Bronchiolitis obliterans (BO) is a severe respiratory illness due to the obstruction of the smallest airways of the lungs, the bronchioles. Inhalation of the -diketone diacetyl has been associated with the development of this disease in employees of the microwave popcorn production industry. Exposure of laboratory animals to diacetyl as well as other α-diketones results in airway epithelial injury, ultimately resulting in BO-like lesions. The electrophilic α-diketones interact with arginine residues causing altered structure and functioning of proteins. However, the critical proteins causing the observed toxicity have not yet been identified. Upon severe or repeated exposure to α-diketones the epithelium of the airways becomes severely damaged or the airways become completely denuded. In these injured regions of the airways the intrinsic regenerative capacity of the epithelium, via proliferation of basal cells and subsequent differentiation, is lost. This leads to compensatory proliferation in the adjacent mesenchyme in which fibroblast to myofibroblast transition may take place under the influence of inflammatory signals. Another possible cause of fibrogenesis is through the occurrence of epithelial-mesenchymal transition (EMT) within the injured airway epithelium. Excessive proliferation of fibrotic cells leads to the occlusion of the bronchioles resulting in dry cough, wheezing, shortness of breath and a strongly reduced lung function, the symptoms of BO.
This AOP is linked to EU-ToxRisk case study “ RDT: Popcorn Lung – read-across on diketones” in which the effects of α-diketone exposures are investigated using ex-vivo human precision cut lung slices and primary human bronchial epithelial cells cultured at the air-liquid interface.