HLA phenotype and exposure in development of occupational asthma

Mechanistic and Therapeutic Approaches to Occupational Exposure-Associated Allergic and Non-Allergic Asthmatic Disease

PURPOSE OF REVIEW

Occupational lung disease, including asthma, is a significant cause of disability worldwide. The dose, exposure frequency, and nature of the causal agent influence the inflammatory pathomechanisms that inform asthma disease phenotype and progression. While surveillance, systems engineering, and exposure mitigation strategies are essential preventative considerations, no targeted medical therapies are currently available to ameliorate lung injury post-exposure and prevent chronic airway disease development.

RECENT FINDINGS

This article reviews contemporary understanding of allergic and non-allergic occupational asthma mechanisms. In addition, we discuss the available therapeutic options, patient-specific susceptibility and prevention measures, and recent scientific advances in post-exposure treatment conception. The course of occupational lung disease that follows exposure is informed by individual predisposition, immunobiologic response, agent identity, overall environmental risk, and preventative workplace practices. When protective strategies fail, knowledge of underlying disease mechanisms is necessary to inform targeted therapy development to lessen occupational asthma disease severity and occurrence.

Progress in Occupational Asthma

Occupational asthma (OA) represents one of the major public health problems due to its high prevalence, important social and economic burden. The aim of this review is to summarize current data about clinical phenotypes, biomarkers, diagnosis and management of OA, a subtype of work-related asthma. Most studies have identified two phenotypes of OA. One is sensitizer-induced asthma, occuring after a latency period and caused by hypersensitivity to high- or low-molecular weight agents. The other is irritant-induced asthma, which can occur after one or more exposures to high concentrations of irritants without latency period. More than 400 agents causing OA have been identified and its list is growing fast. The best diagnostic approach for OA is a combination of clinical history and objective tests. An important tool is a specific inhalation challenge. Additional tests include assessments of bronchial hyperresponsiveness to methacholine/histamine in patients without airflow limitations, monitoring peak expiratory flow at- and off-work, sputum eosinophil count, exhaled nitric oxide measurement, skin prick tests with occupational allergens and serum specific IgE. Treatment of OA implies avoidance of exposure, pharmacotherapy and education. OA is a heterogeneous disease. Mechanisms of its different phenotypes, their diagnosis, role of new biomarkers and treatment require further investigation.

HLA and asthma phenotypes/endotypes: a review

Asthma is a complex chronic inflammatory disease of the airways caused by the interaction of genetic susceptibility with environmental influences. Genome-wide association studies (GWAS) represent the most powerful approach for asthma, that have identified several genes (e.g., IL18R1, IL33, SMAD3, ORMDL3, HLA-DQ and IL2RB loci). HLA super-locus is a genomic region in the chromosomal position 6p21. Since no gene can be considered as an asthma gene, able to reflect the complex etiology and the heterogeneity of the disease the terms 'phenotype' and more recently 'endotype' have been used. This review, according to literature availability, focuses on the relationship between human leukocyte antigens (HLA) region specifically the HLA class II genes and different asthma phenotypes/endotypes, such as allergic asthma/Th2 associated, occupational and aspirin-sensitive asthma. The most common HLA haplotypes in the different asthma phenotypes are HLA-DRB1in allergic asthma, HLA-DQB1in occupational asthma and HLA-DPB1 in aspirin-sensitive asthma. However, it is difficult to study the role of class II genes in vivo because of the heterogeneity of human population, the complexity of MHC, and the strong linkage disequilibrium among different class II genes. Despite the variation and the inconsistency of the HLA haplotypes and alleles in different types of asthma, the association between HLA class II genes and asthma has been demonstrated in the majority of studies.

Current topics in occupational asthma

The study of occupational asthma (OA) provides insights into asthma in general, as the cause is known. The relationships between the cause and response can be measured and modifying factors can be identified and their influence quantified. Developing OA has much more serious consequences for the patient than new onset asthma unrelated to work exposures, as the patient's livelihood is nearly always affected. Many healthcare professionals are more ready to accept and act on asthmatic symptoms when they are unrelated to work than when work may be the cause; antagonism can also occur in the workplace. This article reviews some of the areas where development and controversy enrich the study of OA. It makes no attempt to be comprehensive.

