Molecular Challenges and Advances in Clinical Islet Transplantation

Abstract

The pathophysiology of diabetes is related to the levels of insulin within the body, and the body’s ability to utilize insulin. Patients with diabetes persistently go through life-threatening hypoglycaemia. Consequently, their quality of life gets affected, progressively leading them to micro- and macro-vascular complications. This is an unmanageable happening despite the technology advancements in insulin formulations. Nevertheless, islet transplantation is emerging as an alternative therapeutic option. Our chapter will elaborate on the recent advancements in this field highlighting the present-day challenges of clinical islet cell transplantation. Additionally, details about the advancements in cutting-edge clinical research, bio-molecular signaling with special reference to the pre and post transplant, the need for beta-cell replacement therapies, including the application of induced pluripotent stem cells and mesenchymal stem cells are also mentioned in this chapter.

1. Introduction

1.1 Overview of diabetes: incidence, intensity and implication of innovative therapies

Diabetes is one of the fastest growing health challenges of the twenty-first century, with the number of adults living with diabetes having more than tripled over the past 20 years [1]. Most forms of diabetes witness hyperglycemia, which has its pathogenesis focally in the islet β cell. Known diabetics (diagnosis confirmed) as well as those who are at-risk of developing the disease exhibit impairment or loss of pro-peptide processing and secretory function. For the purpose of prediction, diagnosis or prognostics, biomarkers and genetics are used to understand the state of a biological process, severity of a pathological condition, and response to an intervention. This holds true for both research and clinical settings. The current chapter will focus on the potential utility of genetic markers, circulating molecules, and immune cell phenotyping as β cell’s biomarkers of cellular function. How these biomarkers complement the assessment of β-cell secretory function is also interestingly explained. Having essayed the vitality of islet β cell, the chapter will also touch upon the possible implications of its loss. And henceforth, the β-cell secretory function could itself be considered as a biomarker. However, steadfast advances in the field have set the stage for stem cell-based approaches to take over in the near future. Generation of functional pancreatic islet cells using Human pluripotent stem cells (PSCs), including human embryonic stem cells and induced pluripotent stem cells, are promising cell sources in regenerating pancreatic islets, and this will become possible based on the substantial progress made over the past years. The forthcoming decade will probably witness research driven towards the molecular mechanisms of PSC-derived islet cells and their entailed use in experimental disease treatment. Therapeutic efficacy and safety considerations of patient PSC-derived islet cells and transplantation delivery systems should also be researched upon.

1.2 Diabetes- an understanding at the molecular level

Diabetes is a serious public health challenge, and its types, especially type 1 & 2 are caused by a combination of genetic and environmental risk factors. For instance, an increase in human islet BCL11A expression decreases islet insulin secretion. Thus BCL11A expression is elevated during T2D and chronic hyperglycemia [2]. Being polygenic, they are related to a change, or defect, in multiple genes. This includes variations in important genes which are vital for glucose metabolism (regulation of fasting & postprandial blood glucose levels), insulin function (mainly insulin resistance) & triglyceride metabolism. The following figure, Figure 1 illustrates the pathogenesis of diabetes at a molecular level.

Figure 1.

Understanding diabetes at the molecular level is important.

Diseases like diabetes are now being diagnosed, monitored and treated through an effective, individualized model of care rather than the ‘one size fits all’ approach. Owing to its efficiency in personalized care, precision medicine has gained focus worldwide, and its emerging application has diabetes in the forefront [3, 4]. For instance, the gastrointestinal side effects of the hypoglycaemic drug metformin has been linked to the interaction between the genes encoding the organic cation transporter 1 (OCT1) and the serotonin reuptake transporter (SERT). The number of low-expressing SERT S* alleles increased the odds of metformin intolerance. Likewise, the presence of two deficient OCT1 alleles was associated with over a nine-fold higher odds of metformin intolerance in patients carrying L*L* genotype [5].

