CD8(+)HLA-DR(+)CD27(+) T cells define a population of naturally occurring regulatory precursors in humans.

Regulatory T(reg) cells are essential for immune homeostasis. While CD4 T(reg) cells are well characterized, CD8 T(reg) cells remain less understood and are primarily observed in pathological or experimental contexts. Here, we identify a naturally occurring CD8 regulatory precursor T(rp) cell at the steady state, defined by a CD8(+)HLA-DR(+)CD27(+) phenotype and a transcriptome resembling CD4 T(reg) cells. Multiomics analyses reveal activation of TCF7 and costimulatory and co-inhibitory molecules in CD8 T(rp) cells. CD8 T(rp) cells suppress T cell expansion in vitro and in vivo. In a humanized xenogeneic graft-versus-host disease (GVHD) model, they dampen T cell activation, alleviate GVHD pathology, and prolong survival without impairing antileukemia activity. Mechanistically, CD8 T(rp) cells promote immune regulation by inducing FOXP3 expression in both CD4 T(reg) cells and themselves. Their expansion also correlates with immune homeostasis restoration post-allogeneic stem cell transplantation. These findings establish CD8 T(rp) cells as a naturally occurring regulatory precursor population that promotes transplantation tolerance.