MUC1 and pulmonary alveolar proteinosis: Notably, in eight cases (cases 2, 4, 6, 7, 10, 11, 12, and 13), the increase in KL-6 during immunosuppressive treatment was attributed to the exacerbation of IP rather than the onset of PAP, leading to intensified immunosuppressive treatment and subsequent exacerbation of PAP.