounting for less than 10% of asthmatics, these patients frequently have severe asthma, and they involve a group of patients who account for the majority of health care dollars spent on the treatment of asthma(79). Understanding the mechanisms underlying SR asthma has important clinical implications not only for the management of asthma and allergic diseases, but other chronic inflammatory illnesses such as rheumatoid arthritis, systemic lupus erythematosus, and transplantation rejection, which can be associated with steroid resistance. Patients with SR asthma have a tissue-specific GC insensitivity and are often subjected to continued high dose treatment with GCs, despite the onset of serious adverse GC effects and poor clinical response to GC therapy. Delineation of the molecular basis for GC insensitivity is critical for the development of new treatment approaches for this group of refractory patients, and may provide new insights into the pathogenesis of chronic inflammation.

A number of investigations have revealed evidence of cellular abnormalities in patients with SR asthma (Table 2). These studies demonstrate that GCs inhibit mitogen-induced T cell proliferation and cytokine secretion in vitro by peripheral blood mononuclear cells from SR, but not SS, asthmatics(80). In addition, T cells from the peripheral blood of SR asthmatics, but not SS asthmatics, are persistently activated despite high doses of GC therapy(81).

Table 2 Immunologic responses in SR asthma
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GCs act by binding to a cytoplasmic GCR, which then translocates to the nucleus as a transcription factor(82). Recently, we found that the majority of patients with SR asthma have a reversible defect in T cell GCR ligand and DNA binding affinity, which can be sustained in vitro by the addition of IL-2 and IL-4, but not other cytokines(83). Furthermore, in vitro incubation of peripheral blood T cells from normal subjects with the combination of IL-2 and IL-4 reduces their GCR binding affinity to the level seen in SR asthma(84). Bronchoscopy studies indicate that airway T cells of SR, compared with SS, asthmatics have significantly higher levels of IL-2 and IL-4 gene expression(85). Overall these data suggest that SR asthma results from high level expression of IL-2 and IL-4, which leads to GCR abnormalities and decreased T cell responsiveness to GCs.

The mechanisms by which cytokines induce a decrease in GCR binding is unknown. Cloning of the human GCR cDNA and gene indicate that alternative splicing of the GCR pre-mRNA gives rise to an additional homologous mRNA and protein isoform, termed GCRβ, that is distinct from the ligand-activated classical GCR, GCRα. Both mRNAs contain the first eight exons of the GCR gene(86). The remainder is derived by alternative splicing of the nucleotide sequence encoded by the last exon of the GCR gene, corresponding to either exon 9a or 9b. The two protein isoforms have the same first 727 NH2-terminal amino acids. GCRβ differs from GCRα only in its carboxy terminus with replacement of the last 50 amino acids of GCRα with a unique 15-amino acid sequence. These differences render GCRβ unable to bind GC hormones, thereby antagonizing the transactivating activity of the classic GCRα molecule.

The increased expression of GCRβ could therefore account for SR asthma. Indeed, we have recently found that SR asthma is associated with a significantly higher number of GCRβ-immunoreactive T cells in peripheral blood and bronchoalveolar lavage than SS asthmatics or normal control subjects(87). Furthermore, we found that expression of GCRβ is inducible by the combination of IL-2 and IL-4. Thus, cytokine-induced T cell expression of GCRβ may be directly involved in the development of SR asthma. These observations have general implications as a number of other diseases associated with inflammation-induced steroid resistance may also involve similar mechanisms.

During the past three decades there has been an increase in morbidity and mortality due to chronic asthma. This increase in asthma severity has been attributed to changes in our environment, particularly with regard to allergen exposure and air pollution, both of which stimulate airway inflammation(88, 89). Recent studies also indicate that early treatment with inhaled GCs, to gain control of immune activation and inflammation, is critical for successful response to GCs(90, 91). Our observation that immune activation induces the expression of GCRβ and thereby reduces functional responses to GCs is consistent with the concept that immune activation dampens responses to endogenous and exogenous GCs. An understanding of the mechanisms by which GCs fail to resolve inflammation in asthma will provide important insights into the pathogenesis of asthma, especially as it relates to progressive deterioration from airway remodeling and other chronic changes that may accompany uncontrolled ongoing inflammation(92).

