Viral respiratory illnesses associated with wheezing are extremely common during early life, and remain a frequent cause of morbidity and hospitalization in young children [1] . While many children who wheeze with respiratory viruses during infancy outgrow the problem, most children with asthma and reductions in lung function at school age began wheezing during the first several years of life [2] . Whether symptomatic viral infections of the lower respiratory tract are causal in asthma development or simply identify predisposed children remains a controversial issue. Wheezing illnesses caused by respiratory syncytial virus (RSV), particularly those severe enough to lead to hospitalization, have historically been associated with an increased risk of asthma at school age [3, 4] . However, with the development of molecular diagnostics, human rhinovirus (HRV) wheezing illnesses have been recognized more recently as a stronger predictor of school-age asthma than RSV [5, 6] . In this chapter, we will review the impact of virus infections during early life, focusing primarily on RSV and HRV, and their potential roles in asthma inception.
Infections with respiratory viruses are the leading cause of wheezing during early childhood. In fact, with current molecular diagnostics, a viral pathogen can be identified in at least 90% of wheezing episodes during the first several years of life [6] . All children are infected with respiratory viruses during early life and up to 50% will have a lower respiratory illness with wheezing at least once before school age [2] . A number of environmental factors have been linked to risk of wheezing illnesses during early life. The Tucson Children's Respiratory Study (TCRS), a prospective unselected birth cohort, identified a number of risk factors for wheezing during early childhood. Older siblings and daycare, both associated with increased exposure to respiratory viruses, and tobacco smoke exposure were all associated with increased risk for early life wheezing illnesses, whereas breastfeeding was protective [7] .
The most common viruses identified during early life wheezing illnesses are respiratory syncytial virus (RSV), human rhinovirus (HRV), and multiple viruses. Less common causes include parainfluenza, metapneumovirus, coronavirus, influenza, bocavirus and adenovirus.
RSV is seasonal, with peak infection during winter in the United States. RSV is ubiquitous, with nearly all children infected by two years of age [8] . A subset of these children has more severe illnesses, and infection with RSV leading to bronchiolitis and/or pneumonia is a primary cause of hospitalization during the first year of life. Children hospitalized with RSV tend to be younger than children hospitalized for other respiratory viruses. Other risk factors for RSV hospitalization include prematurity, male gender, daycare attendance, and tobacco exposure [9] .
In contrast to RSV, HRV infections occur throughout the year. HRV is the leading cause of bronchiolitis leading to hospitalization in infants outside of the winter bronchiolitis season [10] . Compared to RSV, children hospitalized with HRV tend to be older and are more likely to have wheezed previously [11] . They also often have more atopic risk factors or characteristics, including eczema, allergic sensitization, and parental asthma.
Traditionally, HRVs have been divided into two groups (A & B) with about 100 known serotypes. Many HRVs do not grow well in traditional cell culture, and this is particularly true for a novel group of HRVs, categorized as Group C HRVs. Molecular diagnostics and characterization of HRVs have clearly demonstrated that the number of HRV species have previously been dramatically underestimated [12] . In fact, numerous recent reports have identified novel Group C HRVs and implicated them in both upper and lower respiratory tract illnesses, frequently including illnesses severe enough to lead to hospitalization [12] [13] [14] [15] [16] .
The role of respiratory syncytial virus (RSV) in asthma RSV lower respiratory tract illnesses, particularly those severe enough to lead to hospitalization, are associated with an increased risk of asthma at school age [3, 17] . This observation has prompted the idea that RSV lower respiratory illness may be causal in asthma development. In this regard, Sigurs et al. utilized a case-control methodology to examine the risk of asthma and allergy development following RSV bronchiolitis severe enough to lead to hospitalization. They found that severe RSV bronchiolitis was associated with a significantly increased risk of both asthma and allergy at 13 years of age [17] , although some have suggested that the rate of asthma and allergy in their control population was lower than expected.
