Simple SummaryMesenchymal Stromal/Stem cells (MSCs) are non-hematopoietic cells of the bone marrow that possess immunomodulatory and regenerative properties. MSCs are widely being tested in clinical trials as a cellular therapy for inflammatory and degenerative disorders. In addition, regulatory authorities in some countries have approved MSC-based cellular therapy for certain ailments such as Graft vs. Host Disease, Crohn’s-disease-associated perianal fistula, and Critical limb ischemia. Knowledge is emerging relating to MSCs’ potency and interaction with host immune components to inform sustained therapeutical benefit. In the present study, we defined the interaction between human bone-marrow-derived MSCs and circulatory dendritic cell populations, which are the key mediators of immunomodulation. This study provides insights into MSC’s modulatory effects on circulating dendritic cell subsets and the associated secretome signature, which could predict MSC’s potency and biomarker evaluation.AbstractMesenchymal Stromal Cells (MSCs) derived from bone marrow are widely tested in clinical trials as a cellular therapy for potential inflammatory disorders. The mechanism of action of MSCs in mediating immune modulation is of wide interest. In the present study, we investigated the effect of human bone-marrow-derived MSCs in modulating the circulating peripheral blood dendritic cell responses through flow cytometry and multiplex secretome technology upon their coculture ex vivo. Our results demonstrated that MSCs do not significantly modulate the responses of plasmacytoid dendritic cells. However, MSCs dose-dependently promote the maturation of myeloid dendritic cells. Mechanistic analysis showed that dendritic cell licensing cues (Lipopolysaccharide and Interferon-gamma) stimulate MSCs to secret an array of dendritic cell maturation-associated secretory factors. We also identified that MSC-mediated upregulation of myeloid dendritic cell maturation is associated with the unique predictive secretome signature. Overall, the present study demonstrated the dichotomy of MSC functionality in modulating myeloid and plasmacytoid dendritic cells. This study provides clues that clinical trials need to investigate if circulating dendritic cell subsets in MSC therapy can serve as potency biomarkers.