Mesenchymal Stromal Cells Reprogram Neutrophils into Immunosuppressive Effectors via Phagocytosis and Extracellular Vesicle Transfer

Background Mesenchymal stromal cells (MSCs) possess potent immunomodulatory properties, but the mechanisms through which they shape innate-adaptive immune interactions remain poorly understood. Given the functional plasticity of neutrophils, we investigated whether MSCs induce an immunoregulatory neutrophil phenotype capable of suppressing autologous T-cell responses and examined the mechanisms involved. Methods Ex vivo models were used to investigate MSC-neutrophil interactions and the capacity of MSC-conditioned neutrophils to modulate activated autologous T-cells. Direct and transwell co-culture systems were employed to evaluate the roles of cell contact and soluble mediators. Results Neutrophils acquired MSC-derived material through an oxygen-dependent phagocytosis process, which induced a regulatory phenotype referred to as “programmed neutrophils”. Compared with untreated controls, these cells exhibited enhanced survival and effectively suppressed autologous T-cell responses by limiting early activation events, as evidenced by reduced CD25 expression prior to HLA-DR upregulation. T-cell suppression required direct neutrophil-T-cell interaction, whereas neutrophil programming occurred independently of physical contact with MSCs. MSC-conditioned medium alone was sufficient to influence neutrophil phenotype, implicating MSC-derived extracellular vesicles (EVs) as mediators of the reprogramming signal. Furthermore, both viable and apoptotic MSCs generated comparable effects, supporting a central role for MSC-driven neutrophil reprogramming in the development of immunosuppressive neutrophils. Conclusions Our study uncovers a novel MSC-neutrophil immunoregulatory pathway in which MSC-programmed neutrophils acquire suppressive activity and regulate adaptive immune responses. This mechanism enables MSCs to indirectly control T-cell activation and highlights neutrophil reprogramming as a central component of MSC immunomodulation, with potential applications in the development of potency assays and biomarkers for MSC-based therapies.