Innate Immune Responses Flashcards
1
Q
How does innate immunity act?
A
Immediately to effect removal of the pathogen without the development of disease in the host.
2
Q
What happens if innate immune responses are overwhelmed/bypassed/evaded by a pathogen?
A
Adaptive immune responses are then required.
3
Q
How does innate immunity recognize a pathogen?
A
Innate immunity has evolved over millions of years to distinguish “dangerous” things from “innocuous” things. Innate immunity involves the use of receptors that recognise parts of the most common pathogens we are likely to encounter. These proteins are known as Pattern Recognition Receptors (PRRs).
4
Q
What are pathogen-associated molecular patterns?
A
Pattern recognition receptors (PRRs) recognise Pathogen-Associated Molecular Patterns (PAMPs).
Important features of PAMPs include:
- They are not produced by the multi-cellular host organism,
- are shared by large groups of pathogens,
- do not undergo frequent mutation,
- are often essential for pathogen’s survival, and
- are recognised by PRRs in the innate response.
5
Q
Provide two examples of PAMPs.
A
- peptidoglycan in Gram-positive bacteria
- lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria.
Examples include: LPS, peptidooglycan, dsRNA (viruses).
6
Q
What are two main groups of cell-assicated membrane PRRs?
A
The two main groups of cell-associated membrane PRRs include:
- phagocytosis receptors (that are used to bring the particle inside the phagocyte) and
- Toll-like Receptors (TLRs, to allow the phagocyte to determine if the particle is dangerous, i.e., a pathogen).
7
Q
What are Toll and Toll-like Receptors (TLRs)?
A
They consist of an extracellular leucine-rich domain that recognises the PAMP, a transmembrane domain, and a cytoplasmic domain that is homologous to the IL-1 receptor (a receptor that binds the cytokine, interleukin-1 (IL-1).
8
Q
What is the complement system?
A
The complement system is a set of plasma proteins that act together to attack extracellular pathogens.
The components of the complement system work in a set order (i.e., some act early in the cascade, whereas others act at a much later stage).
9
Q
Describe the cascade activation of a complement component.
A
Activation of a complement component usually involves the cleavage of the protein to make two smaller proteins. The complement system is activated in a series of reactions, where the product of one reaction catalyzes the next reaction, which catalyzes the next reaction, and so on.
10
Q
What is the classical pathway?
A
Complement proteins are activated when antibodies made during a previous adaptive immune response bind to the pathogen surface.
11
Q
What are antibodies?
A
Antibodies are a type of plasma protein produced by B cells.
12
Q
What is the alternative complement pathway?
A
The alternative C’ pathway is when complement proteins bind directly to the bacteria and initiate the complement activation cascade directly (thereby eliminating the need for antibody recognition to start the cascade).
There is a lot of the complement protein C3 in blood and small amounts spontaneously undergo hydrolysis to result in a form of C3b that binds to the pathogen surface. Additional proteins bind to form an enzyme complex called the C3 convertase.
13
Q
Describe the alternative complement pathway from complement protein C3 (in the blood) to the formation of the membrane attack complex.
A
- Complement protein C3 (in the blood) spontaneously breaks down to form C3a and C3b.
- C3b binds to the pathogen surface and recruits other complement proteins to form a C3 convertase (left panel).
- This results in the formation of additional C3b protein.
- Some of the C3b is used to form additional C3 convertases (left panel), some C3b is used as an opsonin, and some C3b is used to form the C5 convertase.
- The C5 convertase cleaves complement protein C5 into C5a and C5b (right panel).
- C5b is then used to form the membrane attack complex
14
Q
What are the three important consequences of the formation of the C3 convertase?
A
Activation of the inflammatory response
Opsonisation and enhancement of phagocytosis
Formation of the membrane attack complex and bacterial cell lysis
15
Q
What happens if C3a and C5a are produced in large amounts?
A
When produced in large amounts, the effect of C3a and C5a on blood vessel permeability can induce a generalized circulatory collapse, producing a shock-like syndrome known as anaphylactic shock.
These small complement fragments are therefore often referred to as anaphylatoxins.
16
Q
How does the formation of the C3 convertase activate the inflammatory response?
