In vivo osteogenic capability of cultured allogeneic bone in porous hydroxyapatite: immunosuppressive and osteogenic potential of FK506 in vivo.

Fischer or ACI rat marrow cells were obtained from femoral shafts and were cultured to confluence in Eagle's minimal essential medium (EMEM) supplemented with 15% fetal bovine serum. After trypsinization, the cells were subcultured on porous hydroxyapatite (HA; Interpore 500) blocks in the presence of beta-glycerophosphate and 10 nM dexamethasone (Dex). After 2 weeks of subculture, a mineralized bone matrix with osteogenic cells developed on the HA pore surfaces. ACI or Fischer cultured bone tissue/HA constructs were implanted subcutaneously into the backs of Fischer rats and the immunosuppressant FK506 was given to the rats for 4 weeks. Implants were harvested 4 weeks and 8 weeks after insertion. At 4 weeks, the ACI constructs (allografts) showed high levels of osteogenic parameters (alkaline phosphatase [ALP] activity and osteocalcin content) and bone formation was observed together with active osteoblasts without obvious accumulation of inflammatory cells. At 8 weeks, active osteoblasts and progressive bone formation were still observed, while osteogenic parameters remained high and osteocalcin messenger RNA (mRNA) was detected. Without FK506 administration, the allografts showed neither bone formation nor osteocalcin mRNA and there were only trace levels of the osteogenic parameters. In the case of Fischer constructs (isografts), extensive bone formation was detected and all the osteogenic parameters were higher with FK506 than without FK506 at both 4 weeks and 8 weeks. These results indicate that cultured bone tissue/HA constructs possess a high osteogenic potential, even as allografts, and that FK506 not only has an immunosuppressive action, but also promotes bone formation.