Chapter 24- Innate immune system

Question 1

Innate immunity

Answer 1

Defenses that are at the ready (do not need to develop) and are non-specific. Includes epithelial tissue and components, and components of the immune system

Question 2

Acquired immunity

Answer 2

Takes a few days to develop and is very specific and effective. Includes lymphocytes (B and T cells) and antibodies

Question 3

General components of the immune system (4)

Answer 3

1. Phagocytic cells
2. Cytotoxic cells
3. Complement
4. Antigen-presenting cells

Question 4

General mechanisms of defense (4)

Answer 4

1. Epithelial tissues and tight junctions
2. Lysozyme – tears, saliva
3. Microflora
4. Mucosa

Question 5

Epithelial tissues and tight junctions- mechanisms of defense

Answer 5

Epithelial cells have many types of junctions linking the cells together, especially tight junctions. The skin is the most effective barrier (corneocytes). GI, respiratory, urogenital tracts are effective but pathogens can be wily here

Question 6

Mucosa

Answer 6

Mucus itself- microbes may get trapped, and coating with mucus may prevent microbial adherence. In the respiratory tract, mucus + cilia = expulsion of the pathogen

Question 7

Mucosal epithelia

Answer 7

Acidic pH in the stomach and digestive enzymes protect the stomach and upper GI tract. Antimicrobial peptides (cationic): α-defensins and β-defensins are found in different areas of the body. They bind to and disrupt the membranes of pathogens.

Question 8

Antimicrobial peptides (2)

Answer 8

1. α-defensins- Paneth cells of small intestine
2. β-defensins- Epithelial cells of skin and respiratory tract

Question 9

Microflora

Answer 9

Bacteria that colonizes the body. Some of these bacteria produce antimicrobial compounds (like E. coli make colicins). Bacteria in the microflora have a niche that they want to protect

Question 10

Models of antimicrobial peptide function (2)

Answer 10

1. Detergent-carpet model- the antimicrobial peptides work like detergents and break the membrane apart
2. Pore formation in the microbial membrane
   Unclear which model is true

Question 11

Cellular immunity

Answer 11

Includes all of the cells that are important in immune responses. Phagocytes, cytotoxic cells, TC cells (CD8), TH1 cells (CD4), Ag-presenting cells

Question 12

Humoral immunity

Answer 12

Includes the soluble immune components, like proteins, in the serum of the blood. Includes cytokines, chemokines, complement, antibodies (secreted from B cells, TH2 cells (CD4))

Question 13

Cellular players in innate immunity

Answer 13

Phagocytic cells- neutrophils, macrophages, dendritic cells, natural killer (NK) cells. They are important for destroying pathogens and are also important for secreting cytokines/chemokines to attract other immune cells to the site

Question 14

Humoral players in innate immunity

Answer 14

Complement system, chemokines and cytokines are also present here. Includes serum proteins & proteases produced by liver

Question 15

Complement system

Answer 15

Composed of serum proteins and proteases- present in the soluble phase of blood. They are proteolytic protein molecules, so signaling is propagated by protein cleavage. There is a cascade of cleavage events largely confined to surface on which it is initiated (this is usually the pathogen surface). May be specific or non-specific, although usually this system is non-specific like most of the innate immune system. Includes antibodies, sugars, spontaneously hydrolysis

Question 16

3 pathways of the complement system

Answer 16

1. Classical- can be a specific pathway
2. Alternative
3. Mannan-binding lectin

Question 17

3 effects of the complement system

Answer 17

1. Inflammatory cell recruitment
2. Opsonization- increases phagocytosis
3. Formation of a membrane attack complex (MAC)

Question 18

How do the 3 complement pathways differ?

