Human CD8(+)HLA-DR(+) Regulatory T Cells, Similarly to Classical CD4(+)Foxp3(+) Cells, Suppress Immune Responses via PD-1/PD-L1 Axis

We have previously identified a human CD8(+)HLA-DR(+) regulatory T cell subset with the ability to suppress proliferation of autologous PBMCs responder cells through cell contact and CTLA-4 co-inhibitory molecule. The present study characterizes the complete phenotype of CD8(+)HLA-DR(+) Treg cells which showed great similarities with classical CD4(+) cells expressing forkhead box P3 (FOXP3). The shared features included the expression of programmed cell death protein 1 (PD-1), T-cell immunoreceptor with Ig and ITIM domains (TIGIT), C-C chemokine receptor type 4 and 5 (CCR4 and CCR5), low expression of CD127, and a memory and effector-like phenotype. CD8(+)HLA-DR(+) Treg-induced suppression on CD8(+) responder T cells was abrogated by an anti-PD1 neutralizing antibody. Anti-PD-1 did not abrogate the suppressor effect induced on responder CD4(+) T cells. In addition, CD8(+)HLA-DR(+) Treg induced a preferential death on responder CD8(+) T cells. This effect was not reversed by PD-1 neutralization. After activation, most CD8(+)HLA-DR(+) Treg acquire programmed death-ligand 1 (PD-L1) expression. Interestingly, PD-L1 may induce apoptosis through CD80 expressed on activated CD8(+) responder T cells. After PBMCs stimulation, CD8(+)HLA-DR(+) Treg cells showed an increased frequency of IFN-γ and TNFα positive cells and higher degranulation. These data strongly argue against CD8(+)HLA-DR(+) Treg being exhausted cells. Overall, the data presented in this study indicate that CD8(+)HLA-DR(+) Treg and CD4(+)FOXP3(+) Treg share phenotypic and functional features, which may provide cues to similar involvements in the control of antitumor immune responses and autoimmunity.