CpG motifs in bacterial DNA delay apoptosis of neutrophil granulocytes

Abstract

Human neutrophil granulocytes die rapidly, and their survival is contingent upon rescue from programmed cell death by signals from the environment. We now show that a novel signal for delaying neutrophil apoptosis is unmethylated CpG motifs prevalent in bacterial DNA (CpG‐DNA). Human neutrophils express toll‐like receptor 9 that recognizes these motifs. CpG‐DNA, but not mammalian DNA or methylated bacterial DNA, markedly enhanced neutrophil viability by delaying spontaneous apoptosis. Endosomal maturation of CpG‐DNA is prerequisite for these actions and was coupled to concurrent activation of the extracellular signal‐regulated kinase (ERK) and phosphatidylinositol 3‐kinase/Akt signaling pathways, leading to phosphorylation of BAD at Ser¹¹² and Ser¹³⁶, respectively, and to prevention of decreases in mitochondrial transmembrane potential, cytochrome c release and caspase‐3 activation. Consistently, pharmacological inhibition of either ERK or phosphatidylinositol 3‐kinase partially reversed these actions of CpG‐DNA; however, they did not produce additive inhibition. Furthermore, intravenous injection of CpG‐DNA (200 μg/kg) into rats evoked slight decreases in blood pressure and induced a modest leukocytosis, whereas it effectively suppressed neutrophil apoptosis as assayed ex vivo. Our results indicate that unmethylated CpG motifs in bacterial DNA promote neutrophil survival by suppressing the apoptotic machinery and may therefore contribute to prolongation and amplification of inflammation.