Prolonged xenograft survival of islets infected with small doses of adenovirus expressing CTLA4Ig

Adenovirus-Mediated CTLA4Ig or CD40Ig Gene Transfer Delays Pancreatic Islet Rejection in a Rat-to-Mouse Xenotransplantation Model after Systemic but Not Local Expression

Transient costimulation signal blockade of either CD28/CD80–86 interactions and/or CD40/CD154 interactions can prevent islet rejection in some models of both allo- and xenotransplantation. We have used adenoviruses coding for CTLA4Ig or CD40Ig and compared the efficacy of genetic modification of islets to systemic production through either intramuscular (IM) or intravenous (IV) injection of these vectors in a rat-to-mouse islet transplantation model. When gene transfer was performed into islets, a high level of primary nonfunction was induced. Furthermore, transduced functional grafts were rejected with the same kinetics as nontransduced islets. In contrast, IM AdCTLA4Ig and IV AdCD40Ig significantly delayed rejection (mean survival time of 54 ± 26.9 and 67.6 ± 44.9 days, respectively, vs. 24.3 ± 9.7 days for unmodified islets, p < 0.05). Combination of ex vivo AdCTLA4Ig islet transduction and IV AdCD40Ig did not improve graft survival further. In conclusion, islet graft transduction with adenoviruses coding for costimulation inhibitors resulted in local expression with low serum concentrations of CTLA4Ig or CD40Ig and was unable to protect islet xenografts from rejection. In contrast, IM or IV gene transfer resulted in high serum concentrations of these molecules and was highly efficient in prolonging xenograft survival. These results contrast with the efficacy of AdCTLA4Ig we observed in a rat islet allotransplantation model and suggest that islet xenograft rejection might be more difficult to control.

T-cell-mediated immunological barriers to xenotransplantation

Xenotransplantion remains the most viable option for significant expansion of the donor organ pool in clinical transplantation. With the advent of nuclear transfer technologies, the production of transgenic swine has become a possibility. These animals have allowed transplant investigators to overcome humoral mechanisms of hyperacute xenograft rejection in experimental pig-to-non-human primate models. However, other immunologic barriers preclude long-term acceptance of xenografts. This review article focuses on a major feature of xenogeneic rejection: xenogeneic T cell responses. Evidence obtained from both small and large animal models, particularly those using either islet cells or kidneys, have demonstrated that T cell responses play a major role in xenogeneic rejection, and that immunosuppression alone is likely incapable of completely suppressing these responses. Additionally, both the direct and indirect pathway of antigen presentation appear to be involved in these anti donor processes. Enhanced understanding of (i) CD47 and its role in transduced xeno-bone marrow (ii) CD39 and its role in coagulation dysregulation and (iii) thymic transplantation have provided us with encouraging results. Presently, experiments evaluating the possibility of xenogeneic tolerance are underway.

Prolonged xenograft survival induced by inducible costimulator-Ig is associated with increased forkhead box P3(+) cells

BACKGROUND

Blockade of the inducible costimulator (ICOS) pathway has been shown to prolong allograft survival; however, its utility in xenotransplantation is unknown. We hypothesize that local expression of ICOS-Ig by the xenograft will suppress the T-cell response resulting in significant prolonged graft survival.

METHODS

Pig iliac artery endothelial cells (PIEC) secreting ICOS-Ig were generated and examined for the following: (1) inhibition of allogeneic and xenogeneic proliferation of primed T cells in vitro and (2) prolongation of xenograft survival in vivo. Grafts were examined for Tregs by flow cytometry and cytokine levels determined by quantitative reverse-transcriptase polymerase chain reaction.

RESULTS

Soluble ICOS-Ig markedly decreased allogeneic and xenogeneic primed T-cell proliferation in a dose-dependent manner. PIEC-ICOS-Ig grafts were significantly prolonged compared with wild-type grafts (median survival, 34 and 12 days, respectively) with 20% of PIEC-ICOS-Ig grafts surviving more than 170 days. Histological examination showed a perigraft cellular accumulation of Forkhead box P3 (Foxp3(+)) cells in the PIEC-ICOS-Ig grafts, these were also shown to be CD3(+)CD4(+)CD25(+). Survival of wild-type PIEC grafts in a recipient simultaneously transplanted with PIEC-ICOS-Ig were also prolonged, with a similar accumulation of Foxp3(+) cells at the periphery of the graft demonstrating ICOS-Ig induces systemic graft prolongation. However, this prolongation was specific for the priming xenograft. Intragraft cytokine analysis showed an increase in interleukin-10 levels, suggesting a potential role in induction/function of CD4(+)CD25(+)Foxp3(+) cells.

CONCLUSIONS

This study demonstrates prolonged xenograft survival by local expression of ICOS-Ig, we propose that the accumulation of CD4(+)CD25(+)Foxp3(+) cells at the periphery of the graft and secretion of interleukin-10 is responsible for this novel observation.

