Combined mobilization and stimulation of tumor-infiltrating dendritic cells and natural killer cells with Flt3 ligand and IL-18 in vivo induces systemic antitumor immunity.

It was hypothesized that if dendritic cells (DC) could be efficiently manipulated in vivo, this might enable functional maturation and retention of their potent functions and might represent a more promising approach in DC immunotherapy. The present study focused on the modulation of DC in tumor microenvironment using Fms-like thyrosine kinase 3 ligand (Flt3L) combined with interferon-gamma-inducing factor (IL-18). Tumor-inoculated mice were treated with in vivo electroporation (IVE) of expression plasmids carrying complementary DNA of Flt3L. As a combination therapy, mice in the other group were treated with intra-tumoral injection of adenoviral vector carrying IL-18 gene (Ad.IL-18). Significant antitumor effect was observed in mice treated with Ad.IL-18 alone when compared with that of control (P < 0.01). Complete eradication was observed more frequently (100%versus 33%: P < 0.05) in the mice treated with Flt3L and Ad.IL-18 when compared with the mice treated with Ad.IL-18 alone. In un-injected distant tumor, significant antitumor responses were observed only in the mice treated with combination therapy. Lymphoid cells in lymph nodes of mice treated with combination therapy showed significant cytolytic activity against inoculated tumor cells and YAC-1 cells when compared with the lymphoid cells in other groups. In the tumor microenvironment, combination therapy resulted in the recruitment of mobilized DC into the tumor bed, although Flt3L-IVE alone had an effect in the peri-tumoral area. Tumor-infiltrating DC in mice treated with combination therapy showed higher CD86 expression and more potent allogeneic T-cell stimulatory capacity. These results may suggest that local expression of IL-18 combined with in vivo DC mobilization with Flt3L is clinically applicable as a new strategy of DC immunotherapy.