Occupational asthma: current concepts in pathogenesis, diagnosis, and management

Occupational asthma (OA) may account for 25% or more of de novo adult asthma. The nomenclature has now better defined categories of OA caused by sensitizing agents and irritants, the latter best typified by the reactive airways dysfunction syndrome. Selecting the most appropriate diagnostic testing and management is driven by assessing whether a sensitizer is involved, and if so, identifying whether the sensitizing agent is a high-molecular-weight agent such as a protein or a low-molecular-weight reactive chemical such as an isocyanate. Increased understanding of the pathogenesis of OA from reactive chemical sensitizers is leading to development of better diagnostic testing and also an understanding of why testing for sensitization to such agents can be problematic. Risk factors for OA including possible genetic factors are being delineated better. Recently published guidelines for the diagnosis and management of occupational asthma are summarized; these reflect an increasingly robust evidence basis for recommendations. The utility of diagnostic tests for OA is being better defined by evidence, including sputum analysis performed in relation to work exposure with suspected sensitizers. Preventive and management approaches are reviewed. Longitudinal studies of patients with OA continue to show that timely removal from exposure leads to the best prognosis.

Impact of negatively charged patches on the surface of MHC class II antigen-presenting proteins on risk of chronic beryllium disease

Chronic beryllium disease (CBD) is a granulomatous lung disease that occurs primarily in workers who are exposed to beryllium dust or fumes. Although exposure to beryllium is a necessary factor in the pathobiology of CBD, alleles that code for a glutamic acid residue at the 69th position of the HLA-DPbeta1 gene have previously been found to be associated with CBD. To date, 43 HLA-DPbeta1 alleles that code for glutamic acid 69 (E69) have been described. Whether all of these E69 coding alleles convey equal risk of CBD is unknown. The present study demonstrates that, on the one hand, E69 alleloforms of major histocompatibility complex class II antigen-presenting proteins with the greatest negative surface charge convey the highest risk of CBD, and on the other hand, irrespective of allele, they convey equal risk of beryllium sensitization (BeS). In addition, the data suggest that the same alleles that cause the greatest risk of CBD are also important for the progression from BeS to CBD. Alleles convey the highest risk code for E26 in a constant region and for E69, aspartic acid 55 (D55), E56, D84 and E85 in hypervariable regions of the HLA-DPbeta1 chain. Together with the calculated high binding affinities for beryllium, these results suggest that an adverse immune response, leading to CBD, is triggered by chemically specific metal-protein interactions.

Future advances in work-related asthma and the impact on occupational health

Work-related asthma (WRA) and WRA-like conditions refer to all situations in which asthma or asthma-like syndromes occur or worsen in the workplace. This occurs in approximately 10% of all adult-onset asthma subjects. Occupational asthma (OA) is a specific type of WRA that is 'caused' by the workplace, being mediated either by an allergic process with a latency period or by a non-allergic irritant-induced mechanism. Personal asthma can also 'worsen' at work (work-aggravated or exacerbated asthma), the reasons, mechanisms, extent and consequences of this situation being unknown. The author reviews various aspects of WRA with an emphasis on OA (about which more is known) and proposes key issues that need to be further studied, proposed and applied in at-risk workplaces in order to improve recognition, diagnosis and management of this condition. OA represents a unique situation that, unfortunately, is only very rarely provided to health-care providers: affected workers can be cured with minimal impact on quality of life. All efforts should be made to achieve this goal at an affordable socio-economic cost.

What makes a chemical a respiratory sensitizer?

PURPOSE OF REVIEW

In this article we consider the characteristics that are associated with chemical respiratory allergens, and that may be essential for effective sensitization of the respiratory tract.

RECENT FINDINGS

Chemical respiratory allergens share some characteristics with other chemical allergens, specifically chemical allergens that cause skin sensitization and allergic contact dermatitis. The unique and defining characteristic of chemical respiratory allergens, which in most instances distinguishes them from contact allergens, is the ability to provoke the preferential development of T helper 2-type immune responses. There are, in addition, other characteristics, such as the ability to increase matrix metalloproteinase expression or to cause perturbation of redox homeostasis, that may in some instances facilitate the induction or expression of respiratory allergy, but it is not yet clear if these attributes are common or essential properties of all chemical respiratory sensitizers.

