Naturally occurring PD-1⁺ memory phenotype CD8 T cells belong to nonconventional CD8 T cells and are cyclophosphamide-sensitive regulatory T cells

CD8 T cells expressing memory markers exist in naive mice and are thought to be of heterogeneous origin. It was recently reported that among such memory-phenotype (MP) CD8 T cells in naive mice, those expressing programmed death-1 (PD-1) had immune regulatory activity, but their origin and relationship with other regulatory CD8 T cell subsets remain unclear. In the current study, we examined detailed characteristics and functions of PD-1⁺ MP CD8 T cells in naive mice. Their expression pattern of surface molecules resembled that of exhausted CD8 T cells seen in chronic viral infection. However, PD-1⁺ MP CD8 T cells were detected from neonatal periods, even in the thymus; thus, they are naturally occurring. By analyzing bone marrow chimera mice in which β₂-microglobulin-deficient mice were used as the recipients, it was revealed that PD-1⁺ MP CD8 T cells were positively selected by hematopoietic cells, indicating that they belong to nonconventional CD8 T cells. However, in contrast to majority of MP CD8 T cells, PD-1 ⁺ MP CD8 T cells were IL-15 independent. PD-1⁺ MP CD8 T cells showed the fastest cell cycling among various T cell subsets in naive mice, which was consistent with the highest sensitivity to cyclophosphamide (CP) treatment. Importantly, PD-1⁺ MP CD8 T cells were able to suppress delayed-type hypersensitivity response that was augmented by CP treatment. Taken together, our data indicate that the naturally occurring PD-1⁺ MP CD8 T cells in naive mice are a unique subset of nonconventional CD8 T cells and represent the CP-sensitive suppressor CD8 T cells.