Toll like receptors in periodontology
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Toll like receptors in periodontology
- 1. TOLL LIKE RECEPTORS -DR. KRUPALI GANDHI P.G. SECOND YEAR DEPARTMENT OF PERIODONTOLOGY 24/01/2020
- 2. INTRODUCTION • Innate immunity: immediate defense mechanism in organisms against a wide range of pathogens without requiring prior exposure. • Hallmarks of innate immunity: • Recognition • Activation • Adaptive immune response
- 3. HISTORY • The history of TLRs began with the discovery of phagocytic cells and IL-1 in 1940. • Homology between IL-1RI and drosophila Toll was found in 1991 by Janeway and colleagues and this led to the discovery of human Toll in 1997. • The first reported human toll-like receptor was described by Nomura and his colleagues .
- 4. DEFINITION • Toll like receptors (TLRs) are a class of proteins that play a key role in innate immune system. • They are single, membrane- spanning, non- catalytic receptors usually expressed in sentinel cells such as macrophages and dendritic cells, that recognize structurally conserved molecules derived from microbes and few non human cells like human choroidal melanocytes.
- 5. • TLRs are predominantly expressed in tissues involved in immune function, such as spleen and peripheral blood leukocytes, as well as those exposed to the external environment such as lung and the gastrointestinal tract. • In recent years TLRs were identified also in the mammalian nervous system. Members of the TLR family were detected on glia, neurons and on neural progenitor cells. • Once these microbes have breached physical barriers such as the skin or intestinal tract mucosa, they are recognized by TLRs, which activate immune cell responses.
- 6. • TLR recognizes certain Pathogen-Associated Molecular Patterns (PAMP) present on the microorganisms. • Thus, they are a type of pattern recognition receptors.(PRR) • Bacteria can’t mutate the PAMP region to protect itself from TLR as it is integral for its structure. TLR is essential for activation of phagocytes and APC. • Upon ligand binding, TLR activates host defense genes. These pathways are coupled with nuclear translocation of transcriptional factors.
- 7. • Toll and its mammalian homologs are transmembrane proteins, with an ectodomain consisting of leucinerich repeats (LRRs) and one or two cysteine-rich regions. • The intracellular domain of Toll- related receptors contains a Toll/IL- 1receptor (TIR) domain, based on homology of the region with a similar intracellular domain of the IL-1 receptor (IL-1R). • The TIR domain in mammalian Toll- related receptors provides the initial internal scaffold for the interaction of the members of a well-defined signaling cascade.
- 8. LIGANDS • Because of the specificity of toll-like receptors (and other innate immune receptors) they cannot easily be changed in the course of evolution, these receptors recognize molecules that are constantly associated with threats (i.e., pathogen or cell stress) and are highly specific to these threats. • Receptors recognize the following features in pathogens: • Cells surface LPS, lipoproteins, lipopeptides, lipoarabinomannan • Flegellin • Double stranded RNS of viruses • Most TLR, ligand recognition occurs by gene targetting.
- 9. TLR FAMILY Thirteen TLRs (named simply TLR1 to TLR13) have been identified in humans and mice together, and equivalent forms of many of these have been found in other mammalian species. • However, equivalents of certain TLR found in humans are not present in all mammals. For example, a gene coding for a protein analogous to TLR10 in humans is present in mice, but appears to have been damaged at some point in the past by a retrovirus. • On the other hand, mice express TLRs 11, 12, and 13, none of which is represented in humans. Other mammals may express TLRs that are not found in humans.
- 12. TLR IN PERIODONTAL TISSUES
- 13. TLR SIGNALING PATHWAY My-D88 pathway • In the TLR-mediated signalling pathways, MyD88, IRAKs, and TRAF6 play critical roles. • MyD88 is an adaptor protein which produces inflammatory cytokines. • There are four IRAK family members: IRAK-1, IRAK-2, IRAK-M and IRAK-4. • IRAK-4-deficient mice showed almost no inflammatory responses to LPS, peptidoglycan, dsRNA and CpG DNA indicating that it is an important link in the pathway. • Ligation of a TLR promotes dimerization and results in the recruitment of MyD88, which contains two domains: a C-terminal Toll homology, and an N-terminal death domain.
- 15. MYD88 INDEPENDENT PATHWAY • Both TLR3 and TLR4 utilize the TRIF-dependent pathway, which is triggered by viral RNA and LPS, respectively. For TLR3, RNA leads to activation of the receptor, recruiting the adaptor TRIF. • The adaptor molecules for this pathway are Toll-IL-1 receptor domain-containing adaptor inducing interferon-β (TRIF) and TRIF- related adaptor molecules (TRAM).
