Towards curing type 1 diabetes: Prospects and challenges of allogeneic or xenogeneic donor islet cell transplantation

Abstract

Type 1 diabetes (T1D) is a chronic, lifelong, autoimmune disease that is debilitating and life-threatening to those who suffer from severe hypoglycaemic events or the devastating chronic complications. Exogenous insulin replacement, including the artificial pancreas, is the current mainstay of T1D therapy but cannot prevent the chronic vascular complications of the disease. They are also responsible for contributing to severe iatrogenic hypoglycaemia and impaired hypoglycaemic awareness. β-cell replacement with either pancreas or islet allotransplantation can reverse diabetes leading to better glycaemic control, prevention of hypoglycaemic events and improved quality of life for patients. The limited supply of cadaveric organ donors is a major barrier to this therapeutic option. Thus, alternative sources of islets are being actively explored, mainly human pluripotent stem-cell derived islets and xenogeneic porcine islets. Although these sources harbor their own risks and problems, various novel and innovative solutions are being perseveringly investigated across the globe to overcome these in the hopes that safe islet transplantation may one day be available to all T1D patients suffering from severe hypoglycaemic events. This review will concentrate on pre-clinical and clinical studies, in addition to the latest scientific discoveries relevant to T1D transplantation therapy using allogeneic or xenogeneic donor islet cells.

Key Words: Type 1 diabetes mellitus; Human pluripotent stem cells; Islet transplantation; Insulin-secreting β-cells; Xenogeneic; Porcine islets; β-cell replacement therapy; Regenerative medicine; Insulin dependent diabetes mellitus; Pancreatic β-cells

Core Tip: Type 1 diabetes is a lifelong disease that is debilitating and life-threatening due to severe hypoglycaemic events or impaired hypoglycaemic awareness. β-cell replacement with either pancreas or islet allotransplantation can reverse diabetes leading to improved quality of life. The limited supply of cadaveric organ donors is a major barrier to this therapeutic option. Thus, alternative sources of islets are being explored, mainly human pluripotent stem-cell derived islets and xenogeneic porcine islets. This review will concentrate on pre-clinical and clinical studies, in addition to the latest scientific discoveries relevant to type 1 diabetes transplantation therapy using allogeneic or xenogeneic donor islet cells.

INTRODUCTION

Diabetes mellitus (DM) is a metabolic condition of chronic hyperglycaemia due to the absolute deficiency (type 1 DM) or relative deficiency (type 2 DM) of the hormone insulin. Type 1 diabetes (T1D) is an autoimmune disease characterised by the immune mediated loss through CD4⁺ T cells, CD8⁺ T cells and autoantibodies against one’s own insulin-secreting β cells within the islets of Langerhans. The symptoms of hyperglycaemia usually occur following the loss of about 70%-90% β cells[1-3]. In contrast, type 2 diabetes is due to the combination of insulin resistance in peripheral tissues and impaired insulin secretion[4].

The initial presentation of T1D can include polydipsia, polyuria, nocturia, weight loss, lethargy, hunger and blurred vision driven by dysfunctional hyperglycaemia. The chronic hyperglycaemia in DM leads to microvascular and macrovascular complications of end organs such as retinopathy, nephropathy, neuropathy and vascular diseases. These complications can be debilitating and life-threatening, with coronary artery disease, peripheral vascular disease and ischaemic stroke being the main causes of morbidity and mortality in patients with DM.

DM is a significant global health burden that continues to grow in tandem with increasing urbanization and its associated changes in human lifestyles[5]. DM is a growing epidemic affecting 537 million adults, according to the International Diabetes Federation 2021, with that figure expected to grow to 643 million adults by the year 2030, and 783 million by the year 2045[6]. T1D is the more common form of DM in childhood, with almost one-fifth of T1D patients (> 1.2 million) being under 20 years old[6].

DM is a principal cause for end stage renal failure, lower limb amputations and loss of vision[7-9]. The burden of this chronic disease also has significant impacts on a patient’s mental health, with 20%-30% reporting elevated diabetes distress[10]. The economic costs in treating DM and its complications and the associated lost productivity is not an insignificant amount. DM is an extremely costly disease with estimated expenditures of $327 billion by the United States healthcare system in the year 2017 alone[11,12]. T1D patients also have a 3-6 times higher mortality rate compared to non-diabetics across the globe, a positive correlation of increased mortality with higher glycosylated haemoglobin (HbA1c), and a shortened life expectancy by about 12 years[13-17]. To help readers better understand this article, we will first briefly describe some key islet functions and physiology.

ISLETS/β CELLS AND THEIR FUNCTION

The adult human pancreas is about 25 cm long and 45 cm³ in volume, comprising of about 1-3 million islets of Langerhans, each with a diameter of 110 mm. Islets are made up of 57% insulin-secreting β cells (about 1.25 g), 33% α cells that produce glucagon and 10% δ cells that produce somatostatin[18,19]. These pancreatic islets secrete hormones that tightly regulate blood glucose levels (BGLs) to an average of 5.5 mmol/L (100 mg/dL) in response to fluctuating glucose levels following food intake or energy consumption such as exercise, with rapid replenishment of insulin stores.

β cells continuously sense glucose levels and secrete insulin swiftly in a glucose-dependent manner through a K_ATP channel-dependent mechanism[20]. It then transitions to a K_ATP channel-independent mechanism, resulting in biphasic glucose-stimulated insulin secretion (GSIS)[20]. Glucose is transported into β cells and phosphorylated through oxidation and glycolysis to generate ATP, along with the Krebs cycle, which leads to the closing of K_ATP channels and allows sodium to enter and generate membrane depolarisation. Voltage-dependent T-type calcium channels open, and the increase in calcium results in secretory granules containing insulin fuse to the plasma membrane and release insulin into the blood circulation. There are various other stimulatory signals of β cells including glucagon like peptide 1, nutrients such as free fatty acid, amino acids and parasympathetic inputs[20].

Physiologically, insulin secretion is extremely dynamic, responding to changes in glucose levels due to food and physical activities, working closely with the liver and gastrointestinal system. Insulin is secreted into the portal vein before food is swallowed during the cephalic phase from food-related stimulated (first phase) insulin release and returning to basal levels during food absorption. The Nobel prize winning discovery of insulin over a century ago was an important turning point in availing insulin therapy to T1D patients. Since then, T1D was no longer a terminal condition but instead became a chronic condition. However unfortunately for some children in lower income countries without access to insulin replacement therapy, T1D remains a life-limiting disease[21].

