Lipopolysaccharide and CpG DNA synergize for tumor necrosis factor-α production through activation of NF-κB

Unmethylated CpG motifs in bacterial DNA (CpG DNA) activate host innate immune responses synergistically with some other microbial products, such as endotoxins, and may contribute to disease pathogenesis through excessive production of proinflammatory cytokines. Because monocyte-derived tumor necrosis factor (TNF)-alpha is an important mediator of disease, we investigated whether CpG DNA and lipopolysaccharide (LPS) synergize for inducing TNF-alpha biosynthesis. CpG DNA and LIPS synergistically induce TNF-alpha production in RAW264.7 cells and J774 cells through activation of NF-kappaB. Furthermore, transient transfection with a super-repressive mutant of I kappaB alpha (I kappaB alpha -AA) demonstrated that NF-kappaB plays a critical role in CpG DNA-mediated TNF-alpha expression. Like NF-kappaB activation, CpG DNA-induced activation of mitogen-activated protein kinases (MAPK) regulates TNF-alpha production. Both extracellular receptor kinase (ERK) and p38 can regulate TNF-alpha gene transcription induced by CpG DNA. Although CpG DNA at the higher concentration slightly enhanced LPS-mediated phosphorylation of ERK, it did not alter the LPS-mediated activation of c-Jun N-terminal kinase and p38. In addition, CpG DNA showed little or no enhancement of LPS-mediated AP-1 activation. These results suggest that CpG DNA- and LIPS-mediated signals converge at or above the level of NF-kappaB and ERK, and that there are distinct, as well as common, signaling pathways which are utilized by both CpG DNA and LIPS for activating various transcription factors and MAPK.