A 48-year-old man has gradually increasing dyspnea and a 4-kg weight loss over the past 2 years. He has smoked two packs of cigarettes per day for 20 years, but not for the past year. Physical examination shows an increase in the anteroposterior diameter of the chest. Auscultation of the chest shows decreased breath sounds. A chest radiograph shows bilateral hyperlucent lungs; the lucency is especially marked in the upper lobes. Pulmonary function tests show that FEV1 is markedly decreased, but FVC is normal, and the FEV1/FVC ratio is decreased. Which of the following is most likely to contribute to the pathogenesis of his disease?