Chapter 3- Innate Immunity- Induced Response to Infection

Question 1

Paneth cells

Answer 1

The immune cells of the intestine. Paneth cells are the main source of defensins in the intestine- the α-defensins HD5 and HD6, also known as cryptidins are made only by Paneth cells

Question 2

Constitutive regulation

Answer 2

Constitutive genes are always expressed at a basal level

Question 3

Pentraxins

Answer 3

Pentraxins are a family of cyclic multimeric proteins found in blood and lymph. They are effector molecules of innate immunity that have a similar role to the antibodies of adaptive immunity in binding to microbial surfaces and facilitating the phagocytosis of pathogens. Two subfamilies- short and long pentraxins

Question 4

Short vs long pentraxins

Answer 4

Long or short refers to how many amino acids make up the pentraxin

Question 5

Pentraxin structure

Answer 5

Contain a 200-residue pentraxin domain at the C-terminal end of the polypeptide

Question 6

Serum amyloid P component

Answer 6

A short pentraxin produced by liver hepatocytes. Its ligands are bacteria, viruses, fungi, and parasites

Question 7

PTX3

Answer 7

A long pentraxin produced by monocytes, macrophages, dendritic cells, endothelial cells, and epithelial cells. Its ligands are bacteria, viruses, and fungi

Question 8

Pentraxin mechanism

Answer 8

Function as bridging molecules that bind pathogens on one binding site and cell surface receptors of phagocytes to another.

Question 9

Effector cells of the innate immune system (4)

Answer 9

1. Neutrophils
2. Macrophages
3. Dendritic cells
4. NK cells

Question 10

Innate immunity cell receptors function

Answer 10

The cellular receptors of innate immunity distinguish between “non-self” (microbes themselves) and “self” or “altered-self” (infected or cancerous cells) and “self” by recognizing structural features on microbes that are not present on mammalian cells. This includes differences in the macromolecules produced by pathogens. Some features are expressed by entire families of pathogens, allowing a wide range of pathogens to be detected by a much smaller number of receptors. The receptors are expressed by neutrophils, macrophages, NK cells, and other innate immune cells. There are more than 100 different innate immune receptors, and each type of innate cell only expresses a subset of them

Question 11

Macrophage recognition of bacterial carbohydrates

Answer 11

One example of innate cells recognizing “self” vs “non-self”. Macrophages can capture a bacteria using receptors that bind to carbohydrates on the bacterial cell surface but do not bind to the carbohydrates on human cells. In this context the bacterial carbohydrates are perceived as non-self, whereas the human carbohydrates are seen as self. On capturing the bacterium with these receptors, the macrophage is signaled to internalize the bacterium by phagocytosis and then break it down in a phagosome

Question 12

NK cell recognition of viral proteins

Answer 12

One example of innate cells recognizing “self” vs “non-self”. In a virus-infected human cell, the viral genome is transcribed and translated, leading to the presence of viral glycoproteins on the epithelial cell surface. NK cells are another innate immune cell that have surface receptors which bind to the viral glycoproteins. Binding to these receptors instructs the NK cell to kill the infected cell. In this context, the virus-infected cell is perceived as altered-self, whereas the neighboring uninfected cells are seen as self.

Question 13

Tissue macrophages

Answer 13

During embryonic development, macrophage precursors are seeded into every tissue of the human body. All tissues contain resident macrophages which are ready to phagocytose invading pathogens. Tissue macrophages carry a battery of phagocytic and signaling receptors. The receptors work together to bind the pathogens and trigger their phagocytosis by the macrophage. Ligands recognized by phagocytic receptors are mostly bacterial carbohydrates, lipids, proteins, and DNA. Signaling receptors distinguish ‘self’ from ‘non-self’ and induce changes in gene expression and macrophage function

Question 14

Langerhans cells

Answer 14

Resident tissue macrophages of the skin

Question 15

Alveolar macrophages

Answer 15

Resident macrophages of the lung

Question 16

Kupffer cells

Answer 16

Resident macrophages of the liver

Question 17

Microglia

Answer 17

Resident macrophages of the CNS

Question 18

Red pulp macrophages

Answer 18

Resident macrophages of the spleen

Question 19

Osteoblasts

Answer 19

Resident macrophages of the bone

Question 20

Macrophage phagocytic receptors (there are 5)

Answer 20

1. Mannose receptor
2. Dectin-1
3. Scavenger receptors
4. MARCO
5. Complement receptors CR3 and CR4

Question 21

Mannose receptor and dectin-1

Answer 21

Part of the SR-E class of scavenger receptors. Macrophage phagocytic cell-surface receptors and plasma proteins that recognize carbohydrates are called lectins- there are C-type and B-type lectins. Lectins differ on the surface of microbes, identifying them as “non-self”