SUMMARY

Predicting which chemical allergens may selectively induce respiratory sensitization is an important objective, but remains a significant challenge because our understanding of the relevant physicochemical characteristics and biological properties that confer on chemicals respiratory allergenic potential is incomplete.

Occupational asthma

Substantial epidemiologic and clinical evidence indicates that agents inhaled at work can induce asthma. In industrialized countries, occupational factors have been implicated in 9 to 15% of all cases of adult asthma. Work-related asthma includes (1) immunologic occupational asthma (OA), characterized by a latency period before the onset of symptoms; (2) nonimmunologic OA, which occurs after single or multiple exposures to high concentrations of irritant materials; (3) work-aggravated asthma, which is preexisting or concurrent asthma exacerbated by workplace exposures; and (4) variant syndromes. Assessment of the work environment has improved, making it possible to measure concentrations of several high- and low-molecular-weight agents in the workplace. The identification of host factors, polymorphisms, and candidate genes associated with OA is in progress and may improve our understanding of mechanisms involved in OA. A reliable diagnosis of OA should be confirmed by objective testing early after its onset. Removal of the worker from exposure to the causal agent and treatment with inhaled glucocorticoids lead to a better outcome. Finally, strategies for preventing OA should be implemented and their cost-effectiveness examined.

Occupational asthma

PURPOSE OF REVIEW

The scientific literature on occupational asthma is steadily increasing and no longer only describes case reports with prevalence figures and limited physiopathologic and immunologic data. Prospective cohort studies are currently carried out with relevant incidence figures and risk factors. Genetic susceptibility is explored. New diagnostic means are described. Surveillance programs are conducted and preventive measures are proposed.

RECENT FINDINGS

A relevant account was published on the respiratory consequences (irritant-induced asthma, a type of occupational asthma) of exposure of firefighters at the World Trade Center site. Glutathione-S-transferase appears to be an interesting susceptibility gene for occupational asthma as a result of isocyanates. Inflammation caused by neutrophils and not only by eosinophils is more often associated with occupational asthma. Examination of induced sputum and assessment of exhaled nitric oxide are becoming useful diagnostic tools. Cleaners are at increased risk of developing occupational asthma, although the causal agents are unknown. Surveillance programs currently combine strategies that aim to reduce exposure and identify cases early.

SUMMARY

New scientific information provides better, accurate figures on the frequency of occupational asthma and on risk factors, proposes relevant diagnostic tools more directly related to the physiopathology of the disease, and suggests effective surveillance programs in high-risk workplaces.

Clinical phenotypes of asthma

PURPOSE OF REVIEW

Asthma is a phenotypically heterogeneous disorder and, over the years, many different clinical subtypes of asthma have been described. A precise definition of asthma phenotypes is now becoming more and more important, not only for a better understanding of pathophysiologic mechanisms, but in particular to ascertain the specific genes associated with these phenotypes.

RECENT FINDINGS

In children, three asthma phenotypes are now well defined: transient infant wheezing, nonatopic wheezing of the toddler, and IgE-mediated wheezing/asthma. Recently, a fourth phenotype, late-onset childhood asthma has been added to this list. In adults, asthma persisting from childhood into adulthood should be distinguished from asthma starting in adulthood. The phenotypes of adult-onset asthma are still poorly defined. Until now, phenotypic classification has been based primarily on etiologic factors (eg, aspirin sensitivity, persistent respiratory infections, occupational factors, or toxic exposures) or clinical characteristics of the disease (eg, mild, severe, brittle, near fatal, with fixed airflow obstruction, steroid resistant). Novel noninvasive techniques to assess the type and severity of airway inflammation and dysfunction are increasingly used to identify better the different phenotypes.

SUMMARY

The classic phenotype of IgE-mediated asthma starting in childhood is now clearly defined. However, many other phenotypes of asthma in childhood as well in adulthood are being recognized. In particular, asthma starting in adulthood and noneosinophilic asthma constitute an important part of the adult asthma population, and are still poorly defined. A precise definition of these asthma phenotypes is urgently needed because they are likely to be associated with different genotypes, responses to treatment, and prognoses.