Adult porcine islet transplantation in baboons treated with conventional immunosuppression or a non-myeloablative regimen and CD154 blockade

Encapsulation and immune protection for type 1 diabetes cell therapy

Type 1 Diabetes (T1D) involves the autoimmune destruction of insulin-producing β-cells in the pancreas. Exogenous insulin injections are the current therapy but are user-dependent and cannot fully recapitulate physiological insulin secretion dynamics. Since the emergence of allogeneic cell therapy for T1D, the Edmonton Protocol has been the most promising immunosuppression protocol for cadaveric islet transplantation, but the lack of donor islets, poor cell engraftment, and required chronic immunosuppression have limited its application as a therapy for T1D. Encapsulation in biomaterials on the nano-, micro-, and macro-scale offers the potential to integrate islets with the host and protect them from immune responses. This method can be applied to different cell types, including cadaveric, porcine, and stem cell-derived islets, mitigating the issue of a lack of donor cells. This review covers progress in the efforts to integrate insulin-producing cells from multiple sources to T1D patients as a form of cell therapy.

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.

Xenotransplantation of Genetically Modified Neonatal Pig Islets Cures Diabetes in Baboons

Xenotransplantation using porcine donors is rapidly approaching clinical applicability as an alternative therapy for treatment of many end-stage diseases including type 1 diabetes. Porcine neonatal islet cell clusters (NICC) have normalised blood sugar levels for relatively short periods in the preclinical diabetic rhesus model but have met with limited success in the stringent baboon model. Here we report that NICC from genetically modified (GM) pigs deleted for αGal and expressing the human complement regulators CD55 and CD59 can cure diabetes long-term in immunosuppressed baboons, with maximum graft survival exceeding 22 months. Five diabetic baboons were transplanted intraportally with 9,673 – 56,913 islet equivalents (IEQ) per kg recipient weight. Immunosuppression consisted of T cell depletion with an anti-CD2 mAb, tacrolimus for the first 4 months, and maintenance with belatacept and anti-CD154; no anti-inflammatory treatment or cytomegalovirus (CMV) prophylaxis/treatment was given. This protocol was well tolerated, with all recipients maintaining or gaining weight. Recipients became insulin-independent at a mean of 87 ± 43 days post-transplant and remained insulin-independent for 397 ± 174 days. Maximum graft survival was 675 days. Liver biopsies showed functional islets staining for all islet endocrine components, with no evidence of the inflammatory blood-mediated inflammatory reaction (IBMIR) and minimal leukocytic infiltration. The costimulation blockade-based immunosuppressive protocol prevented an anti-pig antibody response in all recipients. In conclusion, we demonstrate that genetic modification of the donor pig enables attenuation of early islet xenograft injury, and in conjunction with judicious immunosuppression provides excellent long-term function and graft survival in the diabetic baboon model.

Advances in Pancreatic Islet Transplantation Sites for the Treatment of Diabetes

Diabetes is a complex disease that affects over 400 million people worldwide. The life-long insulin injections and continuous blood glucose monitoring required in type 1 diabetes (T1D) represent a tremendous clinical and economic burdens that urges the need for a medical solution. Pancreatic islet transplantation holds great promise in the treatment of T1D; however, the difficulty in regulating post-transplantation immune reactions to avoid both allogenic and autoimmune graft rejection represent a bottleneck in the field of islet transplantation. Cell replacement strategies have been performed in hepatic, intramuscular, omentum, and subcutaneous sites, and have been performed in both animal models and human patients. However more optimal transplantation sites and methods of improving islet graft survival are needed to successfully translate these studies to a clinical relevant therapy. In this review, we summarize the current progress in the field as well as methods and sites of islet transplantation, including stem cell-derived functional human islets. We also discuss the contribution of immune cells, vessel formation, extracellular matrix, and nutritional supply on islet graft survival. Developing new transplantation sites with emerging technologies to improve islet graft survival and simplify immune regulation will greatly benefit the future success of islet cell therapy in the treatment of diabetes.

The role of human CD46 in early xenoislet engraftment in a dual transplant model

BACKGROUND

Membrane cofactor protein CD46 attenuates the complement cascade by facilitating cleavage of C3b and C4b. In solid organ xenotransplantation, organs expressing CD46 have been shown to resist hyperacute rejection. However, the incremental value of human CD46 expression for islet xenotransplantation remains poorly defined.

METHODS

This study attempted to delineate the role of CD46 in early neonatal porcine islet engraftment by comparing Gal-knocked out (GKO) and hCD46-transgenic (GKO/CD46) islets in a dual transplant model. Seven rhesus macaques underwent dual transplant and were sacrificed at 1 hour (n = 4) or 24 hours (n = 3). Both hemilivers were recovered and fixed for immunohistochemistry (CD46, insulin, neutrophil elastase, platelet, IgM, IgG, C3d, C4d, CD68, Caspase 3). Quantitative immunohistochemical analysis was performed using the Aperio Imagescope.

RESULTS

Within 1 hour of intraportal infusion of xenografts, no differences were observed between the two types of islets in terms of platelet, antibody, or complement deposition. Cellular infiltration and islet apoptotic activity were also similar at 1 hour. At 24 hours, GKO/CD46 islets demonstrated significantly less platelet deposition (P = 0.01) and neutrophil infiltration (P = 0.01) compared to GKO islets. In contrast, C3d (P = 0.38) and C4d (P = 0.45) deposition was equal between the two genotypes.

CONCLUSIONS

Our findings suggest that expression of hCD46 on NPIs potentially provides a measurable incremental survival advantage in vivo by reducing early thrombo-inflammatory events associated with instant blood-mediated inflammatory reaction (IBMIR) following intraportal islet infusion.

