Immunopathology of the bronchial mucosa in 'late onset' asthma

Recent Insights into the Management of Inflammation in Asthma

The present prevailing inflammatory paradigm in asthma is of T2-high inflammation orchestrated by key inflammatory cells like Type 2 helper lymphocytes, innate lymphoid cells group 2 and associated cytokines. Eosinophils are key components of this T2 inflammatory pathway and have become key therapeutic targets. Real-world evidence on the predominant T2-high nature of severe asthma is emerging. Various inflammatory biomarkers have been adopted in clinical practice to aid asthma characterization including airway measures such as bronchoscopic biopsy and lavage, induced sputum analysis, and fractional exhaled nitric oxide. Blood measures like eosinophil counts have also gained widespread usage and multicomponent algorithms combining different parameters are now appearing. There is also growing interest in potential future biomarkers including exhaled volatile organic compounds, micro RNAs and urinary biomarkers. Additionally, there is a growing realisation that asthma is a heterogeneous state with numerous phenotypes and associated treatable traits. These may show particular inflammatory patterns and merit-specific management approaches that could improve asthma patient outcomes. Inhaled corticosteroids (ICS) remain the mainstay of asthma management but their use earlier in the course of disease is being advocated. Recent evidence suggests potential roles for ICS in combination with long-acting beta-agonists (LABA) for as needed use in mild asthma whilst maintenance and reliever therapy regimes have gained widespread acceptance. Other anti-inflammatory strategies including ultra-fine particle ICS, leukotriene receptor antagonists and macrolide antibiotics may show efficacy in particular phenotypes too. Monoclonal antibody biologic therapies have recently entered clinical practice with significant impacts on asthma outcomes. Understanding of the efficacy and use of those agents is becoming clearer with a growing body of real-world evidence as is their potential applicability to other treatable comorbid traits. In conclusion, the evolving understanding of T2 driven inflammation alongside a treatable traits disease model is enhancing therapeutic approaches to address inflammation in asthma.

The pharmacological modulation of allergen-induced asthma

Aeroallergens are the most common triggers for the development of asthma. Recent birth cohort studies have identified viral infections occurring against a background of aeroallergen sensitization as a potent risk factor for initiation of asthma. Viral infection enhances immunopathogenic potential of pre-existing inhalant allergy via modulating airway mucosal dendritic cells. By using an allergen inhalation challenge clinical model, studies have shown that the late asthma response (LAR) is associated with more pronounced allergen-induced airway inflammation and airway hyperresponsiveness. The degree of airway eosinophilia, regulated by bone marrow progenitor cells and interleukin-5 level, correlates with the magnitude of the LAR and the increase in hyperresponsiveness. Both myeloid and plasmacytoid dendritic cell subsets have been involved in the pathogenesis of allergen-induced LAR. Myeloid dendritic cells are responsible for the allergen presentation and induction of inflammation and plasmacytoid dendritic cells play a role in the resolution of allergen-induced inflammation. A variety of potential new classes of asthma medication has also been evaluated with the allergen inhalation challenge in mild asthmatic subjects. Examples are TPI ASM8, an inhaled anti-sense oligonucleotide drug product, which attenuated both early and LARs via inhibition of the target gene mRNA of chemokine receptor 3, and the common β chain of interleukin-3, interleukin-5 and granulocyte-macrophage colony-stimulating factor receptor. Anti-human antibody interleukin-13 (IM-638) significantly attenuated both early and late allergen-induced asthma response. Pitrakinra, which targets both interleukin-4 and interleukin-13, substantially diminishes allergen-induced airway responses. Allergen-induced airway responses are a valuable way to evaluate the activity of possible new therapies in asthmatic airways.

Epithelial cell proliferation contributes to airway remodeling in severe asthma

RATIONALE

Despite long-term therapy with corticosteroids, patients with severe asthma develop irreversible airway obstruction.

OBJECTIVES

To evaluate if there are structural and functional differences in the airway epithelium in severe asthma associated with airway remodeling.

