Flt3, a member of the tyrosine-kinase receptor family, was initially cloned from fetal liver of cells with hematopoietic stem cell activity [7] . Flt3L is the ligand for Flt3, which is a key regulator for DC commitment and development. Mice administered with Flt3L displayed the expansion of certain subtypes of DCs such as the plasmacytoid DCs (pDCs) and conventional DCs (cDCs) in the spleen [8] . Studies have shown that pDCs play an important role in the induction of Tregs in vivo manifested by that their precursors are able to prolong graft survival [9] . Studies in hematopoietic cell transplantation also revealed that Flt3L is a potent mobilizer to induce murine hematopoietic chimerism, while long-term persistence of donor hematopoietic cells in peripheral lymphoid and non-lymphoid tissues of organ allograft recipients is postulated to be an essential prerequisite for the induction of donor-specific tolerance [10, 11] .