Islet cell transplantation offers a unique therapeutic avenue for patients with type 1 diabetes who experience severe hypoglycemia unawareness or labile glucose control. By infusing donor-derived islets into the liver’s portal vein, the procedure aims to restore endogenous insulin secretion and improve quality of life. However, the same immune system that destroyed the patient’s native beta cells poses a constant threat to the transplanted tissue. Without effective immunosuppression, both allorejection and recurrence of autoimmunity can rapidly eliminate the graft. Over the past two decades, refinements in immunosuppressive regimens have been central to improving islet graft survival and patient outcomes. This article provides a detailed examination of the current immunosuppressive protocols used in islet cell transplantation, the rationale behind each component, and the evolving strategies designed to minimize drug toxicity while maintaining efficacy.

Understanding the Immune Challenge in Islet Transplantation

Islet grafts face a dual immunological attack. The first is allograft rejection, mediated by recipient T cells that recognize donor HLA molecules. The second is recurrence of autoimmunity, in which memory T cells targeting beta-cell antigens (such as GAD, IA-2, or insulin) can infiltrate the graft. Both pathways involve activation of CD4+ and CD8+ T cells, as well as innate immune components such as macrophages and natural killer cells. The initial inflammatory response at the transplant site—often exacerbated by the isolation and infusion process—further amplifies immune recognition. This complex landscape requires a multi-pronged immunosuppressive approach that targets different phases of the immune response.

Early Inflammatory Milieu and Innate Immunity

During the first hours after infusion, the graft is exposed to instant blood-mediated inflammatory reaction (IBMIR). This innate reaction involves activation of complement, coagulation, and neutrophil infiltration. While not strictly adaptive, IBMIR contributes to early islet loss and can prime the adaptive immune response. Therefore, some protocols incorporate anti-inflammatory agents (such as etanercept or anakinra) in the peri‑transplant period, though these are not always classified as traditional immunosuppressive drugs. Understanding the role of innate immunity has led to the development of targeted interventions that are now often combined with conventional immunosuppression.

Induction Therapy: Aggressive Early Immune Suppression

Induction therapy is typically administered around the time of transplantation to achieve rapid and profound suppression of the recipient’s immune system. The goal is to prevent early rejection during the vulnerable period when the graft is establishing blood supply and recovering from isolation stress. Two main categories of induction agents are used in islet transplantation: polyclonal antibodies (e.g., anti-thymocyte globulin) and monoclonal antibodies (e.g., basiliximab or alemtuzumab).

Anti-Thymocyte Globulin (ATG)

ATG is a polyclonal antibody preparation derived from rabbits immunized with human thymocytes. It induces profound T-cell depletion through complement‑dependent lysis and opsonization. In islet transplantation, ATG has been associated with improved long-term graft survival compared to no induction. A typical regimen includes 3–5 doses of 1.5 mg/kg given intravenously over the first few days post‑transplant. Side effects can include cytokine release syndrome, fever, and increased susceptibility to viral infections—especially cytomegalovirus (CMV) and Epstein‑Barr virus (EBV). To mitigate these risks, recipients often receive antiviral prophylaxis and premedication with steroids and antihistamines.

Monoclonal Antibodies: Basiliximab and Alemtuzumab

Basiliximab is a chimeric monoclonal antibody that blocks the IL‑2 receptor alpha chain (CD25) on activated T cells. It does not deplete T cells but prevents their proliferation. Basiliximab is generally well‑tolerated and has a lower infection risk compared to ATG, but it may be less effective in high‑immune‑risk recipients. Alemtuzumab, on the other hand, is a humanized monoclonal antibody against CD52, a protein expressed on T cells, B cells, and natural killer cells. It produces rapid and sustained lymphocyte depletion. Alemtuzumab has been used in islet transplantation as an induction agent, often in combination with a maintenance regimen that includes calcineurin inhibitors and mycophenolate. The trade‑off is a higher incidence of autoimmunity recurrence and later‑onset infections.

Choice of induction therapy depends on the transplant center’s protocol, recipient risk profile, and the source of islets (e.g., deceased donor vs. living donor). Most modern protocols employ ATG or basiliximab, with alemtuzumab reserved for special cases or clinical trials.

Maintenance Immunosuppression: Sustaining Graft Function

Following the induction phase, patients require lifelong maintenance therapy to prevent chronic rejection and control autoimmune recurrence. Maintenance regimens in islet transplantation typically combine agents acting at different points of the immune cascade. The most common backbone consists of a calcineurin inhibitor (CNI) plus an antimetabolite, with or without mTOR inhibitors and corticosteroids.

