New millennium: The conquest of allergy (Supported by a grant from Novartis Pharmaceutical Corp., East Hanover, NJ)

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1 New millennium: The conquest of allergy (Supported by a grant from Novartis Pharmaceutical Corp., East Hanover, NJ) Series editors: Donald Y. M. Leung, MD, PhD, Stanley J. Szefler, MD, and Harold S. Nelson, MD The role of viral infections in the natural history of asthma James E. Gern, MD, a and William W. Busse, MD b Madison, Wis Viral infections have been related to the inception of recurrent wheezing illnesses and asthma in infants and are probably the most frequent cause of exacerbations of established disease in older children and adults. The well-recognized clinical effects of viral infections are mainly caused by virus-induced immune responses. Clinical studies of natural and experimentally induced viral infections have led to the identification of mechanisms of inflammation that could be involved in producing airway obstruction and lower airway symptoms. In addition, host factors that are associated with more vigorous viral replication or severe clinical illness are beginning to be identified. Advances in molecular virology and our understanding of immune responses to viral infections may lead to the development of new strategies for the prevention and treatment of virus-induced respiratory disorders. (J Allergy Clin Immunol 2000;106: ) Key words: Asthma, virus, respiratory syncytial virus, rhinovirus Asthma is a disease characterized by bronchial inflammation, and as such, the natural history of asthma can be strongly influenced by allergens, irritants, or infections that promote inflammation of the small airways. These distinct stimuli induce or exacerbate inflammation through unique pathways and have the potential either to affect the initiation of asthma or trigger acute airway obstruction in affected individuals. Infections with respiratory viruses, particularly respiratory syncytial virus (RSV) and parainfluenza virus, are the principal causes of wheezing illnesses in infants and in some children herald the onset of asthma. In fact, it has been proposed that certain childhood infections provide a sufficient immune response to influence the eventual development of the immune system and modify the risk of subsequent allergy and asthma. The influence of viral infections on asthma continues later in life, and in children and adults with From the Departments of a Pediatrics and b Medicine, University of Wisconsin Medical School, Madison. Support by grants AI40685 and AI34891 from the National Institutes of Health. Received for publication Mar 28, 2000; revised May 12, 2000; accepted for publication May 13, Reprint requests: James E. Gern, MD, H4/438 University of Wisconsin Hospital, Madison, WI Copyright 2000 by Mosby, Inc /2000 $ /1/ doi: /mai Abbreviations used ECP: Eosinophil cationic protein ICAM: Intercellular adhesion molecule IL1-ra: IL-1 receptor antagonist NO: Nitric oxide RSV: Respiratory syncytial virus RV: Rhinovirus established asthma, viral respiratory infections frequently trigger lower respiratory tract symptoms and exacerbations of asthma. This review will discuss the epidemiologic associations between viral infections and asthma, unique features of virus-induced inflammation and their relationship to reduced airway function, and interactions with the pathogenic processes of allergy and asthma. Finally, prospects for the development of specific therapies for virus-induced wheezing illnesses will be explored. DO VIRAL INFECTIONS IN INFANCY ALTER THE RISK OF ALLERGY OR ASTHMA? RSV and bronchiolitis It has long been recognized that infants with wheezing illnesses, typically RSV bronchiolitis, are at increased risk for additional episodes of wheezing in infancy and asthma in childhood. Prospective evaluation of these children, including the measurement of lung function before the onset of illness, has identified risk factors for persistent wheezing and asthma (Table I). 1,2 Lung functions of infants with persistent wheeze are initially normal but are reduced by the age of 6 years. These findings suggest an interaction between virus-induced wheezing and either atopy or allergen-induced inflammation to favor the development of asthma. Because nearly all children contract RSV infections in the first 3 years of life, it is likely that the severity of the infection, and not the infection per se, is related to the subsequent risk of asthma. Several hypotheses have been proposed to explain a link between severe RSV infections and increased asthma. First, RSV infections could promote immune responses that promote allergic inflammation in susceptible individuals. Supporting this 201