A large unselected retrospective cohort study in Tennessee recently reported findings in support of a causal role for RSV bronchiolitis in asthma inception [18] . In this study, Wu et al. reported that children born 120 days prior to the peak of RSV season not only had the greatest risk of hospitalization for lower respiratory tract illness, but also the greatest risk of asthma between 4 and 5.5 years of age.
However, several studies argue against a causal role for RSV in asthma inception. A large twin registry in Denmark was utilized to assess the relationships between severe RSV illness and asthma development [19, 20] . The data from this study suggests that severe RSV illnesses and asthma share a common predisposition, and while RSV can lead to short-term recurrent wheeze, it is not causal in long-term asthma development. Similarly, while the Tucson Children's Respiratory Study identified RSV wheezing illnesses during the first 3 years of life as an independent risk factor for wheezing at 6 years of age, this relationship weakened over time and was no longer significant by 13 years of age [3] . Thus, early RSV wheeze may lead to a recurrent wheeze phenotype, but it is less commonly associated with life-long asthma. In this regard, it will be interesting to determine whether the relationships reported from the Tennessee population based study at 4 to 5.5 years of age persist over time.
Recently, molecular diagnostics have allowed several groups to demonstrate that wheezing illnesses caused by HRV are potentially a more robust predictor of asthma development than episodes caused by RSV. Kotaniemi and colleagues reported that children who were hospitalized with HRV wheezing illnesses during the first 2 years of life were at approximately four-fold increased risk of childhood asthma when compared to children hospitalized with wheezing associated with other viruses [5] .
Two birth cohort studies have recently identified outpatient HRV wheezing illnesses as important predictors of childhood asthma development as well. The Childhood Origins of ASThma (COAST) study, a high-risk birth cohort examining the role of respiratory viruses and immune dysregulation in the development of asthma and other allergic diseases [21] identified HRV wheezing illnesses during the first year of life as significant predictors of wheezing in the third year of life [22] and asthma at age six years [6] . In a similar high-risk birth cohort in Australia, Kusel and colleagues reported that HRV wheezing illnesses during the first year of life were associated with increased asthma risk at age five years, although this finding was restricted to children who developed aeroallergen sensitization by age two years [23] . In the COAST study, while HRV wheezing illnesses during infancy were an independent predictor of asthma development, children who wheezed with HRV and had aeroallergen sensitization during infancy had the greatest risk of asthma at school age [6] .
The COAST study has continued to evaluate specific viral etiology and timing of wheezing illnesses as the children have progressed through early childhood. It appears that the age at which HRV wheezing illnesses occur has significant prognostic value with regard to subsequent asthma risk (Table 1) . Children who wheezed with HRV during the first year of life had about a threefold risk of having asthma at age six years. HRV wheezing during the second year of life was associated with a more pronounced, about a seven-fold, increase in asthma risk, while wheezing with HRV infection during the third year of life was associated with a dramatic (OR about 32) increase in asthma at school age. Remarkably, nearly 90% of children who wheezed with HRV during the third year of life had asthma when they reached school age [6] . Long-term follow up of these children to determine whether these relationships persist into the teenage years and beyond will prove very informative.
Preschool children with virus-induced wheezing utilize a disproportionate amount of healthcare resources compared to older children and adults with asthma [1] in part because they do not respond as well to conventional asthma therapies. This no doubt is also due to the heterogeneity of this population, including children who will 'outgrow' their wheezing as well as children who will continue to wheeze into school age and beyond.
