Alloreactive natural killer cells promote haploidentical hematopoietic stem cell transplantation by expansion of recipient-derived CD4(+) CD25(+) regulatory T cells.

Alloreactive NK cells (Allo-NKs) have been shown to exert advantageous effects on the outcomes of haploidentical hematopoietic stem cell transplantation (Haplo-HSCT) for cancer treatment. However, the mechanisms of action of Allo-NKs remain unclear. We established a novel Haplo-HSCT conditioning regimen composed of Allo-NKs and a low dose of immunosuppressive drugs (Allo-NKs + Chemo) to investigate alternative mechanisms besides direct cytotoxicity. The inhibitory effects of different cell subsets on the donor-recipient mixed lymphocyte reactions (MLRs) were evaluated after Haplo-HSCT. The quantities and functions of CD4(+) CD25(+) regulatory T cells (Tregs) and dendritic cells (DCs) in the spleen and the thymus were examined. Our results showed that the Allo-NKs + Chemo regimen induced systemic tolerance, and that CD4(+) CD25(+) Tregs played a significant role in inducing and maintaining systemic tolerance after Haplo-HSCT. Alloreactive NK cells promoted the expansion of recipient-derived CD4(+) CD25(+) CD127(-) Tregs in the thymus and the spleen which could be amplified in vitro by the immature donor-derived DC subset isolated from the thymus of Allo-NKs + Chemo-treated mice. Our findings suggested that Allo-NKs are capable of inducing systemic tolerance after Haplo-HSCT by assembling donor-derived immature DCs to expand recipient-derived Treg cells in the thymus.