Decreased activation of inflammatory networks during acute asthma exacerbations is associated with chronic airflow obstruction

New insights into the natural history of asthma: primary prevention on the horizon

Recent studies of the natural history of asthma have shifted attention toward viral respiratory tract illness in early life as a major risk factor associated with the development of the most persistent forms of the disease. Although early aeroallergen sensitization is strongly associated with chronic asthma, several trials in which single-aeroallergen exposure in pregnancy and early childhood was successfully accomplished and compared with sham avoidance have failed to show any decrease in asthma incidence. New evidence suggests that complex interactions occur between viral infection and aeroallergen sensitization in genetically susceptible subjects that trigger the immune responses and airway changes that are characteristic of persistent asthma. The finding that exposure to bacterial products among children raised on farms is associated with diminished asthma prevalence during the school years has now been replicated, and experimental studies have suggested that these effects are mediated by the activation of regulatory T cells in the airway. It is thus plausible to hypothesize that primary prevention of asthma could be attained through surrogate therapeutic interventions that activate similar mechanisms in young children at high risk for asthma.

Interferon regulatory factor 7 is a major hub connecting interferon-mediated responses in virus-induced asthma exacerbations in vivo

BACKGROUND

Exacerbations are responsible for a substantial burden of morbidity and health care use in children with asthma. Most asthma exacerbations are triggered by viral infections; however, the underlying mechanisms have not been systematically investigated.

OBJECTIVE

The objective of this study was to elucidate the molecular networks that underpin virus-induced exacerbations in asthmatic children in vivo.

METHODS

We followed exacerbation-prone asthmatic children prospectively and profiled global patterns of gene expression in nasal lavage samples obtained during an acute, moderate, picornavirus-induced exacerbation and 7 to 14 days later. Coexpression network analysis and prior knowledge was used to reconstruct the underlying gene networks.

RESULTS

The data showed that an intricate modular program consisting of more than 1000 genes was upregulated during acute exacerbations in comparison with 7 to 14 days later. The modules were enriched for coherent cellular processes, including interferon-induced antiviral responses, innate pathogen sensing, response to wounding, small nucleolar RNAs, and the ubiquitin-proteosome and lysosome degradation pathways. Reconstruction of the wiring diagram of the modules revealed the presence of hyperconnected hub nodes, most notably interferon regulatory factor 7, which was identified as a major hub linking interferon-mediated antiviral responses.

CONCLUSIONS

This study provides an integrated view of the inflammatory networks that are upregulated during virus-induced asthma exacerbations in vivo. A series of innate signaling hubs were identified that could be novel therapeutic targets for asthma attacks.

Pathophysiology of asthma: what has our current understanding taught us about new therapeutic approaches?

Current asthma therapy is based on the use of adrenergic bronchodilator and anti-inflammatory drugs the specificity, efficacy, duration of action, and safety of which have been derived through classical pharmacology and medicinal chemistry. That asthma is a T(H)2-type inflammatory disorder frequently associated with atopy and allergic comorbidities has led to a concentrated effort to find treatments that act selectively on this pathway. A systematic literature review was undertaken, as well as a review of the Web site Clinicaltrials.gov for ongoing trials. Targets have included T cells themselves and their associated cytokines, chemokines, and receptors mostly targeted with biological agents. With the exception of anti-human IgE, none of these have met the expectations predicted from animal models and human in vitro tests. For most of these new therapies, only a very small subpopulation appears to respond. A case is made for a different approach to drug discovery based on acquiring a greater understanding of asthma stratification, the relevant pathways involved, and the development of appropriate diagnostic tests enabling the targeting of selective treatments to those asthmatic phenotypes most likely to respond. The recognition that asthma is more than allergy mandates improved predictive animal models and an appreciation that many of the environmental insults that initiate, consolidate, and exacerbate asthma operate through an epithelium functioning in a disorderly fashion. An integrated model that places the epithelium at the forefront of asthma pathogenesis suggests that greater emphasis should be placed on therapeutics that increase the airways' resistance against the inhaled environment rather than focusing only on suppression of inflammation.

T regulatory cells in childhood asthma

Asthma is a chronic disease of the airways, most commonly driven by immuno-inflammatory responses to ubiquitous airborne antigens. Epidemiological studies have shown that disease is initiated early in life when the immune and respiratory systems are functionally immature and less able to maintain homeostasis in the face of continuous antigen challenge. Here, we examine the cellular and molecular mechanisms that underlie initial aeroallergen sensitization and the ensuing regulation of secondary responses to inhaled allergens in the airway mucosa. In particular, we focus on how T-regulatory (Treg) cells influence early asthma initiation and the potential of Treg cells as therapeutic targets for drug development in asthma.

Interaction between adaptive and innate immune pathways in the pathogenesis of atopic asthma: operation of a lung/bone marrow axis

Atopic asthma is the most common form of asthma, particularly during childhood, and in many cases it persists into adult life. Although atopy is clearly a risk factor for development of this disease, only a small subset of subjects sensitized to aeroallergens express persistent symptoms, suggesting that additional pathogenic mechanisms are involved. Recent studies have implicated respiratory viral infections as key cofactors in asthma development in atopic patients. In relation to initial expression of the asthma phenotype in early childhood, it has been shown that although both atopic sensitization and early severe lower respiratory tract infections can operate as independent asthma risk factors, the persistence of asthma is most frequent among children who experience both insults, suggesting that the relevant inflammatory pathways interact to maximally drive disease pathogenesis. Importantly, it has been established that both these factors must be operative contemporaneously for these interactions to occur (ie, the interactions are likely to be direct). Recent studies on viral-induced asthma exacerbations in atopic children have provided a plausible mechanism for these interactions. Notably, it has been demonstrated that signals triggered during the innate immune response to the virus can lead to the release of large numbers of migrating high-affinity IgE receptor-bearing bone marrow-derived precursors of mucosal dendritic cells into the blood. The subsequent trafficking of these cells to the infected airway mucosa where dendritic cell turnover is very high provides a potential mechanism for recruitment of underlying aeroallergen-specific T-helper 2 immunity into the already inflamed milieu in the infected airway mucosa.

