TLR2

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Toll-like receptor 2
TLR2.png
PDB rendering based on 1fyw.
Available structures
PDB Ortholog search: PDBe, RCSB
Identifiers
Symbols TLR2 ; CD282; TIL4
External IDs OMIM603028 MGI1346060 HomoloGene20695 ChEMBL: 4163 GeneCards: TLR2 Gene
RNA expression pattern
PBB GE TLR2 204924 at tn.png
More reference expression data
Orthologs
Species Human Mouse
Entrez 7097 24088
Ensembl ENSG00000137462 ENSMUSG00000027995
UniProt O60603 Q9QUN7
RefSeq (mRNA) NM_003264 NM_011905
RefSeq (protein) NP_003255 NP_036035
Location (UCSC) Chr 4:
154.62 – 154.63 Mb
Chr 3:
83.84 – 83.84 Mb
PubMed search [1] [2]

Toll-like receptor 2 also known as TLR2 is a protein that in humans is encoded by the TLR2 gene.[1] TLR2 has also been designated as CD282 (cluster of differentiation 282). TLR2 is one of the toll-like receptors and plays a role in the immune system. TLR2 is a membrane protein, a receptor, which is expressed on the surface of certain cells and recognizes foreign substances and passes on appropriate signals to the cells of the immune system.

Function[edit]

The protein encoded by this gene is a member of the Toll-like receptor (TLR) family, which plays a fundamental role in pathogen recognition and activation of innate immunity. TLRs are highly conserved from Drosophila to humans and share structural and functional similarities. They recognize pathogen-associated molecular patterns (PAMPs) that are expressed on infectious agents, and mediate the production of cytokines necessary for the development of effective immunity. The various TLRs exhibit different patterns of expression. This gene is expressed most abundantly in peripheral blood leukocytes, and mediates host response to Gram-positive bacteria and yeast via stimulation of NF-κB.[2]

In the intestine, TLR2 regulates the expression of CYP1A1,[3] which is a key enzyme in detoxication of carcinogenic polycyclic aromatic hydrocarbons such as benzo(a)pyrene.[4]

Background[edit]

The immune system recognizes foreign pathogens and eliminates them. This occurs in several phases. In the early inflammation phase, the pathogens are recognized by antibodies that are already present (innate or acquired through prior infection; see also cross-reactivity). Immune-system components (e.g. complement) that are bound to the antibodies and kept around in reserve then disable them, and they are phagocytized by scavenger cells (e.g. macrophages). Dendritic cells are likewise capable of phagocytizing but do not do it for the purpose of direct pathogen elimination. Rather, they infiltrate the spleen and lymph nodes, and each presents components of an antigen there, as the result of which specific antibodies are formed that recognize precisely that antigen.

These newly formed antibodies would arrive too late in an acute infection, however, so what we think of as "immunology" constitutes only the second half of the process. Because this phase would always start too late to play an essential role in the defense process, a faster-acting principle is applied ahead of it, one that occurs only in forms of life that are phylogenetically more highly developed.

What are called pattern-recognition receptors come into play here. This refers to receptors that recognize the gross, primarily structural features of molecules not innate to the host organism. These include, for example, lipids with a totally different basic chemical structure. Such receptors are bound directly to cells of the immune system and cause immediate activation of their respective nonspecific immune cells.

A prime example of such a foreign ligand is bacterial endotoxin, whose effects have been known for generations. When it enters the bloodstream it causes systematic activation of the early-phase response, with all the side effects of septic shock. This is known in the laboratory as the Shwartzman phenomenon. The intended effect is to mobilize the organism for combat, so to speak, and eliminate most of the pathogens.

