RSV F binds TLR4:LY96:CD14

Respiratory syncytial virus (RSV) fusion glycoprotein (F) is synthesized as an inactive precursor that is proteolytically cleaved into covalently linked F2 and F1 polypeptides to become fusion competent (González-Reyes et al. 2001). The N-terminal fragment of F1, which includes the fusion peptide, interacts with human lymphocyte antigen 96 (LY96 or MD2), a co-receptor of Toll-like receptor 4 (TLR4) (Rallabhandi et al. 2012). <br><br>This Reactome event shows interaction between the virion-bound F protein and the host TLR4:MD2 complex leading to the production of pro-inflammatory cytokines via the MyD88-dependent or independent pathways. <p>Multiple lines of evidence indicate that F protein binding activates TLR4 signaling, although the precise mechanism is not known (Kurt-Jones et al. 2000; Rallabhandi et al. 2012). Yuan et al. 2018 showed colocalization of RSV (strain Long) F protein with TLR4 but a decrease of TLR4 expression after infection. Increased TLR4 expression upon RSV infection has also been reported (Monick et al. 2003; Gagro et al. 2004). The consensus is also lacking with respect to which form of F protein (virion-associated or purified protein) activates TLR4 signaling (Rallabhandi P et al. 2012; Marr and Turvey 2012; Lizundia et al. 2008).<br><br>Functional TLR4 signaling is required to mount an immune response against RSV. Polymorphisms in the TLR4 and CD14 genes have been associated with severity of RSV infection (Tal et al. 2004; Inoue et al. 2007). Lack of CD14 function leads to impaired TLR4-mediated immune responses in human peripheral blood mononuclear cells (PBMCs) in response to viral F (Kurt-Jones et al. 2000; Besteman et al. 2022), and the CD14-deficient patients develop recurrent RSV bronchiolitis (Besteman et al. 2022). TLR4 knockout mice show persistent RSV infection, impaired cytotoxicity of NK cells, decreased pulmonary CD14+ cell infiltration (Haynes et al. 2001) and reduced expression of inflammatory cytokines (Kurt-Jones et al. 2000; Haynes et al. 2001). Macrophages from LY96-deficient mice fail to express interleukin 1β (IL-1β) upon F protein treatment (Rallabhandi P et al. 2012). Intact TLR4/MyD88 signaling is required for optimal RSV immune response in mice and protection against RSV (Cyr et al. 2009). Intense TLR4 signaling can result in a more severe disease. The interaction of the RSV F protein with TLR4 receptor on the surface of neutrophils may promote release of neutrophil extracellular traps, which may exacerbate inflammation of the airways (Funchal et al. 2015). MEG3 lncRNA, which suppresses TLR4 signaling, has a protective effect against RSV disease severity (Tao et al. 2018). RSV infection can lead to increased TLR4 expression (Monick et al. 2003; Gagro et al. 2004). Increased level of TLR4 may sensitize airway epithelium to bacterial endotoxin LPS (lipopolysaccharide), leading to exacerbated airway inflammation (Monick et al. 2003), possibly through LPS-induced IL6 production in RSV-infected epithelial cells (Xie et al. 2009). In infected infants, LPS exposure and TLR4 genotype influence disease phenotypes (Caballero et al. 2015).<br><br>It is unlikely that the binding of F protein to the TLR4 receptor complex contributes to RSV entry into host cells or that the activation of TLR4 signaling requires viral internalization. While a mouse study has shown that TLR4/Myd88/p38 MAPK signaling cascade may aid in viral internalization (Marchant et al. 2010), other studies have not confirmed this. RSV-mediated, TLR4-dependent NF-kappa-B activation in alveolar macrophages is independent of viral replication (Haeberle et al. 2002). RSV can infect B lymphocytes, but this infection and upregulation of B lymphocyte activation markers is independent of TLR4 (Rico et al. 2010). Infection of macrophages by RSV is TLR4-independent, but differentiation of RSV-infected macrophages into so-called alternatively activated macrophages is dependent on TLR4 signaling (Shirey et al. 2010; Nikonova et al. 2018).

