Mechanisms of occupational asthma

Review of the occupational exposure to isocyanates: Mechanisms of action

Polyurethanes are useful polymers in a large variety of technical and consumer products that are generally made from diisocyanates and polyols or similar compounds. Toluene diisocyanate (TDI), 4,4'-methylenediphenyl diisocyanate (MDI) and 1,6'-hexamethylene diisocyanate (HDI) are useful for polyurethane products. Isocyanates are reactive chemicals that can be handled without problems in manufacturing or technical environments. In general, consumers may only have contact with these chemicals on rare occasions. The objective of this study was to review the mechanisms of action of inhalation of isocyanates. This paper describes, in summary, the potential occupational exposure to isocyanates, the chemistry and reactivity of isocyanates, the results from genotoxicity studies, investigative toxicity studies, metabolism and results from epidemiology studies on isocyanate-exposed workers. The overall conclusion is that because humans are not exposed to high levels of respiratory isocyanate particles, concerns over the possible development of lung tumors should not be relevant. There are many mechanisms of action induced by isocyanates, but those entities are unclear. This is because these mechanisms act simultaneously and are complex.

Relevance of inhalational exposure to food allergens

PURPOSE OF REVIEW

This review discusses the inhalational route as a clinically important route of exposure to food allergens.

RECENT FINDINGS

In childhood, we have recently demonstrated that food allergens can induce both early and late phase bronchial reactions within blinded, placebo-controlled challenges. Additionally, clinically important levels of food allergens have been measured in environmental air samples.

SUMMARY

It is well known that the ingestion of food allergens frequently causes respiratory symptoms and that the mechanism of death in fatal anaphylaxis is usually profound bronchospasm. The mechanism by which ingested food allergens induce bronchial reactions is unclear. There are many case reports of bronchial reactions to aerosolized food allergens. Within the food industry the problems have been examined more systematically. From such work it is possible to gain an impression of the potential prevalence of the problem. With 10% of adult asthma being occupational and 10% of occupational asthma being induced by aerosolized food, inhalational exposure to food allergens plays a major role in at least 1% of adult asthma. For a patient with co-existent food allergy and asthma it is important that both dietary and environmental avoidance be practised. The similar pathophysiology of allergic and occupational asthma and the ability of inhaled food allergens to cause the latter raises the question as to whether aerosolized food could play a role in the pathogenesis of childhood asthma.

A novel mouse model of diisocyanate-induced asthma showing allergic-type inflammation in the lung after inhaled antigen challenge

BACKGROUND

Exposure to diisocyanates, a group of highly reactive, low-molecular-weight compounds, is a major cause of occupational asthma. In contrast to mouse models of atopic asthma, previous mouse models of diisocyanate-induced asthma have failed to show lung inflammation with characteristics of human disease.

OBJECTIVE

Our goal was to establish a novel mouse model of diisocyanate-induced asthma in which lung inflammation reminiscent of that seen in human asthma is generated after inhaled antigen challenge.

METHODS

BALB/c mice were epicutaneously sensitized to hexamethylene diisocyanate (HDI) and then challenged with an HDI-protein conjugate administered by means of an intranasal droplet.

RESULTS

HDI sensitization resulted in development of contact hypersensitivity and HDI-specific antibody production. Most importantly, however, vigorous inflammatory responses with characteristics of human asthma were generated in the lung after inhaled HDI challenge. Challenge of sensitized, but not unsensitized, mice resulted in airway eosinophilia, mucus hypersecretion, and production of T(H)1-type (IFN-gamma) and T(H)2-type (IL-4, IL-5, and IL-13) cytokines by lung inflammatory cells. Despite the mixed T(H)1/T(H)2 response induced by HDI sensitization, use of cytokine-deficient mice revealed that airway eosinophilia was mediated by T(H)2 cytokines and not by IFN-gamma.

CONCLUSION

We report a novel mouse model of diisocyanate-induced asthma that, in contrast to previous models, demonstrates antigen-induced lung inflammation with characteristics of human disease. This model will allow investigation of the immunopathogenesis of diisocyanate-induced asthma and should provide insight into this common form of occupational disease.