Dendritic cells expressing soluble CTLA4Ig prolong antigen-specific skin graft survival

Dendritic cells (DCs) and CTLA4Ig are important in regulating T-cell responses and therefore represent potential therapeutic tools in transplantation. In this study, CTLA4Ig was expressed in a C57BL/6 murine DC line (JAWS II) by lentiviral transduction and these cells were used to examine T-cell immunomodulatory effects in vitro and in vivo. A lower stimulation index to C57BL/6 was observed with splenocytes from BALB/c mice primed with JAWS II-CTLA4Ig compared with control JAWS II-green fluorescent protein (JAWS II-GFP). Mice primed with JAWS II-CTLA4Ig cells had significantly prolonged antigen-specific C57BL/6 skin graft survival compared with either JAWS II-GFP-primed or naïve mice (median 13, 11 and 11 days, respectively, P=0.0001). Furthermore, JAWS II-CTLA4Ig-primed mice that had been previously transplanted with skin grafts were re-transplanted with skin grafts 6 months later without immune manipulation. These mice demonstrated specific prolongation of second-set rejection responses, indicating systemic immune modulation induced by genetically modified DC. The mechanism was not due to expression of indoleamine 2,3-dioxygenase or induction of circulating regulatory T cells as assessed by flow cytometry of the peripheral blood lymphocytes. This potent effect demonstrated with skin grafts and second-set responses highlights the potential use of this strategy for transplantation more generally.

Gene therapy in transplantation

Gene therapy is an exciting and novel technology that offers the prospect of improving transplant outcomes beyond those achievable with current clinical protocols. This review explores both the candidate genes and ways in which they have been deployed to overcome both immune and non-immune barriers to transplantation success in experimental models. Finally, the major obstacles to implementing gene therapy in the clinic are considered.

How not to be seen: immune-evasion strategies in gene therapy

The development of efficient and safe vectors for gene delivery paved the way for evolution of gene therapy as a new modality for treatment of various inherited disorders and for cancer. The current vectors, viral and non-viral, have their limitations. Innate and adaptive immune responses to vector particles and components may restrict the efficiency of gene transfer and the persistence of expression of the transgene. Results from preclinical studies in animals and more recently data from clinical studies have demonstrated the potential impact of the cellular and the humoral immune response on the therapeutic efficacy. Not only the vector components, but also the transgene products may induce an immune response that negatively affects the therapeutic efficacy. The induction of a cytotoxic T-cell response to transgene-encoded peptides, as well as the production of antibodies directed against secreted proteins have been reported in preclinical and clinical studies, and these may thwart those applications that require long-term expression. Here we will review some of the options to blunt the acquired immune responses to transgene-encoded polypeptides.

Lentiviral expression of CTLA4Ig inhibits primed xenogeneic lymphocyte proliferation and cytokine responses

BACKGROUND

Co-stimulatory blockade is known to inhibit lymphocyte responses and to prolong allograft and xenograft survival. The present study examines the effect of transgenic expression of cytotoxic T lymphocyte-associated molecule-4 immunoglobulin (CTLA4Ig) by a porcine endothelial cell line (PIEC) transduced by a lentiviral vector, on primed xenogeneic T-cell proliferative and cytokine responses.

METHODS

Splenocytes from mice primed with PIEC were used as responder cells in a secondary proliferative assay. CTLA4Ig transduced and wild-type PIEC were used as stimulator cells. Responder cells were assayed for proliferation and cytokine production.

RESULTS

Proliferation was profoundly inhibited by CTLA4Ig transduced cells compared with control cells. Cytokine analysis by enzyme linked immunospot demonstrated that production of interferon-gamma, IL4 (interleukin 4) and IL10 was inhibited by CTLA4Ig transduced cells compared with control cells.

CONCLUSION

CTLA4Ig inhibited primed indirect xenogeneic T-cell proliferative and cytokine responses in vitro. Expression of immunomodulatory molecules by xenogeneic tissues has potential therapeutic applications for future xenotransplantation.

Efficient gene delivery to human and rodent islets with double-stranded (ds) AAV-based vectors

Transplantation of allogeneic pancreatic islets is an effective approach to treat type 1 diabetes. To bypass the need for systemic administration of immunosuppression drugs following transplantation, approaches to genetically modify allogeneic islets to express anti-inflammatory, immunosuppressive, or antiapoptotic proteins prior to transplantation are being developed. Adeno-associated viral (AAV) based vectors have been used for gene transfer to islets, but the efficiency of functional transduction is low. Recently, double-stranded (ds) or double-copy (dc) based AAV vectors have been developed that allow for more rapid and efficient AAV-mediated transgene expression following transduction. Here we demonstrate that intact human and murine islets can be transduced with dsAAV2-eGFP efficiently compared to single-stranded AAV2-eGFP. Furthermore, our results demonstrate that murine islets transduced with dsAAV2-eGFP have normal islet glucose responsiveness, viability, and islet insulin content. Transplantation of the dsAAV2-eGFP transduced islet restored normal glycemia in diabetic mice without eliciting an immune response. Significant dsAAV2-mediated eGFP expression was observed in the islet grafts for at least 6 months post-transplant. Finally, we demonstrated that dsAAV serotypes 2, 6, and 8 infect human islets efficiently. Taken together, these results suggest that dsAAV based vectors are highly appropriate for gene transfer to islets to facilitate transplantation.