Understanding relevant genes may not only help determine who is at high risk for developing the disease, but may also be useful in guiding treatment regimens. Beyond treating diabetes, we have set foot in a new era of curing the disease with pancreatic islet cell transplantation. In islet transplantation, the requirement for immunosuppressive drug treatment to protect alloislets from alloimmune rejection and recurrent autoimmunity introduces additional risks that may be specific to the individual immunosuppressive drug agents as well as related more generally to immunosuppression. Thus recognition of inter-individual differences is gaining importance and is becoming possible through integration of pharmacogenetics, pharmacoproteomics, epigenetics, and noncoding RNAs data into clinical practice, thus emphasizing that ‘Precision medicine is vital in transplantation’. A panel of genetic variants for transplant recipients and donors can function as an additional tool at disposition of transplant physicians to provide individualized care.

2. Biological mechanisms that underlie Insulin dependence

Type 1 diabetes is a multifactorial autoimmune disease, which is characterized by T cell mediated damage to the insulin-secreting pancreatic β cells. The initial stages of the disease process feature insulitis, followed by the pancreatic islets’ infiltration by mononuclear immune cells (including dendritic cells, macrophages, and T cells). This detrimental process leads to severe insulin depletion, and consequently hyperglycaemia. Underlying this hyperglycemia are reasons like hepatic overproduction of glucose by glycogenolysis and gluconeogenesis accompanied by a decrease in the cellular uptake of circulating glucose. Insulin’s absence, increases fat breakdown and the consequence of fatty acid oxidation is excessive production of ketones. These metabolic disturbances are serious enough to progressively cause central nervous system depression, coma, and death, if left untreated. Therefore, type 1 diabetes necessitates lifetime treatment with exogenous insulin as a survival-essential. Pancreatic β cell destruction rate shows inter-individual differences, yet, tends to be more aggressive in infants and young children [6]. Type 1 diabetes represents approximately 10% of all cases with diabetes. Its incidence is increasing worldwide at a rate of about 3% per year. The latest edition of the International Diabetes Federation (IDF), Diabetes Atlas shows that 1.1 million children and adolescents under the age of 20 live with type 1 diabetes [1]. Islet cells balance cellular glucose requirement with the glucose supply through blood, and this is illustrated in the following figure, Figure 2 through insulin signaling pathway.

Figure 2.

Islet cells balance cellular glucose requirement with the glucose supply through blood.

For individuals with type 1 diabetes or insulin-deficient forms of pancreatogenic (type 3c) diabetes, isolation of islets from a deceased donor pancreas with intrahepatic transplantation of allogeneic islets can result in amelioration of hypoglycemia, on-target glycemic control, improved quality of life, and insulin independence. Recent progress in techniques for islet isolation, islet culture, and peritransplant management of the islet transplant recipient has resulted in substantial improvements in metabolic and safety outcomes for patients. The metabolic benefits of islet transplantation are dependent on the count of islets transplanted that survive engraftment [7].

3. Molecular semaphore of beta cell in diabetes: integrating biomarkers with functional measures

Circadian regulation of glucose homeostasis and insulin secretion is an important feature to assess whether islets are functional. Molecular clock mechanism is highly conserved among various cell types and is driven by a set of core clock genes that form interrelated transcriptional-translational complex. Thus understanding the molecular mechanism driving the inter relationship between disruption and islet functioning is crucial in context of disease prevention and transplantation.

The molecular mechanism governing this rhythmicity is based on complex program of gene expression. A number of interlocked transcriptional and posttranslational feedback loops are responsible for generation and maintenance of rhythms. It will be interesting to initiate a study to assess the basal levels of glut receptor; Ca2+, glucose kinase and Insulin expression immediately after the purification of islets are done. This will highlight the necessity to understand the molecular and physiological underpinnings responsible for the functionality of islets before the transplant. These basal levels of expression of all the molecular parameters will give the clinician an idea on functionality of islets even before it is transplanted. However after the transplant the levels should vary depending on the success of transplant surgery, circadian regulation and graft function [8, 9, 10, 11, 12, 13].

Once the islets are transplanted to the donor, it is very important to assess the level of relevant molecular signatures in-vivo, which are involved in graft function or damage and these biomarkers decide the fate of the transplant. Optimizing engraftment and early survival after clinical islet transplantation is critical to long-term function. Cell-free circulating DNA (cfDNA), is now recognized as a potential biomarker for a variety of diseases. In humans, the insulin promoter is predominantly unmethylated in islet β-cells and methylated in all other tissues. cfDNA-based estimation of beta cell death 24 hours after islet allotransplantation correlates with clinical outcome and could predict early engraftment [14].