METHODS

In bronchial biopsies from 21 normal subjects, 11 subjects with chronic bronchitis, 9 subjects with mild asthma, and 31 subjects with severe asthma, we evaluated epithelial cell morphology: epithelial thickness, lamina reticularis (LR) thickness, and epithelial desquamation. Levels of retinoblastoma protein (Rb), Ki67, and Bcl-2 were measured, reflecting cellular proliferation and death. Terminal deoxynucleotidyl-mediated dUTP nick end labeling (TUNEL) was used to study cellular apoptosis.

MEASUREMENTS AND MAIN RESULTS

Airway epithelial and LR thickness was greater in subjects with severe asthma compared with those with mild asthma, normal subjects, and diseased control subjects (p=0.009 and 0.033, respectively). There was no significant difference in epithelial desquamation between groups. Active, hypophosphorylated Rb expression was decreased (p=0.002) and Ki67 was increased (p<0.01) in the epithelium of subjects with severe asthma as compared with normal subjects, indicating increased cellular proliferation. Bcl-2 expression was decreased (p<0.001), indicating decreased cell death suppression. There was a greater level of apoptotic activity in the airway biopsy in subjects with severe asthma as compared with the normal subjects using the TUNEL assay (p=0.002), suggesting increased cell death.

CONCLUSIONS

In subjects with severe asthma, as compared with subjects with mild asthma, normal subjects, and diseased control subjects, we found novel evidence of increased cellular proliferation in the airway contributing to a thickened epithelium and LR. These changes may contribute to the progressive decline in lung function and airway remodeling in patients with severe asthma.

Airway remodeling and persistent airway obstruction in asthma

There is growing recognition that some patients with long-standing asthma may possess a component of irreversible airflow obstruction despite optimal therapy. This persistent airflow obstruction is thought to be the result of structural changes in the airways that occur as a result of airway remodeling. The structural changes that lead to chronic obstruction are not known, nor are the intricacies of the remodeling process. Hence airway remodeling and its role in the evolution of irreversible airflow obstruction remain conceptual. Much work has been carried out to better define the histopathologic characteristics of asthma, including the characteristic features of airway inflammation. However, attempts to delineate the physiologic consequences of specific histologic findings are at an early stage of development. The thesis that airway remodeling is driven by chronic inflammatory processes has important implications for the way we make treatment decisions, especially in the patient with mild asthma. Abounding interest in airway remodeling has led to a growing literature on the subject, a literature that is largely speculative and perhaps too tautologic in the sense that remodeling is frequently defined by any observed histologic change, irrespective of its physiologic consequences. Careful attempts to link histologic observations with clinical, demographic, and physiologic findings will be necessary to unravel the causes of remodeling and identify who is at risk for development of irreversible airway obstruction.

Role of bronchoscopy in asthma research

Over the past 15 years, much has been learned about the presence of airway inflammation in asthma through the use of investigative bronchoscopy. It has become quite clear that inflammation is present even in mild asthma. In addition to the eosinophils, T-lymphocytes and a variety of cytokines have been identified to play a prominent role in asthmatic inflammation. The concept of delayed asthmatic response after allergen exposure and its relationship to cellular inflammation and airway hyper-reactivity has become more clearly established. Our understanding of asthmatic airway inflammation, however, is incomplete. As interesting as the database has been so far, investigative FB has not defined a unique profile for patients with asthma. Specifically, lavage or endobronchial biopsy has not identified parameters that help in the diagnosis, assessment of disease severity, prognosis, or likelihood to respond to specific therapies. Also, the exact relationship between parameters in lavage compared with mucosal biopsy and how these are related to airway hyper-reactivity and the clinical syndrome of asthma remains poorly understood. In this regard, it must be confessed that currently FB with lavage and biopsy in asthmatics needs to be considered as a research tool for specimen retrieval to help characterize and express inflammation. Although these techniques have contributed immensely to our understanding of asthma pathogenesis, presently these techniques do not have any practical role or clinical usefulness.

What can we learn from late-onset and occupational asthma?

Late-onset asthma and occupational asthma may provide interesting models of human asthma to compare with the most frequent type of atopic early-onset asthma. The discovery of similarities and discrepancies in the aetiology and pathogenesis of these different diseases might provide new insights on different mechanisms producing the same phenotype and, thus, by recognizing the different underlying mechanisms of the different forms of asthma, may allow better targeting of prevention and treatment. Occupational asthma, in addition to being a late-onset asthma, provides the unique opportunity to study the development of asthma under measurable exposure conditions, and consequently to examine the effect of cessation of exposure which, at variance with allergen avoidance, is possible in most of the cases.

