A comparison of fetal and adult porcine islets with regard to Gal alpha (1,3)Gal expression and the role of human immunoglobulins and complement in islet cell cytotoxicity

The future treatment for type 1 diabetes: Pig islet- or stem cell-derived β cells?

Replacement of β cells is only a curative approach for type 1 diabetes (T1D) patients to avoid the threat of iatrogenic hypoglycemia. In this pursuit, islet allotransplantation under Edmonton's protocol emerged as a medical miracle to attain hypoglycemia-free insulin independence in T1D. Shortage of allo-islet donors and post-transplantation (post-tx) islet loss are still unmet hurdles for the widespread application of this therapeutic regimen. The long-term survival and effective insulin independence in preclinical studies have strongly suggested pig islets to cure overt hyperglycemia. Importantly, CRISPR-Cas9 technology is pursuing to develop "humanized" pig islets that could overcome the lifelong immunosuppression drug regimen. Lately, induced pluripotent stem cell (iPSC)-derived β cell approaches are also gaining momentum and may hold promise to yield a significant supply of insulin-producing cells. Theoretically, personalized β cells derived from a patient's iPSCs is one exciting approach, but β cell-specific immunity in T1D recipients would still be a challenge. In this context, encapsulation studies on both pig islet as well as iPSC-β cells were found promising and rendered long-term survival in mice. Oxygen tension and blood vessel growth within the capsules are a few of the hurdles that need to be addressed. In conclusion, challenges associated with both procedures, xenotransplantation (of pig-derived islets) and stem cell transplantation, are required to be cautiously resolved before their clinical application.

Immunological Challenges Facing Translation of Alginate Encapsulated Porcine Islet Xenotransplantation to Human Clinical Trials

Transplantation of alginate-encapsulated islets has the potential to treat patients suffering from type I diabetes, a condition characterized by an autoimmune attack against insulin-secreting beta cells. However, there are multiple immunological challenges associated with this procedure, all of which must be adequately addressed prior to translation from trials in small animal and nonhuman primate models to human clinical trials. Principal threats to graft viability include immune-mediated destruction triggered by immunogenic alginate impurities, unfavorable polymer composition and surface characteristics, and release of membrane-permeable antigens, as well as damage associated molecular patterns (DAMPs) by the encapsulated islets themselves. The lack of standardization of significant parameters of bioencapsulation device design and manufacture (i.e., purification protocols, surface-modification grafting techniques, alginate composition modifications) between labs is yet another obstacle that must be overcome before a clinically effective and applicable protocol for encapsulating islets can be implemented. Nonetheless, substantial progress is being made, as is evident from prolonged graft survival times and improved protection from immune-mediated graft destruction reported by various research groups, but also with regard to discoveries of specific pathways involved in explaining observed outcomes. Progress in the latter is essential for a comprehensive understanding of the mechanisms responsible for the varying levels of immunogenicity of certain alginate devices. Successful translation of encapsulated islet transplantation from in vitro and animal model testing to human clinical trials hinges on application of this knowledge of the pathways and interactions which comprise immune-mediated rejection. Thus, this review not only focuses on the different factors contributing to provocation of the immune reaction by encapsulated islets, but also on the defining characteristics of the response itself.

Porcine Islet Xenografts: a Clinical Source of ß-Cell Grafts

PURPOSE OF REVIEW

Diabetes is medical and social burden affecting millions around the world. Despite intensive therapy, insulin fails to maintain adequate glucose homeostasis and often results in episodes of hypoglycemic unawareness. Islet transplantation is a propitious replacement therapy, and incremental improvements in islet isolation and immunosuppressive drugs have made this procedure a feasible option. Shortage of donors, graft loss, and toxic immunosuppressive agents are few of many hurdles against making human allogenic islet transplantation a routine procedure.

RECENT FINDINGS

Xenografts-especially pig islets-offer a logical alternative source for islets. Current preclinical studies have revealed problems such as optimal islet source, zoonosis, and immune rejection. These issues are slowing clinical application. Genetically modified pigs, encapsulation devices, and new immune-suppressive regimens can confer graft protection. In addition, extrahepatic transplant sites are showing promising results. Notwithstanding few approved clinical human trials, and available data from non-human primates, recent reports indicate that porcine islets are closer to be the promising solution to cure diabetes.

Human Recombinant Antithrombin (ATryn®) Administration Improves Survival and Prevents Intravascular Coagulation After Intraportal Islet Transplantation in a Piglet Model

Human islet transplantation is a viable treatment option for type 1 diabetes mellitus (T1DM). However, pancreatic islet inflammation after transplantation induced by innate immune responses is likely to hinder graft function. This is mediated by incompatibility between islets and the blood interface, known as instant blood-mediated inflammatory reaction (IBMIR). Herein we hypothesized that portal venous administration of islet cells with human recombinant antithrombin (ATryn®), a serine protease inhibitor (serpin), which plays a central role in the physiological regulation of coagulation and exerts indirect anti-inflammatory activities, may offset coagulation abnormalities such as disseminated intravascular coagulation (DIC) and IBMIR. The current prospective, randomized experiment was conducted using an established preclinical pig model. Three groups were constituted for digested pancreatic tissue transplantation (0.15 ml/kg): control, NaCl 0.9% (n = 7); gold standard, heparin (25 UI/kg) (n = 7); and human recombinant ATryn® (500 UI/kg) (n = 7). Blood samples were collected over time (T0 to 24 h), and biochemical, coagulation, and inflammatory parameters were evaluated. In both the control and heparin groups, one animal died after a portal thrombosis, while no deaths occurred in the ATryn®-treated group. As expected, islet transplantation was associated with an increase in plasma IL-6 or TNF-α levels in all three groups. However, DIC was only observed in the control group, an effect that was suppressed after ATryn® administration. ATryn® administration increased antithrombin activity by 800%, which remained at 200% for the remaining period of the study, without any hemorrhagic complications. These studies suggest that coadministration of ATryn® and pancreatic islets via intraportal transplantation may be a valuable therapeutic approach for DIC without risk for islets and subjects.

Characterizing the Mechanistic Pathways of the Instant Blood-Mediated Inflammatory Reaction in Xenogeneic Neonatal Islet Cell Transplantation

Supplemental digital content is available in the text.

Introduction

The instant blood-mediated inflammatory reaction (IBMIR) causes major loss of islets after transplantation and consequently represents the initial barrier to survival of porcine neonatal islet cell clusters (NICC) after xenotransplantation.

Methods

This study used novel assays designed to characterize the various immunologic components responsible for xenogeneic IBMIR to identify initiators and investigate processes of IBMIR-associated coagulation, complement activation and neutrophil infiltration. The IBMIR was induced in vitro by exposing NICC to platelet-poor or platelet-rich human plasma or isolated neutrophils.

