Bacterial DNA-Induced NK Cell IFN-γ Production Is Dependent on Macrophage Secretion of IL-12

Bacterial DNA (bDNA) activates B cells and macrophages and can augment inflammatory responses by inducing release of proinflammatory cytokines. We found that bDNA stimulation of mouse spleen cells induced NK cell IFN-γ production that was dependent upon the presence of unmethylated CpG motifs, and oligonucleotides with internal CpG motifs could also induce splenocytes to secrete IFN-γ. The bDNA-induced IFN-γ response was strictly macrophage dependent. While splenocytes from SCID mice secreted IFN-γ in response to bDNA, depletion of macrophages eliminated this response. Additionally, purified NK cells did not respond to bDNA; however, addition of macrophages restored the NK cell IFN-γ response. Coculture of NK cells with preactivated macrophages further increased bDNA-induced NK cell IFN-γ production. Anti-IL-12 or IL-10 inhibited bDNA-induced IFN-γ response. Treatment of purified macrophages with bDNA resulted in IL-12 secretion accompanied by an increase in IL-12 p40 mRNA level. Although isolated NK cells did not make IFN-γ in response to bDNA, NK cells costimulated with IL-12 gained the ability to respond to bDNA. These experiments show that bDNA induces macrophage IL-12 production which, in turn, stimulates NK cell IFN-γ production. Macrophage-derived IL-12 renders NK cells responsive to bDNA permitting an even greater IFN-γ response to bDNA.