Human TLR3 Dual Reporter THP-1 Cells (MAVS deficient)

RNA sensor background

To combat viral infection and evasion mechanisms, nature has implemented a multitude of partially overlapping defense strategies. The antiviral response is initiated through the recognition of viral products, such as double-stranded (ds) RNA, by two types of pathogen recognition receptors (PRRs) [1]:

• the RIG-I-like receptors (RLRs) and
• the Toll-like receptors (TLRs).

MDA5 & RIG-I

MDA5 (Melanoma-differentiation-associated gene 5, MDA-5, IFIH1 or Helicard) and RIG-I (retinoic-acid-inducible protein 1, also known as Ddx58) are cytoplasmic RNA helicases belonging to the RLR family. Both sense dsRNA, a replication intermediate of RNA viruses, leading to the production of type I interferons (IFNs) [1]. They recognize a complementary set of cytosolic viral dsRNA. MDA5 recognizes long dsRNA, and accordingly senses the single positive RNA viruses such as the poliovirus. RIG‐I prefers short dsRNA ligands and specifically recognizes most single‐negative RNA viruses which generate lots of short 5′ ppp‐dsRNA during replication (e.g Influenza). Additionally, it is able to sense positive single RNA viruses such as the hepatitis C virus. It was also shown that RIG-I can detect certain DNA viruses and bacteria. On the other hand, both RIG‐I and MDA5 cross‐detect the same viruses, including rota and corona viruses. The synthetic analog of viral dsRNA, transfected Poly(I:C), is also recognized by both sensors [4]. 

MAVS

MAVS (mitochondrial antiviral-signaling protein), also known as IPS‑1, CARDIF, VISA, is an adaptor protein that plays a critical role in the immune response to viral infection. Upon recognition of dsRNA, RIG-I and MDA5 are recruited by MAVS to the outer membrane of the mitochondria leading to the activation of several transcription factors including interferon-regulatory factor 3 (IRF3), IRF7, and NF-κB. IRFs and NF-κB regulate the expression of type I interferons (IFNs) and pro-inflammatory cytokines, respectively [1, 2].

TLR3

Within the large family of TLRs, TLR3 is specialized in sensing viral-derived components and is mainly found in the endosome [4]. Its activation upon viral infection involves several steps, including translocation from the ER (endoplasmic reticulum) to the endosome, proteolytic cleavage and dimerization of TLR3, and finally receptor-ligand binding [6]. In order to start the signaling cascade, activated TLR3 recruits the adaptor protein TRIF (TIR domain-containing adapter-inducing interferon-β). TRIF binds to TRAF3 (TNF receptor-associated factor 3) and TRAF6, activating the transcription factor IRF3 and NF-κB, respectively.  Ultimately, this leads to the production of type I IFNs (interferons) and pro-inflammatory cytokines [5,7].

References

1. Kawai T. et al., 2005. IPS-1, an adaptor triggering RIG-I- and Mda5-mediated type I interferon induction. Nat Immunol. 6(10):981-988.
2. Gebhardt A. et al., 2017. Discrimination of Self and Non-Self Ribonucleic Acids. Journal of Interferon & Cytokine Research 37: 184-97.
3. Pichlmair A. et al., 2006. RIG-I mediated antiviral responses to single-stranded RNA bearing 5’-phosphates. Science 314:997-1001.
X. Vabret N, Blander JM. Sensing microbial RNA in the cytosol. Front Immunol. 2013 Dec 25;4:468.
4. Manuela Sironi, et al., 2012. A Common Polymorphism in TLR3 Confers Natural Resistance to HIV-1 Infection. J Immunol 15; 188 (2): 818–823. 
5. Aluri, J, et al., 2021. Toll-Like Receptor Signaling in the Establishment and Function of the Immune System. Cells, 10, 1374.
6. Chen Y, et al., 2021.  Toll-like receptor 3 (TLR3) regulation mechanisms and roles in antiviral innate immune responses. J Zhejiang Univ Sci B.;22(8):609-632.
7. Komal A, et al., 2021. TLR3 agonists: RGC100, ARNAX, and poly-IC: a comparative review. Immunol Res. 69(4):312-322.