Circulating Surfactant Protein D: A Biomarker for Acute Lung Injury?

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are life-threatening lung diseases in critically ill patients. The lack of prognostic biomarkers has halted detection methods and effective therapy development. Quantitative biomarker-based approaches in the systemic circulation have been proposed as a means of enhancing diagnostic strategies as well as pharmacotherapy in a patient-specific manner. Pulmonary surfactants are complex mixtures made up of lipids and proteins, which are secreted into the alveolar space by epithelial type II cells under normal and pathological conditions. In this review, we summarize the current knowledge of SP-D in lung injury from both preclinical and clinical studies. Among surfactant proteins, surfactant protein-D (SP-D) has been more widely studied in ALI and ARDS. Recent studies have reported that SP-D has a superior discriminatory ability compared to other lung epithelial proteins for the diagnosis of ARDS, which could reflect the severity of lung injury. Furthermore, we shed light on recombinant SP-D treatment and its benefits as a potential drug for ALI, and we encourage further studies to translate SP-D into clinical use for diagnosis and treatment.