Study on the adjuvant activity and mechanism of action of a novel Monophosphoryl Lipid A

In order to develop a novel vaccine adjuvant that is highly efficient, cost-effective, and suitable for widespread application, this study employed synthetic biology techniques to produce a new type of Escherichia coli monophosphate lipid A (N-MPL). Specifically, the phosphate group attached to the C-1 position was removed, and a hydroxyl group was introduced into the 3′-secondary fatty acid chain of the original lipid A structure. This modification aimed to reduce toxicity while enhancing water solubility. Two formulations of N-MPL were prepared and their immunological and adjuvant activities were evaluated. Results demonstrated that both formulations could promote the proliferation of immune cells and the maturation of bone marrow-derived dendritic cells (BMDCs). In an adjuvant immunization experiment using H1N1 vaccine in ICR mice, N-MPL exhibited superior adjuvant effects compared to aluminum adjuvants and commercially available Monophosphoryl Lipid A (Synthetic) (PHAD™). Through comprehensive analysis of splenic lymphocytes, antibodies, cytokines, and immune factors in immunized mice, it was discovered for the first time that different formulations of N-MPL adjuvants could regulate cellular and humoral immunity, albeit with distinct foci of induced immune responses. The oil-in-water formulation of N-MPL primarily induced cellular immune responses in mice, whereas the aqueous solution formulation predominantly elicited humoral immune responses. Mechanistic studies revealed that N-MPL regulates the Th1/Th2 response via the TLR4-MyD88-NF-κB signaling pathway. Overall, the novel N-MPL adjuvant exhibits high efficiency and low cost, demonstrating significant potential for vaccine adjuvant development.