Conclusions
These data demonstrate radiation-induced ACE activation within the immune compartment promotes the pathogenesis of radiation pneumonitis, while ACE inhibition suppresses activation of proinflammatory immune cell subsets. Mechanistically, our in vitro data demonstrate radiation directly activates the ACE/type 1 angiotensin receptor pathway in immune cells and promotes generation of ROS via NADPH oxidase 2.
Purpose
Radiation-induced lung injury is a major dose-limiting toxicity for thoracic radiation therapy patients. In experimental models, treatment with angiotensin converting enzyme (ACE) inhibitors mitigates radiation pneumonitis; however, the mechanism of action is not well understood. Here, we evaluate the direct role of ACE inhibition on lung immune cells.
Results
In both the partial body irradiation and fractionated radiation therapy models, radiation increased ACE activity in lung immune cells. Treatment with lisinopril improved survival during radiation pneumonitis (P = .0004). Lisinopril abrogated radiation-induced increases in bronchoalveolar lavage fluid monocyte chemoattractant protein 1 (chemokine ligand 2) and MIP-1a cytokine levels (P < .0001). Treatment with lisinopril reduced both ACE expression (P = .006) and frequency of CD45+ CD11b+ lung myeloid cells (P = .004). In vitro, radiation injury acutely increased ACE activity (P = .045) and reactive oxygen species (ROS) generation (P = .004) in human monocytes, whereas treatment with lisinopril blocked radiation-induced increases in both ACE and ROS. Radiation-induced ROS generation was blocked by pharmacologic inhibition of either NADPH oxidase 2 (P = .012) or the type 1 angiotensin receptor (P = .013). Conclusions: These data demonstrate radiation-induced ACE activation within the immune compartment promotes the pathogenesis of radiation pneumonitis, while ACE inhibition suppresses activation of proinflammatory immune cell subsets. Mechanistically, our in vitro data demonstrate radiation directly activates the ACE/type 1 angiotensin receptor pathway in immune cells and promotes generation of ROS via NADPH oxidase 2.