SOLUBLE PATTERN RECOGNITION RECEPTORS.pptx

Pattern recognition receptors are type of receptors that plays a major role in innate immunity by recognizing conserved molecular components of the pathogen called pathogens- associated molecular patterns (PAMPs).There are different kinds of PRRS such as soluble pattern recognition receptors and membrane associated PRRs that recognises different kinds of PAMPs such as Carbohydrates,Proteins, lipids and nucleic acids and thereby eliminating the pathogen through different mechanisms.

1. SOLUBLE PATTERN RECOGNITION RECEPTORS (PRRs) IMMUNOLOGY KESSIYA T PETER  MSc MICROBIOLGY DEPT. OF BIOSCIENCE IGCAS, KOTHAMANGALAM

2. INTRODUCTION Vertebrates are protected by both innate and adaptive immunity. Innate immunity consists of the defenses against infection that are ready for immediate action when a host is attacked by a pathogen. The major function of innate immunity is the recognition of foreign substances or organisms that have penetrated our outer defenses such as skin epithelium. The elimination of such agents are carried out by different cells and molecules of immune system. Thus a critical role of immune system is to determine what is foreign or non self from what is normally present in body.

3. INNATE IMMUNITY AND PATTERN RECOGNITION RECEPTORS (PRRs) The cells that participate in  innate immunity  recognize  conserved   molecular components  of pathogen through  cell surface or intracellular receptors called pattern recognition receptors (PRRs). The innate immune system uses these receptors to recognize conserved molecular structures that are produced by microbial pathogens called Pathogen- associated molecular patterns (PAMPs) and also endogenous molecules that are produced and released by damaged and dying cells called Damaged –associated molecular patterns (DAMPs) . Pattern recognition receptors are expressed mainly by immune cells such as dendritic cells, macrophages, monocytes, neutrophiles and non-immune cells such as epithelial cells, they also expressed on the surface, in phagocytic vesicles and in the cytosol of various cell types. When PAMPs and DAMPs bind to the pattern recognition receptors they activate  signal transduction pathways that promote the antimicrobial and proinflammatory functions of the cells in which they are expressed. In addition ; many proteins present in the blood and extracellular fluids recognize PAMPs and facilitate the clearance of microbes from blood.

4. PATHOGEN-ASSOCIATED MOLECULAR PATTERNS (PAMPs) PAMPs are only produced by microbes and not by its hosts. Different types of microbes ( eg ; viruses, gram-negative bacteria, gram-positive bacteria, fungi) express different PAMPs. Table 1 ; examples of PAMPs                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             Pattern – associated molecular patterns (PAMPs) Type of microorganism Nucleic acids ssRNA Virus dsRNA Virus CpG Virus, bacteria Proteins  Pilin  Bacteria  Flagellin  Bacteria  Cell wall lipids  LPS Gram negative bacteria Lipoteichoic acid Gram positive bacteria Carbohydrates  Mannan  Fungi , bacteria  Glucans  Fungi  CpG -cytosine-guanine-rich oligonucleotide, LPS- lipopolysaccharide, 

5. DAMAGE- ASSOCIATED MOLECULAR PATTERNS (DAMPs) DAMPs are produced as a result of cell damage caused by infections, it also indicate sterile injury to cells such as chemical toxins, burns, trauma or loss of blood supply In some cases, endogenous molecules such as cytokines produced by healthy cells are released when cells are damaged or die and then stimulate innate responses. These molecules are subset of DAMPs and are called  alarmins . Damage- associated molecular patterns Stress induced proteins   HSPs Crystals  Monosodium urate Proteolytically cleaved extracellular matrix  Proteoglycan peptides  Mitochondrial components found outside mitochondria  Extracellular formylated peptides and ATP Nuclear proteins or nucleic acids found outside nucleus Extracellular HMGB1, histones, cytoplasmic dsDNA. HSPs-heat shock protein, HMG1-high-mobility group

