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We hypothesized that hematopoietic stem cell (HSC) niches in bone marrow (BM) stromal environment of donor derived vascularized osteomyocutaneous allografts (VOMA) will enable sustained donor stem cell (HSC) renewal, and facilitate tolerance to concomitant donor matched renal allografts through establishment of mixed chimerism (MC). Our goal was to investigate this novel non-myeloblative tolerance strategy in a non-human primate (NHP) renal allograft – VOMA model.Full MHC mismatched, ABO matched, 4-5 year-old (3-4.5 kg) cynomolgus male-female (donor-recipient) NHP pairs (n=4) were used for concomitant donor-matched renal-VOMA transplantation (distal 1/3 of femur). Induction consisted of ATG (2 doses, Day -4 and -1) and total body irradiation (TBI) (200 cGy, Day -1). Maintenance therapy consisted of tacrolimus (TAC, i.m., b.i.d) with a trough target range of 20- 25 ng/mL or Cyclosporin A (CSA, i.m., b.i.d) with target range of 400-600 ng/ml. Donor MC was monitored by RT-PCR using NHP Y-chromosome (SRY) specific primers.All recipients developed clinical (edema, erythema) and histologic (Banff Grade 2) signs of acute rejection (AR) by 7-10 days in the skin component of the VOMA. TAC levels during AR were 3-6ng/ml. Following bolus steroid (Solumedrol, 40 mg, IV, 5 days) rescue to reverse AR, recipients were convered from TAC to Cyclosporin A (CSA) (b.i.d, target trough range: 400-600ng/mL). Skin component of VOMA in 2 recipients progressed to Banff 3-4 AR (epidermolysis, necrosis) with coincident AR of renal grafts. CSA trough levels in these animals ranged between 50-100ng/ml as compared to the two other recipients with reversed AR of VOMA allografts. Serum CSA levels in these recipients were maintained at 400-600 ng/ml for two months, and then dose-tapered to 50-100ng/mL over the next two months, with total CSA withdrawal on 120 days post Tx. One recipient had increasing Cr levels after 150 days post Tx. Exploratory laparotomy at 180 days confirmed obstructive uropathy. Serum Cr levels were normal in other recipient until 550 days post Tx. The skin of the VOMA was normal in both recipients. Vascular patency was maintained in all grafts until end point by ultrasound and CT imaging. Stable donor MC was detectable on RT-PCR up to 60 days post -Tx. This is an ongoing study with further Tx underway.We established a novel NHP model that combines solid organ (renal) and vascularized composite allograft (VOMA) (including distal 1/3 of femur) transplantation. The skin component of the VOMA can serve as a surrogate for renal allograft monitoring. Our preliminary results support the role of vascularized BM HSC niches in a donor-matched VOMA in sustaining MC mediated tolerance towards combined renal-VOMA transplants.National Natural Science Foundation of China. Wuhan University Zhongnan Hospital Science Foundation.