Truncated Plasminogen Activator Inhibitor-1 Protein Protects From Pulmonary Fibrosis Mediated by Irradiation in a Murine Model

Purpose: To determine whether the delivery of recombinant truncated plasminogen activator inhibitor-1 (PAI-1) protein (rPAI-1₂₃) would protect from the development of radiation-induced lung injury. Methods and Materials: C57Bl/6 mice received intraperitoneal injections of rPAI-1₂₃ (5.4 μg/kg/d) or vehicle for 18 weeks, beginning 2 days before irradiation (IR) (5 daily fractions of 6 Gy). Cohorts of mice were followed for survival (n=8 per treatment) and tissue collection (n=3 per treatment and time point). Fibrosis in lung was assessed with Masson-Trichrome staining and measurement of hydroxyproline content. Senescence was assessed with staining for β-galactosidase activity in lung and primary pneumocytes. Results: Hydroxyproline content in irradiated lung was significantly reduced in mice that received rPAI-1₂₃ compared with mice that received vehicle (IR+vehicle: 84.97 μg/lung; IR+rPAI-1₂₃: 56.2 μg/lung, P=.001). C57Bl/6 mice exposed to IR+vehicle had dense foci of subpleural fibrosis at 19 weeks, whereas the lungs of mice exposed to IR+rPAI-1₂₃ were largely devoid of fibrotic foci. Cellular senescence was significantly decreased by rPAI-1₂₃ treatment in primary pneumocyte cultures and in lung at multiple time points after IR. Conclusions: These studies identify that rPAI-1₂₃ is capable of preventing radiation-induced fibrosis in murine lungs. These antifibrotic effects are associated with increased fibrin metabolism, enhanced matrix metalloproteinase-3 expression, and reduced senescence in type 2 pneumocytes. Thus, rPAI-1₂₃ is a novel therapeutic option for radiation-induced fibrosis.