Abstract Bone is one of the most frequently transplanted tissues, with allografts and autografts accounting for more than 80% of total grafts. Synthetic biomaterials in association with autologous or allogeneic mesenchymal stromal cells (MSCs) represent a valid alternative in orthopaedic and maxillofacial surgery. Aim of REBORNE consortium is to perform clinical trials using standardized protocols based on advanced biomaterials and MSCs. Aim of our Unit was to assess MSC immune modulatory properties in presence of a novel biomaterial consisting of hydroxyapatite and tricalcium-phosphate (HA/TCP, Biomatlante, France) as scaffold for MSC delivery. We assessed immune modulatory properties, in terms of immunophenotype, inhibitory and anti-apoptotic effects, of MSCs of different origin when associated with the HA/TCP biomaterial, including bone marrow-derived mesenchymal stromal cells (BM-MSCs), adipose-derived MSCs (ASCs) and cord blood-derived MSCs (CB-MSCs). The culture of all MSCs with HA/TCP, did not modulate the anti-apoptotic and suppressive features of all MSCs toward sorted-T, B and NK lymphocytes as compared to standard culture setting. When exposed to inflammatory cytokines, such as IFN-γ and TNF-α, all MSCs were induced to acquire the suppressive phenotype, characterized by MHC-1 and CAM molecules up-regulation, and de novo expression of HLA-DR molecules. The long-term culture of BM-MSCs with HA/TCP induced an osteoblast-like phenotype, as indicated by the up-regulation of osterix and osteocalcin transcription. The differentiation process is promoted in all MSC types, but especially in BM-MSCs, by dexamethasone and, to a higher extent, BMP-4 treatment. In view of using MSCs for advanced therapies in allogeneic setting, we evaluated the immunogenicity and immune modulatory features of pre-differentiated MSCs in association with HA/TCP towards both innate and adaptive immune cells, as well as the molecular pathways involved in MSC-mediated immune regulation. MSC-derived osteoblasts did not induce immune cell activation, as demonstrated by co-culture of unstimulated immune effector cells with pre-differentiated MSCs. We found a lower suppressive capability of pre-differentiated MSCs towards stimulated T and NK cells. However, some inhibitory effects could be exerted by BMP-4 treated-MSCs, and could be related to the presence of undifferentiated MSCs in culture, which could inhibit lymphocyte proliferation. The balance between the number of bone depositing osteocyte-like MSCs and immunosuppressive undifferentiated MSCs could be useful, in vivo, to promote bone healing and to reduce local inflammation, respectively. We then investigated in pre-differentiated MSCs the molecular mechanisms involved in the inhibition of T cell proliferation, by interfering with IDO and COX-2 activation. We found that pre-differentiated MSCs switched on their suppressive machinery by activating both IDO, which plays a central role in immune regulation, and COX-2, whose role is not significant in undifferentiated MSCs. COX-2 is produced rapidly as inflammation mediator after fracture, and recent data highlighted the role of COX-2 in improving fracture healing. In conclusion, this study increases the knowledge on MSC biology and gives pre-clinical data concerning the use of allogeneic MSC in combination with ceramic scaffolds to treat bone defects. Disclosures: No relevant conflicts of interest to declare.