The Role of Costimulation Blockade in Solid Organ and Islet Xenotransplantation

Pig-to-human xenotransplantation offers a potential bridge to the growing disparity between patients with end-stage organ failure and graft availability. Early studies attempting to overcome cross-species barriers demonstrated robust humoral immune responses to discordant xenoantigens. Recent advances have led to highly efficient and targeted genomic editing, drastically altering the playing field towards rapid production of less immunogenic porcine tissues and even the discussion of human xenotransplantation trials. However, as these humoral immune barriers to cross-species transplantation are overcome with advanced transgenics, cellular immunity to these novel xenografts remains an outstanding issue. Therefore, understanding and optimizing immunomodulation will be paramount for successful clinical xenotransplantation. Costimulation blockade agents have been introduced in xenotransplantation research in 2000 with anti-CD154mAb. Most recently, prolonged survival has been achieved in solid organ (kidney xenograft survival > 400 days with anti-CD154mAb, heart xenograft survival > 900 days, and liver xenograft survival 29 days with anti-CD40mAb) and islet xenotransplantation (>600 days with anti-CD154mAb) with the use of these potent experimental agents. As the development of novel genetic modifications and costimulation blocking agents converges, we review their impact thus far on preclinical xenotransplantation and the potential for future application.

Pig-to-Primate Islet Xenotransplantation: Past, Present, and Future

Islet allotransplantation results in increasing success in treating type 1 diabetes, but the shortage of deceased human donor pancreata limits progress. Islet xenotransplantation, using pigs as a source of islets, is a promising approach to overcome this limitation. The greatest obstacle is the primate immune/inflammatory response to the porcine (pig) islets, which may take the form of rapid early graft rejection (the instant blood-mediated inflammatory reaction) or T-cell-mediated rejection. These problems are being resolved by the genetic engineering of the source pigs combined with improved immunosuppressive therapy. The results of pig-to-diabetic nonhuman primate islet xenotransplantation are steadily improving, with insulin independence being achieved for periods >1 year. An alternative approach is to isolate islets within a micro- or macroencapsulation device aimed at protecting them from the human recipient's immune response. Clinical trials using this approach are currently underway. This review focuses on the major aspects of pig-to-primate islet xenotransplantation and its potential for treatment of type 1 diabetes.

Safe use of anti-CD154 monoclonal antibody in pig islet xenotransplantation in monkeys

Anti-CD154mAb is a powerful co-stimulation blockade agent that is efficacious in preventing rejection, even in xenogeneic settings. It has been used in the majority of successful long-term pig-to-non-human primate islet transplantation models. However, its clinical use was halted as a result of thromboembolic complications that were also observed in preclinical and clinical organ transplantation models. An anti-CD154mAb was administered to 14 streptozotocin-induced diabetic cynomolgus monkey recipients of porcine islets, some of which received the agent for many months. Monkeys were monitored for complications, and blood monitoring was carried out frequently. After euthanasia, multiple biopsies of all organs were examined for histological features of thromboembolism. Anti-CD154mAb prevented rejection of genetically engineered pig islets in all monkeys. No significant complications were attributable specifically to anti-CD154mAb. There was no evidence of thromboembolism in multiple histological sections from all major organs, including the brain. Our data suggest that in diabetic monkeys with pig islet grafts, anti-CD154mAb would appear to be an effective and safe therapy, and is not associated with thromboembolic complications.

Persistence of Indirect but Not Direct T Cell Xenoresponses in Baboon Recipients of Pig Cell and Organ Transplants

We investigated the contributions of direct and indirect T cell antigen recognition pathways to the immune response to porcine antigens in naïve baboons and baboon recipients of pig xenografts. In naïve baboons, in vitro culture of peripheral blood T cells with intact pig cells (direct xenorecognition pathway) or pig cell sonicates and baboon antigen-presenting cells (indirect xenorecognition pathway) induced the activation and expansion of xenoreactive T cells producing proinflammatory cytokines, interleukin-2 and interferon-γ. Primary indirect xenoresponses were mediated by preexisting memory T cells, whose presence is not typically observed in primary alloresponses. Next, baboons were conditioned with a nonmyeloablative regimen before short-term immunosuppression and transplantation of xenogeneic peripheral blood progenitor cells and a kidney, heart, or pancreatic islets from a miniature swine. All transplants were rejected acutely within 30 days after their placement. Posttransplantation, we observed an inhibition of the direct xenoresponse but a significant expansion of indirectly activated proinflammatory T cells. These results suggest that additional treatment to suppress indirect T cell immunity in primates may be required to achieve tolerance of pig xenografts through hematopoietic chimerism.

The pathobiology of pig-to-primate xenotransplantation: a historical review

The immunologic barriers to successful xenotransplantation are related to the presence of natural anti-pig antibodies in humans and non-human primates that bind to antigens expressed on the transplanted pig organ (the most important of which is galactose-α1,3-galactose [Gal]), and activate the complement cascade, which results in rapid destruction of the graft, a process known as hyperacute rejection. High levels of elicited anti-pig IgG may develop if the adaptive immune response is not prevented by adequate immunosuppressive therapy, resulting in activation and injury of the vascular endothelium. The transplantation of organs and cells from pigs that do not express the important Gal antigen (α1,3-galactosyltransferase gene-knockout [GTKO] pigs) and express one or more human complement-regulatory proteins (hCRP, e.g., CD46, CD55), when combined with an effective costimulation blockade-based immunosuppressive regimen, prevents early antibody-mediated and cellular rejection. However, low levels of anti-non-Gal antibody and innate immune cells and/or platelets may initiate the development of a thrombotic microangiopathy in the graft that may be associated with a consumptive coagulopathy in the recipient. This pathogenic process is accentuated by the dysregulation of the coagulation-anticoagulation systems between pigs and primates. The expression in GTKO/hCRP pigs of a human coagulation-regulatory protein, for example, thrombomodulin, is increasingly being associated with prolonged pig graft survival in non-human primates. Initial clinical trials of islet and corneal xenotransplantation are already underway, and trials of pig kidney or heart transplantation are anticipated within the next few years.

