IMI 2: The Innate Immune System Flashcards
1
Q
Observe the learning outcomes of this session
A
2
Q
Who were awarded the Nobel prize in 1908 ‘in recognition of their work on immunity’?
A
- Elie Metchnikoff
- Paul Ehrlich
3
Q
What was Ehrlich’s theory?
A
- After the demonstration of bacteriolysis (the rupture of bacterial cells) by Richard Pfeiffer and haemolysis (the rupture of red blood cells) by Jules Bordet, Ehrlich and Julius Morgenroth broadened their theories by postulating that lysis is based on a mechanism involving recognition of cells by antibodies as can be seen in the figure below of Ehrlich’s original drawings
4
Q
What are antibodies and what do they do?
A
- they are large Y-shaped proteins produced by plasma B-cells which neutralise pathogens
5
Q
What cells can engage with phagocytosis?
A
- neutrophils
- macrophages
- dendritic cells
6
Q
What is the complement system?
A
- a theory suggesting that antibodies can cause lysis of cells by recruiting and activating distinct molecules, which complement the specific action of the antibodies themselves
- it also connects innate immunity to adaptive immunity by increasing the presentation of bacterial fragments to T-cells
7
Q
What are the three key roles of the complement system?
A
- To tag foreign surfaces - a process called opsonisation
- To puncture (permeabilise) the membranes of foreign pathogens/cells, leading to cell lysis.
- To signal to the cellular immune system where there is a foreign invader and trigger the recruitment of cells.
8
Q
Define opsonisation
A
- the process of marking a foreign entity with substances that made them more visible to the cells of the immune system
- This makes it easier for cells such as neutrophils or macrophages to attach to the pathogen, and destroy it
- The pathogen can then be processed for antigen presentation
- This is an example of positive feedback loop, which means that the complement can amplify a very small initial sensing event into a very big effect pretty quickly!
9
Q
Describe the complement system
A
- over 30 different proteins that work together to fight off pathogens
- there are 13 quintillions of them in our body fluid
- antibodies activate the complement system
- our cells have tools to prevent complement from accidentally attacking them
- they float around passively until they are activated and change their shape
- complement proteins cause a cascade
- complement attack begins with C3, switching to its active state, either randomly, via an antibody or through other proteins
10
Q
Describe how the complement cascade works with bacterial or viral infection
A
- C3 breaks into two smaller proteins: C3a, C3b
- C3b:
- seeks bacteria, fungi and viruses: it has a fraction of a second to find one or it will be neutralised by water molecules
- if it finds a target, it anchors itself tightly and then its protein shape changes again
- it is now about to grab other proteins and start a cascade
- finally it transforms itself into a recruiting platform known as C3 convertase
- it activates more C3 proteins and an amplification loop begins
- soon thousands of proteins surround the bacteria, crippling it
- C3a:
- is a distress beacon and passive immune cells notice C3a
- complement guides reinforcements to where it is needed the most
- phagocytes then arrive and the complement is able to act as a glue to help phagocytes grab onto bacteria
- another cascade begins
- C3 convertase is converted to C5 convertase to form a bigger structure, the membrane attack complex
- new proteins shaped like long spears rip a hole into the bacteria
- they bleed to death
- complement is also able to intercept and cripple viruses
- however, due to viruses adapting, it forces it to produce a protein that shuts the complement activation down
- the virus creates safe zones around the cells it infects
- or some bacteria can grab certain molecules from the blood that keep the complement system calm and make themselves invisible
11
Q
How is the complement cascade initiated?
- in terms of the C3 protein
A
- through a series of protein digestion steps, cutting inert complement proteins produced by the liver into two active components
- C3b: covalently attached to the offending surface
- C3a: floating off, a bit like a cytokine
- C3 can catalyse the attachment of more complement protein
- this positive feedback loops means complement can amplify very small initial sensing event into a very big effect pretty quickly
12
Q
What is the protein activation of the complement cascade similar to?
A
- the blood clotting process also involves a similar cascade of protein cleavage events
13
Q
What are the three main pathways that can activate the complement cascade?
