Host-Derived AV Bundles Maintain Bone Blood Flow in Experimental Vascularized Bone Allotransplants after Withdrawal of Short-Term Immunosuppression

At present, long-term immunosuppressive therapy is required to maintain tissue perfusion of allogeneic tissue, with potential serious complications. The authors hypothesized that living bone allografts could maintain perfusion and osteocyte viability when host-derived arteriovenous (AV) bundles are implanted at the time of microsurgical tissue transfer, combined with short-term immunosuppression. Survival would depend on development of a neoangiogenic circulation from host-derived vessels after rejection-mediated thrombosis of the bone nutrient system occurs.

Vascularized allogenic bone transplantation was performed between DA female and PVG male rats, resulting in an MHC mismatch. In addition to the microvascular pedicle anastomoses, a saphenous AV bundle from the recipient animal was implanted in the medullary canal of the transplanted bone. Ninety-seven rats were randomly allocated to one of six groups: 1) no immunosuppression, patent AV bundle; 2) no immunosuppression, ligated AV bundle; 3) 2 weeks FK506, patent AV bundle; 4) 2 weeks FK506, ligated AV bundle; 5) 2 weeks FK506, 4-week survival and patent AV bundle; 6) 2 weeks FK506, 4-week survival and ligated AV bundle (n = 11). Under anesthesia at sacrifice, the microvascular anastomosis and AV bundle patency were evaluated and bone blood flow determined using hydrogen washout. Following sacrifice, a colored microangiographic material was infused. Portions of each allograft were cleared to measure capillary density, and osteocyte counts performed to assess organ viability.

Patent AV bundles underwent neoangiogenic capillary formation, which increased significantly between 2 and 4 weeks to invade cortical bone. In non-immunosuppressed animals, capillary density was significantly greater with AV bundle patency. A similar beneficial effect of the AV bundle was seen in the 4-week groups. Unexpectedly, a lower incidence of spontaneous AV bundle thrombosis was found in animals treated with FK-506, suggesting a rejection-mediated mechanism rather than technical error or low blood flow. Bone blood flow was significantly greater in immunosuppressed animals with patent AV bundles, perhaps by a similar mechanism. Similarly, osteocyte counts were superior at 4 weeks when the AV bundle was patent.

In this short-term rat model, the authors investigated the possible role of vascular bundle implantation combined with short-term immunosuppression to maintain blood flow in vascularized allogeneic bone transplants. These preliminary data suggest that the method has merit for further investigation, providing an alternative to tolerance induction or long-term immunosuppression.