IMMUNE REGULATION AND TOLERANCE II

C. Tan-tam¹, V. Nguyen², P. Stock³, Q. Tang^2
1Surgery, University of California, San Francisco, San Francisco/CA/UNITED STATES OF AMERICA, ²Surgery, University of California, San Francisco, San Francisco/UNITED STATES OF AMERICA, ³, University of California, San Francisco, San Francisco/CA/UNITED STATES OF AMERICA

Body: Introduction: The success of islet transplantation is currently impeded by early loss of the grafted islets, and later alloimmune and recurrent autoimmune-mediated rejection despite immunosuppression. Thus, islet transplantation may be improved by creating a milieu that promotes repair of islets injured during isolation, and protects islets from immune insults. Mesenchymal stem cells (MSC) have been shown to contribute to tissue repair by promoting vascularization and matrix deposition. In addition, MSCs are reported to be hypoimmunogenic and promote immune tolerance by secretion of immunomodulatory factors. In this study, we characterized MSC immunogenicity in vivo and their effect on syngeneic and allogeneic islet grafts. Methods:Bone marrow-derived MSCs were isolated and cultured in our lab, or obtained from Texas A&M. The MSCs were characterized using flow cytometry and differentiation assays. In vivo proliferation of TCR transgenic CD8 and CD4 T cells that are specific for BALB/c alloantigens were used to assess the immunogenicity of MSCs. To track the persistence of MSCs using in vivo imaging, we transduced MSCs with firefly luciferase before subcutaneous transplantation, and monitored luciferase signal over time using Xenogene imaging system. Transplantation of islets into the renal capsule of streptazocin-induced diabetic mice was used to determine if MSCs could prevent early loss of syngeneic islet grafts or to prevent rejection of allogeneic islets grafts. MSCs were infused or co-transplanted with syngeneic or allogeneic islet grafts and the graft function was determined by following blood glucose levels. Results: The MSCs had the cell surface phenotype of CD45-CD34-CD105+CD73+ and variable level of CD90, which decreased with increasing passages. They were able to differentiate into adipocytes, osteoblasts, and chondrocytes in vitro demonstrating that they are functional MSCs. The BALB/c MSCs did not induce proliferation of CD8 T cells specific for BALB/c alloantigen H-2L^d in vivo in B6 mice. In contrast, the cells were able to induce a moderate proliferative response of CD4 T cells that are specific for a BALB/c allopeptide presented on B6 MHC IA^b. Co-transplantation or infusion of syngeneic MSCs with or without prior exposure to IFNg with renal capsule syngeneic islet transplants did not reduce the minimal islet mass required to normalize blood glucose, suggesting that the MSCs were unable to prevent early graft loss. In addition, allogeneic islets co-transplanted with syngeneic MSCs were rejected without delay demonstrating that MSCs were not able to control allogeneic graft rejection in this model. Moreover, some mice developed tumors at the transplant site. Conclusion: MSCs are unable to trigger direct immune recognition and T cell proliferation, but they are able to induce moderate proliferation of T cells that indirectly recognize alloantigens MSCs express. This weak immunogenicity was sufficient to induce their rejection in immune competent hosts. Syngeneic MSCs failed to protect islet grafts from early graft failure and alloimmune rejection. Our results suggest that MSCs may be heterogeneous and their immunological properties should be carefully evaluated in vivo.

Disclosure: All authors have declared no conflicts of interest.