Long-term control of diabetes in immunosuppressed nonhuman primates (NHP) by the transplantation of adult porcine islets

Pig islets are an alternative source for islet transplantation to treat type 1 diabetes (T1D), but reproducible curative potential in the pig-to-nonhuman primate (NHP) model has not been demonstrated. Here, we report that pig islet grafts survived and maintained normoglycemia for >6 months in four of five consecutive immunosuppressed NHPs. Pig islets were isolated from designated pathogen-free (DPF) miniature pigs and infused intraportally into streptozotocin-induced diabetic rhesus monkeys under pretreatment with cobra venom factor (CVF), anti-thymocyte globulin (ATG) induction and maintenance with anti-CD154 monoclonal antibody and low-dose sirolimus. Ex vivo expanded autologous regulatory T cells were adoptively transferred in three recipients. Blood glucose levels were promptly normalized in all five monkeys and normoglycemia (90-110 mg/dL) was maintained for >6 months in four cases, the longest currently up to 603 days. Intravenous glucose tolerance tests during the follow-up period showed excellent glucose disposal capacity and porcine C-peptide responses. Adoptive transfer of autologous regulatory T cells was likely to be associated with more stable and durable normoglycemia. Importantly, the recipients showed no serious adverse effects. Taken together, our results confirm the clinical feasibility of pig islet transplantation to treat T1D patients without the need for excessive immunosuppressive therapy.

Safety of Intraperitoneal Injection of Adipose Tissue-Derived Autologous Mesenchymal Stem Cells in Cats

Background

Chronic inflammatory diseases are common in cats and mesenchymal stem cells (MSC) are a promising therapeutic approach for management of these disorders. The purpose of this study was to evaluate the safety of intraperitoneal injection of MSC in cats.

Hypothesis

Intrapertioneal injection of autologous MSC in cats is safe.

Animals

Ten healthy adult purpose‐bred cats.

Methods

Mesenchymal stem cells were isolated from subcutaneous adipose tissue collected during ovariohysterectomy and characterized for expression of CD90, CD105 and CD44 and trilineage differentiation. Three weeks postoperatively a complete blood count, serum chemistry profile, urinalysis, and abdominal ultrasound were performed. Five cats then received 1 × 10⁶ of autologous MSC/kg of body weight intraperitoneally with ultrasound guidance; 5 additional cats were sham injected. Cats were monitored for 6 weeks with daily physical examinations and weekly clinicopathological evaluations. Abdominal ultrasonography was repeated at weeks 1 and 5 after injection.

Results

Serious adverse effects were not observed in any MSC‐injected cat. Two animals developed transient lethargy and decreased activity. Jejunal lymph node size was increased in MSC‐injected cats compared to controls at weeks 1 (1.38 ± 0.25 versus 0.88 ± 0.25 cm²; P = .036) and 5 (1.75 ± 0.82 versus 0.79 ± 0.12 cm²; P = .047). A hyperechoic renal segmental cortical lesion was observed in 1 MSC‐injected cat.

Conclusions and Clinical Relevance

Intraperitoneal MSC injection was well tolerated with only mild, self‐limiting adverse effects being observed in 2 cats. This route provides a safe means of administration for cell‐based treatment in cats.

Engraftment and reversal of diabetes after intramuscular transplantation of neonatal porcine islet-like clusters

BACKGROUND

Intraportal infusion is currently the method of choice for clinical islet cell transplantation but suffers from poor efficacy. As the liver may not represent an optimal transplantation site for Langerhans islets, we examined the potential of neonatal porcine islet-like clusters (NPICCs) to engraft in skeletal muscle as an alternative transplantation site.

METHODS

Neonatal porcine islet-like clusters were isolated from 2- to 5-day-old piglets and either transplanted under the kidney capsule (s.k.) or injected into the lower hindlimb muscle (i.m.) of streptozotocin-diabetic NOD-SCID IL2rγ(-/-) (NSG) mice. Survival, vascularization, maturation, and functional activity were analyzed by intraperitoneal glucose tolerance testing and immunohistochemical analyses.

RESULTS

Intramuscular transplantation of NPICCs resulted in development of normoglycemia and restored glucose homeostasis. Time to reversal of diabetes and glucose tolerance (AUC glucose and AUC insulin) did not significantly differ as compared to s.k. transplantation. Intramuscular grafts exhibited rapid neovascularization and graft composition with cytokeratin-positive ductal cells and beta cells at post-transplant weeks 2 and 8 and after establishment of normoglycemia was comparable in both groups.

CONCLUSIONS

Intramuscular injection represents a minimally invasive but efficient alternative for transplantation of NPICCs and, thus, offers an attractive alternative site for xenotransplantation approaches. These findings may have important implications for improving the outcome and the monitoring of pig islet xenotransplantation.

Dual islet transplantation modeling of the instant blood-mediated inflammatory reaction

Islet xenotransplantation is a potential treatment for diabetes without the limitations of tissue availability. Although successful experimentally, early islet loss remains substantial and attributed to an instant blood-mediated inflammatory reaction (IBMIR). This syndrome of islet destruction has been incompletely defined and characterization in pig-to-primate models has been hampered by logistical and statistical limitations of large animal studies. To further investigate IBMIR, we developed a novel in vivo dual islet transplant model to precisely characterize IBMIR as proof-of-concept that this model can serve to properly control experiments comparing modified xenoislet preparations. WT and α1,3-galactosyltransferase knockout (GTKO) neonatal porcine islets were studied in nonimmunosuppressed rhesus macaques. Inert polyethylene microspheres served as a control for the effects of portal embolization. Digital analysis of immunohistochemistry targeting IBMIR mediators was performed at 1 and 24 h after intraportal islet infusion. Early findings observed in transplanted islets include complement and antibody deposition, and infiltration by neutrophils, macrophages and platelets. Insulin, complement, antibody, neutrophils, macrophages and platelets were similar between GTKO and WT islets, with increasing macrophage infiltration at 24 h in both phenotypes. This model provides an objective and internally controlled study of distinct islet preparations and documents the temporal histology of IBMIR.

