Intracellular mammalian DNA stimulates myeloid dendritic cells to produce type I interferons predominantly through a toll-like receptor 9-independent pathway.
OBJECTIVE: Exogenous nucleic acids, including bacterial unmethylated DNA and viral single-stranded RNA, are potent activators of innate immunity through interaction with the Toll-like receptors (TLRs). In contrast, mammalian DNA has been generally thought to have a limited activation effect, or even a suppressive effect, on innate immunity. Since DNA is a major component of dying cells and recent studies indicate that mammalian nucleic acids may be stimulatory under certain conditions, we undertook this study to examine the effect of intracellular mammalian DNA on myeloid dendritic cell (DC) activation. METHODS: Mammalian DNA was introduced into murine bone marrow-derived DCs (BMDCs) by transfection. BMDC activation was determined by flow cytometry (CD40, CD86). Production of tumor necrosis factor alpha and interleukin-6 was measured by enzyme-linked immunosorbent assay, and production of type I interferons (IFNs) by bioassay. Parallel studies were conducted using BMDCs from mice deficient in myeloid differentiation 88 (MyD88), TLR-9, and IFNalpha/beta receptor. RESULTS: Intracellular mammalian DNA activated immature BMDCs, as determined by the up-regulation of CD40 and CD86 as well as by the production of significant quantities of type I IFN. The interferogenic response was shown to be relatively independent of TLR-9, and the TLR adaptor MyD88. The IFN response to intracellular DNA was reduced in BMDCs lacking IFNalpha/beta receptor but was intact in embryonic fibroblasts lacking protein kinase R. CONCLUSION: These results indicate that intracellular DNA stimulates BMDC maturation and IFN production predominantly through a TLR-independent pathway, and support a model whereby inefficient clearance and/or degradation of endogenous DNA may stimulate innate immune responses similar to the TLR-independent response to exogenous (i.e., viral) double-stranded RNA.