A
- Classical pathway: induced by antibody-antibody or immune complexes
- Lectin pathway: induced by PAMP recognition by lectins
- Alternative pathway: induced by spontaneous hydrolysis
14
Q
Describe the classical complement pathway
A
- Antibodies bound to a surface can recruit the first protein in the complement proteolytic cascade, and activate complement to attach it to the local surface of the cell
- IgM antibodies are particularly good at this
- This initially opsonises the surface, but if it is a membrane then this will also initiate the lysis of the cell.
15
Q
Describe the lectin complement pathway
A
- Lectins are a family of PRRs that mostly recognise carbohydrates found on the surface of bacteria, but not our own cells
- This innate sensing mechanism can immediately go to work by initiating the complement cascade, opsonising and lysing invaders
- This is essentially the same process as the classical pathway, but has an innate trigger (Lectin-type PRRs) rather than the adaptive immune trigger (antibodies) used in the classical pathway.
16
Q
Describe the alternative complement pathway
A
- The alternative pathway is conceptually different, as it does not have a specific trigger
- Rather, it can be thought of as a continuous process of scanning all the internal surfaces of the body.
- Complement proteins are a little unstable, and can randomly fall apart, sticking to the nearest surface
- All of our cells carry proteins that will defuse the complement bomb, so this has no effect on our cells
- However, on other cells, this complement remains and recruits more complement to be attached, using the positive feedback loop mentioned earlier to mark invaders for destruction by lysis or phagocytosis.
- This pathway is crucial for rapidly (i.e. before you have had time to make antibodies) destroying cells from other species that do not have obvious PAMPs.
17
Q
Describe anti-microbial peptides (AMPs)
- location
- presence in different organisms
- size
- structure
- function
A
- function:
- kill bacteria and fungi
- some can inhibit virus function
- location:
- found on mucosal surfaces
- blood
- found in all classes of life: e.g. animals, bacteria and plants
- structure:
- small polycationic peptides (7-100 amino acids)
- possess a net positive charge
- they are attracted and incorporated into negatively charged bacterial membranes
18
Q
Describe the mechanism of action and functions of AMPs
A
- their mechanism of action is mostly via membrane disruption resulting in lysis of bacteria
- they can also interfere with DNA and protein synthesis and can function as immunomodulator
- Inactive precursors of AMPs are produced mainly by epithelial cells and neutrophils and after proteolytic cleavage then become effective AMPs
- So again, as the complement system, this part of the host defence needs to be activated (cleaved) before these peptides can exert their function.
19
Q
Which systems are in place in the blood and on barrier surfaces to protect us from infections?
- Complement
- Coagulation
- Anti-microbial peptides
A
- complement and AMPs have a direct effect on pathogens to stop them from infecting us
20
Q
Are the complement and AMPs ready to attack pathogens?
- Yes or No
A
- No, as they are very potent, they need to be cleaved to become activated
21
Q
What is cellular immunity?
Who discovered it?
A
- cellular immunity is driven by cells capable of attacking foreign invaders
- it involves phagocytosis, cytotoxic T cells and cytokines
- instead of the activation of antibodies
22
Q
What is humoral immunity and who discovered it?
A
- humoral immunity is protection from pathogens through soluble factors, involving antibodies
- by Paul Ehrlich
23
Q
Name functions of complement
A
- initiating phagocytosis
- opsonising antigen
- lysing pathogens
24
Q
Name the two founders of immunology
A
- Paul Ehrlich
- Elie Metchnikoff
25
Q
What are white blood cells called?
A
- leukocytes
- different leukocytes are classified based on their embryonic region
26
Q
What are the 3 main kinds of circulating blood leukocytes?
A
- granulocytes
- lymphocytes
- monocytes
27
Q
What are the four main types of granulocytes?
A
- neutrophils
- eosinophils
- basophils
- mast cells
28
Q
What are the three main types of lymphocytes?
A
- T cells
- B cells
- innate lymphoid cells (ILCs)
29
Q
Which leukocytes have myeloid lineage?
