Limitations of Islet Transplantation: Engraftment and Long-Term Function
A typical adult pancreas contains approximately one million islets, which represent around 1–2% of the total mass of the organ. It has been estimated that only 50% will survive the harsh process of isolation, with up to 60–80% of the remaining mass perishing in the immediate posttransplantation period due to inflammatory processes not yet fully understood. For instance, it has been shown that islets express tissue factor which may contribute to early islet loss by stimulating coagulation upon their contact with the blood. Considering the many insults that may invariably result in islet cell death from the time of the pancreas procurement to the actual infusion, the fact that only 10% of the transplanted patients are insulin-free 5 years after the procedure is much less surprising than the observation that up to 80% are insulinfree after 1 year. While we define alternative sources of islets that are either plentiful (xenotransplantation) or self-renewable (stem cells), there is an imperative need to “make every islet count” and to minimize their destruction upon implantation. The field of islet cytoprotection is a fertile one, with a large number of chemical gene-based, and protein transduction strategies proven successful in many experimental models. However, it is still necessary to gather much more information about the basic mechanisms that drive beta cell destruction upon implantation. In this context, another limitation of islet transplantation has been the difficulty to explore in real time the causes of their demise once implanted. An important breakthrough was reported just recently by Speier et al., who transplanted islets in the anterior chamber of the eye of recipient mice. This location was supportive of islet engraftment and function, as evidenced by the reversal of diabetes. More importantly, this system allows investigators to follow in real time events such as vascularization, inflammation, and rejection, which can be studied in living animals by microscopic examination of the eyes where the islets are transplanted. Interventions designed to interfere with physiological responses leading to islet destruction can now be implemented by means of eye drops and monitored as they exert their effects.
Limitations of Islet Transplantation: Immunosuppression and Tolerance
General Considerations About Islet Rejection
As much as islet transplantation improves upon exogenous insulin administration as a way to regulate glucose homeostasis, unless both the allo-rejection and the autoimmune processes are arrested, it will not be considered a cure. The paradox about immunosuppressants is that, while they have made islet allotransplantation possible, they are also diabetogenic, impairing beta cell survival, function, and replication. Thus, the ultimate goal of any cell therapy for type I diabetes is that the replaced cells are accepted as “self” by the recipient, and that the immune system of the patient is reeducated to prevent recurrence of autoimmunity. When T cells contact either a self-antigen or a harmless exogenous antigen, the absence of co-stimulatory signals or the presence of co-inhibitory ones will prevent the unleashing of the immune response. At this point, the T cell may get inactivated or even apoptose. As a growing body of evidence suggests that tolerance or rejection are determined by the balance between positive and negative co-signaling, the receptors involved are potentially useful targets to minimize allo-rejection. As for the mechanisms responsible for the recurrence of autoimmunity, it is known that in the nonobese diabetic mice, a model of type I diabetes, beta cell destruction is mediated by autoreactive T cells. The two major costimulatory pathways involved in this rogue T-cell response are CD28–B7 and CD40 ligand–CD40. In fact, the use of antibodies that interfere with these pathways has been proven effective at preventing the onset of diabetes in this animal model. There is a balance between normal immune regulation and autoimmunity that appears to be mediated by regulatory T cells (Tregs), a subset of T cells characterized by the expression of the master transcriptional regulator Foxp3 and identifiable by the surface expression of CD4 and CD25. These cells confer protection against autoimmune diabetes. Unfortunately, the experimental blockade of the co-stimulatory response that triggers allo-rejection has also a negative effect on Treg function. In this context, the identification of co-inhibitory molecules might offer a safer and more effective therapeutic alternative .
More comprehensive strategies go one step beyond the mere tipping of the balance between autoimmunity and regular immune function. Bone marrow transplantation, for instance, has been shown to cure diabetes in animals, but for the new cells to home, the resident stem cells must be wiped out by lethal or sublethal irradiation. This is obviously not a therapeutic option for diabetes as it is for other fulminant illnesses, as diabetes is a chronic and (to some extent) manageable disease. Hence the search for gentler ways of making room for “healthy” bone marrow cells without completely ablating the recipient’s hematopoietic compartment; and inducing graft-versus-host disease. The latest approach (already discussed in the chapter “Adult stem cells & pancreatic differentiation”) is that of using hematopoietic stem cells extracted from the same patient, who is subsequently treated with a strong immunosuppressive regime prior to stem cell reinfusion. The safety and long-term effectiveness of these treatments are still in question, and more studies will be necessary before they are translated into widely used clinical practice.