Question 22

C-type lectins

Answer 22

Calcium ion coordinates the interaction of carbohydrate ligand with the receptor

Question 23

B-type lectins

Answer 23

Bind sulfated galactosamine residues

Question 24

Scavenger receptors

Answer 24

Macrophage phagocytic receptors. Scavenge damaged molecules of low-density lipoprotein from blood as well as negatively charged microbial products. In the absence of infection, macrophages use their scavenger receptors to remove dead and dying cells and unwanted macromolecules. They also remove cells that have died by apoptosis. SR-A: Gram negative LPS, Gram positive teichoic acid, CpG DNA. SR-B: lipopeptides

Question 25

Macrophage Receptor with Collagenous Structure (MARCO)

Answer 25

Phagocytic scavenger receptor on the surface of macrophages that binds a wide range of bacteria as well as apoptotic cells. It recognizes bacterial LPS, which is the most abundant component of the outer membrane

Question 26

Complement Receptors CR3 and CR4

Answer 26

Macrophage phagocytic receptors that are members of the integrin family of proteins. They bind to complement fragment iC3b and recognize microbial PAMPs. This includes the LPS of Gram-negative bacteria

Question 27

Phagocytic and signaling receptors

Answer 27

The mannose receptor has 10 extracellular domains of 4 types. The receptors work in a cooperative fashion to securely capture the pathogen. Then, macrophages carry out receptor-mediated endocytosis and internalize the pathogen into the phagosome

Question 28

Pathogen-associated Molecular Patterns (PAMPs)

Answer 28

Part of how recognition of pathogens is achieved by WBCs. PAMPS are any structural feature on a microbe that is recognized by Pattern Recognition Receptors (PRRs) expressed on the surface and in subcellaular compartments within leukocytes.

Question 29

The ideal PAMPs should be (3)

Answer 29

1. Conserved (i.e., unlikely to mutate)
2. Shared by a large group of pathogens (i.e., bacteria, fungi, viruses)
3. Clearly distinguishable from self molecules

Question 30

Major families of pattern recognition receptors (PRRs-)

Answer 30

1. Toll-like receptors (TLRs)
2. C-type lectin receptors (CLRs)
3. Nucleotide-binding oligomerization
   domain (NOD)- Leucin Rich Repeats
   (LRR)-containing receptors (NLR)
4. Retinoic acid-inducible gene 1
   (RIG-1)-like receptors (RLR; aka
   RIG-1-like helicases—RLH)

Question 31

Pattern recognition receptors (PRRs)

Answer 31

PRRs recognize PAMPs, allowing for pathogen recognition. Engagement of PRRs on the innate immune cells activate microbicidal and pro-inflammatory responses required to eliminate (or at least to contain) infectious agents. One major result is the secretion of small biologically active proteins (cytokines) that induce inflammation in order to recruit other immune cells to the infected tissue

Question 32

Toll-like receptors

Answer 32

One type of transmembrane PRR located on the cell membrane or within endosomes. They are a family of 10 genes which encode TLRI-10. They have an extracellular domain for recognizing pathogens and a cytoplasmic domain that signals this information to the inside of the cell. TLRs are important in innate cell activation. When pathogens are degraded by macrophages, their nucleic acids are delivered to endosomes for recognition by TLRs

Question 33

Extracellular vs intracellular TLRs

Answer 33

Extracellular TLRs recognize extracellular microbial ligands, while intracellular TLRs recognize intracellular microbial ligands (RNA and DNA)

Question 34

Importance of TLR structure

Answer 34

Toll-like receptor (TLR) polypeptides comprise an extracellular sensor domain shaped like a horseshoe, a transmembrane domain, and an intracellular Toll/interleukin-1 receptor (TIR) domain that sends activating signals to the nucleus of the macrophage. The signaling domain is called TIR (Toll Interleukin-1 Receptor). Functional TLRs can form homodimers or heterodimers

Question 35

Extracellular TLR domain

Answer 35

Horseshoe shaped and responsible for pattern recognition. It consists of a leucine-rich repeat region (LRR), a sequence motif of 20-29 amino acids and is rich in leucine.

