Bone Abstracts

β-tricalcium phosphate (β-TCP) biomaterials have been approved for the repair of osseous defects. However, in large defects, the substitution of biomaterial by authentic bone is inadequate to provide sufficient long-term mechanical stability. We aimed to develop composites of β-TCP ceramics and receptor activator of nuclear factor κ-B ligand (RANKL) to enhance the formation of osteoclasts thereby stimulating material resorption. RANKL was immobilized on β-TCP ceramics by i) superficial adsorption (passive short-term release) and ii) co-precipitated together with calcium phosphate, resulting in an incorporation of the protein into a crystalline layer of calcium phosphate and a cell-mediated long-term release. Murine osteoclasts precursors were seeded onto the ceramics. After 15 days, the formation of osteoclasts was evaluated with tartrate-resistant acidic phosphatase (TRAP) staining and quantified with TRAP-activity. Additionally, the expression of the osteoclast markers calcitonin receptor and cathepsin K were quantified by real-time PCR. Superficially adsorbed RANKL did not induce the formation of osteoclasts on β-TCP ceramics. When co-precipitated onto β-TCP ceramics RANKL induced the formation of osteoclasts as demonstrated by positive TRAP-stainings and a 2-fold increase of TRAP-activity, which was similar to that observed in positive controls. Development of osteoclast lineage cells was further confirmed by an increased expression of cathepsin K and calcitonin receptor. Our study shows for the first time that RANKL immobilized on β-TCP ceramics induces the formation of osteoclasts. However, osteoclast formation requires a long-term release system of RANKL. RANKL co-precipitation may induce osteoclast differentiation due to a residual passive release of the protein. Subsequently, matured osteoclasts mediate the release of RANKL by resorbing the protein containing calcium phosphate layer, thereby perpetuing their differentiation and activation. It remains to be proven whether the formation of osteoclast leads to a stimulation of biomaterial resorption. Experiments focusing on the resorptive activity of osteoclasts formed on β-TCP ceramics are ongoing.