GalT-KO pigs: is the cup half empty or half full?

Comparative Inhibitory Effects of Tacrolimus, Cyclosporine, and Rapamycin on Human Anti-Pig Xenogeneic Mixed Lymphocyte Reactions

BACKGROUND

Long-term immunosuppressive maintenance therapy is necessary to prevent the rejection of xenografts. However, it is still unclear which oral immunosuppressant is most suitable for pig-to-human xenotransplantation .

METHODS

A xenogeneic mixed lymphocyte reaction (MLR) system was established using peripheral blood mononuclear cells (PBMCs) isolated from wildtype (WT) or GTKO/CMAHKO/β4GalNT2KO (TKO) pigs as stimulator cells and human PBMCs as responder cells. Various concentrations of tacrolimus (Tac), cyclosporine (CsA), or rapamycin (Rapa) were added to the MLR system as interventions. The inhibitory effects of the three immunosuppressants on the proliferation and cytokine production of human T cells were studied and compared. The inhibitory effect of anti-CD154 mAb alone or in combination with Tac/CsA/Rapa on xenoreactive MLR was also investigated.

RESULTS

PBMCs from both WT and TKO pigs stimulated significant proliferation of human T cells. Tac had a strong inhibitory effect on human T-cell proliferation stimulated by pig PBMCs. CsA inhibited human T-cell proliferation in a typical dose-dependent manner. When Tac and CsA concentrations reached 5 and 200 ng/mL, respectively, the proliferation rates of CD3+/CD4+/CD8+ T cells were reduced almost to a negative level. Even at high concentrations, Rapa had only a moderate inhibitory effect on xenogeneic MLR. The inhibitory effects of these three immunosuppressants on xenogeneic T-cell responses were further confirmed by the detection of CD25 expression and supernatant cytokines (IL-2, IL-6, IFN-γ, TNF-α, IL-4, IL-10, and IL-17). Although anti-CD154 mAb monotherapy showed only moderate inhibitory effects on xenoreactive T-cell proliferation, low-dose anti-CD154 mAb combined with low-dose Tac, CSA, or Rapa could produce significant synergistic inhibitory effects.

CONCLUSION

Tac is more efficient than CsA or Rapa in inhibiting xenogeneic T-cell responses in vitro. If used in combination with anti-CD154 mAb, all the three immunosuppressants can achieve satisfactory synergistic inhibitory effects.

Selection of a novel AAV2/TNFAIP3 vector for local suppression of islet xenograft inflammation

BACKGROUND

Neonatal porcine islets (NPIs) can restore glucose control in mice, pigs, and non-human primates, representing a potential abundant alternative islet supply for clinical beta cell replacement therapy. However, NPIs are vulnerable to inflammatory insults that could be overcome with genetic modifications. Here, we demonstrate in a series of proof-of-concept experiments the potential of the cytoplasmic ubiquitin-editing protein A20, encoded by the TNFAIP3 gene, as an NPI cytoprotective gene.

METHODS

We forced A20 expression in NPI grafts using a recombinant adenovirus 5 (Ad5) vector and looked for impact on TNF-stimulated NF-κB activation and NPI graft function. As adeno-associated vectors (AAV) are clinically preferred vectors but exhibit poor transduction efficacy in NPIs, we next screened a series of AAV serotypes under different transduction protocols for their ability achieve high transduction efficiency and suppress NPI inflammation without impacting NPI maturation.

RESULTS

Forcing the expression of A20 in NPI with Ad5 vector blocked NF-κB activation by inhibiting IκBα phosphorylation and degradation, and reduced the induction of pro-inflammatory genes Cxcl10 and Icam1. A20-expressing NPIs also exhibited superior functional capacity when transplanted into diabetic immunodeficient recipient mice, evidenced by a more rapid return to euglycemia and improved GTT compared to unmodified NPI grafts. We found AAV2 combined with a 14-day culture period maximized NPI transduction efficiency (>70% transduction rate), and suppressed NF-κB-dependent gene expression without adverse impact upon NPI maturation.

CONCLUSION

We report a new protocol that allows for high-efficiency genetic modification of NPIs, which can be utilized to introduce candidate genes without the need for germline engineering. This approach would be suitable for preclinical and clinical testing of beneficial molecules. We also report for the first time that A20 is cytoprotective for NPI, such that A20 gene therapy could aid the clinical development of NPIs for beta cell replacement.

β Cell Replacement Therapy: The Next 10 Years

β cell replacement with either pancreas or islet transplantation has progressed immensely over the last decades with current 1- and 5-year insulin independence rates of approximately 85% and 50%, respectively. Recent advances are largely attributed to improvements in immunosuppressive regimen, donor selection, and surgical technique. However, both strategies are compromised by a scarce donor source. Xenotransplantation offers a potential solution by providing a theoretically unlimited supply of islets, but clinical application has been limited by concerns for a potent immune response against xenogeneic tissue. β cell clusters derived from embryonic or induced pluripotent stem cells represent another promising unlimited source of insulin producing cells, but clinical application is pending further advances in the function of the β cell like clusters. Exciting developments and rapid progress in all areas of β cell replacement prompted a lively debate by members of the young investigator committee of the International Pancreas and Islet Transplant Association at the 15th International Pancreas and Islet Transplant Association Congress in Melbourne and at the 26th international congress of The Transplant Society in Hong Kong. This international group of young investigators debated which modality of β cell replacement would predominate the landscape in 10 years, and their arguments are summarized here.

Xenograft tolerance and immune function of human T cells developing in pig thymus xenografts

Transplantation of xenogeneic thymus tissue allows xenograft tolerance induction in the highly disparate pig-to-mouse model. Fetal swine thymus (SW THY) can support the generation of a diverse human T cell repertoire that is tolerant of the pig in vitro. We demonstrate that SW THY generates all human T cell subsets, including regulatory T cells (Tregs), in similar numbers as fetal human thymus (HU THY) grafts in immunodeficient mice receiving the same human CD34(+) cells. Peripheral T cells are specifically tolerant to the mouse and to the human and porcine donors, with robust responses to nondonor human and pig Ags. Specific tolerance is observed to pig skin grafts sharing the THY donor MHC. SW THY-generated peripheral Tregs show similar function, but include lower percentages of naive-type Tregs compared with HU THY-generated Tregs. Tregs contribute to donor-pig specific tolerance. Peripheral human T cells generated in SW THY exhibit reduced proportions of CD8(+) T cells and reduced lymphopenia-driven proliferation and memory-type conversion, accelerated decay of memory-type cells, and reduced responses to protein Ags. Thus, SW thymus transplantation is a powerful xenotolerance approach for human T cells. However, immune function may be further enhanced by strategies to permit positive selection by autologous HLA molecules.

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.