One interesting observation from the Preventing Early Asthma in Kids (PEAK) study, a trial comparing treatment with low-dose fluticasone to placebo in high-risk children with recurrent wheezing, was that for those children who did not outgrow their wheezing, there was a progressive increase in lower respiratory symptom burden as these children advanced through the preschool years [24] . Importantly, multiple cohort studies have demonstrated that there are long-lasting effects on lung function for children who begin wheezing before three years of age and develop persistent disease. In the TCRS, children who began wheezing before age three and continued to wheeze at school age had reductions in lung function at age six years [2] that persisted at least to the teenage years [25] . Illi and colleagues, through careful assessment of allergic sensitization and exposure, demonstrated that children who wheezed during the preschool years, were sensitized to aeroallergens, and continued to wheeze, had impairment in lung function present to at least age thirteen years [26] .
However, due in part to the invasive nature of the procedures necessary to directly study the lower airway, there is a paucity of data to help us understand the underlying pathogenesis of loss of lung function in these children who start with recurrent virus-induced wheezing but develop persistent disease. Nonetheless, a few studies may shed some light on the inflammatory processes that occur in these children. Krawiec and colleagues performed bronchoscopy and bronchoalveolar lavage (BAL) on children, median age 15 months, with recurrent wheezing, in addition to a group of healthy controls. These studies demonstrated that children with persistent wheezing had increased inflammatory cells and markers of inflammation compared to healthy controls, but did not have the characteristic eosinophil predominance seen in older children and adults with asthma [27] . Similarly, Saglani and colleagues performed bronchoscopy and biopsy in children, median age 12 months, with persistent wheezing, and were unable to demonstrate either eosinophilic inflammation or reticular basement membrane (RBM) thickening, both common features of asthma seen in older children and adults [28] . In contrast, in further studies of older children with recurrent wheezing, median age 29 months, they were able to demonstrate the presence of both eosinophilic inflammation and RBM thickening when compared to a group of controls. This suggests that inflammatory changes and remodeling typical of asthma develop between one and three years of age in recurrent wheezing children, and highlights the potential importance of environmental exposures, such as virus infections, during this period.
A number of animal models have suggested mechanisms by which virus infection in early life could cause asthma inception. In fact, the COAST study was based on a rat model of virusinduced airway dysfunction in which a histological and physiologic asthma phenotype could be induced only in a genetically predisposed Th2-biased strain when they were infected at a critical time in the development of both the lung and/or the immune system (weanling age) [29] . However, there is limited data in humans demonstrating distinct mechanisms for causality of virus infections in asthma inception. Several recent human studies suggest plausible mechanisms by which HRV could be involved in airway remodeling and asthma inception.
Leigh and colleagues demonstrated that in vitro infection of airway epithelial cells with HRV led to increased production of several mediators involved in airway remodeling, including amphiregulin, activin A, and VEGF [30] . Importantly, the authors also found increased VEGF in nasal lavage fluid of volunteers with natural HRV infections. In another epithelial cell model, Zhu and colleagues found that HRV infection led to TLR-3-dependent mucin production and up-regulation of epidermal growth factor receptor, a prominent component of epithelial repair [31] . Finally, epithelial cell gene expression studies have identified a number of potential novel pathways involved in the response to HRV infection. Proud and colleagues found that HRV infection led to up-regulation of many genes, perhaps most notably in interferon and airway repair and remodeling pathways [32] . Bochkov and colleagues similarly demonstrated that HRV infection led to up-regulation of a number genes involved in interferon and airway remodeling pathways [33] . Notably, in this study, differences between asthmatics and controls were similar prior to and during HRV infection. In sum, while data that demonstrates causality of HRV infection in asthma inception is limited, a number of plausible mechanisms by which recurrent HRV infection could lead to airway damage and remodeling have been identified, and are deserving of further study.
If viruses are causal in asthma inception, it is likely that this is particularly true in susceptible hosts (Figure 1) . A so-called "two-hit" hypothesis has been proposed [21] whereby children with immune dysregulation (favoring an allergic phenotype) who develop a lower respiratory viral illness during a critical time in development, go on to an asthma phenotype. Allergic sensitization during early childhood has been identified by a number of studies as an important risk factor for asthma development [6, 23, 26] . In older children with asthma, allergic sensitization is a risk factor for more severe upper and lower respiratory viral illnesses [34, 35] . A number of observations may help explain host susceptibility to virus-induced wheezing and asthma exacerbations.
