THE INNATE IMMUNE SYSTEM
C. PATTERN-RECOGNITION RECEPTORS (PRRs)
The overall purpose of this Learning Object is:
1) to learn how the innate immune system uses pattern-recognition receptors (PRRs) to detect pathogen-associated molecular patterns (PAMPs)in order to detect microbial invasion and initiate innate immune defenses; and
2) to learn the different categories of PRRs and their functions in inducing innate immunity.
LEARNING OBJECTIVES FOR THIS SECTION
Pattern-Recognition Receptors (PRRs) (def)
In order to recognize PAMPs (def) , various body cells have a variety of corresponding receptors called pattern-recognition receptors or PRRs (see Fig. 5) capable of binding specifically to conserved portions of these molecules. Cells that typically have pattern recognition receptors include macrophages (def), dendritic cells (def), endothelial cells (def), mucosal epithelial cells, and lymphocytes (def).
Many pattern-recognition receptors are located on the surface of these cells where they can interact with PAMPs on the surface of microbes. Others PRRs are found within the phagolysosomes (def) of phagocytes where they can interact with PAMPs located within microbes that have been phagocytosed. Some PRRs are found in the cytosol (def) of the cell.
There are two functionally different major classes of pattern-recognition receptors: endocytic pattern-recognition receptors and signaling pattern-recognition receptors.
a. Endocytic Pattern-Recognition Receptors (def)
Endocytic pattern-recognition receptors are found on the surface of phagocytes and promote the attachment of microorganisms to phagocytes leading to their subsequent engulfment and destruction. They include:
1. mannose receptors
Mannose receptors on the surface of phagocytes bind mannose-rich glycans (def), the short carbohydrate chains with the sugar mannose or fructose as the terminal sugar that are commonly found in microbial glycoproteins and glycolipids but are rare in those of humans. Human glycoproteins and glycolipids typically have terminal N-acetylglucosamine and sialic acid groups. C-type lectins found on the surface of phagocytes are mannose receptors (see Fig. 6).
2. scavenger receptors (def)
Scavenger receptors found on the surface of phagocytic cells bind to bacterial cell wall components such as LPS, peptidoglycan and teichoic acids (see Fig. 7). There are also scavenger receptors for certain components of other types of microorganisms, as well as for stressed, infected, or injured cells . Scavenger receptors include CD-36, CD-68, and SRB-1.
3. opsonin receptors
Opsonins (def) are soluble molecules produced as a part of the body's immune defenses that bind microbes to phagocytes. One portion of the opsonin binds to a PAMP on the microbial surface and another portion binds to a specific receptor on the phagocytic cell. Examples include:
- Acute phase proteins (def) circulating in the plasma, such as:
- mannose-binding lectin (also called mannose-binding protein) that recognize mannose-rich glycans (def); and
- C-reactive protein (CRP) that binds to phosphorylcholine in bacterial membranes and phosphatidylethenolamine in fungal membranes.
- Complement pathway proteins (def), such as C3b (see Fig. 8) and C4b recognize a variety of PAMPS.
- Surfactant proteins in the alveoli of the lungs, such as SP-A and SP-D are opsonins.
- During adaptive immunity, the antibody molecule (def) IgG can function as an opsonin (see Fig. 16).
4. N-formyl Met receptors
N-formyl methionine is the first amino acid produced in bacterial proteins since the f-met-tRNA in bacteria has an anticodon complementary to the AUG start codon (see Fig. 17). This form of the amino acid is not typically seen in mammalian proteins. FPR and FPRL1 are N-formyl receptors on neutrophils and macrophages. Binding of N-formyl Met to its receptor promotes the motility and the chemotaxis of these phagocytes. It also promotes phagocytosis.
b. Signaling Pattern-Recognition Receptors (def)
Signaling pattern-recognition receptors bind a number of microbial molecules: LPS, peptidoglycan, teichoic acids, flagellin, unmethylated cytosine-guanine dinucleotide or CpG sequences from bacterial and viral genomes; lipoteichoic acid, glycolipids, and zymosan from fungi; double-stranded viral RNA, and certain single-stranded viral RNAs. Binding of microbial PAMPs to their PRRs promotes the synthesis and secretion of intracellular regulatory molecules such as cytokines that are crucial to initiating innate immunity and adaptive immunity.
