TLR8 dimer binds TLR7, TLR8 ligands

Endosomal recognition of viral single stranded (ss) RNA occurs by means of toll-like receptor 7 (TLR7) and TLR8. TLR7 and TLR8 detect GU-rich ssRNA sequences from the viral genomes of orthomyxovirus (influenza A), lentivirus (human immunodeficiency virus-1, HIV-1), vesiculovirus (vesicular stomatitis virus, VSV), enterovirus (coxsackie B virus), betacoronavirus (severe acute respiratory syndrome-associated coronavirus type 1, SARS-CoV-1 and SARS-CoV-2) and flavivirus (HCV and WNV) (Hemmi H et al. 2002; Jurk M et al. 2002; Diebold SS et al. 2004; Heil F et al. 2004; Cervantes-Barragan L et al. 2007; Li Y et al. 2013; Scheuplein VA et al. 2015; Campbell GR et al. 2021; reviewed in Lester SN & Li K 2014). Specifically, GU-rich ssRNA oligonucleotides derived from HIV-1, for example, stimulate dendritic cells (DC) and macrophages to secrete interferon-alpha and proinflammatory, as well as regulatory, cytokines (Heil F et al. 2004). This has been found to be mediated by TLR7, as well as TLR8. Similarly, ssRNA derived from SARS‑CoV‑1 induced mononuclear phagocytes to release considerable levels of pro‑inflammatory cytokines TNF‑a, IL‑6 and IL‑12 via TLR7 and TLR8 (Li Y et al. 2013). Bioinformatics scanning techniques showed that the SARS‑CoV‑2 genome contains a large number of fragments that could be recognized by TLR7/TLR8 (Moreno‑Eutimio MA et al. 2020). In human macrophages, GU-rich RNA derived from SARS-CoV-2, SARS-CoV-1, and HIV-1 triggered TLR8-mediated pro-inflammatory responses leading to activation of the NLRP3 inflammasome and IL-1β secretion (Campbell GR et al. 2021). Upon engagement of ssRNAs in endosomes, TLR7 and TLR8 initiate the MyD88‑dependent pathway, leading to production of type I and type III IFNs and proinflammatory mediators via activation of IRF7 and NF‑κB, respectively (reviewed in Lester SN & Li K 2014). In addition, imidazoquinoline compounds (e.g. imiquimod and R‑848, a small‑molecule immune response modifier that can induce the synthesis of interferon‑alpha) were reported to be ligands of TLR7 and TLR8 (Hemmi H et al. 2002; Jurk M et al. 2002; Diebold SS et al. 2004). Structural analyses have revealed that both TLR7 and TLR8 possess two binding sites (Zhang Z et al. 2016). Binding site 1 is highly conserved between TLR7 and TLR8 and binds nucleosides (guanosine (G) for TLR7 and uridine (U) for TLR8) or base analogs. Binding site 2 of TLR7 and TLR8 is less conserved and binds ssRNA with U(U) and U(G) motifs. TLR7 acts as a dual receptor for G and U‑containing ssRNAs. Binding of ssRNA to the site 2 of TLR7 strongly enhances the interaction of TLR7 and G at the first site leading to subsequent receptor dimerization. Conversely, chemical ligands are sufficiently potent to induce TLR7 dimerization by binding to the first site alone (Zhang Z et al. 2016, 2018). Further, upon ligand stimulation, the TLR8 dimer was reorganized such that the two C termini were brought into proximity (Tanji H et al. 2013). The loop between leucine‑rich repeat 14 (LRR14) and LRR15 was cleaved, but the N‑ and C‑terminal halves remained associated and contributed to ligand recognition and dimerization, which enables the downstream signaling activation of IRF and NF‑κB (Tanji H et al. 2013, 2016). Although TLR7 and TLR8 share structural homology and both sense viral GU-rich ssRNA, their binding affinities to TLR7/TLR8 ligands may differ (Li Y et al. 2013; Campbell GR et al. 2021). TLR7‑specific ligands generally induce IFN‑regulated cytokines, but TLR8‑specific ligands lead primarily to the production of proinflammatory cytokines (Gorden KB et al. 2005). This Reactome event shows binding between TLR8 and its ligand.