CURRENT STANDARD INSULIN THERAPY FOR T1D

The current standard management of T1D, and for some insulin requiring type 2 diabetes, requires regular blood glucose monitoring and multiple daily subcutaneous insulin injections to control BGLs via either an insulin pen, syringe or pump. The daily insulin regimen can comprise of a combination of rapid-acting, short-acting, intermediate-acting and long-acting insulin; most commonly in a basal bolus regimen or twice daily injections, in a relatively crude attempt to maintain normoglycaemia. However, insulin therapy does not prevent long-term vascular complications, instead turning T1D from a fatal disease into a chronic condition.

Insulin therapy has undergone many developments and iterations. Insulin was initially sourced from pigs for many years, until the production of recombinant insulin DNA in 1982[22], and the first generation of recombinant insulin analogues was mass-manufactured in 1996[23]. Since the development of long-acting and rapid-acting insulin analogues with improved stability and enabling basal and mealtime peak coverage, T1D patients were able to achieve better glucose targets as well as reduce, but not eliminate, the risk of severe hypoglycaemia. However, this also introduced the risk of nocturnal iatrogenic hypoglycaemia (Figure 1A)[24].

Figure 1 Problems of current insulin replacement therapy. A: Insulin therapies are associated with iatrogenic hypoglycaemia (blood glucose concentration < 70 mg/dL, 80%); B: Sensor-enhanced insulin pumps (known as artificial pancreas). Insulin pumps and continuous glucose monitoring only reduce but do not eliminate severe hypoglycaemic events/episodes (blood glucose concentration < 54 mg/dL); C: The incidence of severe hypoglycaemia associated with insulin therapies including artificial pancreas is approximate 7% in type 1 diabetes and insulin-requiring type 2 diabetes patients; D: Over 90% type 1 diabetes patients wish for insulin-independence. T1D: Type 1 diabetes; T2D: Type 2 diabetes.

DIABETES COMPLICATIONS

In addition to the chronic complications of hyperglycaemia or acute cases of diabetic ketoacidosis, T1D patients are also at risk of acute complications of iatrogenic hypoglycaemia due to inappropriately high insulin levels from insulin therapy, the delayed action of insulin and defective counterregulatory response to raised BGL (Figure 1A). Hypoglycaemia is defined by a blood glucose of < 3.9 mmol/L (70 mg/dL), warranting treatment with carbohydrate ingestion. Serious hypoglycaemia known as brittle hypoglycaemia or labile diabetes is a BGL of < 3.0 mmol/L (54 mg/dL) and can lead to cognitive impairment, loss of consciousness, cardiac arrhythmia, seizures, coma and death. The insulin pump improves quality of life but unfortunately does not significantly reduce anamnesis hypoglycaemia (about 80% compared to insulin injections) based on continuous glucose monitoring (CGM) records[25].

Impaired awareness of hypoglycaemia can develop with intense insulin therapy due to defective counterregulatory response to hypoglycaemia. The resultant reduced hypoglycaemia symptoms and inappropriate response in correcting BGL leads to increased risk of recurrent, severe hypoglycaemia and its acute life-threatening complications[26]. About 25% of people with T1D are affected by impaired awareness of hypoglycaemia[27]. Approximately 7% of T1D adults experienced severe hypoglycaemia leading to seizure or loss of consciousness in the prior 3 months, regardless of their HbA1c (Figure 1C)[28]. About 4%-10% of deaths in T1D patients is due to severe hypoglycaemia[29]. Therefore, it is not surprising that over 90% people with T1D wish to be insulin-independent (Figure 1D)[30], which cannot be achieved with current insulin therapies.

DIABETES DEVICES

The artificial pancreas showed superior glycaemic control, with less hypoglycaemic episodes, compared to conventional multiple daily injections in diabetic patients[31,32]. However, glycaemic control remains inadequate as the artificial pancreas is a hybrid device that still requires user input with insulin boluses at mealtimes, as there is no machine cephalic phase insulin release. There is also delay in insulin absorption associated with the subcutaneous route of administration. The artificial pancreas continues to be developed with research being conducted on inclusion of other hormones into the system such as glucagon within a coupled glucagon infusion pump, or amylin. A 1.3% reduction in time in moderate hypoglycaemia has been observed with the addition of glucagon[33,34].

LIMITATIONS OF CURRENT DIABETES TREATMENT

Despite the benefits offered by diabetes devices, they remain inconvenient to use, imprecise and expensive for some patients[28]. Only 63% uptake of insulin pumps and 30% use of CGM were recorded in a United States registry[28]. Barriers to adopting diabetes devices include 45% patients finding the technology troublesome to wear, 35% disliking devices on their body, 61% due to cost of supplies and 57% due to cost of the device itself[35]. Young T1D adults (18-25 years old) have the lowest adoption rates but the highest diabetes distress and HbA1c levels[35]. About 30% of young T1D patients discontinued the use of their artificial pancreas due to difficulties with technology such as error alerts, alarms (particularly at night time that may disturb sleep), amount of work required for calibrations, disappointment in glycaemic control results and high costs, as well as problems with skin irritation, contact dermatitis and sensor adhesiveness in young children[36,37]. Furthermore, the continuous subcutaneous insulin infusion pump may improve quality of life but does not significantly reduce anamnesis hypoglycaemia based on CGM records (Figure 1B)[25].

Although the current insulin therapy has advanced significantly since the development of CGM, only 1 in 5 T1D adults can attain the target HbA1c < 7.0% needed to prevent vascular complications[28]. Not only that, healthy non-diabetic individuals are able to maintain 5.4-5.5 mmol/L (98-99 mg/dL) BGL with little variability, as measured with CGM[38], which is much tighter than the diabetic target range of 3.9-10.0 mmol/L (70-180 mg/dL) set out by consensus guidelines[39]. Tight glycaemic control can reduce risks of microvascular complications, as demonstrated in a prospective randomised control trial comparing intensive and standard insulin therapy in T1D[40]. Although the artificial pancreas has been shown to improve glycaemic control, it is not able to achieve the physiological glycaemic control seen in non-diabetic individuals[41-43]. A significant amount of T1D patients do not even meet guideline glycaemic targets, especially young adults[44]. It is imperative to lower BGL and HbA1c towards normal as it is positively associated with reduced risks of microvascular and macrovascular complications, as well as reduced mortality rates[45,46]. In summary, exogenous insulin therapy cannot replicate the normal physiology of insulin secretion, as subcutaneous injected insulin has about a 1-hour delay to reach peak levels in the bloodstream and does not follow the physiological portal vein route. Additionally, islets continuously sense and respond to BGLs, as opposed to intermittent glucose monitoring of capillary blood self-performed every few hours by T1D patients on exogenous insulin therapy.