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.

Control of IBMIR in neonatal porcine islet xenotransplantation in baboons

The instant blood-mediated inflammatory reaction (IBMIR) is a major obstacle to the engraftment of intraportal pig islet xenografts in primates. Higher expression of the galactose-α1,3-galactose (αGal) xenoantigen on neonatal islet cell clusters (NICC) than on adult pig islets may provoke a stronger reaction, but this has not been tested in the baboon model. Here, we report that WT pig NICC xenografts triggered profound IBMIR in baboons, with intravascular clotting and graft destruction occurring within hours, which was not prevented by anti-thrombin treatment. In contrast, IBMIR was minimal when recipients were immunosuppressed with a clinically relevant protocol and transplanted with NICC from αGal-deficient pigs transgenic for the human complement regulators CD55 and CD59. These genetically modified (GM) NICC were less susceptible to humoral injury in vitro than WT NICC, inducing significantly less complement activation and thrombin generation when incubated with baboon platelet-poor plasma. Recipients of GM NICC developed a variable anti-pig antibody response, and examination of the grafts 1 month after transplant revealed significant cell-mediated rejection, although scattered insulin-positive cells were still present. Our results indicate that IBMIR can be attenuated in this model, but long-term graft survival may require more effective immunosuppression or further donor genetic modification.

Islet cell xenotransplantation: a serious look toward the clinic

Type I diabetes remains a significant clinical problem in need of a reliable, generally applicable solution. Both whole organ pancreas and islet allotransplantation have been shown to grant patients insulin independence, but organ availability has restricted these procedures to an exceptionally small subset of the diabetic population. Porcine islet xenotransplantation has been pursued as a potential means of overcoming the limits of allotransplantation, and several preclinical studies have achieved near-physiologic function and year-long survival in clinically relevant pig-to-primate model systems. These proof-of-concept studies have suggested that xenogeneic islets may be poised for use in clinical trials. In this review, we examine recent progress in islet xenotransplantation, with a critical eye toward the gaps between the current state of the art and the state required for appropriate clinical investigation.

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.

Encapsulated islets for diabetes therapy: history, current progress, and critical issues requiring solution

Insulin therapy became a reality in 1921 dramatically saving lives of people with diabetes, but not protecting them from long-term complications. Clinically successful free islet implants began in 1989 but require life long immunosuppression. Several encapsulated islet approaches have been ongoing for over 30 years without defining a clinically relevant product. Macro-devices encapsulating islet mass in a single device have shown long-term success in large animals but human trials have been limited by critical challenges. Micro-capsules using alginate or similar hydrogels encapsulate individual islets with many hundreds of promising rodent results published, but a low incidence of successful translation to large animal and human results. Reduction of encapsulated islet mass for clinical transplantation is in progress. This review covers the status of both early and current studies including the presentation of corporate efforts involved. It concludes by defining the critical items requiring solution to enable a successful clinical diabetes therapy.

Caprine pancreatic islet xenotransplantation into diabetic immunosuppressed BALB/c mice

BACKGROUND

Type 1 diabetes mellitus is a devastating disease for which there is currently no cure, but only lifetime management. Islet xenotransplantation is a promising technique for the restoration of blood glucose control in patients with diabetes mellitus. The purpose of this study was to explore the potential use of caprine (goat) islet cells as xenogeneic grafts in the treatment for diabetes in a mouse model.

METHODS

Caprine pancreases were harvested and transported to the laboratory under conditions optimized to prevent ischemia. Islets were isolated, purified, and tested for functionality. Caprine islets (2000 islet equivalent) were transplanted beneath the kidney capsules of diabetic BALB/c mice under thalidomide-induced immunosuppression. Blood glucose and insulin levels of grafted mice were evaluated by glucometer and enzyme-linked immunosorbent assay kit, respectively. The functionality and quality of caprine pancreatic islet grafts were assessed by intraperitoneal glucose tolerance tests.

RESULTS

The viability of purified islet cells exceeded 90%. Recipient mice exhibited normoglycemia (<11 mM glucose) for 30 days. In addition, weight gain negatively correlated with blood glucose level. The findings verified diabetes reversal in caprine islet recipient mice. A significant drop in non-fasting blood glucose level (from 23.3 ± 5.4 to 8.04 ± 0.44 mM) and simultaneous increase in serum insulin level (from 0.01 ± 0.001 to 0.56 ± 0.17 μg/l) and body weights (from 23.64 ± 0.31 to 25.85 ± 0.34 g) were observed (P < 0.05). Immunohistochemical analysis verified insulin production in the transplanted islets.

CONCLUSIONS

Purified caprine islets were demonstrated to successfully sustain viability and functionality for controlling blood glucose levels in an immunosuppressed mouse model of diabetes. These results suggest the use of caprine islets as an addition to the supply of xenogeneic islets for diabetes research.

Macro- or microencapsulation of pig islets to cure type 1 diabetes

Although allogeneic islet transplantation can successfully cure type 1 diabetes, it has limited applicability. For example, organs are in short supply; several human pancreas donors are often needed to treat one diabetic recipient; the intrahepatic site may not be the most appropriate site for islet implantation; and immunosuppressive regimens, which are associated with side effects, are often required to prolong survival of the islet graft. An alternative source of insulin-producing cells would therefore be of major interest. Pigs represent a possible alternative source of beta cells. Grafting of pig islets may appear difficult because of the immunologic species barrier, but pig islets have been shown to function in primates for at least 6 mo with clinically incompatible immunosuppression. Therefore, a bioartificial pancreas made of encapsulated pig islets may resolve issues associated with islet allotransplantation. Although several groups have shown that encapsulated pig islets are functional in small-animal models, less is known about the use of bioartificial pancreases in large-animal models. In this review, we summarize current knowledge of encapsulated pig islets, to determine obstacles to implantation in humans and possible solutions to overcome these obstacles.

