CpG oligodeoxynucleotides induce IL-8 expression in CD34⁺ cells via mitogen-activated protein kinase-dependent and NF-κB-independent pathways

To elucidate the role of Toll-like receptor 9 (TLR9) activation along with the intracellular signaling pathways triggered by CpG DNA in CD34⁺ cells, we investigated whether synthetic oligodeoxynucleotides (ODNs), containing unmethylated CpG motifs, could induce IL-8 expression in CD34⁺ cells through mitogen-activated protein kinase (MAPK) or nuclear factor-κB (NF-κB) pathway. We demonstrated evidence for the first time that CD34⁺ cells constitutively expressed TLR9. Exposure of the cells to CpG ODN resulted in a time- and dose-dependent increase of IL-8 expression, and activation of phosphorylated ERK1/2 and phosphorylated p38. In addition, CpG ODN stimulated AP-1, but not NF-κB, signals. Moreover, inhibitors of MAPK (U0126 and SB203580) significantly reduced the IL-8 production, while the inhibition of NF-κB (pyrrolidinedithiocarbamate and retrovirus containing dominant-negative IκBα plasmid) did not affect the IL-8 expression increased by CpG ODN. Moreover, co-stimulation with LPS and CpG synergistically up-regulates IL-8 in CD34⁺ cells. These results suggest that CpG DNA, acting on TLR9, activates CD34⁺ cells to express IL-8 through MAPK-dependent and NF-κB-independent pathways.