Importance of inflammatory and immune components in a mouse model of airway reactivity to toluene diisocyanate (TDI)

BACKGROUND

Nearly 9 million individuals are exposed to agents in the workplace associated with asthma, and isocyanates represent the most common cause of occupationally induced asthma.

OBJECTIVES

Nonetheless, the immunological mechanisms responsible for isocyanate-induced asthma are not clear. A murine model for toluene diisocyanate (TDI) asthma is described and employed to examine inflammatory and immune components that may be involved in the disease.

METHODS

Groups (n = 6) of C57BL/6J and athymic mice were sensitized by subcutaneous injection (20 microl on day 1, 5 microl on days 4 and 11), and 7 days later challenged by inhalation (100 p.p.b., days 20, 22 and 24) with TDI. Twenty-four hours following the last challenge the tracheae and lungs were examined for histological changes as well as for the expression of Th1, Th2 and pro-inflammatory cytokines. Mice were also examined for airway reactivity to methacholine challenge and for specific and total IgE and IgG antibodies.

RESULTS

TDI sensitization resulted in increased reactivity to methacholine challenge as well as a significant inflammatory response in the trachea and nares of wild-type mice, but not in the athymic mice nor in the lungs of the C57BL/6J mice. Airway inflammation was characterized by inflammatory cell influx, goblet cell metaplasia and epithelial damage. Histological changes in the trachea were accompanied by increased mRNA expression of interleukin (IL)-4, tumour necrosis factor alpha, lymphotoxin beta, lymphotactin and Rantes, as well as TDI-specific IgG antibodies and elevated levels of total IgE. IgE-specific antibodies were not detected with this exposure regimen but were produced when the TDI concentrations were increased.

CONCLUSIONS

These studies provide a unique murine model for occupational asthma that generates both inflammatory and immune mediators similar to those occurring in TDI-induced asthma in humans.

Changes in sputum cell counts after exposure to occupational agents: what do they mean?

BACKGROUND

Exposure to occupational agents can induce eosinophilic inflammation in subjects with occupational asthma (OA). It might also induce nonspecific changes in airway inflammation in subjects without OA.

OBJECTIVES

We sought to investigate the changes in airway inflammation induced by exposure to occupational agents in subjects with and without OA and to determine which changes in sputum eosinophil numbers and bronchial responsiveness to methacholine should be regarded as clinically significant for predicting a 20% fall in FEV(1).

METHODS

We performed specific inhalation challenges (SICs) in 3 groups of subjects: subjects reporting a history consistent with OA with a positive SIC response (n = 17); subjects reporting a history consistent with OA with a negative SIC response (n = 14); and asthmatic subjects without any history of OA (n = 10). Induced sputum and methacholine challenges were performed at the end of the control day and again at the end of the last day of exposure; the last day of exposure was always performed in the laboratory.

RESULTS

There was an increase in median sputum eosinophil and neutrophil numbers in subjects with positive SIC responses. Cell counts remained unchanged after exposure in asthmatic subjects without OA. A combination of a greater than 0.26 10(6)/mL increase in sputum eosinophil numbers and a decrease in the concentration of methacholine inducing a 20% fall in FEV(1) of at least 1.8-fold compared with baseline values predicted a 20% fall in FEV(1) in 96% (95% CI, 70%-99%) of patients.

CONCLUSION

Exposure to occupational agents per se does not induce airway inflammation. Changes in both sputum eosinophil counts and methacholine responsiveness are satisfactory predictors of a significant bronchial responsiveness to occupational agents.

Subclinical immunologic and physiologic responses in hexamethylene diisocyanate-exposed auto body shop workers

BACKGROUND

Diisocyanates are potent sensitizing agents and currently the most commonly identified cause of occupational asthma in industrialized countries. However, diisocyanate asthma is difficult to diagnose and exposure and host risk factors are unclear. Auto body shops, one of the most common hexamethylene diisocyanate (HDI) exposure settings, are particularly difficult to study due to their small size and episodic exposures. Surveillance studies of such workers are limited.

