Developing and improving vaccine candidates against existing and emerging pathogens is crucial. Recombinant subunit vaccines are safe and effective and contain an antigen and an immunostimulatory adjuvant. Understanding the mechanisms of action of adjuvants in humans is essential for ongoing development, however this is often studied in animal models and poorly characterized in humans. We have developed a novel in situ human lymph node (LN) explant model as a tool to investigate adjuvant function in this primary organ where vaccines initiate immunity.
Slices of human lymph nodes are exposed in situ to vaccine adjuvants. Advanced high-parameter flow cytometry and cytokine bead-based assays are then used to investigate adjuvant uptake, cellular maturation and activation, and cytokine responses. As this human model utilizes intact LN slices, the natural sequence of immune events is preserved. Adjuvants can sufficiently penetrate the LN to initiate their immune responses, where we can track uptake into specific immune cell subsets and characterize the immune mechanisms involved.
We have investigated various adjuvants including TLR ligands R848 and Pam2Cys. Both these adjuvants induce maturation of dendritic cell (DC) subsets, with R848 particularly activating plasmacytoid DCs. A range of pro-inflammatory cytokines are also detected, with both adjuvants significantly inducing IL-1β, IL-6, IL-23, MCP-1, TNF-α, IFN-α and IFN-γ. R848 additionally induces IL-12p70, whereas Pam2Cys also induces IL-8 and IL-10. We have also investigated liposome uptake as a model for lipid nanoparticle vaccines, which is relevant to mRNA vaccines. Here, LN sinus lining macrophages preferentially take up liposomes.
Overall, our human LN explant model serves as a novel tool for studying adjuvant immunology in humans and is now being applied to the adjuvant AS01B, as well as RNA vaccines. Our model can further serve as a preclinical screening platform to streamline the vaccine development process for investigating new vaccine candidates in humans.