Gary P. Anderson

Endotyping asthma: new insights into key pathogenic mechanisms in a complex, heterogeneous disease

Clinical asthma is very widely assumed to be the net result of excessive inflammation driven by aberrant T-helper-2 (Th2) immunity that leads to inflamed, remodelled airways and then functional derangement that, in turn, causes symptoms. This notion of disease is actually poorly supported by data, and there are substantial discrepancies and very poor correlation between inflammation, damage, functional impairment, and degree of symptoms. Furthermore, this problem is compounded by the poor understanding of the heterogeneity of clinical disease. Failure to recognise and discover the underlying mechanisms of these major variants or endotypes of asthma is, arguably, the major intellectual limitation to progress at present. Fortunately, both clinical research and animal models are very well suited to dissecting the cellular and molecular basis of disease endotypes. This approach is already suggesting entirely novel pathways to disease-eg, alternative macrophage specification, steroid refractory innate immunity, the interleukin-17-regulatory T-cell axis, epidermal growth factor receptor co-amplification, and Th2-mimicking but non-T-cell, interleukins 18 and 33 dependent processes that can offer unexpected therapeutic opportunities for specific patient endotypes.

A protective role for protease-activated receptors in the airways.

The protection of cells in the upper intestine against digestion by pancreatic trypsin depends on the prostanoid prostaglandin E2 (PGE2) and is mediated by protease-activated receptors in the epithelium,. As the airway epithelium is morphologically similar and also expresses one of these receptors, PAR2 (ref. 3), and is a major source of PGE2 (ref. 4), we reasoned that bronchial epithelial PAR2 might also participate in prostanoid-dependent cytoprotection in the airways. Here we show that activation of PAR2, which co-localizes immunohistochemically with trypsin(ogen) in airway epithelium, causes the relaxation of airway preparations from mouse, rat, guinea-pig and humans by the release of a cyclooxygenase product from the epithelium. This physiological protective response in isolated airways also occurred in anaesthetized rats, where activation of PAR2 caused a marked and prolonged inhibition of bronchoconstriction. After desensitization of PAR2, the response to trypsin recovered rapidly by mechanisms dependent on de novo synthesis and trafficking of proteins. Our results indicate that trypsin released from the epithelium can initiate powerful bronchoprotection in the airways by activation of epithelial PAR2.

How cigarette smoke skews immune responses to promote infection, lung disease and cancer

A complex and multilayered immune defence system protects the host against harmful agents and maintains tissue homeostasis. Cigarette smoke ex posure markedly impacts the immune system, compromising the hosts ability to mount appropriate immune and inflammatory responses and contributing to smoking-related pathologies. These adverse effects on the immune system not only occur in active smokers, but also in those exposed to smoke passively in contaminated environments, and may persist for decades after exposure has ended.

Neutrophils Ameliorate Lung Injury and the Development of Severe Disease during Influenza Infection

The clinical response to influenza infection ranges from mild disease to severe pneumonia and it remains unclear whether the inflammatory response to infection is protective or pathogenic. We have defined a novel role for neutrophils in ameliorating lung injury during influenza infection, thereby limiting development of severe disease. Infection of neutrophil-depleted mice with influenza virus HKx31 (H3N2) led to rapid weight loss, pneumonia, and death. Neutropenia was associated with enhanced virus replication in the respiratory tract; however, viral titers were declining at the time of death, leading us to investigate other factors contributing to mortality. In addition to thymic atrophy, lymphopenia, and viremic spread, depletion of neutrophils led to exacerbated pulmonary inflammation, edema, and respiratory dysfunction. Thus, while it is well established that neutrophils contribute to lung injury in a range of pathological conditions, reduced numbers or impaired neutrophil function can facilitate progression of mild influenza to severe clinical disease.

Murine models of asthma.

In vivo animal models can offer valuable information on several aspects of asthma pathogenesis and treatment. The mouse is increasingly used in these models, mainly because this species allows for the application in vivo of a broad range of immunological tools, including gene deletion technology. Mice, therefore, seem particularly useful to further elucidate factors influencing the response to inhaled allergens. Examples include: the role of immunoregulatory mechanisms that protect against T‐helper cell type 2 cell development; the trafficking of T‐cells; and the contribution of the innate immunity. However, as for other animal species, murine models also have limitations. Mice do not spontaneously develop asthma and no model mimics the entire asthma phenotype. Instead, mice should be used to model specific traits of the human disease. The present task force report draws attention to specific aspects of lung structure and function that need to be borne in mind when developing such models and interpreting the results. In particular, efforts should be made to develop models that mimic the lung function changes characteristic of asthma as closely as possible. A large section of this report is therefore devoted to an overview of airway function and its measurement in mice.

Neutrophils, interleukin-17A and lung disease.