Calcineurin Inhibitors: Tacrolimus vs. Cyclosporine

CNIs block T-cell activation by inhibiting the phosphatase calcineurin, which is necessary for nuclear factor of activated T cells (NFAT) translocation. This reduces interleukin‑2 transcription and subsequent T‑cell proliferation. Tacrolimus has largely replaced cyclosporine in islet transplantation due to its superior graft survival outcomes and lower incidence of acute rejection. However, both agents carry significant off‑target toxicities. Tacrolimus is associated with nephrotoxicity, new‑onset diabetes after transplantation (NODAT), neurotoxicity, and hypertension. Cyclosporine also causes nephrotoxicity, hypertension, and cosmetic side effects like hirsutism and gingival hyperplasia. Dosing is carefully monitored through trough levels to balance efficacy and toxicity. In islet recipients, the goal is often to maintain trough levels between 5–10 ng/mL for tacrolimus, depending on the phase and combination therapy.

Antimetabolites: Mycophenolate Mofetil and Azathioprine

Mycophenolate mofetil (MMF) is the preferred antimetabolite in islet transplantation. It inhibits inosine monophosphate dehydrogenase, blocking de novo purine synthesis in lymphocytes. MMF is synergistic with CNIs and allows for lower CNI exposure, thus reducing nephrotoxicity. Typical dosing is 1–1.5 g twice daily, adjusted for gastrointestinal intolerance or leukopenia. Azathioprine, a purine analogue, is used less frequently today because of its weaker efficacy and higher myelosuppression profile.

mTOR Inhibitors: Sirolimus and Everolimus

Sirolimus (rapamycin) and everolimus inhibit the mammalian target of rapamycin (mTOR) pathway, blocking T‑cell proliferation in response to growth signals. These agents were used as primary immunosuppressants in early islet trials, but they were later found to be associated with significant toxicity, including hyperlipidemia, thrombocytopenia, delayed wound healing, and a risk of hepatic artery thrombosis. Moreover, sirolimus alone does not adequately prevent allorejection when used without a CNI. In current protocols, mTOR inhibitors are sometimes added as a third agent to allow CNI reduction, or they are reserved for patients who cannot tolerate CNIs. Their role in islet transplantation remains controversial and is primarily investigational.

Corticosteroids: Balancing Efficacy and Metabolic Harm

Steroids have broad anti‑inflammatory and immunosuppressive effects. They were once a cornerstone of maintenance therapy, but their use in islet transplantation declined sharply after studies showed that steroids directly impair islet function and promote insulin resistance. Most modern protocols avoid steroid maintenance, using them only for short‑term induction or acute rejection treatment. When steroids are necessary, low‑dose prednisone (≤5 mg/day) may be used temporarily, but steroid‑free or steroid‑avoidance protocols are now standard in many centers. The Edmonton protocol, which established a landmark improvement in islet transplant success, used a steroid‑free regimen consisting of sirolimus, low‑dose tacrolimus, and daclizumab (an anti‑CD25 antibody).

Challenges and Side Effects of Current Immunosuppressive Regimens

The success of islet transplantation is tempered by the substantial burden of immunosuppressive‑related adverse effects. Managing these complications is a critical part of post‑transplant care.

Nephrotoxicity

Calcineurin inhibitors are the leading cause of chronic kidney disease after transplantation. Islet recipients often have pre‑existing diabetic nephropathy, making them particularly vulnerable. Up to 30% of recipients may develop stage 3 or 4 chronic kidney disease within five years post‑transplant. Strategies to minimize nephrotoxicity include using the lowest effective CNI dose, converting to mTOR inhibitors, or employing CNI‑free protocols. However, CNI‑free regimens have been associated with higher rejection rates, so the risk‑benefit balance must be individualized.

Infectious Complications

Immunosuppression increases the risk of opportunistic infections. CMV reactivation is common, particularly after ATG induction. EBV‑related post‑transplant lymphoproliferative disorder (PTLD) is a rare but serious complication. Bacterial infections, especially wound infections and line infections, also occur. Viral reactivation can sometimes be prevented with prophylactic valganciclovir and careful screening. Fungal infections (e.g., oral candidiasis) are less common but can arise with prolonged steroid or multi‑drug therapy.

Metabolic Consequences

Tacrolimus is diabetogenic, contributing to NODAT or worsening pre‑existing diabetes. This is especially problematic in islet transplantation, where the goal is to restore normoglycemia. The combination of tacrolimus and steroids further impairs insulin sensitivity and beta‑cell function. Some patients may require additional insulin despite a functioning graft. Everolimus and sirolimus also cause hyperglycemia through insulin resistance. Careful monitoring of glucose tolerance and liberal use of C‑peptide testing help differentiate graft failure from drug‑induced hyperglycemia.

Malignancy and Other Long‑Term Risks

Chronic immunosuppression elevates the risk of non‑melanoma skin cancer, lymphoma, and other malignancies. Regular dermatologic screening and vigilance for lymphadenopathy are recommended. Additional side effects include hypertension, hyperlipidemia, gastrointestinal disturbances (common with MMF), and neurologic symptoms (tremor, headache) from calcineurin inhibitors.

