Emerging concepts in the science of vaccine adjuvants

Abstract

Adjuvants are vaccine components that enhance the magnitude, breadth and durability of the immune response. Following its introduction in the 1920s, alum remained the only adjuvant licensed for human use for the next 70 years. Since the 1990s, a further five adjuvants have been included in licensed vaccines, but the molecular mechanisms by which these adjuvants work remain only partially understood. However, a revolution in our understanding of the activation of the innate immune system through pattern recognition receptors (PRRs) is improving the mechanistic understanding of adjuvants, and recent conceptual advances highlight the notion that tissue damage, different forms of cell death, and metabolic and nutrient sensors can all modulate the innate immune system to activate adaptive immunity. Furthermore, recent advances in the use of systems biology to probe the molecular networks driving immune response to vaccines ('systems vaccinology') are revealing mechanistic insights and providing a new paradigm for the vaccine discovery and development process. Here, we review the 'known knowns' and 'known unknowns' of adjuvants, discuss these emerging concepts and highlight how our expanding knowledge about innate immunity and systems vaccinology are revitalizing the science and development of novel adjuvants for use in vaccines against COVID-19 and future pandemics.

Fig. 1. Molecular targets of adjuvants.

a | Toll-like receptors (TLRs) TLR1, TLR2, TLR4, TLR5 and TLR6 are expressed on the cell surface, whereas TLR3, TLR7, TLR8 and TLR9 are expressed in endosomes. TLR1 and TLR6 heterodimerize with TLR2 and signal through the myeloid differentiation primary response 88 (MyD88) pathway to activate NF-κB and MAP kinases, leading to secretion of pro-inflammatory and anti-inflammatory cytokines. TLR4 and TLR5 function as homodimers and signal through the MyD88 pathway. TLR7 and TLR9 also use the MyD88 pathway, but rapidly activate IRF7 to induce type I interferons. TLR3 uses TIR domain-containing adapter-inducing IFNβ (TRIF) signalling to induce type I interferons through IRF3. b | Cytosolic pattern recognition receptors (PRRs) are sensors of pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) present inside the cytoplasm of the cell. Nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) are cytosolic sensors of bacterial PAMPs but also recognize multiple cellular products including ATP, uric acid and K⁺ to activate the NF-κB pathway and induce cytokines driving T helper 2 (T_H2) cell differentiation. Retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated protein 5 (MDA5) are intracellular viral sensors that drive type I interferon response through IRF3 and IRF7. The cGAS–stimulator of interferon genes (STING) pathway recognized double-stranded DNA (dsDNA) to induce the NF-κB pathway. c | C-type lectin receptors (CLRs) are cell surface molecules expressed on multiple myeloid cell subsets. Dectins 1 and 2 and MINCLE recruit SYK1 and activate NF-κB through the CARD9–BCL-10–MALT1 complex. Furthermore, dectin 1 has been shown to induce the NFAT and AP1 pathways in macrophages and dendritic cells (DCs) and in in vitro experimental models, respectively. Dectins are also specialized in inducing antifungal immunity. Dendritic cell-specific ICAM3-grabbing non-integrin 1 (DC-SIGN) activates NF-κB via acetylation of p65; however, the resulting gene expression is poorly understood although IL-10 expression has been shown to be induced. DEC205 and DNGR1 are known to induce cross-presentation but the signalling pathways are unknown. ASC, apoptosis-associated speck-like protein containing a CARD; CpG, cytosine phosphoguanosine; dsRNA, double-stranded RNA; LPS, lipopolysaccharide; ssRNA, single-stranded RNA; TDM, trehalose-6,6-dimycolate; T_reg cell, regulatory T cell.

Fig. 2. Beyond pattern recognition receptors.

Recent studies have yielded insights into novel pathways that could be targeted for adjuvant activity. a | Tissue damage caused by trauma, infection and autoimmunity results in release of a multitude of damage-associated molecular patterns (DAMPs), including nucleic acids, uric acid, ATP and proteins such as high mobility group box 1 (HMGB1), that activate the innate immune system. b | Cell death induced by different stimuli also releases DAMPs. Of particular interest is the mechanism of cell death induced by different stimuli. Specialized cell death pathways, such as necroptosis and pyroptosis, can activate innate immune cells. Small molecules that induce specific cell death pathways could be effective adjuvants. c | Cellular metabolism is the third concept that is emerging as a central regulatory network of immune responses. Immune cells, such as dendritic cells (DCs), have a distinct metabolic state in different tissues. The insights stemmed from the systems analysis of yellow fever vaccine-induced immune responses in humans, in which the amino acid sensor GCN2 emerged as an early correlate of lasting CD8⁺ T cell responses. GCN2 activation in DCs by the yellow fever vaccine enhances antigen presentation to T cells via autophagy. Furthermore, the central metabolic regulator mTOR is shown to have various effects on innate immune responses, especially of DCs. d | Vaccines such as Bacillus Calmette–Guérin (BCG) and pathogen-associated molecular patterns (PAMPs) such as β-glucan induce epigenetic changes that maintain the innate immune system at an alarming state for extended periods. Small molecules targeting appropriate cell types offer attractive components of novel adjuvants. H3K4me3, histone H3 trimethylated at Lys4; H3K27ac, histone H3 acetylated at Lys27; MHC, major histocompatibility complex; NK cell, natural killer cell; P, phosphorylation; TF, transcription factor; T_H1 cell, T helper 1 cell; uORF, upstream open reading frame.

Fig. 3. Beyond the innate/adaptive paradigm, continuing education by adjuvants.

The innate immune system sensing adjuvants and programming the ensuing adaptive immune responses is the current model of how adjuvants function. Activated dendritic cells (DCs) present antigens to naïve antigen-specific CD4⁺ T helper cells (T_H cells) in T cell areas. Some activated T_H cells upregulate CXCR5, which mediates their migration to the interface between the B cell follicle and the T cell area, where they express IL-21 and CD40L that stimulate the clonal expansion of antigen-activated B cells. Although some antigen-specific B cells migrate to the medullary cords and differentiate into short-lived plasma cells, other activated B cells migrate into B cell follicles to form germinal centres (GCs). B cells in GCs can subsequently differentiate into memory B cells that recirculate, or long-lived plasma cells (LLPCs) that migrate to the bone marrow. Many adjuvants are known to work primarily by targeting DCs to induce their activation and antigen presentation, but emerging studies demonstrate that adjuvants such as Toll-like receptor (TLR) ligands can also target B cells. Therefore, potentially novel adjuvant targets could include B cell subsets in GCs, bone marrow LLPCs and other cell types that aid survival of LLPCs, follicular DCs (FDCs) and T follicular helper cells (T_FH cells). MHC, major histocompatibility complex; TCR, T cell receptor.

Fig. 4. A new framework for development of adjuvants.

a | The current model of developing vaccines containing novel adjuvants represents a linear progression from the systematic testing of novel candidates in mice, to the advancement of promising candidates to testing in non-human primates (NHPs), and the eventual testing in humans in multiple phases of clinical trials. b | The new model we propose relies on a process of iterative testing in mice, organoid cultures, NHPs and humans. We advise the early use of small-scale experimental human trials and the use of systems biology approaches to generate multiparametric immunological read-outs, which enable the generation of novel hypothesis and adjuvant concepts that can be retested in preclinical models.