Objective To investigate the phenotype and the immunoregulatory function of CD8αα+TCRαβ+regulatory T cells in peripheral blood samples from mice.Methods The distribution profile and the phenotype of CD8αα+TCRαβ+regulatory T cells in C57BL/6 mice were detected by flow cytometry.The cytokines released by CD8αα+TCRαβ+regulatory T cells upon the stimulation with anti-CD3 antibody were analyzed by cytometric bead array.The in vitro immunosuppressive activity of CD8αα+TCRαβ+regulatory T cells on activated CD4+T cells was analyzed by using flow cytometry and carboxyfluorescein succinimidyl ester ( CFSE ) .An adoptive cell transfer assay was set up to evaluate the immunoprotective effects of CD8αα+TCRαβ+ regulatory T cells in a mouse model of experimental autoimmune encephalomyelitis ( EAE) .Results CD8αα+TCRαβ+regulatory T cells were detected in liver, spleen and peripheral blood samples collected from na?ve C57BL/6 mice.Compared with CD8αβ+TCRαβ+regulatory T cells, CD8αα+TCRαβ+regulatory T cells showed a memory-activated phenotype of CD25+CD122high CD44high CD62Llow CD69high NK1.1+DX5+.CD8αα+TCRαβ+regulatory T cells could produce IL-2 after 24 hours stimulation with anti-CD3 antibody, followed by producing IFN-γ, TNF-α, IL-4, IL-17A and traces of IL-6 and IL-10. In vitro, CD8αα+TCRαβ+regulatory T cells specifically suppressed the proliferation of activated CD4+T cells ( P<0.01 ).Moreover, they could delay the onset of EAE in mice and reduce clinical score (P<0.01).Conclusion CD8αα+TCRαβ+regulatory T cells were a unique population with immunoregula-tory function, which could be used as a potential therapeutic target in the treatment of autoimmune disease.