Systemic administration of Fms-like tyrosine kinase 3 ligand (FLT3L) has been considered to be a major route of delivery for tumor immunotherapy because expression of its receptor, FLT3, was detected predominantly in hematopoetic progenitor cells. However, several studies indicate that FLT3L locally overexpressed in tumor or dendritic cells (DCs) also shows an anti-tumor effect. In the current study, we found that FLT3 expression is not present in monocytes but is instead induced in DCs through the differentiation process resulting from stimulation by GM-CSF and IL-4. Addition of FLT3L further augmented FLT3 induction and also increased CD40 expression in DCs, leading to enhanced induction of lymphoblastoid cell line-targeted cytotoxic T-lymphocyte response and CD107a mobilization in CD8+ T cells. Furthermore, FLT3L also induced FLT3 expression in peripheral blood NK cells that showed an enhanced response detected by CD107a mobilization. In a murine ovarian cancer model, locally expressed FLT3L showed anti-tumor effects. Collectively, the current study indicates that FLT3L has an immunostimulatory effect on peripheral blood cells and FLT3L targeted to mature peripheral blood cells may serve as a useful tool for cancer immunotherapy.

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