Investigative use of bronchoscopy in asthma

The incorporation of bronchoscopy and bronchoscopic procedures into the investigation of asthma mechanisms, treatment, and in particular, the role of airway inflammation has contributed significantly to the enhanced understanding of this disease. Carefully drafted guidelines have allowed the gradual inclusion of subjects with more severe disease in studies utilizing bronchoscopic investigative tools. Many more questions remained unanswered, including the importance of persistence of airway inflammation in asymptomatic asthma, the specific antiinflammatory effects of new (and old) asthma therapies, the contribution of airway structural changes (subepithelial fibrosis) to nonreversible obstruction, the role of antiinflammatory versus proinflammatory cytokines in the pathogenesis of airway inflammation and the heterogeneity of disease expression in various groups of subjects. We are confident that current and future meticulously designed and executed research studies utilizing bronchoscopic techniques will significantly add to our knowledge of disease mechanisms and lead us to new and improved treatments for asthma.

Expression of growth factors and remodelling of the airway wall in bronchial asthma

BACKGROUND

Bronchial asthma is characterised by airway structural changes, including mucosal inflammatory infiltration and subepithelial collagen deposition, that may represent the morphological basis for the chronicity of the disease. The relationship between airway wall thickness and growth factors in asthma has not been elucidated.

METHODS

Bronchial biopsy specimens were obtained from 21 asthmatic patients and eight healthy subjects and the basement membrane thickness was measured by light microscopy and electron microscopy. At the same time the numbers of eosinophils and fibroblasts were assessed and the expression of transforming growth factor beta 1 (TGF-beta 1), platelet derived growth factor (PDGF), and insulin like growth factor (IGF) I in the bronchial mucosa was examined by immunostaining. The relationship between the degree of thickening of the subepithelial layer and both the clinical data and pulmonary function were also investigated.

RESULTS

The basement membrane of the asthmatic patients was thicker than that of the healthy controls (median 8.09 versus 4.02 microns). Electron microscopic examination of the basement membrane revealed thickening of the subepithelial lamina reticularis; this thickening significantly correlated with the number of fibroblasts in the submucosa in the asthmatic subjects (rs = 0.88) but not in the controls (rs = 0.70). There was a significantly higher number of eosinophils in the airways of the asthmatic subjects than in the healthy subjects (EG1 + cells: 52.0 versus 2.0/mm2, EG2 + cells: 56.0 versus 1.5/mm2). The expression of each growth factor in the bronchial mucosa was similar in asthmatic and healthy subjects (TGF-beta 1: 18.0% versus 16.0%, PDGF: 37.0% versus 32.5%, IGF-I: 15.0% versus 8.0%). A weak but statistically significant correlation was found between the number of fibroblasts and the expression of TGF-beta 1 in asthmatic subjects (rs = 0.50). There was a significant correlation between the thickness of the subepithelial layer in asthmatic subjects and the attack score (rs = 0.58) and a significant inverse correlation between the subepithelial collagen thickness in asthmatic subjects and airway hypersensitivity (rs = -0.65).

CONCLUSIONS

These findings indicate that the thickening of the subepithelial layer in bronchial asthma is due to an increase in fibroblasts, and that the thickness of the subepithelial collagen appears to be linked to an increase in bronchial responsiveness and exacerbation of clinical manifestations.

Extracellular matrix and lung inflammation

Lung injury triggers an acute inflammatory response characterized by increased expression and deposition of extracellular matrix (ECM) components such as fibronectin and collagen. Although the function of newly deposited matrices in injured lungs is unknown, their ability to affect the migration, proliferation, differentiation, and activation state of cells in vitro suggests an important role in the initiation and maintenance of the inflammatory response in vivo. Interactions between immune and nonimmune cells with the lung ECM are mediated via cell surface receptors of the integrin family which link the ECM with intracellular molecules involved in signal transduction. Activation of integrin-mediated intracellular signals may promote inflammation by facilitating leukocyte recruitment and cytokine expression.

