IMI 6: The Immune Response Against Extracellular Pathogens Flashcards
1
Q
Observe the learning outcomes of this session
A
2
Q
What do the first barriers to infection consist of?
Give some examples
A
- physical barriers
- mechanical barriers
- chemical barriers
3
Q
Where do the mucosal membranes cover?
A
- the gastrointestinal tract
- upper and lower respiratory tract
- urogenital tract
4
Q
What is the commensal microbiome?
A
- the collective of microorganisms present on our mucous membranes
5
Q
Describe some key features of commensal microorganisms
A
- The microbiome represents a competitive environment for pathogens.
- Resident commensal bacteria, for example, can secrete antibiotics and antimicrobial peptides called bacteriocins that act against competitor microorganisms, creating an effective first line of defence against invading pathogens.
- Commensal bacteria provide some advantages for the host.
- For example, they produce molecules such as vitamin K and B12, which are essential for the proper functioning of our bodies.
- They are also fundamental in breaking down plant fibres and inactivating toxic substances found in food.
- Some of these harmless bacteria can become pathogenic if they cross the mucosal barrier or relocate to a different anatomical district.
- Staphylococcus epidermidis lives on our skin and is harmless but after damage, for example a cut, it can penetrate the barriers and trigger inflammation;
- Pseudomonas aeruginosa is commonly found in water and most of us can fight it effectively.
- However, it can infect the airways, urinary tract and wounds of vulnerable individuals and cause real harm (e.g. by causing pneumonia and in some cases sepsis).
6
Q
Observe this diagram of the immune system
A
7
Q
How much of the entire immune system do mucosal-associated lymphoid tissues (MALT) represent?
A
- 70%
8
Q
Describe how gut tissue and the gut-associated lymphoid tissue (GALT) is organised
A
- The gut mucosa is composed of a layer of epithelial cells that are in contact with the intestinal lumen and a layer of connective tissue called lamina propria populated by cells of the immune system.
- The epithelial cells are structurally and functionally diverse.
- For example, the enterocytes are covered in microvilli and are specialised in the absorption of digested nutrients, whilst others have an important role in maintaining the immune homeostasis of the gut
- Figure legend: Cellular anatomy of gut mucosa tissue of the small intestine. A single layer of epithelial cells with diverse functions, including enterocytes covered with microvilli, Paneth cells secreting antimicrobial peptides, Goblet cells secreting mucus and anti-microbial peptides and entero-endocrine cells which produce important hormones involved in regulating the digestion. Disseminated within the epithelium are also the intraepithelial lymphocytes (IEL) a subset of T cells that don’t need priming and are able to directly respond to pathogens. Dendritic cells, Macrophages, B cells and T cells reside just below the epithelium in the mucosa. The lumen is populated by commensal bacteria which are kept under control by IgA produced by B cells.
9
Q
Describe the immune anatomy of the lamina propria
A
- The lamina propria includes both resident immune cells and immune cells migrating from other lymphoid organs. Importantly the resident immune cells are organised into distinctive gut structures:
- The Payer patches: have anatomical features that are similar to lymph nodes and they are present in the small intestine.
- Isolated lymphoid follicles (ILFs): are located beneath the epithelial surface of both the small and large intestine and are formed by isolated B cells, mostly producing IgA.
- The location of a variety of immune cells just below the epithelium is fundamental as it offers closer contact with the microorganisms present in the lumen
- Figure legend: Organization of resident immune cells within the lamina propria. Secondary lymphoid tissue associated with the small intestine including Peyer’s patches organised with a structure similar to that of the lymph node and isolated lymphoid follicle.
10
Q
What is immune tolerance?
A
- the absence of an immune response against a particular foreign body
11
Q
What are the steps of the tolerogenic response?
A
- Antigen sampling
- Release of tolerogenic molecules and Treg recruitment
- IgA secretion from plasma cells
12
Q
Describe step 1 ‘antigen sampling’ of the tolerogenic response
A
- How can DCs sample antigens or sense the PAMPs of commensal organisms if they are in the gut lumen, separated from the immune cells by the epithelial layer?
- The figure above illustrates some of the mechanisms involved in antigen sampling in the gut lumen.
- As explained earlier the epithelial layer is formed by a heterogeneous population of cells.
- Some of these cells have evolved the ability to secrete mucus and antimicrobial peptides but can also sample for antigens.
1. Microfold or M cells are often associated with the Peyer’s patches and isolated lymphoid follicles. - Both M cells and Goblet cells move soluble antigens via transcytosis from the apical side to the basal side and release them in the lamina propria where dendritic cells are conveniently located.
2. IgA-bound antigens (from IMI3 you should remember that this antibody subtype is secreted abundantly in the mucosal surfaces) are captured by an FcR𝝰 receptor present on the surface of epithelial cells and released to dendritic cells residing in the lamina propria via receptor-mediated transcytosis.
3. Dendritic cells can directly sample for antigens in the intestinal lumen by extending cell processes called dendrites between epithelial cells.
