Innate Immunity
The Immune System, Third Edition
2009 Garland Science Publishing
Questions
2–1 Explain why the type of innate immune response used by the host during an infection is dependent upon the particular location in which the pathogen resides. 2–2 Explain three mechanisms by which pathogens exert damaging effects upon host cells during the course of an infection, and provide an example of a pathogen for each effect. 2–3 Which of the following does not accurately describe complement components? a. soluble proteins b. made by the spleen c. located in extracellular spaces d. some function as proteases once activated e. activated by a cascade of enzymatic reactions. 2–4 Explain why a genetic deficiency of C3 leads to a type of immunodeficiency characterized by recurrent and severe infections. 2–5 The lectin pathway of complement activation is induced by: a. Creactive protein b. antibodies bound to pathogens c. mannosebinding lectin d. C3Bb e. terminal components of the complement pathway. 2–6 Which of the following is the membranebound form of C3 convertase of the alternative pathway of complement activation? a. iC3b. C3a c. C3b d. iC3Bb e. C3bBb. 2–7 Explain how the alternative C3 convertase on pathogen cell surfaces is (A) formed and (B) stabilized. 2–8 Why is it important to expose the hydrophobic sites of C7 and C8 during the formation of the membraneattack complex? 2–9 The plasma proteins that counteract the activity of factor P by inactivating C3 convertase through the cleavage of C3b are: a. factor B and factor H b. factor H and factor I c. factor B and factor I d. decayaccelerating factor and factor H e. decayaccelerating factor and membrane cofactor protein. 2–10 The membranebound proteins on human cells that dissociate and inactivate alternative C3 convertase to avoid complement activation are: a. factor B and factor H b. factor H and factor I c. factor B and factor I d. decayaccelerating factor and factor H e. decayaccelerating factor and membrane cofactor protein. 2–11 Explain the similarities between MCP, factor H, and CR1 in terms of their complement control properties. 2–12 Explain how the anaphylatoxins C3a and C5a contribute physiologically to inflammation during complement activation. 2–13 Which of the following complement components is an opsonin that binds to complement receptor 1 (CR1) on macrophages? a. C3b 2
b. C3a c. Bb d. Ba e. C3bBb. 2–14 Which of the following polymerizes to form a transmembrane channel that compromises the integrity of cell membranes? a. C5 b. C6 c. C7 d. C8 e. C9. 2–15 Which of the following are important in anchoring the membraneattack complex to the membrane? a. C3 and C5 b. C5 and C6 c. C6 and C7 d. C7 and C8 e. C8 and C9. 2–16 Which of the following does not contain a glycosylphosphatidylinositol (GPI) lipid tail? a. decayaccelerating factor (DAF) b. homologous restriction factor (HRF) c. membrane cofactor protein (MCP) d. protectin (CD59) e. all of the above contain a GPI tail. 2–17 The ligand for CR3 and CR4 formed by the cleavage of C3b by the combined action of factors H and I is called: a. C3bBb b. C3a c. C3b2Bb d. iC3b e. C5b. 2–18 Which of the following does not describe the actions of the coagulation system?