In contrast, the revolutionised β-cell replacement therapy can provide a curative solution for T1D by improving glycaemic control, quality of life and life expectancy, preventing hypoglycaemia, and even curing DM. The β-cell replacement therapy has been progressively developed over the last several decades, firstly through whole pancreas, and later through donor islet transplantation (Figure 2)[47,48]. Mechanistically, β-cell replacement can restore islet function and achieve greater physiological glycaemic control compared to exogenous insulin[48].

Figure 2 Graphic view of the journey towards the cure of type 1 diabetes. The scientific and clinical sectors have begun the endeavour to develop a curative therapy for type 1 diabetes after the discovery of insulin in 1921. The first human pancreas transplantation was successfully performed in 1960, the first alloislet transplantation was carried out in 1980 and the Edmonton protocol was first reported in 2000. The first pig xenoislet clinical trial was performed in 1994 and first human pluripotent stem cell-derived insulin-secreting cells underwent clinical trials in 2015. For more details, please read the relevant texts.

β-cell REPLACEMENT THERAPY

β-cell replacement therapies aim to re-establish physiological glycaemic control that is independent and relieves the burden of exogenous insulin therapy on patients. There are two main methods for allogeneic β-cell replacement, either through whole pancreas or islet transplantation.

Pancreas transplantation

Whole pancreas transplantation to treat DM was first performed in 1966 by Kelly and Lillehei (Figure 2). With improvements to surgical techniques and advancements in immunosuppression over the next few decades, pancreas transplantation developed into the current gold standard therapy for complicated insulin-requiring DM. The procedure has a graft survival of 90% at one year, 70% at five years and 45% at 10 years post-transplant[49]. Following simultaneous pancreas-kidney transplant, there is improvement in glycaemic control with normalisation of glucose concentration and HbA1c, diabetes-related quality of life, and reduction of macrovascular disease and peripheral neuropathies[50]. However, pancreas transplantation is associated with the inherent risks of major surgery and serious complications, most commonly acute rejection and cytomegalovirus infection[49].

Islet transplantation

In contrast, islet transplantation (Figure 3) is a minimally invasive procedure whereby islets cells are injected intraportally under radiological guidance, avoiding the risks of major surgery and offering the convenience of being a day procedure for most patients. In addition to this, islets have the advantage for the potential to be modulated prior to transplant to reduce risks of rejection and even forgo the need for immunosuppression.

Figure 3 Graphic presentation of clinical islet transplantation. A-C: Donated islets are purified from donor pancreas (A) and transplanted via the portal vein of the recipient patient (B). A total of 2170 patients received clinical islet transplantation worldwide over 20 years. The donated islets reside in the liver to perform the regulatory roles of glucose homeostasis. Nevertheless, due to the profound shortage of donated pancreases, over 99% of the 5 million patients with labile diabetes were unfortunately not able to receive the life-saving clinical islet transplantation therapy (C).

Although initially islet transplantation was not as effective as whole pancreas transplantation, the efficacy of the procedure has continuously been improved upon over the last four decades. Islet transplantation as a β-cell replacement therapy has been shown to attain insulin independence. Najarian et al[51] were able to demonstrate this through islet autotransplantation via portal vein infusion in 10 patients who underwent pancreatectomy for chronic pancreatitis in 1980. Shapiro et al[52] showed long-term glycaemic stability following cadaveric islet transplantation into the liver of T1D patients with severe hypoglycaemia unawareness in 2000, known as the Edmonton protocol (Figure 3A and B).

The Edmonton protocol uses a large islet mass of > 13000 islet equivalents (IEQ)/kg body weight, usually requiring more than one islet transplant from multiple donors[52]. The Edmonton protocol uses a corticosteroid-free immunosuppression regime consisting of an anti-CD25/anti-interleukin-2 antibody daclizumab to inhibit T-cell activation, with calcineurin inhibitor tacrolimus and mechanistic target of rapamycin inhibitor sirolimus. The development of the Edmonton protocol was a turning point for the islet transplantation procedure, and is the most widely used β-cell replacement therapy approach so far[53]. Since then, islet transplantation has been able to achieve physiological glucose control similar to non-diabetic individuals, with persistent graft function as demonstrated by 43% recipients remaining insulin independent at the 3 years follow-up, reduction in average HbA1c from 8.3% to 6.7% with mean glucose levels of 6.2 mmol/L (standard deviation 1.8)[54]. The long-term survival and function of islet grafts was 56%, 49% patients maintained HbA1c < 7.0%, > 90% were free from severe hypoglycaemia, and over half maintained insulin independence at 8 years in the CIT-08 study, a prospective trial that included 48 islet-alone transplant recipients[55]. An Italian 20-year retrospective single-centre study of 78 islet transplant patients found median graft survival of 3.9 years and 44% maintained insulin independence for median of 6 years, which increased to 9.7 years and 73% respectively in patients who received > 10000 IEQ/kg islets with basiliximab, tacrolimus and rapamycin immunosuppression, with estimated graft survival rates of 40% at 20 years[56]. In 2023, the United States Food and Drug Administration approved donislecel (trade name Lantidra) by CellTrans Inc., an intraportal allogeneic islet cell therapy from deceased donor pancreases[57].

Islet transplantation is superior to exogenous insulin therapies in maintaining tighter, physiological glucose control and thereby preventing the long-term vascular complications of DM[58,59]. Importantly, islet transplantation prevents life-threatening hypoglycaemic events associated with the insulin therapies such as severe recurrent hypoglycaemia, hypoglycaemic unawareness and glycaemic lability[52,60]. A randomised control trial showed metabolic control was superior in islet transplantation compared with intensive insulin therapy over 6 months, and one patient in the intensive insulin therapy group died due to hypoglycaemia[61]. Islet transplantation can also prevent progression of complications such as diabetic retinopathy, neuropathy and microangiopathy, and subsequently enhance quality of life[62,63]. Within 10 years of transplant, the cost of the transplant would be more economical than exogenous insulin therapy, as well as showing immediate clinical effect[64].

