Interactions between allergic inflammation and respiratory viral infections

Signal transduction around thymic stromal lymphopoietin (TSLP) in atopic asthma

Thymic stromal lymphopoietin (TSLP), a novel interleukin-7-like cytokine, triggers dendritic cell-mediated inflammatory responses ultimately executed by T helper cells of the Th2 subtype. TSLP emerged as a central player in the development of allergic symptoms, especially in the airways, and is a prime regulatory cytokine at the interface of virus- or antigen-exposed epithelial cells and dendritic cells (DCs). DCs activated by epithelium-derived TSLP can promote naïve CD4+ T cells to adopt a Th2 phenotype, which in turn recruite eosinophilic and basophilic granulocytes as well as mast cells into the airway mucosa. These different cells secrete inflammatory cytokines and chemokines operative in inducing an allergic inflammation and atopic asthma. TSLP is, thus, involved in the control of both an innate and an adaptive immune response. Since TSLP links contact of allergen with the airway epithelium to the onset and maintainance of the asthmatic syndrome, defining the signal transduction underlying TSLP expression and function is of profound interest for a better understandimg of the disease and for the development of new therapeutics.

TLR3- and Th2 cytokine-dependent production of thymic stromal lymphopoietin in human airway epithelial cells

Thymic stromal lymphopoietin (TSLP) is elevated in asthma and triggers dendritic cell-mediated activation of Th2 inflammatory responses. Although TSLP has been shown to be produced mainly by airway epithelial cells, the regulation of epithelial TSLP expression has not been extensively studied. We investigated the expression of TSLP in cytokine- or TLR ligand-treated normal human bronchial epithelial cells (NHBE). The mRNA for TSLP was significantly up-regulated by stimulation with IL-4 (5.5-fold) and IL-13 (5.3-fold), weakly up-regulated by TNF-alpha, TGF-beta, and IFN-beta, and not affected by IFN-gamma in NHBE. TSLP mRNA was only significantly up-regulated by the TLR3 ligand (dsRNA) among the TLR ligands tested (66.8-fold). TSLP was also induced by in vitro infection with rhinovirus. TSLP protein was detected after stimulation with dsRNA (120 +/- 23 pg/ml). The combination of TNF-alpha and IL-4 produced detectable levels of TSLP protein (40 +/- 13 pg/ml). In addition, TSLP was synergistically enhanced by a combination of IL-4 and dsRNA (mRNA; 207-fold, protein; 325 +/- 75 pg/ml). The induction of TSLP by dsRNA was dependent upon NF-kappaB and IFN regulatory factor 3 (IRF-3) signaling via TLR3 as indicated by a study with small interfering RNA. The potent topical glucocorticoid fluticasone propionate significantly suppressed dsRNA-dependent TSLP production in NHBE. These results suggest that the expression of TSLP is induced in airway epithelial cells by stimulation with the TLR3 ligand and Th2 cytokines and that this response is suppressed by glucocorticoid treatment. This implies that respiratory viral infection and the recruitment of Th2 cytokine producing cells may amplify Th2 inflammation via the induction of TSLP in the asthmatic airway.

Viral infections and allergies

Respiratory viral infections have been implicated in the origin of, protection from and exacerbation of allergy-related symptoms in a variety of ways. Viral infections are closely linked to infantile wheezing. Severe bronchiolitis in early infancy may predispose to chronic childhood asthma as well as allergic sensitization; alternatively it could represent a marker of susceptible individuals. In contrast, repeated mild infections in early life may have a protective role in the development of asthma or atopy by driving the immune system towards Th1 responses. However, evidence on this hypothesis is not consistent as far as respiratory viruses are concerned. Several factors, including the presence of an atopic environment, timing of exposure and severity of the infection, interactively contribute to the allergy-infection relationship. In the present report, recent data on the role of viral infections in the development and progression of allergy and asthma are reviewed.

Airway epithelial cells produce B cell-activating factor of TNF family by an IFN-beta-dependent mechanism

Activation of B cells in the airways is now believed to be of great importance in immunity to pathogens, and it participates in the pathogenesis of airway diseases. However, little is known about the mechanisms of local activation of B cells in airway mucosa. We investigated the expression of members of the B cell-activating TNF superfamily (B cell-activating factor of TNF family (BAFF) and a proliferation-inducing ligand (APRIL)) in resting and TLR ligand-treated BEAS-2B cells and primary human bronchial epithelial cells (PBEC). In unstimulated cells, expression of BAFF and APRIL was minimal. However, BAFF mRNA was significantly up-regulated by TLR3 ligand (dsRNA), but not by other TLR ligands, in both BEAS-2B cells (376-fold) and PBEC (224-fold). APRIL mRNA was up-regulated by dsRNA in PBEC (7-fold), but not in BEAS-2B cells. Membrane-bound BAFF protein was detectable after stimulation with dsRNA. Soluble BAFF protein was also induced by dsRNA (> 200 pg/ml). The biological activity of the epithelial cell-produced BAFF was verified using a B cell survival assay. BAFF was also strongly induced by IFN-beta, a cytokine induced by dsRNA. Induction of BAFF by dsRNA was dependent upon protein synthesis and IFN-alphabeta receptor-JAK-STAT signaling, as indicated by studies with cycloheximide, the JAK inhibitor I, and small interfering RNA against STAT1 and IFN-alphabeta receptor 2. These results suggest that BAFF is induced by dsRNA in airway epithelial cells and that the response results via an autocrine pathway involving IFN-beta. The production of BAFF and APRIL by epithelial cells may contribute to local accumulation, activation, class switch recombination, and Ig synthesis by B cells in the airways.

