The two branches of the immune system, innate and adaptive, work together within an intricate network in order to respond to infectious threats while not destroying healthy self-tissue. As previously discussed, the innate immune system utihzes germline encoded receptors PRRs, to specifically recognize viral and bacterial PAMPs. Toll-like receptors (TLRs) are one category of PRRs involved in the early detection of pathogenic bacteria.
TLRs are highly evolutionarily conserved; the Drosophila gene, toll, was first described as an essential component for the determination of dorsoventral polarity during embryonic development, and later found to be critical for antifungal responses. There have been 11 human and 13 mouse TLRs identified to date. TLR 1-TLR 9 are conserved among both species, mouse TLR 10 is nonfunctional whereas TLRs 11-13 have been lost from the human genome. TLRs are type I membrane proteins with an extracellular domain containing leucine rich repeats (LRR) which recognize the PAMPs. The intracellular signaling domain is homologous to the cytoplasmic domain of the IL-1 receptor and is referred to as the Toll/Interleukin-1 receptor (TIR) domain. The LRR domains consist of 19-25 LRR each of which are 24-29 amino acids in length and consisting of a 6-strand and an a-helix connected by loops. TLRs can be categorized into two main groups, those which are present on the cell surface and recognize extracellular pathogens, such as TLRs 1, 2, 4, 5, 6, and 11, and those localized to intracellular vesicles, TLRs 3, 7, 8, and 9, and recognize viral pathogens such as nucleic acid.
Cell surface TLRs
Of the surface TLRs, TLR 4 is by far the most-well characterized. TLR 4 is able to bind several ligands such as, respiratory syncicial virus (RSV), mouse mammary tumor virus (MMTV) envelope proteins, S. pneumonia pneumolysin, and paclitaxel. However, lipopolysaccharide (LPS), the major component of the cell wall of Gram negative bacteria, is the most well studied ligand for TLR 4. The lipid portion of LPS, termed “lipid A,” is responsible for most of the pathogenic activity of LPS and it’s binding to TLR 4. TLR 4 complexes with MD2 on the cell surface to facilitate binding of LPS. In addition to MD2, LPS binding protein (LBP) and CD 14 also interact with the LPS-TLR 4 complex. LBP, present in the blood, binds to LPS and CD 14 on the cell surface, while CD14 is responsible for bringing LPS-LBP to the TLR 4-MD2 complex. Five of the six lipid chains of LPS bind to the hydrophobic pocket of MD2, whereas the sixth chain binds directly to the extracellular domain of TLR 4. The negative charged phosphate groups of LPS form electrostatic interactions with positive charged residues on TLR 4.
Unlike other cell surface TLRs, TLR 2 forms heterodimeric complexes with either TLR 1 or TLR 6 to facilitate binding to either diacylated or triacylated proteins. The TLR 1/2 complex binds to triacylated proteins, such as PAM3CSK4 which has two lipid chains that bind directly to TLR 2 while the third hpid chain binds to the hydrophobic pocket within TLR 1. Diacylated proteins, on the other hand, bind to TLR 2/6 heterodimer presumably because TLR 6 lacks the hydrophobic pocket that TLR 1 has for binding of the third lipid chain. Simflarly to the adaptor molecules which facliitate TLR 4 signaling, it is believed that CD36 aids in the recognition of some TLR 2 agonists. In addition to bacterial Upoproteins, TLR 2 is important for the recognition of cell wall components of Gram-positive bacteria, such as lipoteichoic acid (LTA) and peptidoglycan.
TLR 5 and TLR 11 are related molecules; TLR 5 binds to flagellin, the major protein component of bacterial flagella. Flagellin contains N- and Cterminal a-helix chains (DO), central helix chains (Dl), and a hypervariable central region of B-sheets (D2 and D3), TLR 5 binds to the conserved Dl region. TLR 5 is expressed on epithelial cells, monocytes, and immature DCs with particularly high expression on lamina propria DCs, which are unique in their ability to promote the differentiation of Thl7 cells.
Signaling through TLRs induces the expression of inflammatory cytokines such as TNFct, IL-6, IL-16, and IL-12. TLR signaling also aids in the maturation of dendritic cells (DCs) through the upregulation of MHC II as well as costimulatory molecules CD80/CD86 and chemokine receptor CCR7 allowing the migration of DCs from the periphery to the lymphoid organs. Upon receptor-ligand binding the key output is the tightening of intracellular TIR domains, leading to a conformational change which allows the recruitment of adaptor proteins. There are two pathways utilized in the signaling of all TLRs, MyD88-dependent or independent. The MyD88-dependent pathway results in the activation of MAPKs and NF-κB . The MyD88-independent pathway relies on the use of TRIF and leads to downstream activation of IRF3 and NF-κB. Both pathways are capable of inducing the production of inflammatory cytokines and type-I interferon.
Stasya Zarling. 4-lBB IN THE MODULATION OF INNATE AND ADAPTD7E IMMUNITY