Answer 18

They differ in how the complement is recruited and in their initial signaling steps

Question 19

Classical pathway

Answer 19

Require that antibodies are bound to antigens on the pathogen surface first. This is the only pathway in which the complement can be specific. Then, the antibodies recruit the initial complement factor, which is C1q. That initiates the cascade in which an enzyme called C3 convertase is formed

Question 20

Lectin pathway

Answer 20

Lectins are molecules that bind sugars. The complement binds directly to sugars on pathogen surfaces. MBL (mannin binding lectin) has an affinity for a sugar called mannose, which tends to be present in bacterial cells instead of eukaryotic cells. Binding initiates the rest of the pathway until a C3 convertase is formed

Question 21

Alternative pathway

Answer 21

The complement can bind to any membrane in this pathway. The complement factor is C3b/Factor B

Question 22

C3 convertase

Answer 22

An enzyme that needs to be made in order to have the necessary outcome for the complement system. It is central to all 3 of the complement pathways. The convertase cleaves the complement component C3 and chops it into 2 pieces: C3b (potent opsonin, coats the pathogen surface) and C3a (potent inflammatory mediator- recruits inflammatory cells)

Question 23

C5 convertase

Answer 23

Made in cases where we have the membrane attack complex. Formed by C3b (which was made by C3 convertase) binding to C3 convertase itself, creating a dimer. C5 convertase chops complement C5 into 2 pieces. Produces C5a (very potent inflammatory mediator) and C5b (which initiates polymerization reactions leading to formation of the MAC)

Question 24

MAC formation (5)

Answer 24

1. The C5b complement component will associate with complement components C6 and C7
2. C5b67 complexes bind to the pathogen membrane via C7
3. Complement component C8 is recruited, it binds to the complex and inserts itself into the cell membrane
4. Recruits C9- these molecules bind to the complex and polymerize
5. 10-16 molecules of C9 bind to form a pore in the membrane

Question 25

What prevents complements from attacking our cells?

Answer 25

1. Lack of autoreactive antibodies- no Ab:Ag complex, no classical pathway 2. Membrane receptors 3. Soluble proteins/proteases

Question 26

How do membrane receptors prevent the complement from attacking our cells?

Answer 26

In the alternative pathway, the complement can bind to any membrane, so it needs to be inhibited by our own cells. There are some endogenous receptors located in cell membranes. Complement receptor 1 (CR1) competes with Bb for binding to C3b. CD55 also competes with Bb for binding to C3b. CD59 is a membrane receptor that binds to the α chain of C8, b domain of C9. It prevents the oligomerization of the MAC

Question 27

How to soluble proteins or proteases prevent complement from attacking our own cells?

Answer 27

There are some soluble proteins that act as endogenous inhibitors of complements. Factor H competes with Bb for binding to C3b, prevents formation of C3 convertase. C4BP competes w/ C2b for binding to C4b, prevents formation of a different C3 convertase. Factor I cleaves C3b to iC3b, destroying it

Question 28

CD59 mechanism (4)

Answer 28

CD59 is a membrane receptor 1. In the normal pathway, the MAC (C8, C7, C6, and C5b) forms and recruits multiple molecules of C9, which oligomerize 2. All mammalian cells have CD59 in their membranes. CD59 binds to C8 3. This means that C8 and the MAC can't really form at the membrane 4. If MAC can't form, it can't recruit C9

Question 29

CD55 mechanism

Answer 29

CD55 is a membrane receptor 1. In the alternative pathway, Bb and C3b form a dimer and form C3 convertase 2. CD55 binds to Bb and prevents Bb from ever associating with C3b 3. Therefore, no C3 convertase can form

Question 30

Outcomes of each complement pathway

Answer 30

1. Classical- C4b and C2b dimerize to form C3 convertase 2. Alternative- Bb and C3b dimerize to form C3 convertase 3. Lectin- C4b and C2b dimerize to form C3 convertase

Question 31

C4BP

Answer 31

C4 binding protein which binds to C4b in the classical and lectin pathway. This prevents C3 convertase (the final goal of each pathway) from forming

Question 32

Factor H

Answer 32

Prevents Bb from associating with C3b in the alternative pathway, which means C3 convertase doesn't form

Question 33

Neutrophils

Answer 33

An important player in the innate immune response. They are referred to as polymorphonuclear cells because they have multi-lobed nuclei. They only have one nucleus, although it can look like they have many. They are the first responders in inflammation and are specialized for a short-lived immune response. These cells are phagocytic, they circulate and are recruited to tissues during inflammation. They destroy the ingested pathogens and die shortly thereafter. Pus that forms during infections is just dead neutrophils

