Human lung allografts experience persistent fibrogenic shift following acute cellular rejection

RATIONALE: Acute cellular rejection (ACR) remains a significant challenge in lung transplantation, with incomplete understanding of its molecular mechanisms and pathways linking ACR to chronic lung allograft dysfunction (CLAD). OBJECTIVES: To characterize the cellular and molecular mechanisms underlying ACR in lung allografts using single cell genomics and identify potential therapeutic targets for CLAD. METHODS: Single cell RNA-sequencing of freshly collected lung tissue was performed across 8 pediatric and adult patients with ACR, Resolved ACR, and surveillance biopsies without ACR. Validation included gene microarray analysis, immunofluorescence, and single cell ATAC-seq. MEASUREMENTS AND MAIN RESULTS: Gene set enrichment analysis revealed persistent TGF-β signaling and PI3K/AKT/mTOR pathway activation in both ACR and Resolved samples, validated by immunofluorescence showing sustained elevation of mTOR activation marker phosphorylated-S6 ribosomal protein and COL3A1. Fibrogenic cells exhibited myofibroblast gene signatures via mesenchymal state transitions rather than epithelial- or endothelial-to-mesenchymal transition. Cell communication analysis showed increased Type II Interferon signaling, with Jak/Stat pathway activation in endothelial and basal cells, and reduced VE-Cadherin staining in ACR. Compositional analysis revealed increased cytotoxic, memory T cells and dendritic cells, with persistent reduction of natural killer cells in ACR and Resolved. Donor/recipient analysis revealed predominantly recipient-derived immune cells in ACR. CONCLUSIONS: Persistent TGF-β and mTOR pathway activation following histologic ACR resolution provides molecular insight into ACR-CLAD linkage and suggests mTOR inhibition and TGF-β blockade as potential therapeutic mechanisms to prevent CLAD.