Ability of donor splenocytes with costimulation blockade to induce mixed hematopoietic chimerism and transplantation tolerance

We reported stable mixed chimerism and specific tolerance to a fully allogeneic graft after a minimally myelosuppressive regimen including costimulation blockade (CB), donor bone marrow cells (BMC), and busulfan (Bu), a chemotherapeutic conditioning agent that makes niches for engraftment of BMC. For clinical application, the strategy may have the limitation of the number of donor BMC when a deceased donor offers transplants to multiple recipients. Herein, we examined whether donor splenocytes can serve as an alternative source to induce mixed chimerism and tolerance. When a C57BL/6 (H-2b) recipient was treated with CB (CTLA4-Ig and anti-CD154 mAb, on days 0, 2, 4, 6) and donor BALB/c (H-2d) BMC (2 x 10(7) cells on day 0) in the absence of Bu, survival of BALB/c skin graft was remarkably prolonged but not indefinite (median survival time [MST]: 138 days). The recipients never showed durable chimerism. When the recipient was treated with CB and donor splenocytes ([DST] 2 x 10(7) cells on day 0), survival was not indefinite either (MST: 114 days). When the dose of DST was increased to 2 x 10(8) cells, survival was further prolonged; two of six recipients had indefinite survival (MST: 132 days). Moreover, one recipient showed a low level of chimerism. When treated with CB, donor DST (2 x 10(7) cells on day 0) and Bu (20 mg/kg, day -1), six of seven recipients showed a stable, high level of chimerism and enjoyed tolerance of skin allografts. DST combined with CB and Bu may be an alternative source of hematopoietic stem cells to induce mixed chimerism and transplantation tolerance in our model.

Can we make surrogate beta-cells better than the original?

Insufficient pancreatic beta-cell mass is fundamental to the pathogenesis of both types 1 and 2 diabetes and constitutes the basis for the goal of beta-cell replacement therapy. Current methods for isolating islets from organ donor pancreases do not come close to supplying all in need, thus providing a compelling need to find new sources of insulin-producing cells. Possible sources include generation of cells from embryonic stem cells (ESC), adult stem/precursor cells, transdifferentiation of other cell types and xenodonors. Bioengineering can be used to improve secretory performance and strengthen cells to better withstand the challenges of transplantation. Strategies include protection against hypoxia, inflammation, and immune attack.

Bicistronic adenovirus-mediated gene transfer of CTLA4Ig gene and CD40Ig gene result in indefinite survival of islet xenograft

Blockade of CD40-CD154 costimulatory pathway in mice and primates with anti-CD154 monoclonal antibodies results in prolonged survival of vascularized organs and islet grafts. CD40Ig, a recombinant fusion protein comprised of the extracellular domain of human CD40 molecule in frame fused with the site-mutated human IgG1 Fc region, abrogated the cognate interaction of CD40-CD154 pathway by binding the CD154 molecule. In this study, replication-defective adenovirus containing the CD40Ig gene was prepared by homologous recombination and used to infect freshly isolated islets from LEW rats (RT-1(1)) in vitro using a titered dose. The islet transfectants (500 per recipient) were transplanted under the left kidney capsule of streptozocin-rendered diabetic C57BL/6 mouse recipient (H-2(b)). The mean survival time of AdCD40Ig-transfected islet grafts was significantly prolonged, while mock-infected grafts and AdEGFP-transfected grafts were rejected in normal fashion. Additionally, dose-dependent prolongation of islet graft survival was observed in mice receiving AdCD40Ig-transfected grafts. In conclusion, local production of Cd40Ig via adenoviral-mediated gene transfer induced dose-dependent prolongation of LEW --> Balb-c islet xenografts.

Transplantation immunology 2003: simplified approach

Transplantation has been performed clinically for four decades and has become the standard of care for end-stage organ failure. Understanding of the immunobiology of transplantation has made tremendous advances, but knowledge still lags behind the clinical use. As a result, nonspecific immunosuppression remains the standard therapy. This article presents an overview of current knowledge of the immunobiology of solid organ transplantation, with emphasis on T-cell activation (antigen presentation, CoS) and cellular allograft (transplantation) immunity. The molecular events of T-cell activation, with some emphasis on the sites of action of modern immunosuppression, are reviewed. A simplified approach to understanding the immunobiology and strategy of maintenance immunosuppression is discussed. Key early and late steps in T-cell activation and the sites of action of immunosuppressive agents are reviewed. The required cellular interactions for the alloresponse and the targets of biologic agents used in transplants are reviewed. Special considerations for the immunology in neonates, infants, and children as recipients are provided. Understanding the immunobiology of transplantation is key to making decisions about children with transplants, developing better protocols, and creating tolerance in the future.

Simultaneous administration of a low-dose mixture of donor bone marrow cells and splenocytes plus adenovirus containing the CTLA4Ig gene result in stable mixed chimerism and long-term survival of cardiac allograft in rats

T-cell costimulatory blockade combined with donor bone marrow transfusion may induce mixed chimerism, rendering robust tolerance in transplanted organs and cells. However, most protocols entail high doses of donor bone marrow cells (BMCs) or repeated administration of costly agents that block costimulatory pathways, thus delaying clinical development. To circumvent these shortcomings, we developed a strategy in which the dosage of donor BMCs was reduced but compensated by donor splenocytes (SPLCs). Furthermore, repeated administration of costly agents was replaced with a single injection of adenovirus expressing a gene of interest. In rat cardiac transplantation models, cardiac allografts from DA (RT-1a) rats were transplanted heterotopically into the abdomen of LEW (RT-11) recipient rats. Immediately after cardiac transplantation, an adenovirus vector (AdCTLA4Ig; 5 x 10(9) plaque-forming units) containing the gene for CTLA4Ig was administered to recipients (n = 6) simultaneously with a low dose of donor BMCs (1 x 10(8)/rat) and SPLCs (5 x 10(7)/rat) via the portal vein. The treated LEW recipient rats developed long-lasting mixed chimerism (>10% at >100 days) and exhibited long-term cardiac allografts (mean survival time of > 200 days) compared with control recipients. Moreover, recipients displaying long-lasting mixed chimerism accepted subsequent donor skin allografts while promptly rejecting third-party skin allografts. These results suggest that blockade of the CD28-B7 pathway, using adenovirus-mediated CTLA4Ig gene transfer, in concert with a low dosage of donor BMCs and SPLCs, may represent a feasible strategy to induce stable mixed chimerism and permit long-term survival of cardiac allografts.

