8. Immunology of the gut mucosa 1 Flashcards
Why is understanding the immunology of mucosal surfaces important?
It allows the design of vaccines and immunotherapies that are more effective.
What does mucosal immunity do?
It is there to protect internal epithelial surfaces.
Why do epithelial surfaces need protection?
Because they are exposed to a wide variety of infectious agents on a daily basis.
What are the mucosal surfaces?
- GI tract
- Respiratory tract
- Urinogenital tract
- It also includes the glands and draining lymph nodes linked to these tissues.
- Salivary, lachrymal, pancreas and mammary glands.
What else is a good barrier to infection?
the skin
What did the skin evolve from?
A mucosal tissue
What factors challenge the respiratory tract and mucosal surface?
- Particulates
- Pollutants
- Allergens
- Airborne pathogens
What factors challenge the gastrointestinal tract and mucosal surface?
- Food
- Commensals
- Ingested pathogens
What factors challenge the Urogenital tract and mucosal surface?
- Introduced pathogens
- Commensals
- Sperm
What can enter the blood from food?
- Small amounts of whole proteins.
- In most people this doesn’t trigger an immune response.
- But in some people, it does
What can help us treat food allergies?
Understanding how tolerance to food antigens is regulated
What is the experimental protocol for inducing oral tolerance in mouse models of mucosal immunity?
- 1 mouse is fed the protein antigen OVA for 14 days and the control mice are fed PBS.
- After 14 days the mice are injected with OVA and adjuvant.
- The antibody and T cell response are measured.
What did the experiment
about inducing oral tolerance in mouse models of mucosal immunity show?
- If an animal is fed an antigen and then immunised against that antigen, the immune response reduces and shows something happens to induce tolerance and dampen the immune response.
- If no antigen is fed to the animal and then its immunised it develops a full antibody and T cell response
- If an animal is fed antigen A and then immunised with antigen B, it develops an immune response to the antigen that was immunised. (antigen B)
What replicates were done to prove oral induction of mucosal tolerance?
- The antigen feeding experiments were repeated in 3 different strains of mice.
- All three strains showed a reduction in response to the antigens.
- This shows there is a kind of regulation and tolerance induction at mucosal surfaces.
In what other animals has oral induction of tolerance been demonstrated?
- Pigs
- Feed pigs soya.
- Immunised fed and control pigs with soya.
- Soya fed pigs showed a reduction in serum antibodies compared to control pigs.
How new is the concept of oral tolerance?
- Not new in historical context
- People used to dose themselves with small doses of poison to build up tolerance.
- This is not exactly oral tolerance but it demonstrates the point and shows that it is possible.
Are mucosal and systemic immunity the same thing?
no
What is the mucosal immune system?
- It is a specialised branch of the immune system that protects mucosal surfaces exposed to the external environment.
- It is characterised by the secretion of IgA.
- It provides the first line of defence while also maintaining tolerance to harmless antigens.
What is the systemic immune system?
- It is a network of organs, tissues and cells that protect internal non-mucosal tissues from pathogens.
- It includes primary lymphoid organs like the bone marrow and thymus and secondary lymphoid organs like the spleen and peripheral lymph nodes.
- It relies on circulating immune cells, cytokines and mostly IgG antibodies to detect and eliminate infection throughout the body’s internal compartments.
What else is considered part of the mucosal immune system?
The lactating breast
What are the main innate immune defences of mucosal immunity?
- Physical barriers
- Chemical barriers
- Resident microbiota
- Innate immune cells
What is the main function of the mucosal immune system?
To protect against infections entering through these tissues
Innate immune defences: Physical barriers
- They are the separation between the non-sterile external environment and sterile internal body.
- They are covered in a mucus layer to trap pathogens.
- Epithelial tight junctions between cells creates a physical seal to block microbial entry.
- The respiratory tract uses cilia to propel mucus and trapped microbes outwards.
Innate immune defences: Chemical barriers
- Low pH in the stomach and acidic vaginal environment inhibits microbial growth.
- Antimicrobial peptides (AMPs) like defensins that disrupt microbial growth
- Lysozymes which are found in tears, saliva and mucus to break down bacterial cell walls.
- Molecules like lactoferrin to sequester iron to limit bacterial growth.
- These molecules are often secreted into the lumens of mucosal tissues.