- 16. TLR ACTIVITIES TLR protects against variety of diseases like • TLR3- herpes simplex encephalitis • TLR2- leprosy, Lyme disease, Tuberculosis, Colorectal cancer • TLR5- Legionnaire’s disease, resistance to SLE • TLR7- Antitumor and antiviral properties
- 17. ROLE IN PERIODONTAL HEALTH AND DISEASE
- 18. TLR TARGETED THERAPEUTICS TLR agonists as Vaccine adjuvants • TLR used in antiviral therapy • Used to treat neurotoxic infection. • Used to treat atherosclerosis, • To treat Parkinson’s and alzheimer’s disease • To treat multiple sclerosis and many more neurodegenrative diseases
- 19. EFFECTS OF TLR • Following activation by ligands of microbial origin, several reactions are possible. • Immune cells can produce signalling factors called cytokines, which trigger inflammation. • In the case of a bacterial factor, the pathogen might be phagocytosed and digested, and its antigens presented to CD4+ T cells. • In the case of a viral factor, the infected cell may shut off its protein synthesis and may undergo programmed cell death (apoptosis). • Immune cells that have detected a virus may also release anti- viral factors such as interferons.
- 20. • Toll-like receptors have also been shown to be an important link between innate and adaptive immunity through their presence in dendritic cells. • The TLRs 3 and 4 are present on the surface of monocyte derived dendritic cells and use the Myd88- dependent pathway to produce interleukins 12 and 18 which signal naive T-cells to mature into type 1 helper T cells. • TLRs 7 and 9 are present on the endosome of plasmacytoid dendritic cells. These proteins solely make use of the Myd88 dependent pathway to produce interleukins for the maturation of naive T-cells to type 1 helper T-cells.
- 21. REFERENCES • Toll Receptors: a Central Element in Innate Immune Responses Thierry Vasselon and Patricia A. DetmersMerck Research Laboratories, Rahway, New Jersey • Toll-Like Receptors Review,2012 • Adreem A Ulevitch R.J 2000, Toll like receptors in induction of innate immune receptors, Nature 406, 782-787 • Terry K. Meansa, Douglas T. Golenbockb, Matthew J. Fenton, The biology of Toll-like receptors
Notes de l'éditeur
- • Hallmarks of innate immune responses include the ability to (i) recognize structures that are present in large groups of microorganisms and are distinct from self, (ii) activate effector mechanisms that will destroy within hours most microorganisms encountered in life, and (iii) activate and orientate an adaptive immune response that, through clonal expansion of lymphocytes, will be aimed specifically at persistent microorganisms .
- pattern recognition receptor –PRR are: • Mannose receptor • Scavenger receptors • Also recognition of bacteria also occur in absence of cells via: • Complement • C-reactive protein • Surfactant protein A in lungs
- Signaling cascade involves activation of ikk inturn activates ika, which releases itself from nfkb. Activating nfkb
- TLR 1,2,6 recognizes lipoteichoic acid from Gram positive organisms and lipoarabinomannan from Mycobacterium . • TLR 2 in specific recognizes Lipopolysaccharides and it cooperates with TLR 1 and 6. • Macrophages from TLR 6 deficient mice did not produce CD36 on its surface. Similarly they reacted with Triacyl lipopeptide • TLR 1 deficient mice reacted with diacyl but not triacyl lipopeptide. Thus TLR 1 and 6 associate with TLR 2 to discriminate diacyl and triacyl lipopeptide. TLR 3: • It recognizes dsDNA produced during viral replication. Type 1 interferon is produced. TLR 4: • It is the LPS receptor and transduces its signals. • LPS-LBP complex is formed and this is associated with CD14 on macrophages. • MD2, a secreted protein reacts with extracellular region of TLR 4. TLR 5: • Flagellin in flagellated bacteria acts as a ligand to activate TLR 5. TLR 7: • It is used in the treatment of infectious diseases as its used as an antiviral Imidazoquinolones that is against HPV. TLR 9: • It helps in the recognition of CpG DNA . CpG DNA recognizes endosomes after non-specific uptake into cells. It is NOT A CELL-SURFACE RECEPTOR unlike others which indicates a different mechanism. TLR 10: • It has an anti-inflammatory reactions unlike other TLR. It is very helpful in suppressing cytokines. • TLR 10 mechanism was found to suppress NF-κB and MAP kinase signalling. • It is found in spleen, lymph nodes, B cell surface but not on T cell surface. TLR 11: • It is present on monocytes, macrophages and dendritic cells. • When an infection of Toxoplasma gondii occurs profilin from it acts as a ligand for TLR 11 to activate Dendritic cells to induce T cell production.
- The stereotypic inflammatory response provoked by toll Like- Receptor activation has prompted speculation that endogenous activators of toll-like receptors might participate in autoimmune diseases. TLRs have been suspected of binding to host molecules including fibrinogen (involved in blood clotting), heat shock proteins (HSPs), extracellular matrix components and self DNA (it is normally degraded by nucleases, but under inflammatory and autoimmune conditions it can form a complex with endogenous proteins, become resistant to these nucleases and gain access to endosomal TLRs as TLR7 or TLR9). These endogenous ligands are usually produced as a result of non-physiological cell death.
