Longitudinal single-cell profiling of peripheral blood and bronchoalveolar lavage fluid reveals compartment-specific immune remodeling associated with fatal ARDS

Acute respiratory distress syndrome (ARDS) is clinically heterogeneous, but the systemic and lung-local immune programs associated with fatal outcomes remain incompletely defined. We investigated longitudinal immune remodeling across peripheral blood and bronchoalveolar compartments in ARDS. The single-cell ARDS cohort comprised 14 patients (10 survivors and 4 non-survivors): 11 contributed paired longitudinal peripheral blood mononuclear cells (PBMC) samples (8 survivors and 3 non-survivors), 13 contributed paired longitudinal bronchoalveolar lavage fluid (BALF) samples (9 survivors and 4 non-survivors), and 10 contributed both compartments at both sampling time points (T1 and T2). Six healthy controls were included for single-time-point peripheral blood reference profiling. Clinical trajectories were integrated with single-cell RNA sequencing of PBMC and BALF, followed by analyses of cell composition, differential expression, functional enrichment, module scores, and predicted cell-cell communication. Non-survivors showed less favorable longitudinal profiles of oxygenation and systemic recovery, accompanied by outcome-associated differences in inflammatory and neutrophil-related markers at selected time points. In PBMC, fatal outcome was associated with a longitudinal increase in the relative abundance of T cells and a decrease in monocytes, resulting in a sustained T cell–monocyte imbalance. Monocytes from non-survivors were enriched for oxidative phosphorylation (OXPHOS), cellular respiration, ATP synthesis, mitochondrial translation, antigen processing, and proteasome-related programs, whereas T cells showed enrichment of cell-cycle, DNA-replication, and proliferative pathways. In BALF, outcome-associated remodeling was concentrated in macrophages and neutrophils and was accompanied by transcriptional programs related to inflammation, chemotaxis, immune regulation, and tissue injury or repair. Module scoring revealed compartment-specific functional organization: inflammatory and metabolic programs in PBMC were mainly carried by circulating myeloid cells, whereas hypoxia, complement, OXPHOS, and stress-related programs in BALF were concentrated in macrophages and epithelial cells. Module-score visualization suggested a higher oxidative-phosphorylation pattern in PBMC monocytes from non-survivors, whereas TNF-α/NF-κB and BALF module patterns were less consistent. CellChat analysis further indicated distinct predicted communication architectures, with CXCL/MIF-related signaling prominent in PBMC and MIF/SPP1-related signaling in BALF. Overall, fatal ARDS was associated with related but nonredundant, compartment-specific immune remodeling across the circulation and lung. Longitudinal peripheral T cell–monocyte imbalance, monocyte metabolic reprogramming, and heterogeneous airway-local myeloid remodeling represent candidate biological features for future validation in larger independent cohorts and mechanistic studies.