Sulaiman A. Nanji

Clinical islet transplant: current and future directions towards tolerance

Summary:  The ultimate goal of islet transplantation is to completely correct the diabetic state from an unlimited donor source, without the need for chronic immunosuppressive drug therapy. Although islet transplantation provides an opportunity to develop innovative strategies for tolerance in the clinic, both alloimmune and autoimmune barriers must be controlled, if stable graft function is to be maintained long‐term. After islet extraction from the pancreas, the cellular graft may be stored in tissue culture or cryopreserved for banking, providing an opportunity not only to optimally condition the recipient but also to allow in vitro immunologic manipulation of the graft before transplantation, unlike solid organ grafts. As such, islets may be considered a ‘special case.’ Remarkable progress has occurred in the last three years, with dramatic improvements in outcomes after clinical islet transplantation. The introduction of a steroid‐free, sirolimus‐based, anti‐rejection protocol and islets prepared from two (or rarely three) donors led to high rates of insulin independence. The ‘Edmonton Protocol’ has been successfully replicated by other centers in an international multicenter trial. A number of key refinements in pancreas transportation, processing, purification on non‐ficoll‐based media, storage of islets in culture for two days and newer immunological conditioning and induction therapies have led to continued advancement through extensive collaboration between key centers. This review outlines the historical development of islet transplantation over the past 30 years, provides an update on current clinical outcomes, and summarizes a series of unique opportunities for development and early testing of tolerance protocols in patients.

ADVANCES IN PANCREATIC ISLET TRANSPLANTATION IN HUMANS

With recent advances in methods of islet isolation and the introduction of more potent and less diabetogenic immunosuppressive therapies, islet transplantation has progressed from research to clinical reality. Presently, several international centres have demonstrated successful clinical outcomes with high rates of insulin independence after islet transplantation. Ongoing refinements in donor pancreas procurement and processing, developments in islet isolation and purification technology, and advances in novel immunological conditioning and induction therapies have led to the acceptance of islet transplantation as a safe and effective therapy for patients with type 1 diabetes. This review provides a historical perspective of islet transplantation, outlines the recent advances and current clinical outcomes, and addresses the present challenges and future directions in clinical islet transplantation.

Multiple Combination Therapies Involving Blockade of ICOS/B7RP-1 Costimulation Facilitate Long-Term Islet Allograft Survival

In recent years a series of novel costimulatory molecules have been identified, including inducible costimulator (ICOS). In a fully major histocompatibility complex (MHC)‐mismatched mouse model of islet transplantation, we demonstrate that while monotherapy with CTLA4‐Ig, CD40 ligand monoclonal antibody (CD40L mAb) or rapamycin each improves islet allograft survival, graft rejection eventually develops. Immunohistologic analysis of rejected grafts revealed increased ICOS expression, suggesting a role for this costimulatory molecule as an alternate pathway for T‐cell activation. The combination of a blocking anti‐ICOS mAb with each of the above therapies resulted in significantly improved islet allograft survival, confirming the importance of ICOS signaling in islet allograft rejection. Mechanistic studies conducted in mice treated with anti‐ICOS mAb and rapamycin demonstrated a lack of donor‐specific immunological tolerance and an absence of regulatory T‐cell activity. However, a dramatic effect was seen on acute anti‐donor responses whereby anti‐ICOS mAb and rapamycin significantly reduced the initial expansion and function of alloreactive T cells. These data demonstrate that blockade of the ICOS/B7RP‐1 pathway has potential therapeutic benefit given its role in enhancing islet allograft survival and regulating acute alloresponses in vivo.

Islet Transplantation in Patients with Diabetes Mellitus

Islet transplantation offers patients with type 1 diabetes mellitus freedom from long-term insulin therapy and a degree of metabolic control that is far superior to injected insulin. The hope is that near-perfect glucose control sustained over time will prevent progression of secondary diabetic complications. The selection of optimal immunosuppressive agents for islet transplantation has been a formidable challenge, given the need to overcome both autoimmune and alloimmune barriers, as well as the potential toxicity of immunosuppressive agents on transplanted islets. Early strategies relied on protocols that had proven success in solid organ transplantation and consisted of azathioprine, cyclosporine and corticosteroids. Under these protocols, fewer than 10% of patients were able to achieve insulin independence. The development of the “Edmonton Protocol’ dramatically transformed clinical outcomes in islet transplantation in recent years through the introduction of a more potent, less diabetogenic, and corticosteroid-free immunosuppressive regimen consisting of sirolimus, low-dose tacrolimus, and induction anti-interleukin-2 receptor antibody. While insulin independence rates under this protocol have been highly successful, patients must be maintained on lifelong immunosuppression. While the risk of malignancy, post-transplant lymphoma and sepsis have been low and diminishing in transplanted patients to date, fears of these complications and a host of drug-related adverse effects have precluded broader application. Patients undergoing islet transplantation today must exchange insulin for chronic immunosuppressive therapy, and therefore the procedure can only be justified in patients with very unstable forms of diabetes, or in those with another solid organ allograft who already endure the risks of immunosuppression. Advances in more specific and less toxic immunosuppressive agents together with progress in better understanding the biology of diabetes will lead to more suitable strategies to control both alloimmune and recurrent autoimmune reactions. These protocols, ultimately aimed at establishing tolerance, are an essential pre-requisite to move towards providing islet transplantation earlier in the course of the disease, including transplantation in children. This review addresses the evolution of immunosuppressive strategies in islet transplantation, and highlights some novel agents in pre-clinical development or in early clinical trials that may offer considerable promise in facilitating the induction of tolerance.

