Abstract Background Granulocyte-monocyte apheresis (GMA) with Adacolumn® is a non-pharmacological therapy used for ulcerative colitis (UC). Despite its clinical efficacy, the precise immunological mechanisms of GMA have not been completely characterized. This study aimed to uncover the immune cell composition and signalling changes induced by GMA using single-cell RNA sequencing (scRNA-Seq) of peripheral blood mononuclear cells (PBMCs) in UC patients. Methods Patients with steroid-dependent UC receiving GMA were included. scRNA-Seq was performed on PBMCs obtained at a) immediately pre-treatment, b) post-first session (outflow catheter), and c) 1 month after induction GMA. We applied an in-house standard pipeline for quality control, including batch correction and data integration, followed by differential gene expression analyses to ascertain the alterations in immune cell profiles and regulatory pathways induced by GMA. Results Five patients were included and 12 samples were analysed, averaging 8,997 cells and 19,424 genes each. We observed pervasive modulations in immune cell composition, most notably reductions in classical monocytes and expansions in plasmacytoid dendritic cells (pDCs) and Natural Killer (NK) cells. The pathway analyses highlighted downregulation of NF-κB and IL6/JAK/STAT3 signalling across multiple immune cell types, implying widespread decrease in inflammatory activity. This downregulation was particularly pronounced in samples collected immediately after the first session (outflow catheter), suggesting an acute suppression of inflammatory signalling. Classical monocytes showed reduced expression of key pro-inflammatory genes, including TNF, NFKBIZ, and NFKBIA, indicating suppressed pro-inflammatory potential. In turn, non-classical monocytes displayed upregulation of anti-inflammatory markers such as IL1R2, suggesting a reorientation toward balanced immune control. By the second visit (one month after induction), we observed a marked return to homeostasis. This included increased expression of immunoglobulin-related genes, such as IGHA1 and IGKC, across various T cell subsets, such as regulatory T cells (Tregs) and tissue-resident memory T cells (Trm). These patterns suggest a heightened capacity for immune regulation and improved balance within the immune response over time. Conclusion This first single-cell transcriptomic analysis of GMA in UC patients reveals selective impacts on immune cell composition and signalling, with sustained reductions in inflammation and enhanced regulatory balance over time. Our work underscores the potential of GMA as a targeted therapeutic approach to managing immune dysregulation in UC, paving the way for future exploration into its application and optimization in immune-mediated diseases.
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