SP-D Serum Levels Reveal Distinct Epithelial Damage in Direct Human ARDS

Acute respiratory distress syndrome (ARDS) is a heterogeneous syndrome with multiple underlying diseases. Particularly epithelial damage results from direct (e.g., pneumonia) rather than indirect lung injury (e.g., nonpulmonary sepsis), which is more likely associated with endothelial damage. Hence, targeting ARDS patients based on their molecular phenotypes is a promising approach to improve outcome. With regard to distinct inflammatory responses and subsequent lung damage in direct ARDS due to the causing pathogen, we quantified markers of epithelial and endothelial damage and pro-inflammatory cytokines in patients with ARDS triggered by bacterial, viral, and atypical pathogen pneumonia or indirect ARDS. The serum levels of interleukin-6 (IL-6) and interleukin-8 (IL-8), lung epithelial injury markers surfactant protein D (SP-D), and soluble receptor for advanced glycation end-products (sRAGE) as well as endothelial injury marker angiopoietin-2 (Ang-2) from 49 patients with distinct types of ARDS were analyzed by multiplex immunoassay. Epithelial damage marker SP-D was significantly higher in direct ARDS caused by viral and atypical pathogens in contrast to ARDS caused by typical bacterial pneumonia and nonpulmonary sepsis. In contrast, sRAGE levels did not differ due to the causing pathogen. Patients with atypical pathogen pneumonia related ARDS showed significantly lower Ang-2 levels compared to patients with viral and indirect ARDS. Patients with viral and atypical pneumonia related ARDS possessed significantly lower serum IL-6 levels compared to bacterial pneumonia related ARDS and IL-6 levels in atypical pneumonia related ARDS were significantly lower than in indirect ARDS. Current findings report a potential difference in ARDS biomarkers due to the underlying disease and pathogen.