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Cationic liposomes, specifically 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) liposomes, serve as successful carriers for guanine-quadruplex (G4) structure-based cytosine-guanine oligodeoxynucleotides (CpG ODNs). The combined benefits of CpG ODNs forming a G4 structure and a non-viral vector carrier endow the ensuing complex with promising adjuvant properties. Although G4-CpG ODN-DOTAP complexes show a higher immunostimulatory effect than naked G4-CpG ODNs, the effects of the complex composition, especially charge ratios, on the production of the pro-inflammatory cytokines interleukin (IL)-6 and interferon (IFN)-α remain unclear. Here, we examined whether charge ratios drive the bifurcation of cytokine inductions in human peripheral blood mononuclear cells. Linear CpG ODN-DOTAP liposome complexes formed micrometer-sized positively charged agglomerates; G4-CpG ODN-DOTAP liposome complexes with low charge ratios (0.5 and 1.5) formed ~250 nm-sized negatively charged complexes. Notably, low-charge-ratio (0.5 and 1.5) complexes induced significantly higher IL-6 and IFN-α levels simultaneously than high-charge-ratio (2 and 2.5) complexes. Moreover, confocal microscopy indicated a positive correlation between the cellular uptake of the complex and amount of cytokine induced. The observed effects of charge ratios on complex size, surface charge, and affinity for factors that modify cellular-uptake, intracellular-activity, and cytokine-production efficiency highlight the importance of a rational complex design for delivering and controlling G4-CpG ODN activity.

Unmethylated cytosine-phosphate-guanosine oligodeoxynucleotides (CpG ODNs) induce inflammatory cytokines and type I interferons (IFNs) to activate the immune system. To apply CpG ODNs as vaccine adjuvants, the cellular uptake and stability of phosphodiester-based, non-modified ODNs require further improvement. Previously developed new CpG ODNs forming guanine-quadruplex (G4) structures showed higher nuclease resistance and cellular uptake than linear CpG ODNs; however, the complex formation of G4-CpG ODNs with antigen proteins is necessary for their application as vaccine adjuvants. In this study, we utilized a cationic polymer, ε-poly-L-lysine (ε-PLL), as a carrier for G4-CpG ODNs and antigen. The ε-PLL/G4-CpG ODN complex exhibited enhanced stability against nucleases. Cellular uptake of the ε-PLL/G4-CpG ODN complex positively correlated with the N/P ratio. In comparison to naked G4-CpG ODNs, the ε-PLL/G4-CpG ODN complex induced extremely high levels of interleukin (IL)-6, IL-12, and IFN-β. Relative immune cytokine production was successfully tuned by N/P ratio modification. Mice with the ε-PLL/G4-CpG ODN/ovalbumin (OVA) complex showed increased OVA-specific immunoglobulin (Ig)G, IgG1, and IgG2c levels, whereas total IgE levels did not increase and weight gain rates were not affected. Therefore, ε-PLL can serve as a safe and effective phosphodiester-based, non-modified CpG ODN delivery system, and the ε-PLL/G4-CpG ODN/antigen complex is a highly promising candidate for vaccine adjuvants and can be further used in clinical research.