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. 2021 Oct 6;11:745016. doi: 10.3389/fcimb.2021.745016

Figure 1.

Figure 1

Adjuvant candidates and their respective receptors. TLR4 and TLR5 are expressed on the cell surface, while TLR3, TLR4, TLR7, TLR8, and TLR9 are expressed in endosomes. TLR4, TLR5, TLR7, TLR8, and TLR9 initiate signaling through the MyD88 pathway to activate NF-κB, which induces the production of proinflammatory cytokines. Activated Syk elicits the production of proinflammatory cytokines through the NF-κB signaling pathway. RIG-1 and MDA-5 are intracellular nucleic acid sensors that induce both proinflammatory cytokines and type-1 interferons via MyD88 and IRF signaling pathways. cGAS produces cGAMP that activates the cGAS-stimulator STING located on the ER to induce production of proinflammatory cytokines through the NF-κB signaling pathway. Inflammasomes (AIM2, NLRP1, NLRP3, and NLRC4) cleave pro-caspase 1 into caspase 1 that subsequently cleaves pro-IL-1β, pro-IL-18, and Gasdermin D into their mature forms, respectively. N-terminal Gasdermin D oligomerizes to form pores, inducing pyroptosis. Blue text represents adjuvant candidates discussed in this review.