Question 36

Toll Interleukin-1 Receptor (TIR)

Answer 36

The signaling domain of TLRs. It sends activating signals to the nucleus of the macrophage

Question 37

How does TLR4 recognize bacterial lipopolysaccharides? (There are 4 steps)

Answer 37

• Bacterial LPS is recognized using a complex of TLR4, MD2, and CD14.
• LPS from Gram Negative bacteria is bound by a protein on macrophage surface called CD14 which is a co-receptor of TLR4. Alternatively, LPS can be bound to LPS-binding Protein (LBP) and delivered to CD14 on the macrophage surface.
• The TLR4 dimer associates with a protein MD2 to form a complex with CD14 and LPS. This extracellular recognition of the pathogen (LPS) causes the cytoplasmic TIR domain of TLR4 to initiate signaling.
• Recognition of LPS leads to activation of transcription factor NF-KB and synthesis of pro-inflammatory cytokines

Question 38

Transcription factors

Answer 38

They regulate (turn on/off) genes in order to make sure that they are expressed in the right cell at the right time and in the right amount. They bind to regions of DNA adjacent to the genes that they regulate. Stabilizes RNA polymerase and can make DNA more/less accessible to transcription via histone acetylation/deacetylation

Question 39

Synthesis of pro-inflammatory cytokines by TLRs (8 steps)

Answer 39

• On recognizing LPS, TLR4 activates the transcription factor NFκB, which instructs the macrophage to produce inflammatory cytokines. These cytokines induce a state of inflammation in the infected tissue.
• The cytoplasmic tail of activated TLR4 binds the adaptor protein MyD88
• MYD88 binds the protein kinase IRAK4, which self-phosphorylates.
• IRAK4 now binds and phosphorylates TRAF6, which initiates a kinase cascade that activates the kinase IKK.
• In the absence of a signal, NFκB is bound by the IκB inhibitor, which prevents NFκB from entering the nucleus.
• In the presence of a signal, activated IKK phosphorylates IκB, causing NFκB to be released from the complex; IκB is degraded.
• NFκB enters the nucleus to activate the genes encoding inflammatory cytokines.
• Messenger RNA directs the synthesis of the cytokines in the cytoplasm, and the cytokines are secreted via the endoplasmic reticulum (ER)

Question 40

Nemo deficiency symptoms

Answer 40

Fever, lethargy, weight loss, abnormal conical teeth, thin hair, thick skin, eczematous rashes, and reduced sweat glands. These patients also experience frequent infections which may be severe

Question 41

NEMO deficiency

Answer 41

An incredibly rare genetic disease caused by impairment of NF-κB activation. Children lack one of the subunits of inhibitor of κB kinase (IKK). The gene for kinase subunit IKKγ or NF-κB essential modulator (NEMO) is on X-chromosome; so disease more frequent in boys. Patients are highly susceptible to pyogenic (pus-inducing) bacteria, such as Pneumococcus sp. and Staphylococcus sp. Treatment includes IV antibody infusions and antibiotic therapy

Question 42

Cytokines

Answer 42

Small, soluble proteins used as a means of communication between cells. Innate cells respond to infection through activation (interaction with PAMPs) and secreting inflammatory cytokines in response. Cytokines are involved in regulating the development of immune effector cells and regulating the intensity/duration of an immune response. One cell type secretes a cytokine that binds to a specific cytokine receptor on the surface of another target cell. This induces signaling in the second cell.

Question 43

Interleukins (IL)

Answer 43

Secreted by some leukocytes to act on other leukocytes

Question 44

5 types of pro-inflammatory cytokines

Answer 44

TNF-α, IL-6, IL-12, IL-1β, CXCL8

Question 45

Autocrine action

Answer 45

When a cytokine binds to, and has a biological effect on, the same cell that secreted it.

Question 46

Paracrine action

Answer 46

When a cytokine binds to receptors on target cells in close proximity to the original producer cell.

Question 47

Endocrine action

Answer 47

When a cytokine travels through the circulation and binds to target cells in distant parts of the body.

Question 48

Nod-like receptors

Answer 48

A family of proteins that serve as danger signals for the host cell. They are intracellular sensors of bacterial infections. They recognize bacterial degradation products in the cytoplasm. When bacteria is phagocytosed in the lysosome of the macrophage, the bacteria’s cell components are delivered to the cytoplasm, where they are detected. NLRs have a central domain called NOD (Nucleotide-binding Oligomerization Domain). Two types- NOD1 and NOD2

Question 49

Nod-like receptors mechanism

Answer 49

1. Upon recognition of bacterial products, the CARD domain of NOD receptors dimerizes with the CARD domain of the kinase RIPK2
2. RIPK2 phosphorylates the kinase TAK1 which in turn activates IKK
3. Activated IKK mediates degradation of IκBs (Inhibitor of κB)
4. NF-κB travels to the nucleus and regulates gene expression in order to activate macrophages