A
The C3 convertase cleaves additional C3 into C3a and C3b; after binding, C3b recruits additional proteins to form the enzyme complex C5 convertase that cleaves C5 into C5a and C5b.
17
Q
How do C3a and C5a activate an inflammatory response?
A
C3a and C5a are small fragments that bind to specific receptors on blood vessels. This results in an increased permeability of the blood vessels and induces the expression of adhesion molecules that allow leukocytes such as neutrophils and monocytes to attach to the blood vessels.
C5a and C3a also bind to receptors on the resident mast cells and resident macrophages to release additional histamine and TNF-α, respectively.
In addition, C5a also is a powerful chemoattractant for neutrophils and monocytes.
18
Q
Define: Opsonisation
A
Opsonisation is defined as the alteration of the pathogen surface or particle surface so that phagocytic cells can engulf it more efficiently.
19
Q
How does C3b bound to the pathogen surface function as an opsonin?
A
The coating of a pathogen with C3b enhances phagocytosis by neutrophils and macrophages.
C3b binds to a receptor on the surface of the phagocytic cell. This results in an increase number of contact points between the phagocytic cell and the pathogen.
Phagocytic cells have receptors that can bind to C3b on to the pathogen surface
20
Q
How is the membrane attack complex formed?
A
After C5b binds to the pathogen surface, other proteins are recruited to form a membrane attack complex (assembly of the late (“terminal”) complement components) that results in the formation of a pore in the pathogen’s cytoplasmic membrane, causing bacterial cell lysis and death of the pathogen.
21
Q
Describe basic characteristics of the innate immune response.
A
The innate immune response is fast, non-specific, and triggered by components of the pathogen.
22
Q
How are pathogens recognized as ‘foreign’?
A
Pathogens are recognised as “foreign” or “non-self” by pathogen-associated molecular patterns (PAMPs).
PAMPs bind to pattern recognition receptors (PRRs) on the innate immune cells, causing phagocytosis of the pathogen and cytokine production by the immune cell.
23
Q
When does inflammation begin?
A
Inflammation begins after damage to the tissue.
24
Q
Where is histamine released?
A
Mast cells release histamine.
25
What does histamine do?
After damage to tissue, inflammation begins. Mast cells release histamine that increases the permeability of blood vessels.
Cells, proteins, and fluid leak into the tissues, causing swelling.
Pathogens activate macrophages, which release alarm cytokines and recruit more cells from the blood.
Macrophages and neutrophils phagocytose and destroy pathogens.
Coagulation proteins seal off the inflamed area, and complement activation increases recruitment, phagocytosis, and pathogen destruction.
26
How can the complement system be activated?
Via the classical pathway or the alternate pathway
27
How are the classical and alternative pathways different?
The classical pathway uses antibodies generated by a previous adaptive immune response to the pathogen.
The alternate pathway is spontaneous and requires no previous exposure.
28
Reiterate the three important consequences of the complement cascade?
The complement cascade has three important consequences: it activates the inflammatory response, it allows opsonisation of the pathogen, and it can cause pathogen lysis through formation of the membrane attack complex.
29
What does activation of dendritic cells do?
Initiates the adaptive response.
30
What does the ability to mount an adaptive immune response depend on?
Dendritic cells link the innate and adaptive immune systems since the activation of dendritic cells initiates the adaptive response.
The ability to mount an adaptive response is dependent on the ability to mount an effective innate immune response.
31
What are some examples of barriers in our bodies, and how do barriers function to exclude invading microbes?
Natural barriers include the skin, mucous membranes, tears, earwax, mucus, and stomach acid.
Skin and mucosa provide an effective immune barrier between the internal and external environment.
32
What molecules do phagocytes use to identify an invading microbe versus a host cell?
Recognition often takes place by the use of phagocyte receptors that bind molecules commonly found on pathogens, known as pathogen-associated molecular patterns (PAMPs).
33
What are the functions of cell-membrane associated Pattern Recognition Receptors (PRRs)?
Pattern Recognition Receptors (PRRs) are proteins capable of recognizing molecules frequently found in pathogens (Pathogen-Associated Molecular Patterns—PAMPs), or molecules released by damaged cells (Damage-Associated Molecular Patterns—DAMPs).