Question 34

Neutrophil-extracellular traps (NETs)

Answer 34

Neutrophils produce a "net" that trap pathogens. The traps are made up of ejected DNA and granule (lysosome) contents. The ejected DNA is compromised of chromatin and attached histones that form sticky web, entrapping pathogen. The nets serve as physical barrier/restraint that stop the pathogens from moving. Kills pathogens extracellularly- granule contents from the lysosome (elastase, hypochlorous acid) then digest the pathogens. Provides for high local concentration of antimicrobial components. Generally harsher than neutrophils and efficiently destroy pathogens once they are activated by pathogen interaction. May also be activated by the cytokine interferon gamma. Macrophages are also responsible for cytokine signaling themselves

Question 35

Macrophages

Answer 35

Referred to as mononuclear cells, as they look like they only have one nucleus. They are also phagocytic cells but are long-lived. They initially circulate in the blood as an undifferentiated cell type called monocytes. They then differentiate into macrophages once they migrate to tissues at the site of infection

Question 36

Permanent resident macrophages

Answer 36

Some macrophages are permanent residents in different tissues. Kupffer cells (liver), osteoclasts (bone), microglia (brain) – to name a few. These cells detect pathogens and initiate an inflammatory response. They are the first cells that "sound the alarm" to the immune system.

Question 37

Pattern-recognition receptors (PRRs)

Answer 37

Recognize pathogen-associated molecular patterns (PAMPs). Mannose receptors are one type of these receptors that recognizes the PAMP of mannose- this is a common bacterial glycosylation. There is also a subgroup called scavenger receptors- recognize bacterial cell wall components like LPS, peptidoglycan, teichoic acid. Toll-like receptors (TLRs) recognize shared antigens and mediate phagocytosis, and they can also induce signaling in the phagocytic cells

Question 38

Pathogen-associated molecular patterns (PAMPs)

Answer 38

Very common antigens that have repeating units. They are found in many pathogens and are not very complex. They are recognized by pattern recognition receptors

Question 39

Opsonin receptors

Answer 39

Recognize opsonized pathogens. There are 2 types of receptors: complement receptors (recognize pathogens that have been opsonized with complement) and Fc receptors (pathogens that have been opsonized with antibodies). Classically, this is Fcg. There is also Fce, and more recently Fcm

Question 40

Pseudopods

Answer 40

Extension of the cytoplasm- extend from macrophages and capture pathogens or senescent cells

Question 41

Types of phagocytosis (3)

Answer 41

1. Reaching phagocytosis- formation of pseudopods, most common type 2. Sinking phagocytosis- resembles endocytosis 3. Triggered phagocytosis- meant to take in small molecules

Question 42

Process of phagocytosis (7)

Answer 42

1. Receptor binding to the pathogen 2. Exocytosis- vesicles are delivered to the membrane, delivers lipids so the membrane can expand and make pseudopods 3. Rho or Rac GTPases activated 4. PI kinase activated – accumulation of PI(4,5)P2 5. Actin rearrangements to form pseudopods 6. PI 3-kinase converts PIP2 to PI(3,4,5)P3, the phagosome closes 7. Sealing endosome

Question 43

PI(4,5)P2

Answer 43

Allows for docking of the N-WASP protein during phagocytosis. N-WASP activates the Arp2/3 complex. The complex can rearrange actin and therefore make pseudopods

Question 44

Arp 2/3 complex

Answer 44

One of the actin nucleating complexes. Composed of 2 actin-like proteins and accessory proteins. It is activated by a protein called N-WASP and can begin nucleation at this point. Nucleation of an actin filament occurs from the minus end outward, analogous to gamma-TURC. The ARP 2/3 complex attaches to the side of another actin filament while bound to the minus end of a nucleated filament- this helps the complex to nucleate more effectively. In contrast to formins, the Arp 2/3 complex creates a treelike filament web

Question 45

Pseudopod formation (3)

Answer 45

1. Production of PI(4,5)P2, which serves as a docking site for N-WASP 2. N-WASP then activates Arp 2/3 3. The actin is rearranged to form pseudopods

Question 46

Production of chemokines/cytokines

Answer 46

When a pathogen is phagocytosed, it might induce signaling in the cell, leading to the production of chemokines and cytokines

Question 47

What is the fate of phagocytosed pathogens?