Mechanisms of human eosinophil activation by complement protein C5a and platelet-activating factor: similar functional responses are accompanied by different morphologic alterations

The complement system is an important amplification system for the propagation of allergic as well as pseudoallergic inflammatory reactions. In the present study, the effect of the major anaphylatoxin C5a was compared with that of platelet-activating factor (PAF) on highly purified eosinophils (> or = 95%) by functional as well as morphologic criteria. Upon stimulation with C5a, eosinophils maintained their spheric structure, developing short, pseudopodia-like protrusions, whereas PAF induced the generation of a number of digitating protrusions. As shown by functional and ultrastructural assay systems, both stimuli provoked significant extracellular and intracellular H2O2 production in eosinophils, which was inhibited by cytochalasin B. With C5a, a pronounced H2O2 production was detected within the small cytoplasmic vesicles, whereas PAF-induced H2O2 production was observed on the outer surface of the plasma membrane in the contact zones between adjacent cells. Morphologic signs of degranulation induced by C5a and PAF were accompanied by the significantly increased release of eosinophil cationic protein and eosinophil peroxidase in the presence of cytochalasin B. Like PAF, C5a induced a significant production of reactive oxygen species in eosinophils, as measured by lucigenin-dependent chemiluminescence (CL) responses in eosinophils. Maximal responses, comparable with those of interleukin-5 (100 U/ml), were observed at concentrations of 10(-5)-10(-6) and 10(-7)-10(-8) M for PAF and C5a, respectively. Separation of eosinophils by discontinuous density gradients revealed the existence of two hypodense eosinophil populations, one of them showing significantly reduced CL responses upon stimulation with C5a and PAF. In addition, CL responses upon stimulation with C5a and PAF were abrogated by cytochalasin B, staurosporine, and wortmannin, and were almost completely blocked by pertussis toxin. In conclusion, these data indicate that C5a induces events in human eosinophils comparable to those induced by PAF in the assay systems tested. Thus, C5a, generated after activation of the complement system, may be of major importance for the eosinophil activation observed in eosinophil-related disease.

Modulation of bronchial inflammation: corticosteroids and other therapeutic agents

We have shown that treatment with inhaled corticosteroid acts to improve asthma-induced changes in the airway mucosa to such an extent that cell type and structure are normalized. More specifically, such treatment increases the number of intraepithelial nerves, decreases the total number of inflammatory cells, and eliminates epithelial eosinophils, as well as decreasing the number of endothelial gaps in the postcapillary venules. In contrast, these effects do not seem to be found following treatment with beta 2-agonists.

The safety aspects of fiberoptic bronchoscopy, bronchoalveolar lavage, and endobronchial biopsy in asthma

We have documented the physiologic effects of fiberoptic bronchoscopy with bronchoalveolar lavage (BAL) and endobronchial biopsy performed under local anesthesia in 20 asthmatic subjects, 8 healthy nonatopic control subjects, and 8 atopic nonasthmatic subjects. Premedication consisted of nebulized albuterol (2.5 mg; except for the study of atopic nonasthmatic subjects), ipratropium bromide (500 micrograms), and intramuscular atropine (0.6 mg). Intravenous midazolam was given for mild sedation, and oxygen was delivered via a nasal cannula. FEV1 was measured before and after premedication, immediately postbronchoscopy, and after 2 h recovery. There was a significant fall in mean (+/- SD) FEV1 immediately postbronchoscopy in both the asthmatic (26.2 +/- 16.7%; p less than 0.001) and normal (9 +/- 4.7%, p less than 0.05) groups, which in the asthmatic subjects correlated inversely with the concentration of methacholine provoking a 20% fall in FEV1 (PC20) measured 5 days prebronchoscopy (r = -0.74, p less than 0.001) but not with symptom scores, albuterol use, or peak expiratory flow (PEF) variation recorded during 2 wk before the investigation. There was significant arterial hemoglobin O2 desaturation during biopsy in the asthmatic subjects (median 3%, range -1 to 17% fall from baseline; p less than 0.01), which was not related to any of the measured indices of asthma severity. PC20, measured 5 days before and 5 days after bronchoscopy in the asthmatic subjects and 2 days before and 1 day after bronchoscopy in the atopic nonasthmatic subjects was not significantly affected by the procedure. We conclude that fiberoptic bronchoscopy with BAL and endobronchial biopsy can be conducted safely in asthmatic subjects, but requires caution in those with very responsive airways.(ABSTRACT TRUNCATED AT 250 WORDS)

