G-Quadruplex Structure Improves the Immunostimulatory Effects of CpG Oligonucleotides

Single-strand oligodeoxynucleotides (ODNs) containing unmethylated cytosine-phosphate-guanine (CpG) are recognized by the toll-like receptor 9, a component of the innate immunity. Therefore, they could act as immunotherapeutic agents. Chemically modified CpG ODNs containing a phosphorothioate backbone instead of phosphodiester (PD) were developed as immunotherapeutic agents resistant to nuclease degradation. However, they cause adverse side effects, and so there is a necessity to generate novel CpG ODNs. In the present study, we designed a nuclease-resistant nonmodified CpG ODN that forms G-quadruplex structures. G-quadruplex formation in CpG ODNs increased nuclease resistance and cellular uptake. The CpG ODNs designed in this study induced interleukin-6 production in a human B lymphocyte cell line and human peripheral blood mononuclear cells. These results indicate that G-quadruplex formation can be used to increase the immunostimulatory activity of CpG ODNs having a natural PD backbone.