Mechanism of action of MPL-QS21 adjuvant combination

The National Institute of Allergy and Infectious Diseases awarded the University of Sydney $736,873 on July 10, 2026, under the Allergy and Infectious Diseases Research program (CFDA 93.855) to investigate the mechanism of action of the MPL-QS21 adjuvant combination in response to RFA-AI-25-006: Molecular Mechanisms of Combination Adjuvants (MMCA).

The research addresses how monophosphoryl lipid A (MPL), a TLR4 agonist, and QS21, a saponin, work alone and in combination to boost immune responses to vaccine antigens. The work encompasses detailed human and murine investigations of the MPL-QS21 combination's immunological effects, including uptake into lymph node macrophages and downstream cascades of innate and adaptive immune cell activation leading to CD4 T cell stimulation and durable neutralizing antibody production. The project aims to improve understanding of adjuvant mechanisms of action and support optimal matching of adjuvants to vaccine antigens in preclinical and clinical vaccine development, particularly for recombinant protein vaccines such as recombinant zoster vaccine and adjuvanted influenza vaccines.

Performance occurs at the University of Sydney's Camperdown, New South Wales campus in Australia. The project runs through June 30, 2031.

Abstract

/summary adjuvants boost specific immune responses to vaccine antigens. Optimal matching of adjuvants to vaccine antigens is an unmet medical challenge in the long, expensive pre-clinical and clinical stages of the vaccine translational pipeline. The impact of this proposal will be to assist this match-making and speed future vaccine design while maintaining safety, by improving our understanding of adjuvant mechanisms of action (MOA) and possibly toxicity. Adjuvants are especially important as recombinant antigens are favored over whole live or whole or split inactivated vaccines. Use of adjuvants with protein vaccines, such as recombinant zoster vaccine (RZV, shingrixtm) and the adjuvanted influenza vaccine (FLUADTM) have improved efficacy, especially in the aging and immune compromised. In particular, RZV has remarkable and durable efficacy in recipients aged >70, with 90% and 82% efficacy at 1 and 11 years. Modelling suggests efficacy for 20 years. The combination of two adjuvants monophosphoryl lipid a (MPL), a TLR4 agonist, and QS21, a saponin, into cholesterol-containing liposomes as AS01 are responsible for the RZV immunogenicity, which also persists and plateaus over >10 years. No vaccines in clinical use utilize MPL or QS21 singly. The MPL-QS21 combination has shown synergy in innate immune and effector cell functions in mouse models and in our human lymph node (LN) explant model. In this response to RFA-AI-25-006: molecular mechanisms of combination adjuvants (MMCA), we propose detailed human and murine investigations of the MOA of MPL and QS21 alone and in combination. AS01 liposomes are taken up into LN sinus-lining macrophages and results in a cascade of interacting innate and adaptive immune cell events leading to stimulation of naïve and memory ifnγ/IL-2 polyfunctional CD4 T cells and durable NAB. The efficacy of AS01 is dependent on cell-to-cell interactions within the intact lymph node, informing our approach. Our preliminary data indicate key roles for macrophages and inflammasome components followed by macrophage pyroptosis and liberation of IL1Β and IL18. In aim 1 we compare primary LN human explants and genetically defined mice treated with AS01 single and combination components to further define MOA synergy, using single cell, spatial, and pathway inhibitor approaches. Aim 2 centers on a novel study design of explanted human draining LN after RZV administration that retains immune cell interactions in the LN structure. We probe the initiating MOA and the progression to GE-specific acquired immunity with: 24-48 hour LN specimens examining AS01 action on interacting macrophages, DCS and NK cells; 4-6 day post-vaccine specimens focusing on DC stimulated antigen-specific effector CD4 T cell activation (including polyfunctional T cells and TFH), defined by T cell receptor (TCR) matching to known vaccine-specific CD4 T cells. Aim 3 defines the phenotype of the remarkably durable and effective RZV CD4 T cell population in pbmc of long-term vaccinees using an existing biobank and single cell technologies to determine the biosignature of effective, adjuvant-enabled immunity.