34
Person A was born with a genetic defect that results in an inability to produce complement component C3. Person B was born with a different genetic defect and is unable to produce complement component C5. Who would have a more difficulty in eliminating a bacterial infection? Why?
The C3 complement immune deficiency is more severe since it is required to trigger opsonization and inflammation.
C5 is a late stage ('terminal') complement protein. Deficiency of C5 would mean no formation of a membrane attack complex.
35
What cells create inflammation?
Mast cells (via release of histamine)
36
Where do leukocytes not have open access?
The brain and spinal cord.
37
What are the most abundant type of white blood cell?
Neutrophils (~60% of WBCs)
Neutrophils consume the 'foreign' particle and then die, resulting in the formation of pus.
38
How many pathogens can macrophages consume before death?
Macrophages can consume ~100 pathogens before they die.
39
Where are dendritic cells found?
Dendritic cells are present in those tissues that are in contact with the external environment, such as the skin (where there is a specialized dendritic cell type called the Langerhans cell) and the inner lining of the nose, lungs, stomach and intestines. They can also be found in an immature state in the blood.
40
What is an antigen?
The immune system protects the body from possibly harmful substances by recognizing and responding to antigens.
Antigens are substances (usually proteins) on the surface of cells, viruses, fungi, or bacteria.
41
What is the most common complement deficiency?
C2
42
How many proteins make up the complement system?
20
43
What is the terminal pathway?
The terminal pathway is common to the classical, alternative, and lectin complement pathways.
Leads to formation of the membrane attack complex causing lysis.
44
What is the critical step of the complement system?
Activation of C3
45
Where are the complement proteins synthesized?
C1 - intestine
C2, C4 - macrophage
C5, C8 - spleen
C3, C6, C9 - liver
46
Which complement proteins are considered anaphylatoxins?
C3a, C4a, and C5a
47
C5-9 activate bacterial lysis. What are the consequences of deficiency of these complement proteins?
Recurrent infections by capsulated organisms (eg: Pneumococcus)
48
Which complement protein is responsible for chemotaxis?
C5a - activates lypooxygenase pathway
49
What is chemotaxis?
Chemotaxis is the movement of an organism in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment.
50
C3b promotes phagocytosis. What is the consequence of a C3b deficiency?
Recurrent pyogenic infections
51
What causes inflammation?
Inflammation is caused by a physical or chemical insult or by infection with microorganisms.
52
What is acute inflammation?
Acute inflammation is typically of short duration and is the initial response to an infectious agent.
It causes very little tissue damage to the host.
53
What is chronic inflammation?
Chronic inflammation is of longer duration (months to years) in the presence of a persistent infectious agent and is characterized by the involvement of activated macrophages and T cells.
Chronic inflammation often causes considerable tissue destruction due to persistent release of reactive oxidizing species (a.k.a. free radicals) such as oxygen metabolites, nitric oxide (NO), and proteases by inflammatory cells.
54
Describe the inflammatory response.
There are five steps in the inflammatory response.
Bacterial infection initiates a series of responses through activation of the alternative complement pathway as well through stimulation of tissue-resident macrophages that detect bacterial-derived PAMPs.
This results in the recruitment of additional innate cells and proteins that work together to eliminate the infection.
55
What is the first step of the inflammatory response?
* The bacteria in the wound are immediately recognised as by the tissue resident macrophages.
* A phagocytic pattern recognition receptor (PRR) would bind to a structure on the bacterium to trigger phagocytosis.
* A Toll-like receptor (TLR) would bind to pathogen-associated molecular pattern (PAMP); the interaction initiates a signaling cascade that results in the activation of a transcription factor called NF-κB.
* NF-κB activation results in the transcription of genes encoding tumor necrosis factor alpha (TNF-α), Interleukin-1 (IL-1), and Interleukin-6 (IL-6).
* **TNF-α, IL-1 and IL-6** are called pro-inflammatory cytokines because they promote the inflammatory response.
* CXCL8 is a chemokine that acts as a chemoattractant to guide recruited phagocytic cells to the site of infection.
56
What are the three pro-inflammatory cytokines?
TNF-α, IL-1 and IL-6 are called pro-inflammatory cytokines because they promote the inflammatory response.
57
What is CXCL8?
