dr. geahlen hans 431a email: [email protected]
TRANSCRIPT
The Immune System
Second Edition
Chapter 8
The Body’s DefensesAgainst Infection
Copyright © 2005 by Garland Science Publishing
Peter Parham
The body’s defenses against infection-Chpt. 8 To combat infections, the body uses both the innate and adaptive immune systems. • the innate immune system contains the initial infection while the adaptive immune system clears the infection.
Figure 8.1
• It is probable that the innate immune system eliminates the vast majority of infections before any symptoms begin.
The type of immune response required to combat the infection depends on where the pathogens live and replicate in the host.
Figure 8.4
1. Extracellular pathogens - accessible to antibodies and complement, phagocytosis
2. Intracellular - replicate in cytosol - killed by CD8 and NK cells
3. Intracellular - replicate in macrophage vesicles - killed by TH1 and
NK cell-mediated macrophage activation.
Ganz, T. (2002) Science, 298, 977-979.
Innate immunity• Surface epithelia (e.g., skin and mucosal epithelial linings) forms a barrier to keep pathogens out.• Also contain a variety of chemical agents to fight infections• lysozyme in tears and saliva • acid and hydrolytic enzymes in stomach• epithelial cells and neutrophils secrete defensins –
antimicrobial peptides (- and -defensins).• nonpathogenic commensal microorganisms compete with pathogenic microorganisms. e.g., antibiotic-associated diarrhea and C. difficile.
Defensins function by disrupting the membranes of bacteria, fungi and enveloped viruses by forming ion channels.
Figure 8-7
•The first cells to respond to extracellular pathogens are macrophages already resident within the infected tissue.
•Macrophages have two major roles in the innate immune response:
•phagocytosis of invading pathogens
•release of cytokines.
•Macrophages recognize infectious organisms by a couple of important mechanisms:
•complement receptors - recognize C3B or iC3B bound to infectious organism
•pattern recognition receptors - recognize common components of infectious organisms (e.g., polysaccharides, etc.)
Phagocytosis by complement receptors CR1, CR3, CR4
Lysis by membrane-attack complex
Recruitment of phagocytes (e.g., neutrophils) and activation of mast cells by anaphylatoxins C5a, C4a, C3a.
C3b
•Complement system - complement activation occurs mainly via the alternative pathway. This is the non-antibody mediated pathway.
•initial recognition of the pathogen involves component C3b binding covalently to any one of several polysaccharides on the surfaces of pathogens.
•The deposition of C3b on the pathogen cell surface serves three major functions:
Figure 8-8
C3 contains a thioester bond
Figure 8-10
Phagocytosis by complement receptors CR1, CR3, CR4
Lysis by membrane-attack complex
Recruitment of phagocytes (e.g., neutrophils) and activation of mast cells by anaphylatoxins C5a, C4a, C3a.
C3b
•Complement system - complement activation occurs mainly via the alternative pathway. This is the non-antibody mediated pathway.
•initial recognition of the pathogen involves component C3b binding covalently to any one of several polysaccharides on the surfaces of pathogens.
•The deposition of C3b on the pathogen cell surface serves three major functions:
Figure 7-44 part 2 of 2
Pattern recognition receptors - receptors for common components of multiple different types of pathogens.
Pattern recognition receptors recognize common features of several different ligands. These common features are referred to as pathogen-associated molecular patterns (PAMPs).
PAMP
Pattern recognition receptor
PAMP’s include things like LPS, peptidoglycans from bacterial cell walls, mannose, flagellin, pillin, glycolipids and zymosan from fungi, bacterial DNA (nonmethylated CpG), double-stranded RNA (from viruses), etc.
There are two functional classes of pattern-recognition receptors:
1. endocytic pattern-recognition receptors – promotes phagocytosis.
Includes:
•mannose receptor (a sugar common on the end of bacterial polysaccharides, but not human ones)
•scavenger receptor (binds sialic acid-rich molecules) •CD14 – receptor of LPS and LPS-binding protein complexes
2. Signaling pattern recognition receptors – binding of ligand promotes production of cytokines. Includes a family of receptors referred to as Toll-like receptors.
Humans have 10 different toll-like receptors. Each binds a different class of PAMP.
Different TLRs directly or indirectly bind different microbial molecules.
TLR-2 recognizes peptidoglycan and lipoproteinsTLR-3 recognizes double-stranded RNATLR-4 recognizes lipopolysaccharide TLR-5 recognizes bacterial flagellin TLR-9 recognizes bacterial DNA.