Current islet transplant procedure

The current gold standard site for human islet transplantation is through the portal vein route, as it can be accessed percutaneously without need for open surgery or general anaesthesia. Another advantage is that this site allows the islets to mimic physiological insulin secretion by the pancreas to the liver, which is a primary target organ of insulin, rather than a systemic insulin supply[65]. This site remains the gold standard decades after it was first demonstrated in 1972 to ameliorate hyperglycaemia, polyuria and glycosuria when pancreatic islets were injected into the portal vein of the liver of diabetic rats[66], showing superior results compared to transplantation into the subcutaneous tissue or peritoneum[67]. The technique for the intraportal route of islet transplantation has been improved upon over the years, with the use of soluble haemostatic pastes to occlude and obliterate the catheter track to prevent intraperitoneal bleeding during the withdrawal of the catheter[68]. Additionally, therapeutic heparin is administered into the portal vein at 3-5 units/kg/hour, in conjunction with limiting packed cell volume to < 5 mL, to prevent portal vein thrombosis[69]. There is also a restriction of 15 mL of > 20000 IEQ/mL islet tissue volume to be infused, thus purification of islets is important for increasing the success of the graft, however, purification itself reduces islet yield with 18% of the islets lost in the process[70].

Nevertheless, pancreas transplantation is still apparently superior to islet transplantation, though conclusive data remains unavailable. In a large single clinical centre analysis, pancreas transplant resulted in superior results for insulin independence, graft survival and blood glucose control. However, pancreas transplantation is associated with higher rates of mortality due to procedural complexity and immunosuppression risks, procedural complications and post-surgery readmissions[71]. Although patients can become free of exogenous insulin post pancreas transplantation, their lifelong immunosuppression medications are associated with risks such as infections, malignancy, osteoporosis, hypertension, dyslipidaemia, insulin resistance and affects reproduction[50]. Thus, pancreas transplant is usually reserved for patients who already require immunosuppression for another reason such as end-stage diabetic nephropathy, and it is performed simultaneously or sequentially to the renal transplant. Pancreas transplantation alone would be indicated in diabetics who suffer from frequent and severe metabolic complications of hypo-, hyper-glycaemia or ketoacidosis, consistent failure of insulin-based therapy, or in patients who have had a total pancreatectomy[50,72].

Unfortunately, β cell replacement therapy has yet to become a standard therapy due to severe lack of supply of donor insulin producing islets for transplant and the risks associated with immunosuppression. Due to this, there is stringent criteria on eligibility for islet allotransplantation, and only 20%-30% T1D patients suffering from severe hypoglycaemic events who have already exhausted all other treatments were recommended islet allotransplantation[73]. Currently the indication for islet transplantation are brittle type 1 diabetic patients at risk of severe recurrent hypoglycaemic coma despite adherence to an optimal insulin regime, or patients with refractory chronic pancreatitis undergoing pancreatectomy - although they usually receive autologous islet transplantation rather than allogenic islets[74,75]. However, there is a higher incidence of hypoglycaemia related mortality in patients who were referred but did not receive islet transplantation (Figure 4), even taking into consideration of the risks associated with the procedure and immunosuppression[76].

Figure 4 Preclinical proof-of-concept of human pluripotent stem cell-derived insulin secreting cells for blood glucose normalisation. Using transplantation of human foetal pancreas tissues in mice as a positive control[95], human pluripotent stem cell-derived pancreatic endoderm precursors required 110-140 days for blood glucose normalisation in diabetic mice[96], whereas human pluripotent stem cell-derived insulin secreting cells (known as SC-β cells or islets) required approximately 92-123 days for blood glucose normalisation[97]. hPSC: Human pluripotent stem cell; PEP: Pancreatic endoderm precursor.

LIMITATIONS TO ISLET TRANSPLANTATION

Current limitations of islet transplantation include, in addition to donor scarcity, the need to take immunosuppression drugs to prevent alloimmune and xenoimmune rejection, as well as immune attack by the autoantibodies of the recipient’s immune system on graft secreted insulin and other graft antigens. Immunosuppression drugs have various side effects such as mouth ulcers, hypercholesterolaemia, nephrotoxicity, risks of infections and malignancies including lymphomas. Not only that, immunosuppressive drugs such as tacrolimus and cyclosporine are diabetogenic, directly toxic to β cells and inhibit β cell regeneration[77,78].

As islets are thrombogenic, following intraportal transplantation, the islets embolise to the portal sinusoidal capillaries, leading to islet cell death. Furthermore, the liver environment differs from the endogenous islet microenvironment in terms of oxygen content and blood supply, thus leading to further islet graft loss due to hypoxia and ischemia[79-82]. About 50%-70% donor islets are lost within hours of transplantation due to the recipient’s immune response[74,83-86]. Metabolic stress from chronic hyperglycaemia, hypoxia, ischaemia and immunosuppression further increases islet graft failure. Islets are vulnerable to damage by fatty acids[87], amyloid deposition[88,89], reactive oxygen species and oxidative stress, pro-inflammatory cytokines such as tumor necrosis factor alpha mediated apoptosis, free radicals H₂O₂ and peroxynitrite, and superoxide dismutases[90].

As a result of islet losses described above, large numbers of donor islet cells are required for transplantation. This leads to the other major limitation, which is severe lack of supply of donor islets for transplantation. In addition to these issues, the exorbitant cost of islet transplantation is another barrier to becoming a standard therapy. For example, the first United States Food and Drug Administration approved donor islet replacement therapy donislecel (Lantidra) was estimated to cost approximately United States $300000 per treatment per patient[91]. This is not including the cost of lifelong immunosuppression medications or treatment of complications associated with this treatment. Furthermore, the loss of islet grafts post-transplantation due to immune responses or deficient nutrition and oxygen supply to the grafts at the site of engraftment contribute to graft failure and the need for further transplants.

In summary, islet transplantation has the following three main barriers to becoming a widespread, mainstream therapy for T1D, which are: (1) A severe shortage in supply of functional insulin-producing donor islets. Less than 0.5% T1D patients could be treated with an islet allotransplantation due to the limited supply of cadaveric organs, especially when more than one donor pancreas is required per recipient, as a large number of islets are needed for successful transplantation (Figure 3C)[74]; (2) Side-effects of lifelong immunosuppression required to protect transplanted cells from immune rejection and autoantibodies that damage the function of grafted donor β cells. This topic is not the major theme of this review; and (3) Optimal transplantation site with sufficient nutritional supply to the graft whilst supporting timely insulin secretion in response to fluctuating BGL, that is additionally accessible for retrieval and biopsy if required in cases of suspected graft failure and avoids islet losses associated with the intraportal route. Alternative sources of donor islets may ensure an adequate supply, such as human pluripotent stem cell (hPSC)-derived allogeneic β-cells or xenogeneic sources such as porcine islets.