Long-Term Survival of Xenografted Neonatal Cardiomyocytes by Adenovirus-Mediated CTLA4-Ig Expression and CD40 Blockade

BACKGROUND

Prolonged survival of xenografted neonatal cardiomyocytes was achieved by blocking the CD28/B7 costimulatory pathway via CTLA4-Ig gene transfer. We examined the long-term survival of xenografted neonatal cardiomyocytes by adenovirus-mediated CTLA4-Ig expression and transient CD40 blockade with anti-CD40L monoclonal antibody (MR1).

METHODS AND RESULTS

Neonatal cardiomyocytes derived from Dark Agouti rats were infected with CTLA4-Ig-expressing adenovirus vectors and injected directly into the normal myocardium of C3H/He mice. Mice were also given an intraperitoneal injection of 500 microg MR1 (CTLA+MR group, n=30) or control immunoglobulin (CTLA group, n=30) 1 hour before and 1, 3, and 7 days after cardiomyocyte implantation. As a control, cells infected with beta-Gal-expressing adenovirus vector (RL group, n=15) and cells without infection (control group, n=15) were injected into additional mice. Mice from all groups were killed 2, 4, and 8 weeks after xenotransplantation, and mice from the CTLA+MR and CTLA groups were killed 4 and 6 months after xenotransplantation. Neonatal cardiomyocytes were successfully infected by adenovirus vectors. Immunohistochemical analysis showed that the xenografted cardiomyocytes survived and expressed CTLA4-Ig for 6 months in all mice from the CTLA+MR and CTLA groups. A gap junction between the xenografted and host cardiomyocytes was also confirmed. Conversely, neonatal cardiomyocytes did not survive for even 2 weeks after xenotransplantation in the mice from the RL and control groups.

CONCLUSIONS

Long-term survival of xenografted neonatal cardiomyocytes was achieved by adenovirus-mediated CTLA4-Ig expression and transient CD40 blockade.

Mammary gland-specific secretion of biologically active immunosuppressive agent cytotoxic-T-lymphocyte antigen 4 human immunoglobulin fusion protein (CTLA4Ig) in milk by transgenesis

A major challenge in the field of transplantation is to prevent graft rejection and prolong graft survival. Tolerance induction is a promising way to achieve long-term graft survival without the need for potent immunosuppression and its associated side effects. The recent success of co-stimulatory blockade by the chimeric protein CTLA4Ig in the modulation of the recipient's immune system and the prolongation of graft survival in animal models suggests a possible application of CTLA4Ig in clinical transplantation. To produce sufficient amounts of CTLA4Ig for future clinical application, we sought to use the mammary gland as a bioreactor and produce CTLA4Ig in the milk of transgenic farm animals. Prior to the generation of transgenic farm animals, we tested our strategy in mice. Using the promoter of the sheep beta-lactoglobulin gene, we expressed our CTLA4Ig chimeric gene in the mammary gland of transgenic mice. The yield of CTLA4Ig was fivefold higher in transgenic milk than that from transfected cells. Purified milk-derived CTLA4Ig is biologically active and suppresses T cell activation. We showed that the production of CTLA4Ig in the milk has no adverse immunosuppression effect on the transgenic animals and the offsprings that were fed with the transgenic milk. The findings suggest that the approach to produce CTLA4Ig in milk by transgenesis is feasible; further studies involving farm animals are warranted.

CTLA4Ig adenoviral gene transfer induces long-term islet rat allograft survival, without tolerance, after systemic but not local intragraft expression

Genetic engineering using recombinant adenoviruses offers an opportunity to modify islet grafts in order to prevent allograft rejection. We have used an adenovirus coding for CTLA4Ig to compare its efficacy in preventing islet rejection depending on local or systemic production after gene transfer either into the islets or intramuscularly, respectively. Islet allograft survival was also evaluated using recombinant CTLA4Ig administered intraperitoneally or incubated ex vivo with islets prior to transplantation. Transduction of islets with 10(3) or 10(4) plaque-forming units (pfu) per islets of AdCTLA4Ig prolonged islet survival (mean +/- standard deviation [SD] days = 19.5 +/- 5.8 and 19.5 +/- 5.6, respectively, vs. 10.6 +/- 2.4 in control islets, p < 0.001), with low levels of circulating CTLA4Ig. In contrast, long-term survival (>60 days) was obtained after intramuscular injection of AdCTLA4Ig that resulted in sustained high levels of circulating CTLA4Ig. Islets incubated in vitro with CTLA4Ig did not show prolonged survival (10.3 +/- 2.5 days). Graft rejection was delayed after one injection of CTLA4Ig (23 +/- 7.6 days, p < 0.003 vs. control). Recipients of long-term surviving grafts after intramuscular AdCTLA4Ig gene transfer were not tolerant because second islet grafts of donor origin were rejected. These recipients also had a strong inhibition of humoral responses against nominal antigens, whereas animals receiving transduced islets showed normal responses. These data demonstrate that local production of CTLA4Ig after gene transfer was as efficient as a single injection of CTLA4Ig in preventing graft rejection. Furthermore, intramuscular gene transfer of CTLA4Ig was the most efficient way to induce long-term islet graft survival but no donor-specific tolerance was induced.

