Human in vitro modeling characterizes age-specific mechanism of action of vaccine adjuvant formulations for global open access

Abstract

Adjuvants enhance vaccine immunogenicity but their mechanism of action is often incompletely understood, hampering rapid applicability for pandemic vaccines. Herein, we characterized the cellular and molecular activity of several adjuvant formulations available for pre-clinical evaluation, including several developed for global open access. We applied four complementary human in vitro platforms to assess individual and combined adjuvants in soluble, oil-in-water, and liposomal formulations. Of all formulations tested, liposomal co-formulation of MPL and QS-21 was most potent in promoting dendritic cell maturation, selective production of Th1-polarizing cytokines, differentiation of mature dendritic cells in a microphysiological 3D tissue construct, and activation of antigen-specific CD4+ and CD8+ T cells in a SARS-CoV-2 spike antigen-specific co-culture assay. Thus, human in vitro modeling provides insight into the mechanism of action of adjuvanted vaccine formulations which may advance public health by accelerating development of affordable and scalable adjuvants for vaccines tailored to vulnerable populations.

Children demonstrate distinct immunity in early life including diminished Th1-polarizing cytokine production contributing to high susceptibility to respiratory viral infections. This challenge also pertains to pediatric vaccine discovery and development, and could be overcome by developing adjuvantation systems with well characterized mechanisms of action (MOA) that are tailored to enhance age-specific immunogenicity. To this end, we employed novel age-specific human in vitro assays to characterize cellular and molecular activities of a selection of adjuvants in children aged between 2-4 years in soluble, oil-in-water and liposomal formulations available for pre-clinical evaluation, including several that were developed for global open access. In a whole blood assay, which predicts the reactogenicity potential of adjuvants, liposomal co-formulation of adjuvants monophosphoryl lipid A (MPL, a TLR4 agonist) and the soap bark tree-derived saponin Quillaja saponaria (QS)-21 (LMQ) induced a modest cytokine production compared to soluble and oil-in-water formulations. LMQ particularly enhanced production of CXCL8 and IL-6, markers of Th2 and Th17 responses that distinguish early life immunity from that of adults. Additionally, the double mutant heat-labile enterotoxin (dMLT) induced production of CXCL1 and IL-1[beta], and TLR7/8 agonist in oil-in-water (EM) and TLR7/8 agonist + alum (AL) induced production of IFN[alpha]. In a monocyte-derived dendritic cell assay, which dissects the mechanism by which adjuvants activate differentiation of T helper (Th) cell subsets, LMQ induced a robust TNF response, indicative of Th1 immunity important for anti-viral host defense. In a dendritic cell-T cell interface assay that demonstrates antigen processing and presentation, activation of influenza HA antigen-specific CD4+ T cells was driven by MPL and that of CD8+ T cells was induced by QS-21. Insight into the MOA of adjuvanted vaccine formulations via human in vitro modeling may advance global health by accelerating and de-risking development of affordable and scalable precision-adjuvanted vaccines.