TLR7 & TLR8: fraternal twins
Toll-like receptors (TLRs) play a pivotal role in the structural analyses, along with a report describing initiation of anti-infectious immune responses. Distinct human TLR8 as a key sensor of bacterial viability pathogen-associated molecular patterns (PAMPs) are through the recognition of bacterial RNA7. recognized by different TLRs, at the cell surface or in endosomes. TLR7 and TLR8 are endosomal receptors TLR7 and TLR8 exhibit different expression patterns. that share structural homology and sense viral single TLR7 is essentially expressed by plasmacytoid stranded (ss) RNA as well as synthetic base analogs. dendritic cells (pDCs) but is also found in B cells and However, there are functional differences between myeloid cells1. TLR8 is absent from pDCs and B cells, SUMMARY : these two TLRs. and is highly expressed by myeloid cells1. This suggests that TLR7 and TLR8 have evolved to mediate distinct The endosomal distribution of TLR7 and TLR8 allows immune responses upon microbial encounters. Viral REVIEW them to scan for the presence of microbial RNA in the infections trigger TLR7-mediated production of IFN-α phagocytic cargo. Their activation leads to NF-κB-, in pDCs1,6. However, in monocytes, TLR7 and TLR8 TLR7 & TLR8: fraternal twins AP1- and IRF-mediated production of type I IFNs activation induces the expression of TH17- and TH1- (IFN-α/β) and pro-inflammatory cytokines1. Structural skewing cytokines, IL-1β and IL-12, respectively8. Upon analyses have revealed that both TLR7 and TLR8 bacterial infection, TLR7 drives IFN-α production by possess two binding sites which do not share the same pDCs, but its role in myeloid cells remains obscure6. On PRODUCTS specificities. Site 1 is highly conserved between TLR7 the other hand, TLR8 seems to be the ‘best-fit’ sensor and TLR8 and binds nucleosides (guanosine (G) for for bacterial RNA in myeloid cells6,7. Species-driven TLR7 and TLR8 TLR7 and uridine (U) for TLR8) or base analogs. The ligand preference for TLR7 and TLR8 is explained There is converging evidence for TLR8 to be the missing responses by the presence of specific residues in Site 1. Site 2 is link between empirical use of live attenuated microbes TM • HEK-Blue hTLR7 Cells less conserved and binds ssRNA with U(U) and U(G) in vaccines and the known necessity for TH1- and TFH- 2,3 TM motifs, respectively . Of note, Site 1 occupancy allows • HEK-Blue mTLR7 Cells driven humoral immunity to reach superior vaccine the receptor dimerization, and signaling with ad hoc efficiencies. Live bacteria or bacterial RNA, but not TM • HEK-Blue hTLR8 Cells ligand concentration. ssRNA-binding to Site 2 is not dead bacteria, induce the TLR8-dependent production • HEK-BlueTM mTLR8 Cells sufficient for the formation of a signaling competent of IL-12 by human monocytes, thereby promoting TFH TLR dimer but it strongly enhances the binding affinity differentiation6. Moreover, epidemiological analyses • CU-CPT9a: TLR8 specific inhibitor of Site 12,3. Thus, TLR7 and TLR8 appear to sense indicate that the gain-of-function of human TLR8 distinct RNA-degradation products rather than full- through single nucleotide polymorphism is linked Cytosolic sensing of intermediate length ssRNAs3. to better protective immunity in response to a live metabolites of LPS bacteria vaccine (i.e. BCG)7. Lastly, TLR8 seems to play TLR8 has been less studied than TLR7 as it was initially a unique role in neonatal immunity as human neonatal • ADP-Heptose thought to be non-functional in mice4. Of note, this phagocytes are only responsive to TLR8 ligands9. • HEK-BlueTM KO-ALPK1 Cells does not hold true when using TL8-506, an analog of
TM the synthetic agonist VX-2337 (see inside). Further, A better comprehension of the functional differences • HEK-Blue KO-TIFA Cells TLR13 has been suggested as a murine TLR8 homolog5. between TLR7 and TLR8 should allow the development • HEK-BlueTM Null1-v Cells Thus, findings regarding mouse TLR7 and TLR8 are not of more potent, specific and less toxic molecules as transposable to their human counterparts4,6. However, stand-alone drugs or adjuvants for the treatment of Preventing contamination of primary there is a renewed interest in TLR8 supported by the inflammatory, autoimmune and cancerous diseases. cell cultures
TM 1. Georg P. & Sander L.E., 2019. • Primocin e doso e Microbial Innate sensors that regulate vaccine degradation degradation ssRNA responses. Curr. Op. Immunol. 59:31. 2. Zhang Z. et al., 2018. Structural Guanosine Uridine analyses of Toll-like receptor 7 reveal Guanosine-containing ssRNA Base Uridine-containing ssRNA U detailed RNA sequence specificity and recognition mechanism of analog U U U Base analog agonistic ligands. Cell Rep. 25:3371. 3. Tanji H. et al., 2015. Toll-like Site 1 Site 1 receptor 8 senses degradation products of single-stranded RNA. USA Site 2 TLR7 Site 2 TLR8 Nat. Struct. Mol. Biol. 22:109. 10515 Vista Sorrento Pkwy TLR7 TLR8 4. Heil F. et al., 2004. Species- San Diego, dimer dimer specific recognition of single-straned CA, 92121 MyD88 RNA via Toll-like receptor 7 and 8. MyD88 Science. 303:1526. 5. Choo M.K. et al., 2017. TLR sensing of bacterial IRF3 IRF3 IRF7 IRF7 MAPK NF-κB spore-associated RNA triggers host immune responses with detrimental effects. J. Exp. Med. 214:1297. 6. Eigenbrod T. & Dalpke A.H., κ ISRE3 2015. Bacterial RNA: an AP-1 NF- B T : +1 888 457 5873 cle s ISRE7 β underestimated stimulus for innate β F : +1 858 457 5843 α α immune respones. J. Immunol. 195:411. 7. Ugolini M. et al., 2018. Recognition of microbial viability via TLR8 drives TFH cell differentiation and vaccine responses. Nat. Immunol. 19:386. 8. De Marcken M. IFN-α B cell et al., 2019. TLR7 and TLR8 activate [email protected] Anti-viral cytokine + distinct pathways in monocytes www.invivogen.com IL-23 LT CD4 TH1 IL-12 during RNA virus infection. Sci. T-helper 1 Signaling. 12:eaaw1347. 9. Levy O. IL-1β + + LT CD4 TFH LT CD4 TH17 et al., 2006. Unique efficacy of Toll- T-helper 17 Follicular like receptor 8 agonists in activating FEBRUARY 2020 T-helper human neonatal antigen-presenting cells. Blood. 108:1284. Species-driven TLR7 and TLR8 differential responses
InvivoGen offers a series of HEK293-derived reporter cells to assess the cellular responses upon stimulation of TLR7 or TLR8, either human or murine. These cell lines individually display distinct response profiles. TLR7 and TLR8 mediate different responses depending on the stimulatory ligand. Moreover, for the same TLR (7 or 8) activated by the same ligand, discrepancies can be observed between the two species (human and mouse).