Prolongation of liver xenograft survival by adenovirus vector-mediated CTLA-4Ig gene transfer

Cytotoxic T lymphocyte-associated antigen-4/immunoglobulin fusion products (CTLA-4Ig), a structural homologue of CD28, has been shown to inhibit cellular and humoral immune responses. In this study, we investigated the efficacy of an adenovirus vector containing the CTLA-4Ig gene (AdCTLA-4Ig) on recipient survival after hamster-to-rat liver xenografting. AdCTLA-4Ig was administrated intravenously immediately after grafting. Gene expression was achieved a maximum of 7 days after vector injection and continued for more than 4 weeks. The proportion of CD25(+) T-cells in recipient lymph nodes was significantly reduced 7 days after administration of AdCTLA-4Ig, compared to a group given an adenoviral vector containing LacZ gene (AdLacZ) or to an untreated control group. AdCTLA-4Ig markedly reduced CD2(+) T-cell infiltration into the graft and significantly prolonged recipient survival time (9.2+/-4.12 days), compared to the untreated group (5.4+/-0.78 days) (P<0.001) and the AdLacZ-treated group (5.2+/-0.28 days) (P<0.001). These results indicate that a blockade of T-cell co-stimulation by AdCTLA-4Ig inhibited T-cell activation and attenuated CD2(+) T-cell infiltration into the xenograft, resulting in significant prolongation of recipient survival time. Thus, AdCTLA-4Ig therapy may provide a novel approach to immune regulation.

Development and retroviral transduction of porcine neonatal pancreatic islet cells in monolayer culture

To learn more about the potential of neonatal porcine pancreatic duct and islet cells for xenotransplantation, the development of these cells when cultured as monolayers was evaluated. Immunostaining for islet hormones and cytokeratin-7 revealed that day eight monolayers consisted of approximately 70% duct cells and less than 10% beta cells. Using Ki-67 immunostaining as a proliferation marker, the fraction of beta cells in the cell cycle was shown to decrease from 20% at day three to 10% at day eight, and for duct cells from 36 to 19%. Insulin secretion increased 2.4-fold upon glucose stimulation, and 38-fold when 10 mm theophylline was added, showing the responsiveness of the neonatal beta cells. Reaggregated monolayers consisted mostly of duct cells, but 4 weeks after transplantation, grafts contained predominantly endocrine cells, with duct cells being almost absent, suggesting in vivo differentiation of duct cells to endocrine cells. Monolayer susceptibility to retroviral transduction was also investigated using a Moloney Murine Leukemia Virus-based vector. Approximately 60% of duct cells but less than 5% of beta cells expressed the transgene, indicating that precursor duct cells are better targets for transgene expression. These results show that porcine neonatal pancreatic cells can be cultured as monolayers in preparation for transplantation. Furthermore, in such a culture setting, precursor duct cells have a high rate of proliferation and are more efficiently transduced with a retrovirus-based reporter gene than are beta cells.

Ex vivo and systemic transfer of adenovirus-mediated CTLA4Ig gene combined with a short course of FK506 therapy prolongs islet graft survival

Adenovirus-mediated CTLA4Ig gene transfer has been reported to enhance graft survival in several rodent transplantation models. In this study, we investigated the efficacy of ex vivo and systemic transfer of the CTLA4Ig gene by adenoviral vectors in pancreatic islet allo-transplantation. Islet grafts from BN rats were transplanted to chemically induced diabetic LEW rats. First, ex vivo CTLA4Ig gene transfer into isolated islets was performed prior to transplantation. Survival of transduced grafts under the kidney capsule was slightly prolonged (8.6+/-1.3 days) compared with survival of untransduced grafts (6.7+/-1.2 days); when combined with a short course of FK506, graft survival was further extended (32.6+/-10.7 days vs. 13.7+/-1.0 days with FK506 alone). Secondly, systemic gene transfer was accomplished by intravenous administration immediately after the transplantation procedure. In these animals, islet grafts under the kidney capsule survived longer (15.2+/-3.3 days) than in controls (6.7+/-1.2 days), and when FK506 was administered perioperatively, all the islet grafts survived for more than 100 days. In systemically transduced recipients, the survival of islet grafts transplanted into the liver was not significantly different from that of the grafts placed under the kidney capsule. In order to examine organ-specific immunogenicity, heterotopic BN cardiac grafts were transplanted to LEW rats intra-abdominally, with the virus transferred systemically as in the islet model. In contrast to the islet grafts, all the cardiac grafts were accepted for longer than 100 days, even without FK506 therapy. Finally, the LEW recipients with long-surviving islet or cardiac grafts were re-transplanted with islet grafts from the same donor strain (BN) on day 100. The second islet grafts survived longer than 100 days in half of the cardiac recipients, but consistently failed in the islet recipients. We conclude that in this transplant model, CTLA4Ig gene transfer and FK506 treatment synergistically improved islet graft survival, systemic transfer of the gene was more effective than ex vivo transfer to the islets, and donor-specific tolerance could not be achieved for islet transplantation but was achieved for cardiac transplantation.

