Accessory molecules for Toll-like receptors and their function

Abstract

Toll-like receptors (TLRs) are essential components of the innate immune system. Accessory proteins are required for the biosynthesis and activation of TLRs. Here, we summarize recent findings on TLR accessory proteins that are required for cell-surface and endosomal TLR function, and we classify these proteins based on their function as ligand-recognition and delivery cofactors, chaperones and trafficking proteins. Because of their essential roles in TLR function, targeting of such accessory proteins may benefit strategies aimed at manipulating TLR activation for therapeutic applications.

Figure 1. Accessory molecules mediate ligand binding and delivery to surface and endosomal TLRs

The TLR1-TLR2 heterodimer utilizes CD14 to respond to triacyl lipopetides. The TLR2-TLR6 heterodimer uses CD14 to respond to zymosan and both CD14 and CD36 to respond to lipoteichoic acid (LTA) and diacyl lipopeptides. LPS-binding protein (LBP) binds LPS and presents it to CD14, which is required for TRIF-dependent signaling in response to LPS and at low doses for MyD88-dependent signaling. TLR4 requires MD2 to bind LPS and homodimerize. CD36 is required by the TLR4-TLR6 heterodimer to respond to the altered self-components, amyloid-β and oxidized LDL (oxLDL). Endosomal TLRs utilize cofactors for nucleic acid delivery. CD14 and HMGBs bind to dsRNA, ssRNA, and DNA and mediate their delivery to TLR3, TLR7, and TLR9, respectively. LL37 binds both RNA and DNA and delivers it to TLR7, TLR8 (RNA) and TLR9 (DNA). Progranulin bind just to DNA, and mediate DNA delivery to TLR9. Signaling from TLRs culminates in the activation of the transcription factors AP1, NF-κB, and interferon-regulatory factors (IRFs) and the production of pro-inflammatory cytokines and type I interferons (not shown).

Figure 2. ER chaperones and trafficking and processing factors of TLRs

The ER luminal chaperones gp96 and PRAT4A are responsible for proper folding and function of TLR1, TLR2, TLR4, TLR7 and TLR9, but not TLR3. The ER membrane protein UNC93B1 is required for translocation of endosomal TLR7 and TLR9 to endolysosomes, where these TLRs are cleaved by cathepsins and asparagine endopeptidase (AEP). The cleaved TLRs bind ligand (DNA or RNA) triggering recruitment of signaling components leading to NF-κB-dependent proinflammatory cytokine production. The adaptor protein 3 (AP3) mediates translocation of TLR9 to LAMP2⁺ lysosome or lysosome-related organelles (LRO), where the IRF7 signaling pathway is initiated leading to the expression of type I interferon genes.