Towards xenotransplantation of pig islets in the clinic. Curr Opin Organ Transplant. 2011;16:195-200. [PMID: 21358330 DOI: 10.1097/MOT.0b013e3283449dec]

Bioengineering the Vascularized Endocrine Pancreas: A Fine-Tuned Interplay Between Vascularization, Extracellular-Matrix-Based Scaffold Architecture, and Insulin-Producing Cells

Intrahepatic islet transplantation is a promising β-cell replacement strategy for the treatment of type 1 diabetes. Instant blood-mediated inflammatory reactions, acute inflammatory storm, and graft revascularization delay limit islet engraftment in the peri-transplant phase, hampering the success rate of the procedure. Growing evidence has demonstrated that islet engraftment efficiency may take advantage of several bioengineering approaches aimed to recreate both vascular and endocrine compartments either ex vivo or in vivo. To this end, endocrine pancreas bioengineering is an emerging field in β-cell replacement, which might provide endocrine cells with all the building blocks (vascularization, ECM composition, or micro/macro-architecture) useful for their successful engraftment and function in vivo. Studies on reshaping either the endocrine cellular composition or the islet microenvironment have been largely performed, focusing on a single building block element, without, however, grasping that their synergistic effect is indispensable for correct endocrine function. Herein, the review focuses on the minimum building blocks that an ideal vascularized endocrine scaffold should have to resemble the endocrine niche architecture, composition, and function to foster functional connections between the vascular and endocrine compartments. Additionally, this review highlights the possibility of designing bioengineered scaffolds integrating alternative endocrine sources to overcome donor organ shortages and the possibility of combining novel immune-preserving strategies for long-term graft function.

Ekdahl K, Nilsson B, Teramura Y. Optimization of Islet Microencapsulation with Thin Polymer Membranes for Long-Term Stability

Microencapsulation of islets can protect against immune reactions from the host immune system after transplantation. However, sufficient numbers of islets cannot be transplanted due to the increase of the size and total volume. Therefore, thin and stable polymer membranes are required for the microencapsulation. Here, we undertook the cell microencapsulation using poly(ethylene glycol)-conjugated phospholipid (PEG-lipid) and layer-by-layer membrane of multiple-arm PEG. In order to examine the membrane stability, we used different molecular weights of 4-arm PEG (10k, 20k and 40k)-Mal to examine the influence on the polymer membrane stability. We found that the polymer membrane made of 4-arm PEG(40k)-Mal showed the highest stability on the cell surface. Also, the polymer membrane did not disturb the insulin secretion from beta cells.

Can We Re-Engineer the Endocrine Pancreas?

PURPOSE OF REVIEW

Engineering endocrine pancreatic tissue is an emerging topic in type 1 diabetes with the intent to overcome the current limitation of β cell transplantation. During islet isolation, the vascularized structure and surrounding extracellular matrix (ECM) are completely disrupted. Once implanted, islets slowly engraft and mostly are lost for the initial avascular phase. This review discusses the main building blocks required to engineer the endocrine pancreas: (i) islet niche ECM, (ii) islet niche vascular network, and (iii) new available sources of endocrine cells.

RECENT FINDINGS

Current approaches include the following: tissue engineering of endocrine grafts by seeding of native or synthetic ECM scaffolds with human islets, vascularization of native or synthetic ECM prior to implantation, vascular functionalization of ECM structures to enhance angiogenesis after implantation, generation of engineered animals as human organ donors, and embryonic and pluripotent stem cell-derived endocrine cells that may be encapsulated or genetically engineered to be immunotolerated. Substantial technological improvements have been made to regenerate or engineer endocrine pancreatic tissue; however, significant hurdles remain, and more research is needed to develop a technology to integrate all components of viable endocrine tissue for clinical application.

Pancreas regeneration

The pancreas is made from two distinct components: the exocrine pancreas, a reservoir of digestive enzymes, and the endocrine islets, the source of the vital metabolic hormone insulin. Human islets possess limited regenerative ability; loss of islet β-cells in diseases such as type 1 diabetes requires therapeutic intervention. The leading strategy for restoration of β-cell mass is through the generation and transplantation of new β-cells derived from human pluripotent stem cells. Other approaches include stimulating endogenous β-cell proliferation, reprogramming non-β-cells to β-like cells, and harvesting islets from genetically engineered animals. Together these approaches form a rich pipeline of therapeutic development for pancreatic regeneration.

