Overexpression of interleukin-12 enables dendritic cells to activate NK cells and confer systemic antitumor immunity.

Dendritic cells (DC) are initiators of T cell-mediated immunity. However, less is known about the relationship between DC and natural killer (NK) cells, and direct evidence of their interaction in vivo is scarce. Interleukin (IL)-12 is an activator of both DC and NK cells. We postulated that secretion of IL-12 by DC would enable them to activate NK cells. Bone marrow-derived DC propagated only in granulocyte macrophage colony-stimulating factor did not activate NK cells. In contrast, DC engineered to express IL-12 markedly stimulated NK cells as determined by coculture experiments in vitro, assays of NK cells isolated from treated animals, and survival experiments in a systemic tumor model. Activation depended on both DC-NK cellular interaction and secretion of IL-12. Adoptive transfer of DC expressing IL-12 to mice markedly increased NK cell interferon-gamma production and lytic activity in vivo. Treated mice were also protected against B16 melanoma hepatic metastases. The in vivo effects on NK cells were DC-specific. Administration of IL-12 protein alone or melanoma cells or fibroblasts engineered to secrete IL-12 were only weakly activating. Our findings demonstrate that IL-12 expression by DC enables them to activate NK cells and provide evidence for a substantial DC-NK relationship in vivo.