Reported effects of anti-CD154 treatment on autoimmunity, alloreactivity, and inf lammatory events mediated by macrophages and endothelial cells indicated that it might be an ideal agent for the prevention of intrahepatic islet allograft failure. This hypothesis was tested in MHCmismatched rhesus monkeys. Transplantation of an adequate number of viable islets resulted in engraftment and insulin independence in six of six recipients treated with anti-CD154 (hu5c8) induction plus monthly maintenance therapy (postoperative day >125, >246, >266, >405, >419, >476). Anti-CD154 (hu5c8) displayed no inhibitory effect on islet cell function. For monkeys followed for >100 days, continued improvement in graft function, as determined by first phase insulin release in response to intravenous glucose, was observed after the first 100 days post-transplant. No evidence of toxicity or infectious complications has been observed. All recipients treated with anti-CD154 became specifically nonresponsive to donor cells in mixed lymphocyte reactions. Furthermore, three monkeys are now off therapy (>113, >67, and >54 days off anti-CD154), with continued insulin independence and donor-specific mixed lymphocyte reaction hyporeactivity. In striking contrast to all previously tested strategies, transplantation of an adequate number of functional islets under the cover of anti-CD154 (hu5c8) monotherapy consistently allows for allogeneic islet engraftment and long-term insulin independence in this highly relevant preclinical model.Islet cell transplantation for patients with type 1 diabetes can result in the reversal of hyperglycemia and normalization of metabolic control (1-7). Broad-based application of curative islet cell transplantation has been limited, however, by the inability of current, generalized immunosuppressive reagents to reliably support long-term islet graft survival and function. The CD40-CD154 costimulation pathway has proven to be a critical interaction in the generation of a T-dependent immune response (8-10), and blockade of this pathway has prevented allograft rejection (11-16), graft versus host disease (17-19), and autoimmunity (20-29) in rodent models. Humanized anti-CD154 (30) (hu5c8, Biogen) has been shown to prevent renal allograft rejection in a rigorous non-human primate model (31). Additionally, blockade of the CD40-CD154 costimulation pathway can prevent production of proinflammatory mediators by activated macrophages (32-34) and endothelial cells (35)(36). Blockade of this pathway, therefore, has the potential to prevent allograft rejection, recurrent autoimmunity, and the nonspecific inflammatory events that occur on transplantation of islets into the liver, without the adverse effects of conventional, generalized immunosuppressive drugs on islet function (37). This study was undertaken to determine whether anti-CD154 (hu5c8) monotherapy would prevent the rejection of allogeneic islets in a preclinical, non-human primate model of pancreatectomy-induced diabetes.

Clinical islet cell transplantation has resulted in insulin independence in a limited number of cases. Rejection, recurrence of autoimmunity, and impairment of normal islet function by conventional immunosuppressive drugs, e.g., steroids, tacrolimus, and cyclosporin A, may all contribute to islet allograft loss. Furthermore, intraportal infusion of allogeneic islets results in the activation of intrahepatic macrophages and endothelial cells, followed by production of proinflammatory mediators that can contribute to islet primary nonfunction. We reasoned that the beneficial effects of anti-CD154 treatment on autoimmunity, alloreactivity, and proinflammatory events mediated by macrophages and endothelial cells made it an ideal agent for the prevention of islet allograft failure. In this study, a nonhuman primate model (Papio hamadryas) was used to assess the effect of humanized anti-CD154 (hu5c8) on allogeneic islet engraftment and function. Nonimmunosuppressed and tacrolimus-treated recipients were insulin independent posttransplant, but rejected their islet allografts in 8 days. Engraftment and insulin independence were achieved in seven of seven baboon recipients of anti-CD154 induction therapy administered on days -1, 3, and 10 relative to the islet transplant. Three of three baboons treated with 20 mg/kg anti-CD154 induction therapy experienced delayed rejection episodes, first detected by elevations in postprandial blood glucose levels, on postoperative day (POD) 31 for one and on POD 58 for the other two. Re-treatment with three doses of anti-CD154 resulted in reversal of rejection in all three animals and in a return to normoglycemia and insulin independence in two of three baboons. It was possible to reverse multiple episodes of rejection with this approach. A loss of functional islet mass, as detected by reduced first-phase insulin release in response to intravenous glucose tolerance testing, was observed after each episode of rejection. One of two baboons treated with 10 mg/kg induction therapy became insulin independent post-transplant but rejected the islet graft on POD 10; the other animal experienced a reversible rejection episode on POD 58 and remained insulin independent and normoglycemic until POD 264. Two additional baboon recipients of allogeneic islets and donor bone marrow (infused on PODs 5 and 11) were treated with induction therapy (PODs -1, 3, 10), followed by initiation of monthly maintenance therapy (for a period of 6 months) on POD 28. Rejection-free graft survival and insulin independence was maintained for 114 and 238 days, with preservation of functional islet mass observed in the absence of rejection. Prevention and reversal of rejection, in the absence of the deleterious effects associated with the use of conventional immunosuppressive drugs, make anti-CD154 a unique agent for further study in islet cell transplantation.

Transplantation of pancreatic islets of Langerhans as a therapeutic approach for treatment of type I diabetes offers an alternative to subcutaneous insulin injections. Normalization of blood glucose levels by transplanted islets may prevent the development of diabetes-related complications. Problems related to rejection, recurrence of autoimmunity, and local inflammation upon transplantation of islets into the liver need to be solved before the implementation of islet cell transplantation can be viewed as a justifiable procedure in a large cohort of patients. Islet cell isolation has been quite successful in small animals, but the translation of this approach to nonhuman primates has been less rewarding. One of the main problems encountered in nonhuman primate models is the difficulty of isolating an adequate number of functional islets for transplantation. The aim of the present study was to develop a method for isolating a sufficient number of viable islets from nonhuman primates to allow for reversal of diabetes. By implementing minor modifications in the automated method for human islet isolation we were able to obtain viable, functional islets that responded normally to glucose stimulation in vitro. These islets were also able to reverse diabetes in immunocompromised nude mice, rendered diabetic by streptozotocin. This method of islet cell isolation has enabled us to proceed with protocols of allogeneic islet cell transplantation in preclinical, nonhuman primate models.

Islet cell transplantation can potentially normalize blood glucose levels and stop the progression of clinical complications, and if the transplant is done early in the course of the disease complications may be prevented. Remarkable progress has been made in recent years and islet cell transplantation has resulted in normalization of metabolic control in several patients with Type 1 diabetes in the absence of hypoglycemia. Only a few patients, however, have achieved insulin independence. Issues relating to islet cell engraftment within the liver, prevention of rejection and recurrent autoimmunity, and identification of alternative immunosuppressive drugs that do not adversely affect islet cell function remain to be solved. Thus far, the need for chronic, generalized immunosuppression to prevent rejection of the islets has limited the indication to those patients who have already received another transplant or to those who simultaneously receive islets and another organ (generally a kidney). Identification of immunointervention protocols that allow for engraftment in the absence of deleterious effects on the islets and prevent rejection and recurrent autoimmunity would make this procedure suitable for all patients, including children who have not yet developed long-term complications of the disease.

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