Emerging Strategies to Improve Safety and Efficacy

Despite significant progress, the IS community continues to seek regimens that balance protection of the graft with minimal toxicity. Several innovative approaches are under investigation.

Co‑Stimulation Blockade: Belatacept

Belatacept is a fusion protein that blocks the CD80/86‑CD28 co‑stimulatory pathway, preventing T‑cell activation. It has been successfully used in kidney transplantation as a CNI‑sparing agent and is being evaluated in islet transplantation. Early clinical trials (e.g., NCT02806206) show that belatacept can enable CNI reduction and improve renal outcomes while maintaining graft function. Its main drawback is a higher incidence of lymphoproliferative disorders in EBV‑seronegative recipients, so patient selection is crucial.

Targeted Therapies: Jak Inhibitors and Proteasome Inhibitors

Small molecule inhibitors that interfere with intracellular signaling are gaining attention. Tofacitinib (a JAK3 inhibitor) has been studied in islet transplantation in combination with other agents. In animal models, it prolongs graft survival without the metabolic side effects of CNIs. Proteasome inhibitors like bortezomib target plasma cells and could potentially control antibody‑mediated rejection. Clinical experience in islet transplantation is limited, but these agents offer a new dimension to control both cellular and humoral immunity.

Immune Tolerance and Regulatory T‑Cells

The ultimate goal of transplantation immunology is to induce donor‑specific tolerance, allowing long‑term graft survival without continuous immunosuppression. Strategies include the infusion of ex‑vivo expanded recipient regulatory T cells (Tregs), mixed chimerism induction via donor bone marrow, or the use of costimulatory blockade to promote anergy. Early‑phase clinical trials (such as the ONE Study consortium) have demonstrated feasibility and safety of Treg therapy in kidney transplantation, and similar approaches are being adapted for islet transplantation. Treg therapy could theoretically control both allo‑ and autoimmune responses, offering a paradigm shift in management.

Islet Encapsulation and Alternative Sources

To escape immune detection entirely, researchers are developing encapsulated islets using semi‑permeable membranes that allow glucose and insulin diffusion but block immune cells and antibodies. Encapsulation eliminates the need for systemic immunosuppression. Clinical trials with devices such as ViaCyte’s PEC‑Encap (using stem cell‑derived islet precursors) have shown proof of concept, though challenges remain with fibrosis and oxygen supply. Similarly, the use of genetically engineered pig islets (xenotransplantation) combined with encapsulation could provide a renewable source. Progress in these areas may eventually reduce or eliminate reliance on conventional immunosuppression.

Personalized Regimens and Immune Monitoring

Modern transplant medicine increasingly recognizes that one size does not fit all. The immune system varies widely among individuals, influenced by genetic, epigenetic, and environmental factors. Personalized immunosuppression involves selecting agents and doses based on a patient’s immunologic risk, metabolic status, and tolerance to side effects.

Biomarkers for Rejection and Toxicity

Non‑invasive biomarkers are being developed to detect rejection earlier and guide drug adjustments. These include donor‑derived cell‑free DNA (dd‑cfDNA) in the blood, which rises when graft cells die, and gene expression profiles of peripheral blood cells. Frequent monitoring of HbA1c, C‑peptide, and glucose tolerance tests remains essential. In addition, pharmacogenomic testing (e.g., of CYP3A5 for tacrolimus metabolism) can help predict optimal dosing. Implementing these tools in routine practice can reduce the risk of both under‑immunosuppression (rejection) and over‑immunosuppression (toxicity).

Protocols Based on Recipient Risk Stratification

High‑risk recipients—those with high panel‑reactive antibodies (PRA), pre‑existing autoantibodies, or a history of failed grafts—may require more intensive induction (e.g., ATG) and higher maintenance target levels. Conversely, low‑risk recipients (e.g., negative crossmatch, no DSA) might be candidates for CNI‑sparing or steroid‑avoidance protocols. The choice of regimen also considers the patient’s renal function, lipid profile, and infection history. This tailored approach is increasingly feasible as transplant centers accumulate experience and data.

Conclusion

Immunosuppressive regimens are the backbone of successful islet cell transplantation. Current protocols combine induction agents (ATG or basiliximab) with maintenance drugs such as tacrolimus, mycophenolate, and occasional mTOR inhibitors or steroids to provide robust protection against both allo‑ and autoimmune attack. However, these benefits come at the cost of significant nephrotoxicity, metabolic derangements, infections, and long‑term malignancy risk. Emerging strategies—including co‑stimulation blockade, targeted immunotherapies, Treg‑mediated tolerance, and encapsulation—hold promise for safer and more effective regimens. Personalized immune monitoring and risk‑adapted protocols are already improving outcomes. As research continues, the hope is that islet transplantation can offer durable insulin independence with minimal immunosuppressive burden, ultimately transforming the lives of patients with type 1 diabetes.