ALLOGENEIC HPSC-DERIVED ISLET CELLS

An attractive source for donor islets cells are β cells derived from hPSCs including human embryonic stem cells and human induced pluripotent stem cells, which theoretically have unlimited potential for growth and differentiation to all somatic cell lineages[92]. Pre-clinical transplantation of wide developmental stages of human foetal pancreas tissues demonstrated that experimental diabetic hyperglycaemia can be normalised as early as 28 days post-engraftment with 49 mg pancreatic tissue in mice (Figure 4)[93]. Transplantation of 3-7 million hPSC-derived pancreatic endodermal progenitors per mouse by ViaCyte normalised experimental hyperglycaemia around four months post transplantation[94]. However, post-transplantation of approximately 5 million hPSC-derived (SC)-β cells per mouse only marginally accelerated glucose normalisation to around 90-120 days[95]. However, the transplantation of human foetal pancreas tissue can only represent a partial positive control as we are unable to consider the exact differences amongst inter-laboratory variabilities, mouse strain differences, exact number of grafted β cells per mouse in each laboratory and the developmental stages of the grafted β cells from each laboratory. Nevertheless, these data support the direct comparison by single-cell RNA-seq analysis of in vivo foetal human islet cells with the hPSC derived islets (SC-islets or SC-β cells), clearly indicating that the SC-β cells are similar to foetal human islet cells[96].

SC-islets may bypass the need for islet organ donors[97]. Since 2014, the first SC-islets were demonstrated to have GSIS and normalise diabetes in mice[95,98,99]. Restoration of insulin secretion and improvement of diabetes was shown following transplant of human induced pluripotent stem cell-islets in non-human primate (NHP) macaques[100]. Stem cell-derived β cells produced by several in vitro protocols, although expressing variable maturation markers, are functionally immature both in vitro and in vivo[92]. The immaturity of differentiated cells, cell types, molecular signatures and safety concerns of SC-islets have been reviewed recently in detail[92]. Further maturation of SC-islets involves the complex interplay of various transcription factors, molecular mechanisms and metabolic conditions, with several ex vivo protocols to differentiate hPSCs into islet-like cells with variable expressions of maturation markers[92], and further research is required to develop mature, safe, genuine insulin-secreting β-like cells. In the last decade, human clinical trials of SC-islet transplantation in T1D patients have been underway.

ViaCyte Inc. was the first to generate the human embryonic stem cell-derived insulin-secreting pancreatic endoderm cells (PEC-01) macroencapsulated in a device called Encaptra^TM and has conducted several rounds of clinical trials but has never achieved insulin-independence for participating patients (Figure 5)[101].

Figure 5 World challenge in manufacturing mature, fine-tuned cells for transplantation. A: Clinical trials in xenotransplantation of immature pig islet cells to recipient patients have never achieved insulin independence; B: Clinical trials in allotransplantation of human pluripotent stem cell-derived pancreatic precursor cells to recipient patients never achieved insulin independence; C: Clinical trials in allotransplantation of human pluripotent stem cell-derived insulin-secreting cells (known as SC-islets) are currently ongoing. PSC: Pluripotent stem cell; hPSC: Human pluripotent stem cell; T1D: Type 1 diabetes.

Vertex had transplanted their own SC-islets with systemic immunosuppression called VX-880 in T1D patients with impaired hypoglycaemia awareness or severe hypoglycaemic events in phase I/II clinical trials[102]. The trial was briefly put on hold after two participants died, reportedly unrelated to VX-880[102]. The first patient was insulin independent at day 270 post-transplantation, whereas it was unclear when the second patient who died became insulin-independent[103]. All 12 VX-880 participants who received the full dose had GSIS and elimination of severe hypoglycaemic events by day 90, as well as improved glycaemic control with HbA1c < 7% and time-in-range > 70% on CGM[102]. Three patients achieved the secondary endpoint of insulin independence at the one-year follow-up, and the trial expanded from 17 to 37 participants in 2024 due to the positive results[102]. Despite these positive results, a HbA1c up to 7% does not signify diabetes-free, as 5.7%-6.4% is prediabetes levels and 6.5% or higher is diabetic, as well as the time outside the target glucose range suggests there are ongoing risks of hypoglycaemic and hyperglycaemic complications. Concurrently VX-264 was developed and is in phase I/II trials, which is the encapsulated version of VX-880 without the need for immunosuppression (Figure 5)[104]. Vertex recently announced that it is collaborating with TreeFrog Therapeutics to license its C-Stem^TM technology to scale up and optimise stem cell production[105].

XENOGENEIC ISLET TRANSPLANTATION

Xenotransplantation is the transplant of organs, tissues or cells from one species to another species and is a potential solution to the severe shortage of donor organs for transplant in patients with end-organ failure. Designated pathogen-free healthy, young pigs are a favourable source for clinical transplantation in humans due to their similar anatomy and insulin DNA sequence with only one amino acid difference, and suitability for genetic modification to address genetic differences[106]. Organ supply would not be an issue as pigs are easily bred, and already bred for human consumption, thus posing less of an ethical dilemma compared to primates[107]. A xenotransplantation preclinical study at Seoul National University in South Korea has been able to maintain insulin independence for over 900 days and ongoing, following porcine islet transplantation in five immunosuppressed diabetic rhesus monkeys[108].

There is no consensus on whether adult pig islets or neonatal islet cell clusters (NICC) are superior for transplant[109]. Adult pigs provide a larger islet yield with higher β cell percentage and can provide up to 720000 IEQ, an amount that can be used to transplant into a 60 kg human recipient through isolation with good manufacturing practice grade collagenase AF-1 with thermolysin or neutral protease to increase islet yield, function and viability[110]. Adult pig islets have also shown immediate insulin response and longer survival time, with weaker or no expression of the xenoantigen a-galactose-1,3-galactose that triggers antibody-mediated hyperacute rejection due to pre-formed antibodies, compared to neonatal porcine islets[111,112]. NICC has higher islet yield per gram of pancreas tissue, lower cost per islet (USD $0.02 compared to $0.09 for adult islets) due to shorter duration of pig maintenance, lower isolation cost due to ease of isolation in neonatal pigs, and greater resistance to hypoxia, scalability and proliferative ability[111,113,114]. However, an additional challenge with NICC is that it will likely take longer to ameliorate diabetes than more mature islets, as shown by more mature foetal human pancreas sources leading to shorter time for diabetes reversal in streptozotocin-treated mice[93].

Immune challenges and potential solutions

A key barrier for xenotransplantation is the challenges of the immune barrier and organ rejection, of which there are three main types of rejections - hyperacute xenograft rejection, delayed xenograft rejection (either acute humoral xenograft rejection, cellular rejection or coagulation dysregulation) and chronic rejection[115,116].