A
- granulocytes: neutrophils, eosinophils, basophils and mast cells
- monocytes, which in response to inflammation can differentiate into macrophages
30
Q
Which leukocytes have lymphoid lineage?
A
- T lymphocytes
- B lymphocytes
- innate lymphoid cells or ILCs, including NK cells
- innate-like T cells
31
Q
Describe the general characteristics of granulocytes
- types
- how they were named
- shape
- function
A
- four types:
- neutrophils
- basophils
- eosinophils
- mast cells
- named after H&E staining
- shape:
- multi-lobed nucleus, so they are also called polymorphonuclear cells
- easy to distinguish under the microscope
- function:
- contains vesicles called granules, filled with noxious substances that can be released to fight infections
- the shape of the nucleus allows granulocytes to easily squeeze through gaps between endothelial cells and rapidly migrate from blood or lymphatic vessels into tissues in response to an infection
32
Q
Describe neutrophils
- relative abundance
- structure
- function
A
- relative abundance:
- the most abundant circulating leukocyte
- comprising up to 70% of the total count
- function:
- defend us against bacterial or fungal infection
- circulating neutrophils are relatively inert, but quick to respond to inflammation and migrate to the site of infection
- they are the first immune cell type to arrive from the circulation
33
Q
What do neutrophils do once activated?
A
- has the phagocytic capability to engulf or destroy pathogens
- release noxious substances stored into cytoplasmic granules, called degranulation
- release neutrophil extracellular traps (NETs), a process called NETosis
34
Q
What are the three types of granules present in neutrophils?
Describe them
A
- primary granules:
- also called ‘azurophilic’
- contain enzymes such as:
- proteases: degrades proteins
- myeloperoxidase: produces hypohalous acids with antimicrobial activity
- defensins: an evolutionary ancient class of potent antimicrobial products
- secondary granules:
- the most abundant type
- contains enzymes such as lysozyme (an antimicrobial peptide that can mediate bacterial wall hydrolysis) and lactoferrin (a potent anti-microbial compound)
- tertiary granules:
- contain metalloproteinases
- involved in the degradation of bacterial proteins
- also serve the important function of breaking down the extracellular matrix, facilitating the migration of neutrophils through tissues
35
Q
Describe how neutrophils can fight infection by phagocytosis
A
- Neutrophils can intake pathogens (literally eat them) in a process called phagocytosis
- The phagocytic vacuole called phagosome fuses with the granule content and the combination of free radicals, noxious chemicals and proteolytic enzymes causes the pathogen digestion and killing within the phagosome
36
Q
Describe how neutrophils can fight infection by degranulation
A
- The release of granules content is concurrent to neutrophil activation and is tightly regulated by signalling and cytoskeletal dynamics
- However, the release of primary and secondary granule content, and ROS can often be incidental
- granules can fuse with the plasma membrane during phagocytosis when the engulfing vacuole has not been completely closed causing the release of granule content
- The release of granules can kill pathogens in the extracellular environment but the sustained release of granules can also cause tissue damage.
37
Q
Describe how neutrophils can fight infection by NETosis
A
- Neutrophils extracellular traps (NETs) are the result of controlled cell death triggered by pro-inflammatory cytokines
- This process is called NETosis
- It involves the release of neutrophil’s nuclear DNA associated with histone proteins alongside granules content.
- These NETs have the double function of immobilising the pathogen where antimicrobial peptides have been locally concentrated, making them a potent killing weapon.