a. blood clot formation b. enhancement of dissemination of microbes into lymphatics and bloodstream c. decrease in blood loss and fluid into interstitial spaces in tissues d. release of inflammatory mediators by platelets e. wound healing. 2–19 Which of the following is not a characteristic of mannosebinding lectin? Select all the correct answers. a. acts as an opsonin by binding to mannosecontaining carbohydrates of pathogens b. synthesized by hepatocytes c. induced by elevated IL6 d. a member of the pentraxin family e. triggers the alternative pathway of complement activation. 2–20 Which of the following is not a characteristic of Creactive protein? a. acts as an opsonin by binding to phosphocholine of pathogens b. synthesized by spleen c. induced by elevated IL6 d. a member of the pentraxin family e. triggers the classical pathway of complement activation. 2–21 Damage to tissues triggers a cascade of plasma proteins involving bradykinin and is known as: a. the alternative pathway of complement b. the coagulation system c. the kinin system d. receptormediated endocytosis e. the acutephase response. 2–22 Which of the following does not describe defensins? a. highly conserved with few variants b. contain a large proportion of arginine residues c. contain three intrachain disulfide bonds d. amphipathic, with hydrophobic and hydrophilic regions e. disrupt pathogen membranes by penetrating and disrupting their integrity. 2–23 Describe the two different domains of TLRs and their respective functions. 4
2–24 Explain the consequence of engagement of the TLR4, CD14, and MD2 complex with LPS in macrophages. 2–25 Which of the following TLRs do not use a signal transduction cascade involving MyD88? a. TLR1:TLR2 b. TLR3 c. TLR4 d. TLR2:TLR6 e. TRL7. 2–26 Which of the following TLR3 and TLR4 adaptor proteins participates in the activation pathway that culminates in the synthesis of type I interferons? a. Creactive protein b. MyD88 c. LPSbinding protein d. TRIF and TRAM e. NFB. 2–27 Explain specifically how the systemic release of TNF by macrophages causes (A) septic shock, (B) disseminated intravascular coagulation, (C) organ failure, and (D) hemorrhaging during infection with bloodborne pathogens. 2–28 Which of the following is not a characteristic of septic shock? a. organ failure b. high mortality rate c. compromised blood supply to vital organs d. blood vessel constriction e. disseminated intravascular coagulation. 2–29 Which of the following properties is common to macrophages and neutrophils? a. life span b. anatomical location c. ability to phagocytose d. morphology e. formation of pus.
2–30 Which of the following best describes an endogenous pyrogen? a. cytokines made by pathogens that decrease body temperature b. pathogen products that decrease body temperature c. pathogen products that increase body temperature d. cytokines made by the host that decrease body temperature e. cytokines made by the host that increase body temperature. 2–31 Which of the following is an acutephase protein that enhances complement fixation? a. TNF b. mannosebinding lectin c. fibrinogen d. LFA1 e. CXCL8. 2–32 A. Using the table below, match the local and systemic effects in column A with the appropriate cytokine in column B. Note that more than one answer in column B may be used. B. (i) Which of these cytokines are produced by macrophages? (ii) Which cells produce the other(s)? Column A Column B a. activation of bloodvessel endothelium 1. IL1 b. fever 2. IL6 c. induction of IL6 synthesis 3. CXCL8 d. increase in vascular permeability 4. IL12 e. localized tissue destruction 5. TNF f. production of acutephase proteins by hepatocytes 6. type I interferons g. induction of resistance to viral replication h. activation of NK cells i. leukocyte chemotaxis j. activation of binding by 2 integrins (LFA1, CR3) k. septic shock l. mobilization of metabolites 2–33 6
Describe in chronological order the four steps involved in the extravasation of neutrophils to infected tissue sites during an innate immune response. Use the following terms in your description: rolling adhesion, tight binding, diapedesis, migration, inflammatory mediators, integrins, adhesion molecules, chemokines, Weibel–Palade bodies, Pselectin, Eselectin, sialylLewisx, and basement membrane protease. 2–34 During inflammation, host tissue may be damaged owing to the release of toxic oxygen derivatives produced by activated macrophages and neutrophils. Explain what cellular mechanisms limit these damaging bystander effects. 2–35 Describe the two structural types of NKcell receptor.
