|Home | About | Journals | Submit | Contact Us | Français|
This article, derived from a meeting on advances in transplantation but incorporating new data, focuses on the aspect that was discussed in detail by Professor James Shapiro, of the University of Alberta, Canada.
Diabetes mellitus (DM) is increasing worldwide1 and now affects up to 5% of the population in the UK. Roughly 10% of these patients have type I DM, caused by insulin deficiency secondary to autoimmune destruction of pancreatic islet cells. DM is associated with life-threatening metabolic or vascular complications in 30% of patients2. According to data from the UK Renal Registry, 20% of all new patients in the UK under the age of 65 years requiring dialysis treatment have end-stage renal failure secondary to DM. In addition to renal care, patients with diabetes require a diverse range of services including cardiology and cardiac surgery, vascular surgery and ophthalmology.
The Diabetes Control and Complications Trial (DCCT)3 showed that tight glycaemic control delays and reduces diabetic complications. However, intensive insulin treatment is poorly tolerated by many patients and will decrease the number of patients who develop microvascular complications by no more than 30-40%. Further, a small but substantial number of patients have life-threatening hypoglycaemic episodes despite scrupulous attention to their insulin regimens. Therefore, to improve diabetic care, the need is for treatments that achieve metabolic stability and prevent microvascular complications. Reports from Professor Shapiro's team describe major improvements in the early clinical outcome of patients with type I DM treated with human islet cell transplantation by newly developed protocols. Here we discuss the key areas they report that contribute to improvements in the outcome of islet cell transplantation, particularly the use of novel immuno-suppressive strategies.
Until recently, islet cell transplantation was less successful than whole pancreatic transplantation, which achieves insulin independence at one year in 80% of patients. However, the latter is an invasive procedure with a substantial morbidity4. In the UK it is performed in only a few centres and usually in patients with end-stage renal failure who receive a simultaneous kidney transplant. As a result, less than 10% of available cadaveric pancreases are transplanted. In 1999 just 78 patients were on the waiting list for pancreatic transplantation, and 69 of these were listed to receive simultaneous kidney and pancreas transplants5.
Results from the International Islet Transplant Registry6, held in Giessen, Germany, show why, until recently, there has been little enthusiasm for establishing islet cell transplantation in routine clinical practice. 445 adults with type I DM have received transplants, 355 in the past 10 years. Of patients transplanted between 1990 and 1996, 70% lost all islet cell function within the first year. 12.4% of patients remained insulin independent beyond 1 week, 8.2% beyond 1 year and only one recipient for longer than 5 years. The reasons for these poor results have been reviewed by Hering and Ricordi7.
In June 2000 Shapiro and colleagues reported their results with islet cell transplantation by percutaneous transhepatic portal venous embolization and a new clinical protocol. They had treated seven patients (median age 44 years; range 29-54; type I DM for a median of 35 years) who were troubled by repeated episodes of severe hypoglycaemia. At the time of reporting, follow-up was 11.9 months (range 4.4-14.9). All seven patients became insulin independent, although two briefly required insulin during intercurrent illnesses. To achieve insulin independence, all had required a second islet infusion from another donor pancreas a median of 29 days (range 14-70) after the first procedure and one, the most obese, had required a third. The patients were in hospital for a median of 2.3 days (range 0.5-14.7). After transplantation, glycosylated haemoglobin levels did not rise and serum C-peptide concentrations did not decrease over time. No patient experienced further episodes of life-threatening hypoglycaemia.
The Edmonton group have performed islet cell transplants according to this protocol in sixteen patients with a one-year insulin independence rate of 80% (Shapiro J, personal communication). In addition four have received single-donor transplants with a protocol including infliximab. There is some evidence that this anti-TNFα antibody improves engraftment; all four of these patients are positive for C-peptide.
Why should these early results from this group be so much better than those in the registry data? Several changes were introduced simultaneously, so their individual contributions cannot be assessed, but Shapiro and colleagues believe the key factors to be as follows.
Donor islet cells are prepared in xenoprotein-free medium to avoid targeting by preformed antibodies that facilitate cell destruction by complement activation or antibody-dependent cellular cytotoxicity. Cells are transplanted less than 12 hours after harvesting of the donor pancreas. In the past, islets were often transplanted after several days in culture. In solid organ transplantation there is good evidence that long ischaemic times are associated with worse graft outcome.
The minimum number of islets infused is based on recipient body weight. The Edmonton patients received a mean of 11 547 islet equivalents per kg body weight (IE/kg), with a minimum of 5000 IE/kg for the first transplant and at least one further infusion of fresh islets to ensure a cumulative total of at least 10 000 IE/kg. In some patients a third transplant was required.
An increase in the range of immunosuppressive agents has allowed the Edmonton group to avoid glucocorticoids, with their diabetogenic effects. The aim is to prevent both allograft rejection and autoimmune recurrence of diabetes without inhibiting islet cell function, and the successful regimen comprises daclizumab, sirolimus and low-dose tacrolimus. (The combination of low-dose tacrolimus and sirolimus is highly efficacious in solid organ transplantation for prophylaxis of acute allograft rejection9.)