Question 50

Structure of Nod-like receptors

Answer 50

They all have a central domain called NOD (Nucleotide-binding Oligomerization Domain). Their pathogen-recognition domain is made up of LRRs on the C-terminal side of the NOD domain. Their Caspase-recruitment domain (CARD) is on the amino-terminal side

Question 51

IL-1β

Answer 51

A major cytokine. Macrophages amplify the innate immune response by increasing the production of IL-1β, which is responsible for acute phase response and promotes inflammation. IL-1β is very potent and its production must be tightly regulated. Overexpression observed in autoinflammatory syndromes, autoimmune disorders, & degenerative diseases. Regulation is accomplished by the inflammasome

Question 52

Inflammasome

Answer 52

A large multi-protein complex that controls when, where, and how much IL-1β is secreted by macrophages

Question 53

Components of the NLRP3 inflammasome

Answer 53

NLRP3 molecule, Adaptor protein ASC, and Protease Caspase-1

Question 54

NLRP3 inflammasome function

Answer 54

Enables activated macrophages to produce IL-1β from stored pro-IL-1β.

Question 55

Damage-associated molecular patterns (DAMPs)

Answer 55

Some receptors of innate immunity can detect damage to cells or tissue that does not involve an infection. In this case, the receptor recognizes a damage-associated molecular pattern

Question 56

NLRP3 inflammasome formation

Answer 56

Before sensing any infection, the tissue macrophage makes and stores a large quantity of pro-IL-1β in its cytoplasm. On sensing infection (exposure to PAMPs or DAMPs), NOD-like receptor protein 3 engages its microbial ligands. This stimulates each NLRP3 molecule to recruit each of the 3 necessary units, which oligomerize to form the structure of the inflammasome. Caspase-1 is synthesized as inactive pro-caspase-1 which must undergo cleavage to become active. Active IL-1β is generated after inactive pro- IL-1β is cleaved by caspase-1

Question 57

Mutations in the genes encoding inflammasome components may lead to

Answer 57

Autoinflammatory diseases are caused by mutations in the genes encoding sensors and other inflammasome components.

Question 58

How do inflammasomes amplify the innate immune response?

Answer 58

They amplify the innate immune response by increasing the production of IL-1Β. Exposure to IL-1β can induce macrophages to produce more inactive pro-IL-1β to be cleaved by active caspase-1. Bacterial products, ROS, uric acid crystals, and ATP can all result in the formation of an active inflammasome

Question 59

How do resident macrophages respond to IL-1β?

Answer 59

By making and secreting inflammatory cytokines. TNF-α, IL-6, CXCL8, CCL2, and IL-12 are five crucial inflammatory cytokines that are secreted by macrophages after the release of IL-1β by an inflammasome. The cytokines recruit effector cells and plasma proteins to the infected tissue, where they work together to create a state of inflammation.

Question 60

Inflammation

Answer 60

A localized reaction of tissue to irritation, injury, or infection, characterized by pain, redness, swelling, heat and sometimes loss of function- can be acute or chronic.

Question 61

Why does inflammation cause pain?

Answer 61

The inflamed area is likely to be painful, especially when touched. Chemicals that stimulate nerve endings are released, making the area much more sensitive.

Question 62

Why does inflammation cause redness?

Answer 62

This is because the capillaries are filled up with more blood than usual. Swelling may also result from the accumulation of fluid from blood. More blood in the affected area can also make it feel hot to the touch

Question 63

Initiation of inflammation of infection of the skin (3)

Answer 63

1. A surface wound introduces bacteria, which activate resident effector cells to secrete cytokines
2. Vasodilation and increased vascular permeability allow fluid, protein, and inflammatory cells to leave the blood and enter tissue
3. The infected tissue becomes inflamed, causing redness, heat, swelling, and pain

Question 64

Neutrophils

Answer 64

Also called polymorphonuclear leukocytes. Short-lived and dedicated phagocytes. They are granulocytes with numerous granules in the cytoplasm. Neutrophils are the first cells recruited to sites of infection by inflammatory mediators such as CXCL8. Neutrophils are potent killers of pathogens and are themselves programmed to die. They have a range of phagocytic receptors that recognize microbial products. They also have complement receptors that facilitate phagocytosis by opsonization and complement fixation

Question 65

CXCL8

Answer 65

An inflammatory cytokine made by pathogen-activated macrophages, which recruits neutrophils from the blood to the site of infection in the tissue. CXCL8 is considered a chemokine since it directs the flow of leukocyte traffic throughout the body.