Immunogenicity of human embryonic stem cell-derived beta cells

AIMS/HYPOTHESIS

To overcome the donor shortage in the treatment of advanced type 1 diabetes by islet transplantation, human embryonic stem cells (hESCs) show great potential as an unlimited alternative source of beta cells. hESCs may have immune privileged properties and it is important to determine whether these properties are preserved in hESC-derived cells.

METHODS

We comprehensively investigated interactions of both innate and adaptive auto- and allo-immunity with hESC-derived pancreatic progenitor cells and hESC-derived endocrine cells, retrieved after in-vivo differentiation in capsules in the subcutis of mice.

RESULTS

We found that hESC-derived pancreatic endodermal cells expressed relatively low levels of HLA endorsing protection from specific immune responses. HLA was upregulated when exposed to IFNγ, making these endocrine progenitor cells vulnerable to cytotoxic T cells and alloreactive antibodies. In vivo-differentiated endocrine cells were protected from complement, but expressed more HLA and were targets for alloreactive antibody-dependent cellular cytotoxicity and alloreactive cytotoxic T cells. After HLA compatibility was provided by transduction with HLA-A2, preproinsulin-specific T cells killed insulin-producing cells.

CONCLUSIONS/INTERPRETATION

hESC-derived pancreatic progenitors are hypoimmunogenic, while in vivo-differentiated endocrine cells represent mature targets for adaptive immune responses. Our data support the need for immune intervention in transplantation of hESC-derived pancreatic progenitors. Cell-impermeable macro-encapsulation may suffice.

Mesothelial cell transplantation: history, challenges and future directions

Mesothelial cells line the surface of the pleura, pericardium, peritoneum and internal reproductive organs. One of their main functions is to act as a non-adhesive barrier to protect against physical damage, however, over the past decades their physiological and pathological properties have been revealed in association with a variety of conditions and diseases. Mesothelium has been used in surgical operations in clinical settings, such as omental patching for perforated peptic ulcers and in glutaraldehyde-treated autologous pericardium for aortic valve reconstruction. Various methods for mesothelial cell transplantation have also been established and developed, particularly within the area of tissue engineering, including scaffold and non-scaffold cell sheet technologies. However, the use of mesothelial cell transplantation in patients remains challenging, as it requires additional operations under general anesthesia in order to obtain enough intact cells for culture. Moreover, the current methods of mesothelial cell transplantation are expensive and are not yet available in clinical practice. This review firstly summarizes the history of the use of mesothelial cell transplantation in tissue engineering, and then critically discusses the barriers for the clinical application of mesothelial cell transplantation. Finally, the recent developments in xenotransplantation technologies are discussed to evaluate other feasible alternatives to mesothelial cell transplantation.

Islet cell transplantation from Göttingen minipigs to cynomolgus monkeys: analysis of virus safety

BACKGROUND

Xenotransplantation using pig cells, tissues or organs may be associated with the transmission of porcine zoonotic micro-organisms. Hepatitis E virus (HEV), porcine cytomegalovirus (PCMV) and porcine endogenous retroviruses (PERVs) are potentially zoonotic micro-organisms which do not show clinical symptoms in pigs and which are due to the low expression level difficult to detect. Göttingen Minipigs (GöMP) are often used for biomedical investigations and they are well characterized concerning the presence of numerous bacteria, fungi, viruses and parasites and therefore may be used for islet cell transplantation.

METHODS

Islet cells derived from three GöMP were transplanted into four healthy, non-diabetic cynomolgus monkeys using a macroencapsulation device. PCR, nested PCR, real-time PCR, real-time RT-PCR and Western blot analyses were used to estimate the presence of PERV, PCMV and HEV in the donors and recipients.

RESULTS

Using sensitive detection methods, no HEV was found in the donor pigs and in the pig islet cell preparations. Antibodies against PERV, PCMV and HEV were not found in all cynomolgus monkeys with exception of one monkey showing an immune response against HEV. Using real-time PCR, no PCMV and HEV were found in the sera of all monkeys.

