Acute transient inflammation is crucial for initiation of bone healing, osseointegration of implants, osteogenic differentiation of MSCs, and immunomodulation. However, harmful chronic inflammation and bone loss (osteolysis) are induced by adverse stimuli including wear byproducts from joint replacements (JR). Nuclear Factor kappa B (NF-κB) is a critical transcription factor in both macrophages (mϕ) and MSCs that regulates inflammation, bone formation/remodeling, and aging. We showed that preconditioning MSCs with lipopoly- saccharide (LPS) and tumor necrosis factor-α (TNF-α) to induce acute transient activation of NF-κB synergistically enhances osteogenesis, and modulates mϕ polarization from a pro-inflammatory (M1) to an anti-inflammatory pro-regenerative (M2) phenotype. Furthermore, inhibition of persistent NF-κB signaling using an NF-κB decoy OligoDeoxyNucleotide (ODN) was shown to mitigate inflammatory bone loss. We have genetically modified MSCs to sense NF-κB activation and then increase production of the anti-inflammatory pro-regenerative cytokine IL-4. The purpose of this grant is to accelerate net bone formation and mitigate chronic inflammation and osteolysis via NF-kB driven immunomodulation using preconditioned or genetically modified MSCs transplanted to the local environment. Specific Aim #1a -To define the critical immunomodulatory interactions of preconditioned vs. NF-κB sensing and IL-4 secreting MSCs on mϕ exposed to wear byproducts from orthopaedic implants. Specific Aim #1b - To enhance osteogenesis by using preconditioned vs. NF-κB sensing and IL-4 secreting MSCs in an MSC/mϕ co-culture model exposed to wear byproducts from orthopaedic implants. Specific Aim #2a - To demonstrate the therapeutic effects of transplanted preconditioned MSCs on bone during the acute inflammatory stage (simulating the stage of initial implantation of a prosthesis and osseointegration) using the murine continuous femoral particle infusion model. Specific Aim #2b - To demonstrate the therapeutic effects of preconditioned vs. NF-κB sensing and IL-4 secreting MSCs on net bone formation during the chronic inflammatory stage (simulating established wear particle disease) using the murine continuous femoral particle infusion model. Specific Aim #3 - To demonstrate that the above principles of modulating acute and chronic inflammation are valid irrespective of animal sex (male vs. female mice) or age (young vs. elderly mice) in vivo. We hypothesize that NF-κB preconditioning or NF-κB sensing and IL-4 secreting MSCs will enhance the osteogenic and immunomodulatory signaling of MSCs, enhancing bone formation via crosstalk by mϕ and MSCs. NF-kB driven immunomodulation of MSCs should mitigate particle-induced inflammation, osteoclast activation, and enhance bone formation in young and aged, male and female mice. NF-kB driven immune modulation of MSCs is novel, mechanistic, and directly translational to inflammatory disorders in other organ systems.