Results

We found that xenogeneic IBMIR was characterized by rapid, platelet-independent thrombin generation, with addition of platelets both accelerating and exacerbating this response. Platelet-independent complement activation was observed as early as 30 minutes after NICC exposure to plasma. However, membrane attack complex formation was not observed in NICC histopathology sections until after 60 minutes. We demonstrated for the first time that NICC-mediated complement activation was necessary for neutrophil activation in the xenogeneic IBMIR setting. Finally, using the Seahorse extracellular flux analyzer, we identified substantial loss of islet function (up to 40%) after IBMIR with surviving NICC showing evidence of mitochondrial damage.

Conclusions

This study used novel assays to describe multiple key pathways by which xenogeneic IBMIR causes islet destruction, allowing further refinement of future interventions aimed at resolving the issue of IBMIR in xenotransplantation.

Expression of human leukocyte antigen class I in endocrine and exocrine pancreatic tissue at onset of type 1 diabetes

The cause of type 1 diabetes remains unknown. To dissect the link between hyperexpression of human leukocyte antigen (HLA) class I on the islet cells, we examined its expression in subjects with recent-onset type 1 diabetes. IHC showed seemingly pronounced hyperexpression in subjects with recent-onset type 1 diabetes, as well as in some nondiabetic subjects. In all subjects, HLA class I expression on exocrine tissue was low. However, no difference in the level of HLA class I expression was found between islet and exocrine tissue using Western blot, flow cytometry, real-time quantitative PCR, or RNA sequencing analyses. Also, the level of HLA class I expression on the messenger level was not increased in islets from subjects with recent-onset type 1 diabetes compared with that in nondiabetic subjects. Consistently, the HLA class I specific enhanceosome (NLRC5) and related transcription factors, as well as interferons, were not enhanced in islets from recent-onset type 1 diabetic subjects. In conclusion, a discrepancy in HLA class I expression in islets assessed by IHC was observed compared with that using quantitative techniques showing similar expression of HLA class I in islets and exocrine tissue in subjects with recent-onset type 1 diabetes, nor could any differences be found between type 1 diabetic and nondiabetic subjects. Results presented provide important clues for a better understanding on how this complex disease develops.

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.

Acceleration of wound healing by α-gal nanoparticles interacting with the natural anti-Gal antibody

Application of α-gal nanoparticles to wounds and burns induces accelerated healing by harnessing the natural anti-Gal antibody which constitutes ~1% of human immunoglobulins. α-gal nanoparticles present multiple α-gal epitopes (Galα1-3Galβ1-4GlcNAc-R), the carbohydrate ligand of anti-Gal. Studied α-gal nanoparticles were comprised of glycolipids with α-gal epitopes, phospholipids, and cholesterol. Binding of anti-Gal to α-gal nanoparticles in wounds activates the complement cascade, resulting in formation of chemotactic complement cleavage peptides that induce rapid recruitment of many macrophages. The Fc/Fcγ receptors interaction between anti-Gal coating α-gal nanoparticles and the recruited macrophages activates macrophages to produce cytokines/growth factors that promote wound healing and recruit stem cells. Studies of wound healing by α-gal nanoparticles were feasible in α1,3galactosyltransferase knockout mice and pigs. In contrast to other nonprimate mammals, these mice and pigs lack the α-gal epitope, and thus they are not immunotolerant to it and produce anti-Gal. Treatment of skin wounds and burns with α-gal nanoparticles resulted in 40-60% decrease in healing time in comparison with control wounds treated with saline. This accelerated healing is associated with increased recruitment of macrophages and extensive angiogenesis in wounds, faster regrowth of epidermis, and regeneration of the dermis. The accelerated healing further decreases and may completely eliminate fibrosis and scar formation in wounds. Since healing of internal injuries is mediated by mechanisms similar to those in external wound healing, it is suggested that α-gal nanoparticles treatment may also improve regeneration and restoration of biological function following internal injuries such as surgical incisions, myocardial ischemia following infarction, and nerve injuries.

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.

Gal/non-Gal antigens in pig tissues and human non-Gal antibodies in the GalT-KO era

Our knowledge regarding Gal and non-Gal antigens in GalT-KO pig tissues can be summarized as α3Galactosyl-tranferase gene knock out eliminates the Galα3Galβ4GlcNAc-R antigen expression in pig tissues as well as anti-Gal antibody binding. Other Galα-terminating saccharides (e.g. iGb3 glycolipids and Galα2 determinants) may be present but have not been documented. α3Galactosyl-tranferase gene knock out slightly changes the carbohydrate antigen expression but no "new" antigens recognized by the human immune system have been found. Non-Gal antigens are both of protein and carbohydrate nature but their exact chemical structures are poorly defined. Regarding human non-Gal antibodies our knowledge is as Non-Gal antibodies exist naturally and increase in humans/non-human primate (NHP) receiving WT or GalT-KO pig grafts. Non-Gal antibodies with new antigen epitope recognition can be induced in humans/NHP after challenge by WT or GalT-KO pig grafts. Non-Gal antibodies react with both carbohydrates and proteins. Part of the protein reactivity is directed to glycoprotein carbohydrates chains. Non-Gal antibodies reacting with neuraminic acid terminated saccharides (both N-Acetyl and N-Glycoloyl variants) are present in humans/NHP. Anti-neuraminic acid antibodies are increased, as well as induced, after grafting pig organs into humans/NHP. Non-Gal antibodies does not cause hyperacute xenorejection but can be cytotoxic and cause xenoorgan damage. If humans sensitized to HLA antigens are at a higher risk of rejecting pig xenograft compared with non-sensitized individuals is not fully clarified. Clinical trials are needed to evaluate the relevance of non-Gal antigens/antibodies and for the xenofield to advance.

Xenotransplantation: an overview of the field

Xenotransplantation, the transplantation of cells, tissues, or organs between different species, has the potential to overcome the current shortage of human organs and tissues for transplantation. In the last decade, the progress made in the field is remarkable, suggesting that clinical xenotransplantation procedures, particularly those involving cells, may become a reality in the not-too-distant future. However, several hurdles remain, mainly immunological barriers, physiological discrepancies, and safety issues, making xenotransplantion a complex and multidisciplinary discipline.

Islet xenotransplantation using gal-deficient neonatal donors improves engraftment and function

Significant deficiencies in understanding of xenospecific immunity have impeded the success of preclinical trials in xenoislet transplantation. Although galactose-α1,3-galactose, the gal epitope, has emerged as the principal target of rejection in pig-to-primate models of solid organ transplant, the importance of gal-specific immunity in islet xenotransplant models has yet to be clearly demonstrated. Here, we directly compare the immunogenicity, survival and function of neonatal porcine islets (NPIs) from gal-expressing wild-type (WT) or gal-deficient galactosyl transferase knockout (GTKO) donors. Paired diabetic rhesus macaques were transplanted with either WT (n = 5) or GTKO (n = 5) NPIs. Recipient blood glucose, transaminase and serum xenoantibody levels were used to monitor response to transplant. Four of five GTKO versus one of five WT recipients achieved insulin-independent normoglycemia; transplantation of WT islets resulted in significantly greater transaminitis. The WT NPIs were more susceptible to antibody and complement binding and destruction in vitro. Our results confirm that gal is an important variable in xenoislet transplantation. The GTKO NPI recipients have improved rates of normoglycemia, likely due to decreased susceptibility of xenografts to innate immunity mediated by complement and preformed xenoantibody. Therefore, the use of GTKO donors is an important step toward improved consistency and interpretability of results in future xenoislet studies.