To understand on the beta cell function and beta cell mass, high-quality pancreatic islets are essential as they correlate with better post transplantation endocrine function. Islet quality and yield get affected by stress during the isolation process. During isolation, islet-enriched microRNAs (miRNAs) -375 are released and they serve helpful in assessing the extent of islet damage by correlating with post transplantation endocrine function. Assessment of the absolute concentration of miR-375 C-peptide at various islet isolation steps, including digestion, dilution, recombination, purification, and bagging is possible. Measurement of the absolute quantity of miRNA-375 during islet isolation is a useful tool to assess islet damage. The quantity of released miRNA is indicative of post transplantation endocrine function in transplant patients [15, 16, 17].

Better engraftment and functioning of islets needs to be proved by the production of Insulin for the alleviating problematic hypoglycemia before and after islet transplant. This can be done by constantly monitoring the insulin level by various methods. CD30 levels are a predictor marker for acute rejection before and after the islet transplant. Elevated CD30 levels may reflect an immune state detrimental for islet allograft survival. In islet allograft recipients, post-transplant reduction in sCD30 levels can serve as a biomarker to monitor graft function. Furthermore, an insight on how various immunosuppression protocols impact the timing and extent of changes in post-transplant sCD30 levels may facilitate patient-specific tailoring of immunosuppression [18, 19].

HMGB1 is a mediator of immune system during islet transplant. HMGB1 is one of the best-characterized DAMP molecules associated with islet. HMGB1 seemed to be released by damaged islets before and after transplant. HMGB1 will be one of the useful bio signatures detecting islet damage in clinical situation. It is the need of the hour for more research and development of kits in these area to bring the islet product release criteria that screen preparations before and after clinical allogeneic islet cell transplantation that are currently unavailable to predict post-transplant success from failure. More sensitive and reliable islet viability, potency kits and assays that characterize cell composition and molecular profiles will be useful in further defining the islet product and may provide useful information on islet immunogenicity and pro-inflammatory potential to evaluate islet functionality in the clinical setting [20, 21]. The following figure, Figure 3 is a pictorial representation of ‘development of molecular Bio Signatures- pre and post- islet cell transplantation.

Figure 3.

Factors that govern the acceptance or rejection of an islet transplant.

A transplantation outcome that curtails infection and rejection is desirable; still, the present day immunosuppression strategies using prophylactic antimicrobial medications do not guarantee this! Human leukocyte antigen matching is an important aspect of graft survival. However, other factors like extent of immunosuppression, infections and management of comorbidities is also crucial. Considering transplant patient’s predisposing genetic modifiers for risk stratification and as a basis for applying precision pharmacotherapy may improve transplant outcomes [22].

The most important genes deciding the fate of a transplanted cell, tissue, or organ belong to what is termed the MHC (the major histocompatibility complex). The MHC antigens’ primary function is to aid in distinguishing self from no-self through peptide presentation to the immune system. The MHC antigens are also termed as HLA (human leukocyte antigens), and they comprise three regions, namely, class I (HLA-A, B, Cw), class II (HLA-DR, DQ , DP) and class III (no HLA genes). For acute rejection of an islet transplant, there is not any established treatment available. And hence transplantation of alloislets with HLAs reactive against even very low levels of preformed alloantibody in the recipient should be avoided even when T and B lymphocyte crossmatches are negative [23, 24].

Association of genetic assessment with demographic and clinical outcomes in a transplantation can potentially enable individualized risk stratification and immunosuppression through the identification of genetic variants relating to immune-mediated complications, post-transplant disease and also alterations in drug-metabolizing genes. Notably, immunosuppressive drug toxicity is of concern as the risk for impairment in kidney function relates with both calcineurin inhibitors and mTOR inhibitors. The drug Tacrolimus predominates immunosuppression in transplantation, and it is metabolized by the cytochrome P 450 3A (CYP3A) subfamily of enzymes in the liver and small intestine. In CYP3A5 gene, a polymorphism in intron 3 alters its expression affecting the enzyme activity and thereby tacrolimus drug metabolism. Tacrolimus drug level correlated well with presence or absence of CYP3A5 polymorphisms. Acute rejection episodes were more frequent in expressers, and they may require higher doses of tacrolimus. When alloislets are transplanted in patients with type 1 diabetes, the use of low-dose tacrolimus in combination with sirolimus is associated with decline in estimated GFR of ~5 mL/min/y/1.73 m². Therefore, CYP3A5 polymorphism analysis before transplant may help determine the optimal dose of tacrolimus in this population and prevent acute rejection episodes or tacrolimus toxicity [7, 25, 26].