A chemokine that acts as a chemoattractant to guide recruited phagocytic cells to the site of infection.
58
What is the second step of the inflammatory response?
The inflammatory response at the site of infection is initiated by tissue-resident macrophages and tissue-resident mast cells.
59
What does histamine do?
Upon injury to the tissue, mast cells release histamine and other inflammatory molecules. Histamine causes blood vessels to dilate, thus increasing blood flow to the area and resulting in the redness and heat association with inflammation.
60
What causes the swelling and pain associated with inflammation?
The increased vascular permeability (due to histamine released by mast cells) allows components of the blood to leak into the tissue, including fluid, complement proteins, coagulation proteins and antibodies, and cells such as neutrophils and monocytes. The accumulation of fluid in the tissues causes the swelling and pain associated with inflammation.
61
What is extravasation?
Adhesion molecules allow leukocytes such as neutrophils and monocytes to stick to the endothelial cells of the blood vessel and then squeeze through the spaces between the endothelial cells of the blood vessel and into the damaged tissue. This process is known as **extravasation**.
62
What guides leukocytes to the location of the bacteria in tissues?
A gradient of CXCL8 guides the neutrophils, monocytes and dendritic cells to location of the bacteria in the tissues.
63
Compare phagocytosis by neutrophils with that of macrophages.
Neutrophils immediately begin to phagocytose the bacteria. (can only consume a single pathogen before death)
The monocytes must mature into macrophages before they become effective phagocytic cells. (can consume many pathogens before death)
64
Describe phagocytosis and the killing of pathogens.
Macrophages and neutrophils express many surface receptors that may bind microbes for subsequent phagocytosis; select examples of such receptors are shown.
Microbes are ingested into phagosomes that fuse with lysosomes to from phagolysosomes.
The microbes are killed by enzymes and several toxic substances produced in the phagolysosomes.
65
What is step three of the inflammatory response?
Large numbers of neutrophils and monocytes are recruited to the site of infection within a few hours after the infection has been detected.
66
How is a phagolysosome formed?
When the neutrophil or macrophage phagocytoses the bacteria, the bacteria are brought into the cell in a special membrane-bound vesicle known as a **phagosome**. The phagosome is then acidified and membrane-bound granules called lysosomes fuse with the phagosome to form a **phagolysosome**.
67
What takes place within a phagolysosome?
In the phagolysosome, the nitric oxide (NO), the superoxide anion (O2–), hydrogen peroxide (H2O2), cationic peptides (defensins) and proteases work together to kill the bacteria and degrade it into smaller fragments.
68
What is step four of the inflammatory response?
Clotting mechanisms and skin repair mechanisms are activated. These help wall off the site of wound/infection and immobilize the bacteria.
69
What is step five of the inflammatory response?
Pus is formed. Pus is an aggregation of macrophages, neutrophils, live and dead skin cells, dead and dying bacteria, and plasma (blood fluid).
70
Briefly list the five steps of the inflammatory response.
1. The bacteria in the wound are immediately recognised as by the tissue resident macrophages.
2. The inflammatory response at the site of infection is initiated by tissue-resident macrophages and tissue-resident mast cells.
3. Large numbers of neutrophils and monocytes are recruited to the site of infection within a few hours after the infection has been detected.
4. Clotting mechanisms and skin repair mechanisms are activated.
5. Pus is formed.
71
How are macrophages activated?
Pathogens activate the macrophages, which release alarm cytokines and recruit more cells from the blood.
72
How is the inflamed area sealed?
Coagulation proteins seal off the inflamed area, and complement activation increases recruitment, phagocytosis, and pathogen destruction.
73
What are the four steps of Leukocyte Extravasation?
1. Rolling
2. Activation
3. Adhesion
4. Transendothelial migration
74
What is an oxidative burst?
Neutrophils kill microbes by the production of reactive oxidzing species/free radicals.
The generation of H2O2 and other oxygen radicals involves a rapid increase in the consumption of O2; this is known as the oxidative burst.
75
What does binding of PAMPs to TLRs initiate?
Toll-like receptors (TLRs) is a different type of PRR: the binding of the PAMPs initiates intracellular signals that result in:
* release of cytokine
* up-regulation of MHC class II proteins
* up-regulation of B7 co-stimulatory molecule (important in the development of the adaptive immune responses
76
Innate immune responses to pathogens include both a cellular component and a protein component. What are the two important types of proteins that are part of innate responses?