Myofibroblasts and subepithelial fibrosis in bronchial asthma

A thickened bronchial epithelial basement membrane has long been regarded as a histopathologic characteristic of bronchial asthma. As we had previously demonstrated that this phenomenon is due to the deposition of interstitial collagens and fibronectin, we have now sought to determine the nature of the cell responsible for this process by studying endobronchial biopsies from eight normal and seven asthmatic volunteers by immunohistochemistry and electron microscopy. Biopsies were stained with PR 2D3, a monoclonal antibody to myofibroblasts of the pericrypt sheath of the colon and a monoclonal antibody to alpha-smooth muscle actin. The thickness of the subepithelial collagen and the organelle content of the cells therein were determined by electron microscopy. The subepithelial collagen thickness in the normal subjects ranged from 2.16 to 6.26 microns, while that in the asthmatic subjects ranged from 3.75 to 11.1 microns (Mann-Whitney test; P = 0.05). Elongated cells in the collagen layer were identified by staining with PR 2D3. As this antibody also stains smooth muscle, consecutive frozen sections were stained for alpha-smooth muscle actin and the number of positive cells per millimeter of basement membrane was subtracted from the count for PR 2D3. This yielded a count of 4.9 to 9.4 cells/mm in the normal subjects and 11.9 to 20.6 cells/mm in the asthmatics (P = 0.001). There was a highly significant correlation between the depth of subepithelial collagen and the number of PR 2D3-positive, alpha-smooth muscle actin-positive cells (Spearman rank correlation; r = 0.764 and P = 0.006). Electron microscopy confirmed the myofibroblastic nature of these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Mucosal inflammation in asthma

Over the past decade, it has become increasingly recognized that airways inflammation is one of the major components of asthma. Until recently, measurements of bronchial responsiveness and mediators of allergic reactions were the only methods of studying pathogenetic mechanisms in asthma. With improved diagnostic procedures such as fiberoptic bronchoscopy, it has become possible to investigate these mechanisms and the resulting inflammatory changes in situ. BAL has highlighted the presence of mast cells and eosinophils and has given proof of their mediator participation in airways inflammation and hyperresponsiveness. Endobronchial biopsies have so far yielded results that are similar to those obtained from postmortem studies, although it appears that there are varying degrees of inflammation in living asthmatics. Even in mild disease, the histopathologic features of bronchial asthma are consistent with chronic inflammation. Indirect evidence obtained from allergen challenge leading to increased bronchial hyperresponsiveness during LAR, and direct evidence of inflammatory cells and their mediators in the airway mucosa and lumen after allergen challenge argue for an active role of cells in bringing about inflammatory changes. At present, however, it is not possible to relate precisely the findings obtained by bronchoscopy to the clinical presentation and progression of asthma. Cell activation with production of potent mediators of inflammation may be more relevant to inflammation than the simple presence of these cells in the airways. Almost all the inflammatory cells present in the bronchial wall and lumen have been implicated in the pathogenesis of mucosal inflammation in asthma, but with our current state of knowledge, none can be singled out as the most important contributor. The mast cell was the first to be investigated in depth, and despite the accumulation of large amounts of data concerning its ultrastructure and function, it remains uncertain to what extent this cell is involved in inflammatory responses. Thus, while its main role appears to be that of initiator of allergen-induced responses, the eosinophil has attracted more attention as a proinflammatory cell rather than as an antiinflammatory cell with a capacity to be selectively recruited from the circulation in response to IgE-dependent signals. The eosinophil secretes potent mediators that cause damage to the bronchial epithelium and lead to bronchoconstriction. The role of other cells is at present not as well defined.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunologic lung disease

The term "immunologic lung disease" comprises a broad spectrum of disease. I have covered a few entities in which recent studies have been particularly helpful in elucidating pathophysiology though not in uncovering the inciting cause. Common to all of these entities is the problem of finding appropriate methods of defining disease activity and response to treatment. As exemplified by the improved outlook for Goodpasture's syndrome with elucidation of its underlying immunopathology, it is likely that better understanding of the immunologic basis of sarcoid and interstitial disease may be helpful in planning more effective treatment strategies.