Mechanism[edit]

As a membrane surface receptor, TLR2 recognizes many bacterial, fungal, viral, and certain endogenous substances. In general, this results in the uptake (internalization, phagocytosis) of bound molecules by endosomes/phagosomes and in cellular activation; thus such elements of innate immunity as macrophages, PMNs and dendritic cells assume functions of nonspecific immune defense, B1a and MZ B cells form the first antibodies, and specific antibody formation gets started in the process. Cytokines participating in this include tumor necrosis factor-alpha (TNF-α) and various interleukins (IL-1α, IL-1β, IL-6, IL-8, IL-12). Before the TLRs were known, several of the substances mentioned were classified as modulins. Due to the cytokine pattern, which corresponds more closely to Th1, an immune deviation is seen in this direction in most experimental models, away from Th2 characteristics. Conjugates are being developed as vaccines or are already being used without a priori knowledge.

A peculiarity first recognized in 2006 is the expression of TLR2 on Tregs (a type of T cell), which experience both TCR-controlled proliferation and functional inactivation. This leads to disinhibition of the early inflammation phase and of specific antibody formation. Following a reduction in pathogen count, many pathogen-specific Tregs are present that, now without a TLR2 signal, become active and inhibit the specific and inflammatory immune reactions (see also TNF-β, IL-10). Older literature that ascribes a direct immunity-stimulating effect via TLR2 to a given molecule must be interpreted in light of the fact that the TLR2 knockouts employed typically have very few Tregs.

Functionally relevant polymorphisms are reported that cause functional impairment and thus, in general, reduced survival rates, in particular in infections/sepsis with Gram-positive bacteria.

Signal transduction is depicted under Toll-like receptor.

Expression[edit]

TLR2 is expressed on microglia, Schwann cells, monocytes, macrophages, dendritic cells, polymorphonuclear leukocytes (PMNs or PMLs), B cells (B1a, MZ B, B2), and T cells, including Tregs (CD4+CD25+ regulatory T cells). In some cases, it occurs in a heterodimer (combination molecule), e.g., paired with TLR-1 or TLR-6. TLR2 is also found in the epithelia of air passages, pulmonary alveoli, renal tubules, and the Bowman's capsules in renal corpuscles. In the skin, it is found on keratinocytes and sebaceous glands; spc1 is induced here, allowing a bactericidal sebum to be formed.

Agonists[edit]

The following ligands have been reported to be agonists of the toll-like receptor 2:

Agonist Organism
Lipoteichoic acid Gram-positive bacteria
atypical LPS Leptospirosis and Porphyromonas gingivalis
MALP-2 and MALP-404 (lipoproteins) Mycoplasma
- Chlamydophila pneumoniae
OspA Borrelia burgdorferi (Lyme disease)
Porin Neisseria meningitidis Haemophilus influenzae
Antigen mixtures Propionibacterium acnes
LcrV Yersinia
Lipomannan Mycobacterium: Mycobacterium tuberculosis
GPI anchor Trypanosoma cruzi
Lysophosphatidylserine Schistosoma mansoni
Lipophosphoglycan (LPG) Leishmania major
Glycophosphatidylinositol (GPI) Plasmodium falciparum
Zymosan (a beta-glucan) Saccharomyces cerevisiae
- Malassezia (commensal yeast)
Antigen mixtures Aspergillus fumigatus, Candida albicans
hsp60, as peptide transporter and adjuvant for antigen presentation -
- Herpes simplex virus
- Varicella zoster virus
- Cytomegalovirus (CMV)
Hemagglutinin Measles

Interactions[edit]

Protein-Protein Interactions[edit]

TLR 2 has been shown to interact with TLR 1[5] and TOLLIP.[6]

Protein-Ligand Interactions[edit]

TLR2 resides on the plasma membrane where it responds to lipid-containing PAMPs such as lipoteichoic acid and di- and tri-acylated cysteine-containing lipopeptides . It does this by forming dimeric complexes with either TLR 1 or TLR6 on the plasma membrane.[7] TLR2 interactions with malarial Glycophosphatidylinositols of Plasmodium falciparum was shown [8] and a detailed structure of TLR–GPI interactions was computationally predicted.[9]

Gene polymorphisms[edit]

Various single nucleotide polymorphisms (SNPs) of the TLR2 have been identified [10] and for some of them an association with faster progression and a more severe course of sepsis in critically ill patients was reported.[11] No association with occurrence of severe staphylococcal infection was found [12]

