Novel Nanoadjuvant for Subunit Vaccines: Cyclic Seven-Membered Tertiary Amine-Based Polymer-Grafted Ethoxy-Acetalated Dextran Nanoparticles Encapsulating CpG Oligodeoxynucleotides

CpG oligodeoxynucleotides (ODNs) are synthetic Toll-like receptor 9 (TLR9) agonists that promote Th1-biased immune responses. However, their clinical utility is limited by rapid nuclease degradation and poor cellular uptake in antigen-presenting cells (APCs). To overcome this, we developed a pH-responsive nanoadjuvant, Ace-Dex-PC7A@CpG, composed of a cyclic seven-membered tertiary amine-based polymer (PC7A) grafted onto ethoxy-acetalated dextran (Ace-Dex) encapsulating CpG ODN 1668. In vitro, this nanoadjuvant significantly increased the levels of TNF-α, IL-6, IFN-β, and CXCL10 in RAW macrophages, indicating the simultaneous activation of both the TLR9 and stimulator of interferon genes (STING) pathways. In vivo, Ace-Dex-PC7A@CpG significantly enhanced the immunogenicity of the receptor-binding domain (RBD) of SARS-CoV-2, eliciting robust humoral responses with mean anti-RBD IgG titers exceeding 300,000, a 15-fold increase over free RBD (20,000). Sera from immunized mice showed >50% inhibition of RBD-hACE2 binding, far surpassing the 6% inhibition observed with free RBD. Furthermore, in a tumor-associated carbohydrate antigen (TACA) vaccine model, GM2-conjugated bacteriophage MX1 (GM2-MX1) virus-like particles (VLPs) adjuvanted with Ace-Dex-PC7A@CpG produced 10-fold higher anti-GM2 IgG titers than GM2-MX1 alone. More importantly, vaccination with 9NHAc-GD2-mQβ VLPs (a leading TACA vaccine candidate) plus Ace-Dex-PC7A@CpG significantly suppressed liver metastasis in an EL4 lymphoma model. These findings establish Ace-Dex-PC7A@CpG as a versatile nanoadjuvant for subunit vaccines against infectious diseases and cancer.