CONCLUSIONS AND CLINICAL IMPLICATIONS

Although many questions remain, there has been considerable progress in elucidating underlying mechanisms of allergic responses at both the cellular and molecular level. Based on these insights, it should be possible to rationally dissect the pathogenesis of allergic diseases so that novel therapies based on mechanisms of disease can be developed. Recent studies particularly in severe asthma indicate the complex heterogeneity of inflammatory responses that lead to a common clinical phenotype, e.g. wheezing, and therefore the importance of developing new therapies based on disease mechanisms. It is clear that early treatment of inflammation is one important clinical take-home message for pediatricians, and long standing poorly controlled inflammation with associated airway remodeling in asthma or lichenification in AD can result in refractoriness to conventional therapy. In the future, new techniques are needed to monitor the response to therapy, as current approaches using clinical symptom scores or physiologic techniques do not accurately reflect the qualitative nature, magnitude, or extent of allergic inflammation.

The management of chronic allergic diseases requires a multipronged approach that includes the control of environmental factors which trigger illness, pharmacologic therapy, monitoring responses to therapy, and education of patients and their parents to encourage adherence to the management plan. Exposure to allergens is known to induce the secretion of TH2-like cytokines. Therefore identification and elimination of allergens from the patient's environment or diet can be considered an immunomodulatory approach toward reducing host production of TH2 cytokines. Furthermore, immunotherapy to aeroallergens have been demonstrated to increase the production of TH1 cytokines while decreasing TH2 cytokines(93).

Antiinflammatory agents, particularly topical GCs, form the cornerstone of therapy in the management of allergic diseases. The primary mechanism by which GCs act to inhibit inflammation likely relates to their capacity to effectively inhibit the gene expression and production of multiple proinflammatory cytokines and chemokines(94). Certain patients however, fail to respond to combined topical and/or systemic GC treatment(78). As discussed above, the majority of patients with steroid resistance have high level immune activation with certain cytokines, which alter GCR binding affinity of their T cells. These patients require the development of alternative therapeutic approaches for control of their ongoing allergic inflammation.

Many of these emerging therapeutic strategies are directed at either down-regulation of TH2 cytokines, such as IL-4 and IL-5, or augmentation of TH1 cytokines, such as IFN-γ and IL-12, that inhibit allergic responses (Table 3). The recent development of humanized anti-IgE antibodies, which are nonanaphylactogenic, also offers the hope that it may be possible to eliminate or reduce IgE responses(95). However, eliminating the IgE response may have less importance in patients with ongoing cell-mediated responses. Thus, it may be necessary to combine several approaches to effectively interrupt the complex inflammatory pathways associated with allergic diseases. In any case, it is clear from Table 3 that we are entering into an exciting era which holds enormous promise for fundamental changes in the treatment of this group of illnesses.

Table 3 Potential therapeutic approaches targeting the allergic response*
Full size table

In closing, I would like to thank and acknowledge my many collaborators over the years listed as co-authors on my cited papers. I would particularly like to thank my colleagues at National Jewish Medical and Research Center in Denver who have made important contributions to our work discussed in this review. These include Erwin Gelfand who kindly nominated me for this prestigious award, Stanley Szefler who has been a close collaborator in our studies on steroid resistance, and Mark Boguniewicz for his assistance in our studies on atopic dermatitis. A special thanks also goes to Qutayba Hamid in Montreal for his help in many of our studies on the immunopathology of allergic diseases, and Raif Geha who first attracted me to the field of allergy-immunology and served as my early mentor in Boston. I would also like to thank my family, particularly my parents, Moo Kit Tsui and Kwok Choy Leung, who first taught me the importance of hard work and education, as well as my wife Susan who has been a constant source of support and encouragement throughout my career. Finally, this review is dedicated to all our patients who have served as a “reality check” for those exciting observations we make at the laboratory bench and move us to exceed even our own expectations!