2 202 Gern and Busse J ALLERGY CLIN IMMUNOL AUGUST 2000 TABLE I. Risk factors for persistent wheezing Eczema Rhinitis apart from colds Male sex Elevated total serum IgE (age 9 mo) Maternal smoking Maternal asthma TABLE II. Predictive value of immune responses during bronchiolitis Observation Outcome RSV-specific IgE 11 ECP (serum or nasal secretions) 12,13 IFN-γ secretion (PBMC, ex vivo) 14 Eosinophilia or lack of eosinopenia 15 Transient IgE 15 Risk of recurrent wheezing in infancy Persistent wheezing age 6 y hypothesis are studies to demonstrate that a particular RSV protein (protein G) can induce a T H 2-like immune response, 3,4 that viral activation of the antiviral protein kinase in B cells can induce isotype switching to IgE in vitro, 5 and that viral infections can sometimes promote allergen sensitization in animals. 6 However, studies to determine whether RSV infections enhance allergy in humans have yielded conflicting results Second, it could be that RSV and other severe lower respiratory tract infections promote asthma because of their anatomic location. The severe lower airway inflammation produced by these infections could affect lung development, initiate airway remodeling, or somehow target allergic inflammation to the lower airway in atopic individuals. Third, it is possible that children with severe RSV infections have an underlying immune system defect that has two consequences: (1) promotion of allergen sensitization and (2) relatively ineffective antiviral responses, leading to increased viral replication and more severe lower respiratory involvement during an RSV infection. Although there are no studies to prospectively evaluate immune responses before RSV infection and compare them with the severity of subsequent disease, there is evidence that immune responses during bronchiolitis are distinct in those infants who go on to have recurrent wheezing or asthma (Table II) These observations raise the question of whether these abnormal immune responses, which have also been associated with atopy, were preexisting or were caused by the acute viral infection. Influence of other infections on the development of asthma A consistent finding that has been noted in multiple epidemiologic surveys is that the risk of allergen sensitization is inversely related to the number of older siblings in the family, and these findings have also been extended to include asthma. Because large family size, and having multiple older siblings in particular, presumably increases the exposure to infectious diseases in early childhood, studies have been conducted to test the hypothesis that exposure to certain infectious diseases in early childhood can reduce the risk of allergy and asthma, perhaps by promoting the development of a protective T H 1-like immune response (Fig 1). 17 One of the first of these studies found that Japanese schoolchildren with a strong positive tuberculin skin test response after BCG vaccination, possibly signifying exposure to tuberculosis, also have reduced rates of allergy and asthma. 20 In addition, long-term follow-up of survivors of a measles epidemic in an African village indicated that measles infection in early childhood was associated with reduced risk of allergen sensitization, 21 although this finding has not been reproduced in two studies of measles infections in western societies. 22,23 Finally, serologic evidence of hepatitis A in Italian military recruits was associated with a reduced rate of allergy and asthma. 24 Attendance in day care, which clearly increases exposure to infectious diseases, may have complex effects on wheezing illnesses in childhood. Initially, it is clear that day care attendance increases the risk of recurrent wheezing episodes in infants and young children. 25,26 However, long-term followup of children who attended day care before the age of 6 months suggests that the risk of asthma in later childhood may actually be reduced. 27 If confirmed, these findings would suggest that the effects of day care on wheezing illnesses may depend on both the age of exposure and the age at follow-up. Together, these epidemiologic studies suggest that potential effects of viral infections on the developing immune system could depend on several factors, including type of viral infection, infection of the respiratory tract, age, and factors related to the host immune response. To determine the true relationships between viral infections, antiviral immune responses, and the development of asthma will require prospective evaluation of these factors in a controlled study and the development of new antiviral interventions to enable the associations identified in epidemiologic studies to be tested. VIRAL INFECTIONS AS TRIGGERS FOR ESTABLISHED ASTHMA Epidemiology In children, adults, or both with existing asthma, the association between viral infections and acute wheezing was first demonstrated more than 25 years ago by detecting viruses by using either serology or culture during wheezing episodes. 28 However, many respiratory viruses, particularly rhinovirus (RV), are difficult to detect by using standard virologic methods, and thus these initial studies likely underestimated the effect of respiratory viral infections on wheezing. In more recent studies the use of extremely sensitive RT-PCR assays 29,30 has established the dominating importance of respiratory infections, particularly RV, in causing exacerbations of asthma. By using RT-PCR in addition to standard techniques to