Pig-to-nonhuman primates pancreatic islet xenotransplantation: an overview

The therapy of type 1 diabetes is an open challenging problem. The restoration of normoglycemia and insulin independence in immunosuppressed type 1 diabetic recipients of islet allotransplantation has shown the potential of a cell-based diabetes therapy. Even if successful, this approach poses a problem of scarce tissue supply. Xenotransplantation can be the answer to this limited donor availability and, among possible candidate tissues for xenotransplantation, porcine islets are the closest to a future clinical application. Xenotransplantation, with pigs as donors, offers the possibility of using healthy, living, and genetically modified islets from pathogen-free animals available in unlimited number of islets. Several studies in the pig-to-nonhuman primate model demonstrated the feasibility of successful preclinical islet xenotransplantation and have provided insights into the critical events and possible mechanisms of immune recognition and rejection of xenogeneic islet grafts. Particularly promising results in the achievement of prolonged insulin independence were obtained with newly developed, genetically modified pigs islets able to produce immunoregulatory products, using different implantation sites, and new immunotherapeutic strategies. Nonetheless, further efforts are needed to generate additional safety and efficacy data in nonhuman primate models to safely translate these findings into the clinic.

Towards xenotransplantation of pig islets in the clinic

PURPOSE OF REVIEW

Porcine islet xenotransplantation into humans faces two major hurdles - safety issues related to xenosis and xenorejection of the transplants. The former has been overcome mainly by selection of a suitable disease-free source herd.

RECENT FINDINGS

Four strategies have been employed to date to overcome the rejection, all of which have shown some efficacy in animal models.

SUMMARY

Immune suppression, Sertoli cell co-transplantation and microencapsulation have been tried in type 1 diabetic humans with some clinical benefit derived reported from the latter two. Unaware hypoglycemia in particular seems amenable to the microencapsulation approach.

Unexpected immunoresponse to Gal and APA antigens in diabetic type 1 patients receiving neonatal pig islets after 6 years

Cotransplantation of porcine islets and Sertoli cells into preimplanted subcutaneous devices improve metabolic control in type 1 diabetic patients, and survive grafted for more than 4 years. We report here, further assessment of the endocrine and porcine nature of the surviving cells and the immune responses elicited toward Gal alpha(1,3)-Gal beta(1,4)-GlcNAc (Gal) antigen in patients who received a second and third transplants. No immunosuppressive drugs were administered. We were able to immunostain insulin- and glucagon-positive cells in all biopsies of patients and Sertoli cell markers in 60.9% of biopsies. Additionally, all biopsies tested, amplified the porcine COII gene. Patients demonstrated an increase in antipig antibodies in response to the first transplant with a decreasing response toward the second and third transplants. In all transplants, the IgG levels promptly returned to basal values after 3-4 months. The long-term survival of porcine cells and the reduced humoral immune response to multiple transplants indicate a form of tolerance. We have not been able to find CD25-positive cells, indicating that it is probably an immune accommodation of the graft.

Gal knockout and beyond

Recently, Galalpha1-3Galbeta1-4GlcNAc (Gal) knockout (k/o) pigs have been developed using genetic cloning technologies. This remarkable achievement has generated great enthusiasm in xenotransplantation studies. This review summarizes the current status of nonhuman primate experiments using Gal k/o pig organs. Briefly, when Gal k/o pig organs are transplanted into primates, hyperacute rejection does not occur. Although graft survival has been prolonged up to a few months in some cases, the overall results were not better than those using Gal-positive pig organs with human complement regulatory protein transgenes. Gal k/o pig kidneys rapidly developed rejection which was associated with increased anti-non-Gal antibodies. Although the precise mechanisms of Gal k/o pig organ rejection are not clear, it could result from incomplete deletion of Gal, up-regulation of new antigen (non-Gal antigen) and/or production of non-Gal antibodies. Future work in xenotransplantation should place emphasis on further modification of donors, such as combining human complement regulatory genes with Gal k/o, deleting non-Gal antigens and adding protective/surviving genes or a gene that inhibits coagulation. Induction of donor-specific T- and B-cell tolerance and promotion of accommodation are also warranted.

Pig-to-nonhuman primate islet xenotransplantation: a review of current problems

Islet allotransplantation has been shown to have potential as a treatment for type 1 diabetic patients. Xenotransplantation, using the pig as a donor, offers the possibility of an unlimited number of islets. This comprehensive review focuses on experience obtained in pig-to-nonhuman primate models, particularly with regard to the different types of islets (fetal, neonatal, adult) and isolation procedures used, and the methods to determine islet viability. The advantages and disadvantages of the methods to induce diabetes (pancreatectomy, streptozotocin) are discussed. Experience in pig-to-nonhuman primate islet transplantation studies is reviewed, including discussion of the possible mechanisms of rejection and the immunosuppressive regimens used. The research carried out to date has led to workable animal models to study islet xenotransplantation, but several questions regarding methodology remain unanswered, and details of these practicalities require to be adequately addressed. The encouraging porcine islet survival reported recently provides an indicator for future immunosuppressive regimens.

Pancreatic islet transplantation for treating diabetes

Pancreatic islet transplantation is one of the options for treating diabetes and has been shown to improve the quality of life of severe diabetic patients. Since the Edmonton protocol was announced, islet transplantation have advanced considerably, including islet after kidney transplantation, utilisation of non-heart-beating donors, single-donor islet transplantation and living-donor islet transplantation. These advances were based on revised immunosuppression protocols, improved pancreas procurement and islet isolation methods, and enhanced islet engraftment. Further improvements are necessary to make islet transplantation a routine clinical treatment. To synergise efforts towards a cure for type 1 diabetes, a Diabetes Research Institute (DRI) Federation is currently being established to include leading diabetes research centres worldwide, including DRIs in Miami, Edmonton and Kyoto among others.

Identification of the V genes encoding xenoantibodies in non-immunosuppressed rhesus monkeys

The major immunological barrier that prevents the use of wild-type pig xenografts as an alternative source of organs for human xenotransplantation is antibody-mediated rejection. In this study, we identify the immunoglobulin variable region heavy (IgV(H)) chain genes encoding xenoantibodies to porcine heart and fetal porcine islet xenografts in non-immunosuppressed rhesus monkeys. We sought to compare the IgV(H) genes encoding xenoantibodies to porcine islets and solid organ xenografts. The immunoglobulin M (IgM) and IgG xenoantibody response was analysed by enzyme-linked immunosorbent assay and cDNA libraries from peripheral blood lymphocytes were prepared and sequenced. The relative frequency of IgV(H) gene usage was established by colony filter hybridization. Induced xenoantibodies were encoded by the IGHV3-11 germline progenitor, the same germline gene that encodes xenoantibodies in humans mounting active xenoantibody responses. The immune response to pig xenografts presented as solid organs or isolated cells is mediated by identical IgV(H) genes in rhesus monkeys. These animals represent a clinically relevant model to identify the immunological basis of pig-to-human xenograft rejection.