All toll like receptors signal through common pathways to regulate the expression of cytokine genes
Figure 8-14
E. The activated macrophages secrete a battery of cytokines and chemokines that function to initiate the inflammatory response and to recruit other effector cells to the site of infection.
cytokines (IL-1, IL-6, IL-8 (CXCL8), IL-12, TNF-)
bacteria lipid mediators (prostaglandins, leukotrienes, etc.)
tissue macrophage chemokines (e.g., CXCL8 (IL-8))
complement C3a, C4a, C5a recruit neutrophils
mast cells histamine, lipids, TNF-
TLR
Due to the release of these various inflammatory mediators, blood vessels near site of infection increase in diameter (vasodilation) and exhibit enhanced permeability (capillary endothelial cells contract and fluid enters site of inflammation) causing swelling and redness (inflammation). They trigger platelet activation and clotting to restrict the infection to a limited area. They also attract other effector cells (more monocytes and neutrophils) to the site of infection.
TNF- acts on vascular endothelial cells to increase vascular permeability.
TNF- induces vascular endothelial cells to make platelet-activating factor, which promotes blood clotting to block local blood vessels to restrict pathogen from entering blood.
The excessive release of TNF- by macrophages into the blood stream, as can occur in response to blood-borne pathogens (e.g., a bacterial infection that enters the wider circulation), can produce a dangerous condition known as sepsis.
Instead of local blood vessel dilation, systemic vasodilation can occur with massive leakage of fluid into tissues and widespread blood clotting. Causes septic shock and can lead to organ failure and death.
CXCL8 is an example of a chemoattractant or chemokine.
Small, 90-130 residue polypeptides. Some are produced all the time and some are produced selectively during an infection to help determine where a cell will go in the body. Over 50 chemokines and 15 chemokine receptors.
Classified based on the sequence around cysteines as the CC group (Cys’s are adjacent) or the CXC group (one amino acid separates the two Cys) (there are also minor groups of C and CXXXC chemokines). Receptors for the CC group are termed CCR’s (e.g., CCR1, CCR2, etc.) and for the CXC group are termed CXCR’s (e.g., CXCR1, CXCR2, etc.). Chemokine receptors are G-protein coupled receptors.
IL-8 is a CXC chemokine that binds CXCR1 and CXCR2 to recruit neutrophils. C C
C C CXC
C C
E. The activated macrophages secrete a battery of cytokines and chemokines that function to initiate the inflammatory response and to recruit other effector cells to the site of infection.
cytokines (IL-1, IL-6, IL-8 (CXCL8), IL-12, TNF-)
bacteria lipid mediators (prostaglandins, leukotrienes, etc.)
tissue macrophage chemokines (e.g., CXCL8 (IL-8))
complement C3a, C4a, C5a recruit neutrophils
mast cells histamine, lipids
TLR
Neutrophils are the most abundant of the cells recruited from the blood stream to the site of an infection.
neutrophils – most abundant of the white blood cells – also known as polymorphonuclear leukocytes (odd shaped nuclei). Found in blood and not in normal tissue, are recruited into infected tissues by cytokines released by activated macrophages.
neutrophils are short-lived cells that die within hours causing the formation of pus at sites of infection. Bacteria that efficiently recruit neutrophils are pyogenic or pus-forming.
The recruited neutrophil must move from the blood stream, through the wall of the blood vessel, into the tissue. This process is known as extravasation –
Two features are important for the cell to move from the blood stream into a particular organ or tissue:
1) The cell must recognize where it wants to bind to the wall of the blood vessel and then bind there and then
2) it must cross through the wall into the tissue.
Extravasation occurs in four steps:
1) rolling 2) arrest and adhesion3) transendothelial migration and 4) movement to central region of infection
Figure 8-19 part 2 of 2
Figure 8-19 part 1 of 2
Cell adhesion molecules involved in extravasation
1. Rolling – selectins on neutrophils bind sialyl-Lewisx carbohydrates on endothelial cells.
2. Tight binding – integrins (heterodimers of and integrin subunits) are activated by chemoattractants binding to receptors on the cell surface. Activated integrins like LFA-1 or CR3 on neutrophils (Mac-1 on monocytes) bind to adhesion molecules like ICAM-1 (intercellular cell adhesion molecule-1) and VCAM-1 (vascular cell adhesion molecule-1). ICAM-1 and VCAM-1 expression are induced by TNF- and IL-1.
Figure 8-19 part 3 of 3
Neutrophils can phagocytose and kill bacteria, fungi and even some enveloped viruses using Fc receptors (once Ab’s are made) or complement receptors. Contain phagosomes with lots of degradative enzymes and other toxic substances like NO, oxygen radicals, etc. They cannot replenish these and they die by undergoing apoptosis.
Systemic effects of IL-1, IL-6 and TNF-a.
Known as endogenous pyrogens, they act on the hypothalamus to increase body temperature (stimulate production of the prostaglandin PGE2 in the brain).
Stimulate the acute-phase response.
Act on hepatocytes (liver cells) to produce acute-phase proteins like C-reactive protein and mannose-binding lectin.
C-reactive protein and mannose-binding protein bind to a wide variety of microorganisms. C-reactive protein recognizes phosphorylcholine groups on multiple bacterial capsular polysaccharides. Mannose-binding protein recognizes mannose residues. Can opsinize a bacterium to stimulate phagocytosis via Fc receptors and activate complement for lysis or uptake by phagocytic cells.