Targeted gene therapy with CD40Ig to induce long-term acceptance of liver allografts

BACKGROUND

The purpose of this study was to modulate the immune response of rat liver transplant recipients by adenovirus-mediated gene transfer of CD40Ig, a secretable fusion protein designed to block the CD40-CD154 T-cell costimulation pathway.

METHODS

CD40Ig complementary DNA was created by joining the reverse transcriptase-polymerase chain reaction complementary DNA products for the extracellular domain of murine CD40 to the Fc portion of murine IgG2a. AdCD40Ig and AdSIg (IgG2a-Fc control) recombinant adenoviruses were used to transduce donor liver grafts before nonarterialized orthotopic rat liver transplantation. Donor specific unresponsiveness was examined with skin transplants.

RESULTS

All rats (n = 6) that received liver allografts transduced with AdCD40Ig survived >100 days with normal liver histology. Serum levels of CD40Ig at 10, 30, 60, and 100 days after transplantation ranged from 100 to 500, 100 to 250, 5 to 40, and 2 to 10 microg/mL, respectively. Mean survival of rats (n = 4) that received liver allografts transduced with AdSIg control adenovirus was 9.25 +/- 2.9 days. Long-term survivors were rechallenged with skin grafts 100 days after liver transplantations. Survival was 72, >100 (x4) days for donor specific allogeneic skin grafts and 14, 14, 18, 19, and 21 days for third-party allogeneic skin grafts.

CONCLUSIONS

Adenovirus-mediated gene transfer of CD40Ig into cold-preserved liver allografts before transplantation results in high levels of transgene expression with resultant long-term survival of hepatic allografts and donor specific unresponsiveness.

Gene therapy in transplantation

Gene transfer and gene therapy represent a relatively new field that has grown and expanded enormously in the last 5-10 years. The application of gene transfer and gene medicines to transplantation is currently in its infancy. Consideration for gene medicines in transplantation requires delivery of vectors, either to the graft or to the immune system. Delivery of vectors to the graft provides a choice of potential immunologic targets including: costimulatory signals; inhibitory cytokines; adhesion molecules; and molecules relating to apoptosis. In addition, non-immunologic targets, that increase graft protective mechanisms by reducing ischemic and immunologic damage, represent significant targets for gene transfer. Delivery of vectors to the immune system includes potential targets to modify the immune system, and results in tolerance. Other considerations for gene therapy include the development of additional technologies, such as gene conversion or transgenesis coupled with xenotransplantation, which may provide genetically modified organs. Another important aspect of gene transfer relates to regulation of the transgene expression. A variety of issues concerning innate immunity, adaptive immunity, response to vector components, response to transgene products, and entry of vectors into the antigen presentation and processing pathway require further investigation and refinement of approaches. Lastly, regulatable promoters and the understanding of their interaction with individual cells, tissues and organs, and their interaction with innate and adaptive immunity, are of paramount importance to improving the efficacy and utility of gene transfer. There is no doubt that there is much exciting basic and translational science to be accomplished in the next decade in order to solve these potential barriers and advance gene medicines into the clinical realm in transplantation.

Immunomodulation with CTLA4-Ig in islet transplantation

The need for permanent, nonspecific, and potentially harmful immunosuppression remains a major obstacle for islet transplantation. The response of a type 1 diabetic recipient to an islet graft includes a specific allogenic immune response and the recurrence of autoimmunity. Free or encapsulated in an immunoisolation device, islet cells are exposed to immune aggression, initiated by donor antigen-presenting cells or by indirect, host antigen-presenting cell-mediated antigen presentation. CTLA4-Ig is a genetically engineered fusion protein of human CTLA4 and the IgG 1 Fc region. It prevents T-cell activation by binding to human B7, which costimulates T cells through CD28. Interesting data were reported in experimental islet transplantation, suggesting that CTLA4-Ig may be slightly but significantly beneficial to islet allograft survival, although studies in autoimmune diabetes are scarce. The main limitations include transient and low levels of expression when CTLA4-Ig is delivered locally, a predominant effect on the direct recognition pathway, and the lack of effect on memory cells. Clinical trials in islet transplantation could be discussed in nonuremic patients, with steroid-free and anticalcineurin-free regimens, in combination with another costimulation blocker, rapamycin, and an anti-interleukin 2 receptor antibody, and with a strategy directed against the recurrence of autoimmunity.

Neonatal porcine pancreatic cell clusters as a potential source for transplantation in humans: characterization of proliferation, apoptosis, xenoantigen expression and gene delivery with recombinant AAV

Neonatal porcine islets are characterized by reproducible isolation success and high yields, sizable advantages over adult islets. In this work we have analyzed selected phenotypic and functional characteristics of porcine neonatal islets relevant to their possible use for transplant in humans. We show that porcine islet cells proliferate in culture, and synthesize and store islet-specific hormones. Proliferating beta cells can be easily identified. Implant of cultured neonatal islets in immunodeficient rodents results in the reversal of diabetes, albeit with delay. We also show that measurable apoptosis occurs in cultured neonatal porcine islets. Further, antigens recognized by human natural antibodies are expressed in a dynamic fashion over the culture period analyzed and are not limited to the alpha-Gal epitope. Lastly, we demonstrate that a recombinant Adeno-Associated virus can be used to efficiently deliver a reporter gene in porcine islets. This characterization might be helpful in the definition of the potential use of neonatal porcine islets for human transplantation.

