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The immature plasmacytoid dendritic cell (PDC) is identical with the principal type I IFN‐producing cell upon viral infection. Oligodeoxynucleotides which contain unmethylated CpG motifs (CpG ODN) are recognized by the vertebrate immune system. Previously, we described CpG ODN that strongly activate human B cells and human blood dendritic cells. Here we describe distinct CpG‐containing oligonucleotide sequences which, in contrast to previously described CpG ODN, induced high amounts of IFN‐α and IFN‐β in peripheral blood mononuclear cells (PBMC). Intracellular staining for IFN‐α revealed that within PBMC CpG ODN‐induced IFN‐α is produced exclusively by PDC. Unlike IFN‐α, TNF‐α is up‐regulated in PDC by all CpG ODN tested. Purified PDC responded to CpG ODN, demonstrating direct activation of PDC by CpG ODN. The most active sequence induced the production of up to 5 pg IFN‐α per …