Hypothalamus PGE2 feverLocal inflammatory IL-1, IL-6, TNF-Response
Liver CRP, MBL
CRP can bind C1q to initiate the classical pathway of complement activation in the absence of antibody. MBL activates MBL-associated protease (MASP) to cleave C4 and C2 to initiate complement activation. Known as the lectin pathway of complement activation.
Innate immune responses to viral infections
The initial defense against viral infections also includes innate immune responses, which mostly involve interferons and NK cells.
Many cells, when infected by a virus, produce IFN- and IFN- in response to a double-stranded RNA binding to TLR3.
IFN-/ binds to the IFN receptors on neighboring cells and induces a state of viral resistance by inhibiting specific protein synthesis.
Figure 8-25
Oligoadenylate synthetase dsRNA-dependent protein kinase (PKR)
dsRNA
ATP 2’-5’A(n) eIF-2 (active) eIF-2-P (inactive)
ATP ADP
RNaseLi RNaseLa
mRNA Protein Synthesis
Figure 8-26
NK or natural killer cells are large lymphocytes distinct from T or B cells important for fighting viral infections. Function both by killing virally infected cells and by producing cytokines. Killing function enhanced by IFN-/. Cytokine production (IFN-) is enhanced by macrophage-derived IL-12. IFN- activates macrophages. NK cells turned off by IL-10 from cytotoxic T cells
Virally infected cell NK cell IL-12 macrophage
IFN/
IFN-
IL-10Cytotoxic activity
Cytotoxic T cell
Figure 8-28
NK cell mediated killing is a function of inhibitory and activating signals received by NK cell receptors from ligands on the target cell.
Two main classes:
1). Killer cell immunoglobulin-like receptors (KIRs). Those that activate cell killing are referred to as natural cytotoxicity receptors (NCRs), 2). NK-cell lectin-like receptors – NKG receptors.
Activating and inhibitory receptors are present in both structural classes.
NK cell activating receptors trigger cell killing. NK inhibitory receptors prevent cell killing.
Usually engagement of an inhibitory receptor will prevent killing by an engaged activating receptor.
Figure 8-29
Figure 8-31Both KIRs and NKGs are polymorphic, especially the KIRs. NKGs are encoded by genes in the natural killer complex (NKC) while KIR genes are encoded by genes in the leukocyte receptor complex (LRC).
Different receptors are expressed nearly randomly on surfaces of NK cells from same individual leading to multiple subpopulations. Must have at least one inhibitory receptor that can prevent killing of autologous cells.
Ligands for activating receptors
1) NKG2D binds MICA and MICB – class 1-like heavy chains encoded by genes in the HLA region. Not expressed on normal cells, but are expressed in response to stress. Often a marker of infected, damaged or cancerous cells. (This is a rare exception in that NKG2D is actually dominant over inhibitory receptors.)
2) KIR2DS –binds HLA-C molecules with specific amino acid substitutions. May enhance killing of cells with a certain HLA-C allotype
3) NKp30, 44, 46 – main activating NCRs. Normal ligands (e.g., for killing cancer cells) are unknown. NKp44 and NKp46 can bind viral hemagglutinins
Ligands for inhibitory receptors
1). Recognize MHC class I ligands. Both KIRs and NKGs recognize MHC I-peptide complexes; but usually the peptide itself is irrelevant since most of it doesn’t contact the receptor.
2) KIR2DL binds HLA-C molecules of a certain allotype.
3) NKG2A functions as a heterodimer with CD94 and binds HL-E that has bound a peptide derived from the leader sequence of HLA-A, B
Figure 8-32
Figure 8-33
Specialized T and B cells with a limited antigen receptor repertoire are poised at major sites of possible infection to ward off pathogens.
1. T cells have TCR’s composed of and chains of very limited diversity. They are found in highest abundance in epithelial tissues (in any given tissue, these cells have pretty much the same TCR’s). Recognize nonpeptide microbial antigens like prenylpyrophosphates and alkylamines and MICA and MICB, autologous proteins upregulated by transformed, infected or stressed epithelial cells.
2. NK T cells – subpopulation of T cells bearing both T cell and NK cell markers. Have a very limited TCR () repertoire that recognizes lipid antigens presented by CD1D, a class I-type molecule. NK T cells are known for their ability to secrete lots of IL-4 (stimulates TH2 responses) and IFN- (TH1 responses). Recent evidence suggests that they are present normally in an “activated” state due to presentation of endogenous ligands and then respond quickly to IL-12 production from macrophages or dendritic cells encountering foreign PAMPs through toll-like receptors. They likely provide a crucial link between the innate and adaptive immune systems.
B-1 lineage B cells have a limited set of variable regions and recognize general polysaccharide antigens in a T cell-independent fashion. Often found in body cavities rather than lymph nodes – generate rapid IgM responses.