Innate and Acquired Immunity

Question 1

What are defensins?

Answer 1

• An array of antimicrobial peptides that protect almost every part of our bodies from a wide range of pathogens; ubiquitous, and produced by many cell types
• At a minimum, their production increases the minimum infectious dose of microorganisms
  1. The higher inoculum that is needed to cause a disease, the less likely an organism is to cause symptomatic infection

Question 2

Describe the shape of the defensins. How does this shape determine their MOA?

Answer 2

• Amphipathic, alpha-helical structure (hydrophobic AA
  side chains on one side and hydrophilic AA side chains on other side): rich in + charged AA (ARG and LYS)
• Multiple copies can multimerize via hydrophobic interaction, and overall + charge causes them to insert across (-) charged outer bacterial envelope (more effective in G- bugs)
• Once inserted, the hydrophobic faces of each peptide monomer interact w/the hydrophobic lipids of the outer
  envelope, while the hydrophilic faces come together to form a pore in the surface of the pathogen (bactericidal)

Question 3

What is the primary source of defensins in the gut?

Answer 3

Paneth cells

Question 4

What are cathelicidins?

Answer 4

• A type of antimicrobial peptide (distinct from defensins) that is produced in lysosomes of macrophages, neutrophils, and epithelial cells
• Secondary structure of alpha helical and beta sheet motifs

Question 5

Is the MAC more effective against G- or G+ bacteria?

Answer 5

• Much more effective as a defense against G- (compared with G+) bacteria because of the differences in their outer envelop construction

Question 6

Explain macrophage-mediated initiation of inflammation.

Answer 6

1. Injury to the skin w/bacterial pathogen in tissue
2. Resident macrophage recognizes the bacterium via
   PRRs and produces inflammatory cytokines to activate vascular endothelium, resulting in movement of fluid/inflammatory cells into tissue
3. Newly recruited phagocytes take up bacteria they encounter and produce more inflammatory cytokines (IL-1, IL-6, and TNF-alpha)
4. Mvmt of fluid into tissues results in flow of fluid via the lymphatics toward the draining lymph nodes where APCs can present antigen to naïve T cells
5. In many cases, innate phagocyte-mediated response can completely clear bacterial infection before symptoms
6. If not, an amplified innate response will be produced to hold infection down until acquired response primed

Question 7

What is NETosis?

Answer 7

• A type of cell death that neutrophils can undergo that results in what is essentially the opposite of apoptosis
• Instead of the genome being digested (like in apoptosis), it is expelled from the cell in undigested form, allowing it to create a web or net of chromatin material that can entrap pathogens, preventing their dissemination
• Also results in expulsion of cytoplasmic contents, including digestive enzymes, anti-microbial peptides, etc.

Question 8

What are endocytic PRRs?

Answer 8

• Promote phagocytosis of microorganisms by phagocytes without relaying an IC signal
  1. Primarily recognize carbohydrates, i.e., mannose, glucan, and scavenger receptors

Question 9

What are signaling receptors? Briefly describe the two types.

Answer 9

• Toll-like receptors (TLRs): recognize microbial constituents (LPS, flagellin, peptidoglycans, etc.), and mediate signaling that initiates cytokine production
• NOD-like receptors: cytoplasmic proteins that have affinity for microbial products; initiate signaling that results in production of inflammatory cytokines

Question 10

Identify four specific TLRs, and their primary functions.

Answer 10

• TLR2/6: heterodimers that recognize lipoteichoic acid (G+ bacteria) and zymosan (fungi)
• TLR3: bind to double-stranded viral RNA
• TLR4: heterodimers serve as receptor for LPS (G- bacteria)
• TLR7/8: recognize single-stranded viral RNA

Question 11

What are secreted PRRs? Provide at least 2 examples.

Answer 11

• PRRs secreted from host cells (e.g., complement receptors, serum amyloid, c-reactive protein, mannose-binding protein)
  1. MBL: secreted PRR and acute phase reactant involved in initiation of the Lectin Complement Pathway
  2. C-reactive protein: binds to phosphocholine residues to initiate the Classical Complement Pathway (also an acute phase reactant)

Question 12

List 5 of the cytokines released by macrophages in the innate response, and briefly describe their functions.

Answer 12

1. IL-6: fever, acute-phase reactant production by liver
2. TNF-alpha: activates vascular endothelium and increases vascular permeability locally, and also induces fever, mobilization of metabolites, and shock systemically
3. IL-1beta: activates vascular endothelium and lymphocytes locally, and induces fever and produces IL-6 systemically
4. CXCL8: chemotactic factor for neutro/basophils
5. IL-12: activates NK cells

Question 13

How do IL-1, IL-6, and TNF-alpha released by macrophages work synergistically to initiate the innate immune response?

Answer 13

1. Initiation of the acute phase response: stimulate the production of C-reactive protein and MBL
2. Initiate neutrophil mobilization by acting on bone marrow endothelium, resulting in increased phagocytosis and killing of the pathogen.
3. Act on both the hypothalamus and on fat and muscle to increase body temperature

Question 14

What is fibrinogen?

Answer 14

A critical acute phase protein important for the clotting cascade

Question 15

How does TNF-alpha localize inflammation and help limit the infection?

Answer 15

• Activates vascular endothelium, causing leakage of fluids into the tissues, increasing diapedesis of phagocytes from the circulation into the tissues and platelet
  adhesion to the walls of small blood vessels
• This results in occlusion of the blood vessels and prevents the bacteria form disseminating via the
  blood, allowing time for local phagocytes to engulf and kill the organisms

Question 16

How can macrophage-derived TNF-alpha provoke septicemia?