The role of IL-15 deficiency in the pathogenesis of virus-induced asthma exacerbations

Rhinovirus infections are the major cause of asthma exacerbations. We hypothesised that IL-15, a cytokine implicated in innate and acquired antiviral immunity, may be deficient in asthma and important in the pathogenesis of asthma exacerbations. We investigated regulation of IL-15 induction by rhinovirus in human macrophages in vitro, IL-15 levels in bronchoalveolar lavage (BAL) fluid and IL-15 induction by rhinovirus in BAL macrophages from asthmatic and control subjects, and related these to outcomes of infection in vivo. Rhinovirus induced IL-15 in macrophages was replication-, NF-κB- and α/β interferon-dependent. BAL macrophage IL-15 induction by rhinovirus was impaired in asthmatics and inversely related to lower respiratory symptom severity during experimental rhinovirus infection. IL-15 levels in BAL fluid were also decreased in asthmatics and inversely related with airway hyperresponsiveness and with virus load during in vivo rhinovirus infection. Deficient IL-15 production in asthma may be important in the pathogenesis of asthma exacerbations.

We previously reported deficiency in interferon production in asthma, which correlated with disease severity and viral load during experimental rhinovirus infection. Here we show that macrophages produce IL-15 upon rhinovirus infection and that IFN-β plays an important role in IL-15 production. In asthmatic subjects, there is a deficiency in rhinovirus-induced production of IL-15 by macrophages, which indicates immunodeficiency in asthma is surprisingly broad, also involving IL-15, an important cytokine that bridges innate and acquired immunity. These results show that IFN-β therapy in asthma exacerbations could be effective not only due to direct anti-viral effects of IFN-β, but also by inducing IL-15 production. We also show induction of IFN-β and IL-15 to be NF-kB dependent, an important finding which has implications for NF-kB inhibitor drug development programmes as these drugs have potential to worsen rather than improve asthma exacerbation severity, by further enhancing deficiencies of IL-15 and IFN-β. This study investigating the role of IL-15 in rhinovirus infection and asthma has also major implications in other diseases, for example pandemic influenza, where asthma is a major risk factor for severe disease and death, and COPD and cystic fibrosis where IFN-β deficiency is also present.

Role of innate immunity in the development of allergy and asthma

PURPOSE OF REVIEW

Asthma is essentially a developmental disease, in which the normal growth and development of the respiratory and immune systems are affected by environmental exposures acting on underlying genetic predispositions. The purpose of this review is to examine the role of innate immunity in the lungs in the development of allergy and asthma.

RECENT FINDINGS

Both the innate and adaptive arms of the immune system are immature at birth and undergo prolonged periods of postnatal maturation. As such, they are vulnerable to adverse environmental exposures, both before and after birth. Both genetic predispositions and environmentally induced epigenetic changes in gene expression are likely to contribute to the risk of asthma; however, the relative contributions are unclear. Increasing interest is focused on deficient innate responses of the respiratory epithelium to viral infections and how these may increase the risk of asthma. However, definitive proof that these are primary and not secondary effects is lacking. Although most research has concentrated on the role of respiratory viral infections in increasing the asthma risk, the recent suggestion that the lung has a resident bacteriome and potentially important viral-bacterial interactions in the lungs broadens research scope in this area.

SUMMARY

Classic risk factors for asthma include a family history of asthma and allergies, early and persistent allergic sensitization and viral lower-respiratory infections in early life. However, these factors do not fully explain the risk. Perhaps, the resident pulmonary microbiome and the immune response that this generates during respiratory viral infections will provide the 'missing link' in the epidemiology.

Animal models of asthma: value, limitations and opportunities for alternative approaches

Asthma remains an area of considerable unmet medical need. Few new drugs have made it to the clinic during the past 50 years, with many that perform well in preclinical animal models of asthma, failing in humans owing to lack of safety and efficacy. The failure to translate promising drug candidates from animal models to humans has led to questions about the utility of in vivo studies and to demand for more predictive models and tools based on the latest technologies. Following a workshop with experts from academia and the pharmaceutical industry, we suggest here a disease modelling framework designed to better understand human asthma, and accelerate the development of safe and efficacious new asthma drugs that go beyond symptomatic relief.

Genome-virome interactions: examining the role of common viral infections in complex disease

New technologies have widened our view of 'complex diseases': those with both genetic and environmental risk factors. In this Review, we explore recent genetic and virological evidence implicating host-virus interactions in three diseases: type 1 diabetes, inflammatory bowel disease and asthma. The viruses implicated in these diseases cause mucosal infections that affect most of the population but are asymptomatic or mild in many hosts. These findings place a new emphasis on common viral infections as important environmental factors in the pathogenesis of complex diseases, and they compel the field to pursue a better understanding of host interactions with the human virome.