Oligonucleotide Adjuvants in Vaccinology: Current Classification, Mechanisms of Action, and Development Trends

Introduction. Modern vaccines require adjuvants that can induce not only a humoral immune response but also a cellular one. CpG oligonucleotides are among the most promising candidates. They are synthetic TLR9 agonists that imitate bacterial DNA. Despite the approval of products such as HEPLISAV-B® and SpikoGen®, information on the classification, mechanisms of action, and modification strategies of CpG adjuvants remains fragmented. This review aims to systematise current data on CpG oligonucleotide adjuvants, including their structure–function characteristics, chemical modifications, and delivery strategies. Materials and methods. A review of the scientific literature indexed in PubMed, PubMed Central, and eLibrary was conducted. The search covered a 20-year period, reflecting the availability of seminal studies on CpG-mediated immunostimulation published in the early 2000s. In addition, recent publications from the past 5–7 years were selected to assess current trends. The search strategy included the following keywords: CpG oligonucleotides, TLR9 agonists, vaccine adjuvants, oligonucleotide synthesis, and phosphorothioate modifications. Results. The major classes of adjuvants were systematically reviewed, and the position of oligonucleotides within the overall classification was defined. CpG oligonucleotides of classes A, B, C, and P were characterised in detail, with particular attention to their structural features and immunological profiles. Phosphorothioate backbone modifications were shown to be critical for improving the stability and efficacy of CpG oligonucleotides. The main barriers to the broader use of CpG adjuvants were identified as targeted delivery challenges and potential reactogenicity. Current strategies to overcome these limitations were analysed, including ligand conjugation, such as GalNAc, encapsulation in nanoparticles, including liposomes, PLGA, metallic nanoparticles, and hybrid OMV-based platforms, as well as incorporation into combined adjuvant systems. Discussion and conclusion. The most promising directions for future research include optimising the chemical structure of CpG oligonucleotides to improve their pharmacokinetic properties; developing targeted delivery systems, including nanoscale carriers and ligand conjugates; incorporating CpG oligonucleotides into multicomponent adjuvant systems to achieve synergistic effects; and applying them in cancer immunology to reprogramme tumour-associated macrophages and overcome resistance to immune checkpoint inhibitors. Further research in this field may pave the way for the development of next-generation vaccines against socially significant infectious diseases and cancer.