Alternative immunomodulatory strategies for xenotransplantation: CD40/154 pathway-sparing regimens promote xenograft survival

Immunosuppressive therapies that block the CD40/CD154 costimulatory pathway have proven to be uniquely effective in preclinical xenotransplant models. Given the challenges facing clinical translation of CD40/CD154 pathway blockade, we examined the efficacy and tolerability of CD40/CD154 pathway-sparing immunomodulatory strategies in a pig-to-nonhuman primate islet xenotransplant model. Rhesus macaques were rendered diabetic with streptozocin and given an intraportal infusion of ≈ 50 000 islet equivalents/kg wild-type neonatal porcine islets. Base immunosuppression for all recipients included maintenance therapy with belatacept and mycophenolate mofetil plus induction with basiliximab and LFA-1 blockade. Cohort 1 recipients (n = 3) were treated with the base regimen alone; cohort 2 recipients (n = 5) were additionally treated with tacrolimus induction and cohort 3 recipients (n = 5) were treated with alefacept in place of basiliximab, and more intense LFA-1 blockade. Three of five recipients in both cohorts 2 and 3 achieved sustained insulin-independent normoglycemia (median rejection-free survivals 60 and 111 days, respectively), compared to zero of three recipients in cohort 1. These data show that CD40/CD154 pathway-sparing regimens can promote xenoislet survival. Further optimization of these strategies is warranted to aid the clinical translation of islet xenotransplantation.

T-cell activation and transplantation tolerance

Transplantation of allogeneic or "nonself" tissues stimulates a robust immune response leading to graft rejection, and therefore, most recipients of allogeneic organ transplants require the lifelong use of immune suppressive agents. Excellent outcomes notwithstanding, contemporary immunosuppressive medications are toxic, are often not taken by patients, and pose long-term risks of infection and malignancy. The ultimate goal in transplantation is to develop new treatments that will supplant the need for general immunosuppression. Here, we will describe the development and application of costimulation blockade to induce transplantation tolerance and discuss how the diverse array of signals that act on T cells will determine the balance between graft survival and rejection.

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.

CD40-specific costimulation blockade enhances neonatal porcine islet survival in nonhuman primates

The widespread clinical implementation of alloislet transplantation as therapy for type 1 diabetes has been hindered by the lack of suitable islet donors. Pig-to-human islet xenotransplantation is one strategy with potential to alleviate this shortage. Long-term survival of porcine islets has been achieved using CD154-specific antibodies to interrupt the CD40/CD154 costimulation pathway; however, CD154-specific antibodies seem unlikely candidates for clinical translation. An alternative strategy for CD40/CD154 pathway interruption is use of CD40-specific antibodies. Herein, we evaluate the ability of a chimeric CD40-specific monoclonal antibody (Chi220) to protect islet xenografts. Neonatal porcine islets (~50,000 IEQ/kg) were transplanted intraportally into pancreatectomized diabetic macaques. Immunosuppression consisted of induction therapy with Chi220 and the IL-2 receptor-specific antibody basiliximab, and maintenance therapy with sirolimus and the B7-specific fusion protein belatacept. Chi220 effectively promoted xenoislet engraftment and survival, with five of six treated recipients achieving insulin-independent normoglycemia (median rejection-free survival 59 days; mean 90.8 days, maximum 203 days). No thromboembolic phenomena were observed. CD40 represents a promising alternative to CD154 as a therapeutic target, and the efficacy of CD40-specific antibodies in islet xenotransplantation warrants further investigation.

Alginate macroencapsulation of pig islets allows correction of streptozotocin-induced diabetes in primates up to 6 months without immunosuppression

BACKGROUND

This study assessed the capacity of alginate-encapsulated islets to reverse diabetes in a pig-to-primate model.

METHODS

Adult pig islets were encapsulated in microcapsules implanted under the kidney capsule (n=4) or in a subcutaneous macrodevice (n=5) in diabetic primates. Fasting blood glucose (FBG), insulin, porcine C-peptide, glycosylated hemoglobin (HbA1C), and cellular and humoral responses were followed.

RESULTS

Nonencapsulated pig islets were rejected within 7 days. A transient decrease of FBG was observed only during the 2 weeks after microencapsulated pig islet implantation under the kidney capsule. After subcutaneous transplantation of a macrodevice, diabetes was corrected up to a maximum of 6 months in five animals: FBG less than 107 mg/dL and HbA1C at 8% ± 1.4%. Two of the five animals received a new macrodevice between 25 and 35 weeks after the first graft dysfunction (HbA1C ≥ 13), and diabetes was controlled for an additional 18 weeks in these animals. Although a strong humoral response was elicited after transplantation of encapsulated islets, a total impermeability of alginate 3% wt/vol to IgG was demonstrated before and up to 20 weeks after transplantation of the subcutaneous macrodevice.

CONCLUSIONS

Pig islets encapsulated in a subcutaneous macrodevice can control diabetes up to 6 months without immunosuppression.

Beta-5 score to evaluate pig islet graft function in a primate pre-clinical model

BACKGROUND

We developed a composite scoring system to accurately assess pig islet function in pre-clinical primate studies.