3 J ALLERGY CLIN IMMUNOL VOLUME 106, NUMBER 2 Gern and Busse 203 FIG 1. The hygiene hypothesis. 17 According to this theory, immune responses to viruses and perhaps other organisms that generate T H 1-like cytokines, such as IL-12 and IFN-γ, suppress T H 2 responses that are predominant in the neonatal immune system. When many siblings are present, repeated infections, through the activities of T H 1-like cytokines, would help T-cell immune responses to mature into a T H 1-like phenotype that would be less likely to favor allergen sensitization. identify respiratory viruses, viruses have been detected in up to 85% of wheezing episodes in children and in approximately half of exacerbations in adult asthmatic subjects; the virus most commonly detected was RV Virus-induced exacerbations of asthma may be severe; indeed, seasonal patterns of upper respiratory virus prevalence correlate closely with hospital admissions for asthma, especially in children. 35,36 Additional epidemiologic studies conducted in acute care settings indicate that viral infections and markers of atopy have independent and synergistic effects on the risk for wheezing illnesses in childhood. In studies evaluating wheezing infants and children in a hospital emergency department, wheezing children older than 2 years of age were more likely to have evidence of atopy (positive RAST test, nasal eosinophilia, or elevated eosinophil cationic protein [ECP] in nasal secretions) or evidence of an RV infection (nasal secretions tested positive by using RT-PCR) when compared with children without wheezing. 34,37 Moreover, the strongest odds for wheezing were in children with both risk factors, suggesting that there are synergistic interactions either between allergy or eosinophilic inflammation and viral infection in the pathogenesis of wheezing with respiratory illnesses. Clinical effects of viral infections in asthma The upper respiratory manifestations of common cold viruses are quite similar, regardless of the presence of asthma, and usually begin with a sore throat, followed by rhinorrhea and nasal congestion, sneezing, and cough. Lower airway symptoms and reduced peak flow in asthmatic subjects begin soon after the onset of the cold and last an average of 14 days. 31 There is evidence that changes in lower airway physiology caused by viral infections may have some unique features compared with the effects of poor asthma disease control. Reddel et al 38 measured peak flow and calculated diurnal variability on 43 patients whose asthma was poorly controlled on entry into the study. These subjects were treated with inhaled budesonide for 18 months, and the effects of the therapy and of asthma exacerbations on peak flow measurements were analyzed. Forty exacerbations were noted in 26 subjects, and although viral cultures were not performed, all but two exacerbations were preceded by symptoms of an upper respiratory infection. Patterns of peak flow readings were distinct during the run-in period, when the asthma was poorly controlled, and during exacerbations of asthma (Fig 2). Poor asthma control caused decreased lung function and increased peak flow variability (21.3%), and the authors point out that increased diurnal variability has also been observed when inhaled corticosteroid therapy has been withdrawn and after allergen exposure. In contrast, diurnal variability was not increased during exacerbations of asthma (7.7% vs 5.3% during stable asthma), although both morning and evening peak flow readings were lower. Furthermore, β-agonist responsiveness was demonstrated during poorly controlled asthma, but the bronchodilator response to β-agonist was diminished during the exacerbations. Assuming that most of the exacerbations were in fact caused by viral infections, these findings indicate that viral infections induce distinct perturbations of lower airway physiology that include reduced peak flow, normal variability, and impaired response to β-agonist. The mechanisms of these effects were not determined but could include pronounced induction of airway edema or mucus production or interference with β-adrenergic receptor function. Regardless of the mechanism, the observed unresponsiveness to β-agonist therapy under-

