Regulatory T cells modulate bone marrow stromal cell osteogenesis by activating ROCK-myosin axis and cell contractility

Summary Bone regeneration depends not only on the intrinsic signaling in bone marrow-derived mesenchymal stromal cells (BMSC), but also on immune cell-derived cues. Regulatory T cells (Treg) are emerging as immune regulators of tissue repair, yet their direct impact on BMSC osteogenesis remains unexplored. Here, we showed that direct human BMSC-Treg co-culture induced more pronounced osteogenic gene expression and cytokine modulation in BMSC than indirect co-culture. This contact-dependent interaction temporally enhanced BMSC osteogenic gene expression, alkaline phosphatase activity and matrix mineralization by interacting with ROCK-myosin signaling pathway. Exploratory proteomic profiling further revealed enrichment of cytoskeletal, mechanosensitive and osteogenic regulators in BMSC after direct Treg co-culture. Pharmacological inhibition of cell contractility impaired osteogenesis yet Treg co-culture partially restored cytoskeletal integrity and differentiation. These findings identify novel mechanisms behind Treg as direct modulators of BMSC mechanobiology and osteogenesis, highlighting their therapeutic potential in bone regeneration.