Monocytes Seeded on Engineered Hypertrophic Cartilage Do Not Enhance Endochondral Ossification Capacity.

Engineered hypertrophic cartilage (HC) represents an attractive bone substitute material, capable to induce bone formation by endochondral ossification. Since bone formation by HC depends on factors released from the extracellular matrix, in this study, we hypothesized that HC seeding with monocytes committed to osteoclastogenesis could enhance its remodeling, improve chemotaxis of skeletal and vascular cells, and consequently enhance bone formation. This would be particularly relevant for devitalized HC, which currently exhibits only limited osteoinductivity. Living or devitalized HC engineered from human bone marrow-derived mesenchymal stromal cells (MSCs) was seeded or not with human monocytes in the presence of macrophage colony-stimulating factor and RANK-ligand, cultured for up to 15 days, or implanted ectopically in nude mice. Monocytes seeded on devitalized, but not living, HC induced its in vitro resorption, resulting in 30-fold higher release and 2.7-fold lower content of glycosaminoglycans compared with unseeded samples. In vitro, supernatants from monocyte-seeded devitalized HC attracted more monocytes compared with unseeded samples, but did not enhance chemotaxis of MSCs or human umbilical vein endothelial cells. In vivo, however, neither remodeling nor invasion by osteoclasts, endothelial cells, and mouse MSCs were significantly affected by the seeding with monocytes. Finally, in vitro priming of living or devitalized HC by monocytes did not enhance their bone-forming capacity. Further investigations should test the proposed approach on HC engineered to prevent rapid degradation and support osteoclastogenesis, or identify alternative strategies to enhance engineered HC remodeling and bone-forming capacity.