Pig islet xenotransplantation: activation of porcine endogenous retrovirus in the immediate post-transplantation period

BACKGROUND

Porcine endogenous retroviruses (PERV) are considered as the main infectious barrier in islet xenotransplantation. PERV has been shown to infect, but not to cause symptomatic disease in mice after islet transplantation. In vivo activation of PERV have so far not been examined. Expression of PERV was examined in adult and fetal porcine islets with or without the presence of known retroviral inducers or after transplantation to rats.

METHODS

Isolated adult and fetal porcine islets were cultured under normal conditions or in the presence of dexamethasone or 5-azacytidine and 5-iodo-2-deoxyuridine. PERV mRNA content was analyzed by real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and culture supernatants were analyzed for the presence of retroviral RT. Also, fetal islets were transplanted under the kidney capsule of immunocompetent or nude athymic rats. Expression of PERV mRNA in the grafts was evaluated by real-time quantitative RT-PCR. Infiltration of immunocompetent cells were evaluated by immunohistochemistry.

RESULTS

Both fetal and adult islets in culture produced small or even undetectable amounts of PERV mRNA and retroviral RT. PERV expression was not enhanced by retroviral inducers. In contrast, activation of PERV expression was observed the first day after transplantation of fetal islet-like cell clusters in both athymic and normal rats. PERV expression peaked after 1 to 3 days and was then rapidly returned to background levels. PERV expression neither correlated with the innate immune response seen in athymic rats nor with the specific process of rejection in normal rats.

CONCLUSION

Both fetal and adult islets produce low amounts of PERV mRNA in culture. After transplantation PERV expression is induced, seemingly independent of both the unspecific inflammatory response and the specific T-cell-mediated rejection process. It is speculated that PERV expression is correlated with the level of hypoxia in the islet xenograft.

Is the expression of Gal-alpha1,3Gal on porcine pancreatic islets modified by isolation procedure?

AIM

The aim of this study was to study the Galactosyl alpha(1,3) Gal expression and the vascular tissue distribution prior to and after isolation of porcine pancreatic islets.

METHODS

Biopsies of native pancreas were carried out in young (12-15 weeks; n = 4) and adult Landrace pigs (2 years old; n = 7). These pancreases were then digested (Liberase Porcine Islets [PI]) to obtain isolated pancreatic islets from each pancreas. Alpha Gal-specific biotinylated BS-1 isolectin B4 and von Willebrand's Factor (vWF) staining were performed for Galactosyl and vascular structure analysis, respectively. Quantitative Galactosyl expression as well as location of the vascular structure were determined using image analysis in pig islets of different sizes.

RESULTS

Vascular structures and Galactosyl expression varied following the islet sizes but not the pig age. In large islets (>100 microm), capillaries were mainly located within the islets, whereas in small islets (<100 microm), 4-fold more vessels were situated at the periphery of the islets. Galactosyl expression followed a comparable distribution than vascular tissue in small and large islets. After isolation, a significant decrease of Gal staining (-49%) was observed, but Galactosyl expression remained positive within both small and large islets.

CONCLUSIONS

Galactosyl expression is maintained within pancreatic islets after isolation procedure. Gal knock-out pigs could represent the solution to this hurdle.

The endothelial cells in islets of langerhans

The blood vessels of the pancreatic islets are of crucial importance for oxygen and metabolite supply, and dispersal of secreted hormones. In addition to this, endothelial cells have an important role in the revascularization process after islet transplantation. Studies have reported signs of poor engraftment of transplanted islets, presumably due to impaired revascularization. The aims of this study were to investigate islet endothelial cells and the revascularization process of transplanted islets. The lectin Bandeiraea simplicifolia was found to consistently stain endothelium of both endogenous and transplanted pancreatic islets. By using this marker, we investigated the vascular density of both endogenous and transplanted islets of C57BL/6 mice. One month post-transplantation, a time point when the implants are assumed to be completely revascularized, the graft vascular density was decreased at all investigated implantation sites when compared to endogenous islets. Furthermore, most of the blood vessels were located in the graft connective tissue stroma. Similar results were obtained six months post-transplantation and in cured diabetic animals after one month. In order to evaluate the function of intraportally transplanted islets, we developed a method to retrieve such islets. Enzymatic and mechanic treatment of the liver enabled us to re-isolate the transplanted islets for further in vitro studies. These islets had decreased insulin release, insulin content and glucose oxidation rate when compared to non-transplanted control islets. To understand the role of islet endothelium in the revascularization of transplanted islets we performed angiogenesis microarray studies on islet endothelial cells, from non-cultured, cultured and transplanted islets. We found that the islet endothelium expressed mRNA for both inhibitors and inducers of angiogenesis, and that this expression differed with time. In conclusion, these results provide a useful platform for further studies on the islet endothelium.

Reversal of diabetes in non-immunosuppressed rhesus macaques by intraportal porcine islet xenografts precedes acute cellular rejection

BACKGROUND

The functional response and immunobiology of primarily non-vascularized islet cell xenografts remain poorly defined in non-human primates.

METHODS

We transplanted 20,000 adult porcine islet equivalents/kg (purified and cultured for 48-h) intraportally into six streptozotocin-diabetic and two non-diabetic rhesus macaques. Two recipients were killed at various intervals post-transplant for histologic examination of livers bearing xenografts.

RESULTS

Plasma glucose levels in diabetic recipients averaged 94 mg/dl at 12 h, 92 mg/dl at 24 h, 147 mg/dl at 48 h, and 157 mg/dl at 72 h post-transplant. Serum porcine C-peptide was present in eight of eight recipients at 12 h, in five of six at 24 h, in four of four at 48 h, and in one of two at 72 h post-transplant. C3a and SC5b-9 plasma levels increased at 12 h post-transplant and returned to pre-transplant levels by 24 h. IgG, IgM anti-pig and anti-Gal IgG serum antibody levels did not increase post-transplant. Rejection was initiated by IgM and complement deposition on islets. Neutrophils dominated the cellular infiltrate at 12 h; CD4+ and CD8+ T cells were the main infiltrating cells at 24, 48, and 72 h; and macrophages increasingly infiltrated xenografts starting at 24 h post-transplant. Numerous xenoislets were present at all time points; their proportion without intraislet infiltrates decreased from 65% at 24 h to 17% at 72 h post-transplant.

CONCLUSIONS

Pig-to-primate intraportal islet xenografts reverse diabetes and the majority of intraportally transplanted xenogeneic islets are not subject to hyperacute rejection. They undergo acute cellular rejection mediated by CD4+- and CD8+ T cells and macrophages.