4 204 Gern and Busse J ALLERGY CLIN IMMUNOL AUGUST 2000 FIG 2. Distinct peak expiratory flow (PEF) trends in poorly controlled asthma compared with asthma exacerbation associated with upper respiratory tract infection (URI). ICS, Inhaled corticosteroid therapy. Adapted from Reddel et al. 38 scores the need to develop specific therapies for virusinduced exacerbations of asthma. Lessons from experimental infection with RV Experimental infection with RV and other respiratory viruses has provided a convenient means to study the pathogenesis of wheezing with viral respiratory infections while controlling for confounding factors, such as the time and dose of inoculation and the type of virus producing the illness. There are, however, important differences between natural and experimental infections with RV. For example, natural infections produce a broader range of illness severity, and it is likely that some or all of the viruses used to inoculate volunteers are attenuated and produce relatively mild clinical illnesses. Perhaps, as a result of this, it is unusual for subjects with asthma to experience clinically significant exacerbations of asthma or changes in FEV 1, 39 although aerosol inoculation with RV has been shown to increase peak flow variability. 40 Despite these limitations, experimental inoculation with viruses, such as RV16, have enabled the analysis of viral effects on upper and lower airway physiology, as well as a system to evaluate potential interactions between allergen exposure and viral infection. Do viruses that trigger asthma infect the lower airway? There is little doubt that some respiratory viruses (ie, influenza, RSV, and parainfluenza virus) infect lower airway tissues and cause tissue inflammation and lower airway obstruction. Several lines of evidence suggest that RV infections can also extend into the lower airway and that effects on asthma are initiated by ensuing lower airway inflammation. For example, case reports and epidemiologic studies have linked RV to lower airway syndromes, such as bronchitis, bronchiolitis, and pneumonia In addition, experimental RV inoculation increases lower airway inflammation, as indicated by increased submucosal lymphocytes and epithelial eosinophils in bronchial biopsy specimens, 47 and increases in the number of neutrophils in bronchial lavage fluid. 48 Furthermore, RV can replicate in cultured bronchial epithelial cells and at temperatures approximating those found in the lower airway. 49 Finally, although RV is difficult to culture from lower airway secretions obtained through bronchoalveolar lavage, 50 RV RNA was detected in bronchoalveolar lavage cells from volunteers after experimental inoculation with RV Interpretation of these findings is limited, however, by the necessity of passing the bronchoscope through the upper airway to get lower airway samples. Nevertheless, these data suggest that, at least under some conditions, RV viral infections extend into the lower airway and induce bronchial inflammation that could contribute to virus-induced exacerbations of asthma. Viral effects on airway responsiveness. One of the cardinal features of asthma is airway hyperresponsiveness, which is defined as the increased sensitivity of the airways to bronchoconstriction to irritants or allergen. It is therefore of interest that several types of viral infections, including experimental infection with certain strains of RV (RV16 but not RV39 or RV Hanks), influenza, and RSV, can cause changes in airway responsiveness to histamine, methacholine, or allergen. 52 For example, Cheung et al 53 inoculated 14 subjects with mild asthma either with RV16 (type 16 rhinovirus) or placebo and found that airway responsiveness transiently increased during the acute infection and returned to baseline levels by 1 week after the inoculation. In addition to increasing the sensitivity of the airway, RV16 infection also increased the maximal response to inhaled methacholine for up to 15 days after the acute infection. Furthermore, there is evidence that experimental infection with RV16 induces greater changes in airway responsiveness in volunteers with respiratory allergy 54,55 or mild allergic asthma, 47 suggesting a potential mechanism for the greater severity of the lower airway effects of naturally acquired RV infections in patients with asthma. Interactions between viral infections and allergic inflammation. Several studies have been conducted to test the hypothesis that there are specific interactions

5 J ALLERGY CLIN IMMUNOL VOLUME 106, NUMBER 2 Gern and Busse 205 FIG 3. RVs approaching ICAM-1 receptors on the surface of an airway epithelial cell. This figure shows the molecular surface structure of RV14, as solved by x-ray crystallography, approaching an artist s rendition of an epithelial cell (gray) with several ICAM-1 molecules (gold) on the surface. The dark grooves that radiate out from the 5-fold axis of symmetry depict canyons that contain the ICAM-1 binding sites. The computer graphic representations of RV14 and ICAM-1 were provided by Dr Jean Yves Sgro (Institute of Molecular Virology, University of Wisconsin-Madison), with artwork by Robert J. Gordon (Department of Pediatrics, University of Wisconsin-Madison). between allergen- and virus-induced inflammation. In one set of studies, lower airway responses to allergen in subjects with allergic rhinitis were evaluated before, during, and after experimental inoculation with RV. 54,56-59 RV infections increased airway responsiveness to histamine, methacholine, and allergen and increased the probability of development of a late-phase allergic response after whole lung antigen inhalation. 