Answers
2–1 The innate immune response must be of the appropriate form to access the particular compartment of an ongoing infection. If a pathogen exists in extracellular spaces or surfaces, then soluble molecules such as complement and antimicrobial peptides are effective in eradicating the pathogen. Some pathogens, however, reside in intracellular compartments, such as the nucleus, cytosol, or vesicles. If the pathogens are enclosed within a vesicle, the innate immune response instructs the infected cell to direct antimicrobial mechanisms to the interior of the vesicle to eradicate viable microbes. Macrophages carry out this type of antimicrobial activity. If the pathogens are replicating free in the nucleus or cytosol, then innate mechanisms are more aggressive and are aimed at killing the infected cell. Natural killer (NK) cells mediate this type of activity. 2–2 (i) Pathogens secrete exotoxins that interact with a specific receptor on host cell surfaces and damage or kill the cell. An example is the cholera exotoxin produced during infection with Vibrio cholerae. (ii) Some pathogen components act as endotoxins. When released from the pathogen and bound to macrophages, endotoxins induce the production of cytokines that act locally and systemically. An example is the endotoxin released from Yersinia pestis, the etiological agent of the plague. (iii) Some pathogens grow inside host cells and damage them directly. An example is the influenza virus, the cause of influenza. 2–3: b 2–4 C3 is a key element in the initiation of the complement cascade in all three pathways of complement activation, namely the alternative, lectin, and classical pathways. Its cleavage into C3a and C3b occurs early in the complement cascade. C3a acts as an inflammatory mediator and recruits inflammatory cells to the site of infection. C3b becomes fixed to the pathogen surface and facilitates the opsonization of pathogens by phagocytes and the assembly of complement components for perforation of the pathogen membrane. In the absence of C3, all three pathways of complement activation would be arrested and extracellular pathogens would escape immune detection until adaptive immune mechanisms develop fully many days later. 2–5: c 2–6: e 2–7(A) Spontaneous hydrolysis of C3 without cleavage exposes its highly reactive thioester bond, forming iC3. Factor B binds to iC3, is cleaved by factor D, and consequently releases a small fragment called Ba. The larger fragment, Bb, remains associated with iC3 to form iC3Bb, a soluble C3 convertase, which cleaves C3 into C3a and C3b. The reactive thioester bond of C3b is attacked by R-OH and R-NH2 groups on the surface of
the pathogen, where it becomes anchored and binds to factor B. Factor D then cleaves factor B, releasing fragment Ba and forming C3bBb on the pathogen surface.
(B) Factor P (properdin) binds to C3 convertase (C3bBb) bound to the pathogen surface, and inhibits the proteolytic degradation of C3bBb. This stabilizes the C3 convertase and enhances the rate of C3b deposition on the pathogen surface.
2–8
The hydrophobic sites of C7 and C8 enable anchoring of these two complement components into the membrane of the pathogen. Once anchored in the membrane, the hydrophobic site of C8 facilitates C9 polymerization, which completes the formation of the membrane-attack complex.
2–9: b 2–10: e
2–11
MCP, factor H, and CR1 all bind to C3b and render it susceptible to proteolytic cleavage by factor I. All three contain complement control protein (CCP) modules and are
therefore considered regulators of complement activation (RCA).
2–12
G-protein-coupled receptors for the anaphylatoxins C3a and C5a are found on
phagocytes, mast cells, and the endothelial cells of blood vessel walls. Anaphylatoxin bound to mast cells causes them to degranulate, releasing inflammatory mediators such as histamine and leading to increased vascular permeability. Through their action on
endothelial cells, anaphylatoxins exert vasoactive effects on blood vessels, contributing to increased vascular permeability and increased blood flow, which facilitate the
extravasation of plasma proteins, such as complement proteins and antibodies, and the recruitment of cells to infected tissues through increased adherence and chemotaxis. Phagocytic activity is enhanced by anaphylatoxins, which bring about increased levels of CR1 and CR3 and microbicidal activity. All these activities enhance inflammation.