39
Q
Describe eosinophils
- relative abundance
- location
- function
A
- abundance:
- 2-4% of leukocytes
- with numbers rising in response to allergy or parasitic infections
- location:
- found in the mucosa, contributing to allergy symptoms as well as infections
- function:
- they can migrate to a damaged or infected tissue in response to pro-inflammatory cytokines
- they contain several types of granules filled with noxious substances
- when they encounter a parasite, the granule contents are released, damaging the pathogen’s surface, eventually killing it
- the granules contain lipids such as:
- prostaglandin
- leukotrienes
- these promote an inflammatory response and appear important in allergy and asthma
- smaller pathogens can be potentially phagocytoses by eosinophils
40
Q
Describe basophils
- abundance
- function
A
- abundance:
- represent a small proportion of leukocytes
- accounting for 5% of the total amount
- function:
- they are non-phagocytic granulocytes
- like eosinophils, they play an important role in controlling parasitic infection and contribute to allergic response
- mainly respond to components of the complement cascade, mannose-binding proteins or IgE (a specific subclass of antibody)
- once activated, they release granule contents to kill pathogens
- granules contain:
- histamine: causing dilation of blood vessels
- heparin: an anticoagulant that can inhibit blood clotting
- the release of these molecules promotes the trafficking of leukocytes into sites of tissue infection and inflammation
41
Q
Describe mast cells
- function
- structure
A
- they have long been considered the tissue-resident counterpart of circulating basophils as they share many features and functions
- however, it has now been established that they have a distinct origin
- they play a role in allergy and immune responses of parasitic infection
- they release histamine, like basophils
42
Q
Describe monocytes
- abundance
- structure
- categories
- origin
A
- abundance:
- represent 2-12% of the total circulating leukocytes
- originate in the bone marrow
- two categories:
- patrolling monocytes:
- present in blood and have a role in maintaining blood vessels’ homeostasis but can also phagocytose blood-borne pathogens.
- inflammatory monocytes:
- can quickly respond to chemotactic gradients formed by pro-inflammatory cytokines and chemokines released at the site of infection
- This allows them travel into infected tissues
- Once there, monocytes secrete inflammatory mediators and can differentiate into macrophages, the main phagocytic cell specialised in fighting infections
43
Q
Describe macrophages
- structure
- types
A
- two main categories:
- tissue-resident macrophages
- monocyte-derived macrophages
- they share several key functions, but also have fundamental differences
44
Q
Describe tissue-resident macrophages
A
- Tissue-resident macrophages migrate to their home tissue during their early embryonic life
- Their role is to maintain tissue homeostasis and they can self-renew (unlike most circulating immune cells)
- The tissue-resident macrophages have been given different names depending on their location in the body
- For example: in the brain they are called microglia, in the liver Kupfer cells, and in the bone they are called osteoclasts
- Alongside other tissue-resident immune cells, tissue-resident macrophages work as tissue ‘sentinels’ being the first to sense infection (PAMPs) or tissue damage (DAMPs) and alert more distant cells of the immune system such as neutrophils and monocytes by sending off molecular messages in the form of cytokines and chemokines.
- Tissue-resident macrophages have also an important housekeeping role within the tissues, removing and digesting apoptotic cells, and contributing in restoring tissue homeostasis after an infection has resolved.
46
Q
Describe dendritic cells
- function
- structure
A
- Dendritic cells (DCs) are innate immune cells and play the role of messengers to the adaptive immune system.
- In fact, they are specialised in patrolling tissues in search of antigens – bits of foreign material – and bring it to T cells to activate them to fight infection, a process called antigen presentation
- This sampling occurs both within tissues and on the outside of tissues that are in contact with external environments, such as the skin, lungs, intestines and other mucosal surfaces, which are rich in foreign antigens.
- Dendritic cells are characterised by long cellular projections (processes) similar to branches of a tree called dendrites (déndron being Greek for “tree”). The dendrites increase the overall cell surface allowing an efficient scanning of the surrounding environment.
47
Q
Describe how antigen presentation works
A
- Dendritic cells are exceptionally good at internalising pathogens by phagocytosis
- Once the phagocytised material is digested, it is loaded onto membrane-bound molecules called major histocompatibility complex (MHC) that show the material to T cells
- Activated DCs migrate to secondary lymphoid organs such as lymph nodes where they can present MHC-bound antigens to naïve T cells, (T cells that have never encountered an antigen before)
- This process, called antigen presentation is the way innate immune cells educate the adaptive immune system about an ongoing infection and activate them to help fight the infection
- image depicts dendritic cells presenting an antigen (red)
48
Q
Describe lymphocytes
- types
- abundance
- location
- function
A
- includes B cells, T-cells and innate lymphoid cells (ILCs)
- location:
- found in lymphoid organs, particularly secondary lymphoid organs
- e.g. lymph nodes and in tissues
- they traffic around the body through blood or lymphatic circulation
- abundance:
- 20-40% of leukocytes
- 99% of cells in lymph
- function: T and B cells are central to adaptive immunity
61
Q
What is phagocytosis?