Answer 16

• Once a bacterial infection becomes septic, macros in the liver and spleen make TNF-alpha that is secreted directly into the bloodstream
• TNF-alpha now acts systemically on the vasculature, inducing systemic leakage of fluids from the vasculature and adherence of platelets to the walls of small vessels
• Small vessels become occluded due to a combination of reduced blood volume and platelet adhesion, and many of them collapse
• This is particularly problematic in organs and can result in multiple organ failure, septic shock, and ultimately, death

Question 17

How do type 1 interferons mediate the innate response to viral infection?

Answer 17

• Virus-infected cells release IFN-alpha and IFN-beta, provoking the interferon response
  1. Induce upregulation of MHC I/inhibitory ligand expression on surrounding cells
  2. Activate NK cells to kill virally infected cells (MIC proteins expressed by stressed cells)
  3. Increase expression of ligands for receptors on NK cells (NK cells are programmed to kill MHC I-deficient cells; do not need to express activation ligand)

Question 18

Describe the concept of antigen threshold, and its purpose.

Answer 18

• Immune system is designed to respond to a wide range of antigens, but very low levels of antigen (because not indicative of infective process that would involve pathogen replication) -> a minimum threshold of antigen must be met before immune system will respond
• Once that threshold is met, innate response initiates inflammatory responses that control pathogen replication and promote antigen presentation to develop acquired response

Question 19

How do dendritic cells sample antigen in the gut lumen?

Answer 19

They can extend from underlying mucosal surfaces into the lumen

Question 20

What percentage of our body’s lymphocytes reside in the MALT?

Answer 20

More than 50%

Question 21

What are the two primary effector functions of CD4 effector cells?

Answer 21

1. Supplying the second signal of activation to B cells
2. Activation of macrophages

Question 22

What are memory T cells?

Answer 22

• A clonally expanded population of cells specific for a determinant of a pathogen that will live a very long time in the secondary lymphoid tissues
• Re-exposure to the pathogen will result in a much more rapid and effective T-cell response

Question 23

What are the 4 cytotoxic effector molecules produced by CTLs?

Answer 23

1. Perforin
2. Granzymes
3. Granulysin
4. Fas-L

Others: IFN-gamma, LT-alpha, and TNF-alpha

Question 24

What are the 5 macrophage-activating effector molecules made by TH1 cells?

Answer 24

1. IFN-gamma
2. GM-CSF
3. TNF-alpha
4. CD40 ligand
5. Fas-L

Others: IL-3, LT-alpha, CXCL2

Question 25

What are the 4 B-cell activating effector molecules of TH2 cells?

Answer 25

1. IL-4 2. IL-5 3. IL-13 4. CD40 Others: IL-3, GM-CSF, IL-10, TGF-beta, CCL11 (eotaxin), CCL17

Question 26

What are the 3 neutrophil recruiting effector molecules of TH17 cells?

Answer 26

1. IL-17A 2. IL-17F 3. IL-6 Others: TNF, CXCL1

Question 27

What are the two suppressive cytokines released by Treg cells?

Answer 27

1. IL-10 2. TGF-beta Others: GM-CSF

Question 28

What drives differentiation of TH0 cells into TH1 and TH2?List two for each pathway.

Answer 28

1. TH1: IL-12 and IFN-gamma 2. TH2: IL-4 and IL-6

Question 29

What cytokines are released by dendritic cells and NK cells to promote TH0 cell differentiation?

Answer 29

1. Viruses and some bacteria: a. Dendritic cells- IL-12 b. NK cells- IFN-gamma 2. Other pathogens (e.g., worms): a. NK cells- IL-4

Question 30

What are the 3 primary effector functions of TH1 cells?

Answer 30

1. Macrophage activation 2. Production of IL-2 that can help drive proliferation of activated T cells 3. Production of cytokines/chemokines that promote production of macrophages and neutrophils, and their migration to the inflammatory site

Question 31

What are the 6 primary effector molecules released by TH1 cells? Describe their actions. Hint: some are released as pairs.

Answer 31

1. IFN-gamma and CD40 ligand: activate macrophages to destroy engulfed bacteria 2. Fas-L or LT: kill chronically infected macros, releasing bacteria to be destroyed by healthy macros 3. IL-2: induces T-cell proliferation, increasing number of T cells 4. IL-3 and GM-CSF: induces macro differentiation in the bone marrow 5. TNF-alpha and LT: activates endothelium to induce macro adhesion and exit from blood vessel at site of infection 6. CXCL2: causes macros to accumulate at site of infection

Question 32

What two activation stimuli are require for full B-cell activation?

Answer 32

1. Recognition of cognate antigen through Ig component of BCR 2. TH cell that includes CD-40 ligand binding to CD40 on B cell and cytokines derived from helper T cell

Question 33

What Ab isotypes are most important for neutralization?

Answer 33

IgG1-G4, IgA

Question 34

What Ab isotypes are best for sensitization for killing by NK cells (initiating ADCC)?

Answer 34

IgG1 and IgG3

Question 35

What Ab isotypes are best for activation of the complement system?

Answer 35

IgM (+++), IgG3 (+++), and IgG1 (++)

Question 36

What Abs are best for transport across the placenta?

Answer 36

IgG1 (+++), IgG3 (++), IgG4 (++), and IgG2 (+)

Question 37

What are 4 ways of recognizing and killing virus infected host cells?

Answer 37

there are 4 mechanisms of recognition and killing of virus infected host cells: 1) recognition of high ratio of MIC to MHC class I by NK cells, 2) recognition of MHC class I-deficient cells by NK cells, 3) IgG1 and or IgG3 bound to surface proteins of infected cells [that will typically be viral envelope proteins] resulting in ADCC by NK cells, and 4) CTL recognition of cognate virus-derived peptides bound to MHC class I molecules via their TCR.