References[edit]

  1. ^ Rock FL, Hardiman G, Timans JC, Kastelein RA, Bazan JF (January 1998). "A family of human receptors structurally related to Drosophila Toll". Proc. Natl. Acad. Sci. U.S.A. 95 (2): 588–93. doi:10.1073/pnas.95.2.588. PMC 18464. PMID 9435236. 
  2. ^ "Entrez Gene: TLR2". 
  3. ^ Do KN, Fink LN, Jensen TE, Gautier L, Parlesak A (2012). "TLR2 controls intestinal carcinogen detoxication by CYP1A1". PLoS ONE 7 (3): e32309. doi:10.1371/journal.pone.0032309. PMC 3307708. PMID 22442665. 
  4. ^ Uno S, Dalton TP, Dragin N, Curran CP, Derkenne S, Miller ML, Shertzer HG, Gonzalez FJ, Nebert DW (April 2006). "Oral benzo[a]pyrene in Cyp1 knockout mouse lines: CYP1A1 important in detoxication, CYP1B1 metabolism required for immune damage independent of total-body burden and clearance rate". Mol. Pharmacol. 69 (4): 1103–14. doi:10.1124/mol.105.021501. PMID 16377763. 
  5. ^ Takeuchi O, Sato S, Horiuchi T, Hoshino K, Takeda K, Dong Z, Modlin RL, Akira S (July 2002). "Cutting edge: role of Toll-like receptor 1 in mediating immune response to microbial lipoproteins". J. Immunol. 169 (1): 10–4. doi:10.4049/jimmunol.169.1.10. PMID 12077222. 
  6. ^ Zhang G, Ghosh S (March 2002). "Negative regulation of toll-like receptor-mediated signaling by Tollip". J. Biol. Chem. 277 (9): 7059–65. doi:10.1074/jbc.M109537200. PMID 11751856. 
  7. ^ Botos, Istvan, David M. Segal, and David R. Davies (2011). "The structural biology of Toll-like receptors". Structure 19 (4): 447–59. doi:10.1016/j.str.2011.02.004. PMC 3075535. PMID 21481769. 
  8. ^ Zhu, J., Krishnegowda, G., Li, G., & Channe Gowda, D (2011). "Proinflammatory responses by glycosylphosphatidylinositols (GPIs) of Plasmodium falciparum are mainly mediated through the recognition of TLR2/TLR1". Experimental parasitology 128 (3): 205–11. doi:10.1016/j.exppara.2011.03.010. PMC 3100359. PMID 21439957. 
  9. ^ Durai, Prasannavenkatesh, Rajiv Gandhi Govindaraj, and Sangdun Choi (2013). "Structure and dynamic behavior of Toll‐like receptor 2 subfamily triggered by malarial glycosylphosphatidylinositols of Plasmodium falciparum". FEBS Journal 280 (23): 6196–212. doi:10.1111/febs.12541. PMID 24090058. 
  10. ^ Schroder NW (2005). "Single nucleotide polymorphisms of Toll-like receptors and susceptibility to infectious disease.". Lancet Infect Dis 5 (3): 156–64. doi:10.1016/S1473-3099(05)01308-3. PMID 15766650. 
  11. ^ Nachtigall I (Feb 2014). "Polymorphisms of the toll-like receptor 2 and 4 genes are associated with faster progression and a more severe course of sepsis in critically ill patients.". J Int Med Res 42 (1): 93–110. doi:10.1177/0300060513504358. PMID 24366499. 
  12. ^ Moore CE (2004). "Lack of association between Toll-like receptor 2 polymorphisms and susceptibility to severe disease caused by Staphylococcus aureus.". Clin Diagn Lab Immunol 11 (6): 1194–7. doi:10.1128/CDLI.11.6.1194-1197.2004. PMC 524778. PMID 15539529. 

Further reading[edit]

This article incorporates text from the United States National Library of Medicine, which is in the public domain.