Innate immune defences: Resident microbiota
- They compete with pathogens for nutrients and attachment sites.
- They produce metabolites that enhance epithelial integrity and immune regulation.
Innate immune defences: Innate immune cells
- Macrophages and dendritic cells which sample antigens and initiate immune responses.
- Neutrophils are rapidly recruited to infection sites to clear pathogens.
- Innate lymphoid cells are important for early cytokine responses for immunity and tissue repair
What are the main adaptive immune defences of mucosal immunity?
- Mucosa associated lymphoid tissues.
- Regional lymph nodes
- T and B cells
Adaptive immune defences: Mucosa associated lymphoid tissue
- aka MALT
- There are different types of MALT for different mucosal surfaces.
- Nasal associated lymphoid tissue eg tonsils
- Bronchial associated lymphoid tissue
- Gastrointestinal-associated lymphoid tissue eg peyers patches
Adaptive immune defences: Regional lymph nodes
- Draining for the tissue
- help initiate the immune response
- Retropharyngeal, submaxillary and parotid lymph node.
- Mediastinal lymph node
- Mesenteric lymph node
Adaptive immune defences: T and B cells
- B plasma cells secrete lots of IgA, some IgM and some IgG.
- All the normal T cell subtypes, Th1, Th2, Th17 and Treg
What makes up mucosal associated lymphoid tissue?
- A T/B cell follicle. Sometimes both.
- Follicle-associated epithelium
- High endothelial venule
What is a peyers patch?
- Lymphoid aggregates that make up the main part of the GALT.
- They are big yellow structures that are present in all of us.
- There can be multiple follicles in a peyers patch.
- If you have HIV, you have depleted CD4 T cells, and you lack Peyers patches and are vulnerable to infection.
What different structures can make up mucosal associated lymphoid tissue?
- Peyer’s patches
- Isolated lymphoid follicles
- Cryptopatches
What are isolated lymphoid follicles?
- These are generally smaller then a peyers patch.
- They exist in a range of sizes.
- They display characteristics of a germinal centre.
What are crytopatches?
- They are very small structures.
- They develop into single B-cell follicles.
- They contain mainly innate lymphoid cells.
What are mesenteric lymph nodes?
- A lymph node associated with the GALT.
- Contain T and B cell aggregates.
- help prevent infection through the gut
What is mucus made of?
- glycoproteins called mucins.
- It also contains water, lipids, DNA, and antimicrobial compounds.
- There are 21 different types of mucin proteins in humans.
- Glycans can be enzymatically attached to the mucin proteins
What cells produce mucus?
- Goblet cells.
- They sit in the epithelium and secrete vesicles full of mucus.
What is the function of the mucus layer on the epithelium?
- It provides an extra layer of defence by trapping bacteria so they don’t come into contact with the epithelium.
- Bacteria can live in the mucus but stay separated from the epithelium.
- In normal intestines, the outer mucus layer provides a habitat for commensal bacteria and the inner mucus layer prevents bacteria coming close to the epithelium
What does inflammation in the colon cause?
- The inner mucus layer becomes penetrated with bacteria.
- The bacteria come into contact with the intestinal epithelium.
What antibody dominates the systemic immune response?
IgG
What antibody dominates the mucosal immune response?
IgA
What form does IgA take in the plasma?
Monomeric form
What form does secretory IgA take at mucosal surfaces?
A dimeric form
What happens to most of the IgA antibodies produced at mucosal surfaces?
They are actively secreted into the lumen of that tissue
What functions can IgA have at mucosal surfaces?
- Neutralising antibodies that bind to surface structures or toxins.
- They can bind to invading pathogens once they have crossed the epithelium and translocate them back out into the lumen.
What can influence the production of IgA at mucosal surfaces?
- The local microbiome
- They cause the production of IgA specific to them.
- This selective response shapes the composition of the microbiota and main balance between host and commensals
What are antimicrobial peptides?
- These are small cationic peptides produced by epithelial cells and immune cells
- they are normally less then 100 amino acids.
- They play an important role in mucosal immunity.
- 2 main families are defensins and cathelicidins.
What are defensins?
- Small cystein-rich cationic peptides.
- They disrupt microbial membranes and modulates immune responses
- Split into class alpha and beta based on structure and expression
- Produced by Paneth cells
What are cathelicidins?
