Cyclic di-nucleotides: new era for small molecules as adjuvants.
Subunit vaccines were then introduced as more refined formulations, exhibiting improved safety profiles.In the 1990s, immunologists found that pathogens could be sensed as 'danger signals' by receptors recognizing conserved motifs.Some of the latest players arrived to this field are the cyclic di-nucleotides, which are ubiquitous prokaryotic intracellular signalling molecules.
Affiliation: Department of Vaccinology and Applied Microbiology, Helmholtz Centre for Infection Research, 38124 Braunschweig, Germany. email@example.com
- Adjuvants, Immunologic/pharmacology*
- Nucleotides, Cyclic/pharmacology*
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f2: Putative intracellular cascades activated by cyclic diânucleotides. In this schematic representation, TLRs are separated in two major groups, those associated to the membrane and those located in the endosomal compartment. For the sake of clarity, in this scheme there is no discrimination between the different TLR at either the membrane or endosomal compartment (for details in TLRs pathways see review by Kawai and Akira, 2011). The membraneâbound TLRs (TLRâ1, TLRâ2/1, TLRâ2/6, TLRâ4 and TLRâ5) detect PAMPs and DAMPs on the cell surface and bind to specific TIR domain containing adapters, such as TRIF, MyD88, TIRAP and TRAM. Other TLRs, such as TLRâ3, TLRâ7 and TLRâ8, are localized in intracellular vesicles and recognize RNA, whereas the intracellular TLRâ9 recognizes DNA. The important players downstream in these signalling cascades are TRAF6, TAK1 and TBK1, which in turn phosphorylates IRFâ3 and IRFâ7, leading to their homoâdimerization and translocation into the nucleus, where they drive transcription of IFNs. This signalling cascades result in the activation of NFâÎșB and MAPK, which in turn are leading to the production of proâinflammatory cytokines and type I IFN. On the other hand, TLRâindependent recognition of PAMPs is mediated by the intracellular receptors NLRs (NODs) and RLRs (RIGâI, MDAâ5) present in the cytosol, which after activation trigger a subset of responses, which are similar to those promoted by TLRs. Exogenous or viral dsRNA is recognized by the RNA helicase RIGâI (or MDAâ5), and signals through the mitochondrial antiviral signalling adaptor MAVS (also known as IPSâ1), which activates TBK1, thereby leading to phosphorylation of IRFâ3, NFâÎșB release, translocation of the transcriptional regulators and gene induction. DNA is sensed by DAI, leading to activation of the same TBK1/IRF pathway as RIGâI/MDAâ5. The ERâlocalized STING protein was shown to be critical for regulating the production of IFN in response to cytoplasmic DNA virus. In vitro and in vivo studies suggest that câdiâGMP and câdiâAMP are sensed through a cytosolic pathway leading to type I IFN induction. The induction of type I IFNs by câdiânucleotides is dependent on TBK1/IRFâ3 signalling, although it is independent of known cytosolic receptors. The adaptor molecule STING also seems to be required for the type I IFN responses induced by câdiânucleotides. Preliminary studies suggest that the adjuvanticity of câdiâGMP relies on activation by IRFâ3/IRFâ7. However, it is still unclear if câdiânucleotides need to reach the cytosol to exert their activity or they are acting via upâtoânow uncharacterized surface receptors. It is also unknown to which extend the induction of type I IFN is sufficient to explain the complex and pleiotropic adjuvant properties of these molecules. Additional information is also needed, to assert the molecular processes responsible for the observed differences between the biological activities of different câdiânucleotides. Red lines: putative câdiânucleotide driven pathways for which strong experimental evidence exists. Green lines: Presumptive pathway for which preliminary data is available. Black lines: non câdiânucleotide driven pathways.
Interestingly, there are in vitro studies showing the induction of similar transcriptional profiles in cells stimulated by cyclic diânucleotides and DNA. Both are able to trigger type I IFNs and coâregulated genes via induction of Tankâbinding kinase 1 (TBK1) and its substrate the IRFâ3, as well as nuclear factor NFâÎșB and MAP kinases (Ishii etâal., 2008; McWhirter etâal., 2009; Woodward etâal., 2010). On the other hand, in vivo studies showed that câdiâGMP activates both IRFâ3 and IRFâ7 (McWhirter etâal., 2009). However, cylic diânucleotides are not signalling through the cytosolic DNA sensor DAI (DNAâdependent activator of IRFs) (McWhirter etâal., 2009; Trinchieri, 2010), as it is the case for DNA (Fig.â2).