Human and murine TLR7 or TLR8 reporter cells Human and mouse TLR7- or TLR8-induced responses • HEK-BlueTM hTLR7 Cells • HEK-BlueTM mTLR7 Cells A. 0 TLR7 human B. 0 TLR8 human mouse Figure 1: TLR7 and TLR8 mouse TM • HEK-Blue hTLR8 Cells induction15 in HEK293- e e derived reporter cells.
TM s s 0 TM • HEK-Blue mTLR8 Cells a a HEK-Blue hTLR7 or mTLR7 e e TM r r (A), and HEK-Blue hTLR8 c c 10 n n
TM i i or mTLR8 (B) were cultured HEK-Blue hTLR7, mTLR7, hTLR8, or mTLR8 cells are derived from the human TM d d l 10 l in HEK-Blue Detection embryonic kidney (HEK293) cell line. They express the corresponding TLR and o o F F medium5 with 1 µg/ml R848 an NF-κB/AP1-inducible SEAP (secreted embryonic alkaline phosphatase) (TLR7/8 agonist), 3 µg/ml reporter gene. SEAP levels produced upon TLR7 or TLR8 stimulation can be Imiquimod (TLR7 agonist), TM TM 0 5 0µg/ml ssRNA40/LyoVec readily determined by performing the assay in HEK-Blue Detection medium. / / 48 od 40 06 (referred8 as humanod TLR840 6 TM R8 im A -5 84 im A -50 These cells are selectable with Blasticidin and Zeocin . u N TM 8 R u N TM 8 miq ssR TL agonist), mori q1 µg/mls TL8-506sR TL I Vec I Vec Lyo (TLR8 agonist, VTX-2337Lyo ssRNA40/ analog). After 24h incubation, R848 Imiquimod TM TL8-506 Response profiles A. 0 TLR7 LyoVec human B. 0 TLR8 human TLR7- or TLR8-induced mouse mouse NF-κB/AP1 responses were TLR7/8 TLR7 TLR8 TLR8 assessed by measuring SEAP TM 15 levels in the supernatant by
HEK-Blue hTLR7 cells e e s 0 s reading the OD at 630 nm. a a
TM e e OD fold increase over non- HEK-Blue mTLR7 cells r * * r c c 10 induced cells is shown. n n i i
TM
HEK-Blue hTLR8 cells d d l 10 l o o HEK-BlueTM mTLR8 cells ** F ** F 5 1. Liu J. et al. 2009. A five-amino- * mTLR7 and mTLR8 share strong homology and may have evolved to detect 0 0 acid motif in the undefined a broad overlapping range of ligands. 8 d 40/ 6 8 d 0/ 6 region of the TLR8 ectodomain 84 imo A -50 84 imo A4 -50 is required for species-specific R u N TM 8 R u N TM 8 ** The addition of poly(dT) rescues the responsesm toiq ssRNA40,ssR and variousTL miq ssR TL I Vec I Vec ligand recognition. Mol. Immunol. TLR7/8 agonists1. Lyo Lyo 47:1083.. www.invivogen.com/hek-blue-tlr Specific TLR8 inhibitor • CU-CPT9a NEW CU-CPT9a inhibition of TLR8-induced responses InvivoGen offers CU-CPT9a, a potent and selective inhibitor of TLR82,3 (Fig2). CU-CPT9a binds to and stabilizes the TLR8 dimer in its resting state, Figure 2: Specific thereby preventing its conformational change. This TLR8 antagonist blocks .0 inhibition of human TLR8 by CU-CPT9a. the activation of TLR8 and the subsequent activation of NF-κB without TM HEK-Blue hTLR8, hTLR7 or impacting the responses induced by other TLRs, especially the closely related ) 1.5 mTLR7 cells were incubated 2 TLR7 (Fig2). m with 1 µM CU-CPT9a for 3 n www.invivogen.com/cucpt9a 0 hours before adding 10 µg/ml