Question 66

Chemokine

Answer 66

A molecule that directs the flow of leukocyte traffic throughout the body. This movement of leukocytes between blood and tissue is mediated by interactions between adhesion molecules

Question 67

Which adhesion molecules are necessary for adhesion of leukocytes to the vascular endothelium? (4)

Answer 67

Vascular addressins, selectins, integrins, and proteins having immunoglobulin-like domains. Each family of adhesion molecules has a distinctive structure

Question 68

Selectin-mediated adhesion

Answer 68

Allows for weak adhesion of leukocytes to the vascular endothelium, and leukocytes can roll along the endothelium. Movements of leukocytes between circulation and tissue is determined by interactions of complementary adhesion molecules. Inflammation causes endothelial cells to increase expression of selectins, ICAM-1, and ICAM-2. In the same way, neutrophils express corresponding molecules like LFA-1 and sialyl-Lewis x (carbohydrate groups of glycoproteins on neutrophil surfaces)

Question 69

4 stages of extravasation

Answer 69

1. Rolling adhesion
2. Tight binding
3. Diapedesis
4. Migration

Question 70

Which interactions allow for strong neutrophil adhesion?

Answer 70

CXCL8 binds to CXCR1 and CXCR2, chemokine receptors on neutrophils. Upon entering the tissue, neutrophil’s gene expression changes, making it more phagocytic

Question 71

Diapedesis

Answer 71

When neutrophils squeeze between the endothelial cells of the blood vessels, guided by a chemokine.

Question 72

Extravasation mechanism

Question 73

Neutrophil secretory granules

Answer 73

During their development in the bone marrow, neutrophils are loaded with four types of secretory granule. The granules are loaded in the order azurophilic, specific, and gelatinase granules, followed by secretory vesicles.

Question 74

Neutrophil killing of bacteria

Answer 74

For neutrophils to kill bacteria, the phagosome must fuse with 3 types of secretory granules. Upon engulfing, degradative enzymes and toxic substances are released to rapidly kill the pathogen. The bacterium is engulfed to form an endosome that then fuses with the azurophilic, specific, and gelatinase granules of the neutrophil. The components of NADPH oxidase provided by the specific granules facilitate a respiratory burst. This raises the pH of the vesicle, which now becomes a phagosome. The antimicrobial proteins and peptides are activated by the change in pH, and they kill the bacterium. The phagosome fuses with lysosomes containing acid hydrolases, which lowers the pH and activates the hydrolases to degrade the bacterium completely. The neutrophil dies and its remains are phagocytosed by a macrophage. One neutrophil will kill many bacteria before its arsenal of weaponry and capacity for phagocytosis are exhausted.

Question 75

3 types of preformed neutrophil granules

Answer 75

1. Primary (or azurophilic)- has myeloperoxidase, lysozyme, defensins, cathepsin G
2. Secondary (or specific)- has lactoferrin protein, lysozyme, components of NADPH oxidase
3. Tertiary (or gelatinase)- has gelatinase enzyme

Question 76

Respiratory burst

Answer 76

Neutrophils rely on respiratory burst to kill bacteria. Occurs when O2 consumed by phagocytes is converted within phagosomes to toxic reactive oxygen species (ROS). Occurs via action of the enzyme NADPH phagosome oxidase

Question 77

How does the respiratory burst kill pathogens?

Answer 77

These reactions raise the pH of the phagosome to 7.8-8.0 within a couple minutes, and antimicrobial peptides get activated at this pH. The pH slowly goes down, reaching 7.0 after 10-15 min after which phagosome-lysosome fusion occurs

Question 78

Neutrophil extracellular tarps (NETs)

Answer 78

Extracellular fibers, primarily composed of DNA from neutrophils, which bind pathogens to prevent dissemination. The pathogens are also exposed to antimicrobial proteins. In this way, neutrophils protect against pathogens even after they die (they are short lived, and cannot replenish their enzymes after they are used)

Question 79

Death of neutrophils

Answer 79

Mature neutrophils cannot replenish their granule contents; once they are used up, they die. Some die by apoptosis and are then phagocytosed by a macrophage. Others die by a process called NETosis

Question 80

TNF-alpha

Answer 80

Induces blood vessels to be more permeable, enabling cells, fluid, and soluble effectors to enter infected tissue

