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Bacterial CpG DNA or synthetic oligonucleotides (ODNs) that contain unmethylated CpG motifs (CpG ODN) can directly activate antigen-presenting cells (APCs) to secrete various cytokines through the intracellular receptor TLR9. Cytokine profiles elicited by the actions of stimulatory CpG DNA on TLR9 expressed APCs are crucial to the subsequent immune responses. To date, cytokine profiles in APCs upon CpG ODN stimulation in vitro are not fully investigated. In the present study, vector-based siRNA was used to downregulate TLR9 expression. Cytokine profiles were observed in murine macrophage cell line RAW264.7 transfected with TLR9-siRNA plasmid upon CpG ODN stimulation. We found that not all the cytokine expressions by the macrophage were decreased while TLR9 was downregulated. IL-12, TNF-α, IFN-γ and IL-1β expressions were significantly decreased, but IL-6,IFN-β and IL-10 expressions were not affected. Interestingly, the level of IFN-α was even increased. This alteration of cytokines produced by TLR9-downregulated APCs upon CpG ODN stimulation might indicate that the role of CpG DNA is more complicated in the pathogenesis and prevention of diseases. Cellular & Molecular Immunology.2005;2(2):130-135.