However, donor pig-human recipient compatibility can now be increased following genetic editing of pigs by using targeted genome editing techniques such as CRISPR-Cas9[117]. For example, genetic editing can be used to remove the carbohydrate pig antigen a-galactose-1,3-galactose, as well as two other carbohydrate xenoantigens N-glycolylneuraminic acid and Sda, to produce triple-knockout pigs and prevent antibody-mediated rejection in human recipients[118,119]. Inclusion of human transgenes such as human complement-regulatory genes CD46, CD55, CD59 and thrombomodulin can reduce coagulation and inflammation in xenotransplantation[114]. Tissue factor pathway inhibitor (TFPI), CD39, HLA-E and programmed death-ligand 1 could increase physiological compatibility between the porcine xenograft and human recipient[120,121]. Genetic engineering of grafts to express human complement and coagulation regulatory proteins are important for protecting against coagulation dysfunction and graft survival[121,122]. There are also methods to inactivate porcine endogenous retrovirus (PERV), eliminate several xenoantigens and express human antigens to improve porcine-human immunological and coagulation compatibility through a combination of CRISPR-Cas9 and transposon technology, which is a promising advancement towards viable and safe porcine xenotransplantation in humans[123,124].

Xenotransplantation of genetic modified porcine islets was shown to function for up to 1 year in streptozotocin induced diabetic NHP[125]. The expression of the human complement-regulatory protein (hCD46) limits antibody-mediated rejection and reduced the amount of immunosuppression required to prevent loss of islet cell mass[125]. This allowed sustained normoglycaemia, although it did not reduce loss from Instant Blood Mediated Immune Reaction (IBMIR)[125]. Furthermore, the development of multi-transgenic pigs with 4 modified genes - alpha1,3-galactosyltransferase gene-knockout (GTKO), hCD46, human tissue factor pathway inhibitor (hTFPI) and CTL4-Ig - were able to achieve preserved islet mass and long-term islet engraftment up to 1 year post-transplant[126]. The addition of hTFPI helps provide anti-thrombosis and anti-inflammatory effects, whilst CTL4-Ig helps inhibit cellular immune response[126]. Thus, multi-gene donor pig islets have been shown to be an effective source of islets for NHP transplants[127].

Xenoislet preclinical and clinical studies

A preclinical study on xenotransplantation of genetically modified GTKO/CD55-CD59-HT porcine NICC with costimulation blockade immunosuppression into 5 baboons showed promising results for graft survival against immune rejection[128]. Baboons share similar temperature and immune system to humans in contrast to macaques that were used in most other preclinical studies. The baboon recipients were able to achieve and maintain insulin independence for a mean of 87 ± 43, and 397 ± 174 days post-transplant respectively, and was able to avoid pig antibody response and IBMIR[128]. The immunosuppression was largely well tolerated with most recipients gaining weight, except one baboon who lost weight after developing lymphoma likely due to over-immunosuppression and genetic predisposition from the closed colony source; another recipient developed ulcerative skin lesions of undetermined cause[128].

In pioneering xenotransplantation clinical trials, NICCs were intraportally injected, or placed under the kidney capsule of T1D patients who also received a combination of immunosuppression drugs[129]. Interestingly, pig C-peptide was detectable in urine for 200-400 days, though the amount of exogenous insulin required was not reduced for glycaemic control[129]. In New Zealand, biopsies of the alginate-encapsulated porcine islets 9.5 years after they were transplanted into the peritoneum of a T1D man showed a small amount of GSIS remaining[130]. The registered clinical trials in New Zealand began in 2009, and fourteen participants who suffered from severe unaware hypoglycaemia underwent xenotransplantation with alginate-microencapsulated neonatal porcine islets (DIABECELL^®) and followed for 52 weeks[131]. The results demonstrated a reduction of unaware hypoglycaemia events, positive porcine C-peptide levels and four participants achieved HbA1c < 7%, however, it lacked clinical benefit due to ongoing exogenous insulin requirements with only a small decrease in doses[131]. Further clinical trials on another 8 patients at two doses over two transplants, with the higher dose (total 20000 IEQ/kg) received by 4 of the patients showed improved HbA1c < 7% for over 600 days and reduced unaware hypoglycaemia, however the HbA1c did not normalise[132].

In Mexico, a 4-year study wherein neonatal porcine islets and Sertoli cells within a novel subcutaneous stainless steel tube encapsulation device were transplanted into twelve patients showed positive insulin staining cells at the 3 years follow-up, and 2 participants achieving insulin independence for several months[133]. At the 4 years follow-up, six participants maintained reduced insulin requirements and three participants showed porcine insulin secretion in response to glucose challenge[133]. The long-term longitudinal study of 23 participants demonstrated improved HbA1c, detection of porcine C peptide and lower incidence of chronic diabetic complications compared to those of similar age and duration of the disease[134].

Further islet xenotransplantation clinical trials are required to improve clinical outcomes, however supply of clinical-grade pigs is reported to be limited with currently only two clinical-grade facilities worldwide in China and New Zealand[135]. The Medical Porcine Development Organization in Japan was recently created to develop technologies for increased production of clinical-grade pigs[135].

Combined islet-kidney xenotransplantation for patients with renal insufficiency due to diabetic nephropathy is a future possibility with continued advancements in porcine islet xenotransplantation and by addressing each of the barriers. A preclinical study of sequential porcine kidney-then-islet transplantation into five diabetic baboons has achieved insulin independence and normal creatinine for 180 days[136]. This sequential kidney-then-islet xenotransplantation may have the additional benefit of reducing IBMIR through anti-pig antibody absorption by the kidney graft first. Adjunctive immunomodulation by vascularised thymic lobe co-transplantation could reduce the number of islets required to reverse hyperglycaemia - 12500 IEQ/kg compared to allotransplantation of the 16117 IEQ/kg purified human pancreatic islets after kidney transplant required in a phase 3 clinical trial on 24 patients[136,137].

There has not been any detection of PERV or anti-PERV antibodies thus far in preclinical and clinical studies, which could be due to low PERV expression in pig pancreas and thus low probability of viral release[138,139]. The use of good manufacturing practice and designated pathogen-free facilities also minimise pathogen transmission[140]. It would be prudent to screen for and eradicate viruses such as picornavirus and rotaviruses from donor pigs, as these viral infections can impact on islet function and insulin secretion[141].