1. signaling PRRs found on cell surfaces (see Fig. 5):
A series of signaling pattern-recognition receptors known as toll-like receptors (TLRs) are found on the surface of a variety of defense cells and other cells. These TLRs play a major role in the induction of innate immunity and contribute to the induction of adaptive immunity.
The binding of a microbial PAMP to its TLR (or other PRR) transmits a signal to the host cell's nucleus inducing the expression of genes coding for the synthesis of intracellular regulatory molecules called cytokines. The cytokines, in turn, bind to cytokine receptors on other defense cells.
Different combinations of TLRs appear in different cell types and may occur in pairs. Different TLRs directly or indirectly bind different microbial molecules. For example:
a. TLR-2 - recognizes peptidoglycan, bacterial lipoproteins, lipoteichoic acid (LTA), and porins;
b. TLR-4 - recognizes lipopolysaccharide (LPS) from gram-negative cell wall, fungal mannans, viral envelope proteins, parasitic phospholipids, heat-shock proteins;
c. TLR-5 - recognizes bacterial flagellin;
d. TLR-1/TLR-2 pairs - bind uniquely bacterial lipopeptides and glycosylphosphatidylinositol (GPI)-anchored proteins in parasites;
e. TLR-2/TL6 pairs - bind lipoteichoic acid (LTA) from gram-positive cell walls, bacterial lipopeptides, and peptidoglycan.
Many of the TLRs, especially those that bind to bacterial and fungal cell wall components, stimulate the transcription and translation of inflammatory cytokines (def) such as interleukin-1 (IL-1), tumor necrosis factor-alpha (TNF-alpha), and interleukin-12 (IL-12), as well as chemokines (def) such as interleukin-8 (IL-8), MCP-1, and RANTES. These cytokines trigger innate immune defenses such as inflammation (def), fever, and phagocytosis in order to provide an immediate response against the invading microorganism (see Fig. 9). Because cytokines such as IL-I, TNF-alpha, and IL-12 that trigger an inflammatory response, they are often referred to as inflammatory cytokines. Chemokines are a group of cytokines that enable the migration of leukocytes from the blood to the tissues at the site of inflammation. To counter inflammation, anti-inflammatory cytokines such as IL-1 receptor antagonist, IL-4, and IL-10 are produced.
Another cell surface PRR is CD14. CD14 is found on monocytes, macrophages, and neutrophils and promotes the ability of TLR-4 to respond to LPS. LPS typically binds to LPS-binding protein in the plasma and tissue fluid. The LPS-binding protein promotes the binding of LPS to the CD14 receptors. At that point the LPS-binding protein comes off and the LPS-CD14 bind to TLR-4. Interaction of LPS and CD14 with TLR-4 leads to an elevated synthesis and secretion of inflammatory cytokines such as IL-1, IL-6, IL-8, TNF-alpha, and platelet-activating factor (PAF). These cytokines then bind to cytokine receptors on target cells and initiate inflammation and activate both the complement pathways and the coagulation pathway (see Fig. 9).
The signaling process for the CD14 and TLR-4 response to LPS is shown in Fig. 15.
TLRs also participate in adaptive immunity by triggering various secondary signals needed for humoral immunity (the production of antibodies (def)) and cell-mediated immunity (the production of cytotoxic T-lymphocytes (def), activated macrophages, and additional cytokines (def)). Without innate immune responses there could be no adaptive immunity.
a. T-independent (TI) antigens (def) allow B-lymphocytes (def) to mount an antibody (def) response without the requirement of interaction with effector T4-lymphocytes (def). The resulting antibody molecules are generally of the IgM isotype and do not give rise to a memory response. There are two basic types of T-independent antigens: TI-1 and TI-2. TI-1 antigens are pathogen-associated molecular patterns (PAMPs) such as lipopolysaccharide (LPS) from the outer membrane of the gram-negative cell wall and lipoteichoic acids from the gram-positive cell wall. These antigens activate B-lymphocytes by binding to their specific toll-like receptors rather than to B-cell receptors (see Fig. 11). Antibody molecules generated against TI-1 antigens are often called "natural antibodies" because they are always being made against bacteria present in the body.
b. The activation of naive T-lymphocytes requires co-stimulatory signals involving the interaction of accessory molecules on antigen-presenting cells (def) or APCs with their corresponding ligands on T-lymphocytes. These co-stimulatory molecules are only synthesized when toll-like receptors on APCs bind to pathogen-associated molecular patterns of microbes (see Fig. 12).