59 In additional studies with bronchoscopy, RV infection caused an increased release of histamine into the lower airways after segmental allergen challenge and augmented the recruitment of both total leukocytes and eosinophils into the airway 48 hours after allergen challenge. 57 These effects were noted only in allergic individuals, indicating that RV infection specifically enhanced allergen-induced responses in the airway. The possibility that exposure to allergen might enhance the severity of viral illness has also been evaluated in two separate studies. 60,61 Neither nasal challenge with allergen nor natural exposure to ragweed during the peak of the season accentuated the severity of experimentally induced infections with RV16 and influenza, respectively. In fact, nasal allergen challenge delayed the onset and shortened the duration of RV16-induced colds. Although the mechanism for this effect is uncertain, it is conceivable that a component of the allergen-induced immune response has antiviral activity, and in support of this theory, it has been demonstrated that eosinophilderived neurotoxin, a granular protein released on eosinophil activation, has RNase activity and can inhibit replication of RSV. 62 Alternately, it is possible that the preceding allergen-induced inflammation led in turn to the activation of anti-inflammatory mechanisms (eg, secretion of IL-10) and that the presence of these factors at the time of inoculation caused attenuation of the viral illness. Whatever the mechanism, the findings of this study do not suggest that allergic inflammation leads to more severe respiratory viral infections or that this mechanism contributes to the pathogenesis of virus-induced exacerbations of asthma. Host factors that contribute to the severity of respiratory viral infections. Interestingly, despite the use of standardized lots of viral inoculum, inoculation techniques, and the enrollment of subjects with no evidence of prior

6 206 Gern and Busse J ALLERGY CLIN IMMUNOL AUGUST 2000 FIG 4. Effects of respiratory virus infection on airway tissues (see text). AHR, Airway hyperresponsiveness. infection with the serotype used, there has been considerable individual variability in the severity of respiratory symptoms and in the amount of virus detected in nasal secretions after experimental inoculation. 54,56,59 This variability suggests that there are host factors, other than the quantity of virus-specific antibody, that contribute to the outcomes of viral respiratory infections. Because T-cell responses have been linked to the severity of viral infections in the response of viruses in animal models, 66 it seems likely that variability in T-cell responses could be influential in human subjects as well. To test for relationships between virus-induced lymphocyte responses and the outcomes of viral infections, PBMCs from seronegative volunteers were tested for virus-specific proliferation and IFN-γ secretion in tissue culture. 67 After the blood test, the volunteers were inoculated with RV16, and quantitative viral cultures were performed on samples of nasal lavage fluid obtained during the acute cold. The results of this study showed that vigorous RV-specific lymphocyte responses before the cold (either proliferation or IFN-γ secretion) were associated with reduced viral shedding after inoculation. These findings suggest that variations in virus-induced T-cell responses contribute to the individual variability in viral shedding during experimentally induced and perhaps naturally acquired RV infections in subjects with respiratory allergy or asthma. Additional studies are under way to determine whether T- cell responses and IFN-γ secretion to viral infection are different in patients with asthma and whether this could help to explain the increased lower airway sequela of RV infections in asthma. Virus-induced inflammation Although the precise mechanisms by which respiratory viruses, such as RV, cause symptoms are unknown, there is evidence to suggest that the immune response to the virus plays a major role in symptom pathogenesis. For example, RV infections do not cause extensive epithelial cell destruction, even when severe cold symptoms are present. 68 Second, the severity of respiratory symptoms correlates closely with the influx of inflammatory cells and increases in cytokines and mediators in nasal secretions. 48,69-72 Whether these factors are participating in symptom pathogenesis or are markers of severe disease has not yet been established. Third, studies in rodents have demonstrated that morbidity of viral infections can be amplified by the passive transfer of certain T-cell subsets or clones. 66,73-75 Although the immune responses to viruses are complex and involve multiple airway cells, cytokines, and mediators, there are a few cells and mediators that are likely to play key roles in this process. For example, the airway epithelial cell is the principal host cell for most respiratory viruses. Interestingly, RV can also infect airway smooth muscle cells and submucosal gland cells in tissue culture. 76,77 If these findings are confirmed in vivo, it would raise the possibility that viral infection could directly affect muscle cell responsiveness and airway mucus secretion. Unlike RSV and influenza, which can destroy large numbers of epithelial cells in vivo or in vitro, only a small subset of epithelial cells becomes infected with