IMMUNOPROTECTION BY ENCAPSULATION

Encapsulation devices could help overcome the challenge of high immunogenicity of xenografts, in addition to traditional immunosuppression or genetic engineering to evade the recipient’s immune system. A preclinical study of macroencapsulated porcine islet xenografts in a device called betaO₂ bioartificial pancreas transplanted into three diabetic NHP without immunosuppression showed safety with no transmission of PERV or porcine cytomegalovirus, maintained graft function and avoidance of hypoglycaemic events up to 6 months before the device was retrieved[142]. However, it did not show insulin independence, likely related to the NHP model and diffusion limitations of the encapsulation device[142].

On a smaller scale, microencapsulation is another potential option for immune protection/immunoisolation. Neonatal porcine islet with alginate microencapsulation was transplanted intraperitoneally without immunosuppression in 8 T1D patients leading to improved HbA1c and unaware hypoglycaemia, however, it did not reverse diabetes[132]. This strategy may be improved with an alternate site of transplant such as the omentum, which has greater vascularisation, instead of the intraperitoneal site used[132].

Due to the severe shortage of high-quality human donor islets, there is less data on encapsulation of allogeneic islets and its survival and function compared to xenogeneic islets. In a 10-month study with one patient, allogeneic islets were macroencapsulated in an oxygenated chamber and transplanted into the subperitoneal space of a 63-year-old male patient[143]. There was maintained graft survival and function at 10 months with no graft rejection, improvement in HbA1c and modest reduction in insulin requirements from 52 ± 5.8 IU/day to 43 ± 4.9 IU/day post-transplant[143]. There was low C peptide levels, which may be due to smaller islet mass of 2100 IEQ/kg transplanted, the transplant site and diffusion limitations[143].

Various encapsulation methods such as altering biomaterial chemistry, biologics, cell co-transplantation, as well as promoting neovascularisation have been studied[144]. A challenge to encapsulation is that it lacks the vascular structures, innervation and beneficial extracellular matrix molecules of native islets, thus it is important to consider and mimic these functions[145]. Another issue is that the recipient’s immune system can detect the cytokine and damage associated membrane products released due to injured encapsulated donor islets. The damage associated membrane products attract the recipient’s innate immune macrophages, neutrophils, dendritic cells that lead to proinflammatory cytokines production and damage to the encapsulated islets and trigger the recipient’s foreign body response[146]. Currently several strategies have been studied to address local inflammation and fibrosis. These include immunomodulating biomaterials on the islet surface or microcapsules, or coencapsulation with immunomodulating cells such as regulatory T cells, mesenchymal stem cell, human amniotic epithelial cells, dendritic or Sertoli cells to improve long-term survival of grafts[53,146].

Another encapsulation method is nanoencapsulation. Nanoencapsulation is the uniform coating of polymer films such as polyethylene glycol based biomaterials onto the surface of islets or using a layer-by-layer technique using alternating positive and negative electrons to form a nanomembrane of polyamino acids, polysaccharides or synthetic polymers[147]. Nanoencapsulation has the advantage of reduced distance for passive diffusion, increasing the efficiency of nutrient and hormone exchange[148,149]. Another advantage of using nanoencapsulation is the reduced transplant volume for intraportal injection compared to microencapsulation, thus reducing the risks of thrombosis[148,149]. Furthermore, nanoencapsulation may be useful to prevent IBMIR and other functions[147]. The disadvantage of nanoencapsulation is that it does not fully shield the islet, is vulnerable to mechanical and biochemical stressors and the coating may degrade over time[150]. Both nano- and micro-encapsulation devices are also difficult to retrieve[151].

ALTERNATIVE SITES FOR ISLET TRANSPLANTATION

Alternative sites to the liver portal vein route have been explored to combat the issue of IBMIR and reduce the amount of donor pancreatic islet mass needed for successful transplant, accessibility for biopsy, graft retrieval and imaging. Such sites include the peritoneum, the pancreas, omental pouch, liver, spleen, skeletal muscle, renal subcapsule, subcutaneous tissue, gastric submucosa, bone marrow, thymus, testis and anterior eye chamber - with varying results[152-157].

Intramuscular grafts failed to function and developed fibrosis and necrosis at the transplantation site[153,154]. Islet transplant into the renal subcapsule space in streptozotocin-induced diabetic mice demonstrated less islets and less days needed to achieve normoglycaemia, although this is yet to be validated in non-murine models[158]. And although the renal subcapsule represents a retrievable site for transplant, it has limited capacity and thus would not be a suitable site for encapsulated islets in humans[159]. Subcutaneous transplants are simple surgical procedures with small surgical risks, free from IBMIR and would allow for monitoring and graft retrieval; however, there is inadequate nutrient and oxygen supply due to lack of blood supply, which can be further hampered by fibrosis[47]. Although the thymus, testis and anterior eye chamber sites are immunoprivileged sites, their size limits the transplantation of adequate volume of islets needed to achieve normoglycaemia, and implanting into the eye has the risk of impairing vision[160].

The retroperitoneal retrocolic space is an attractive alternative site that could accommodate a larger volume of donor islets. Encapsulated islets from rat, neonatal porcine or human transplanted into the retrocolic space of diabetic mice showed longer duration of normoglycaemia up to 275 days (rat islets) and better preserved structure on retrieval of the grafts compared to the kidney capsule or peritoneum[161]. Another advantage of this site is the access to the portal system, however, further investigation in large animal studies is required[161].

The omental pouch may represent another fair alternative, with large surface area and vascularisation supportive of oxygenation and metabolic exchange. This site is also favourable in terms of being accessible as a laparoscopic procedure, able to support large graft volumes including unpurified or encapsulated islets, and delivery of secreted insulin to the portal vein[162]. There is an ongoing phase I/II clinical trial investigating transplantation of allogeneic islet cells at this site achieving and maintaining insulin independence at 17 days and 12 months post-transplant respectively in a 43 years old woman[163]. Further research with a large cohort is required to determine if the omentum can be the preferred alternative site for human islet transplantation. Despite the advantages of alternative sites, issues with oxygen/nutrient supply, increased number of islets needed, and more invasive procedures means intraportal infusion to the liver is currently still the preferred site for islet allotransplantation and xenotransplantation.

