The toll-like-receptor (TLR) 9 is a prominent member of the TLR family recognizing pathogen-associated molecular patterns. TLR 9 is expressed in particular on the surface of plasmacytoid dendritic cells (PDCs) and B cells and binds specifically unmethylated CpG motifs in bacterial DNA leading to activation of immune cells.
These effects can be mimicked by short synthetic oligodeoxynucleotides (ODNs) containing unmethylated CpG motifs. Several classes of CpG ODNs have been identified and can be distinguished by their effects on certain cell types with regard to stimulation and cytokine production.
The recently discovered P-class ODNs induce a strong IFN-α production as well as cell activation and thus show similar but superior properties to C-class ODNs.