A
- the process of ingesting (endocytosing) potentially hazardous material
65
Q
Give examples of phagocytic receptors
A
- C-type lectin receptors (CLRs):
- such as the mannose receptor, or scavenger receptors (receptors capable of binding components of bacterial wall) are examples of PRRs able to trigger phagocytosis
- Another important class of phagocytic receptors are the opsonin receptors.
- These are the complement receptors (CRs) that bind to the forms of complement covalently attached to surfaces, and the Fc receptors, which bind to antibodies
- some of the phagocytic receptors are PRRs that recognise PAMPs or DAMPs
66
Q
What happens once phagocytic receptors are activated?
A
- Upon activation, the phagocytic receptors trigger actin filaments reorganisation to form extensions that envelop and internalise the pathogen
- this newly formed structure is called phagosome and is the place where the material ingested is degraded
- At the same time as phagocytosis is triggered, the respiratory burst activation in the cell’s mitochondria causes the formation of reactive oxygen species (ROS) which are carried in small vesicles that fuse with the phagosome.
- Peroxisomes ( but also primary granules in neutrophils) can also produce ROS.
- The material internalised in the phagosome is then digested by proteolytic enzymes but the modalities can slightly vary depending on the type of phagocytes (to be further explained)
70
Q
Observe the stages of phagocytosis of a bacterium by macrophages
A
71
Q
What are the 5 key steps of phagocytosis?
A
73
Q
What activates macrophages and how does this make them different?
A
- Macrophages will be activated by the environment in which they are and will therefore exert different functions
80
Q
Observe this diagram of the interaction between macrophages, neutrophils and monocytes during an immune response
A
- Macrophages, neutrophils and monocytes interaction in the immune response to a pathogen
- In the image M2 macrophage (or alternatively activated macrophages) is playing the role of the tissue sentinel a role played mostly by resident macrophages
- G-CSF (granulocytes colony stimulation factor) is a growth factor that increases the life span of neutrophils.
87
Q
What are the four steps in the leukocyte adhesion cascade and extravasation?
A
- Rolling
- Activation
- Arrest / Adhesion
- Transmigration or Diapedesis
- Each step involves the interaction between molecules present on the leukocytes surface and corresponding ligands (binding partners) expressed on the surface of the endothelial cells lining the blood or lymphatic vessels.
- Some of the molecules expressed by endothelial cells and leukocytes can be induced by chemokines released during inflammation, which is why leukocytes normally just pass by when the tissue is not inflamed
88
Q
Describe the rolling step of the leukocyte adhesion cascade and extravasation
A
- Selectins on the leucocytes surface interact with cell adhesion molecules (CAMs) on the endothelial cell surface allowing the capture of leukocytes by the endothelium
- However, the selectin/CAM interaction is relatively weak and the leukocytes in this phase do not stop but rather slow down their forward movement similar to a ball rolling on a rough surface.
92
Q
What is transcellular and paracellular diapedesis?
A
- Other modalities of migration such as transcellular, where the leukocyte goes across the body of an endothelial cell
- and paracellular where the extracellular junction between endothelial cells are disrupted and a new bond with the migrating cells are formed to allow the transmigration have also been observed.
- figure legend:
C) Paracellular diapedesis. Leukocytes and endothelial cells coordinately disassemble endothelial cell-cell junctions and open up a gap between two or more endothelial cells (Muller, 2003). (D) Transcellular diapedesis. Leukocytes migrate directly through individual endothelial cells via a transient transcellular pore that leaves endothelial cell-cell junctions intact. Note that the two individual endothelial cells in C and D are distinguished by different shades of pink