Prolonged survival of xenograft fetal cardiomyocytes by adenovirus-mediated CTLA4-Ig expression

BACKGROUND

Experimental allografting of fetal cardiomyocytes has been performed successfully. In this study, we attempted to transplant rat fetal cardiomyocytes into the hearts of mice by blocking the CD28/B7 costimulatory pathway via CTLA4-Ig gene transfer.

METHODS

Fetal cardiomyocytes derived from Dark Agouti rat were infected with CTLA4-Ig-expressing adenovirus vectors (AdCTLA) and injected directly into the normal myocardium of C3H/He mice (n=15). For control, cells infected with beta-Gal-expressing adenovirus vector (AdRL) and cells without infection were injected into additional mice (n=30). Mice were killed at 2 (n=5), 4 (n=5), and 8 (n=5) weeks after xenotransplantation. Transplanted fetal cardiomyocytes were examined for survival by immunostaining with anti-rat atrial natriuretic peptide and anti-CTLA4-Ig antibodies.

RESULTS

Fetal cardiomyocytes were successfully infected by AdCTLA and AdRL. The cardiomyocytes infected with AdCTLA expressed CTLA4-Ig and survived to 8 weeks after xenotransplantation in all of these mice. However, cardiomyocytes infected with AdRL and noninfected cells were not detected even 2 weeks after xenotransplantation.

CONCLUSION

Survival of xenografted fetal cardiomyocytes is prolonged by adenovirus-mediated CTLA4-Ig expression.

Glucose-induced toxicity in insulin-producing pituitary cells that coexpress GLUT2 and glucokinase. Implications for metabolic engineering

We have shown that intermediate lobe (IL) pituitary cells can be engineered to produce sufficient amounts of insulin (ins) to cure diabetes in nonobese diabetic mice but, unlike transplanted islets, ILins cells evade immune attack. To confer glucose-sensing capabilities into these cells, they were further modified with recombinant adenoviruses to express high levels of GLUT2 and the beta-cell isoform of glucokinase (GK). Although expression of GLUT2 alone had negligible effects on glucose usage and lactate production, expression of GK alone resulted in approximately 2-fold increase in glycolytic flux within the physiological (3-20 mm) glucose range. GLUT2/GK coexpression further increased glycolytic flux at 20 mm glucose but disproportionately increased flux at 3 mm glucose. Despite enhanced glycolytic fluxes, GLUT2/GK-coexpressing cells showed glucose dose-dependent accumulation of hexose phosphates, depletion of intracellular ATP, and severe apoptotic cell death. These studies demonstrate that glucose-sensing properties can be introduced into non-islet cells by the single expression of GK and that glucose responsiveness can be augmented by the coexpression of GLUT2. However, in the metabolic engineering of surrogate beta cells, it is critical that the levels of the components be closely optimized to ensure their physiological function and to avoid the deleterious consequences of glucose-induced toxicity.

Long-term acceptance of allografts by in vivo gene transfer of regulatable adenovirus vector containing CTLA4IgG and loxP

CTLA4IgG was shown to inhibit the costimulatory signal for T cell activation by interfering with the ligation of CD28 and B7-1 or B7-2. To inhibit various immune responses including acute cellular rejection of allografts, a certain level of serum CTLA4IgG should be maintained for an appropriate period. We previously reported on an adenovirus vector containing CTLA4IgG, which we designated Adex1CACTLA4IgG. Adex1CACTLA4IgG was able to maintain a significant level of serum CTLA4IgG for a long period on intravenous injection, which in turn inhibited various immune responses including protective immunity against infectious agents. To overcome the inhibitory effect, we constructed a new adenovirus vector, Adex1CALoxCTLA4IgGLox, by cloning CTLA4IgG cDNA between two loxP sequences under the control of the CAG promoter. We demonstrated that the administration of adenovirus vector containing Cre recombinase gene (Adex1CACre) at the desired time induced Cre-mediated recombination within a gene derived from Adex1CALoxCTLA4IgGLox vector, and the cDNA of CTLA4IgG was excised from the transduced gene and terminated the expression of CTLA4IgG in vitro and in vivo. More importantly, we also demonstrated that the long-term acceptance of allografts was achieved after the termination of CTLA4IgG expression, while the immune response against adenovirus was restored.

Rat xenograft survival in mice treated with donor-specific transfusion and anti-CD154 antibody is enhanced by elimination of host CD4+ cells

BACKGROUND

Treatment with a donor-specific transfusion (DST) and a brief course of anti-mouse CD154 (anti-CD40-ligand) monoclonal antibody (mAb) prolongs the survival of both allografts and rat xenografts in mice. The mechanism by which allograft survival is prolonged is incompletely understood, but depends in part on the presence of CD4+ cells and the deletion of alloreactive CD8+ T cells. Less is known about the mechanism by which this protocol prolongs xenograft survival.