6. SPECIFICITY OF INNATE AND ADAPTIVE IMMUNITY innate immunity adaptive immunity Specificity   For structures shared by classes of microbes (PAMPs) For structural details of microbes ;antigens  No of microbial molecules recognized About 1000 molecular patterns >10^7 antigens Receptors  Encoded in germline; limited diversity (PRRs) Encoded by genes produced by somatic recombination  No and types of receptors  <100 different types of invariant receptors  Only two types of receptors (IgG and TCR) Distribution of receptors  Non clonal; identical receptors are present in cells of same lineage Clonal ; clones of lymphocytes with distinct specificities express different receptors

7. FUNCTIONS OF PPRs Opsonization Phagocytosis Activation of complement Activation of proinflammatory pathways Induction of apoptosis CLASSES OF PATTERN RECOGNITION RECEPTORS (PRRs) There are different types of pattern recognition receptors such as; Soluble PRRs Membrane associated PRRs Based upon the location of Pattern recognition receptors ;   Extracellular PRRs – plasma membrane – binding activates proinflammatory activity              Toll- like receptors (TLRs)              C-type lectin receptors (CLRs)  2 .   Endosomal PRRs – detect ingested microbes- mediate microbial uptake by phagocytosis             Toll-like receptors( TLRs)  3.    Cytosolic PRRs- detects microbes in the cytoplasm- activate complement proteins            NOD- like receptors (NLRs) and Inflammasome            RIG-I- like receptors (RLRs) 

8. SOLUBLE PATTERN RECOGNITION RECEPTORS Extracellular soluble PRRs are important part of non-specific immunity. Hepatocytes  of liver and respiratory tract airway epithelial cells are major source of SRRRs. SPPRs also contribute to phagocytosis by recognizing the soluble protein that bound to the microbial surfaces  and enhance opsonization . These  soluble proteins are called opsonins bind to conserved components in the surface of microbes. Opsonins are recognized by membrane opsonin receptors on phagocytes activating phagocytosis. SPPRs also eliminate pathogens through complement activation, aggregation and neutralization of inflammatory regulation. Extracellular soluble PRRs are composed of different molecular families, mainly including           Pentraxin           Collectin           Ficolin  

9. 1.PENTRAXIN Pentraxin are aggregates of five molecules; includes two families of short molecules and long molecules. The family of short molecules is called acute phase proteins represented by C-Reactive protein and Serum amyloid P component. These molecules were produced by the stimulation of inflammatory signals and interleukins. C- REACTIVE PROTEIN It is discovered in the serum of patient with acute inflammation . CRP is a plasma protein and its concentration increases during inflammatory states. CRP bind and precipitate c polysaccharide contained in pneumococcal cell wall in calcium dependent manner. It also recognize phosphocholine and carbohydrates on bacteria, fungi and parasites. It is then bound to fc receptors ( FcRs ) for IgG found in phagocytes.

10. 2.COLLECTINS Collectins are collagen containing  C-type lectins. Mannose-binding lectins (MBL) Surfactant protein A and D A.MANNOSE-BINDING LECTINS (MBL) Mannose binding lectins are found in plasma and alveoli. It recognizes large array of pathogens and activates classical pathway of complement. It recognizes carbohydrates with terminal mannose and fucose and various microbial structures. STRUCTURE     MBL is   composed   of 3 identical   32kDa polypeptide chains, which associate to a hexamer. The individual polypeptide chains are composed of a C- terminal fibrinogen like recognition or carbohydrate binding domain (CDRs) and an N-terminal cysteine rich neck domain which is followed by a collagen domain. The end of CDRs can identify sugar structures of pathogens such as mannose, fucose , glucose etc. Yeast , parasites, gram bacteria and so on. In circulation MBL is complexed to MBL –associated serine protease MASP-2 and MASP-3. The MASPs then activate the C1 complement pathways  .