Natural antibodies prevent in vivo transmission of porcine islet-derived endogenous retrovirus to human cells

The discovery of porcine endogenous retroviruses (PERV) has raised concerns regarding the safety of porcine xenotransplantation. However, transmission of PERV had not been observed in humans exposed to porcine tissue. We examined whether PERV derived from porcine pancreatic islet cells could infect human cells in vivo and the role of natural antibodies in inhibiting PERV infection. In vivo infective potential of PERV was studied in SCID mice reconstituted with human peripheral blood leucocytes. Porcine islets were transplanted under the kidney capsule. PERV infection was determined by analyzing PERV gene expression in graft infiltrating lymphocytes (GIL) harvested 21 days posttransplantation. Mice were administered normal human serum prior to and 2 days posttransplantation to study their role in protection of human cells against PERV infection. PERV genes were expressed in all porcine tissues examined, including purified porcine islets. PERV expression was detected in GILs from three of five human-SCID mice. Administration of human serum blocked PERV infection in GILs in five of five human-SCID mice. These results indicate that PERV from porcine islets can infect human cells in vivo. Normal human serum blocks transmission of retrovirus in vivo, suggesting that natural xenoreactive antibodies can prevent PERV infection.

Gal alpha 1,3Gal expression on porcine pancreatic islets, testis, spleen, and thymus

Gal alpha 1,3Gal (Gal) is the first target in antibody-mediated rejection of pig-to-non-human primate xenograft. Its expression may vary between organs and constituents of organs. Gal expression was studied in pancreas, testis, spleen and thymus of 22 pigs, with ages ranging from 1 to 22 months. The immunoperoxidase technique using the biotinylated lectin, Griffonia simplicifolia (IB4), was used. In the pancreas, neither endocrine (islet cells) nor exocrine cells expressed Gal. The Sertoli cells in the testis were negative. The spleen capsule and trabeculae did not stain for Gal, although both splenic T and B lymphocytes expressed Gal (B > T). Thymocytes were weakly positive, whereas thymic epithelial cells were negative for Gal. No age-related differences were seen in any tissues. Porcine islets of Langerhans, Sertoli cells, and the splenic and thymic structural frameworks did not express Gal, and therefore, should be relatively resistant to anti-Gal antibody-mediated rejection. The availability of pigs deficient in Gal as a source of islets may therefore not be beneficial in extending islet graft survival in non-human primate models.

Biological activity of pig islet-cell reactive IgG antibodies in xenotransplanted diabetic patients

BACKGROUND

The IgG antibody response in type I diabetic patients, transplanted with fetal pig islet-like cell-clusters, was investigated using purified immunoglobulin G (IgG) fractions from sera collected 7 to 9 yr after transplantation. From our earlier studies, we knew that the immunological specificities of xenoreactive IgG1 and IgG2 antibodies are different, and that IgG1 antibodies, in contrast to the IgG2 population, are mainly directed against non-Galalpha1,3Gal epitopes. In this study our goal was to establish whether xenoreactive IgG1 and IgG2 antibodies react with pig islet cells and, if so, to identify the target cell type, the biological function as well as the specificity of such antibodies for islet cell antigens. Sera from xenotransplanted patients were compared with those of patients with diabetes, selected for high titres of islet-cell specific autoantibodies.

METHODS

IgG antibody fractions from patient sera were purified on a protein G column. Surface expression of target antigens was studied using flow cytometry as well as immunofluorescence microscopy. The biological function of islet-cell reactive sera was tested using antibody dependent cellular cytotoxicity with both xenogeneic adult pig islet cells and allogeneic human islet cells as targets. Antibody specificity was assessed using 2D Western blots with both fetal and adult pig islet as well as human islet cell antigenic preparations.

RESULTS

Some of the diabetic patients, who have been transplanted with xenogeneic fetal pig islet cells, continue to produce xenospecific IgG1 and IgG2 antibodies for 7 to 9 yr post-transplantation. A separate analysis of IgG1 and IgG2 antibodies showed that IgG1 antibodies react with pig islet beta cells, whereas IgG2 antibodies mainly react with non-endocrine pig cells. Such antibodies are xenospecific, as they were found to mediate antibody-dependent cellular cytotoxicity of adult pig, but not human islet target cells. The reverse was true for antibodies from non-transplanted diabetic patients with high titres of autoantibodies against beta cells. Fluorescence analysis as well as 2D gel Western blots revealed that the reactivity was variable between patient samples, indicating that target antigens for non-Galalpha1,3Gal-specific antibodies are heterogeneous.

CONCLUSION

Thus, xenotransplantation of diabetic patients induces islet-beta cell reactive xenospecific IgG1 antibodies, which are biologically active and can mediate antibody-dependent cellular cytotoxicity of pig islet cells.

Importance of the Gal alpha1-3 Gal antigen in discordant islet xenotransplantation: immunosuppression, which inhibits porcine islet xenograft rejection in ordinary mice, is equally effective in Gal-knockout mice

BACKGROUND

Islet xenotransplantation will most likely be performed in diabetic patients treated with immunosuppressive drugs. The importance of the galactosyl alpha(1-3) galactose (Galalpha1-3Gal) antigen in immunosuppressed islet xenograft recipients has not been studied.

METHODS

Fetal porcine islet-like cell clusters (ICCs) were transplanted into the renal subcapsular space of both Gal-knockout mice and ordinary mice. Transplantations were performed in untreated mice and mice immunosuppressed with cyclosporine A (CsA) plus 15-deoxyspergualin (DSG). Studies were also performed in immunosuppressed Gal-knockout mice that had been actively immunized against Galalpha1-3Gal. Evaluation was performed 12 days after transplantation using morphologic techniques. The levels of serum immunoglobulin (Ig)G and IgM to the Galalpha1-3Gal antigen or to the ICCs were determined.

RESULTS

No difference in the morphologic appearance could be seen between ordinary mice and Gal-knockout mice. No deposits of IgG, IgM, or C3 could be detected. Almost no difference could be seen between immunosuppressed Gal-knockout mice and immunosuppressed ordinary mice. In immunosuppressed, immunized Gal-knockout mice, the results were similar. In ordinary mice treated with CsA+DSG, the levels of anti-Gal IgM were lower than they were in untreated mice, whereas the levels of anti-Gal IgG were similar. In Gal-knockout mice (including immunized animals) treated with CsA+DSG, the levels of anti-Gal IgG and IgM were lower than they were in untreated Gal-knockout mice.

CONCLUSIONS

After renal subcapsular transplantation, antibodies against Galalpha1-3Gal have no major influence on islet xenograft rejection in the pig-to-mouse model. Immunosuppression, which inhibits rejection in the pig-to-mouse model, is equally effective when transplantation is performed across the Galalpha1-3Gal barrier.

Evidence that tilapia islets do not express alpha-(1,3)gal: implications for islet xenotransplantation

BACKGROUND

Cell therapy for diabetes using teleost fish islet tissue has emerged as an intriguing alternative to the use of islet tissue from mammalian pancreases. The islet tissue, called Brockman bodies (BBs), is anatomically distinct from the pancreatic exocrine tissue and can be easily identified and isolated. Islets harvested from Nile tilapia (Oreochromis niloticus), when transplanted into streptozotocin-diabetic nude mice, produce long-term normoglycemia and achieve mammalian-like glucose tolerance profiles. We asked whether tilapia express the alpha-(1,3)gal epitope, the immunodominant target of human xenogeneic responses.

