Cell therapeutic strategies to improve pig islet graft survival after xenotransplantation

Abstract

Transplantation of the whole pancreas organ or isolated Langerhans‐islets is the only methods to cure Type 1 diabetes. Transplantation is largely limited by the lack of organ donors and considerable side effects of immunosuppressive therapies. Porcine pancreas represents a promising cell source to overcome the shortage of islets. Due to the strong rejection of xenoantigens novel strategies such as the generation of transgenic pig islets are needed. Another complementary alternative is the co‐transplantation with immunomodulatory cells that mediate only local immunosuppression at the transplantation site.Mesenchymal stem cells (MSCs) are unique cells residing in different tissues which possess pleiotropic immunoregulatory activities. We isolated murine and human MSCs from bone‐marrow (CD73+, CD105+) and used them for in vitro experiments (mixed lymphocyte reaction after mitogen, allogenic and xenogenic stimulation) and co‐transplantation studies in diabetic C57BL/6 mice.Both murine and human MSCs exert strong immunosuppressive effects on lymphocyte proliferation (up to 75% and 80% respectively). Interestingly, there were major differences in the underlying mechanisms. We were able to demonstrate for the first time that mouse MSCs suppressed MLR by a combination of prostaglandin E2 production and adenosine formation. Only human but not mouse MSCs do express the tryptophan degrading enzyme indoleamine 2,3‐dioxygenase (IDO) upon stimulation with proinflammatory cytokines. Our features such as TGFβ and hepatocyte growth factor (HGF) production also contribute to the immunoregulatory effect. When human lymphocytes were challenges with porcine antigens there was a strong proliferative immune response which can be suppressed in a dose‐dependent manner by the addition of hMSCs (inhibition up to 95%). Co‐transplantation of allogenic islets with MSCs favoured islet cell function and survival. This effect was dependent on the MSC source as demonstrated by the fact that the use of donor MSCs resulted in longer survival rate as compared to syngenic MSCs. In current experiments we test the effect of hMSCs in the NOD SCID‐IL2R–/–mouse model to study immune response of human PBMCs towards porcine islets.In conclusion, our results strongly support and extend the concept that MSCs are potent candidates to control cellular immune rejection. We observed surprising differences in the immunosuppressive properties between mouse and human MSCs. Since human MSCs also strongly suppress immune response against pig antigens, they are promising novel candidates to modulate the local microenviroment for islet cell xenotransplantation. The next step is to analyse survival of transgenic pig islets (CTLA‐4 Ig) with and without hMSC in diabetic NOD SCID‐IL2R–/–mice.Supported by the Deutsche Forschungsgemeinschaft (Transregio Forschergruppe ‘‘Xenotransplantation’’, FOR 535).