It is now established that an excessive and sustained mobilisation of neutrophils is a hallmark of several chronic inflammatory lung disorders, including severe obstructive lung disease. This article reviews evidence that the cytokine interleukin (IL)-17A is a major orchestrator of sustained neutrophilic mobilisation. Current evidence suggests that IL-17A is produced by T-lymphocytes, and that it exerts an orchestrating effect on the accumulation and associated activity of neutrophils in the bronchoalveolar space indirectly, through an induced release of specific cytokines and colony-stimulating factors in resident lung cells. Although the involvement of IL-17A in inflammatory lung disorders is supported by several recent studies, its causative role is still uncertain. However, the unique position of interleukin-17A at the interface between acquired and innate immunity puts this cytokine forward as an important signal for the reinforcement of host defence; it also implies that interleukin-17A may constitute a useful target for pharmacotherapeutic intervention.

GM-CSF Biology.

Granulocyte macrophage-colony stimulating factor (GM-CSF) was originally defined by its ability to generate in vitro granulocyte and macrophage colonies from bone marrow precursor cells. Apart from its physiological role in the control of alveolar macrophage development, it now appears more likely that its major role lies in its ability to govern the properties of the more mature myeloid cells of the granulocyte and macrophage lineages, particularly during host defence and inflammatory reactions. This review summarizes the in vivo evidence to support this proposition. This evidence includes both the findings obtained by administration of GM-CSF, e.g. as an adjuvant, and also includes those observed in depletion studies, e.g. during inflammatory reactions where GM-CSF can be shown to have a proinflammatory action.

Activation of the Fas receptor on lung eosinophils leads to apoptosis and the resolution of eosinophilic inflammation of the airways.

While considerable progress has been made in understanding the events by which eosinophils accumulate in various pathophysiological conditions, the mechanisms controlling the resolution of eosinophilic inflammation are poorly understood. In the present study, we demonstrate that lung eosinophils obtained by bronchoalveolar lavage (BAL) after aerosol allergen provocation of immunized mice expressed the Fas receptor. Stimulation of purified eosinophils in vitro with a monoclonal anti-Fas mAb (1 ng-1 microg/ml) induced a dose/time dependent loss of cell viability from 24-72 h. Measurement of DNA fragmentation with propidium iodide confirmed that anti-Fas induced eosinophil death by apoptosis. While incubation with IL-3, IL-5, or GM-CSF prevented spontaneous apoptosis, these factors failed to prevent anti-Fas induced apoptosis. Administration of anti-Fas mAb to the lungs after the induction of a lung eosinophilia increased the number of peroxidase positive macrophages in BAL fluid 4-12 h later which was followed by a marked reduction in the number of eosinophils in the airways. Importantly, Fas-mediated resolution of eosinophilic inflammation occurred in the absence of any overt secondary inflammatory changes in the lungs. We speculate that defects in this pathway may at least in part explain the chronic eosinophilic inflammation often observed in the lungs of asthmatic individuals.

TH2 and ‘TH2-like’ cells in allergy and asthma: pharmacological perspectives

Recently, considerable progress has been made in understanding the molecular basis by which cytokines released from CD4+ helper T cells contribute to allergic disease. A subset of CD4+ helper T cells, termed TH2 cells, produce interleukin 4 (IL-4) and IL-5, but not interferon gamma. IL-4 has a critical role in causing B-cell immunoglobulin-isotype switch leading to IgE synthesis, and IL-5 governs eosinophilic inflammation of airway tissue. Studies on the mechanisms whereby TH2 cells, and non-T cells such as metachromatic cells, produce a highly restricted panel of cytokines has revealed molecular mechanisms that may affect our views on the induction and treatment of asthma, and these are discussed in this review by Gary Anderson and Anthony Coyle. TH2 cytokine biology may enable pharmacologists to design better, and perhaps even preventative, therapies for the treatment of asthma and allergy. Surprisingly IL-4, rather than IL-5, is emerging as a critical drug target owing to its central role in the regulation of CD4+ helper T cell phenotype commitment.

Cigarette Smoke Inhibits Lipopolysaccharide-Induced Production of Inflammatory Cytokines by Suppressing the Activation of Activator Protein-1 in Bronchial Epithelial Cells

Chronic smoking is characterized by immunosuppressive changes in the airways, leading to chronic colonization with bacteria, which in turn may contribute to the chronic obstructive pulmonary disease. The mechanisms causing this immunosuppression, however, are poorly characterized. This study evaluated whether cigarette smoke can inhibit endotoxin (LPS)-induced inflammatory cytokine production in bronchial epithelial cells and, if so, what the mechanisms are behind this effect. Pretreatment with cigarette smoke extract (CSE) concentration dependently inhibited the LPS-induced GM-CSF and IL-8 protein release, which was accompanied by decreased expression of mRNA in human bronchial epithelial cells (Beas-2B). The increase of neutrophil chemotaxis induced by conditioned medium from LPS-treated Beas-2B cells was also suppressed by CSE. In addition, the activity of LPS-induced transcription factor AP-1, but not NF-κB, was down-regulated by CSE. Notably, at the concentrations used, CSE had no effect on number or viability of Beas-2B cells. These data indicate that cigarette smoke possesses immunosuppressive properties by down-regulating the bacterial pathogen-induced neutrophil-mobilizing cytokine production via suppression of AP-1 activation in the airways. Hence, this study suggests a novel mechanism by which cigarette smoke may contribute to chronic colonization and chronic obstructive pulmonary disease in smokers.