Discovery of inulin acetate as a novel immune-active polymer and vaccine adjuvant: synthesis, material characterization, and biological evaluation as a toll-like receptor-4 agonist

Vaccine adjuvants are an essential part of modern vaccine design, especially against intracellular pathogens such as M. tuberculosis, malarial parasite, HIV, influenza virus and Ebola. The present work offers a unique approach to designing novel vaccine adjuvants by identifying polymers that mimic pathogen associated molecular patterns (PAMPS) and engineering an immune-active particulate vaccine delivery system that uses the polymer. By using this strategy, we report the discovery of the first plant polymer based toll-like receptor-4 (TLR-4) agonist, inulin acetate (InAc). InAc was synthesised from the plant polysaccharide inulin. Inulin acetate as a polymer and particles prepared using InAc were characterised using various physicochemical techniques. The TLR-4 agonistic activity of InAc was established in multiple immune, microglial, dendritic, peripheral blood mononuclear (human and swine) and genetically modified epithelial cells (HEK293) that exclusively express TLR-4 on their surface. InAc activated all the above-mentioned cells to release proliferative cytokines; however, InAc failed to activate when the were cells either pre-incubated with a TLR-4 specific antagonist or isolated from mice deficient in adapter proteins involved in TLR signalling (Mal/MyD88). Antigen encapsulated microparticles prepared with TLR-4 agonist InAc mimicked pathogens to offer improved antigen delivery to dendritic cells compared to soluble antigen (47 times) or antigen encapsulated poly(lactic-co-glycolic acid) (PLGA) particles (1.57 times). In conclusion, InAc represents a novel polymer-based modern vaccine adjuvant targeting specific signalling pathways of the innate immune system, which could be formulated into a platform vaccine delivery system against cancer and viral diseases.