METHODS

We measured rat islet and skin xenograft survival in euthymic and thymectomized mice treated with combinations of DST, anti-CD154 mAb, anti-CD4 mAb, and anti-CD8 mAb. Recipients included C57BL/6, C57BL/6-scid, C57BL/6-CD4null, and C57BL/6-CD8null mice.

RESULTS

Pretreatment with a depleting anti-CD4 mAb markedly prolonged the survival of both skin and islet xenografts in mice given DST plus anti-CD154 mAb. Comparable prolongation of xenograft survival was obtained in C57BL/6-CD4null recipients treated with DST and anti-CD154 mAb. In contrast, anti-CD8 mAb did not prolong the survival of either islet or skin xenografts in mice treated with DST and anti-CD154 mAb. Thymectomy did not influence xenograft survival in any treatment group. Adoptive transfer of splenocytes from C57BL/6-CD4null recipients treated with DST and anti-CD154 mAb and bearing long-term skin xenografts revealed the presence of residual xenoreactive cells.

CONCLUSIONS

These data suggest that treatment with DST and anti-CD154 mAb induces a state of "functional" transplantation tolerance. They also support the hypothesis that both the induction and maintenance of graft survival based on this protocol depend on different cellular mechanisms in allogeneic and xenogeneic model systems.

A self-immunomodulating myoblast cell line for erythropoietin delivery

The transplantation of genetically engineered cells faces limitations associated with host immunity. Allogeneic cells are typically rejected in response to inherent histo-incompatibility. Even autologous cells can induce an immune response toward antigenic molecules expressed following transfer of foreign genes. The goal of the present study was to investigate the ability of immunomodulating molecules co-expressed with biotherapeutic factors to overcome these limitations both in syngeneic and allogeneic cell transplantation. The C(2)C(12) mouse myoblast cell line was engineered to express CTLA4Ig, a soluble factor blocking T cell costimulation, in conjunction with erythropoietin (Epo), a reporter biotherapeutic protein. In syngeneic C3H mice, myoblasts expressing only mouse Epo were mostly rejected within 2 weeks, as indicated by the transient increase in host hematocrit. In allogeneic recipients, the same cells induced only a 1-week increase in the hematocrit reflecting an acute rejection process. CTLA4Ig expression significantly extended the survival of mouse Epo-secreting myoblasts in approximately half of syngeneic hosts, whereas it led only to a 1-week improvement effect in allogeneic recipients. When combined with a transient anti-CD154 treatment, CTLA4Ig expression prevented Epo-secreting C(2)C(12)myoblasts from being rejected in allogeneic DBA/2J recipients for at least 1 month. In contrast, the same anti-CD154 treatment alone induced only a 1 week improvement. These results demonstrate that CTLA4Ig co-expression associated with a transient anti-CD154 treatment can prolong the delivery of recombinant proteins via transfer of ex vivo modified cells in allogeneic recipients.

Hepatocyte transplantation in rats with decompensated cirrhosis

Hepatocyte transplantation improves the survival of laboratory animals with experimentally induced acute liver failure and the physiological abnormalities associated with liver-based metabolic deficiencies. The role of hepatocyte transplantation in treating decompensated liver cirrhosis, however, has not been studied in depth. To address this issue, cirrhosis was induced using phenobarbital and carbon tetrachloride (CCL(4)) and animals were studied only when evidence of liver failure did not improve when CCL(4) was held for 4 weeks. Animals received intrasplenic transplantation of syngeneic rat hepatocytes (G1); intraperitoneal transplantation of syngeneic rat hepatocytes (G2); intraperitoneal transplantation of a cellular homogenate of syngeneic rat hepatocytes (G3); intraperitoneal transplantation of syngeneic rat bone marrow cells (G4); or intrasplenic injection of Dulbecco's modified Eagle medium (DMEM) (G5). After transplantation, body weight and serum albumin levels deteriorated over time in all control (G2-G5) animals but did not deteriorate in animals receiving intrasplenic hepatocyte transplantation (G1) (P <.01). Prothrombin time (PT), total bilirubin, serum ammonia, and hepatic encephalopathy score were also significantly improved toward normal in animals receiving intrasplenic hepatocyte transplantation (P <. 01). More importantly, survival was prolonged after a single infusion of hepatocytes and a second infusion prolonged survival from 15 to 128 days (P <.01). Thus, hepatocyte transplantation can improve liver function and prolong the survival of rats with irreversible, decompensated cirrhosis and may be useful in the treatment of cirrhosis in humans.

Xenotransplantation

BACKGROUND

The success of clinical transplantation has led to a large discrepancy between donor organ availability and demand; considerable pressure exists to develop an alternative source of organs. The use of animal organs for donation is a possible solution that is not yet clinically applicable.

METHODS AND RESULTS

A literature review was performed based on a Medline search to find articles on xenotransplantation. Keywords included hyperacute, acute vascular, xenograft rejection combined with concordant and discordant. Additional references cited in these articles from journals not included in Medline were obtained from the British Library. Limited information on unpublished, preliminary work has been included from sources known to the authors, based on their research work in the field. One hundred and forty-six references and four personal communications have been included in this review article.

CONCLUSION

A greater understanding of the pathogenesis of xenograft rejection is developing rapidly. Strategies to abrogate hyperacute rejection have proved successful, but control of antibody-driven acute vascular rejection has not yet been achieved. The safety and viability of xenotransplantation as a therapeutic modality are still unproven.