METHODS

Two scoring methods that have been clinically validated in human islet allotransplantation were tested in six non-diabetic and nine streptozotocin (STZ)-induced diabetic primates: (i) SUITO index=[1500 × fasting C-peptide (ng/ml)]/[fasting blood glucose (FBG, mg/dl) - 63] and (ii) CP/G ratio =[fasting C-peptide (ng/ml) × 100]/FBG (mg/dl). Both scores were analysed as a function of the β-cell mass of the native primate pancreas. Next, a proposed β5 score based on FBG values, daily glycosuria, post-prandial glycosuria, polydipsia, and polyuria was validated on the same primates. Ranges of normal and pathologic values for each parameter were assessed during 5 months in non-diabetic and diabetic primates, respectively. Finally, scores were tested on the nine STZ-induced diabetic primates, four of which were transplanted with microencapsulated pig islets and five with macroencapsulated pig islets. All parameters required for each score were measured prior to transplantation and up to 12 weeks post-transplantation. For the CP/G ratio after transplantation, primate C-peptide was replaced by porcine C-peptide.

RESULTS

The Suito index was not correlated with the pancreatic β-cell mass in contrast to the CP/G ratio (R(2) = 0.17, P = 0.645 vs. R(2) = 0.76, P = 0.003; respectively). The internal consistency of the parameters implied by the β5 score was confirmed by a Cronbach's alpha test of 0.97. Diabetes was confirmed by a significant decrease in the CP/G ratio and the β5 score before and after diabetes induction, respectively. After transplantation, a significant correlation was found between the CP/G ratio and the β5 score, which reflected the functionality of pig islet xenografts and diabetes control. In addition, the CP/G ratio and β5 score were correlated with the glycosylated hemoglobin course after transplantation and diabetes correction with macroencapsulated pig islets.

CONCLUSION

The proposed β5 score provides a valid tool to accurately assess islet transplantation in a primate pre-clinical model.

Pig islets for clinical islet xenotransplantation

PURPOSE OF REVIEW

Allogeneic islet transplantation faces difficulties because organ shortage is recurrent; several pancreas donors are often needed to treat one diabetic recipient; and the intrahepatic site of islet implantation may not be the most appropriate one. Another source of insulin-producing cells, therefore, would be of major interest, and pigs represent a possible and serious source for obtaining such cells.

RECENT FINDINGS

Pig islet grafts may appear difficult because of the species barrier, but recent studies demonstrate that pig islets may function in diabetic primates for at least 6 months.

SUMMARY

Pig islet xenotransplantation, however, must still overcome the selection of a suitable pig donor to translate preclinical findings into clinical applications. This review summarizes the actual acquired knowledge of pig islet transplantation in primates to select the 'ideal' pig donor.

The effect of composite pig islet-human endothelial cell grafts on the instant blood-mediated inflammatory reaction

Instant blood-mediated inflammatory reaction (IBMIR) causes rapid islet loss in portal vein islet transplantation. Endothelial cells are known to protect against complement-mediated lysis and activation of coagulation. We tested composite pig islet-human endothelial cell grafts as a strategy to overcome IBMIR. Porcine islets were cocultured with human endothelial cells in specially modified culture medium composed of M199 and M200 for 1-9 days. A positive control group, negative control group, and the endothelial cell-coated group were examined with an in vitro tubing loop assay using human blood. The endothelial cell-coated group was subdivided and analyzed by degree of surface coverage by endothelial cells (< or = 50% vs. > 50%) or coculture time (< 5 days vs. > or = 5 days). Platelet consumption and complement and coagulation activation were assessed by platelet count, C3a, and thrombin-antithrombin complex (TAT), respectively. After 60-min incubation in human blood, the endothelial cell-coated group showed platelet consumption inhibition and low C3a and TAT assay results compared to uncoated controls. When the endothelial cell-coated group was subdivided by degree of surface coverage, the < or = 50% coated group showed less platelet consumption and less activation of complement and coagulation compared with the positive control (uncoated) group. On analysis by coculture time, only the subgroup cocultured for < 5 days showed the same protective effect. Human endothelial cell-coated pig islets, especially the partially coated and short-term cocultured pig islet-human endothelial cell composites, reduced all components of IBMIR. If the optimal endothelial cell-islet coculture method could be identified, human endothelial cell coating of pig islets would offer new strategies to improve xenogenic islet transplantation outcomes.

Cellular xenotransplantation

PURPOSE OF REVIEW

Expectations are high on cellular therapy. Being fundamental to elucidate organogenesis, it is unlikely that embryonic stem cells will be used for clinical purposes. Postembryonic stage, developing cells are, therefore, the front-runner for regenerative medicine. In addition to autologous cells, both allogeneic and xenogeneic cells are hypothetical candidates to treat specific diseases. This review summarizes the current knowledge on immunological and functional aspects of xeno(allo)-cellular transplantation for cardiomyopathy, diabetes, liver failure, neural diseases, and bone regeneration.

RECENT FINDINGS

Xenocellular transplantation is promising for tissue repair in immunologically privileged sites such as the central nervous system or nonvascularized tissues in which no or moderate immunosuppression is required. In vascularized organs, major immune responses are present when cells are transplanted without additional conditioning. Positive results from encapsulation methods that protect cells from the immune system should further stimulate preclinical research. Also, conditioning immunosuppression could be used to circumvent the initial immune response. Transgenic pigs cells are probably the best xenogeneic substitute for human application, although basic research on innate and noninnate immunity toward pig cells is still required.

SUMMARY

In several fields of medicine, cellular xenotransplantation is slowly emerging as a potential therapeutic tool.

