Dolph Houben

17 Introduction and Outline of the Thesis 1 Yucatan miniature swine as large animal transplantation model The majority of the VCA research conducted by the microvascular research laboratory at Mayo Clinic has been performed in small animal models. Laboratory rats and rabbits have been most commonly used, due to their defined immunology, easy handling, and availability. These studies have shown promising results in maintaining VCA viability by surgical angiogenesis and short- term immunosuppression [87, 105, 126-136] . For translatable results, a porcine model is of pre-clinical importance since they have distinct advantages for allogenic tissue transplantations research. Their size, anatomy, physiology, and immunology are well known and comparable to man. Most importantly, both blood type and the major histocompatibility haplotypes (swine leukocyte antigen, SLA) have been well defined. SLA haplotypes can be determined by pre-operative DNA sequencing [137-139] . This allows transplantation over a major mismatch in histocompatibility while animals are matched for blood type, size, and age. Evaluating transplantation chimerism is possible if a sex-mismatched experimental set-up is used [140] . Regular blood draws allow long-term monitoring of systemic immune responses after transplantation by cytokine detection and blood cell counts. For bone and joint, orthotopic reconstruction of a segmental defect uses surgical techniques and implants identical to clinical use. Their physiology, including the rate of new bone formation, is nearly identical to man [141] . A number of VCA studies have taken place using miniature swine, including skin, muscle, bone, bone marrow and composite knee joint allotransplants [56, 142-148] . All of these studies used an subcutaneous inguinal pocket in which the allotransplant was transplanted. Orthotopical allotransplantation of bone and joint is unknown in large animal models. A large animal tibia defect model has been developed in Yucatan miniature swine for more translatable results [149, 150] . This Yucatan miniature swine model has proven to be a great asset. This pre-clinical model allows orthotopical microsurgical transplantation of bone VCA, long-term monitoring of the transplanted tissue and host immune response while resulting in minimal morbidity for the animal [151] . Significance Segmental bone or joint loss is a serious and challenging clinical problem after primary or metastatic bone tumor resection, severe trauma, infection, congenital pseudarthrosis, or failed primary reconstruction. Current methods used to reconstruct these defects have significant problems. Cryopreserved bone allografts (CBAs) are widely used since they are available in the right size and shape, and they provide immediate stability and sufficient bulk to the reconstruction. Complications associated with this type of reconstruction include non-union, infection, and late stress-fractures due to their avascular status. Vascularized bone autografts (VBGs) contain their own intrinsic blood supply due to microvascular repair of the nutrient vessels. Osteocyte viability is therefore maintained, resulting in improved strength, faster union times, and hypertrophy potential. Despite these favorable properties, due to the lack of donor sites VBGs often mismatch the defect in size and shape. Additionally, VBGs are associated with donor site morbidity. Due to the complications and limitations of current reconstructive methods, innovation is needed. Microsurgical transplantation of living allogenic bone and/or joint may be a future alternative for the treatment of segmental defects. Currently, VCAs require multi-level immune modulation to prevent rejection of the allotransplant. Immunosuppressive regimens for VCA are similar to those in solid organ transplantation. Complications of life-long IS include, opportunistic infections, malignancy,