13
Q
Describe step 2 ‘release of tolerogenic molecules and Treg recruitment of the tolerogenic response
A
- Epithelial cell interaction with commensal organisms via PRR or via IgA bound antigens also promotes the release of tolerogenic molecules, for example, TGF-beta, that program dendritic cells to release IL-10.
- IL-10 is a potent anti-inflammatory cytokine that is produced when the mucosa is healthy, un-breached and populated by commensal organisms rather than pathogens.
- When DCs migrate to the regional lymph node or the Peyer’s patches to present antigens to CD4+ helper T cells, they release IL-10.
- This promotes the differentiation of helper T cells into regulatory T cells (Treg).
- Tregs maintain the immune homeostasis producing more IL-10 and suppressing the immune response.
14
Q
Describe Step 3 ‘IgA secretion from plasma cells’ in the tolerogenic response
A
- The tolerogenic environment, particularly the presence of TGF-beta and IL-10, promotes also plasma cells present in the Peyer’s patches and in the ILFs (isolated lymphoid follicles), to switch class from IgM to IgA (if you need to refresh your memory on the mechanism of class switch recombination have a look at IMI3).
- This can happen in a T cell-dependent or independent manner. In fact, some of the cytokines released by epithelial cells and dendritic cells can directly promote the production of IgA from B cells.
- IgA dimers are transcytosed from the lamina propria to the intestinal lumen via polymeric Ig receptors on epithelial cells.
- Once in the lumen IgAs bind antigens on the surface of commensal organisms.
15
Q
What do IgA, antimicrobial peptides and mucus produced by Goblet cells do?
A
- they keep commensal microorganisms at a safe distance
- making the symbiotic relationship with the microbiome possible
16
Q
What is an important difference between commensal organisms and pathogens?
A
- pathogens release molecules aimed at damaging the barrier and penetrating it.
- These include toxins, and lytic enzymes able to degrade the extracellular matrix and damage the epithelial layer allowing bacteria to penetrate, but also hair-like structures such as fimbriae and pili that make bacteria adhere to epithelial cells.
17
Q
What is the first step pathogens make in the gut?
A
- adhesion is sometimes the first step before the damage
18
Q
What are the steps of the gut immune response to pathogens
A
- Inductive phase
- Effector phase
19
Q
Describe Step 1 ‘Inductive phase’ of the gut immune response to pathogens
A
- In the presence of a pathogen, PRRs on both epithelial cells and antigen-presenting cells will sense the presence of PAMPs that differ from those of commensal bacteria;
- one of the consequences is the activation of the inflammasome and the secretion of inflammatory cytokines (IMI4).
- An example of a PAMP is pilin: a protein present in bacterial fimbriae.
- The shaping of the immune response mounted by antigen-presenting cells is called the inductive phase: after capturing the pathogen’s antigen DCs travel to the Peyer’s patches to alert naïve T helper cells of the danger.
- Here they not only present the antigen but also release inflammatory cytokines that polarise helper T cells into the most appropriate subtype to fight the infection.
- Examples include Th1, Th2, Th9 and Th17 depending on the type of pathogen invading the barrier.
20
Q
Describe Step 2 ‘Effector phase’ of the gut immune response to pathogens
A
- The most appropriate subtype of T helper cells will orchestrate the immune response by recruiting and activating other cells of the immune system to fight the infection.
- This phase is called the effector phase.
- One important event in the effector phase is the class switch of immunoglobulins in B cells to produce IgG (IMI3).
- This class of antibodies is in fact found in all body fluid and provide full protection in case of a systemic infection.
21
Q
Why do some epithelial cells express TLRs at the basolateral rather than the apical surface of the cell?
A
- In this way, they are able to sense bacteria that have crossed the epithelium.
- Thus they can tell the difference between invading bacteria and those that are localised in the gut lumen.
- This is important to check for commensal organisms that do not pose any harm when they are confined in the gut lumen but can be potentially dangerous once they have crossed the barrier.
22
Q
Watch this video which summarises the key points of mucosal immune responses
23
Q
What is the role of microfold cells (the so-called M-cells)?
A
- transcytosis of antigens
- They transport soluble substances, stimulate the production of IgA (although they do not produce the antibodies themselves!), uptake antigen via endocytosis and move them via transcytosis.
24
Q
A
25
Q
How do we define an extracellular pathogen?
A
- An extracellular pathogen invades the human body by crossing its protective barriers and spends some of its life-cycle (if not all) outside the cells of its host.
- We are naturally prone to consider extracellular pathogens to be organisms like bacteria and in fact, most of this module will focus on these organisms. But let’s think again of the wide array of pathogens that can infect our body and their life cycle.
- viruses are obligate intracellular pathogens, but before infecting the cell (step 1) and when they break out to infect new cells or new individuals (step 6) they are exposed to the extracellular environment and as such, they are potentially vulnerable to all the immune defences aimed at extracellular pathogens.
26
Q
Describe some bacterial species that can live inside the cell
A
- some bacterial species have evolved to live hidden inside the cell.