Pig-to-nonhuman primate islet xenotransplantation

Type 1 diabetes continues to present a therapeutic challenge. The restoration of normoglycemia and insulin independence in immunosuppressed type 1 diabetic recipients of human islet allografts has highlighted the potential of cell-based diabetes therapy. The unlimited and on-demand availability of pig islets from healthy, young, living, designated pathogen-free, and potentially genetically modified donors presents unique opportunities for improving the availability and outcomes of islet replacement therapies in diabetes. One of the fundamental prerequisites for initiating clinical research is a favorable benefit-over-harm determination in the stringent preclinical transplant model in nonhuman primates. To date, xenotransplants of pig islet cell therapy products have been reported by 15 institutions in 181 NHPs, including xenotransplants in 72 non-diabetic and 109 diabetic recipients. These studies have demonstrated the feasibility of successful preclinical islet xenotransplantation and have provided insights into the critical events operative in the immune recognition and destruction of islet xenografts in nonhuman primates. Particularly promising is the recent achievement of prolonged insulin independence in this model by means of several distinct islet xenotransplantation products, implantation sites, and immunotherapeutic strategies. Further progress appears likely and the development of suitable source pigs will position the scientific community to translate these findings safely to the clinic.

Dissecting the instant blood-mediated inflammatory reaction in islet xenotransplantation

BACKGROUND

A massive destruction of transplanted tissue occurs immediately following transplantation of pancreatic islets from pig to non-human primates. The detrimental instant blood-mediated inflammatory reaction (IBMIR), triggered by the porcine islets, is a likely explanation for this tissue loss. This reaction may also be responsible for mediating an adaptive immune response in the recipient that requires a heavy immunosuppressive regimen.

MATERIALS AND METHODS

Low molecular weight dextran sulfate (LMW-DS) and the complement inhibitor Compstatin were used in a combination of in vitro and in vivo studies designed to dissect the xenogeneic IBMIR in a non-human primate model of pancreatic islet transplantation. Adult porcine islets (10,000 IEQs/kg) were transplanted intraportally into three pairs of cynomolgus monkeys that had been treated with LMW-DS or heparin (control), and the effects on the IBMIR were characterized. Porcine islets were also incubated in human blood plasma in vitro to assess complement inhibition by LMW-DS and Compstatin.

RESULTS

Morphological scoring and immunohistochemical staining revealed that the severe islet destruction and macrophage, neutrophilic granulocyte, and T-cell infiltration observed in the control (heparin-treated) animals were abrogated in the LMW-DS-treated monkeys. Both coagulation and complement activation were significantly reduced in monkeys treated with LMW-DS, but IgM and complement fragments were still found on the islet surface. This residual complement activation could be inhibited by Compstatin in vitro.

CONCLUSIONS

The xenogeneic IBMIR in this non-human primate model is characterized by an immediate binding of antibodies that triggers deleterious complement activation and a subsequent clotting reaction that leads to further complement activation. The effectiveness of LMW-DS (in vivo and in vitro) and Compstatin (in vitro) in inhibiting this IBMIR provides the basis for a protocol that can be used to abrogate the IBMIR in pig-human clinical islet transplantation.

The Baboon in Xenotransplant Research

If cross-species transplantation is ever to become a reasonable therapeutic modality for human beings, it will be because the potential for success has been demonstrated in a nonhuman primate model. The imperative has always been to select a primate research subject from a species that is plentiful, is not endangered, readily procreates in a managed environment, and mimics the human response (immunologic homology) to both organ transplantation and potential transfer of infectious disease. Several Papio subspecies of baboons, including Papio hamadryas anubis (olive baboon), meet these important criteria. These animals remain ubiquitous throughout sub-Saharan Africa and have adapted well to the managed environments of major primate centers worldwide. A list of United States-based primate centers housing breeding colonies of baboons can be found in Table 19.1. The Surgical Research Laboratory at Loma Linda University, for instance, has maintained a salutary relationship with the Southwest National Primate Research Center in San Antonio, Texas, for the procurement of juvenile baboon research subjects.

The coagulation barrier in xenotransplantation: incompatibilities and strategies to overcome them

PURPOSE OF REVIEW

Dysregulated coagulation is now recognized as a major contributor to graft loss in xenotransplantation. This review summarizes recent data on putative mechanisms of pathogenic coagulation in xenotransplantation and discusses progress on strategies to overcome them.

RECENT FINDINGS

Evidence continues to grow that the primary cause of failure of pig cardiac and renal xenografts is probably antibody-mediated injury to the endothelium, leading to development of microvascular thrombosis. Several factors that may exacerbate the problem will remain, even in the absence of a humoral response. These include molecular incompatibilities that affect the control of coagulation - in particular the failure of pig thrombomodulin to activate the primate protein C pathway - and platelet reactivity. Expression of anticoagulant and antiplatelet molecules within the graft is a potential solution that has been successfully tested in rodent models and will soon be applied to the pig-to-primate model. This strategy, in parallel with physical methods such as encasing islets in a protective layer, also holds promise for reducing the thrombogenicity of pig islet xenografts.

SUMMARY

Thrombosis is a barrier to long-term survival and function of porcine xenografts, which may eventually be overcome by various combinations of genetic and physical manipulation.

Pig islet xenotransplantation into non-human primate model

Allogeneic islet transplantation faces difficulties because (1) organ shortage is recurrent; (2) several pancreas donors are often needed to treat one diabetic recipient; and (3) the intrahepatic site of islet implantation may not be the most appropriate site. Another source of insulin-producing cells, therefore, would be of major interest, and pigs represent a possible and serious source for obtaining such cells. Pig islet grafts may seem difficult because of the species barrier, but recent reports demonstrate that pig islets may function in primates for at least 6 months. Pig islet xenotransplantation, however, must still overcome several hurdles before becoming clinically applicable. The actual consensus is to produce more preclinical data in the pig-to-primate model as a necessary requirement to envisage any pig-to-human transplantation of islets; therefore, a summary of the actual acquired knowledge of pig islet transplantation in primates seemed useful and is summarized in this overview.