1. Complement.
2. Some type of soluble PRRs (synthesized by liver) can activate complement.
77
Describe the formation of a membrane attack complex.
* C5b binds C6 and C7
* C5b67 complexes bind to membrane via C7
* C8 binds to the complex and inserts into the cell membrane
* C9 molecules bind to the complex and polymerize
* 10-16 molecules of C9 bind to form a pore in the membrane.
78
What is the step in both the classical and alternative pathways that leads to the formation of C3 convertase?
Classical pathway - Binding of antibody (or soluble PRR) to pathogen, Recruit early component proteins
Alternative pathway - Spontaneous breakdown of C3 Deposit of C3b on pathogen surface
79
What are the other roles for C3a?
* contributes to inflammatory response,
* stimulates macrophage to release more TNF-α.
* a = attracts and activates other cells*
* b = binds the bacterial surface*
80
What are the other roles for C3b?
* acts as an opsonin (enhances phagocytosis of the pathogen by the phagocytic cell).
* a = attracts and activates other cells*
* b = binds the bacterial surface*
81
What are the other roles for C5a?
* contributes to inflammatory response,
* stimulates mast cells to release more histamine.
82
Discuss the severity of a deficiency of the complement protein C2.
* would not be able to activate complement with antibodies or soluble PRRs, but could still activate complement with alternative pathway.
83
Discuss the severity of a deficiency of the complement proteins C5 and C9.
* would not make membrane attack complex
* no C5a to reinforce the inflammatory response either
84
Discuss the severity of a deficiency of the complement protein C3.
* no C3b for opsonization, so C3a/C5a for inflammatory response
* also no membrane attack complex.
85
Define: pathogen.
A disease causing microorganism
86
What are the two phases of the innate immune response?
**Immediate** - this involves preformed proteins in the blood/tissues and phagocytic cells already in the tissues (like resident macrophages),
**Induced phase** - this involves the recruitment of phagocytic cells from the bloodstream and into the tissue (site of infection).
87
For induced innate responses to develop, what needs to happen?
1. Need resident macrophages to recognize the bacteria and send out the “alarm” to recruit more phagocytic cells.
2. Need changes in the blood vessel wall of the vein so that neutrophils and monocytes and get to the site of infection.
3. Need to get phagocytic cells to migrate towards the bacteria in the tissue and engulf them and kill them.
*This is called the “inflammatory response.”*
88
What causes chronic inflammation?
Chronic inflammation due to microorganisms usually involves special pathogens that have extraordinary abilities to evade the host’s immune response.
89
What kind of junction do endothelial cells lining blood vessel walls contain?
The endothelial cells that line the blood vessel wall (i.e., vein) have **tight junctions.**
90
What do resident macrophages release?
Resident macrophages release **proinflammatory mediators** that include the cytokines TNF-α, IL-1 and IL-6, and a number of chemokines (e.g., IL-8).
91
What do alarm cytokines do?
The alarm cytokines (IL-1, IL-6, TNF-α) act on the endothelial cells of the veins - this causes the vein to become slightly “leaky” and “sticky.”
This helps to recruit neutrophils and monocytes to the infection site, allows them to slow down and stop at the infection site
92
How do neutrophils and monocytes enter the damaged tissue?
The changes in the endothelial cells allow neutrophils and monocytes to bind to adhesion molecules - this causes them bind to and “roll” on the blood vessel wall before eventually stopping at the site.
The weakening of the junctions between endothelial cells allow the neutrophils and monocytes to squeeze (extravasate) between the endothelial cells and move into the tissue.
93
How do leukocytes move towards the infection site?
The neutrophils move towards the infection site by following the **IL-8 gradient** (made by the macrophages) and begin to phagocytose the bacteria.
The monocytes also follow the IL-8 gradient and start to mature into macrophages once they are in the tissue.
Dendritic cells follow too.
94
How is the pathogen phagocytosed by the neutrophil (or macrophage) and killed?
* Bacterium bind to neutrophil, macrophage or dendritic cell via phagocytic PRRs.