Diabetes Induces Rapid Suppression of Adaptive Immunity Followed by Homeostatic T-cell Proliferation

Surprisingly, the effect of acute diabetes on immunity has not been examined in detail. We, herein, show for the first time that untreated acute diabetes causes rapid lymphopenia followed by homeostatic T‐cell proliferation. The diabetes‐induced lymphopenia was associated with an immunosuppressed state that could be sufficiently strong to allow engraftment of fully allogeneic β‐cells or block rejection of islet transplants. In contrast, homeostatic proliferation and recovery of T‐cell numbers were associated with islet rejection. Thus, the timing of islet transplant challenge in relation to diabetes induction was critical in determining whether islets were accepted or rejected. In addition, we tested whether diabetes‐related immunosuppression could result in an overestimation of the efficacy of a tolerance‐inducing protocol. Consistent with this possibility, a protocol targeting CD40L and ICOS that we have shown induces tolerance in diabetic recipients was unable to induce tolerance in non‐diabetic recipients. The data uncover a previously unrecognized suppressive effect of diabetes on adaptive immunity. Furthermore, they suggest that the standard methods of testing new tolerance‐inducing protocols in islet transplantation require modification and that diabetes itself can contribute to homeostatic proliferation, a process associated with autoimmunity and a resistance to tolerance induction.

Robust tolerance to fully allogeneic islet transplants achieved by chimerism with minimal conditioning.

Background. Whether mixed chimeras induced by nonmyeloablative conditioning are tolerant to challenge with donor allogeneic islet grafts is unknown. Here we investigate whether our nonmyeloablative, costimulation blockade-free and sirolimus (SRL)-based protocol could facilitate mixed chimerism via bone marrow transplantation (BMT) and induce islet allograft tolerance. Methods. After low dose (1–3 Gy) total body irradiation (TBI, day –1), with or without prior lymphocyte depletion, C57BL/6 mice were transfused with 40 × 106 BALB/c bone marrow cells (day 0) and received SRL (3 mg/kg/day) for 4 weeks. Chimerism was monitored by flow cytometry and the recipients were rendered diabetic chemically and challenged with donor islets. Results. Mixed chimerism was achieved in mice treated with TBI 3Gy/SRL but it declined over time in 60% (9/15) of them. Long-term stable chimerism was established in 100% of recipients over 50 weeks with either antilymphocyte serum (ALS, 9/9), anti-CD4 (4/4), or anti-CD4 plus anti-CD8 (5/5) prior to BMT. TBI conditioning could be reduced to 1 Gy, with 90% (9/10) maintaining chimerism in the long-term. When TBI was substituted with cyclophosphamide (CTX) or busulfan (BUS), all mice remained chimeric in the long-term. The chimeras showed no proliferative response to donor antigen and accepted both first and second donor-specific islet grafts indefinitely while rejecting third-party grafts. Conclusions. This data provides the first evidence that stable fully allogeneic chimeras induced with BMT after nonmyeloablative conditioning with SRL and lymphocyte-depleting antibodies exhibit robust donor-specific tolerance to islet grafts.

Simultaneous resection of primary colorectal cancer and synchronous liver metastases: a population-based study

Background Simultaneous resection of primary colorectal cancer (CRC) and synchronous liver metastases (LM) is gaining interest. We describe management and outcomes of patients undergoing simultaneous resection in the general population. Methods All patients with CRC who underwent surgical resection of LM between 2002 and 2009 were identified using the population-based Ontario Cancer Registry and linked electronic treatment records. Synchronous disease was defined as having resection of CRCLM within 12 weeks of surgery for the primary tumour. Results During the study period, 1310 patients underwent resection of CRCLM. Of these, 226 (17%) patients had synchronous disease; 100 (44%) had a simultaneous resection and 126 (56%) had a staged resection. For the simultaneous and the staged groups, the mean number of liver lesions resected was 1.6 and 2.3, respectively (p < 0.001); the mean size of the largest lesion was 3.1 and 4.8 cm, respectively (p < 0.001); and the major hepatic resection rate was 21% and 79%, respectively (p < 0.001). Postoperative mortality for simultaneous cases at 90 days was less than 5%. Five-year overall survival and cancer-specific survival for patients with simultaneous resection was 36% (95% confidence interval [CI] 26%–45%) and 37% (95% CI 25%–50%), respectively. Simultaneous resections are common in the general population. A more conservative approach is being adopted for simultaneous resections by limiting the extent of liver resection. Postoperative mortality and long-term survival in this patient population is similar to that reported in other contemporary series. Conclusion Compared with a staged approach, patients undergoing simultaneous resections had fewer and smaller liver metastases and underwent less aggressive resections. One-third of these patients achieved long-term survival.