Dendritic cells (DCs) are potent antigen-presenting cells which play pivotal roles in immunological response. The clinical application of DCs induced from peripheral monocytes in vitro has been initiated as a promising immuno-logical therapy against cancer. If the same type of immuno-stimulator could be achieved without in vitro manipulation, it might be very convenient in clinical settings. In this study, we performed systemic gene transfer of Flt3L using in vivo electroporation of Flt3L plasmid DNA (Flt3L-IVE) in pretibial muscles in order to determine the effects on DCs in situ. After Flt3L-IVE, Flt3L was detected in the serum for 10 days after IVE at significant levels. The peak concentration of 5326±920 pg/ml in the serum was observed 4 days after Flt3L-IVE. The number of DCs was significantly increased and showed highly co-stimulatory molecule expressions both in spleen and bone marrow after Flt3L-IVE compared to those of control groups. Immunohistochemical evaluation revealed that not only DCs but also CD8 and CD4 positive cells were significantly infiltrated into the local tumor site compared with those of control and remained in the tumor 21 days after a single Flt3L-IVE. However, anti-tumor effects of Flt3L-IVE were not significant in the MCA205 established tumor. Most of the tumor infiltrating DCs had immature phenotype. Only a small number of DCs in the peripheral areas had the mature phenotype. These results suggest that Flt3L gene transfer using in vivo electroporation could mobilize DCs into tumor site. Additional means to induce maturation of these DCs could have a positive impact on anti-tumor effects of this strategy.

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