7 J ALLERGY CLIN IMMUNOL VOLUME 106, NUMBER 2 Gern and Busse 207 Image available in print only. FIG 5. Two mechanisms for virus-induced lymphocyte activation. Respiratory viruses, such as RV, can bind to antigenpresenting cells, such as macrophages, B cells, or dendritic cells, through specific receptors (eg, ICAM-1 and the lowdensity lipoprotein receptor [LDLr]). Innate responses can be activated through the secretion of soluble factors, such as IFN-α, IL-12, and/or IL-18, leading to increased CD69 expression and IFN-γ secretion by T cells and natural killer cells. In addition, antigen presentation can activate a smaller subset of antigen-specific T cells. (Adapted from Gern JE, Vrtis R, Kelly EAB, Dick EC, Busse WW. Rhinovirus produces nonspecific activation of lymphocytes through a monocyte-dependent mechanism. J Immunol 1996;157: Copyright Used with permission.) RV. 78,79 By using in situ hybidization and RV-specific probes, it has been demonstrated that specialized nonciliated cells that overlie lymphoid follicles in adenoidal tissue express large amounts of intercellular adhesion molecule (ICAM) 1, the receptor used by over 90% of RV serotypes (Fig 3), and are especially susceptible to RV infection. 80 Determining whether the level of ICAM-1 expression or some other factor conveys susceptibility to these cells has yet to be determined. Viral replication within epithelial cells is a central event in the initiation of airway immune responses and inflammatory processes (Fig 4). Viral replication in the epithelial cell triggers intracellular signaling pathways, including proteolysis of IκB and activation of NFκB, leading to increases in the secretion of multiple cytokines, chemokines, and adhesion molecules. This is one area where virus- and allergen-induced inflammation overlap in that cytokines (TNF-α, G-CSF, and IFN-γ) and chemokines (IL-8, macrophage inflammatory protein 1α, and RANTES) that are increased in airway secretions during viral infections 48,72,84-86 can recruit and activate inflammatory cells (neutrophils, eosinophils, and activated T cells) 87,88 that have been linked to asthma. There is evidence that endothelial cells are also activated early during the course of upper respiratory tract infections and could contribute to airway dysfunction during respiratory illnesses through several pathways. First, upregulation of adhesion molecules by cytokines, such as TNF-α, is an important factor in the recruitment of inflammatory cells. Perhaps of equal or greater importance is the transudation of plasma proteins from the vascular tissue of the nasal mucosa, leading to increased nasal secretions and congestion. 89 Peak levels of plasma proteins, such as albumin and IgG, coincide with the time of maximal cold symptoms 89 and increases in bronchial responsiveness, 53 suggesting that similar processes occur in the lower airway. Activation of kinins has been advanced as a possible mechanism for cold-induced increased endothelial permeability 69,90 ; however, a bradykinin antagonist NPC 567 did not improve cold symptoms in a clinical trial. 91 Finally, vasodilation is likely to contribute to edema of the nasal mucosa and possibly the lower airway during viral respiratory infections. Leukotrienes are increased in nasal secretions during viral infections, 86 and studies are under way to determine whether leukotriene receptor blockers can relieve cold-related nasal congestion. Immediately after the transudation of fluids into the airway, products of goblet cells and mucin glands are increased. 89,92 Mechanisms of mucin gene upregulation and goblet cell degranulation are now being elucidated, and potent stimuli include cytokines (TNF-α, and IL-9) and bacterial cell wall components. 93,94 Activation of the epithelial growth factor receptor has been identified as a key mechanism for the differentiation and degranulation of goblet cells and may be activated by a number of different stimuli. 95 Because secreted mucus is a major contributor to upper and lower airway obstruction during viral respiratory infections, developing specific inhibitors of this process could be an important advance in the therapy of exacerbations of asthma and other disorders characterized by overproduction of mucus. As a result of epithelial-derived chemokines and increased expression of endothelial adhesion molecules, leukocytes are recruited into airway secretions during the acute stages of viral infections. Neutrophils are the main cells found in nasal and lower airway secretions during acute viral infections, 68,96 and increases in blood and nasal neutrophils correlate with cold and asthma symptom scores and cold-induced changes in airway hyperresponsiveness. 72 Once recruited to the airway, respiratory viruses can activate neutrophil inflammatory functions,