Answer 47

Bound material is internalized in phagosomes and broken down in phagolysosomes. 1. Pathogen is taken into the phagosome- the phagosome undergoes acidification, and its pH decreases 2. Lysosomes fuse w/ phagosome to form a phagolysosome. Acid hydrolases (digestive enzymes) are delivered. Lysosomal NADPH oxidases, which produce ROS, are also delivered. H2O2, O2-, NO attack pathogens in a respiratory burst 3. Myeloperoxidase forms

Question 48

Myeloperoxidase

Answer 48

An enzyme forms during pathogen degradation. It produces hypochlorous acid (HOCl, bleach) from H2O2 and Cl- in phagocytic cells. Partially dissociates into hypochlorite (OCl-) in aqueous solution. Ultimately, the pathogen is destroyed

Question 49

Types of pattern-recognition receptors (3)

Answer 49

1. Membrane bound- toll like receptors- typically located at the cell surface 2. Cytosolic- nod-like receptors 3. Extracellular (soluble)- LPS binding protein

Question 50

Toll-like receptors (TLRs)

Answer 50

Bind various PAMPs via a portion of the receptor called the leucine-rich repeat (LRR). All TLRs recognize different PAMPs. Toll-like receptors can be located on the myeloid cell surface and within endosomes. All TLRs dimerize upon ligand binding (some are homodimers and some are heterodimers). Some have co-receptors. They participate in triggered phagocytosis and pro-inflammatory signaling induction. They do this by interacting with downstream signaling proteins via an adaptor protein called MyD88

Question 51

MyD88

Answer 51

An adaptor protein that activates the protein TAK1, which phosphorylates IKK – phosphorylates IkB (degrades) – eventually the transcription factor NFkB migrates to the nucleus to start the express the genes for chemokines and cytokines

Question 52

TLR4- toll-like receptor mechanism (8)

Answer 52

1. LPS binding protein binds to LPS 2. This complex then binds to CD14, the lipid-anchored TLR4 co-receptor 3. CD14 interacts with TLR4. TLR4 must dimerize 4. TGF beta-activated kinase 1 (TAK1) binds to MyD88 5. TAK1 binding proteins 2 and 3 bind to the TAK1 complex 6. The TAK1 complex passes the signal over to the IKK complex (which contains the NEMO protein). The beta component is phosphorylated to pass the signal along and activate the complex 7. The phosphates are then moved to the next molecule- I-kappa-beta. This is an inhibitor of NF kappaB. The inhibitor is constitutively bound unless the signaling takes place 8. IKB is ubiquitylated and degraded, NFKB goes to the nucleus and expresses pro-inflammatory, pro-survival cytokines and chemokines

Question 53

NLRP3- nod-like receptor mechanism (5)

Answer 53

1. A pathogen has already been phagocytosed for this receptor to be activated, and degraded PAMPs are located in the phagolysosome 2. PAMPs are released and are sensed by the LRR. Once they are sensed, protein interactions can occur (ASC, pro-caspase 1). ASC has a pyrin domain, so it can associate with NLRP3. Pro-caspase 1 has a CARD that associates with the CARD of ASC 3. Once the three proteins associate with each other, they form a pinwheel shaped protein complex (an inflammasome) 4. The inactive caspases are in close proximity, so they will cleave one another and become activated- caspase 1 is activated by proximity 5. Caspase 1 will go on to encourage the secretion of particular cytokines from the immune cell

Question 54

NLRP3 structure

Answer 54

Nod-like receptor- these receptors are located in the cytoplasm and detect intracellular PAMPs. This receptor has 2 pieces- the leucine rich repeat (LRR) which detects the PAMPs and the pyrin domain (PYD)

