A continuous infusion of a minor histocompatibility antigen–immunodominant peptide induces a delay of male skin graft rejection

Advances on Non-CD4 + Foxp3+ T Regulatory Cells: CD8+, Type 1, and Double Negative T Regulatory Cells in Organ Transplantation

The overwhelming body of research on T regulatory cells (Treg) has focused on CD4 + CD25 + Foxp3+ T cells. However, recent years have witnessed a resurgence in interest in CD4 - CD8+, CD4 - CD8- (double negative [DN]), and CD4 + Foxp3- type 1 Treg (Tr1) Treg and their role in controlling autoimmune diseases and in promoting the survival of organ allografts and xenografts. CD8+ and DN Treg can arise spontaneously (natural Treg) or can be induced in situ. Both CD8+ and DN Treg have been shown to enhance the survival of organ allografts and xenografts. Additionally, both can suppress alloimmune responses by contact-dependent mechanisms by either inducing apoptosis or mediating direct cytolysis of effector T cells. CD8+, DN, and Tr1 Treg can also act in a contact-independent manner by elaborating soluble immunosuppressive factors, such as TGF-β and IL-10. Applying CD8+, DN, and Tr1 Treg for enhancing the survival of organ allografts and xenografts is still in its infancy but holds significant potential. Furthermore, there is a need for a more comprehensive understanding of how current immunosuppressive therapies applied to organ transplantations affect the wide array of Treg populations.

The role of co-inhibitory signals in spontaneous tolerance of weakly mismatched transplants

The immune system of female H-2(b) (C57BL/6) mice is a strong responder against the male minor-H antigen. However rejection or acceptance of such weakly mismatched grafts depends on the type of tissue transplanted. The mechanism responsible for such spontaneous graft acceptance, and its relationship to the natural mechanisms of tolerance of self antigens is unknown. Co-inhibitory molecules negatively regulate immune responses, and are important for self tolerance. We examined whether co-inhibitory molecules play a critical role in "spontaneous" allograft tolerance. Naïve or donor sensitized diabetic female C57BL/6 (B6) wild type (WT), PD-1(-/-), and BTLA(-/-) mice were transplanted with freshly isolated syngeneic male islet grafts. The role of co-inhibitors during priming of anti-donor responses and graft challenge was also assessed using monoclonal antibodies targeting co-inhibitory receptors. Among the co-inhibitor (CTLA-4, PD-1) specific antibodies tested, only anti-PD-1 showed some potential to prevent spontaneous acceptance of male islet grafts. All BTLA(-/-) and almost all PD-1(-/-) recipients maintained the ability to spontaneously accept male islet grafts. While spontaneous graft acceptance in naïve recipients was only weakly PD-1 dependent, tolerance induced by the accepted islets was found to be highly PD-1 dependent. Furthermore, spontaneous graft acceptance in pre-sensitized recipients showed an absolute requirement for recipient PD-1 but not BTLA. Thus, the PD-1 pathway, involved in self tolerance, plays a critical role in spontaneous tolerance induced by weakly mismatched grafts in naïve recipients and spontaneous graft acceptance in pre-sensitized recipients.