To determine a potential role for this immune checkpoint protein in lung fibrosis, we employed a humanized mouse model carrying exon 2 and 3 of the human CTLA4 gene (hCTLA4 mice; C57BL/6 background) encoding an epitope containing its extracellular ligand binding and transmembrane domains (Figure 4A); this epitope is known to bind the FDA-approved anti-CTLA4 monoclonal antibody, ipilimumab (19). Here, CTLA4 is linked to pulmonary fibrosis.