Islet transplantation: learning from the Edmonton experience

Figure 1. Schematic representation of the basic steps of islet transplantation.

1. Retrieval of donor pancreas and isolation of islets by enzymatic and mechanical digestion. 2. Purification of islets using density gradient centrifugation. 3. Infusion of purified islets into the portal vein via percutaneous catheterization. 4. Implantation of infused islets in hepatic sinusoids.

Modified from the University of Alberta Clinical Islet Transplant Program [101].

Figure 2. Survival analysis for C-peptide secretion (A) and insulin independence (B) over time for all those who completed islet transplant procedures between April 1999 and November 2004 at the University of Alberta.

The survival curves are dated from the time of the final transplant.

^©2005 American Diabetes Association, adapted from [10].

Islet transplantation for Type 1 diabetes has made significant progress over recent years since it was reported in 2000 that a series of seven islet transplant recipients became insulin independent after undergoing the procedure [1]. Since the introduction of steroid-free immunosuppressive regimens in the late 1990s, the 1-year insulin independence rate observed in islet transplant recipients has risen from less than 10 to approximately 80% [2]. These results represent a marked improvement in the clinical outcome of islet transplantation and have encouraged further study of β-cell replacement as a viable therapy for selected individuals with Type 1 diabetes.

Since this initial success, substantial efforts have been made to establish islet transplant programs worldwide. However, preliminary results from the Immune Tolerance Network multicenter trial suggest that successful islet transplantation may require a degree of laboratory and clinical experience that can be difficult to achieve for many centers [3]. Also, the costs of establishing an islet isolation laboratory, which must meet stringent Good Manufacturing Practice (GMP) guidelines, have limited the widespread proliferation of islet transplant centers. An alternative approach that might allow greater access to islet transplantation would be to use experienced regional islet isolation laboratories to provide transplantable islets to surrounding clinical centers. This approach has been tried in at least one circumstance with some success [4].

The principle impact of islet transplantation on individual recipients has been a marked stabilization of blood glucose control in people who were previously plagued by severe glycemic lability and hypoglycemia. This glycemic stability has been observed consistently in recipients with functioning islet grafts (i.e., who are C-peptide positive), whether or not they use exogenous insulin. However, as is often the case with clinical research, progress has not been without difficulty. Rates of insulin independence have tended to decline with time, although most islet recipients remain C-peptide positive. Also, immunosuppressive drugs, used to avoid graft rejection, have adverse side effects that must be balanced with the benefits of transplantation for each individual. Nevertheless, after 6 years of uninterrupted clinical experience, investigators understand more than ever before about the benefits and risks of islet transplantation and the need for further progress in this field.

Islet transplant procedure

The islet transplant procedure occurs in several steps beginning with the retrieval of a donor pancreas (Figure 1). In the islet isolation laboratory, the pancreatic duct is cannulated and perfused with a prepared enzyme blend. The pancreas is then placed in a steel (or acrylic) chamber (ricordi chamber) and enzymatically and mechanically dissociated using automated or manual agitation. The liberated islets are then purified by density gradient centrifugation and may be placed into culture for 12–24 h to facilitate a planned daytime procedure.

Once the islets are isolated, an ABO blood group-matched recipient is admitted to the hospital radiology department where the portal vein is cannulated under fluoroscopic guidance. Islets are then infused by gravity into the portal circulation and flow with the blood to lodge in the hepatic sinusoids. Portal pressure is monitored periodically throughout the procedure to ensure that it does not rise significantly. To minimize the risk of bleeding, after the infusion is complete, the catheter tract is plugged using a combination of coils and fibrinogen paste. The survival of islets after transplantation has been estimated to be only 10–20% [5]. This low rate of islet survival is probably due to a number of factors, including cellular hypoxia and the initiation of an inflammatory cascade, including the so-called instant blood-mediated inflammatory reaction (IBMIR), which may trigger the release of tissue factors detrimental to islets [6,7].