Finally, another area of research aims at encapsulating the transplanted islets with a physical barrier that will protect them from both allo- and auto-rejection (including immune-effector cells, complement system, and immunoglobulins), while allowing the free transit of insulin, sugar, nutrients, and oxygen. Considering the sophistication of the cellular and molecular strategies described above to interfere with the immune response, this could be considered a “brute force” approach. If successful, however, it could even open the door to xenotransplantation of nonhuman islets, which could potentially be a valid alternative to that of using stem cells for directed differentiation into pancreatic endocrine cells. The different strategies presented thus far (intravascular implant, macroencapsulation, and microencapsulation) are posted here. Several materials have already been tested with variable degrees of success. The workhorse of these studies is alginate, derived from kelp, a member of the brown algae (Phaeophyceae) group.
Antigens are detected by T cells by means of their interaction with antigen-presenting cells (APCs). At the molecular level, such interaction occurs between a T-cell receptor (TCR) and a major histocompatibility complex (MHC). Without any other signal, optimal activation does not occur, and may even induce tolerance or anergy. However, secondary interactions (costimulatory signals) cannot activate the T cell by themselves. A combination between the primary TCR-MHC and a co-stimulatory signal are necessary for optimal response. Below is represented the balance between co-stimulation and co-inhibition. The net result of these opposing forces will ultimately determine the response of the T cell .
General encapsulation strategies. The intravascular implant (also known as biohybrid artificial pancreas) is a perfusion chamber directly connected to the blood vessels through an arteriovenous shunt. Islets are within the diffusion area of influence of the blood vessel, while protected by a membrane. However, implantation is not a straightforward procedure, and there is a significant risk of clotting. More conventional strategies involve the protection of islets by means of immunobarriers (macroencapsulation and microencapsulation) not in direct contact with existing vasculature .
This compound is already in clinical trials, and offers the advantage that it can be conjugated to materials such as polyethylene glycol and poly-lysine to reduce plasma adsorption and decrease the formation of fibrotic tissue around the capsule (recently reviewed by Beck et al ).
Polysulphone is another promising material for islet encapsulation, although it needs to be chemically modified in order to minimize insulin adsorption. All of these mechanisms, however, suffer from two important limitations, namely: their inability to prevent the circulation of cytokines across the barrier (which would be especially problematic in xenotransplantation settings) and nutrient deprivation and hypoxia. The latter has already been acknowledged as one of the main determinants of islet cell death upon transplantation, and islet cell encapsulation can only exacerbate this problem. Conventional approaches induce large void volumes where the islet ( ~ 150 m m) is engulfed in a much larger structure (400–800 m m). Considering the ensuing delay in nutrient and oxygen delivery, as well as the unusually high metabolic demands of islets, it is hardly surprising that encapsulated islets tend to exhibit starvation-induced apoptosis and/or impaired function. In an effort to retain the critical advantages of encapsulation while maximizing transport of oxygen and nutrients across the barrier, a promising area of research is that of designing thin, conformal polymeric layers that can be “coated” directly onto the islet surface. Doing so may reduce the diffusion distance between the islet and the capsule by up to 1,000- fold thus improving the chances for islet survival in the immediate posttransplantation period.
In a paradoxical reversal of the “miniaturization” trend, the recent development of a subcutaneously implantable biocompatible device may also help address many of the problems of immune rejection. In preliminary studies in rats, a cylindrical stainless-steel mesh was implanted and allowed to be vascularized for 40 days. Islets were then placed in the device, where they supported reversal of diabetes in a syngeneic setting. Although a device of this nature is not designed to prevent the access of the immune effectors (cells, complement, and immunoglobulins), a major advantage over noncontained transplantation sites is that immunosuppression could be potentially delivered in a local fashion. Systemic medication to prevent rejection is based on the administration of very large doses of immunosuppressants, due to the need of reaching biologically active concentrations at the site of the graft. The well-known side effects of this treatment include higher incidence of infection and cancer. With this system, much smaller doses could be delivered locally in the site of the graft, allowing for immunoprotection with no or little systemic effects. In fact, this approach might allow for the delivery of potentially powerful drugs not approved for systemic administration.
Tags: beta cell destruction, brute force approach, Engraftment and Long-Term Function, field of islet cytoprotection, graft-versus-host disease, hematopoietic compartment, immune-effector cells, immunoglobulins, Immunosuppression and Tolerance, inflammation, master transcriptional regulator Foxp3, posttransplantation period, regulatory T cells Tregs, vascularization