Scaffold-guided subchondral bone repair: implication of neutrophils and alternatively activated arginase-1+ macrophages.

BACKGROUND: Microfracture and drilling elicit a cartilage repair whose quality depends on subchondral bone repair. Alternatively activated (AA) macrophages express arginase-1, release angiogenic factors, and could be potential mediators of trabecular bone repair.

HYPOTHESIS: Chitosan-glycerol phosphate (GP)/blood implants elicit arginase-1+ macrophages in vivo through neutrophil-dependent mechanisms and improve trabecular bone repair of drilled defects compared with drilling alone.

STUDY DESIGN: Controlled laboratory study.

METHODS: Bilateral trochlear cartilage defects were created in 15 rabbits, microdrilled, and treated or not with chitosan-GP/blood implant to analyze AA macrophages, CD-31+ blood vessels, bone, and cartilage repair after 1, 2, or 8 weeks. Neutrophil and macrophage chemotaxis to rabbit subcutaneous implants of autologous blood and chitosan-GP (+/-blood) was quantified at 1 or 7 days. In vitro, sera from human chitosan-GP/blood and whole blood clots cultured at 37 degrees C were analyzed by proteomics and neutrophil chemotaxis assays.

RESULTS: Chitosan-GP/blood clots and whole blood clots released a similar profile of chemotactic factors (PDGF-BB, IL-8/CXCL8, MCP-1/CCL2, and no IL-1beta or IL-6), although chitosan clot sera attracted more neutrophils in vitro. Subcutaneous chitosan-GP (+/-blood) implants attracted more neutrophils (P < .001) and AA macrophages than whole blood clots in vivo. In repairing subchondral drill holes, chitosan-GP/blood implant attracted more AA macrophages at 1 and 2 weeks and more blood vessels at 2 weeks compared with drilled controls. Treatment elicited a more complete woven bone repair at 8 weeks than controls (P = .0011) with a more uniform, integrated collagen type II+ cartilage repair tissue.

CONCLUSION AND CLINICAL RELEVANCE: AA macrophages may play a role in the regeneration of subchondral bone, and chitosan-GP can attract and transiently accumulate these cells in the repair tissue. The resulting improved subchondral repair could be advantageous toward enhancing integration of a restored chondral surface to the subchondral bone.