PD-1+CD4+ T cells with reduced proliferative capacity and increased TGF-β/IL-17A expression were detected in IPF, sarcoidosis, and bleomycin CD4+ T cells. PD-1+ Th17 cells are the predominant CD4+ T cell subset expressing TGF-β. Coculture of PD-1+CD4+ T cells with human lung fibroblasts induced collagen-1 production. Here, PDCD1 is linked to idiopathic pulmonary fibrosis.