KNOWLEDGE GAPS

As described in the previous sections, SC-islets and pig NICC in clinical trials are all functionally immature, reflected by a protracted time for graft maturation post-transplantation in recipients’ bodies, with limited and disappointing outcomes. This is because stem cell maturation biology has been vastly understudied and is one of the latest frontiers in regenerative medicine. Along the maturation biological pathway, a number of transcription factors have been discovered over the last two decades, some of which are involved in β cell-maturation in vivo that were reviewed recently[92]. Whether, and how, to utilise those transcription factor signaling pathways to mature immature human SC-β cells and pig NICC is a significant knowledge gap. For instance, the thyroid hormone receptors were discovered to be expressed on human mature islets[164]. The administration of thyroid hormone triiodothyronine (T3) or overexpression of the transcription factor MAF bZIP transcription factor A (MafA) was shown to increase human foetal islet-like clusters, insulin secretion at 16.8 mmol/L glucose and proinsulin-to-insulin processing[165]. Chromatin immunoprecipitation study visualised the binding of thyroid receptors to MafA promoter, thereby confirming that T3 directly regulates the expression of MafA[92].

Secondly, as there is high prevalence of acquired cancer-related mutations in hPSC lines and their differentiated derivatives[166], it would be critically important to understand whether the mutated hPSC lines and their differentiated derivatives have any unique biological, biochemical and/or cell surface markers. These markers can subsequently be used to remove the mutated cells from the transplant products. Thirdly, as SC-islets may contain various percentages of hPSC-derived enterochromaffin cells[167], discovery of methods to enrich hPSC-derived genuine islet progenitors for further differentiation would be critically important too.

Although not the focus of this review, transplantation immunology has made remarkable progress in the last few decades on the molecular and cellular mechanisms of chronic immune rejection. How to minimise and eventually eliminate immune rejection of allogeneic and xenogeneic grafts is an area that is relatively understudied. This understudied area needs to be significantly strengthened as the prevalence of end-stage organ failure including T1D have been growing exponentially in recent decades. Further understanding of these mechanisms could significantly improve the outcomes of long-term graft survival and quality of life in transplantation recipient patients.

The survival and functionality of transplanted donor islets in different sites require better understanding. Further long-term studies directly comparing graft survival and function in promising alternative sites of transplantation, which could include the comparison of various encapsulation methods to prevent immune rejection, would be invaluable to identify an optimal transplantation site. For example, direct comparative studies designed to assess the long-term survival and function of grafts transplanted via the current gold standard hepatic portal vein route compared with the promising alternative retroperitoneal retrocolic space and/or omental pouch sites in animal models before it proceeding to clinical trials.

Finally, it is well known that genetic engineering on GGTA1[168], NeuGc[169] and SDa[170] gene-knockout pigs may minimise up to 95% antibody-initiated xenogeneic rejection against pig cells[171,172]. However, the molecular identities of the remaining 5% xenogeneic antigens and their role in immune rejection remains largely understudied. On the other hand, the basal insulin secretion of isolated GTKO pig islets was higher than in wild type pig islets[173]. Whether this and other gene knockout and the addition of other human transgenes in as many as 69 genomic-edited pigs[174] would affect the long-term survival and function of pig islets in humans also requires further research.

FUTURE CHALLENGES

The safety of hPSC-derived islet-like cells is of clear concern; a recent study discovered a high prevalence of acquired cancer-related mutations in 146 hPSC lines and their differentiated derivatives after analysing over 2200 transcriptomics[166]. In other words, the mutated differentiated SC-β cells, off-target differentiation and a low frequency of undifferentiated PSC could form teratoma in patients after transplantation. Indeed, a recent report described the development of large numbers of immature teratomas in patients after transplantation of iPSC-derived β cells[175]. Furthermore, 2 patients died in early 2024 after being transplanted with Vertex’s VX880 SC-β cells, although their deaths were reported to be unrelated to the transplanted cells. VX880 SC-β cells had been selected with the cell surface marker CD49a (also known as ITGA1) from cultured products[176]. However, it is unknown whether the hPSC line or lines that were used to give rise to VX880 SC-β cells acquired any cancer-related mutations, similar to that of any 2200 transcriptomic datasets[166].

Selection and determination of genuine islet progenitor-differentiated islet-like cells would be critically important for developing a durable stem cell therapy for T1D. SC-islets, according to single nucleus multi-omics analysis, are not only clearly different from donated human islets, but also only gradedly and not distinctively, different from hPSC-derived enterochromaffin cells[167], confirming that the current differentiation protocols for production of SC-islets have clearly generated a various percentage of off-target differentiated cells including enterochromaffin cells. This is because the embryonic pancreas and small intestine are derived from the neighbouring posterior foregut endodermal domain. Indeed, developing foetal human small intestine contains insulin-expressing enteroendocrine cells[177].

However, it is currently unknown whether the lifespan of enteroendocrine cells is similar to that of gut epithelial cells, which are approximately only 3-5 days. The high proportion of SC-derived enterochromaffin cells in VX880 SC-β cell products may partially explain the delay in achieving insulin independence for many months[102], in addition to allogeneic immune rejection, islet autoantibodies against and cytotoxicity of immunosuppressants on grafted cells. Therefore, after removing mutated hPSC and their derivatives, selection of hPSC-derived genuine islet progenitors for further differentiation and maturation into matured islet-like cells may help significantly progress this critical area for a curative allogeneic T1D therapy.

Given hPSC- and pig-derived donor islets are currently functionally immature, the discovery of maturation methods for both types of islets is another future research direction, and the research design can be based on knowledge from islet maturation in vivo. The maturation methods and their effects on insulin secretion capabilities, and duration of time to achieve insulin-independence would need to be directly compared to identify effective methods.

The key barrier for xenotransplantation of pig islets is overcoming the immune barrier and chronic rejection. Engineered donor pigs that carry as many as 69 genome modifications have been produced[174]. Eliminating swine glycan antigens and overexpressing selected human transgenes would significantly reduce immune rejection and prolong graft survival time. Inactivating pig endogenous retroviruses would significantly improve the safety profile of donor tissues[174]. This genomic-modified pig line and its further optimal lines may further help address the problem of long-term chronic xenogeneic rejection of pig islets after transplantation in human recipients. Better mechanistic understanding of the immune rejections on xenotransplantation would assist in addressing the problems of acute and chronic xenogeneic rejections. Additionally, better encapsulation devices or methods would also improve the long-term safety and function of xenogeneic islets. When these problems are addressed, pig islet xenotransplantation would be a promising treatment option for people suffering with debilitating T1D.

CONCLUSION

In conclusion, after successfully addressing the technological limitations and challenges, bridging knowledge gaps and avoiding the need for long-term immunosuppression, the cure for T1D could then be successfully realised. The widespread adoption of allogeneic and/or xenogeneic β cell replacement therapy would save the lives of millions of diabetic patients who suffer from severe hypoglycaemic events, help restore their physiological glucose homeostasis and achieve insulin independence.