CONCLUSION

Although the donor islet cells and the recipients were negative for HEV using PCR and Western blot analysis, in one recipient, antibodies against HEV were found, indicating infection in a single case. All recipients were negative for antibodies against PERV, and all were negative for PCMV, indicating absence of infection. As HEV was not detected in the donor pig before transplantation, a more complex and regular screening of the animals using highly sensitive methods is required to avoid virus transmission.

Islet cell transplant: Update on current clinical trials

In the last 15 years clinical islet transplantation has made the leap from experimental procedure to standard of care for a highly selective group of patients. Due to a risk-benefit calculation involving the required systemic immunosuppression the procedure is only considered in patients with type 1 diabetes, complicated by severe hypoglycemia or end stage renal disease. In this review we summarize current outcomes of the procedure and take a look at ongoing and future improvements and refinements of beta cell therapy.

In vitro exposure of pig neonatal isletlike cell clusters to human blood

BACKGROUND

Pig islet grafts have been successful in treating diabetes in animal models. One remaining question is whether neonatal pig isletlike cell clusters (NICC) are resistant to the early loss of islets from the instant blood-mediated inflammatory reaction (IBMIR).

METHODS

Neonatal isletlike cell clusters were harvested from three groups of piglets-(i) wild-type (genetically unmodified), (ii) α1,3-galactosyltransferase gene-knockout (GTKO)/CD46, and (iii) GTKO/CD46/CD39. NICC samples were mixed with human blood in vitro, and the following measurements were made-antibody binding; complement activation; speed of islet-induced coagulation; C-peptide; glutamic acid decarboxylase (GAD65) release; viability.

RESULTS

Time to coagulation and viability were both reduced in all groups compared to freshly drawn non-anticoagulated human blood and autologous combinations, respectively. Antibody binding to the NICC occurred in all groups.

CONCLUSIONS

Neonatal isletlike cell clusters were subject to humoral injury with no difference associated to their genetic characteristics.

Optimal pig donor selection in islet xenotransplantation: current status and future perspectives

Islet transplantation is an attractive treatment of type 1 diabetes mellitus. Xenotransplantation, using the pig as a donor, offers the possibility of an unlimited supply of islet grafts. Published studies demonstrated that pig islets could function in diabetic primates for a long time (>6 months). However, pig-islet xenotransplantation must overcome the selection of an optimal pig donor to obtain an adequate supply of islets with high-quality, to reduce xeno-antigenicity of islet and prolong xenograft survival, and to translate experimental findings into clinical application. This review discusses the suitable pig donor for islet xenotransplantation in terms of pig age, strain, structure/function of islet, and genetically modified pig.

Pig-to-monkey islet xenotransplantation using multi-transgenic pigs

The generation of pigs with genetic modifications has significantly advanced the field of xenotransplantation. New genetically engineered pigs were produced on an α1,3-galactosyltransferase gene-knockout background with ubiquitous expression of human CD46, with islet beta cell-specific expression of human tissue factor pathway inhibitor and/or human CD39 and/or porcine CTLA4-lg. Isolated islets from pigs with 3, 4 or 5 genetic modifications were transplanted intraportally into streptozotocin-diabetic, immunosuppressed cynomolgus monkeys (n = 5). Immunosuppression was based on anti-CD154 mAb costimulation blockade. Monitoring included features of early islet destruction, glycemia, exogenous insulin requirement and histopathology of the islets at necropsy. Using these modified pig islets, there was evidence of reduced islet destruction in the first hours after transplantation, compared with two series of historical controls that received identical therapy but were transplanted with islets from pigs with either no or only one genetic modification. Despite encouraging effects on early islet loss, these multi-transgenic islet grafts did not demonstrate consistency in regard to long-term success, with only two of five demonstrating function beyond 5 months.

Progress in pig-to-non-human primate transplantation models (1998-2013): a comprehensive review of the literature

BACKGROUND

The pig-to-non-human primate model is the standard choice for in vivo studies of organ and cell xenotransplantation. In 1998, Lambrigts and his colleagues surveyed the entire world literature and reported all experimental studies in this model. With the increasing number of genetically engineered pigs that have become available during the past few years, this model is being utilized ever more frequently.

METHODS

We have now reviewed the literature again and have compiled the data we have been able to find for the period January 1, 1998 to December 31, 2013, a period of 16 yr.

