CD8α+ plasmacytoid precursor DCs induce antigen-specific regulatory T cells that enhance HSC engraftment in vivo.

CD8-positive/T-cell receptor-negative (CD8(+)/TCR(-)) graft facilitating cells (FCs) are a novel cell population in bone marrow that potently enhance engraftment of hemopoietic stem cells (HSCs). Previously, we showed that the CD11c(+)/B220(+)/CD11b(-) plasmacytoid-precursor dendritic cell (p-preDC) FC subpopulation plays a critical but nonredundant role in facilitation. In the present study, we investigated the mechanism of FC function. We report that FCs induce antigen-specific CD4(+)/CD25(+)/FoxP3(+) regulatory T cells (Tregs) in vivo. The majority of chimeric Tregs were recipient derived. Chimeric Tregs harvested at ≥ 4 weeks after transplantation significantly enhanced engraftment of donor- and recipient-derived HSCs, but not third-party HSCs, in conditioned secondary recipients, demonstrating antigen specificity. Although Tregs were present 2 and 3 weeks after transplantation, they did not enhance engraftment. In contrast, week 5 and greater Tregs potently enhanced engraftment. The function of chimeric Tregs was directly correlated with the development of FoxP3 expression. Chimeric Tregs also induced significantly stronger suppression of T-cell proliferation to donor antigen in vitro. Removal of p-preDC FCs resulted in impaired engraftment of allogeneic HSCs and failure to produce chimeric Tregs, suggesting that the CD8α(+) p-preDC subpopulation is critical in the mechanism of facilitation. These data suggest that FCs induce the production of antigen-specific Tregs in vivo, which potently enhance engraftment of allogeneic HSCs. FCs hold clinical potential because of their ability to remain tolerogenic in vivo.