- This is the case of Mycobacterium leprae which causes leprosy (see figure on the left).
- Yersinia pestis, the infectious agent that causes the bubonic plague (or black death), the pandemic with the largest death toll ever recorded (75-200 million people) can live both inside and outside the cell, a strategy to elude the immune system!
32
Q
What are the functions of complement proteins?
A
- Complement proteins can be covalently attached to any extracellular pathogen.
- The first attachment creates a positive feedback loop cleaving and attaching more and more complement molecules to that surface.
- This results in the pathogen opsonzation (B), allowing clearance by professional phagocytic cells (neutrophils and macrophages) through their complement receptors.
- The release of cleavage byproducts of complement acts as a potent chemoattractant (C), potentiating the activation and migration of phagocytic cells and other cells of the immune system to the site of infection.
- Finally, the membrane attack complex (MAC) inserted into the pathogen’s membrane causes direct damage and lysis (A) of the target extracellular pathogen
34
Q
Describe the membrane attack complex (MAC) and how it differs between gram-positive and gram-negative bacteria
A
- The membrane attack complex (MAC) can directly damage pathogens by perforating their outer layer.
- However, it is worth mentioning that the susceptibility of gram-positive and gram-negative bacteria to the MAC is very different because of the fundamental structural differences between these two groups of bacteria.
- The MAC is able to penetrate the outer (and sometimes the inner) membrane of gram-negative bacteria and enveloped viruses (IMI7) causing effective damage, but cannot get through the thick peptidoglycan wall of most gram-positive bacteria to reach the membrane.
46
Q
Describe the difference between MHC class I and class II
A
- The key difference between MHC class I and MHC class II (the most relevant to this session) is their source of antigens.
- MHC class II is loaded with extracellular antigen sampled and processed by APCs.
- In contrast, MHC class I is loaded with endogenous antigens: those that have been produced and/or degraded inside the cell cytosol. This (and a specific exception to this rule) have been discussed in IMI5 and will be discussed further in the next module (IMI7).
52
Q
What are superantigens?
A
- A Superantigen is a protein that is able to bind the outside surfaces (ie not the part that binds antigen) of both MHC class II and TCR complexes, forming a cross-linking bridge between the two.
- This tricks the T cell into thinking its TCR has bound an antigen, inducing activation regardless of the specificity of the TCR for a suitable antigen.
55
Q
Describe the function of circulating B cells
A
- Circulating B cells have a particularly important mechanism of sampling and presenting antigens.
- They take up and process soluble extracellular pathogen antigens that are bound by the B cell receptor (BCR - see IMI3 if you have forgotten what this is).
- The antigen is then internalised by receptor-mediated endocytosis, degraded in the lysosomal compartment, and presented on MHC class II, in much the same way as other antigens, as depicted in the image below.
- The presentation from B cells to T helper cells, which was first explained to you in IMI3 is really important during B cell maturation.
- The B cell will therefore receive T cell help only if there is a T cell that regards what its BCR has bound as a foreign antigen.
- Conversely, the resulting activation of the B cell will make it release cytokines to polarise the T helper cell into T follicular helper cells, which feedback to B cells with the signals necessary to travel to a germinal centre, and mature into an antibody-producing B cell (ie a plasma cell) or memory B cell.
- Plasma cells produce a large number of antibodies to fight the infection.
- Memory B cells (more in IMI7) will be stored in case of future encounters with the same antigen.
- These cells both recognise the antigen more specifically, can be rapidly activated, and subsequently proliferate and differentiate into plasma cells for a much faster response.
56
Q
What are the functions of the antibody
A
- opsonisation:
- Antibodies bind the target antigen with their variable region, while their constant region (Fc) is free to bind to Fc receptors on the surface of phagocytic cells, which can then internalise the antigen for MHC class II presentation.
- neutralisation:
- Neutralisation is a process whereby an antibody binds to a pathogen protein and blocks its function.
- The figure shows examples of antibodies preventing a toxin (left), a virus (middle) and a bacterium (right) from binding its target.
- This can stop viruses/bacteria from binding and entering a cell, or can stop toxins from binding and affecting their target proteins.
- complement activation:
- Antibodies activate the classical complement pathway. This is the only complement pathway that needs to use adaptive immune molecules for its activation.
58
Q
A naive B cell
A
60
Q
Which of these are roles of DCs in the Peyer’s patch?
A
- The correct answer is E-F-G-H-.
- The Payers’ patches are secondary lymphoid organs that are structured similarly to the lymph node.
- DCs sample antigen from the gut lumen either directly or indirectly (ie via M cells).
- They then travel to the Peyers’ patches where naive T cells reside.
- Here DCs present the sampled antigens and release IL-10.
- By doing so they activate T cells and polarise them into T regulatory cells or T regs. T regs are now aware of the type of microorganisms present in the gut lumen and by secreting IL-10 they send a message of no danger.
- T regs, in turn, will produce more IL-10 and TGF-Beta wich downplay the immune response.