RESULTS

The data are presented for transplants of the heart (heterotopic and orthotopic), kidney, liver, lung, islets, neuronal cells, hepatocytes, corneas, artery patches, and skin. Heart, kidney, and, particularly, islet xenograft survival have increased significantly since 1998.

DISCUSSION

The reasons for this are briefly discussed. A comment on the limitations of the model has been made, particularly with regard to those that will affect progression of xenotransplantation toward the clinic.

Microbiological safety of the first clinical pig islet xenotransplantation trial in New Zealand

BACKGROUND

Xenotransplantation using pig cells, tissues, or organs may be associated with the transmission of porcine microorganisms and the development of zoonoses. Among all porcine microorganisms porcine endogenous retroviruses (PERVs) represent a special risk because they are integrated in the genome of all pigs and able to infect human cells. In previous preclinical and retrospective clinical trials of xenotransplantation, no transmission of PERV was observed. The first clinical trial of (alginate-encapsulated) porcine islet cell transplantation in New Zealand, which was approved by the New Zealand Government as an open-label phase I/IIa safety/efficacy trial, offers the possibility to analyze microbiological safety in a prospective clinical study.

METHODS

Before the trial started, a multilevel testing strategy was used to screen for 26 microorganisms in donor pigs of the Auckland Island strain and the islet cell preparations used for treatment. Donor testing was performed using molecular methods including multiplex real-time PCR. Blood samples from 14 pig islet cell recipients were also investigated by molecular biological methods at weeks 1, 4, 8, 12, 24, and 52 post-transplant for the transmission of porcine microorganisms. Sera were also monitored at these time points for antibodies against PERVs.

RESULTS

Beginning in 2009, fourteen patients with severe unaware hypoglycemia were treated with one of four different dosages of alginate-encapsulated porcine islets ranging from 5000-20,000 islet equivalents delivered in a single dose. No transmission of either PERVs or other porcine microorganisms was detected by PCR and immunological methods.

CONCLUSION

These findings support previous results and strongly indicate the safety of xenotransplantation as performed here.

Cell replacement strategies aimed at reconstitution of the β-cell compartment in type 1 diabetes

Emerging technologies in regenerative medicine have the potential to restore the β-cell compartment in diabetic patients, thereby overcoming the inadequacies of current treatment strategies and organ supply. Novel approaches include: 1) Encapsulation technology that protects islet transplants from host immune surveillance; 2) stem cell therapies and cellular reprogramming, which seek to regenerate the depleted β-cell compartment; and 3) whole-organ bioengineering, which capitalizes on the innate properties of the pancreas extracellular matrix to drive cellular repopulation. Collaborative efforts across these subfields of regenerative medicine seek to ultimately produce a bioengineered pancreas capable of restoring endocrine function in patients with insulin-dependent diabetes.

Islet xenotransplantation from genetically engineered pigs

PURPOSE OF REVIEW

Pigs have emerged as potential sources of islets for clinical transplantation. Wild-type porcine islets (adult and neonatal) transplanted into the portal vein have successfully reversed diabetes in nonhuman primates. However, there is a rapid loss of the transplanted islets on exposure to blood, known as the instant blood-mediated inflammatory reaction (IBMIR), as well as a T-cell response that leads to rejection of the graft.

RECENT FINDINGS

Genetically modified pig islets offer a number of potential advantages, particularly with regard to reducing the IBMIR-related graft loss and protecting the islets from the primate immune response. Emerging data indicate that transgenes specifically targeted to pig β cells using an insulin promoter (in order to maximize target tissue expression while limiting host effects) can be achieved without significant effects on the pig's glucose metabolism.

SUMMARY

Experience with the transplantation of islets from genetically engineered pigs into nonhuman primates is steadily increasing, and has involved the deletion of pig antigenic targets to reduce the primate humoral response, the expression of transgenes for human complement-regulatory and coagulation-regulatory proteins, and manipulations to reduce the effect of the T-cell response. There is increasing evidence of the advantages of using genetically engineered pigs as sources of islets for future clinical trials.