METHODS AND RESULTS

Immunostaining with the alpha-(1,3)gal-specific IB4 lectin on tilapia BB, liver, heart, spleen, and head kidney was negative, as was staining with murine anti-alpha-gal-specific monoclonal antibodies. Absence of alpha-gal-specific binding of IB4 or murine anti-gal mAbs to dispersed BBs was confirmed by fluorescent-activated cell sorter analysis. Tilapia BB cell membranes failed to reduce binding of anti-alpha-(1,3)gal-specific mAb in an enzyme-linked immunosorbent assay (ELISA) inhibition assay, while porcine and murine tissue lysates did. Tilapia BB cell lysates were shown to be devoid of alpha-1,3 galactosyltransferase activity by ELISA. Transplantation of tilapia BBs into diabetic alpha-gal knockout (gal KO) mice was not associated with accelerated xenograft rejection when compared with wild type control recipients (mean survival time 6.5 days vs. 7.2 days). Tilapia BBs failed to induce a rise in anti-gal IgG and IgM titers in gal KO mice, while the transplant of wild type mouse islets into gal KO mice caused a significant rise in anti-gal IgG and IgM antibodies.

CONCLUSIONS

We conclude that tilapia BBs are devoid of alpha-gal expression, and may offer an alternative to swine as a donor species for islet xenotransplantation.

T-cell-specific immunosuppression results in more than 53 days survival of porcine islets of langerhans in the monkey

BACKGROUND

Transplantation of islets of Langerhans can restore insulin production in diabetic patients. Because of the shortage of human donor organs, transplantation of porcine islets may be an alternative solution. The present study was aimed at the characterization of rejection mechanisms of porcine islets transplanted into eight nondiabetic monkeys under the kidney capsule.

METHODS

Cultured adult pig islets were used, which showed no expression of the galactose(alpha1,3)galactose epitope, major histocompatibility complex class II, or CD45, and no binding of antibodies or complement after exposure to monkey serum. Immunosuppression consisted of cyclophosphamide, cyclosporine A (CsA), and steroids (group 1); or antithymocyte globulin, anti-interleukin-2 receptor antibody, CsA, and steroids (group 2). In three animals of group 2, islets were also transplanted in the portal vein.

RESULTS

Although all monkeys had preformed anti-pig antibodies, no correlation was found between antibody titers and rejection and no deposition of antibodies or complement was observed in the grafts. Group 1 showed islets up to day 11, followed by T-cell infiltration and rejection at approximately day 14. In group 2, two monkeys showed infiltrates consisting predominantly of T cells starting at approximately day 29, whereas two monkeys showed well-preserved islets without infiltration up to day 53. In the livers of the three monkeys that also received islets intraportally and were resectioned on days 21, 33, and 49, no islets could be detected.

CONCLUSIONS

This study demonstrates that cultured adult pig islets can survive in the monkey for more than 53 days without signs of rejection under standard immunosuppression.

Xenogeneic transplantation of porcine islets: an overview

The extreme demand for human organs or tissues for transplantation has driven the search for viable alternatives. Pigs are considered a possible source of tissue for a number of reasons including shared physiology, plentiful supply, short gestation, and, more recently, the generation of transgenic animals. Porcine islets show promise as a source of islets for the treatment of type 1 diabetes mellitus. Porcine islets regulate glucose levels in the same physiologic range as humans, and porcine insulin has been used for years as an exogenous source of insulin for glucose control. In this review, we discuss the advantages and disadvantages of the use of adult or neonatal porcine islets, the immunologic challenges facing transplantation of xenogeneic islets, and the concerns regarding transmission of infectious agents between species. Porcine islets isolated from both adult and neonatal pigs are capable of restoring euglycemia in experimental animal models of diabetes. Adult islets are more difficult to isolate, whereas neonatal islets have great proliferation potential but require several weeks to function posttransplantation. Xenogeneic islets are susceptible to complement-mediated lysis after the binding of preformed natural antibodies and cellular immunity involving both macrophages and CD4+ T cells. In addition, the potential for transmission of porcine endogenous retroviruses, porcine cytomegalovirus, and porcine lymphotropic herpesvirus type 1 are all concerns that must be addressed. Despite the challenges facing xenotransplantation, the extreme need for donor organs and tissues continues to drive progress toward overcoming the unique issues associated with transplantation between species.

Adenovirus-mediated expression of human CD55 or CD59 protects adult porcine islets from complement-mediated cell lysis by human serum

BACKGROUND

Protection against complement activation may reduce acute islet damage in pig-to-human islet xenotransplantation. Expression of the human complement regulatory proteins decay-accelerating factor (DAF, CD55) or CD59 was induced on intact adult porcine islets (APIs) by adenoviral transduction. The functional capacity of the transgenes was examined in vitro after exposure to fresh human serum.

METHODS

Intact APIs were transduced with adenoviral vectors Ad.hDAF or Ad.hCD59 or a control vector. After 3 days, the islets were trypsin dissociated to a single-cell suspension. A cytotoxicity assay was performed in which the islet cells were incubated with human complement active AB serum. Flow cytometry and immunohistochemistry were used to evaluate transgene expression.

RESULTS

APIs could be transduced to express hDAF or hCD59. Flow cytometry analysis of islet single cells revealed that only a fraction of the cells expressed the transgene; immunohistochemical staining of transduced islets demonstrated that mainly cells located in the periphery of the islets were expressing the protein. Cells from nontransduced islets or islets expressing the control protein were sensitive to lysis in human sera (66+/-4.0% and 73+/-3.7% cytotoxicity, respectively). Single cells from islets transduced with hDAF and hCD59 were partially protected from lysis. Islet cells expressing hCD59 were slightly less sensitive to lysis (33+/-3.3%) than cells expressing hDAF (45+/-3.5%).

CONCLUSIONS

These data show that intact pig islets can be transduced to express human regulators of complement activation on the surface and that pig islet cells expressing hDAF or hCD59 are less sensitive to complement-mediated lysis.

Clinical xenotransplantation: pigs might fly?

Xenotransplantation has the potential to deliver an unlimited supply of organs for transplantation. However, this promise has yet to translate into clinical application, despite substantial research efforts in the last decade. Although increasing numbers of studies are being performed in relevant pre-clinical (pig-to-primate) transplantation models, so far these have highlighted the apparent elusiveness of long-term xenograft survival. Humoral rejection remains the main obstacle to success, but control of T cell-mediated rejection will be a problem in the future and there are major concerns about the possible transmission of porcine endogenous retroviruses (PERV) and other infectious agents. This article reviews recent advances in the understanding of acute vascular rejection (AVR), acute T cell-mediated rejection and PERV transmission and highlights some of the strategies that may prove successful in overcoming these problems. Although progress has been slow, the promise of an inexhaustible supply of organs is sufficient reason to continue research in these areas. Assuming the specific problem of AVR can be ameliorated by one of a number of strategies currently under investigation, there are grounds to believe that xenotransplantation will become a clinical reality. Pig xenografts, currently grounded, might eventually fly!