Indications for islet transplantation

Based on our current experience, the selection of potential islet transplant candidates is critical if the estimated risk–benefit ratio for each individual is to favor transplantation. In many respects, the current selection criteria are similar to those for solitary pancreas transplantation [8], consisting of frequent episodes of severe, undetected hypoglycemia, severe glycemic lability or progressive diabetic complications, despite attempts to optimize insulin injection therapy. However, since the greatest impact of islet transplantation is in stabilizing blood glucose and since no long-term data are yet available regarding the impact of islet transplantation on diabetic complications, the progressive complications indication rarely justifies transplantation without another indication also being present.

Clinical & metabolic outcomes

Since 1999, the University of Alberta has performed 169 islet transplant procedures on a total of 88 recipients (33 men and 55 women, as of March 2006). The mean age of the recipients is 44 years (range: 24–64) and the mean duration of diabetes is 28 years (range: 5–52). Insulin independence in most cases requires at least two separate islet infusions from different pancreas donors. The mean cumulative number of transplanted islets in individuals receiving at least two infusions (n = 68) is 826,405 ± 28,843. After the first islet infusion, daily insulin requirements are reduced by a mean of 52% (range: 12–100) and glycemic stability is usually greatly improved. However, recipients do not generally achieve insulin independence before receiving a second islet infusion for a total transplanted islet mass of at least 12,000 islets/kg [9]. It appears that those recipients who demonstrate a reduction of their daily insulin dose by more than 50% after the first islet infusion are much more likely to achieve insulin independence after two islet infusions than those whose daily insulin requirements decline by less than 50%.

A major potential benefit of islet transplantation is freedom from regular insulin injections. This can be achieved in the majority of recipients in the first 1–2 years after transplantation. The 12-month insulin independence rate for recipients who have received at least two islet infusions is approximately 80% by Kaplan–Meier survival analysis. However, insulin independence rates tend to decline with time, so that by 5 years after transplantation, less than 10% of recipients are free from insulin (Figure 2) [10]. The reason for this decline is not clear, but may involve direct immunosuppressive toxicity, allo- or autoimmune rejection or islet cell apoptosis, potentially reflecting the islet’s natural life span [11]. Complete loss of C-peptide secretion after transplantation has occurred in 11% of recipients. Mean glycated hemoglobin is normalized in insulin-independent recipients and near-normal in islet transplant recipients who remain C-peptide positive.

Other than insulin independence, the most immediate and long-lasting benefit of islet transplantation is stabilization of blood glucose, which occurs in almost all recipients, usually after a single islet infusion. A recent study using continuous glucose monitoring to compare glycemic control in insulin-independent islet transplant recipients versus those who are C-peptide positive, but use exogenous insulin, showed that C-peptide-positive recipients who used exogenous insulin continued to benefit from excellent, stable blood glucose control, while eliminating or significantly reducing the incidence of hypoglycemia [12]. This finding is important because it suggests that, although insulin independence is an important goal for most islet transplant recipients, it is not necessary in order to obtain benefits from islet transplantation in the form of markedly improved glycemic control and hemoglobin A₁C with total or near-total elimination of severe hypoglycemia.

Another potential benefit of islet transplantation may be a reduction of long-term secondary complications of diabetes. Some preliminary studies have suggested a benefit for both cardiovascular function and diabetic retinopathy [13,14]. However, no randomized, controlled trials have been performed. Consequently, a reduction in secondary diabetic complications remains only a theoretical benefit and will require much more detailed, long-term investigation to be fully known.

Complications of islet transplantation

As with other transplant procedures, islet transplantation is not without risks. The most common procedure-related adverse events are abdominal pain and nausea. Intraperitoneal hemorrhage has occurred in approximately 10–15% of recipients, however, recent improvements in sealing the catheter tract after islet infusion have greatly reduced the risk of bleeding [15]. Portal hypertension can occur acutely during islet infusions, but portal pressures tend to normalize after the acute phase of transplantation [16]. Portal vein thrombosis has occurred in 4% of recipients but has generally been limited to branch veins and has resolved with appropriate anticoagulation. Post-transplant elevation of liver enzymes (54%) and catheter-related puncture of the gallbladder (3%) can also occur, although these tend to be self-limited [9].

