Background: Donor-specific xenograft tolerance can be achieved by grafting fetal porcine thymus tissue to thymectomized (ATX) mice treated with natural killer (NK) and T-cell-depleting monoclonal antibodies plus 3 Gy of total body irradiation (TBI). Grafting of neonatal, instead of fetal, thymus, along with neonatal pig spleen, leads to a lower level of mouse CD4 cell reconstitution, with less reliable tolerance induction. For a number of reasons, it would be advantageous to use neonatal rather than fetal pigs as donors. We therefore investigated the possibility that grafting larger amounts of neonatal porcine thymus tissue to different sites could allow improved outcomes to be achieved.
Materials and methods: Multiple or single fragments of neonatal porcine thymus tissue were grafted with a splenic fragment to different sites (mediastinum, mesentery, and kidney capsule) of ATX B6 mice treated with T- and NK-cell-depleting antibodies and 3Gy TBI. Mice also received an intraperitoneal injection containing 1 x 10(7) donor splenocytes. Donor-specific skin graft tolerance was evaluated, and CD4 reconstitution and mouse anti-donor xenoantibodies were followed by flow cytometry.
Results: Peripheral repopulation of CD4+ cells occurred by 7 weeks after transplantation in mice grafted with four fragments of neonatal porcine tissue in either the mediastinum or the mesentery, but not in mice grafted under both kidney capsules with the same amount of tissue. The level of CD4 reconstitution correlated with skin graft tolerance and an absence of induced anti-donor xenoantibodies. Seventy-five percent of mice with >20% of CD4+ cells among peripheral blood lymphocytes (PBL) by 13 weeks posttransplantation accepted donor porcine skin, while rejecting either non-donor neonatal porcine or mouse BALB/c skin allografts. In contrast, only 29% of grafted mice with <20% CD4+ cells in the peripheral blood at 13 weeks accepted donor porcine skin. Grafted mice with poor reconstitution showed either low or high levels of anti-pig xenoantibodies of the IgM, IgG1, and IgG2a isotypes. Grafted mice with >20% CD4+ cells all had low levels of anti-pig xenoantibodies of these isotypes and displayed mixed lymphocyte reaction (MLR) tolerance to donor pig major histocompatibility complex (MHC), with responsiveness to allogeneic mouse stimulators.
Conclusion: Grafting neonatal porcine thymus into either the mediastinum or mesentery provides earlier and more efficient reconstitution of the CD4 compartment than does grafting under the kidney capsule. Good CD4 reconstitution was associated with optimal donor-specific skin graft tolerance and avoidance of the anti-donor xenoantibody responses observed in mice with poor CD4 reconstitution. These results also suggest that there is a suppressive component to the porcine xenograft tolerance induced with this approach.