OBJECTIVES

We have initiated a cross-sectional field epidemiologic study, Survey of Painters and Repairers of Auto bodies by Yale (SPRAY), to characterize the effects of diisocyanate exposures on actively employed auto body shop workers. Methods and Results We present here questionnaire, physiologic, immunologic, and exposure data on 75 subjects enrolled in the study. No overt cases of clinically apparent diisocyanate asthma were identified based on spirometry, methacholine challenge, peak flows, and symptoms. HDI-specific lymphocyte proliferation was present in 30% of HDI-exposed workers and HDI-specific IgG in 34% of HDI-exposed workers, but they were not associated. HDI-specific IgE was detected in two workers. HDI-specific lymphocyte proliferation, increased methacholine responsiveness, and symptoms of chest tightness and shortness of breath were more common in the most heavily HDI-exposed workers, the painters. More long-term follow-up of this cohort should clarify the significance of these HDI-specific immunologic responses, physiologic changes, and symptoms.

CONCLUSIONS

These findings demonstrate the presence of HDI-specific immune responses in a large proportion of healthy HDI-exposed workers.

T cell involvement in persulfate triggered occupational contact dermatitis and asthma

BACKGROUND

Ammonium and potassium persulfates may induce a variety of cutaneous and respiratory diseases. The precise underlying mechanisms, however, are unclear.

OBJECTIVE

To describe a hairdresser, who developed contact dermatitis, rhinoconjuntivitis, and bronchial asthma of delayed onset after occupational exposure to hair bleaches containing persulfate salts and to provide evidence for a common T-cell mediated mechanism responsible for the clinical manifestations.

METHODS

We performed skin testing, routine histologic and immunohistochemical examination of the skin reaction after prick testing, lymphocyte proliferation analysis, nasal challenge test, and pulmonary function testing.

RESULTS

The causative role of bleaching powder and ammonium persulfate was demonstrated by case history, skin tests, and a nasal challenge test. Patch tests produced a delayed cutaneous reaction to ammonium persulfate confirming contact sensitization. Prick tests with bleaching powder and ammonium persulfate were negative at 15 minutes but revealed a late skin reaction with a papule at the prick sites after 24 hours. Histologic examination of this late reaction demonstrated a perivascular infiltration comprising predominantly T lymphocytes. Further, a significant proliferation of T cells to bleaching powder was reproducibly found by a lymphocyte proliferation analysis. Nasal challenge test with bleaching powder showed a significant reduction of air flow after 24 hours.

CONCLUSION

Our findings suggest that immunologic mechanism with direct involvement of T cells may not only play an important role in the pathogenesis of the cutaneous but also in the respiratory and rhinoconjunctival reactions.

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.

Are chronic obstructive pulmonary disease (COPD) and asthma different diseases?

In the past it has been hypothesized that asthma and chronic obstructive pulmonary disease (COPD) are related diseases. As both show high prevalences and as smoking may occur in both groups of patients the simultaneous presence of both diseases in the same subject may well occur. The most characteristic features of asthma are short-term changes in airway caliber, reversible airflow limitation, bronchial hyperresponsiveness as well as eosinophilic and lymphocytic airway inflammation. The common definition of COPD implies long-term changes in airway calibre, less or irreversible airflow obstruction, no or less common bronchial hyperresponsiveness and neutrophilic airway inflammation. By analysing the results of epidemiology, risk factors, airway morphology, lung function and markers of airway inflammation in both diseases, it becomes clear that asthma and COPD share some similarities. However, their pathological presentations differ considerably and this provides evidence that the two syndromes are, in fact, different diseases. Furthermore, the analysis of data on lung volumes, diffusion capacity and aerosol-derived airway morphology shows important differences between asthma and COPD. It is suggested that the specific type of airway inflammation in both diseases is responsible for the peculiar pattern of lung function abnormalities.