Pig-islet xenotransplantation: recent progress and current perspectives

Islet xenotransplantation is one prospective treatment to bridge the gap between available human cells and needs of patients with diabetes. Pig represents an ideal candidate for obtaining such available cells. However, potential clinical application of pig islet still faces obstacles including inadequate yield of high-quality functional islets and xenorejection of the transplants. Adequate amounts of available islets can be obtained by selection of a suitable pathogen-free source herd and the development of isolation and purification method. Several studies demonstrated the feasibility of successful preclinical pig-islet xenotransplantation and provided insights and possible mechanisms of xenogeneic immune recognition and rejection. Particularly promising is the achievement of long-term insulin independence in diabetic models by means of distinct islet products and novel immunotherapeutic strategies. Nonetheless, further efforts are needed to obtain much more safety and efficacy data to translate these findings into clinic.

Treating diabetes with islet transplantation: lessons from the past decade in Lille

Type 1 diabetes (T1D) is due to the loss of both beta-cell insulin secretion and glucose sensing, leading to glucose variability and a lack of predictability, a daily issue for patients. Guidelines for the treatment of T1D have become stricter as results from the Diabetes Control and Complications Trial (DCCT) demonstrated the close relationship between microangiopathy and HbA1c levels. In this regard, glucometers, ambulatory continuous glucose monitoring, and subcutaneous and intraperitoneal pumps have been major developments in the management of glucose imbalance. Besides this technological approach, islet transplantation (IT) has emerged as an acceptable safe procedure with results that continue to improve. Research in the last decade of the 20th century focused on the feasibility of islet isolation and transplantation and, since 2000, the success and reproducibility of the Edmonton protocol have been proven, and the mid-term (5-year) benefit-risk ratio evaluated. Currently, a 5-year 50% rate of insulin independence can be expected, with stabilization of microangiopathy and macroangiopathy, but the possible side-effects of immunosuppressants, limited availability of islets and still limited duration of insulin independence restrict the procedure to cases of brittle diabetes in patients who are not overweight or have no associated insulin resistance. However, various prognostic factors have been identified that may extend islet graft survival and reduce the number of islet injections required; these include graft quality, autoimmunity, immunosuppressant regimen and non-specific inflammatory reactions. Finally, alternative injection sites and unlimited sources of islets are likely to make IT a routine procedure in the future.

Long-term IgG response to porcine Neu5Gc antigens without transmission of PERV in burn patients treated with porcine skin xenografts

Acellular materials of xenogenic origin are used worldwide as xenografts, and phase I trials of viable pig pancreatic islets are currently being performed. However, limited information is available on transmission of porcine endogenous retrovirus (PERV) after xenotransplantation and on the long-term immune response of recipients to xenoantigens. We analyzed the blood of burn patients who had received living pig-skin dressings for up to 8 wk for the presence of PERV as well as for the level and nature of their long term (maximum, 34 y) immune response against pig Ags. Although no evidence of PERV genomic material or anti-PERV Ab response was found, we observed a moderate increase in anti-αGal Abs and a high and sustained anti-non-αGal IgG response in those patients. Abs against the nonhuman sialic acid Neu5Gc constituted the anti-non-αGal response with the recognition pattern on a sialoglycan array differing from that of burn patients treated without pig skin. These data suggest that anti-Neu5Gc Abs represent a barrier for long-term acceptance of porcine xenografts. Because anti-Neu5Gc Abs can promote chronic inflammation, the long-term safety of living and acellular pig tissue implants in recipients warrants further evaluation.

Lectin array analysis for wild-type and α-Gal-knockout pig islets versus healthy human islets

PURPOSE

We performed lectin microarray analyses of islets from wild-type (WT) pigs and α1-3galactosyltransferase gene knockout (GKO) pigs and compared the results with the corresponding values for islets from healthy humans.

METHODS

Islets were isolated from the pancreas. After sonication and centrifugation, the proteins in the supernatant from each islet were labeled with Cy3 and applied to a lectin array.

RESULTS

Despite negligible expression of the Gal antigen on the adult pig islets (APIs), GKO-islets showed weaker signals, not only for GS-I-B4 but also for PNA, WFA, PTL-I, and GS-I-A4, than the WT islets, indicating reduced contents of α-linked GalNAc and Galβ1-3GalNAc. In comparing the islets of pigs vs. humans, human islets showed stronger signals for UEA-I, AAL, TJA-II, EEL, WFA, HPA, DBA, SBA and PTL-I, indicating that besides ABO blood type antigens, high levels of fucose and α-linked GalNAc are present. On the other hand, the high mannose form was very rich in the APIs.