Infections and asthma

A new paradigm is developing in regard to the interaction between infection and asthma. This paradigm comprises the acute exacerbations seen in asthma and also asthma chronicity. Viral infections have been commonly evaluated in acute exacerbations, but findings suggest viral-allergen and viral-bacterial interactions are important for chronicity. Most recently, studies are also invoking atypical bacterial infections, Mycoplasma pneumoniae and Chlamydia pneumoniae, as factors in both acute exacerbation and chronic asthma.

Duration of postviral airway hyperresponsiveness in children with asthma: effect of atopy

Background

Respiratory viruses induce asthma exacerbations and airway hyperresponsiveness (AHR). Atopy is an important risk factor for asthma persistence.

Objective

We sought to evaluate whether atopy is a risk factor for prolonged AHR after upper respiratory tract infections (URIs).

Methods

Twenty-five children (13 atopic and 12 nonatopic children) with intermittent virus-induced asthma were studied. Clinical evaluation, skin prick tests, methacholine bronchoprovocation, questionnaires, and a nasal wash specimen were obtained at baseline. For 9 months, subjects completed diary cards with respiratory symptoms. During their first reported cold, a nasal wash specimen was obtained. Methacholine provocation was performed 10 days and 5, 7, 9, and 11 weeks later. In case a new cold developed, the provocation schedule was followed from the beginning.

Results

Viruses were detected in 17 (68%) of 25 patients during their first cold, with rhinovirus being most commonly identified (82%). AHR increased significantly 10 days after the URI, equally in both groups (P = .67), and remained so up to the fifth week. Duration of AHR in subjects experiencing a single URI ranged from 5 to 11 weeks, without a significant difference between groups. In the duration of the study, atopic children experienced more colds and asthma exacerbations than nonatopic children. Thus for duration of AHR, significant prolongation was noted in the atopic group when assessed cumulatively.

Conclusion

In asthmatic children the duration of AHR after a single natural cold is 5 to 11 weeks. However, an increased rate of symptomatic cold and asthma episodes in atopic children is associated with considerable cumulative prolongation of AHR, which might help explain the role of atopy as a risk factor for asthma persistence.

Susceptibility to allergic lung disease regulated by recall responses of dual-receptor memory T cells

BACKGROUND

Microbial infections are associated with the initial susceptibility to and flares of asthma. However, immunologic mechanisms whereby infections might alter the asthmatic phenotype are lacking.

OBJECTIVE

To test the hypothesis that memory T cells specific both for a viral antigen and an allergen could influence the pathogenesis of allergic disease in vivo .

METHODS

We developed a system in which 2 distinct T-cell receptors coexist on the T-cell surface, 1 specific for a virus and the other for an inhaled antigen.

RESULTS

We show that a population of dual-receptor T cells, polarized through a virus-specific T-cell receptor to contain T(H)1 or T(H)2 cells, can be reactivated through an unrelated T-cell receptor in recall responses in vivo . Quiescent memory cells derived from a T(H)1-polarized effector population blocked the development of airway hyperreactivity in a model of allergic lung disease, in association with decreased induction of chemokines and eosinophil recruitment. Conversely, reactivation of quiescent T(H)2 cells after inhalation of antigen or virus infection was sufficient to lead to the development of airway hyperresponsiveness and allergic pulmonary inflammation in mice whose lungs were previously normal.

CONCLUSION

These data provide evidence that dual-receptor memory T cells can regulate allergic disease susceptibility and suggest that they may play a role in mediating the influence of microbes on asthma pathogenesis.

Viral respiratory infections in association with asthma and sinusitis: a review

OBJECTIVE

Viral respiratory infections (VRIs) commonly precede asthma exacerbations in both children and adults. Likewise, VRIs may affect the paranasal sinuses, predisposing infected individuals to the development of subsequent acute bacterial sinusitis. This article discusses the role that viruses play in both the development of asthma and in acute asthma exacerbations. Mechanisms by which viral infections provoke asthma exacerbations are reviewed, and treatment of such episodes is discussed. The pathogenesis of sinusitis and association with VRIs is reviewed along with treatment recommendations.

DATA SOURCES

Relevant articles in the medical literature were reviewed with sources including randomized, controlled clinical trials, review articles, epidemiologic studies, and standard textbooks in allergy and immunology.

CONCLUSIONS

This review highlights the prominent role that viral pathogens (especially rhinovirus) play in exacerbation of asthma and in the development of sinus disease. The specific mechanisms whereby viral infection leads to an acute asthma exacerbation or to subsequent bacterial sinusitis are described. Treatment options are outlined including the potential future application of antiviral compounds.