PIG-TO-PRIMATE ISLET XENOGRAFTS REVERSE DIABETES BEFORE UNDERGOING ACUTE CELLULAR REJECTION

Abstract

31 The functional response and immunobiology of primarily non-vascularized islet cell xenografts remain poorly defined in relevant pre-clinical models. This project was therefore aimed to delineate the initial functional response and basic mechanisms operative in the rejection process of adult porcine islets transplanted to a clinically applicable site in non-immunosuppressed non-human primates. Six streptozotocin-diabetic and two non-diabetic female Rhesus monkeys were used as recipients and adult pigs served as donors. After pretreatment in culture for two days, 20,000 islet equivalents/kg body weight were transplanted into the portal vein of heparinized, non-immunosuppressed recipient animals. Plasma glucose, fasting serum porcine C-peptide, complement activation products (C3a, sC5b-9), xenoreactive natural antibodies (XNAb), and serum nitrite were measured pre-transplant and at intervals post-transplant. Two monkeys each were sacrificed at 12, 24, 48, and 72 hrs after transplantation. Livers bearing islet xenografts were examined by routine histology and immunohistochemistry. Mean fasting plasma glucose levels in diabetic recipients were 75, 42, 85, and 157 mg/dl at 12, 24, 48 hrs, and 72 hrs posttransplant. Porcine C-peptide, negative in all animals pretransplant, was present in 8/8, 5/6, 4/4, and 1/2 recipients at 12, 24, 48, and 72 hrs posttransplant, respectively. Levels of C3a and sC5b-9 showed no evidence of complement activation. Pretransplant IgM XNAb serum levels varied between animals. On day 1, 2, and 3 after xenograft, IgM XNAb levels remained unchanged compared to pretransplant levels in the same primates. Nitrite levels increased significantly (38 mM) in 2 primates on day 2. Histopathological findings varied considerably between recipient animals. Embolized islets were discernable at each time point. Insulin-positivity was however significantly reduced at 48 and 72 hrs. At 12 and 24 hrs, islets were found associated with thrombi. Strong immunostaining for IgG, IgM, C3, C5, and C9 was seen at 12 and 24 hrs, and minimal at 72 hrs. Neutrophils and few T cells were adherent to the islets as early as 12 hrs. At 48 hrs, neutrophils had disappeared, and macrophages became increasingly discernable. However, T cells were the predominant cells from 24 to 72 hrs posttransplant. Only few B cells were noted, whereas NK cells remained absent in the developing granulomatous lesions. Pig-to-Rhesus monkey intraportal islet xenografts normalized blood glucose for 48 hrs. Antibody and complement deposition may not be solely due to the intravascular location of the graft. Islet xenografts succumbed to acute cellular rejection, initially dominated by T cells followed by granulomatous lesions.