Scratching the Surface Takes a Toll: Immune Recognition of Viral Proteins by Surface Toll-like Receptors

Abstract

Early innate viral recognition by the host is critical for the rapid response and subsequent clearance of an infection. Innate immune cells patrol sites of infection to detect and respond to invading microorganisms including viruses. Surface Toll-like receptors (TLRs) are a group of pattern recognition receptors (PRRs) that can be activated by viruses even before the host cell becomes infected. However, the early activation of surface TLRs by viruses can lead to viral clearance by the host or promote pathogenesis. Thus, a plethora of research has attempted to identify specific viral ligands that bind to surface TLRs and mediate progression of viral infection. Herein, we will discuss the past two decades of research that have identified specific viral proteins recognized by cell surface-associated TLRs, how these viral proteins and host surface TLR interactions affect the host inflammatory response and outcome of infection, and address why controversy remains regarding host surface TLR recognition of viral proteins.

Keywords: TLR agonist; TLR antagonist; TLR2; TLR4; Toll-like receptors; cytokines; innate immunity; interferons; viral proteins; viruses.

Figure 1

Toll-like Receptors (TLR) 2 and 4 Signaling Pathways: TLR2/1 recognizes triacylated lipoproteins. TLR2/6 recognizes diacylated lipoproteins. The ligand for TLR2/10 remains unknown. CD14 and CD36 cooperate in the transfer of lipoproteins to TLR2. TLR4 recognizes lipopolysaccharide (LPS) that is transferred to TLR4 by a series of proteins including LPS-binding protein (LBP), CD14, and myeloid differentiation factor 2 (MD-2). TLR2/1, TLR2/6, and TLR4 all signal via myeloid differentiation primary response 88 (MyD88) and Toll-interleukin-1 receptor (TIR) domain-containing adaptor protein (TIRAP/Mal) for the activation of a proinflammatory response through a mitogen-activated protein kinase (MAPK) or nuclear factor-κB (NF-κB)-dependent pathway. Endocytosis of TLR4 results in TLR4 signaling from the endosome via TRIF-related adaptor molecule (TRAM) and TIR-domain-containing adapter-inducing interferon-β (TRIF). TLR4 endosomal signaling activates type I interferons (IFNs) through interferon regulatory factors (IRF). TLR2 endosomal signaling has been suggested to occur via TRAM and MyD88 signaling for the induction of a type I IFN response. Signaling pathways poorly established in the literature are indicated by both dashed lines and question marks.

Figure 2

Baltimore Classification I viruses recognized by surface Toll-like Receptors (TLRs): within Baltimore Class I, 5 viruses were identified as activating a surface TLR response including cytomegalovirus (CMV), Epstein–Barr virus (EBV), herpes simplex viruses (HSV), human papillomavirus (HPV), and Varicella-zoster virus (VZV). CMV has been demonstrated to activate an inflammatory response via TLR2/1 in addition to CD14 and both gH and gB have been demonstrated to activate surface TLR signaling. EBV has been demonstrated to activate both an inflammatory and anti-inflammatory response via TLR2, with several potential ligands being identified including gp350/220, gB, gH, and dUTPase. HSV has several potential agonistic ligands including gH/gL, gB, dUTPase and antagonistic ligands including ICP0 and US3. These HSV ligands potentially interact, either agonistically or antagonistically with TLR2 with CD14, TLR1/6, TLR4, and TLR9 for an inflammatory, anti-inflammatory, and/or IFN response. HPV L1 composed VLPs induced an inflammatory and anti-inflammatory response via TLR4. VZV dUTPase activated a TLR2/1-dependent inflammatory and anti-inflammatory response, but VZV may have alternatively activated TLR9.

Figure 3

Baltimore Classification II viruses recognized by surface TLRs: for Baltimore Class II, Porcine Parvovirus (PPV) activated an inflammatory response via TLR2-NF-κB signaling, likely involving NS1.

Figure 4

Baltimore Classification IV viruses recognized by surface TLRs: Baltimore Class IV include dengue viruses (DENV), hepatitis C virus (HCV), severe acute respiratory syndrome coronavirus (SARS), and yellow fever virus (YFV). All DENV serotypes were demonstrated to activate a surface TLR response. DENV was demonstrated to activate TLR2/6 and TLR4 for the induction of an inflammatory and IFN response, with NS1 being the potential ligand. HCV VLP, core, NS3, and NS5A were all demonstrated to activate different surface TLR responses including TLR2/1, TLR2/6, and TLR4 for an inflammatory, anti-inflammatory, and/or IFN response. SARS-CoV-2 ligands including S, E, and N were all demonstrated to activate an inflammatory, anti-inflammatory, and/or IFN response via TLR2/1 or TLR2/6 and TLR4. YFV NS1 activated an IFN response, and both the attenuated vaccine (indicated by the needle and syringe), YF-17D and NS1 activated an inflammatory response.

Figure 5

Baltimore Classification V viruses recognized by surface TLRs: the viruses in Baltimore Class V include Ebola virus (EBOV), lymphocytic choriomeningitis (LCMV), measles, and respiratory syncytial virus (RSV). The primary ligand identified for EBOV is GP, and VLPs composed of VP40, and GP have been demonstrated to activate an inflammatory, anti-inflammatory, and IFN response via TLR4. Specific ligands for LCMV have not been identified, but LCMV has been demonstrated to activate TLR2 for the induction of an inflammatory and IFN response. Wild-type measles HA activates TLR2 with CD14, while the HA of the measles vaccine strain (indicated by the needle and syringe) does not (indicated by an X), which is believed to be due to a single amino acid difference at position 481, an asparagine to tyrosine change. RSV F protein and VLP have been demonstrated to activate TLR2/6 along with CD14, and TLR4 via MyD88-NF-κB signaling for an inflammatory and anti-inflammatory response.

Figure 6

Baltimore Classification VI viruses recognized by surface TLRs: Baltimore Class VI include human immunodeficiency virus (HIV) and mouse mammary tumor virus (MMTV). HIV infection was shown to be blocked by soluble TLR2 (sTLR2) with HIV ligands being shown to act agonistically and antagonistically. HIV ligands including gp120, gp41, p17, and p24 either activated or inhibited TLR2/1, TLR2/6, and/or TLR4 for the activation or inhibition of an inflammatory response. MMTV is unique in that it ‘steals’ host associated TLR4, MD-2, LPS binding protein (LBP), and CD14 to associate with LPS to activate host TLR4 for both an inflammatory and anti-inflammatory response. In addition to this, MMTV has been suggested to activate TLR4 via its Env protein.

Figure 7

Baltimore Classification VII viruses recognized by surface TLRs: Baltimore Class VII includes hepatitis B virus (HBV). HBV ligands were agonistic and antagonistic including HBsAg, HBcAg, and HBeAg. HBsAg was found to both activate and inhibit TLR2 signaling and inhibit TLR4 signaling. HBcAg was found to activate TLR2 signaling and inhibit TLR4 signaling. HBeAg was shown to inhibit both TLR2 and TLR4 signaling by colocalizing with Mal and TRAM at the subcellular level.