Question 81

IL-6

Answer 81

Induces fat and muscle cells to metabolize, generate heat, and raise the temperature in infected tissue. It is involved in the acute-phase response, changing the spectrum of plasma proteins made by the liver. Concentrations increase for 30 proteins that contribute to innate immunity, but decrease for other plasma proteins like albumin

Question 82

CCL2

Answer 82

Recruits monocytes from the blood and directs them to the infected tissue

Question 83

IL-12

Answer 83

Recruits and activates natural killer cells to secrete cytokines that strengthen macrophages’ response to infection

Question 84

Chronic granulomatous disease

Answer 84

Genetic syndrome caused by defective forms of genes encoding the NADPH oxidase subunits. No respiratory burst → No killing of pathogens. These patients are more susceptible to certain infections

Question 85

Granulomas

Answer 85

Chronic infections are characterized by granulomas- small areas of inflammation in tissue. This is a way to wall off bacteria the body is unable to clear. Found in tuberculosis, A. fumigatus, and other bacterial/fungal infections

Question 86

Time course of inflammation

Answer 86

Edema begins in less than a day, neutrophils respond in around a day, and monocytes, macrophages peak around 2-3 days, taking longer to disperse

Question 87

Chronic inflammation

Answer 87

Prolonged duration (weeks or months) in which active inflammation, tissue destruction, and attempts at repair are proceeding simultaneously. It arises during persistent infections (HIV), prolonged exposure to toxic agents, or autoimmunity. Attempts at healing lead to fibrosis (scaring).
Macrophages and the secretion of pro-inflammatory cytokines are implicated. Inflammation is connected to several human diseases. The immune response is specific (where the immune response is activated).

Question 88

Acute vs chronic inflammation

Answer 88

Acute inflammation is short term, over a period of days, while chronic inflammation is long term (weeks to months). Acute inflammation is nonspecific while chronic inflammation is specific. In acute inflammation, there is active vasodilation and increased permeability, while chronic inflammation causes new vessel formation (granulation tissue).

Question 89

Pyrogens

Answer 89

Molecules that induce fever (IL-1β, IL-6, TNF-α and other molecule)

Question 90

Effects of IL-1β, IL-6, and TNF-α on organs (4)

Answer 90

1. Liver- acute phase proteins (C-reactive protein and mannose binding lectin) cause activation of complement and opsonization
2. Bone marrow endothelium- neutrophil mobilization, causing phagocytosis
3. Hypothalamus- increased body temperature, leading to decreased viral and bacterial replication
4. Fat and muscle- protein and energy mobilization to generate increased body temperature

Question 91

Acute-phase response

Answer 91

The cytokine-induced increase in the concentration of around 30 proteins within hours after inflammatory stimulus. Causes a systemic inflammatory response and increases the supply of the pathogen-recognition receptors of innate immunity. Proteins that change their concentration by 25% or more are called acute-phase proteins. C-reactive protein (CRP) and serum amyloid A’s concentrations rise by several hundredfold during the response. This effect is so strong and predictable that CRP is measured clinically to test for infection, inflammation, and tissue damage

Question 92

C-reactive protein (CRP)

Answer 92

Member of the pentraxin family of proteins. It is a pentameric molecule with a hole in the middle. CRP binds to bacteria, fungi, yeasts, and parasites, and acts as an opsonin, triggering the classical pathway of complement activation

Question 93

Serum amyloid A protein

Answer 93

A 100 amino acid protein that associates with HDL proteins. It interacts with TLRs and scavenger receptor to activate cells to produce cytokines, and data suggests it can activate the inflammasome. Induces chemotaxis of phagocytic cells

Question 94

Mannose-binding lectin (MBL)

Answer 94

A C-type lectin that binds to mannose-containing carbohydrates of pathogens- it is a member of a family of soluble proteins called collectins. It is an acute phase protein that triggers the lectin pathway upon binding to the pathogen. Can act as an opsonin, facilitating the uptake of bacteria by monocytes

Question 95

Mannose-binding lectin (MBL) structure

Answer 95

Structure resembles a flower and each stalk is a triple helix made of 3 identical polypeptides. The stalks are rigid collagen-like triple helices, with one site that can flex; each bud or flower comprises three carbohydrate-binding domains. Each polypeptide contributes a carbohydrate-recognition domain. MBL circulates in plasma as a complex with 2 zymogens: MBL-associated serine protease (MASP) 1 and 2

Question 96

Activation of the lectin pathway by mannose-binding lectin

Answer 96

When MBL complex binds to mannose-containing macromolecules at a pathogen surface, MASP-2 gets activated (protease). Activated MASP-2 cleaves C4 into C4a and C4b. Some C4b covalently binds to the microbial surface. Activated MASP-2 will also cleave C2 to C2a and C2b

Question 97

C2a function in the lectin pathway

Answer 97

After C2 is cleaved by MASP-2 into C2a and C2b, C2a binds to surface C4b, forming the classical C3 convertase C4b2a

Question 98

C4b2a

Answer 98

A C3 convertase of the lectin pathway. Binds C3 and cleaves it to C3a and C3b. C3b binds covalently to the microbial surface.