- Myloid dependent primary response protein. In the MyD88-dependent pathway, TLR signaling is mediated by adaptor molecule MyD88, which either alone or in combination with another adaptor protein, Toll-IL-1 receptor domain-containing adaptor protein (TIRAP), activates interleukin-1 receptor-associated kinase-4 (IRAK-4). This IRAK-4 then unites with tumor necrosis factor receptor-associated factor 6 (TRAF6), leading to activation of two distinct signaling pathways. One pathway leads to activation of activator protein 1 (AP-1) through activation of mitogen-activated protein kinase (MAK). The other pathway activates transforming growth factor-β-activated kinase 1 (TAK1), which enhances the activity of inhibitor of the nuclear factor-κB kinase complex. This leads to degradation of the inhibitor of nuclear factor-κB and release of nuclear factor- κB (NF-κB), which translocates to the nucleus. Both the end products of these signaling pathways, AP-1 and NF-κB, induce the expression of pro-inflammatory cytokines and chemokines. TLR 7, 8 and 9 recognize viral nucleic acid through another pathway, which is also MyD88-dependent. MyD88 associates with interleukin-1 receptor associated kinase-1 (IRAK1), which in turn phosphorylates interferon-regulatory factor-7 (IRF7). The phosphorylated IRF7 translocates to the nucleus and regulates the expression of type I interferon. TLR signaling through the MyD88-independent pathway results in induction of type 1 interferon through interferon-regulatory factor-3
- In the MyD88-dependent pathway, TLR signaling is mediated by adaptor molecule MyD88, which either alone or in combination with another adaptor protein, Toll-IL-1 receptor domain-containing adaptor protein (TIRAP), activates interleukin-1 receptor-associated kinase-4 (IRAK-4). This IRAK-4 then unites with tumor necrosis factor receptor-associated factor 6 (TRAF6), leading to activation of two distinct signaling pathways. One pathway leads to activation of activator protein 1 (AP-1) through activation of mitogen-activated protein kinase (MAK). The other pathway activates transforming growth factor-β-activated kinase 1 (TAK1), which enhances the activity of inhibitor of the nuclear factor-κB kinase complex. This leads to degradation of the inhibitor of nuclear factor-κB and release of nuclear factor- κB (NF-κB), which translocates to the nucleus. Both the end products of these signaling pathways, AP-1 and NF-κB, induce the expression of pro-inflammatory cytokines and chemokines. TLR 7, 8 and 9 recognize viral nucleic acid through another pathway, which is also MyD88-dependent. MyD88 associates with interleukin-1 receptor associated kinase-1 (IRAK1), which in turn phosphorylates interferon-regulatory factor-7 (IRF7). The phosphorylated IRF7 translocates to the nucleus and regulates the expression of type I interferon. TLR signaling through the MyD88-independent pathway results in induction of type 1 interferon through interferon-regulatory factor-3
- Aka trif dependent pathway MyD88-independent activation of the LPS-TLR4 signalling pathway is evident in several aspects. Dendritic cells from MyD88-deficient, but not from TLR4- deficient, mice showed functional maturation in response to LPS. Lipopolysaccharide stimulation induced caspase-1-dependent cleavage of the IL-18 precursor into its mature form. Lipopolysaccharide stimulation of MyD88-deficient macrophages led to the induction of several IFN-inducible genes. In addition to LPS, dsRNA induced activation of NF-kB in MyD88-deficient mice. LR 3 induces MyD88 independent pathway and activates IRF3 and IFN-β.
- TLR-mediated effects of PAMPs on cells of the periodontium and their interactions. ICAM-1: intercellular adhesion molecule-1; LFA-1: ligand for lymphocyte function-associated antigen-1; IL-8: interleukin-8; MMPs: matrix metalloproteinases; PDL: periodontal ligament; LPS: bacterial lipopolysaccharides; TNF-α: tumor necrosis factor-α; IL-1: interleukin-1; RANKL: receptor activator of nuclear factor kappa B ligand; M-CSF: macrophage colonystimulating factor
- Significant progress has been made over the past years in the understanding of TLR function . • TLRs are essential receptors in host defense against pathogens by activating the innate immune system, a prerequisite to the induction of adaptive immune responses. • Although TLR-mediated signaling is paramount in eradicating microbial infections and promoting tissue repair, the regulation must be tight. TLRs are implicated in a number of inflammatory and immune disorders and play a role in cancer . • Many single nucleotide polymorphisms have been identified in various TLR genes and are associated with particular diseases. Several therapeutic agents targeting the TLRs are now under preclinical and clinical evaluation . However, the complexity lies in that TLRs act as doubleedged swords either promoting or inhibiting disease progression. Furthermore, therapeutic agents targeting the TLRs must be able to antagonize the harmful effects resulting without affecting host defense functions.