AP-1 activation in TLR signaling mostly mediated by p30, mitogen activated protein kinase (MAPK), and IκK. TLR7 and TLR9 orchestrate antiviral responses by upregulating gene transcription for IFN-α and IFN-β. Recruitment of IRF5 then leads to induction of inflammatory cytokines IL6, IL12, p40, and tumour necrosis factor (TNF)-α, but not type I IFN. TLR3 and TLR4 stimulation can lead to IFN-α and IFN-β production via the TRIF pathway, leading to IκK (non-canonical IkB kinase) and TBK1 (TANK-binding kinase 1) activation that in turn phosphorylate IRF3 and lead to transcription of IRF3-dependent
genes.[30, 31] TLR3 and TLR4 agonists activate TRIF, which in turn can also activate NFκB. TRIF is the only adaptor for TLR3 to activate NFκB pathway. However, TLR4-induced NFκB activation occurs via both TRIF and MyD88. Because of the potentially deleterious effect of an unchecked pro-inflammatory Panobinostat ic50 state, negative feedback exists for TLR signaling and is a critical component of immune activation and modulation. Perturbation of TLR function can occur at multiple levels in the Selleck Alisertib signaling cascade, including synthesis and expression of signaling receptors and proteins, through proteins that negatively interact
with signaling and enhanced ubiquination and degradation of signaling proteins. Another important mechanism of negative feedback is via tolerance or reduced subsequent responses from repeated TLR stimulation after initial stimulation of one TLR type. Cross-tolerance also occurs, whereby activation of one TLR pathway can cross-inhibit another via negative feedback. Potentially, both negative feedback and tolerance can be manipulated by viral infections such as HCV in order to prevent immune clearance. Hepatitis C is a positive strand RNA enveloped flavivirus that was first
cloned in 1989. HCV virions bind to the cell surface and enter cells via receptor-mediated endocytosis. The structure of HCV is outlined in Figure 2. The core and non-structural proteins shown in the diagram are important sequences recognized by PRRs, including TLRs. They are also important inhibitors medchemexpress of TLR signaling.[35, 36] In order to understand the context of TLR immune responses in HCV infection, it is necessary to consider general features of the immune response against HCV. Fundamentally, T-cell responses to HCV are critical for viral eradication and also response to HCV therapy.[37-39] The balance between Th1 antiviral and Th2 viral-permissive T-cell responses determines viral clearance or persistence, and the degree of inflammation and disease progression.[40-43] CD4+ T cells have a protective effect against liver disease progression in chronic HCV infection, and effective CD4+ T-cell responses to HCV are required to mount an active cytotoxic CD8+ T-cell response for viral eradication.