8 208 Gern and Busse J ALLERGY CLIN IMMUNOL AUGUST 2000 as indicated by enhancement of superoxide responses, chemotaxis, and adhesion. 97 In addition, proteases released by activated neutrophils are potent secretagogues for airway submucosal glands. 98 Although the neutrophil is the predominant cell in nasal secretions during acute viral infections, eosinophil granular proteins have also been detected in the nasal secretions of children with wheezing illnesses caused by RV or RSV. 84,99,100 In addition, increases in sputum ECP during the acute phase of experimentally induced RV infection correlated with increased airway responsiveness in a group of adult asthmatic subjects after experimental inoculation with RV Experiments conducted in vitro indicate that RV does not activate eosinophils directly, 102 and therefore it is likely that eosinophil activation is secondary to the activity of virus-induced mediators or cytokines secreted by T cells, epithelial cells, or other airway cells. 103 Mononuclear cells are also recruited to the upper and lower airway during acute viral infections. 47,104 Macrophages, which predominate in lower airway secretions, can bind RV in vitro and secrete cytokines that have antiviral (IFN-α) effects, proinflammatory (IL-1 and TNF-α) effects, or both. 105,106 Virus-activated macrophages and lymphocytes are likely to help clear viruses and virus-infected cells from the airway. Lymphocyte responses may be of particular importance because of their central role in orchestrating both allergic inflammation 107 and antiviral responses. Viruses can activate T cells either through innate or antigen-specific pathways (Fig 5). Innate immune responses to virus can occur rapidly (within hours) after exposure to virus and may be caused by soluble mediators, such as IFN-α, secreted by antigen-presenting cells. 103,108,109 Virus-specific T-cell responses are generally not detectable in the airway or blood until 7 to 10 days after inoculation with virus; however, T-cell responses to either influenza or RV can be cross-reactive among different viral serotypes Because different serotypes either of RV or influenza virus can cause many infections in the same host over a lifetime, T-cell reactivity to a particular strain of virus can be detected in some seronegative individuals. 67 As discussed previously, these mechanisms to rapidly activate T cells and natural killer cells may serve to enhance antiviral responses and limit viral replication. Finally, perhaps of equal importance to proinflammatory mechanisms are the pathways to downregulate virusinduced immune response to promote healing of the airway and lung tissue. Nitric oxide (NO), which is increased both in patients with asthma and patients with upper respiratory tract infections, could have complex effects on airway physiology. On the one hand, NO is a potent vasodilator and could increase airway wall edema and thereby increase airway obstruction in asthma. Effects of NO that could be beneficial include inhibition of viral (including RV) replication 113 and bronchodilation through relaxation of airway smooth muscle. The overall effect of NO may be beneficial because greater NO production during experimentally induced RV infections correlated with a smaller increase in airway responsiveness. 114 In addition, virus-induced secretion of cytokines with antiviral properties has been investigated. Respiratory viruses can stimulate IL-10 secretion, which can in turn downregulate antigen presentation by monocytes. 115 In addition, IL-10 secretion from PBMCs was impaired in allergic individuals after experimentally induced infection with influenza. 116 This suggests that allergic individuals may have reduced capacity to suppress virusinduced inflammation after the acute phase of infection. Finally, IL-1 receptor antagonist (IL-1ra) has been detected in the nasal secretions of volunteers infected with RV. 117 IL-1ra binds to IL-1 receptors without causing activation and thereby blocks the proinflammatory effects of active forms of IL-1. Interestingly, RV infection induces secretion of a brief and modest peak of IL- 1β 2 days after inoculation, whereas IL-1ra is secreted in much greater amounts for up to 3 days after inoculation. These findings suggest that IL-1ra may be an important regulator of inflammatory effects mediated by the IL-1 receptor during common cold infections. Effects of viral infections on neural regulation of the airway. Although neural mechanisms of virus-induced bronchoconstriction and airway obstruction are of great interest, they are particularly difficult to study in human subjects because definitive experiments often require the disruption of neural tissue. Consequently, much of our understanding has been gained through the use of animal models infected with respiratory viruses other than RV. Viral infections could potentially cause bronchoconstriction and increased airway responsiveness by enhancing parasympathetic bronchoconstrictive responses, by stimulating reflex bronchospasm or neuropeptide release from sensory C fibers, or by interfering with the function of nonadrenergic noncholinergic neurons, which produce the potent bronchodilator nitric oxide. Each of these mechanisms has been explored and is the subject of recent reviews. 