呼吸道合胞病毒（RSV）融合糖蛋白（F）作为一种无活性的前体蛋白合成，经蛋白水解酶切割成共价连接的F2和F1多肽，从而获得融合活性（González-Reyes et al. 2001）。F1蛋白的N端片段，包括融合肽，与人类淋巴细胞抗原96（LY96或MD2）相互作用，而LY96或MD2是Toll样受体4（TLR4）的共受体（Rallabhandi et al. 2012）。<br><br>Reactome事件展示了病毒颗粒结合的F蛋白与宿主TLR4:MD2复合物之间的相互作用，通过MyD88依赖性或非依赖性途径产生前炎症细胞因子。多项证据表明，F蛋白的结合激活了TLR4信号传导，尽管其确切机制尚不清楚（Kurt-Jones et al. 2000；Rallabhandi et al. 2012）。Yuan et al. 2018的研究显示，RSV（Long株）F蛋白与TLR4共定位，但感染后TLR4表达减少。也有报道指出，RSV感染后TLR4表达增加（Monick et al. 2003；Gagro et al. 2004）。关于哪种形式的F蛋白（病毒颗粒结合蛋白或纯化蛋白）激活TLR4信号传导，尚无定论（Rallabhandi P et al. 2012；Marr and Turvey 2012；Lizundia et al. 2008）。<br><br>功能性TLR4信号传导对于发起针对RSV的免疫反应至关重要。TLR4和CD14基因的多态性与RSV感染的严重程度相关（Tal et al. 2004；Inoue et al. 2007）。CD14功能缺失会导致人类外周血单个核细胞（PBMCs）对病毒F蛋白的TLR4介导的免疫反应受损（Kurt-Jones et al. 2000；Besteman et al. 2022），CD14缺陷的病人易患复发性RSV细支气管炎（Besteman et al. 2022）。TLR4敲除小鼠表现出持续的RSV感染，自然杀伤细胞（NK细胞）的细胞毒性受损，肺泡CD14+细胞浸润减少（Haynes et al. 2001）以及炎症细胞因子表达降低（Kurt-Jones et al. 2000；Haynes et al. 2001）。LY96缺陷小鼠的巨噬细胞在F蛋白处理后无法表达白细胞介素1β（IL-1β）（Rallabhandi P et al. 2012）。完整的TLR4/MyD88信号传导对于小鼠的RSV免疫反应和RSV保护至关重要（Cyr et al. 2009）。强烈的TLR4信号传导可能导致更严重的疾病。RSV F蛋白与中性粒细胞表面的TLR4受体的相互作用可能促进中性粒细胞胞外陷阱的释放，这可能会加剧呼吸道炎症（Funchal et al. 2015）。抑制TLR4信号传导的MEG3长非编码RNA（lncRNA）对RSV疾病严重程度的保护作用（Tao et al. 2018）。RSV感染可能导致TLR4表达增加（Monick et al. 2003；Gagro et al. 2004）。TLR4水平升高可能使气道上皮对细菌内毒素LPS（脂多糖）敏感，导致气道炎症加剧（Monick et al. 2003），可能通过RSV感染上皮细胞中LPS诱导的IL6产生（Xie et al. 2009）。在感染婴儿中，LPS暴露和TLR4基因型影响疾病表型（Caballero et al. 2015）。<br><br>不太可能的是，F蛋白与TLR4受体复合物的结合有助于RSV进入宿主细胞，或者激活TLR4信号传导需要病毒内化。虽然一项小鼠研究表明TLR4/Myd88/p38 MAPK信号级联反应可能有助于病毒内化（Marchant et al. 2010），但其他研究并未证实这一点。RSV介导的、TLR4依赖的NF-kappa-B在肺泡巨噬细胞中的激活与病毒复制无关（Haeberle et al. 2002）。RSV可以感染B淋巴细胞，但这种感染和B淋巴细胞活化标记物的上调与TLR4无关（Rico et al. 2010）。RSV感染巨噬细胞是TLR4非依赖性的，但RSV感染巨噬细胞向所谓交替活化巨噬细胞的分化依赖于TLR4信号传导（Shirey et al. 2010；Nikonova et al. 2018）。