Donor-antigen Inoculation in the Testis Promotes Skin Allograft Acceptance Induced by Conventional Costimulatory Blockade via Induction of CD8 + CD122+ and CD4 + CD25+ Regulatory T Cells.

BACKGROUND Immune responses are somewhat suppressed in immune privileged sites, including the testes, which provide a preexisting opportunity to prolong allograft survival. Previous studies have shown that intratesticular islet allografts enjoy extended survival even without any immunosuppression. However, it is unknown if testicular immune privilege can be exploited to prolong the survival of a solid allograft, including the skin, because it is impractical to implant a solid tissue in human testes. METHODS To immunize recipient mice, splenocytes from BALB/c mice were injected into the testis of C57BL/6 recipients 1 week before skin transplantation. CD8 + CD122+ and CD4 + FoxP3+ regulatory T [Treg] cells were quantified by fluorescence-activated cell sorting. RESULTS Although donor-antigen inoculation alone did not delay skin allograft rejection, it significantly extended the allograft survival when combined with CD40/CD40L or B7/CD28 costimulatory blockade and further induced long-term skin allograft acceptance when both costimulatory pathways were blocked. Similarly, donor-antigen inoculation suppressed alloreactive T cell proliferation in draining lymph nodes of skin recipients in the presence of the same costimulatory blockade. Interestingly, donor-antigen inoculation via intratesticular injection increased CD8 + CD122+, but not CD4 + FoxP3+, Treg numbers after transplantation. However, both CD8 + CD122+ and CD4 + CD25+ Treg cells induced by donor-antigen inoculation and the costimulatory blockade were more potent in suppression than that induced without the inoculation. Depletion of CD8+ or CD25+ T cells largely abrogated long-term skin allograft survival induced by donor-antigen inoculation and the costimulatory blockade. CONCLUSIONS Intratesticular inoculation with donor antigens promotes long-term skin allograft survival induced by conventional costimulatory blockade via the induction of both CD8 + CD122+ and CD4 + CD25+ Treg cells.