P1088 Aims: The histological features of xenotransplanted ureters in pig-to-primate renal transplantation have not been described in detail, apart from a few particular cases. The aim of this study was to illustrate the ureteral findings and their clinical implications in 31 cynomolgus monkey recipients of porcine renal xenografts. Methods: Thirty-one consecutive bi-nephrectomized cynomolgus monkeys received a kidney from hDAF transgenic pigs with or without a ureteral stent. The transplanted animals received 5 different immunosuppressive regimens based on induction with cyclophosphamide/methotrexate and maintenance with cyclosporine A, mycophenolate sodium/methotrexate and steroids. Results: Four monkeys were euthanized prematurely after transplantation. In the remaining 27 cases, the mean survival was 24±18 days. Except for one case, where late venous thrombosis was the cause of a diffuse fibrosis with ischemic necrosis, histological examination showed 4 different situations: no rejection of any type (3 cases), acute cellular xenograft rejection (ACXR) alone (1 case), focal acute humoral xenograft rejection (AHXR, 6 cases), diffuse AHXR (the remaining 17 cases). Diffuse AHXR was represented in 2 different patterns. The first (8 cases) consisted in diffuse AHXR associated with wall thickening and a severe reduction of the lumen, particularly in the medial part. There were signs of diffuse mucosal erosion, hemorrhage and blood vessel thrombosis. Immunohistochemical examination revealed the presence of intravascular C5b9, C3, IgG, IgM and fibrin deposits on the wall of many small vessels of all stenotic ureters. Three of these cases developed a ureteral stenosis, while positioning a stent at surgery prevented hydroureteronephrosis in the other 4 cases. The second pattern (9 cases) was characterized by histological and immunohistochemical features of diffuse AHXR associated with homogeneous ureteral wall thickening due to considerable smooth muscle hypertrophy of the external layer of the ureter. In all these 9 cases, a ureteral stent was positioned at the time of transplantation or at resurgery for ureteral stenosis. Apart from one case, there was a close relationship between ureter and kidney in terms of type and severity of rejection. Conclusions: AHXR is a common ureteral complication following xenotransplantation of hDAF pig organs into non-human primates. While renal AHXR leads to a slow and progressive decline in renal function, ureteral AHXR causes the rapid onset of renal failure due to ureteral stenosis and hydroureteronephrosis. In the light of our experience in this delicate and ethically demanding model, we recommend the routine use of suitable surgical tools, e.g. ureteral stent, when renal xenografts are transplanted into small primates.