Decreased vascular density in mouse pancreatic islets after transplantation

An adequate revascularization is crucial for islet survival and function after transplantation. Previous studies have suggested that islet revascularization is concluded within 14 days after transplantation. We investigated if the vascular density of transplanted islets and endogenous pancreatic islets differs. Cultured islets were syngeneically transplanted into the kidney, liver, or spleen of C57BL/6 mice. One month later, the graft-bearing organ was removed, and histological specimens were prepared and stained for endothelium with the lectin Bandeiraea simplicifolia. Pancreata from nontransplanted control animals were prepared similarly. Uniform staining of endothelium within the grafts and endogenous islets was obtained. The vascular density was markedly decreased in transplanted islets at all implantation sites, but preferentially in islets implanted into the spleen. The vascular density in the connective tissue surrounding the transplanted islets was very high compared with that of graft intra-islet capillaries. A much lower vascular density was detected in connective tissue surrounding implanted microspheres of a size similar to the islets, which suggests that the islets per se induced blood vessel formation in their vicinity. We conclude that the vascular density in revascularized transplanted islets is markedly decreased compared with endogenous islets. This has potential implications for islet graft metabolism and function.

Alpha Gal is widely expressed in embryonic porcine stem cells and neural tissue

Fetal porcine neural xenografts are an alternative to human fetal tissue for cell based treatments of a number of neurodegenerative conditions but are currently limited by host immunological rejection. The expression of a major immunological epitope, Galalpha1-3Galbeta1-4GlcNAcbeta-R (alphaGal) was determined on stem cells and primary cells derived from E26 porcine fetal brains. alphaGal was detected on the majority of neural stem cells and cells from primary cell suspensions. The expression of this epitope paralleled the binding of human IgG and IgM to the cells, a binding that was significantly reduced with anti-alphaGal depleted human serum. This study demonstrates that alphaGal expression is extensive in embryonic porcine neural cells and will be of relevance to any clinical trials using this tissue.

Anti-pig antibody levels in naïve baboons and cynomolgus monkeys

Anti-pig antibodies (APA) were analysed in serum from 28 naïve wild-caught baboons (originating from Kenya) and 31 naïve captive-bred cynomolgus monkeys (13 from the Philippines and 18 from Mauritius), using a haemolytic assay with pig erythrocytes (APA), flow cytometry on the porcine lymphoma T-cell cell line L35, and enzyme linked immunosorbent assay (ELISA) using alpha-Gal type II and type VI antigen. This was extended in baboon samples by the evaluation in two laboratories (Imutran, Cambridge, UK and Immerge, Boston, USA), and by antibody absorption using either immobilized alpha-Gal type II or alpha-Gal type VI. Anti-porcine antibodies were demonstrated in all assays with substantial variability within and between the three non-human primate groups. Immunoglobulin (Ig)M antibody levels tended to be similar to or higher than those in a pooled normal human standard serum while IgG levels tended to be lower. Highest antibody levels were recorded in Mauritius cynomolgus monkeys. There were statistically significant correlations between assays for IgM or IgG class anti-Gal antibodies using either alpha-Gal type II or alpha-Gal type VI as antigen, both for different assays and two laboratories involved. Also, significant correlations were observed between the anti-Gal and L35 binding assays. Baboon sera before and after absorption to immobilized alpha-Gal type II or type VI were analysed for anti-Gal type VI or type II antibody: levels were almost undetectable indicating that most anti-Gal antibodies react to epitopes shared between alpha-Gal type II and type VI oligosaccharides. Finally, the relation between APA and outcome of porcine heart xenotransplantation in cynomolgus monkeys and baboons showed no apparent relation between pre-transplant APA levels and the occurrence of hyperacute rejection (HAR) when compared with non-immunological cause of organ/recipient dysfunction or acute humoral xenograft rejection during the first 4 days post-transplantation or survival exceeding 4 days post-transplantation.

Hepatic sinusoidal endothelium upregulates IL-1alpha, IFN-gamma, and iNOS in response to discordant xenogeneic islets in an in vitro model of xenoislet transplantation

BACKGROUND

Data indicate that early islet graft failure is due to nonspecific inflammatory mechanisms that occur prior to T-cell-mediated rejection. The role of the host hepatic endothelium in mediating this immediate islet injury has not been elucidated. The endothelial cell may be important in this process because it is essentially the first cellular barrier encountered by intraportally introduced islets. We have characterized the initial response of hepatic endothelium to xenogeneic islets by measuring the expression of Il-1alpha, TNF-alpha, IFN-gamma, and iNOS in an in vitro dog-to-pig model of xenoislet transplantation.

MATERIALS AND METHODS

Dog islets (500 islet equivalents) were cocultured with either porcine hepatic endothelium or porcine aortic endothelium over a 24-h period in serum-free medium. RNA was extracted at eight time points (0, 1, 2, 4, 6, 8, 12, and 24 h). Reverse-transcriptase polymerase chain reaction was performed on each sample. Polymerase chain reaction was done on the cDNA in order to visualize Il-1alpha, TNF-alpha, IFN-gamma, and iNOS expression. Bands were semiquantitated by comparison to an external standard (GAPDH) using band densitometry.

RESULTS

Hepatic endothelium had early (1 h) expression of IL-1alpha, IFN-gamma, and iNOS. IL-1alpha peaked at 2 h, IFN-gamma at 12 h, and iNOS at 1 and 12 h. Aortic endothelium expressed low levels of IL-1alpha and TNF-alpha, but not IFN-gamma or iNOS.

CONCLUSIONS

We have demonstrated that xenogeneic islets are able to activate host endothelial cells without serum or immune cells. The observed endothelial response corresponds with known islet toxic substances. Furthermore, the response differs between hepatic and aortic endothelial cells, suggesting that these differences may be important in choosing suitable implantation sites for islets. Our findings suggest that host endothelium may play an important part in early injury of islet xenotransplants.

Transgenic pigs designed to express human CD59 and H-transferase to avoid humoral xenograft rejection

Research in pig-to-primate xenotransplantation aims to solve the increasing shortage of organs for human allotransplantation and develop new cell- and tissue-based therapies. Progress towards its clinical application has been hampered by the presence of xenoreactive natural antibodies that bind to the foreign cell surface and activate complement, causing humoral graft rejection. Genetic engineering of donor cells and animals to express human complement inhibitors such as hCD59 significantly prolonged graft survival. Strategies to decrease the deposition of natural antibodies were also developed. Expression of human alpha1,2-fucosyltransferase (H transferase, HT) in pigs modifies the cell-surface carbohydrate phenotype resulting in reduced Galalpha1,3-Gal expression and decreased antibody binding. We have developed transgenic pigs that coexpress hCD59 and HT in various cells and tissues to address both natural antibody binding and complement activation. Functional studies with peripheral blood mononuclear cells and aortic endothelial cells isolated from the double transgenic pigs showed that coexpression of hCD59 and HT markedly increased their resistance to human serum-mediated lysis. This resistance was greater than with cells transgenic for either hCD59 or HT alone. Moreover, transgene expression was enhanced and protection maintained in pig endothelial cells that were exposed for 24 h to pro-inflammatory cytokines. These studies suggest that engineering donor pigs to express multiple molecules that address different humoral components of xenograft rejection represents an important step toward enhancing xenograft survival and improving the prospect of clinical xenotransplantation.