Interferons are known to play an important role in the host response to virus infections, and there is growing data to support a prominent role for interferons in the pathogenesis of virusinduced wheezing and asthma. Gern and colleagues linked immature peripheral blood interferon-γ responses to both mitogens and viruses at birth with recurrent wheezing during infancy [36] . Similarly, Stern and colleagues connected reduced peripheral blood interferonγ responses to mitogen at nine months of age to an increased risk of wheezing in toddlers and school age children [37] . Interestingly, peripheral blood mononuclear cells from adult asthmatics produce less interferon-γ in response to HRV infection than controls [38] and the degree of impairment correlates with airway hyper-responsiveness and degree of airway obstruction [39] .
A number of other investigators, through experiments on airway epithelial cells and in peripheral blood, have identified deficiencies in interferons as an important potential mechanism for asthmatics' susceptibility to more severe HRV illnesses. Wark et al. demonstrated that bronchial epithelial cells from atopic asthmatics infected with HRV in vitro have deficient interferon-β responses, a type I interferon, when compared to controls. In this study, the decreased interferon-β response was associated with both delayed apoptosis and increased viral replication, which was overcome with the addition of exogenous interferon-β [40] . Additionally, lower levels of interferon-α, another type I interferon, responses have been demonstrated in the peripheral blood of both children [41] and adults [42] with atopic asthma versus non-atopic controls. Finally, Contoli and colleagues found that bronchial epithelial cells and alveolar macrophages from atopic asthmatics had impaired interferon-λ, a type III interferon, responses to human rhinovirus infection, and that these responses correlated with FEV 1 decline during exacerbations [43] . While this collection of studies has identified defects in interferon pathways as potential important contributors to early childhood wheezing and asthma, unfortunately, attempts to replicate these findings have not always been successful. Two recent studies found no difference in epithelial cell responses between asthmatics and controls [33, 44] .
Another important component of the innate immune response to virus infection is barrier function. Barrier function of the skin is important in the pathogenesis of atopic dermatitis, with mutations in filaggrin identified in a number of patients with severe atopic dermatitis [45] . Interestingly, ongoing allergic inflammation may lead to a local or acquired filaggrin deficit [46] . Several studies have highlighted how disruption of the airway epithelium could alter host antiviral responses and severity of lower respiratory illnesses. In fact, mutations in filaggrin have been identified as a risk factor for asthma and asthma exacerbations in children [47, 48] . A unique model of airway injury took this observation one step further in demonstrating the potential consequences of impairment in airway barrier function through physical injury. Jakiela and colleagues, utilizing in vitro epithelial cell culture, showed that mechanical injury to the epithelia resulted in increased replication of HRV [49] . This is a potential mechanism by which various host factors or environmental exposures such as allergen, tobacco smoke, and previous virus infection, could lead to impairment in barrier function, increased viral replication, and more severe lower respiratory illnesses.
Respiratory viruses are a major cause of morbidity in young children. Wheezing illnesses caused by viruses during early life are the most common initial presentation of childhood asthma. Recent evidence suggests that wheezing with HRV is the most robust predictor of subsequent asthma. However, whether HRV is causal in asthma inception is an open question. Plausible mechanisms of causality have been identified, but are in need of further study. The most definitive way to prove causality would be to prevent HRV infection and demonstrate reductions in asthma development, but vaccines for HRV have proven elusive, in part due to the nearly 150 existing species of HRV. Studies attempting to identify more virulent or 'asthmagenic' forms of HRV are ongoing and could prove very informative. Novel preventive and therapeutic strategies are sorely needed to prevent or interrupt the progression from virusinduced wheezing to persistent asthma. Immunol Allergy Clin North Am. Author manuscript; available in PMC 2011 November 1.