The TLR family plays a fundamental role in pathogen recognition and activation of innate immunity. TLRs are named for the high degree of conservation in structure and function seen between mammalian TLRs and the Drosophila transmembrane protein Toll. TLR9 recognizes unmethylated CpG sequences in DNA molecules. TLR9 is expressed by numerous cells of the immune system such as B lymphocytes, monocytes, natural killer (NK) cells, and plasmacytoid dendritic cells. TLR9 is expressed intracellularly, within the endosomal compartments and functions to alert the immune system of viral and bacterial infections by binding to DNA rich in CpG motifs. TLR9 signals leads to activation of the cells initiating pro-inflammatory reactions that result in the production of cytokines such as type-I interferon and IL-12.