Cobalt Protoporpyhrin Reduces Caspase-3,-7 Enzyme Activity in Neonatal Porcine Islets, But Does Not Inhibit Cell Death Induced by TNF-α

Apoptotic phenomena observed in vitro following isolation and following transplantation contribute significantly to islet graft loss. Strategies to reduce apoptosis of islet tissue prior to and posttransplantation may improve graft survival and function and reduce the amount of tissue necessary to achieve insulin independence. The expression of cytoprotective proteins is one such strategy that may prolong islet survival. In this light, heme-oxygenase 1 (HO-1) upregulation has been studied in both allo- and xenotransplantation models. In this study, the effect of HO-1 on apoptosis in neonatal porcine islet-like cell clusters (NPICC) was assessed. In in vitro assessments of NPICC apoptosis, NPICC showed a high sensitivity to apoptotic stimulation using a combination of TNF-α and cycloheximide. Stimulation with TNF-α alone was sufficient to induce reproducible apoptotic responses as demonstrated by caspase-3,-7 activation and subdiploid DNA analysis. Dose-dependent, high-level HO-1 protein expression was achieved following culture of NPICC in medium containing either cobalt protoporphyrin (CoPP) or cobalt mesoporphyrin (CoMP). CoPP treatment resulted in the reduction of caspase-3,-7 enzyme activity following TNF-α stimulation. However, such an effect was not associated with a reduction in the levels of cell death. Indeed, the inhibition of caspase enzyme activity resulted in decreased PARP-1 cleavage, which may lead to heightened levels of necrosis in treated NPICC cultures, possibly explaining the observed commitment of NPICC to the death pathway.

Rapid loss of intraportally transplanted islets: an overview of pathophysiology and preventive strategies

Islets isolated from multiple pancreas donors are often necessary to achieve euglycemia in type 1 diabetic patients treated by islet allotransplantation. This increases the burden on the limited pool of donor organs. After infusion into the portal vein, a substantial percentage of islets are lost in the immediate post-transplant period through an inflammatory response termed the instant blood-mediated inflammatory reaction (IBMIR). IBMIR is equally, if not more of a problem after islet xenotransplantation, e.g., using pig islets in non-human primates. Coagulation, platelet aggregation, complement activation, and neutrophil and monocyte infiltration play roles in this reaction. IBMIR is potentially triggered by islet surface molecules, such as tissue factor and collagen residues that are normally not in direct contact with the blood. Also, stress during the islet isolation process results in the expression and production of several inflammatory mediators by the islets themselves. The potential mechanisms involved in this rapid graft loss and treatment options to reduce this loss are reviewed. Preventive strategies for IBMIR can include systemic treatment of the recipient, pre-conditioning of the isolated islets, or, in the case of xenotransplantation, genetic modification of the organ-source pig. Pre-conditioning of islets in culture by exposure to anti-inflammatory agents or by genetic modification harbors fewer risks of systemic complications in the recipient. The future of clinical islet transplantation will, at least in part, depend on the success of efforts made to reduce rapid graft loss, and thus allow islet transplantation to become a more efficient therapy by the use of single donors.

Nutrient control of insulin secretion in perifused adult pig islets

OBJECTIVES

Xenotransplantation of pig islets is a potential solution to the shortage of human islets, but our knowledge of how these islets secrete insulin in response to nutrients is still fragmentary. This was the question addressed in the present study.

METHODS

After 24 h culture adult pig islets were perifused to characterize the dynamics of insulin secretion. Some responses were compared to those in human islets.

RESULTS

Increasing glucose from 1 to 15 mM weakly (approximately 2x) stimulated insulin secretion, which was potentiated (approximately 12x) by the cAMP-producing agent, forskolin. The effect of glucose was concentration-dependent (threshold at 3-5 mM and maximum at approximately 10 mM). The pattern of secretion was biphasic with a small first phase and an ascending second phase, and a paradoxical increase when the glucose concentration was abruptly lowered. Diazoxide abolished glucose-induced insulin secretion and tolbutamide reversed the inhibition. Glucose also increased secretion when islets were depolarized with tolbutamide or KCl. Insulin secretion was increased by leucine+glutamine, arginine, alanine or a mixture of amino acids, but their effect was significant only in the presence of forskolin. Upon stimulation by glucose alone, human islets secreted approximately 10x more insulin than pig islets, and the kinetics was characterized by a large first phase, a flat second phase, and rapid reversibility.

CONCLUSIONS

Compared with human islets, in vitro insulin secretion by adult pig islets is characterized by a different kinetics and a major quantitative deficiency that can be corrected by cAMP.

Isolation outcome and functional characteristics of young and adult pig pancreatic islets for transplantation studies

INTRODUCTION

Pig islets have been proposed as an alternative to human islets for clinical use, but their use is limited by rejection. The availability of genetically modified pigs devoid of alpha1,3-galactosyltransferase might provide islets more suitable for xenotransplantation. To limit the costs involved in the logistics and health care of pigs for clinical xenotransplantation, we have studied whether younger, rather than older, pigs that are typically preferred can be used as islet donors.

METHODS

We utilized pancreases from Yorkshire and White Landrace wild-type pigs and alpha1,3-galactosyltransferase gene-knockout pigs of three main different age and size groups: (i) <6 months, (ii) 6 to 12 months, and (iii) >2 yr of age, inclusive of retired breeders. We compared isolation yield and in vitro and in vivo function of islet cells obtained from these groups.

RESULTS

Islets from adult pigs (>2 yr) offered not only higher islet yields, but retained the ability to preserve intact morphology during the isolation process and culture, in association with high functional properties after transplantation. Following isolation, islet cells from young (<6 m) and young-adult (6 to 12 m) pigs dissociated into small aggregates and single cells, and exhibited inferior functional properties than adult islets both in vitro and in vivo.

CONCLUSIONS

These data support the conclusion that, in view of the large number of islets needed to maintain normoglycemia after xenotransplantation, organ-source pigs need to reach adult age (>2 yr) before being considered optimal islet donors, in spite of the higher costs involved.