Long-Term Survival of Living Bone Allografts without Immunosuppression or Tolerance Induction Using Host-Derived Neoangiogenesis

Large skeletal defects resulting from tumors, infection, or failed arthroplasty present reconstructive challenges. Methods currently available include use of vascularized autografts, non-viable structural allografts, and/or prosthetic components. All have important limitations. Transplantation of living allogenic bone would provide replacements closely matched to the dimensions and mechanical properties of the resected bone, combined with the desirable healing properties of vascularized autografts. Vascularized bone allotransplantation without the health risks of immunosuppressive drugs or tolerance induction would be an important advance. The authors proposed a novel method by which this may be accomplished: microsurgical bone transfer with development of a new host-derived osseous blood supply by surgical neoangiogenesis. Immmunosuppression needs only to allow sufficient time for angiogenesis to occur and donor-specific tolerance is not a prerequisite for bone survival.

Microsurgical femoral transplantation was performed in rats across a major histocompatibility barrier. The graft and recipient femoral vessels were anastomosed and the contralateral recipient saphenous artery and vein were implanted through the medullary canal as an arteriovenous pedicle. A flexible silicone wrap prevented angiogenesis from the surrounding tissue. In Groups 1 and 2 (n = 19 and 8), no immunosuppression was given. In Groups 3, 4, and 5 (n = 13, 9, 15), immunosuppression was maintained for 2 weeks postoperatively with FK506. The arteriovenous pedicle was ligated in Groups 2 and 4. At 18 weeks, skin grafts from graft type, recipient type, and third party rats were placed in Group 5 animals and in Groups 1–4, graft cortical blood flow was measured using the hydrogen washout method. Skin grafts were maintained for 3 weeks. Microangiography after infusion with Microfil and bone clearing provided a measure of neoangiogenesis. Bone viability and degree of rejection were analyzed histologically.

Blood flow and capillary density in the groups with patent a/v pedicles (1 and 3) were significantly higher (p < 0.005). Immunosuppression and a/v pedicle patency combined showed significantly higher blood flow than a/v pedicle patency alone (p < 0.05). All recipient-type skin grafts healed and all donor type and third party grafts were rejected. Severe to moderate necrosis was seen in the non-immunosuppressed groups and mild to moderate necrosis in Groups 3 and 4.

Neoangiogenesis from implanted blood vessels of recipient origin maintains blood flow in a vascularized bone allograft after removal of immunosuppression and without induction of tolerance. This is an important step toward clinical application of living bone allografts without the risks of long-term immunosuppression or tolerance induction.