CTLA4 and idiopathic pulmonary fibrosis: This finding contrasts with our results and is likely explained by 3 major differences: (a) the use of immunocompetent versus severely immunodeficient mice; (b) fibrosis induced by repetitive bleomycin injury versus intravenous injection of IPF cells; and (c) initiation of CTLA4 blockade in the late phases of established fibrosis (5 weeks after the initial bleomycin dose and 3 weeks after a second dose) versus relatively early during fibrosis induction (7 days after intravenous injection of IPF fibroblasts).