2–13: a 2–14: e 2–15: d 2–16: c
2–17: d 2–18: b 2–19: d, e 2–20: b 2–21: c 2–22: a 2–23 The first domain of the TLR is an extracellular domain, also known as the pathogen recognition domain, which contains a hydrophobic, leucinerich repeat region (LRR) which forms a horseshoeshaped structure that binds specifically to arrays on microbial surfaces. The second domain of the TLR is the cytoplasmic signaling domain, also known as the Toll–interleukin receptor (TIR) domain, which facilitates the transmission of information to the interior of the cell. 2–24 When TLR4 on the surface of macrophages is bound to its LPS ligand, a signal transduction cascade is initiated that mediates signaling between the cell surface and the nucleus. The macrophage in turn begins to express particular genes encoding inflammatory cytokines that are important for the initiation of adaptive immune responses. 2–25: b 2–26: d 2–27 A. TNF induces the activation of endothelial cells lining blood vessels. This in turn results in the dilation of blood vessels, increased vascular permeability, and the subsequent leakage of fluid from the blood across the endothelium and into the interstitial spaces of tissue. The fluid consists of complement components and antibodies, which are released in response to TNF to help combat infection in tissues. But when large amounts of TNF are produced throughout the body as the result of a systemic infection, blood pressure is compromised; blood volume decreases, leading to shock associated with low blood pressure (hypovolemic shock). 10
B. TNF also triggers the release of plateletactivating factor from activated endothelium. Plateletactivating factor initiates the bloodclotting cascade, consuming bloodclotting factors and causing localized blood vessel blockage at sites of TNF production by resident macrophages in various organs, for example the Kupffer cells of the liver and splenic macrophages. When this occurs on a large scale, it is known as disseminated intravascular coagulation. C. The blood supply to organs is choked off as a result of disseminated intravascular coagulation, leading to organ failure. D. As bloodclotting factors are consumed by disseminated intravascular coagulation, they are not available elsewhere to help repair broken blood vessels. As a result, hemorrhaging occurs, causing petechiae to develop, which present as small purple spots under the skin. 2–28: d 2–29: c 2–30: e 2–31: b 2–32 A. a. Activation of blood vessel endothelium 1, 5 b. Fever 1, 2, 5 c. Induction of IL6 synthesis 1 d. Increase in vascular permeability 5 e. Localized tissue destruction 1 f. Production of acutephase proteins by hepatocytes 2 g. Induction of resistance to viral replication 6 h. Activation of NK cells 4, 6 i. Leukocyte chemotaxis 3 j. Activation of binding by 2 integrins (LFA1, CR3) 3 k. Septic shock 5 l. Mobilization of metabolites 5 B. (i) The cytokines IL1, IL6, CXCL8, IL12, and TNF are produced by macrophages. (ii) Type I interferons can be produced by many different types of cell when infected with a virus. Specialized cells called interferonproducing cells (IPCs) or natural interferonproducing cells (NIPCs) produce large amounts of interferon. 2–33 Step 1: dilation of blood capillaries, combined with the binding of neutrophil sialyl Lewisx carbohydrate to selectins, a type of adhesion molecule, expressed on activated endothelium, slows down neutrophils as they roll along the endothelium, binding
reversibly to the endothelial selectins, a process called rolling adhesion. Two types of selectin are expressed on activated endothelium in response to inflammatory mediators. The first, Pselectin, is stored in preformed granules called Weibel–Palade bodies and is expressed soon after endothelial activation. Later, Eselectin will appear on the surface of activated endothelium in response to TNF or LPS. Step 2: in response to the chemokine CXCL8, neutrophil integrins LFA1 and CR3 increase their affinity for the endothelial adhesion molecule ICAM1 through conformational changes. Leukocytes are now engaged in tight binding to the endothelium and the rolling stops. Step 3: the neutrophil squeezes between the endothelial cells. When the basement membrane is encountered, proteases are secreted by the neutrophil, the basement membrane is degraded, and the process of diapedesis is complete. Step 4: migration to the focus of infection is mediated through a chemokine (CXCL8) gradient. Neutrophils bearing IL8 receptors will travel toward the highest concentration of IL8 until they arrive at the focus of infection. 2–34 Toxic oxygen species including superoxide, hydrogen peroxide, singlet oxygen, hydroxyl radical, hypohalite, and nitric oxide are produced during the respiratory burst in macrophages and neutrophils. Simultaneous extraphagosomal production of enzymes that neutralize these compounds occurs. Specifically, superoxide dismutase metabolizes superoxide to hydrogen peroxide, which is further metabolized by catalase to innocuous water and molecular oxygen. 2–35 NKcells have two main structural types of extracellular receptor. The NKcell immunoglobulinlike receptor contains immunoglobulin domains that interact with protein ligands on target cells. The NKcell lectinlike receptor resembles the mannose binding lectin structurally, but can interact with protein and carbohydrate ligands on target cells. Activating receptors contain charged amino acid residues spanning the transmembrane region, which associate with intracellular signaling proteins that will initiate the activation of NK cells. Inhibitory receptors contain long cytoplasmic tails bearing an immunoreceptor tyrosinebased inhibitory motif (ITIM), which inhibits NK cell activation. 12