Daclizumab (Zenapax) is a humanized anti-CD25 monoclonal antibody used for prophylaxis against acute rejection in solid organ recipients. Cell surface expression of CD25, the alpha chain of the IL-2 receptor, is a key event in the activation of T cells following alloantigen recognition. Blocking of this target in solid organ transplantation leads to a 40% reduction in the incidence of acute allograft rejection without increasing rates of cytomegalovirus or other tissue invasive infections or post-transplant lymphomas. A chimeric anti-CD25 antibody called basiliximab (Simulect) matches the efficacy of daclizumab in solid organ transplantation. Daclizumab is given immediately before transplantation and every two weeks after transplantation for four more doses10. The Edmonton group repeats this dose regimen for subsequent islet transplants. In the randomized control trials with anti-CD25 antibodies there are no reports of a cytokine release syndrome.
Tacrolimus (FK 506) binds to an intracytoplasmic immunophilin to produce a complex that inhibits calcineurin, a key intracellular signalling protein activated after T cell receptor ligation. This interaction inhibits IL-2 gene expression and subsequent T-cell activation. Solid organ transplant recipients treated with tacrolimus have a reduced incidence of acute rejection and possibly a reduced incidence or delayed onset of chronic graft rejection11. The Edmonton group used low-dose tacrolimus to maintain a trough concentration at 12 hours of 3-6ng/mL. In combination with daclizumab and sirolimus this provided adequate immunosuppression whilst avoiding dose-related complications such as hypertension, diabetes (due to diminished beta cell function), and nephrotoxicity.
Sirolimus (rapamycin) binds to intracytoplasmic binding proteins to form an intracellular complex that prevents T cell proliferation12 by inhibiting a key regulatory kinase. In kidney transplantation the use of sirolimus substantially reduces the occurrence of acute rejection13. Preclinical studies in islet cell transplantation point to extended allograft survival14. In cyclosporin-based regimens, the use of sirolimus increases serum triglycerides and cholesterol in 40-50% of patients. However, of the patients transplanted in Edmonton, only one had an increase in cholesterol (mild). Probably the avoidance of steroids and the use of tacrolimus in low dosage lessens the risk of dyslipidaemia. Although there is concern about nephrotoxicity with sirolimus, it has not been evident with this regimen.
Although initial experience with this immunosuppressive combination is highly encouraging, the precise risks of infection, carcinoma and post-transplant lymphoproliferative disorder are unclear at present. Other immunosuppressive agents recently introduced into clinical practice such as mycophenolate mofetil and FTY720 also require assessment. A better understanding of the mechanisms involved in early islet injury—which is thought to be antigen-independent and mediated by macrophage-derived proinflammatory cytokines, reactive oxygen and nitrogen intermediates15,16—could lead to development of therapeutic agents17,18 that prevent loss of graft function without increasing immunosuppression. Finally, the development of tolerance-inducing therapies is a major focus of transplantation research and may ultimately remove the need for lifelong immunosuppression.
The Edmonton series was confined to patients with metabolic lability/instability, particularly reduced awareness of hypoglycaemia. Future studies will need to address the effectiveness of islet cell transplantation in patients with established end-organ damage. The effect on established nephropathy and retinopathy is of great interest. At present, there is only limited potential for islet cell transplantation in type II DM, which is usually characterized by insulin resistance rather than insulin deficiency. As with any allogenic transplant, the recipient may be sensitized against donor antigens. In Edmonton, donors and recipients were matched for blood group and cross-matched to exclude lymphocytotoxic antibodies. HLA matching was not performed. Since diabetes is a major risk factor for kidney failure, donor sensitization is a potential problem in patients who may subsequently require a renal transplant. However, no patient in the Edmonton series has shown any evidence of sensitization to date.
Islet cell transplantation from pancreases harvested from cadaveric donors will never be possible for more than a fraction of the patients who could benefit. There have been rapid developments in stem cell derived islet cells, and this approach may ultimately provide an unlimited source of islets for transplantation. Ramiya et al.9 have reported the generation of islet cells from pancreatic ductal epithelial cells isolated from prediabetic adult non-obese diabetic mice in long-term cultures. The cultured cells were induced to produce functioning islet cells that responded in vitro to glucose challenge, and reversed insulin-dependent diabetes after implantation into diabetic non-obese mice. In a further study, insulin-secreting cells derived from mouse embryonic stem cells normalized glycaemia in diabetic mice20. Bonner-Weir and colleagues21 have reported the successful expansion of human pancreatic ductal tissue directed to differentiate into glucose responsive islet tissue in vitro. Further, De La Tour et al.22 have developed a human betacell line exhibiting glucose-responsive insulin secretion22.
The Edmonton protocol is now being rolled out to ten other centres in an international multicentre trial supported by the Immune Tolerance Network [www.ucsf-179-35.ucsf.edu/patients/islet/edmonton ]. Six of these centres are in the USA, three in Europe (Germany, Italy, and Switzerland) and one in Canada. Diabetes UK [www.diabetes.org.uk ] is planning to run a separate trial based in several centres. If these studies confirm the safety and efficacy of the Edmonton approach, attention will shift to developing a framework that will introduce islet cell transplantation into regular clinical practice.