Thrombopoietin mimetic reduces mouse lung inflammation and fibrosis after radiation by attenuating activated endothelial phenotypes
Radiation-induced lung injury (RILI) initiates radiation pneumonitis and progresses to fibrosis as the main side effect of lung cancer patients treated with radiotherapy. There is no effective drug for RILI. Sustained vascular activation is a major contributor to the establishment of chronic disease. Here, using a whole thoracic irradiation (WTI) mouse model, we investigated the mechanisms and effectiveness of thrombopoietin mimetic (TPOm) for preventing RILI. We demonstrated that administering TPOm 24 hours before irradiation decreased histologic lung injury score, apoptosis, vascular permeability, expression of pro-inflammatory cytokines, and neutrophil infiltration in the lung of mice 2 weeks after WTI. We described the expression of c-MPL, a TPO receptor, in mouse primary pulmonary microvascular endothelial cells, showing TPOm reduced endothelial cell-neutrophil adhesion by inhibiting ICAM-1 expression. Seven months after WTI, TPOm-treated lung exhibited less collagen deposition, expression of MMP-9, TIMP-1, IL-6, TGF-b, and p21. Moreover, TPOm improved lung vascular structure, lung density, and respiration rate, leading to a prolonged survival time after WTI. Single-cell RNA sequencing analysis of lungs 2 weeks after WTI revealed that TPOm shifted populations of capillary endothelial cells towards a less activated and more homeostatic phenotype. Taken together, TPOm is protective for RILI by inhibiting endothelial cell activation. Overall design: C57BL/6 male mice underwent whole thoracic irradiation with 16.0 Gy  delivered by x-ray irradiator (300 kv and 10 mA, 1mm Cu filtration). Fourteen days post-IR, lungs were collected, and processed into a single-cell suspension after sacrificing mice.