Background and objective: The lack of useful biomarkers reflecting the disease state limits the management of Mycobacterium avium complex lung disease (MAC-LD). We clarified the associations between serum KL-6 level, disease progression and treatment response. Methods: Resected lung tissues from MAC-LD patients were immunostained for KL-6. We compared serum KL-6 levels between MAC-LD and healthy control or bronchiectasis patients without nontuberculous mycobacterial lung disease (NTM-LD). Serum KL-6 level was assessed in a prospective observational study at Keio University Hospital between May 2012 and May 2016. We investigated associations between serum KL-6 level and disease progression and treatment response in patients untreated for MAC-LD on registration (n = 187). Results: The KL-6+ alveolar type 2 cell population in the lung and serum KL-6 level were significantly higher in MAC-LD patients than in controls. Serum KL-6 level in bronchiectasis patients without NTM-LD showed no significant increase. Of the 187 patients who did not receive treatment on registration, 53 experienced disease progression requiring treatment. Multivariable Cox analysis revealed that the serum KL-6 level (aHR: 1.18, P = 0.005), positive acid-fast bacilli smear (aHR: 2.64, P = 0.001) and cavitary lesions (aHR: 3.01, P < 0.001) were significantly associated with disease progression. The change in serum KL-6 (ΔKL-6) was significantly higher in the disease progression group; it decreased post-treatment, reflecting the negative sputum culture conversion. Conclusion: Serum KL-6 level is associated with disease progression and treatment response. Longitudinal assessment combined with AFB smear status and presence of cavitary lesions may aid MAC-LD management.
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