CONCLUSION

GKO reduced alpha-linked GalNAc, despite negligible expression of the Gal antigen on WT-API. On the other hand, the high-mannose form was richer in both APIs than in healthy human islets. These results provide useful information for future studies.

Generating insulin-producing cells for diabetic therapy: existing strategies and new development

Type 1 and 2 diabetes are characterized by a deficiency in β-cell mass, which cannot be reversed with existing therapeutic strategies. Therefore, restoration of the endogenous insulin-producing cell mass holds great promise for curing diabetes in the future. Since the initial induction of insulin-producing cells (IPCs) from embryonic stem (ES) cells in 1999, several strategies and alternative cell sources have been developed to generate β-like cells, including direct differentiation from ES cells or induced pluripotent stem (iPS) cells, proliferation of existing adult β-cells, and reprogramming of non-pancreatic adult stem/mature cells or pancreatic non-β-cells to β-like-cells. However, several barriers persist in the translation of the aforementioned strategies into clinically applicable methods for IPC induction. We briefly review the most relevant studies for each strategy, and discuss the comparative merits and drawbacks. We propose that ex vivo patient-specific IPCs generated from iPS cells may be practical for cell transplantation in the near future, and in situ regeneration of IPCs from cells within the pancreas may be preferable for diabetes therapy.

Generation and characterization of human heme oxygenase-1 transgenic pigs

Xenotransplantation using transgenic pigs as an organ source is a promising strategy to overcome shortage of human organ for transplantation. Various genetic modifications have been tried to ameliorate xenograft rejection. In the present study we assessed effect of transgenic expression of human heme oxygenase-1 (hHO-1), an inducible protein capable of cytoprotection by scavenging reactive oxygen species and preventing apoptosis caused by cellular stress during inflammatory processes, in neonatal porcine islet-like cluster cells (NPCCs). Transduction of NPCCs with adenovirus containing hHO-1 gene significantly reduced apoptosis compared with the GFP-expressing adenovirus control after treatment with either hydrogen peroxide or hTNF-α and cycloheximide. These protective effects were diminished by co-treatment of hHO-1 antagonist, Zinc protoporphyrin IX. We also generated transgenic pigs expressing hHO-1 and analyzed expression and function of the transgene. Human HO-1 was expressed in most tissues, including the heart, kidney, lung, pancreas, spleen and skin, however, expression levels and patterns of the hHO-1 gene are not consistent in each organ. We isolate fibroblast from transgenic pigs to analyze protective effect of the hHO-1. As expected, fibroblasts derived from the hHO-1 transgenic pigs were significantly resistant to both hydrogen peroxide damage and hTNF-α and cycloheximide-mediated apoptosis when compared with wild-type fibroblasts. Furthermore, induction of RANTES in response to hTNF-α or LPS was significantly decreased in fibroblasts obtained from the hHO-1 transgenic pigs. These findings suggest that transgenic expression of hHO-1 can protect xenografts when exposed to oxidative stresses, especially from ischemia/reperfusion injury, and/or acute rejection mediated by cytokines. Accordingly, hHO-1 could be an important candidate molecule in a multi-transgenic pig strategy for xenotransplantation.

Treatment of diabetic rats using islets from a herd of outbred Wuzhishan miniature pigs

AIM: To assess the efficacy of Wuzhishan miniature pig islets in the treatment of diabetic rats.

METHODS: Adult male miniatures pigs and market pigs were used as donors. After collagenase digestion, the islets were purified and intraportally transplanted into diabetic SPF rats, with cyclosporine (20 mg/kg) intramuscularly injected as an immunosuppressive agent. Islet graft survival was monitored by the detection of changes in blood sugar and liver histology in diabetic rats.

RESULTS: Islet yield was 4 608 IEQ/g ± 593 IEQ/g in the miniature pig group and 3 820 IEQ/g ± 718 IEQ/g in the market pig group. On day 1 after transplantation, the blood glucose decreased to normal in 84.6% of diabetic rats in both groups. Islet survival time was 3-5 d (median: 4.5 d) in the miniature pig group and 2-4 d (median: 3.7 d) in the market pig group. Kaplan-Meier analysis showed no significant difference between the two groups.

CONCLUSION: The Wuzhishan miniature pigs are an ideal donor species for islet xenotransplantation due to high islet yield and good islet function.

