Effects of G-Quadruplex Ligands on the Topology, Stability, and Immunostimulatory Properties of G-Quadruplex-Based CpG Oligodeoxynucleotides

We previously reported that the formation of guanine-quadruplex (G4) structures provides phosphodiester oligodeoxynucleotides containing unmethylated cytosine-phosphate-guanine (CpG ODNs) with higher nuclease resistance and cellular uptake, thereby increasing their immunostimulation efficiency through TLR9 activation. CpG ODNs forming G4 structures (G4 CpG ODNs) are thus potential vaccine adjuvants against infectious diseases. However, the G4 structure changes topology depending on the surrounding environment. Recently, G4 ligands, which are small molecules that bind to G4 ODNs with high affinity, were reported to improve the stability of G4. In this study, we propose to increase the stability and function of G4 CpG ODNs using G4 ligands. We show the effects of two G4 ligands, named L2H2-6OTD (L2H2) and L2G2-2M2EG-6OTD (L2G2), on the topology, stability, and immunostimulatory properties of a monomeric hybrid-type G4 CpG ODN containing CpG motifs in the central loop, named GD3. We found that L2H2 helps maintain the hybrid G4 topology of GD3, whereas L2G2 induces parallel G4 formation. Both G4 ligands increase the thermodynamic and nuclease stability of GD3. However, only GD3 associated with L2H2 binds efficiently to TLR9 and evokes a higher immune response from mouse macrophage-like RAW264 cells. GD3 associated with L2G2 does not bind efficiently to TLR9 and elicits lower cytokine production. Our results demonstrate that the potential to enhance immunostimulatory properties depends on the ability of G4 ligands to maintain and stabilize the hybrid G4 of GD3. We anticipate that our findings will facilitate the development of more effective G4 CpG ODN-based vaccine adjuvants against infectious diseases.