* The bacterium, is engulfed into a phagosome.
* The phagosome fuses with a lysosome to become the phagolysosome.
* The phagolysosome is a highly acidic environment with activated proteases, production of nitric oxide (NO), H2O2, and oxygen radicals.
95
What are important immune cells that do phagocytosis?
Dendritic cells
Neutrophils
Macrophages
96
What is the purpose of phagocytosis for the important immune cells that can do it?
For neutrophils, the purpose of phagocytosis is the destruction of the pathogen.
For macrophages and dendritic cells, the purpose of phagocytosis is twofold:
* Destruction of the pathogen.
* After engulfing and digesting, small peptide fragments are “presented” by MHC proteins to T cells.
97
How do macrophages sense danger?
Presence of a PAMP, e.g., LPS or peptidoglycan.
98
What are the alarm cytokines?
IL-1, IL-6 and TFN-α
99
What guided the recruited phagocytes to the infection?
IL-8
100
Neutrophils make up how much of WBC population?
How long do they live?
How much can they pahgocytose?
* comprise ~65% of WBCs
* short-lived (~few days)
* can phagocytose only once, then they die
101
Monocytes make up how much of WBC population?
How long do they live?
How much can they phagocytose?
* ~6% of WBCs
* long-lived cell (months to years), but don’t divide
* can phagocytose and kill about 100 bacteria before dying from the toxic products the macrophage produces.
102
How do dentritic cells activate the adaptive immune responses?
* present in small numbers, highly phagocytic when immature (in tissue)
* once they phagocytose bacteria, they start to mature and migrate to the lymph node to activate the adaptive immune responses.
103
PRRs were first observed in phagocytes in Drosophila where they are called Toll receptors.
True or false?
True
104
Toll-like receptors (TLRs) are a type of PRR that allow innate cells of the immune system to identify something as dangerous to the host.
True or false?
True.
105
Toll-like receptors are found in the plasma membrane of cells as well as endosomal membranes.
True or false?
True
106
Complement fragment C3a can act as an opsonin to improve efficiency of phagocytosis.
True or false?
False
107
Complement proteins are produced by the liver.
True or false?
True
108
Some PRRs recognize structures on pathogens for phagocytosis, other PRRs are involved in cell signalling.
True or false?
True
109
Toll-like receptors are an example of a soluble PRR.
True or false?
False
110
All of the activities below are functions of anaphylatoxins (complement fragments C3a, C4a and C5a):
1. Act as chemokines that recruit leukocytes from the blood into injured tissue.
2. Trigger degranulation, leading to the release of other inflammatory mediators.
3. Induce extravasation through the upregulation of adhesion molecules on neutrophils and endothelial cells.
4. Stimulate macrophages to release pro-inflammatory cytokines.
True or False?
True.
111
Complement protein C3 is constantly and spontaneously hydrolyzing into fragments C3a and C3b.
True or false?
True.
112
Epithelial tissues serve as a barrier to microbial infection.
True or false?
True
113
Infection has no influence on the rate of hematopoiesis.
True or false?
False
114
Toll-like receptors are a type of PRR that allow innate cells of the immune system to identify something as dangerous to the host.
True or false?
True
115
Immediately (i.e.,within minutes) after injury, during acute inflammation, vasodilation occurs to deliver leukocytes to the site.
True or false?
False
116
The inflammatory response can be triggered by complement binding to the surface of microorganisms.
True or false?
True
117
Extravasion (diapedesis) occurs in veins.
True or false?
True
118
Neutrophils respond to infection by releasing antibody proteins into the blood.
True or false?
False
119
Complement fragment C3a can act as an opsonin to improve efficiency of phagocytosis.
True or false?
False
120
The presence of neutrophils in higher than normal numbers in the blood or at a site of inflammation indicates an active response to a current infection.
True or false?
True
121
The innate immune response varies between a first and a second exposure to a pathogen.
True or false?
False
122
The innate mechanisms of host immunity do not play a role in the defense of the body unless the specific immune responses fail.
True or false?
False
123
Dead phagocytes make up much of the material of pus.
True or false
True
124
A person with a deficiency in complement protein C8 would have a more severe immune deficiency than someone lacking complement protein C3.
True or false?
False
125
126
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