Question 99

C4bC2a and C3bBb

Answer 99

The C3 convertase produced by the classical and lectin-mediated pathways is C4bC2a. They are homologous, functionally similar enzymes that cleave C3 to activate its thioester bond. This causes C3b to become covalently bound to the pathogen surface.

Question 100

Complement component C1

Answer 100

C1 molecule consists of a complex of C1q, C1r, and C1s.
C1q contains 6 identical subunits, each with one binding site for the Fc region of antibodies (IgM, IgG). CRP binds to the C1q stalks, leading to the cleavage of C1r and C1s, resulting in activation of classical pathway of complement activation

Question 101

C1s

Answer 101

CRP binds to the C1q stalks, leading to the cleavage of C1r and C1s. C1s then goes on to cleave C4 and C2. The C2a and C4b fragments combine to form the C3 convertase of the classical and lectin pathway of complement activation: C4b2a

Question 102

Toll-like receptors (TLR) sensors that detect viral infections (3)

Answer 102

1. TLR3 senses double-strand (ds) RNA
2. TLR7 and TLR8 sense ssRNA
3. TLR9 senses hypomethylated CpG DNA.

Question 103

Cytosolic DNA sensors that detect viral infections (3)

Answer 103

AIM2 (Absent in Melanoma-2), IFI16, and cGAS

Question 104

Cytosolic RNA sensors that detect viral infections (2)

Answer 104

1. Retinoic acid inducible-I (RIG-I)
2. Melanoma differentiation-associated protein 5 (MDA5)

Question 105

RIG-I and MDA5

Answer 105

Cytosolic RNA virus sensors which signal via a unique adaptor called mitochondrial antiviral signaling (MAVS; also known as IPS). These receptors mediate NF-κB and IRF (IFN regulatory factor)-dependent transcription of inflammatory genes.

Question 106

RIG-1 like receptors (RLRs) mechanism (4)

Answer 106

1. Viral replication in an infected cell produces uncapped RNA with a 5’-triphosphate
2. RIG-1 binding to viral RNA induces association with MAVS
3. MAVS associations initiate signal via TRAG6 that activates IRF3 and IRF7
4. IRF3 and IRF7 turn on the synthesis and secretion of IFN-beta and IFN-alpha, respectively

Question 107

AIM2

Answer 107

Absent in melanoma-2, recognizes viral double-stranded DNA. Induces IL-1β secretion

Question 108

Cyclic GMP-AMP synthase (cGAS)

Answer 108

Recognizes viral DNA. It activates ER protein STRING to trigger type 1 interferon production

Question 109

Interferon-response factor 3 (IRF3)

Answer 109

Works with NF-κβ and AP-1 to turn ON IFN-β synthesis

Question 110

IFN-β actions (2)

Answer 110

1. Autocrine: IRF7 and IFN-α secretion
2. Paracrine: increased resistance to viral infection

Question 111

Plasmacytoid dendritic cells

Answer 111

Plasmacytoid dendritic cells are factories for making large quantities of type I interferons. They combine features of lymphocytes and myeloid dendritic cells and are present in blood and lymphoid tissue. They detect viral infection by using TLR7 and TLR9- MyD88 signaling leads to the activation and translocation of IRF7 to nucleus. Results in transcription of Type I IFN gene. Type I Interferon prevents the spread of infection

Question 112

Important functions of NK cells (3)

Answer 112

1. Kill the cell infected with virus
2. Increase inflammation in the infected tissue
3. Secrete inflammatory cytokines that act on macrophages to make them activated

Question 113

Natural killer (NK) cells

Answer 113

Natural killer cells are the main circulating lymphocytes that contribute to the innate immune response. NK cells are large active lymphocytes that are induced rapidly in response to infection. Abundant in blood (5-25%)

Question 114

Induced innate immune response

Answer 114

The innate immune response to a pathogen that has overcome the immediate defenses of the body. Pathogens that expand their population at a greater rate than the pathogen is being killed or eliminated require the induced innate immune response to be triggered. This induced component can take up to 4 days to be activated

Question 115

3 phases of an immune response to a pathogen

Answer 115

1. Initial phase- begins as soon as the pathogen invades. Sufficient to terminate most infections
2. Induced innate response
3. Adaptive immune response- occurs after 4 days of infection if the immediate and induced responses aren’t effective

Question 116

Phagocytosis by macrophages

Answer 116

Cell-surface receptors in macrophage are used to capture pathogens and deliver them to the acidic endosomes through the process of receptor-mediated endocytosis. From an endosome, the pathogen is taken through a series of vesicles where the pathogen is killed and partly degraded. The lysosome is the ultimate destination, where extreme acidity and degradative enzymes completely degrade the pathogen.

