Efficient induction and expansion of human alloantigen-specific CD8 regulatory T cells from naive precursors by CD40-activated B cells

Although recent studies have focused on CD4⁺ regulatory T cells (Treg), CD8⁺ Treg have also been reported to play important roles in the induction and maintenance of immune tolerance. Adoptive transfer of CD8 ⁺ Treg in rodents or induction of CD8⁺ Treg in humans can prevent or treat allograft rejection and autoimmune diseases. However, no approaches have been reported for the generation of human Ag-specific CD8 ⁺ Treg at a practical scale for clinical use. Here, we found that two novel CD8⁺ T cell subsets with different levels of CD8 surface expression, CD8^high and CD8^low, could be induced from naive CD8⁺ precursors in vitro by allogeneic CD40-activated B cells, whereas only CD8^high T cells were alloantigen-specific Treg with relatively poor alloantigen-specific cytotoxicity. Importantly, alloantigen-specific CD8^high Treg could be induced and expanded from naive CD8⁺CD25^- T cells at a large scale after 3 wk of culture without exogenous cytokines. These induced alloantigen-specific Treg were CD45RO⁺ and CCR7^- memory cells, and they expressed Foxp3, CD25, CD27, CD28, and CD62L. The induction and expansion of CD8 ^high Treg by CD40-activated B cells were dependent on endogenously expressed IFN-γ, IL-2, IL-4, and CTLA-4. This approach may facilitate the clinical application of CD8⁺ Treg-based immunotherapy in transplantation and autoimmune diseases.