The most common adverse effect of immunosuppressive therapy is mucosal ulceration involving the tongue or buccal mucosa, which occurrs in the vast majority (~90%) of recipients. This is presumed to be a consequence of sirolimus therapy and tends to be dose dependent. Increased use of lipid-lowering or antihypertensive agents has occurred in approximately half of all patients. Post-transplant anemia is common (> 50%) and a reduction in total white blood cell count is also frequently seen, but severe neutropenia is uncommon. Post-transplant weight loss is also often seen. The appearance of intrahepatic periportal steatosis has been observed using magnetic resonance imaging in 20% of recipients and may occur in a larger proportion [17,18]. These changes are thought to be benign, owing to the local effects of insulin on surrounding liver parenchyma. They also appear to be reversible, since complete resolution has been seen in one patient whose graft failed completely. However, the impact of fatty changes in the liver on underlying hepatic glucose metabolism is not yet known.

Immunosuppressive agents can be nephrotoxic and may be partially responsible for an elevation of serum creatinine and a reduction in glomerular filtration rate (GFR) after transplantation [19]. Retinal bleeds have also been observed, but because of the limited duration of follow-up, no firm conclusions can be drawn regarding the long-term effects of islet transplantation on microvascular complications. The risk of post-transplant infection and malignancy also exists. So far, among University of Alberta islet transplant recipients, there has only been one case of severe infection: a fungal infection of the lung, requiring intravenous antifungal therapy. This patient experienced mild respiratory symptoms and recovered fully after treatment. One recipient had a well-differentiated thyroid neoplasm resected and another had two basal cell skin cancers removed. There is no clear way of establishing whether or not these neoplasms were related to immunosuppression.

Conclusions

In the 6 years since the initial report of seven Type 1 diabetes patients who became insulin-free after islet transplantation, it has been clearly established that islet transplantation can provide insulin independence for the majority of recipients if an adequate functioning islet mass is established. Insulin independence can persist for at least 1–2 years in most recipients and longer in many. Recipients who remain C-peptide positive after 2–3 years can continue to benefit from their transplant through a stabilization of blood glucose. In such circumstances, the risk–benefit ratio of immunosuppression for each individual must be reassessed on an ongoing basis. Beyond 5 years posttransplant, the results are not yet clear and will require further evaluation.

To build on the recent progress that has been made, a number of issues must continue to be addressed. The current severe lack of transplantable insulin-secreting tissue must be overcome. This may be accomplished by improving isolation techniques or the potential future use of alternative transplantable insulin-secreting tissue (stem cells or animal-derived cells). Other proposed means of increasing functional β-cell mass include the use of possible β-cell proliferative agents, such as glucagon-like peptide (GLP)-1 analogs or growth factors. In addition, further efforts need to be made to better understand the reasons for the gradual decline in insulin independence after transplantation. In addition, the efficacy and side-effect profile of immunosuppressive agents must also continue to be improved to enable consistent islet engraftment with fewer adverse side effects. Finally, the metabolic effects of islet transplantation remain poorly understood and deserve greater exploration.

Despite these challenges, the recent progress in islet transplantation has reinforced the potential of β-cell replacement for the treatment of diabetes. As further improvements are made, it is anticipated that the balance of risks and benefits will become increasingly favorable. Currently, at a minimum, islet transplantation can allow Type 1 diabetes patients with severe, intractable glycemic lability and frequent hypoglycemia to live more normal lives. This benefit should not be underestimated, since islet transplantation is a last resort for many individuals whose daily lives are severely limited by diabetes. In a larger sense, and perhaps equally as important, the promise of islet transplantation continues to give hope to many individuals with diabetes that they may one day be free from the risk of secondary complications and the daily burden of insulin injections.

Acknowledgements

The University of Alberta Clinical Islet Transplant Program is supported by the Juvenile Diabetes Research Foundation and Capital Health, Edmonton, AB, Canada.