Gene transfer of the Bcl-2 gene confers cytoprotection to isolated adult porcine pancreatic islets exposed to xenoreactive antibodies and complement

BACKGROUND

Exposing adult porcine pancreatic islets (PI) to xenoreactive natural antibodies (XNA) induces brisk inflammatory injury that involves activation of the complement system. Gene transfer of Bcl-2 has been shown to protect PI from apoptosis and necrosis in several models. In this study, we investigated the effect of Bcl-2 gene transfer on protection of PI from primate XNA and complement-mediated injury.

METHODS

The PI were isolated from adult female sows. Only islet preparations that exhibited >90% viability and purity were used. Fresh rhesus monkey serum served as the XNA source. Gene transfer of Bcl-2 was achieved with an adenoviral vector (AdBcl-2) at 500 particle forming units (pfu)/cell. The Bcl-2 expression was confirmed by Western blot technique. Untransfected and transfected PI were incubated in 50% fresh complete serum (CS) or heat-inactivated (HI) rhesus serum for 24 hours. The PI viability was analyzed with acridine orange and ethidium bromide staining. Antibody and complement-mediated cytotoxicity were tested by intracellular lactate dehydrogenase (LDH) release. The PI function was assessed in vitro by static incubation studies and in vivo after intraportal transplantation in diabetic severe combined immunodeficiency (SCID) mice.

RESULTS

The AdBcl-2 gene transfer resulted in Bcl-2 gene expression in >90% of PI cells. Following exposure to XNA, <15% of the untransfected cells were viable. Similar results were obtained in PI transfected with a similar recombinant adenovirus encoding the reporter gene E coli beta-galactosidase (AdLacZ), an irrelevant gene. A significant increase in LDH release was observed in control PI after exposure to CS compared with PI that overexpressed Bcl-2 (82.89% +/- 7.78% vs 34.31% +/- 5.4%, P <.005). Higher insulin release was observed in vitro in PI transfected with Bcl-2 compared with untransfected PI or islets transfected with AdLacZ (stimulation index of 0.9 +/- 0.31, 0.9 +/- 0.3 vs 2.67 +/- 0.4, respectively). Only PI treated with AdBcl-2 were able to achieve euglycemia after exposure to XNA and complement after transplantation.

CONCLUSIONS

Transfer of the antiapoptotic and antinecrotic Bcl-2 gene into PI can reduce primate XNA and complement-mediated lysis. Cytoprotection of PI with Bcl-2 has potential to improve survival of PI xenotransplants.

Expression of complement regulatory proteins on islets of Langerhans: a comparison between human islets and islets isolated from normal and hDAF transgenic pigs

BACKGROUND

The expression of regulators of complement activity (RCAs) on islet cells may be of great importance for protecting them against complement-mediated lysis in the immediate posttransplant period after intraportal islet transplantation. We examined porcine and human islet cells for expression of RCA. We also examined to what extent human decay accelerating factor (hDAF) is expressed on adult and fetal islet cells isolated from hDAF transgenic (TG) pigs having a high transgene expression on endothelial cells. Moreover, the susceptibility of the various types of cells to lysis in human serum and blood was investigated.

METHODS

Adult human islets (n=5), normal adult and fetal porcine islets (n=9 and n=8, respectively), and islets from adult and fetal hDAF TG pigs (n=5 and n=6, respectively) were examined. With islet single-cell suspensions and flow cytometry, adult human islet cells were examined for expression of hDAF (CD55), hCD59, and human membrane cofactor protein (hMCP; CD46), while porcine islet cells were examined for expression of pCD59 and pMCP. Islet cells from hDAF TG pigs were also examined for hDAF expression. Porcine peripheral blood lymphocytes, normal and hDAF TG porcine endothelial cell lines, a human endothelial cell line, and the human cell line U937 served as controls. Islet cytotoxicity was assayed after incubation of the islet cells with fresh human serum. Furthermore, adult islets from normal control pigs and hDAF TG pigs were exposed to fresh human blood in vitro for 60 min, and the inflammatory reaction elicited was compared between the different types of islets.

RESULTS

All human islet cell preparations expressed hCD59, two of five expressed hMCP, but none expressed hDAF. Porcine islet cells expressed both pCD59 and pMCP. Normal adult porcine islet cells exposed to fresh human serum resulted in 74+/-5.4% cell lysis (mean+/-SEM, n=16). In comparison, only 1.3+/-2.8% (n=20, P<0.001) of human islet cells were lysed in the human serum. One islet cell preparation from an hDAF TG pig expressed small amounts of hDAF. This preparation from hDAF TG pigs bound significantly less C3c than did normal control islets (mean fluorescence ratio 16+/-2.2 and 58+/-4.3, respectively; P=0.046) and were partially protected from cell lysis in fresh human serum (47+/-10% and 78+/-18% cell lysis, respectively; P=0.046). The other four preparations from hDAF TG pigs were negative for hDAF and were equally susceptible to lysis as normal control islets. All fetal pancreatic islet cells from hDAF TG pigs analyzed were negative for hDAF expression. When exposed to fresh human blood in vitro, adult and fetal islets from hDAF TG pigs elicited equally strong inflammatory changes as did the normal control islets. The inflammatory changes were characterized by activation of the complement and coagulation systems, resulting in islet damage with "dumping" of insulin into the blood.

CONCLUSIONS

Porcine and human islet cells express species-restricted complement regulatory proteins, with the human islet cells expressing mainly hCD59. A low expression of hDAF was detected on islet cells from one of five hDAF TG pigs. These islet cells displayed reduced islet cell cytotoxicity in fresh human serum. We conclude that protection from complement-mediated lysis will be important in the context of intraportal pig-to-human islet transplantation, and expression of a human RCA on islet cells should be beneficial in this context.

Bioartificial pancreas: materials, devices, function, and limitations

The term "bioartificial endocrine pancreas" (BEP) was introduced by Anthony Sun in 1980. It was in 1968, however, that Thomas Chang proposed the use of microencapsulated islets as artificial beta-cells. By applying a semipermeable membrane on the top of microcapsules, a system can be produced that is impermeable to viable islet cells and large effector molecules of the immune system, thus providing a protection for transplanted islets against rejection. Since then, the term BEP has not often appeared in papers. Instead, the term "bioartificial pancreas" (BAP) has gained widespread use. In a broader sense, BAP would include an application of suitable endocrine cells and protective polymeric vehicles, but not necessarily providing a filtration barrier of precisely defined properties (e.g., cells injected into a gel of hyaluronate).