2. Signaling PRRs found in the membranes of the endosomes (def) (phagolysosomes (def)) used to degrade pathogens (see Fig. 5):
a. TLR-3 - binds double-stranded viral RNA;
b. TLR-7 - binds single-stranded viral RNA, such as in HIV, rich in guanine/uracil nucleotide pairs;
c. TLR-8 - binds single-stranded viral RNA;
d. TLR-9 - binds unmethylated cytosine-guanine dinucleotide sequences (CpG DNA) found in bacterial and viral genomes but uncommon or masked in human DNA and RNA.
Most of the TLRs that bind to viral components trigger the synthesis of cytokines called interferons (def) that block viral replication within infected host cells as well as inflammatory cytokines.
3. Signaling PRRs found in the cytoplasm (see Fig. 5):
a. NODs (nucleotide-binding oligomerization domain)
NOD proteins (def), including NOD-1 and NOD-2, are cytostolic proteins that allow intracellular recognition of peptidoglycan components.
1. NOD-1 recognizes peptidoglycan containing the muramyl dipeptide NAG-NAM-gamma-D-glutamyl-meso diaminopimelic acid, part of the peptidoglycan monomer in common gram-negative bacteria and just a few gram-positive bacteria.
2. NOD-2 recognizes peptidoglycan containing the muramyl dipeptide NAG-NAM-L-alanyl-isoglutamine found in practically all bacteria (see Fig. 5).
As macrophages (def) phagocytose either whole bacteria or peptidoglycan fragments released during bacterial growth, the peptidoglycan is broken down into muramyl dipeptides. Binding of the muramyl dipetides to NOD-1 or NOD-2 leads to the activation of genes coding for inflammatory cytokines such as IL-1, TNF-alpha, IL-8, and IL-12 in a manner similar to the cell surface TLRs. Activation of NOD-2 also induces the production of antimicrobial peptides such as defensins as well as microbicidal reactive oxygen species (ROS).
b. CARD-containing proteins
CARD (caspase activating and recruitment domain)-containing proteins, such as RIG-1 (retinoic acid-inducible gene-1) and MDA-5 (melanoma differentiation-associated gene-5), are cytoplasmic sensors that recognize both viral double-stranded and single-stranded RNA molecules produced in viral-infected cells, and trigger the synthesis of cytokines called interferons (def) that block viral replication within infected host cells in a manner similar to the endosomal TLRs. RIG-1 recognizes 5'-PPPs on viral RNAs. The 5'-PPPs on host cell RNAs are either capped or removed and are not recognized by RIG-1.
4. Secreted signaling PRRs found in plasma and tissue fluid
In addition to the PRRs found on or within cells, there are also secreted pattern-recognition receptors. These PRRs bind to microbial cell walls and enable them to activate the complement pathways, as well promote phagocytosis by functioning as opsonins.
For example, mannan-binding lectin (def) -also known as mannan-binding protein - is synthesized by the liver and released into the bloodstream as part of the acute phase response. Here it can bind to the carbohydrates on bacteria, yeast, some viruses, and some parasites (see Fig. 6). This, in turn, activates the lectin complement pathway and results in the production of a variety of activated complement proteins that are able to trigger inflammation, chemotactically attract phagocytes to the infection site, promote the attachment of antigens to phagocytes via enhanced attachment or opsonization, and cause lysis of gram-negative bacteria and infected or transformed human cells.
Other secreted PRRs include C-reactive protein (CRP), surfactant protein A (SP-A), surfactant protein D (SP-D), collectin liver 1 (CL-L1), and ficolins.
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