PDCD1 and pulmonary fibrosis: Studies from clinical samples have shown that upregulation of PD-1 signaling on CD4+T cells promotes the process of lung fibrosis by facilitating signal transducer and activator of transcription 3 (STAT3)-mediated TGF-β1 production (98), and these results suggest that TGF-β can influence the development of lung fibrogenesis by modulating the alteration of T cells from a latent to an activated state.