Glyco-Nanoadjuvants: Impact of Linker Length for Conjugating a Synthetic Small-Molecule TLR7 Ligand to Glyco-Nanoparticles on Immunostimulatory Effects.

Immunotherapy has become a powerful clinical strategy for treating infectious diseases and cancer. Synthetic small-molecule toll-like receptor 7 (TLR7) ligands are attractive candidates as immunostimulatory agents for immunotherapy. TLR7 is mainly localized in intracellular endosomal compartments so that the formulation of their small-molecule ligands with macromolecules enhances endocytic uptake of TLR7 ligands and improves the pharmaceutical properties. Previously, we demonstrated that gold nanoparticles co-immobilized with a TLR7 ligand derivative, that is, a conjugate of synthetic small-molecule TLR7 ligand (1V209) and thioctic acid (TA) via 4,7,10-trioxa-1,13-tridecanediamine, and α-mannose (1V209-αMan-GNPs: glyco-nanoadjuvants) significantly enhances immunostimulatory effects. In the present study, we designed a second-generation glyco-nanoadjuvant that possesses a poly(ethylene glycol) (PEG) chain as a spacer between 1V209 and GNPs and investigated the impact of linker length in 1V209 derivatives on the immunostimulatory activities. We used different chain lengths of PEG (n = 3, 5, 11, or 23) as spacers between 1V209 and thioctic acid to prepare four 1V209-αMan-GNPs. In the in vitro study using primary mouse bone-marrow-derived dendritic cells, 1V209-αMan-GNPs that immobilized with longer 1V209 derivatives, especially the 1V209 derivative possessing PEG23 (1V209-PEG23-TA), showed the highest potency toward induction both for interleukin-6 and type I interferon production than those derivatives with shorter PEG chains. Furthermore, 1V209-αMan-GNPs that immobilized with 1V209-PEG23-TA showed significantly higher adjuvant effects for inducing both humoral and cell-mediated immune responses against ovalbumin in the in vivo immunization study. These results indicate that the linker length for immobilizing small-molecule TLR7 ligand on the GNPs significantly affects the adjuvant activity of 1V209-αMan-GNPs and that 1V209-αMan-GNPs immobilized with 1V209-PEG-23-TA could be superior adjuvants for immunotherapies.

Glyco-Nanoadjuvants: Sugar Structures on Carriers of a Small Molecule TLR7 Ligand Affect Their Immunostimulatory Activities.

Toll-like receptors (TLRs) are pattern recognition receptors that activate innate immunity, and their ligands are promising adjuvants for vaccines and immunotherapies. Small molecule TLR7 ligands are ideal vaccine adjuvants as they induce not only proinflammatory cytokines but also type I interferons. However, their application has only been approved for local administration due to severe systemic immune-related adverse events. In a previous study, we prepared the gold nanoparticles coimmobilized with synthetic small molecule TLR7 ligand, 1V209, and α-mannose (1V209-αMan-GNPs). 1V209-αMan-GNPs were selectively delivered via a cell surface sugar-binding protein, mannose receptor, which enabled selective delivery of TLR7 ligands to immune cells. Besides the mannose receptor, immune cells express various sugar-binding proteins such as macrophage galactose binding lectins and sialic acid-binding immunoglobulin-type lectins and recognize distinct sugar structures. Hence, in the present study, we investigated whether sugar structures on GNPs affect the efficiency and selectivity of intracellular delivery and subsequent immunostimulatory potencies. Five neutral sugars and two sialosides were selected and each sugar was coimmobilized with 1V209 onto GNPs (1V209-SGNPs) and their innate immunostimulatory potencies were compared to that of 1V209-αMan-GNPs. The in vitro study using mouse bone marrow derived dendritic cells (BMDCs) demonstrated that α-glucose, α-N-acetylglucosamine, or α-fucose immobilized 1V209-SGNPs increased interleukin-6 and type I interferon release similar to that of 1V209-αMan-GNPs, whereas galacto-type sugar immobilized 1V209-SGNPs predominantly enhanced type I interferon release. In contrast, sialoside immobilized 1V209-SGNPs did not enhance the potency of 1V209. In the in vivo immunization study using ovalbumin as a model antigen, neutral sugar immobilized 1V209-SGNPs induced comparable T helper-1 immune response to that of 1V209-αMan-GNPs and by 10-fold higher than that of sialoside immobilized 1V209-SGNPs. These results indicate that the sugar structures on 1V209-SGNPs affect their immunostimulatory activities, and functionalization of the carrier particles is important to shape immune responses.

Gold Nanoparticles Coimmobilized with Small Molecule Toll-Like Receptor 7 Ligand and α-Mannose as Adjuvants.

Adjuvants enhance the immune response during vaccination. Among FDA-approved adjuvants, aluminum salts are most commonly used in vaccines. Although aluminum salts enhance humoral immunity, they show a limited effect for cell-mediated immune responses. Thus, further development of adjuvants that induce T-cell-mediated immune response is needed. Toll-like receptors (TLRs) recognizing specific pathogen-associated molecular patterns activate innate immunity, which is crucial to shape adaptive immunity. Using TLR ligands as novel adjuvants in vaccines has therefore attracted substantial attention. Among them a small molecule TLR7 ligand, imiquimod, has been approved for clinical use, but its use is restricted to local administration due to unwanted adverse side effects when used systematically. Since TLR7 is mainly located in the endosomal compartment of immune cells, efficient transport of the ligand into the cells is important for improving the potency of the TLR7 ligand. In this study we examined gold nanoparticles (GNPs) immobilized with α-mannose as carriers for a TLR7 ligand to target immune cells. The small molecule synthetic TLR7 ligand, 2-methoxyethoxy-8-oxo-9-(4-carboxy benzyl)adenine (1V209), and α-mannose were coimmobilized via linker molecules consisting of thioctic acid on the GNP surface (1V209-αMan-GNPs). The in vitro cytokine production activity of 1V209-αMan-GNPs was higher than that of the unconjugated 1V209 derivative in mouse bone marrow-derived dendritic cells and in human peripheral blood mononuclear cells. In the in vivo immunization study, 1V209-αMan-GNPs induced significantly higher titers of IgG2c antibody specific to ovalbumin as an antigen than did unconjugated 1V209, and splenomegaly and weight loss were not observed. These results indicate that 1V209-αMan-GNPs could be useful as safe and effective adjuvants for development of vaccines against infectious diseases and cancer.