The structural components of hRSV are mobilized to the plasma membrane for the assembly and budding of viral particles. The minimum molecular requirement for viral particle assembly are the F, M, N and P proteins, in addition to the genome and anti-genome. The budding of hRSV takes place at the apical membrane in polarized cells. The F protein goes to the apical membrane through the secretory pathway from the endoplasmic reticulum
and Golgi, where it is associated with the lipid raft. The rest of the hRSV Epigenetics Compound Library clinical trial structural proteins and the RNA genome also traffic to the apical membrane from the cytoplasm and from viral inclusion bodies. The matrix protein is localized in the nucleus in early stages after infection, but is mostly cytoplasmic in the late phases of infection. Once in the airways, hRSV is recognized by pattern recognition receptors (PRRs) expressed on epithelial and immune cells that induce the secretion
of innate cytokines and chemokines. These molecules promote inflammation and the recruitment of eosinophils, neutrophils and monocytes into the lungs, as well as the onset of an anti-viral response. To date, there are three types of PRRs identified, which include toll-like receptors (TLRs), retinoic acid-inducible gene (RIG)-I-like receptors (RLRs) and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs), find more all involved in eliciting the immune response against hRSV. Several TLRs are activated by hRSV, including TLR2, TLR3, TLR4 and TLR7.[25, 30-33] As detailed in Fig. 1, TLR2 and TLR4 are expressed in the cell surface and recognize hRSV when associated with the co-receptors TLR6 and CD14, respectively. TLR4 interacts with hRSV F protein, leading to nuclear factor-κB (NF-κB) activation and promotes the secretion of the pro-inflammatory cytokines interleukin-6 (IL-6) and IL-8
by epithelial cells. TLR3 is an intracellular receptor that recognizes dsRNA generated during the viral replication. In response to hRSV, TLR3 activates oxyclozanide NF-κB and interferon regulatory factor 3 (IRF3) through the adaptor protein TRIF, with the subsequent secretion of interferon-β (IFN-β), CXCL10, CCL12 and CCL5. TLR7 is expressed in the endosomal membrane and recognizes ssRNA. Entry of hRSV into the cytosol is detected by TLR7, which regulates the secretion of IL-12 and IL-23 through signalling via MyD88. In addition, RIG-1 is a cytosolic RLR (that belongs to the RNA helicase family) that detects intracellular viral RNAs. Upon hRSV infection, RIG-1 is activated by the 5′ triphosphate structure of viral RNA, which activates the NF-κB and IRF3 pathways using the mitochondrial anti-viral signalling (MAVS) adaptor localized in the mitochondrial membrane, inducing the expression of IFN-β, IP-10 and CCL5 in the airway epithelium. Furthermore, NOD2 is an NLR that belongs to the large cytosolic receptor family.