内体对病毒单链（ss）RNA的识别是通过Toll样受体7（TLR7）和TLR8实现的。TLR7和TLR8能够检测来自正粘病毒（流感A型病毒）、慢病毒（人类免疫缺陷病毒-1，HIV-1）、弹状病毒（水泡性口炎病毒，VSV）、肠道病毒（柯萨奇B病毒）、β冠状病毒（严重急性呼吸综合征相关冠状病毒1型，SARS-CoV-1和SARS-CoV-2）以及黄病毒（HCV和WNV）的病毒基因组中的GU富集的ssRNA序列（Hemmi H等，2002；Jurk M等，2002；Diebold SS等，2004；Heil F等，2004；Cervantes-Barragan L等，2007；Li Y等，2013；Scheuplein VA等，2015；Campbell GR等，2021；Lester SN & Li K，2014年综述）。具体而言，例如，HIV-1来源的GU富集的ssRNA寡核苷酸可刺激树突状细胞（DC）和巨噬细胞分泌干扰素-α和前炎症，以及调节性细胞因子（Heil F等，2004年）。研究发现，这一过程由TLR7以及TLR8介导。同样，SARS-CoV-1来源的ssRNA通过TLR7和TLR8诱导单核吞噬细胞释放大量的前炎症细胞因子TNF-α、IL-6和IL-12（Li Y等，2013年）。生物信息学扫描技术显示，SARS-CoV-2基因组含有大量可被TLR7/TLR8识别的片段（Moreno-Eutimio MA等，2020年）。在人巨噬细胞中，SARS-CoV-2、SARS-CoV-1和HIV-1来源的GU富集RNA通过TLR8介导的前炎症反应导致NLRP3炎症小体的激活和IL-1β的分泌（Campbell GR等，2021年）。当ssRNA在内体中被结合时，TLR7和TLR8启动MyD88依赖性通路，通过IRF7和NF-κB的激活分别导致I型及III型干扰素和前炎症介质的生产（Lester SN & Li K，2014年综述）。此外，咪唑喹啉类化合物（例如咪喹莫特和R-848，一种小分子免疫反应调节剂，能够诱导干扰素-α的合成）被报道为TLR7和TLR8的配体（Hemmi H等，2002年；Jurk M等，2002年；Diebold SS等，2004年）。结构分析揭示，TLR7和TLR8均具有两个结合位点（Zhang Z等，2016年）。结合位点1在TLR7和TLR8之间高度保守，并绑定核苷（对于TLR7是鸟苷（G），对于TLR8是尿苷（U））或碱基类似物。TLR7和TLR8的第二个结合位点保守性较低，并绑定含有U(U)和U(G)基序的ssRNA。TLR7充当G和U含有ssRNA的双重受体。ssRNA与TLR7位点2的结合可显著增强TLR7与G在第一个位点的相互作用，导致随后的受体二聚化。相反，化学配体足够强大，可以通过仅与第一个位点结合来诱导TLR7的二聚化（Zhang Z等，2016年，2018年）。此外，在配体刺激下，TLR8二聚体被重新组织，使得两个C端靠近（Tanji H等，2013年）。在富含亮氨酸重复14（LRR14）和LRR15之间的环被切割，但N端和C端 halves 保持关联，并贡献于配体识别和二聚化，从而激活下游的IRF和NF-κB信号通路（Tanji H等，2013年，2016年）。尽管TLR7和TLR8在结构上具有同源性，并且都能识别病毒中的GU富集的ssRNA，但它们对TLR7/TLR8配体的结合亲和力可能存在差异（Li Y等，2013年；Campbell GR等，2021年）。TLR7特异性配体通常诱导IFN调节的细胞因子，而TLR8特异性配体主要导致前炎症细胞因子的产生（Gorden KB等，2005年）。此Reactome事件显示了TLR8与其配体之间的结合。