3.1 Genes can impact the effectiveness of islet graft

The effectiveness of the islet graft depends both on beta cell function as well as the interaction between the graft and the host, and most importantly, these are governed by the expression of specific islet genes [27]. Components of specific cytokine pathways are upregulated in bad islet preparations (those which failed to reverse diabetes after transplantation). And these include tumor necrosis factor (TNF) machinery such as the TRAIL receptor TNFRSF10B that engage in β-cell death induced by T cells [28, 29]. The FAS and its ligand, FASL, can induce beta cell apoptosis [30, 31], and these are hiked in bad islets, suggesting that islet death-related pathways are already activated in these preparations even before transplantation. Adding on to apoptosis is the activation of NFκB and AP-1 transcription factors, which up-regulate expression of inflammatory cytokines [32]. A local proinflammatory environment is promoted by CCL2 (MCP1), which associates with islet death and diabetes [33, 34]. Also seen in bad islets are higher expressions of the pattern recognition receptor TLR3, which relates with islet dysfunction and increased cytokine expression [35]. An elevated expression of tissue factor (F3) is pro-inflammatory and inhibits islet graft function [33, 36]. On the contrary, the TGFB2 and its receptor TGFBR1, and the IL13 receptor, OSMR, are other elevated chemokines which can initiate protective signals for islet cells [37, 38, 39]. Similarly, SERPINA3, also known as alpha-1-antichymotrypsin is upregulated and may promote wound healing [40, 41]. The SEPT9 gene is upregulated in bad islets and has recently been shown to be upregulated in islets of type 2 diabetics [42]. So it appears that the pathways leading to islet dysfunction are already triggered before transplantation, but that there is also the initiation of some counteractive measures.

A list of genes that were preferentially upregulated in good islet preparations (those which failed to reverse diabetes after transplantation) were relatable with the development and regeneration of pancreas. Hence a prior initiation of repair/regeneration pathways in damaged islets would prove effective after transplantation. Several such genes including ONECUT1 (HNF6) [43, 44], MNX1 (HB9) [45, 46], NKX2-2 [47, 48], INSM1 [49, 50], NKX6-1 [47, 51], FOXA2 [52, 53, 54], and PTCH1 [55, 56] interact in regulatory networks aiding embryonic pancreas development and regeneration following an injury. On the other hand, the NOTCH2 gene, is predominantly expressed in the bad islet preparations, possibly because of its significant role in expansion of the progenitor cell population through suppression of neurogenin3-dependent endocrine cell differentiation [57]. Gene encoding the rectifying potassium channel KCNMA1 which is upregulated in good islets has been shown to be important for repolarization of the membrane following insulin secretion. Loss of KCNMA1 suppresses insulin secretion and increases susceptibility to oxidative stress and apoptosis [58].

In the recipient inducing protective genes like heme oxygenase-1 (HO-1), A20/tumor necrosis factor alpha inducible protein3 (tnfaip3), biliverdin reductase (BVR), Bcl2, and others could synergistically improve islet graft survival and function. A similar effect is seen on administration of one or more of the products of HO-1 to the donor [59].

In heme degradation, HO-1 is the rate-limiting enzyme that produces equal molar amounts of carbon monoxide (CO), biliverdin, and iron. Biliverdin’s rapid conversion by biliverdin reductase to bilirubin sequesters iron into ferritin. Being an ubiquitous stress protein, HO-1 gets induced in several cell types by various stimuli. Evidence piles up supporting HO-1 induction to offer cellular protection against transplant rejection. Induction of HO-1 pharmacologically or via gene transfer protects islets from stress-induced apoptosis in both the in vitro and the in vivo settings. HO-1 induction in β cell lines, or human islets protects against apoptosis induced by TNF-α and cyclohexamide (CHX), interleukin-1β (IL-1β), and Fas. In recepients, HO-1 induction with cobalt protoporphyrin (CoPP) pharmacologically improves islet function in a rodent model with marginal mass islet transplantation, wherein fewer islets are required to achieve normoglycemia when transplanted into a sygeneic recipient, whose been rendered diabetic by streptozotocin (STZ) treatment [60].

