Single-cell ATAC-seq of circulating leukocytes reveals global epigenetic alterations as a prognostic factor in critical illness

Background: While bulk and single cell transcriptomic patterns in circulating leukocytes from trauma patients have been reported, how these relate to changes in open chromatin patterns remain unstudied. Here, we investigated whether single-cell ATAC-seq would provide further resolution of transcriptomic patterns that align with patient outcomes. Methods: We performed scATAC-seq on peripheral blood mononuclear cells from four trauma patients at <4hr, 24hr, 72hr post-injury and four matched healthy controls, and extracted the features associated with the global epigenetic alterations. Three large-scale bulk transcriptomic datasets from trauma, burn and sepsis patients were used to validate the scATAC-seq derived signature, explore patient epigenetic heterogeneity (Epigenetic Groups: EG_hi vs. EG_lo), and associate patterns with clinical outcomes in critical illness. Findings: Patient subsets with gene expression patterns in blood leukocytes representative of a high global epigenetic signature (EG_hi) had worse outcomes across three etiologies of critical illness. EG_hi designation contributed independent of the known immune leukocyte transcriptomic responses to patient prognosis (Trauma: HR=0.62 [95% CI: 0.43-0.89, event set as recovery], p=0.01, n=167; Burns: HR=4.35 [95% CI: 0.816-23.2, event set as death], p=0.085, n=121; Sepsis: HR=1.60 [95% CI: 1.10-2.33, event set as death], p=0.013, n=479; Cox proportional hazards regression). Interpretation: The inclusion of gene expression patterns that associate with global epigenetic changes in circulating leukocytes improves the resolution of transcriptome-based patient classification in acute critical illnesses. Early detection of both the global epigenetic signature and the known immune transcriptomic patterns associates with the worse prognosis in trauma, burns and sepsis. Overall design: We longitudinally single-cell ATAC sequenced circulating mononuclear cells in 4 trauma patients (<4hrs, 24hrs and 72hrs post injury) along with paired healthy controls, total 16 samples.