52,118,119 Further advances in the understanding of virus effects on neural control of the airways await the development of new specific antagonists of neural pathways or the development of new experimental techniques. SUMMARY AND THERAPEUTIC IMPLICATIONS Viral respiratory infections exert considerable influence on airway function and asthma in all age groups. In infancy respiratory viruses, such as RSV, cause episodes of wheezing that may be recurrent but are largely transient. In addition, there are indications that early viral infections may be able to affect the development of the immune system and modify the subsequent risk of allergy and asthma. Finally, in children and adults with established asthma, common cold viruses, such as RV, frequently trigger acute symptoms of asthma. This model suggests that specific antiviral therapies could have a major effect on reducing the morbidity of wheezing illnesses in infancy and of asthma in older

9 J ALLERGY CLIN IMMUNOL VOLUME 106, NUMBER 2 Gern and Busse 209 children and adults. Moreover, strategies to prevent severe respiratory illnesses in infancy, either through vaccination or antiviral medications, could potentially reduce the incidence of asthma in childhood. Vaccination against influenza is widely recommended for patients with asthma, and vaccines for RSV are under development. Administration of a monoclonal neutralizing antibody to RSV (palivizumab) has proved to be an effective strategy to prevent RSV infection in premature and high-risk infants, but this therapy is too expensive for general use. 120 Greater understanding of the role of RSV infection in the initiation of asthma and risk factors for severe RSV infection is needed to design more comprehensive and cost-effective programs to prevent RSVrelated morbidity. Unfortunately, because there are more than 100 RV serotypes, standard vaccination is not an option for prophylaxis of RV-induced exacerbations of asthma. In the absence of an RV vaccine, antiviral agents have the potential to either treat or prevent virus-induced exacerbations of asthma, although there are still a few barriers to overcome. Experience with antiviral medications, such as the influenza neuraminidase inhibitors, has demonstrated that treatment needs to be started early in the disease course to produce clinical benefit, and similar restraints apply to new medications, such as soluble ICAM-1, that prevent RV binding or uncoating. 121 Another potential barrier is cost. Although prophylactic treatment with anti-rv medications during the fall and spring cold seasons may not be practical for normal individuals because of the anticipated high cost of the new medications, this approach could be cost-effective for children and adults at increased risk for RV-related morbidity because of asthma or chronic obstructive pulmonary disease. Whether the new antiviral agents that interfere with viral attachment or enzymes (3C protease inhibitors) 122 can prevent exacerbations of asthma if used preventively or, better yet, if started at the first sign of a cold, remains to be determined. Another critical challenge is to determine why individuals with allergy and lower airway inflammation are so susceptible to the effects of respiratory viruses, such as RV, that cause mild disease in normal individuals. Data in infants infected with RSV suggest that immune responses to this virus are distinct in children who go on to have recurrent wheezing and asthma, and host factors that are associated with greater viral replication in adults are beginning to be identified. Key challenges include establishing whether antiviral immune responses are determined solely by genetics or whether environmental factors play a significant role in their development and to ascertain which elements of the antiviral, or perhaps antiinflammatory, immune responses are impaired in patients with recurrent wheezing. Are immune abnormalities associated with asthma only in response to allergens, or is there also a fundamental defect in the immune response to respiratory viruses? The answers to these questions may provide new therapeutic targets for asthma and respiratory viral illnesses. REFERENCES 1. Martinez FD, Wright AL, Taussig LM, Holberg CJ, Halonen M, Morgan WJ, et al. 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