Abstract Rationale Sepsis is a common trigger of pediatric acute respiratory distress syndrome (PARDS), and patients with sepsis-associated PARDS (SA-PARDS) suffer from disproportionately high rates of morbidity and mortality. Much of the research into the dysfunctional immune response in both has used peripheral blood samples; however, the fidelity of using blood immune cells to analyze lung-specific disease in pediatric sepsis is not known. Given the critical role of neutrophils in the pathogenesis of both sepsis and ARDS, we chose to focus on this cell type. We hypothesized that subjects with SA-PARDS would exhibit unique neutrophil subsets in the lower respiratory tract that were absent in peripheral blood. Methods We enrolled intubated subjects less than 18 years old who were admitted to the PICU and met both PALICC 2 and Phoenix criteria for PARDS and sepsis, respectively. A blood sample and tracheal aspirate were collected on days 1, 3, and 7 if subjects were still intubated. The specimens were then processed for scRNA seq. cDNA libraries were sequenced, aligned, and analyzed using Seurat in R. Results We enrolled two patients and collected five paired blood and tracheal aspirate specimens. We found that the most abundant cell types in both were monocytes and neutrophils. Three neutrophil subsets were present in the lower respiratory tract. Two of the neutrophil subsets were defined by traditional markers of neutrophil identity and activation. The third subset, which we termed intermediate neutrophils, was characterized by low expression of C-X-C motif chemokine receptor 1 (CXCR1) and CXCR2, and increased expression of secretory leukocyte protease inhibitor (SLPI) and peptidase inhibitor 3 (PI3). The proportion of intermediate neutrophils increased in both subjects over time. Three subsets of neutrophils were identified in the blood samples, each with distinct markers of neutrophil identity and activation. None of the neutrophils in the blood had a transcriptome analogous to the intermediate neutrophils found in the tracheal aspirates. Conclusion In two patients with SA-PARDS, a unique population of neutrophils in the lower respiratory tract was characterized by increased expression of SLPI and PI3 and decreased expression of CXCR1 and CXCR2. This subset of neutrophils was not present in matched blood specimens. Our results highlight the need to assess the lung specific immune compartment when investigating the role of sepsis in lung disease and demonstrates the promise of a systems biology approach to evaluating the biology of SA-PARDS. This abstract is funded by: UM1TR004909 and 1K12TR004924
Williams et al. (Fri,) studied this question.