Local production of CTLA4-Ig by adenoviral-mediated gene transfer to the pancreas induces permanent allograft survival and donor-specific tolerance

Avenues for immunomodulation and graft protection by gene therapy in transplantation

Organ transplantation represents the only definitive therapy for many causes of end-organ failure. However, the universal success of this therapy is limited by chronic allograft rejection, the side effects of chronic immunosuppressive therapy, and a severe shortage of donor organs. Presently, the success of solid-organ transplantation depends on the continuous administration of toxic and nonspecific immunosuppressive agents, therapies that present risks for opportunistic infection, malignancy, and a variety of agent-specific side effects. To promote the use of transplantation with limited risk of long-term sequelae, three dominant research challenges emerge: (i) elimination of the need for exogenous immunosuppression by immunological tolerance induction; (ii) prevention of chronic rejection/graft dysfunction; and (iii) expansion of available organs for transplantation. Gene therapy may provide significant advances and solutions in each of these areas. Rejection of the graft in the immediate post-transplant period has been attacked through the transfer of immunomodulatory molecules in addition to tolerance inducing approaches. Chronic graft rejection may be similarly addressed through permanent tolerance induction or alternatively through the introduction of molecules to resist chronic graft damage. Genetic manipulation of stem cells may ultimately produce transgenic animals to serve as tissue donors to overcome the limited donor organ supply. This review will highlight ongoing developments in the translation of gene therapy approaches to the challenges inherent in 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.

Development of Donor-Specific Immunoregulatory T-Cells After Local CTLA4Ig Gene Transfer to Pancreatic Allograft

BACKGROUND

CTLA4Ig gene transfer directly to graft tissue might have the potential to avoid the need for systemic immunosuppression. In our previous studies of bio-breeding (BB) rats, local adenovirus-mediated CTLA4Ig gene transfer protected the pancreas from autoimmune and alloimmune responses. This study investigated the potency of local CD28/B7 costimulatory blockade for induction of donor-specific tolerance and further examined the existing mechanisms.

METHODS

Brown Norway (BN; RT1)-pancreaticoduodenal grafts transfected with Ad.CTLA4Ig via intraarterial ex vivo perfusion were transplanted into streptozotocin-induced diabetic Lewis (LEW; RT1) rats.

RESULTS

Ad.CTLA4Ig transduced grafts combined with a short course of FK506 resulted in indefinitely prolonged survival (>156 days vs. 19.5 days with FK506 alone). CTLA4Ig was predominantly expressed in grafts on day 4. The expression was gradually diminished and was only slightly detectable at day >100. The proliferative responses against BN antigen were remarkably enhanced among recipients with rejected grafts, but the T-cells from tolerant recipients (>100 days) showed poor cytotoxic responses. On adoptive transfer assay, the splenic T-cells of tolerant recipients were able to suppress the rejection of BN, but not third-party Wistar Furth (WF; RT1) hearts in irradiated (480 cGy) LEW recipients. The percentage of CD4CD25 splenic T-cells was significantly increased in tolerant recipients (13.53 +/- 4.06% vs. 6.06 +/- 0.56% in naive rats).

CONCLUSION

CTLA4Ig gene transfer to the pancreaticoduodenal allograft combined with a short course of FK506 induces donor-specific tolerance. The mechanism of maintaining tolerance could be explained by development of splenic T suppressor cells.

Immunological characteristics of pancreas transplantation: review and our experimental experience

Pancreas and islet transplantation is the only treatment that can cure type 1 diabetes mellitus (DM). With the recent advances of operative procedure and immunosuppression, pancreas graft survivals have become better than before, but some problems still remain. It is extremely difficult to establish tolerance and to reverse rejection once it has been initiated because the pancreas graft itself has a strong immunogenicity. It is also known that pancreatic graft failure is sometimes due to autoimmune recurrence. In the clinical situation, however, these immunologic events actually coexist within the pancreatic graft. Thus, it is rather difficult to analyze each of them independently, but possible to delineate each of these immunologic mechanisms with using animal models of type 1 DM such as BB (BioBreeding) rats or NOD (nonobese diabetic) mice. In the current study, we reviewed the immunological characteristics in pancreas transplantation (PTx) based on our experimental experiences together with that of others and investigated the possibility of tolerance induction in PTx.

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.

Modified AdCTLA-4 vector blocks alloimmune response in vitro

BACKGROUND

Gene transfer of the costimulatory blockade molecule CTLA-4Ig into cold-preserved rat liver allografts results in indefinite allograft survival. Despite local delivery, this mode of immunomodulation is associated with systemic immunosuppression. In an effort to restrict immunosuppression to the graft, we have constructed a novel adenoviral vector, AdCTLA-4ex-TAG, in which the Ig sequence of CTLA-4Ig DNA has been deleted to destabilize the gene product to promote rapid extrahepatic degradation while maintaining its immunosuppressive activity within the graft milieu.

METHODS

(1) Vector construction. CTLA-4 extracellular binding domain (CTLA-4ex) was prepared by PCR-based cloning methods and fused in frame to a genetic element encoding an epitope TAG allowing identification of the transgene product CTLA-4exTAG. CTLA-4exTAG was subcloned into a shuttle vector enabling isolation of AdCTLA-4exTAG. (2) In vitro transfection: AdCTLA-4exTAG was transfected into MH(1)C(1) cells and then supernatant was recovered for Western blot analysis. (3) In vitro alloimmune response was characterized by CFSE proliferation assay. (4) Extrahepatic effect of AdCTLA-4exTAG was characterized by the ability to control tumor growth after implantation of a regressive, immune sensitive cancer cell line (REGb) in the skin of BDIX rats after liver transduction with AdCTLA-4exTAG.

RESULTS

Expression and secretion of the recombinant protein were documented by Western blot after infection of the MH(1)C(1) cell line() with AdCTLA-4exTAG. Addition of infected MH(1)C(1) cell supernatant resulted in abrogation of alloimmune response as shown by markedly diminished division of CD4(+) T cells in a CFSE proliferation assay. Extrahepatic tumor regressed normally after liver transduction with AdCTLA-4exTAG.

CONCLUSIONS

These results show efficient in vitro expression of CTLA-4exTAG after transfection with AdCTLA-4exTAG. The modified protein retains its ability to abrogate in vitro alloimmune response. Efficient control of extrahepatic tumor growth after liver-directed delivery of AdCTLA-4exTAG suggests that the immunosuppressive effect of this vector is restricted to the liver. These results set the ground for the utilization of this novel adenoviral vector in the transplant setting.

Gene therapy and transplantation

Advances in molecular biology and in techniques of gene transfer have resulted in the development of practical approaches to human gene therapy. Many applications are of relevance to manipulation of the immune system and have potential in organ and cell transplantation. For example, gene therapy approaches may facilitate the induction of immunological tolerance to a donor organ or protect it locally against the host's immune response. Based on a comprehensive review of the world literature, examples of current research efforts in both allogeneic and xenogeneic transplantation are presented and discussed.

Gene therapy strategies to facilitate organ transplantation

Organ transplantation is now the definitive therapy for many forms of end-organ disease, but chronic allograft rejection, the side effects of chronic immunosuppressive therapy and the severe donor organ shortage continue to limit its success. Gene therapy has the potential to prevent graft rejection by manipulating the immune response in the microenvironment of the graft or by facilitating the induction of tolerance. Genetic manipulation of stem cells to create transgenic and/or knockout animals that could serve as organ or cell donors could be combined with gene therapy approaches to overcome the problem of limited allogeneic donor organ supply.