A20, also known as the TNF-α-induced protein 3 (TNFAIP3), is a zinc-ring finger protein. As a negative regulator of nuclear factor kappa B (NF-κB) activation, A20 is recognized as a central and ubiquitous regulator of inflammation and as a potent antiapoptotic gene in certain cell types, including β cells. Islets can be protected against apoptosis induced by IL-1β/INF-γ and Fas through adenovirus-mediated gene transfer causing overexpression of A20. A higher percentage of cure was seen after transplantation in recipients with suboptimal number of islets overexpressing A20 compared to control islets. Islets expressing A20 preserved functional β cell mass and are resistant to cell death. In β cells, expression of A20 renders a dual-protective effect through antiapoptosis and antiinflammation. The antiapototic effect of A20 attributes to its cytoprotective properties and is dependent on the abrogation of cytokine-induced NO (nitric oxide) production due to transcriptional blockade of iNOS induction [61, 62, 63].

Bilirubin administration reduced apoptosis and improved insulin secretion in an in vitro model in INS-1 cells when challenged with nonspecific inflammation induced by cytokines. Protective genes like HO-1 and bcl-2 were strongly expressed seen in in freshly isolated islets from bilirubin-treated donors. Also noticed was an evident suppression in the proinflammatory and proapoptotic genes including caspase-3, caspase-8, and MCP-1. Such a protective effect rendered by bilirubin reduces β-cell destruction post- transplantation, minimizes macrophage infiltration, and suppresses expression of MCP-1, BID, caspase-3, -8, and -9, TNF-α, iNOS, Fas, TRAIL-R, and CXCL10 in the graft after allogeneic transplantation [64, 65].

Exposing human islets to the nonpeptidyl low molecular weight radical scavenger IAC [bis(1-hydroxy-2,2,6,6-tetramethyl-4-piperidiny) decanedioate dihydrochloride] on isolated human islet cells protected them from isolation and culture-induced oxidative stress. Transduction of NOD islets with the antioxidative gene thioredoxin (TRX, reactive oxygen species scavenger and antiapoptotic) using a lentiviral vector before transplantation prolonged islet graft survival. Anthocyanins present in Chinese Bayberry have the potential to upregulate HO-1, and thus protect β cells against hydrogen-peroxide-induced necrosis and apoptosis. Islets’ viability and function improved with adenoviral transfection as X-linked inhibitor of apoptosis provided protection from inflammatory cytokines [66].

4. Islet transplantation- practical difficulties and resolving approaches

1. Immunosuppressant regimen

   Islet transplantation is emerging as a treatment for type 1 diabetes mellitus in selected patients with inadequate glucose control despite insulin therapy. However, considering the 267 allografts transplanted since 1990, insulin independence was observed in only 12.4 percent for periods of more than 1 week, and merely 8.2 percent could sustain the beneficial outcome for more than a year. Predominantly in these procedures the immunosuppression regimen comprised of antibody induction with an antilymphocyte globulin combined with cyclosporine, azathioprine, and glucocorticoids. Many of these immunosuppression regimens pose a threat of damaging beta cells or induce peripheral insulin resistance. Immunosuppressive drugs that concentrate in the liver can be toxic to the islets, yet must be taken for the lifetime of the graft. Currently, we are stepping into an era of accessing new and more potent immunosuppressive agents that will provide greater immunologic protection without diabetogenic side effects. Such an approach is glucocorticoid-free immunosuppressive protocol that includes sirolimus, low-dose tacrolimus, and a monoclonal antibody against the interleukin-2 receptor (daclizumab) for use in a trial of islet transplantation alone for patients with brittle type 1 diabetes. This has shown to result in insulin independence with excellent metabolic control when glucocorticoid-free immunosuppression is combined with the infusion of an adequate islet mass. Additionally, this immunosuppressive protocol has not clinically evidenced any episodes of graft rejection, and it appears to be effective in preventing autoimmune recurrence of diabetes [67].

   Islet transplantation has emerged as a promising treatment option for type 1 diabetes. Still its progress is challenged by barriers like patient accessibility and long-term graft function. These can be overcome by amalgamating emerging technologies in biomaterials with drug delivery and immunomodulation. The hepatic microenvironment and traditional systemic immunosuppression can stress the vulnerable islets and limit the success rate of transplantation [68].