Mechanisms and techniques of reprogramming: using PDX-1 homeobox protein as a novel treatment of insulin dependent diabetes mellitus

Homeobox proteins are key regulators of stem cell proliferation and differentiation which function as transcription factors and regulate cell fate decisions. Pancreatic Duodenal Homeobox-1 (PDX-1) is a homeobox protein which acts as a key regulator in the development of b cells in the Islets of Langerhans. It plays an important role in maintaining the identity and function of the Islets of Langerhans, and in the development of the pancreas. There is strong evidence that PDX-1 plays a role in activating the insulin promoter and increasing insulin levels in response to glucose. PDX-1 also binds to sequences within β cells and regulates the promoter activity of a number of islet genes including insulin, glut-2 and neurogenin 3. When fused with the VP16 activation sequence, transfection of the PDX-1 gene has been shown to transform liver cells into insulin producing cells. Because homeobox proteins are able to passively translocate through cell membranes, due to an intrinsic transduction domain (penetratin), the use of these proteins to reprogram target cells may help overcome the limiting supply of β cells and be a potential future treatment for Type 1 diabetes.

Advances in research of pig islet xenotransplantation for diabetes

Pig islet xenotransplantation is effective in treating diabetes. Nowadays, the research of islet xenotransplantation is still in the research phase, and its clinical use is mainly restricted by the shortage of functional islets and graft rejection. In recent years, several studies have demonstrated the feasibility of successful preclinical pig islet xenotransplantation. Moreover, promising results concerning prolonged insulin independence were achieved with the improvement of islet isolation technologies, application of novel immunotherapeutic strategies, and the development of transplantation surgery. This review aims to elucidate the advances in the separation and preparation of transplanted pig islet, immunological rejection and treatments, potential safety problems, and clinical studies.

A brief history of cross-species organ transplantation

Cross-species transplantation (xenotransplantation) offers the prospect of an unlimited supply of organs and cells for clinical transplantation, thus resolving the critical shortage of human tissues that currently prohibits a majority of patients on the waiting list from receiving transplants. Between the 17th and 20th centuries, blood was transfused from various animal species into patients with a variety of pathological conditions. Skin grafts were carried out in the 19th century from a variety of animals, with frogs being the most popular. In the 1920s, Voronoff advocated the transplantation of slices of chimpanzee testis into aged men whose "zest for life" was deteriorating, believing that the hormones produced by the testis would rejuvenate his patients. Following the pioneering surgical work of Carrel, who developed the technique of blood vessel anastomosis, numerous attempts at nonhuman primate organ transplantation in patients were carried out in the 20th century. In 1963-1964, when human organs were not available and chronic dialysis was not yet in use, Reemtsma transplanted chimpanzee kidneys into 13 patients, one of whom returned to work for almost 9 months before suddenly dying from what was believed to be an electrolyte disturbance. The first heart transplant in a human ever performed was by Hardy in 1964, using a chimpanzee heart, but the patient died within 2 hours. Starzl carried out the first chimpanzee-to-human liver transplantation in 1966; in 1992, he obtained patient survival for 70 days following a baboon liver transplant. With the advent of genetic engineering and cloning technologies, pigs are currently available with a number of different manipulations that protect their tissues from the human immune response, resulting in increasing pig graft survival in nonhuman primate models. Genetically modified pigs offer hope of a limitless supply of organs and cells for those in need of a transplant.

Current status of immunomodulatory and cellular therapies in preclinical and clinical islet transplantation

Clinical islet transplantation is a β-cell replacement strategy that represents a possible definitive intervention for patients with type 1 diabetes, offering substantial benefits in terms of lowering daily insulin requirements and reducing incidences of debilitating hypoglycemic episodes and unawareness. Despite impressive advances in this field, a limiting supply of islets, inadequate means for preventing islet rejection, and the deleterious diabetogenic and nephrotoxic side effects associated with chronic immunosuppressive therapy preclude its wide-spread applicability. Islet transplantation however allows a window of opportunity for attempting various therapeutic manipulations of islets prior to transplantation aimed at achieving superior transplant outcomes. In this paper, we will focus on the current status of various immunosuppressive and cellular therapies that promote graft function and survival in preclinical and clinical islet transplantation with special emphasis on the tolerance-inducing capacity of regulatory T cells as well as the β-cells regenerative capacity of stem cells.