Question 55

Pro-caspase 1

Answer 55

Has a caspase-recruitment domain (CARD) that can associate with the CARD of ASC in nod-like receptor protein interactions

Question 56

Outcome of inflammasome formation

Answer 56

1. Caspase 1 is cleaved and forms a tetramer to be activated. Caspase 1 then specifically cleaves pro-IL-1β and pro-IL-18 (the immature versions of these molecules). This allows the molecules to be secreted by the cell. Mature IL-1β and IL-18 are secreted, leading to a cell-mediated immune response

Question 57

IL-1β

Answer 57

Potent endogenous pyrogen. Stimulates: fever, leukocyte tissue migration, expression of diverse cytokines & chemokines

Question 58

IL-18

Answer 58

Induces the cytokine IFN-γ production. Important for activation of T cells, macrophages & other cell types

Question 59

Release of IL-1β and IL-18 from the cell (3)

Answer 59

1. Can be secreted through pores in the cell 2. Can be released through the formation of vesicles 3. Can be released through exocytosis

Question 60

Inflammation

Answer 60

Recruitment of other inflammatory cells to the site of infection- the process is typically initiated by tissue (resident) macrophages or complement. They secrete the initial chemokines or cytokines. Results in changes such as an increase in vascular diameter (increased local blood flow), reduction in velocity of blood flow (especially along endothelial wall), and endothelial cell activation to express adhesion molecules, resulting in diapedesis. All of above accounts for the symptoms: pain, redness, heat, and swelling

Question 61

Diapedesis

Answer 61

The migration of leukocytes from the blood vessel, between the endothelial cells, into the underlying tissue. TNF-α, C5a, and other cytokines are involved in this process

Question 62

Lipid-derived inflammatory mediators

Answer 62

Includes prostaglandins, leukotrienes, PAF. These molecules undergo enzymatic cleavage from the macrophage surface (resident macrophages)

Question 63

Cytokines and chemokines

Answer 63

Synthesized & secreted by resident macrophages. Includes TNF-α, which is a potent activator of endothelial cells, induces many other cytokines. Adhesion molecule expression

Question 64

C5a

Answer 64

A complement inflammatory mediator. Increases vascular permeability, induces expression of adhesion molecules – endothelial cells. It is a powerful chemoattractant for neutrophils and monocytes. Activates phagocytes & mast cells and releases histamine & TNF-α (causing more inflammation)

Question 65

Importance of inflammation

Answer 65

All of the effects of inflammation are to recruit & activate immune cells

Question 66

Cells involved in acute inflammation

Answer 66

Primarily neutrophils and monocytes

Question 67

Cells involved in chronic inflammation

Answer 67

Shifts to predominantly lymphocytes

Question 68

2 phases of diapedesis in neutrophils

Answer 68

1. Rolling adhesion of neutrophils. E-selectins (selective lectins that bind to specific sugars) are expressed by endothelial cells. They bind to sugars on the neutrophil surface to slow them down 2. ICAMs are expressed on the surface of endothelial cells. They interact with integrins on the surface of neutrophils. This causes a tight adhesion and totally immobilizes the cell

Question 69

Diapedesis of monocytes (3)

Answer 69

1. The monocyte binds adhesion molecules on the vascular endothelium near the site of infection and receives a chemokine signal 2. The monocyte migrates into the surrounding tissue 3. The monocyte differentiates into a macrophage and migrates to the site of infection

Question 70

Antigen-presenting cells (APCs)

Answer 70

Bridge the gap between innate and acquired immunity. These cells present peptide fragments of antigens on their cell surface to T cells, resulting in T cell activation. T cell activation eventually results in B cell activation and antibody production. All cells can act as antigen presenting cells with pathogens like viruses and intracellular bacteria, because all cells can be infected by these pathogens. However, there are professional antigen presenting cells as well

Question 71

Professional antigen presenting cells

Answer 71

Present the antigens of extracellular and phagocytosed pathogens, although they are also capable of presenting the antigens of other pathogens. Includes dendritic cells, macrophages, and B cells