2. Other challenges like limitation in islet engraftment and function, low oxygen tension, insulin-induced hepatic steatosis, lipotoxicity and inflammation

Intrahepatic transplantation is a minimally invasive portal infusion that results in islet entrapment within hepatic sinusoids. The islet engraftment and function is restrained by hepatic portal vasculature. An instant blood-mediated inflammatory reaction (IBMIR) is a resultant of vascular delivery. Also noticed are activated complement and coagulation cascades, and leucocyte infiltration leading to the loss of nearly two-thirds of the islets within the first few days after transplantation. Islets that survive inside the hepatic portal environment experience high glucose levels, low oxygen tension, and first-pass exposure to metabolites and pharmaceuticals. Intrahepatically transplanted islets may also be lost as a result of localized, insulin-induced hepatic steatosis, lipotoxicity and inflammation. A transformative approach to islet transplantation may be achieved through the adaptation of technologies for locally controlling the transplant microenvironment to promote engraftment and long-term function while minimizing or eliminating systemic non-specific immunosuppression with local immunomodulation or operational tolerance induction [68].

Resolving approaches

• Natural or synthetic biomaterials can be employed to engineer an extrahepatic space to localize islets and control the microenvironment after transplantation. Transplantation at extrahepatic and extravascular site is enabled through the support of biomaterial scaffold, which proves beneficial by avoiding the unfavorable influences on the liver environment and the IBMIR. Interestingly, the extrahepatically implanted biomaterial scaffolds can be retrieved, facilitating the adoption of insulin-producing cells derived from stem cells.

• Porous scaffolds are emerging as a suitable vehicle for islet transplantation. The reason being the ease of seeding islets into the pores as well as the porosity supporting rapid cell infiltration for integration with the host tissue. The composition of microporous scaffolds include biocompatible, biodegradable copolymer of lactide and glycolide (PLG) which are approved by the US Food and Drug Administration. The PLG create and maintain a space for transplanted islets while exerting control on their density and distribution. Similar to this, rapid ingrowth and revascularization can also be supported through degradable hydrogels such as collagen, fibrin and clotted plasma.

• Isolation employing non-degradable natural or synthetic hydrogels such as alginate and polyethylene glycol is an alternate to engraftment. These hydrogels have manifold benefits, including prevention against cellular attack by the immune system, avoidance of cell ingrowth, vascularisation and re-innervation. The aforementioned benefits can potentially target glucose sensing, insulin secretion and long-term beta cell turnover

• The interaction between islet and Extracellular matrix modellling (ECM) crucially determines islet survival signals. During islet isolation, the enzymatic and mechanical processes disrupt the islet cell’s specialized basement membrane of ECM proteins. Applying biomaterial technology to supply ECM proteins to islets may significantly enhance engraftment and function owing to the structural support provided, alongside the binding of cell-surface integrins which mediate adhesion and activate intracellular signaling pathways.

• During the early stages of post-transplantation, the islet cell death is relatable to a loss in integrin signaling, which may trigger apoptosis (also known as anoikis). To enhance engraftment and transplantation, biomaterial surfaces or hydrogels can be formed or modified with ECM molecules, adhesive peptides or other biochemical signals.

• Specific cellular processes stimulated by the trophic factor delivery may associate with islet survival and engraftment to maximize transplant success. The transplanted islet survival and function is enhanced by specific factors including, insulinotropic factors such as IGF-1, prolactin and exendin-4 or anti-apoptotic factors like BCL2-associated X protein (BAX)-inhibiting peptide. Regulation of beta cell mass, its proliferation and regeneration are determined by IGF-1. Exendin-4 is a long-acting glucagon-like peptide-1 agonist that stimulates beta cell proliferation, protects against apoptosis and improves outcomes in islet transplantation. Prolactin signaling during pregnancy is responsible for beta cell proliferation, and prolactin pre-incubation and injection have been shown to improve transplanted islet engraftment and revascularisation. Islet cell apoptosis is a major contributor to islet loss in the early post-transplant period. It is triggered by cues such as loss of ECM contacts, DNA damage, hypoxia and nutrient starvation. Pre-treatment with or induced production of BAX-inhibiting peptide in islet transplants can enhance engraftment by minimizing apoptosis. Delivery of angiogenic factors like vascular endothelial growth factor (VEGF) can enhance islet engraftment and glucose sensing by improving revascularization [68, 69, 70, 71, 72].