11. STRUCTURE AND ACTION OF MBL

12. B. SURFACTANT PROTEIN A AND D Surfactant protein-A and surfactant protein-D are produced and expressed on alveoli type 2 cells and club cell of the distant bronchioles. SP-A proteins are similar to C1q;forms a bouquet like structure. Within the alveolar compartment, SP-A tightly associated to phospholipids. SP-A opsonizes staphylococcus aureus  but also Herpes simplex particles for uptake by alveolar macrophages. In contrast, SP-D has a cruciform  structure, which links its trimeric subunits; they also exists as a nonlipid bound free form especially in alveoli. SP-D also binds to phosphatidyl-inositol and glucosyl ceramide. It opsonizes a number of gram-negative bacteria such as  E.coli and activates respiratory burst in alveolar macrophage and leads to its uptake by dendritic cells followed by T cell activation. SP-A and SP-D contributes to clearance of the fungal respiratory pathogens such as Pneumocytosis caririi , a major cause of pneumonia on HIV patients. After binding to microbes they are recognized by CD91opsonin receptors and promote phagocytosis.

13. STRUCTURE AND ACTION OF SP-A AND D

14. 3.FICOLINS L ficolin, a member of ficolin family that is related to MBL and other collectins . It is found in blood. Binds to acetylated sugars on microbes including some streptococcal species. STRUCTURE Ficolins are lectins   assemble to form oligomeric structures that looks like 'bunch of flowers'. It has high calcium dependent binding specificity for N-acetyl glucosamine, a cell wall component of Aspergillus fumigatus. Primarily function as opsonins , induce phagocytosis of fungi. Contribute to the activation of lectin pathway by interacting and activating the MBL-associated serine proteases. Trigger the production of inflammatory cytokines and nitric oxide by macrophages.

15. STRUCTURE AND ACTION OF FICOLIN

16. 4. ANTIMICROBIAL PROTEINS AND PEPTIDES To provide strong defense the epithelial cells secrete many proteins and peptides The antimicrobial substances can be enzymes such as saliva found in saliva and tears and fluids of respiratory tracts that cleaves peptidoglycan. The major class of antimicrobial components is antimicrobial peptides which is less than 100 amino acid long. Antimicrobial peptides are cysteine- rich, cationic and amphipathic in nature. They interact with acidic phospholipids in lipid bilayer and disrupts the membranes of microbes by inhibiting the synthesis of DNA, RNA or proteins and activating antimicrobial enzymes resulting in cell death. Major types of antimicrobial peptides α and β defensins cathelicidin

17.    ΑLPHA AND BETA  DEFENSINS AND CATHELICIDINS Produced by epithelia ( eg ; oro , nasal, respiratory, intestinal, reproductive tracts, skin keratinocytes, kidney) and NK cells. Human defensins kills a wide variety of bacteria including E.coli, S.aureus , Streptococcus pneumonia, Pseudomonas aeruginosa  and Haemophilus influenzae. They also attack the lipoprotein envelope of viruses such as influenzae  and herpes  viruses. Act on monocytes ,immature dendritic cells and T cells results in cytokine activation. Act on mast cells results in degranulation. Defensins and cathelicidin LL-37 are secreted by epithelial cells as well as stored in neutrophil granules where they contribute to killing phagocytosed microbes. Human α-defensins antimicrobial peptides secreted into the gut by intestinal epithelial  paneth cells, located in crypts between the villi are required for maintaining beneficial bacterial flora. On mucosal and glandular epithelial cells, skin keratinocytes and NK cells the expression of AMPs are induced by particular PRRs (toll-like receptors and NOD NLRs). Macrophages cannot produce AMPs followed by PRRs activation, but there is an indirect pathway for the production of  cathelicidin . Macrophages TLRs binds to the microbial ligand increases the expression of receptors for vitamin D ; this binding activates the macrophages to produce cathelicidin which helps to kill pathogen.

18. ACTION OF AMPs

19. REFERENCES Abul-K.-Abbas-MBBS-Andrew-H.-Lichtman-MD-PhD-Shiv-Pillai-MBBS-PhD-Cellular-and-Molecular-Immunology-Elsevi 2017 Janeways_Immunobiology_9th_Edition Kuby Immunology/7th edition/chapter5/141 Li, D., Wu, M. Pattern recognition receptors in health and diseases.  Sig Transduct Target Ther   6 , 291 (2021). https://doi.org/10.1038/s41392-021-00687-0

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