Deciphering the mechanisms of action of vaccine adjuvants in human lymphoid tissue

Abstract

Vaccine adjuvants are thought to work by stimulating innate immunity at the site of vaccination and the draining lymph node (LN), where vaccine induced immunity is primarily generated. Different adjuvants have unique innate immune functions that can shape the downstream adaptive immune response. It is crucial to understand how different vaccine adjuvants work in humans, for the success of new formulations when transitioning from animal studies to human clinical settings. To bridge data obtained in animals to humans, we have developed a novel in situ human LN explant model for studying innate immune responses and investigating how adjuvants initiate immunity in LNs. Slices of explanted skin-draining LNs were exposed to adjuvants and revealed responses that were not detectable in LN cell suspensions. We used this model to compare the liposome-based AS01 with TLR ligands R848, MPL and Pam2Cys, and the saponin QS-21. Liposomes were predominantly taken up by subcapsular sinus-lining macrophages (SMs), monocytes and dendritic cells. AS01 induced dendritic cell maturation and a strong pro-inflammatory cytokine response in intact LN slices but not in dissociated cell cultures, in contrast to R848. This suggests the onset of the immune response to AS01 requires a coordinated activation of LN cells in time and space. Consistent with the robust immune response observed in older adults with AS01-adjuvanted vaccines, the AS01 response in human LNs was independent of age, unlike R848. Additionally, the molecular pathways of AS01 were investigated in part using the explant model. Blocking the NLRP3 inflammasome in LN slices produced an inhibitory effect on key AS01-induced pro-inflammatory cytokines. Preliminary assessments on isolated LN macrophages revealed the formation of inflammasome specks in response to AS01. This LN explant model is a novel tool for studying the mechanisms of adjuvants in humans and screening new formulations to streamline vaccine development.