Combined stimulation with interleukin-18 and CpG induces murine natural killer dendritic cells to produce IFN-γ and inhibit tumor growth

Natural killer dendritic cells (NKDC) are a novel subtype of dendritic cells with natural killer (NK) cell properties. IFN-gamma is a pleiotropic cytokine that plays an important role in the innate immune response to tumors. Based on our previous finding that the combination of Toll-like receptor 9 ligand CpG and interleukin (IL)-4 stimulates NKDC to produce IFN-gamma, we hypothesized that NKDC are the major IFN-gamma-producing dendritic cell subtype and may play a significant role in the host antitumor response. We found that under several conditions in vitro and in vivo NKDC accounted for the majority of IFN-gamma production by murine spleen CD11c(+) cells. IL-18 alone induced NKDC to secrete IFN-gamma, and the combination of IL-18 and CpG resulted in a synergistic increase in IFN-gamma production, both in vitro and in vivo. NK cells made 26-fold less IFN-gamma under the same conditions in vitro, whereas dendritic cells produced a negligible amount. The mechanism of IFN-gamma secretion by NKDC depended on IL-12. NKDC selectively proliferated in vitro and in vivo in response to the combination of M-18 and CpG. Systemic treatment with IL-18 and CpG reduced the number of B16F10 melanoma lung metastases. The mechanism depended on NK1.1(+) cells, as their depletion abrogated the effect. IL-18 and CpG activated NKDC provided greater tumor protection than NK cells in IFN-gamma-/mice. Thus, NKDC are the major dendritic cell subtype to produce IFN-gamma. The combined use of IL-18 and CpG is a viable strategy to potentiate the antitumor function of NKDC.