A donor PD-1+CD8+ TSCM-like regulatory subset mobilized by G-CSF alleviates recipient acute graft-versus-host-disease

Donor selection determines the occurrence of acute graft-versus-host-disease (aGVHD) following allogeneic hematopoietic stem cell transplantation (allo-HSCT). To optimize the current clinical donor selection criteria and identify putative donor lymphocyte subsets associated with better recipient outcomes, we analyzed the peripheral CD4⁺ and CD8⁺ subsets in 80 granulocyte colony-stimulating factor (G-CSF) mobilized donors and examined the aGVHD incidence of the corresponding 80 haploidentical and identical allo-HSCT recipients. The G-CSF-induced expansion of subsets varied among donors. We discovered a novel PD-1⁺CD8⁺CD45RA⁺CCR7⁺ T lymphocyte subset in suitable donors that was significantly correlated with lower incidence of aGVHD and post-transplant anti-infection. The anti-aGVHD activity of this subset was confirmed in a validation cohort (n = 30). Single-cell RNA sequencing revealed that this T cell subset exhibited transcriptomic features of stem cell-like memory T cell (T_SCM) with both Treg and Teff activities which indicated its dual functions in aGVHD inhibition and graft-versus-leukemia (GVL) effect. Intriguingly, upon G-CSF mobilization, the donor PD-1⁺CD8⁺ T_SCM-like regulatory cells increased the PD-1 expression in a BCL6-dependent manner. Next, we showed that the mouse counterpart of this subset (PD-1⁺CD8⁺CD44^-CD62L⁺) ameliorated aGVHD, and confirmed the existence of this subset in clinical recipients. In summary, we, for the first time, identified a novel donor peripheral T cell subset suppressing aGVHD while promoting the immune reconstitution of recipients. It may serve as an indicator for optimal haploidentical and identical donor selection. Importantly, the dual Treg and Teff function of these T cells makes it a promising treatment for not only aGVHD but also auto-immune diseases.