Modulation and Regulation of Gene Expression by CpG Oligonucleotides

Immune cells are triggered when their Toll-like receptors (TLR) interact with pathogen-associated molecular patterns present on infectious agents. For example, TLR9 recognizes unmethylated CpG motifs present in bacterial DNA, whereas TLR3 recognizes single-stranded RNA present in viruses. Microarrays were used to examine the pattern of gene activation elicited following engagement of TLR9 and TLR3. Two distinct waves of activation were induced by CpG DNA. Each wave was regulated by a small group of major and minor inducers. Included in these waves of gene activation were suppressive networks that served to then downregulate TLR-dependent cellular stimulation. Triggering via TLR3 activated a larger and more complex regulatory network than did TLR9. Synergy was observed when cells were stimulated with both TLRs. This was characterized by the early and persistent activation of multiple regulatory pathways, resulting in a prolonged signaling cascade that increased, magnified, and diversified gene expression.