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+ Tcells. 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.