Acute respiratory distress syndrome (ARDS) is characterized by increased permeability of the alveolar–capillary membrane, a thin barrier composed of adjacent monolayers of alveolar epithelial and lung microvascular endothelial cells. This results in pulmonary edema and severe hypoxemia and is a common cause of death after both viral (e.g., SARS-CoV-2) and bacterial pneumonia. The involvement of the lung in ARDS is notoriously heterogeneous, with consolidated and edematous lung abutting aerated, less injured regions. This makes treatment difficult, as most therapeutic approaches preferentially affect the normal lung regions or are distributed indiscriminately to other organs. In this review, we describe the use of thoracic ultrasound and microbubbles (USMB) to deliver therapeutic cargo (drugs, genes) preferentially to severely injured areas of the lung and in particular to the lung endothelium. While USMB has been explored in other organs, it has been under-appreciated in the treatment of lung injury since ultrasound energy is scattered by air. However, this limitation can be harnessed to direct therapy specifically to severely injured lungs. We explore the cellular mechanisms governing USMB and describe various permutations of cargo administration. Lastly, we discuss both the challenges and potential opportunities presented by USMB in the lung as a tool for both therapy and research.

In acute lung injury, the lung endothelial barrier is compromised. Loss of endothelial barrier integrity occurs in association with decreased levels of the tight junction protein claudin-5. Restoration of their levels by gene transfection may improve the vascular barrier but how to limit transfection solely to regions of the lung that are injured is unknown. We hypothesized that thoracic ultrasound in combination with intravenous microbubbles (USMB) could be used to achieve regional gene transfection in injured lung regions and improve endothelial barrier function. Since air blocks ultrasound energy, insonation of the lung is only achieved at areas of lung injury (edema, atelectasis); healthy lung is spared. Cavitation of the microbubbles achieves local tissue transfection. Here we demonstrate successful ultrasound-microbubble (USMB)-mediated gene transfection in the injured lungs of mice. After thoracic insonation, transfection was confined to the lung and only occurred in the setting of injured (but not healthy) lung. In a mouse model of acute lung injury, we observed down-regulation of endogenous claudin-5 and an acute improvement in lung vascular leakage and in oxygenation after claudin-5-over-expression by transfection. The improvement occurred without any impairment of the immune response as measured by pathogen clearance, alveolar cytokines and lung histology. In conclusion, USMB-mediated transfection targets injured lung regions and is a novel approach in the treatment of lung injury.