5. CYTOKINE ASSOCIATED SIGNALING PATHWAYS
5.1. Toll-like Receptors (TLRs)
TLRs are the major pattern recognition receptors (PRRs) that can identify pathogen-associated molecular patterns (PAMPs) and endogenous damage-associated molecular patterns (DAMPs). The function of TLRs is closely related to process of inflammatory cytokine release. Expressed on monocytes, macrophages, dendritic cells, neutrophils, B cells, T cells, fibroblasts, endothelial cells, epithelial cells, and even on neurons, TLRs are type I transmembrane proteins that contain three structural domains: a leucine-rich repeats (LRRs) motif as an extracellular domain, a transmembrane domain, and a cytoplasmic Toll/IL-1R (TIR) domain. The LRRs motif is responsible for identifying PAMPs, while the TIR domain interacts with signal transduction adaptors to initiate downstream signaling pathways (Kawasaki & Kawai, 2014). .
TLR mediated cell signaling pathway plays a significant role in cytokine storm-associated diseases including severe COVID-19. Upon the stimulation by microbial-associated molecular patterns (MAMPs) of the pathogens or DAMPs of the host cells, TLRs are activated. Bacteria, fungi, protozoa, and viruses all express MAMPs (Goulopoulou, McCarthy & Webb, 2016). After activation, TLRs recruit cytoplasmic TIR domain-containing adaptor proteins, such as myeloid differentiation primary-response 88 (MyD88) and TIR-containing adapter-inducing interferon-β (TRIF), leading to the activation of nuclear factor (NF)-κB, MAPKs, or interferon-regulatory factor (IRF). Subsequently, the transcription of genes that are responsible for synthesis and the release of proinflammatory cytokines are activated, promoting the release of cytokines, such as including TNF-α, and IL-1β, IL-6, IL-18, chemokines, and type I IFNs (Costa et al., 2018; Vogelpoel et al., 2015) (Figure 2).
In SARS-CoV-2 induced cytokine storm, TLR4 seems to play a role (Aboudounya & Heads, 2021). Recombinant proteins of SARS-CoV-2 has been demonstrated to promote the expression of pro-inflammatory cytokines/chemokines and NF-κB signaling activation in human primary peripheral blood mononuclear cells and monocyte-derived macrophages (Sohn et al., 2020). Activation of TLR4 in lung macrophages resulted in a concentration- and time-dependent increase in the production of chemokines and cytokines (Grassin-Delyle, Abrial, Salvator, Brollo, Naline & Devillier, 2020). In patients with COVID-19, the expression of TLR 4 and NF-κB target genes was upregulated (Sohn et al., 2020). Among TLRs, TLR4 is most likely to be involved in recognizing MAMP from SARS-CoV-2 to induce inflammatory responses (Grassin-Delyle, Abrial, Salvator, Brollo, Naline & Devillier, 2020). In addition, TLR4 may bind to and interact with S-protein to promote the expression of ACE2, facilitating the viral entry into the host cells (Choudhury & Mukherjee, 2020). Antagonists of TLR4 may attenuate the cytokine storm and reduce the viral entry in severe COVID-19 patients (Choudhury, Das, Patra & Mukherjee, 2021).