5. Promise and challenges of stem cell derived islet cell transplant- can it become a clinical reality?

People who are insulin dependent require multiple insulin injections, sometimes with an insulin pump, coupled with regular blood glucose monitoring. Diabetes management has improved with the availability of modified insulin’s, each with peaks of activity at varying times and conditions. Transplantation of cadaveric islets coupled with immune suppression has been impressive results leading to insulin independence in patients with Type 1 Diabetes. But again there is not a perfect balance between the available donors populations compared to diabetic load. Continuous availability of beta cells to the patients at an appropriate timelines is the need of the hour in any country. This can be made a reality with the use of pluripotent stem cells, to produce a virtually unlimited and uniform supply of human islet-like clusters by directed differentiation can solve many issues.

Stem cells, being undifferentiated, are capable of self-renewal, and can virtually produce any tissue or organ [73, 74, 75, 76, 77, 78]. Stem cells can be broadly classified on the basis of their origin as embryonic stem cells (ESCs), fetal stem cells (FSCs), adult stem cells (ASCs), and induced pluripotent stem cells (iPSCs). The iPSCs and ESCs are pluripotent stem cells (PSCs), while ASCs are unipotent or oligopotent [79, 80, 81].

Human-induced PSCs (iPSCs) and human embryonic stem cells (ESCs), serve as a reproducible source of human cells even at early developmental stages owing to their potential of forming any cell type in the adult body [82, 83, 84]. From a viewpoint of preservation of β-cells through islet protection and regeneration, the human-induced PSCs (iPSCs), hematopoietic stem cells (HSCs), human cord blood-derived multipotent stem cells (CB-SCs), and MSCs are being used. Additionally, stem cells are capable of re-establishing peripheral tolerance towards β-cells by remodeling of immune responses alongside inhibition of autoreactive T-cell function [85, 86]. Stem cells have the potential to increase islet mass owing to their capability of differentiation to β-cells-like organoids. They inhibit the immune responses of T cell and Th1 cells through TGF-β and inflammatory pathways, and thus reconstitute immunotolerance. Type 1 diabetes being an autoimmune disease is featured by activated immune cells which target and destroy pancreatic β-cells. Thus stem cell therapy for treating T1DM should take into account the stem cell’s immunomodulatory properties of and also its capability of differentiation into insulin-producing cells.

Nevertheless, challenges demanding resolution are plenty, like the ethical problem in using autologous and allogeneic stem cells to preserve the β-cells’ function. Even though research focused on stem cell-derived β-cell replacement is gaining momentum year by year, enormous effort on the interventions with stem cell transplantation is required in the future to aid in achieving remission of T1DM by β-cell replacement.

6. Conclusion and perspectives

A potentially promising therapeutic option for diabetes (especially T1DM) treatment can be stem cell treatment, as it cures the disease rather than treating or managing them. Major advances in research on the derivation of IPCs from hPSCs have improved our chance of reestablishing glucose-responsive insulin secretion in patients with T1DM. There are many questions and technical hurdles that still need to be solved.

The major problems include the following five aspects:

1. Can we generate more mature functional β-like cells in vitro from hPSCs.

2. Is there a possibility to improve the differentiation efficiency of IPCs from hPSCs.

3. How to protect implanted IPCs from autoimmune attack.

4. Need of the hour to generate sufficient numbers of desired cell types for clinical transplantation.

5. How to establish thorough insulin independence to make it a clinical reality.

Despite these obstacles, the application of stem cell-based therapy for T1DM represents the most advanced approach for curing type 1diabetes.

Acknowledgments

Indian Council for Medical Research (ICMR), New Delhi.

Apollo group of Hospitals, Chennai.

Apollo Hospital and Research Foundation, Chennai.

Dr. Viswanadham D, Vice President, IKP.

IKP Knowledge Park, Telangana.

Conflict of interest

We hereby declare that we have no conflict of interest of any form pertaining to our chapter titled, ‘Molecular Challenges and Advances in Clinical Islet Transplantation’.