Question 117

Adaptor protein

Answer 117

Protein in an intracellular signaling pathway that has no enzymatic or other activity in itself but brings two other components of the pathway together so that they can interact

Question 118

Allotypes

Answer 118

A naturally occurring variant of a protein. Allotypes are encoded by different alleles of the same gene

Question 119

Genetic polymorphism

Answer 119

The existence of two or more alleles of a given gene within a population, leading to variation between individuals

Question 120

TLR4 polymorphism

Answer 120

Polymorphism is more common in TLRs that recognize surface determinants of proteins. TLR4 polymorphism influences severity of disease. For example, the rarer TLR4 allotype with glycine 299 confers a greater susceptibility to septic shock than the more common allotype with asparagine 299

Question 121

NOD1

Answer 121

Soluble cytoplasmic receptor. Recognizes a degradation product of the peptidoglycan of gram-negative bacterial cell walls. Expressed in many cell types

Question 122

NOD2

Answer 122

Soluble cytoplasmic receptor. Recognizes a degradation product of bacterial peptidoglycan. Expression is restricted to myeloid and cytoplasmic cells

Question 123

Caspases

Answer 123

A type of protease that is involved in generating some cytokines from their inactive pro-proteins as part of the inflammasome. Caspases also have many non-immune functions

Question 124

Type 1 interferons

Answer 124

A collective name for interferons alpha and beta. They are cytokines produced by virus-infected cells that interfere with viral replication by the infected cell. They also signal to neighboring infected cells to prepare to resist infection. Type 1 interferons have functions similar to those of cytokines

Question 125

Interferon receptors

Answer 125

They are constitutively expressed on human cell surfaces, ready to bind interferon in response to infection

Question 126

Interferons

Answer 126

A general name for a family of cytokines that induce cells to resist viral infection. They signal to neighboring cells to prepare to resist infection. Also, they alert the immune system that infection is present and cause virus-infected cells to become more vulnerable to attack by NK cells. All nucleated human cells can be infected by viruses, and they all make interferon. Interferons are barely detectable in the blood of healthy people but will increase when infection is present

Question 127

Pyroptosis

Answer 127

A type of programmed cell death that results in inflammation and is the means by which large amounts of cytokine IL-1β are released from storage in macrophages. Pores are formed in the cell membrane through which the cytokines and cell contents leak out, resulting in the eventual death of the macrophage. Only a small amount of macrophages in infected tissue will die in this way. The cytokines released by one pyroptotic macrophages are usually enough to activate multiple other macrophages

Question 128

Autoinflammatory diseases

Answer 128

A type of disease characterized by chronic and recurrent bouts of systemic inflammation mediated by cells of innate immunity. These diseases do not involve adaptive immunity.

Question 129

Adhesion molecules

Answer 129

Any cell-surface proteins that allow human cells to bind to each other. Movements of leukocytes between blood and tissue are determined between complementary pairs of adhesion molecules- one is present on the leukocyte surface and the other is present on a tissue-cell surface

Question 130

What triggers neutrophils to interact with the vascular endothelium?

Answer 130

In the absence of infection, neutrophils move rapidly through capillaries and don’t interact with the vascular endothelium. In infected tissue, inflammatory cytokines instruct the blood vessels to dilate, while inducing the endothelial cells to express selectin adhesion molecules. Blood flow decreases, which allows the neutrophils to contact the vascular endothelium within post-capillary venules. There are transient interactions between selectins on the endothelium and the sialyl-Lewisx carbohydrate of glycoproteins on the neutrophil surface. These interactions impede the neutrophils, causing them to roll along the endothelial surface

Question 131

Importance of fever

Answer 131

It impairs the growth and replication of bacteria and viruses. In the presence of fever, tissue cells become more resistant to damage by TNF-alpha , which is an inflammatory cytokine. Other cytokines induce the lethargy, fatigue, and anorexia that come along with fever. This also helps to fight infection by preventing the body from using energy on other functions.