IFIH1 and autoimmune pulmonary alveolar proteinosis: PSL, prednisolone; TAC, tacrolimus; IVCY, intravenous cyclophosphamide; IVIg, intravenous immunoglobulin; MDA-5, melanoma differentiation-associated gene 5; GM-CSF, granulocyte–macrophage colony-stimulating factor; KL-6, Krebs von den Lungen 6; APAP, autoimmune pulmonary alveolar proteinosis