PDCD1 and pulmonary fibrosis: CD28-T cells in IPF patients may promote lung fibrosis, but the immune checkpoint proteins Cytotoxic T-Lymphocyte-Associated Antigen-4 (CTLA-4) and PD-1 limit this effect (66), and it has also been shown that the poor prognosis of IPF patients is associated with downregulation of CD28 on CD4+ T cells in the periphery system (88, 89).