The immunostimulatory activity of unmethylated and methylated CpG oligodeoxynucleotide is dependent on their ability to colocalize with TLR9 in late endosomes.

TLR9 recognizes CpG motifs present in pathogenic DNA and triggers potent immune responses. It is generally accepted that TLR9 distinguishes pathogenic DNA based, in part, on methylation status, where TLR9 binds unmethylated but not methylated CpG. However, we showed that methylated CpG induces potent TLR9-mediated responses when delivered in lipid nanoparticles. In this article, we report that methylation dictates the ability of free CpG DNA to colocalize with TLR9 in late endosomes. However, when delivered in lipid nanoparticles, CpG DNA and TLR9 colocalize, regardless of methylation status. Therefore, it is proposed that the ability of immune cells to distinguish unmethylated pathogenic from methylated mammalian DNA is controlled by a mechanism that regulates TLR9 mobilization and colocalization rather than a differential binding affinity.