Flt3 ligand expands lymphoid progenitors prior to recovery of thymopoiesis and accelerates T cell reconstitution after bone marrow transplantation.

Deficient thymopoiesis and retarded recovery of newly developed CD4(+) T cells is one of the most important determinants of impaired immunocompetence after hemopoietic stem cell transplantation. Here we evaluated whether Fms-like tyrosine kinase 3 (Flt3) ligand (FL) alone or combined with IL-7 affects T cell recovery, thymopoiesis, and lymphoid progenitor expansion following bone marrow transplantation in immunodeficient mice. FL strongly accelerated and enhanced the recovery of peripheral T cells after transplantation of a low number of bone marrow cells. An additive effect on T cell recovery was not observed after coadministration of IL-7. Lineage(-)sca-1(+)c-kit(+)flt3(+) lymphoid progenitor cell numbers were significantly increased in bone marrow of FL-treated mice before recovery of thymopoiesis. Thymocyte differentiation was advanced to more mature stages after FL treatment. Improved T cell recovery resulted in better immunocompetence against a post-bone marrow transplantation murine CMV infection. Collectively, our data suggest that FL promotes T cell recovery by enhanced thymopoiesis and by expansion of lymphoid progenitors.