- Linear antimicrobial peptides.
- Exemplified by LL-37
- They disrupt microbial membranes and regulate inflammation and immune signalling.
- Produced by enterocytes.
What different T cells are present in the mucosa?
- Th2
- Th1
- Th17
- Treg
All do their normal functions
What innate lymphoid cells are present in the mucosa?
- These are similar to T cells but don’t have antigen specific receptors.
- ILC-2
- ILC-1
- ILC-3
What are M cells?
- Microfold cells
- They make up a large part of peyers patches.
- They have a big indent in the baso-lateral side that immune cells can sit inside.
- This aids the sampling of antigens from the lumen and the external environment.
- The indent makes the distance antigen travel shorter so antigen recognition is easier.
- This sampling is mainly done by dendritic cells and T cells.
How else can dendritic cells sample the external environment?
- They can stick a Dendrite between the epithelial cells to sample the environment.
- This can still trigger an immune response
What type of dendritic cell samples antigens?
CD11c+ dendritic cells.
How were CD11c knock out mice made?
- Mice cannot get diphtheria.
- The diphtheria toxin receptor was inserted into dendritic cells under the CD11c promoter.
- This ensures it is always expressed.
- Those DC are susceptible to diphtheria toxin and can be killed by it.
- This specifically causes the apoptosis of DC.
What are CD11c knock out mice used for?
- The role of dendritic cells in the immune response
- This includes the role of DC in the development of tolerance.
What are OT1 and OT2 mice?
- These are transgenic mice used to probe T cell responses.
- They are specific for OVA antigens.
- OT1 mice are MHC1 restricted so used to study CD8 T cell responses.
- OT2 mice are MHC2 restricted so used to study CD4 T cell responses.
What happens if OT2 T cells are put into a CD11c KO mouse?
- control still had DC.
- the peyers patches the mesenteric lymph nodes were observed.
- No DC should mean no immune response.
- In the control with DC and immunised to OVA the immune response remains.
- In the DC KO, the immune response is gone and there is no T cell proliferation to OVA.
What does using OT2 T cells in CD11c KO prove?
Removing CD11c+ dendritic cells stops the induction of mucosal tolerance.
What are the different dendritic cell markers?
CD11c = the general DC marker
CD103 = an integrin marker found in tissues with mucosal surfaces.
CX3CR1 is a chemokine receptor found in the lamina propria Dc of the gut.
Which subset of dendritic cells is important for activating T cells?
CD103 CD11c+ dendritic cells
Which subset of dendritic cells is important for mucosal immune responses?
CD103 CD11c+ dendritic cells
Where are T cells in mucosal immune responses activated?
Peyers patches or mesenteric lymph nodes
How do activated T and B cells get back to the intestine?
- DC don’t just activate T cells, they educate them through cytokines.
- This aids homing signals.
- CCR9 and a4ß7 are induced by cDC on T cells to home them to the gut.
- CCR9 binds to CCL25 and a4ß7 binds to MAdCAM-1.
- These homing signals cannot be induced by blood derived cDCs.
- This homing is important when designing vaccines.
What tolerance mechanism is inducing mucosal tolerance?
Induction of Treg cells.
What are Treg cells?
- They are a specialised sub set of T cells that play a critical role in immune regulation and tolerance.
- Their primary function is to suppress excessive or inappropriate immune responses, preventing autoimmune reactions and controlling inflammation.
- They express CD4 and CD25.
- Foxp3 is a critical transcription factor.
- They secrete IL-10, IL-4 and TGFß.
What are SCID mice?
- They have no T/B/NK cells
- They are very susceptible to infections.
- They need to be kept in sterile environments
How were SCID mice used to show the function of Tregs?
- If SCID mice were just given T cells they get colitis.
- If SCID mice are given IL-10 producing T cells they still got colitis.
- If SCID mice were given IL-10 producing T cells and the antigen these cells were specific for, Colitis was eliminated.
- This shows that Tregs can suppress immunity, but they need to be activated to do so.
What cells are important for inducing Tregs?
- Mesenteric lymph node CD103+ dendritic cells.
- These preferentially induce Tregs.
What is the role of Tregs in controlling mucosal immunity?
- This is done using the diphtheria toxin receptor like DC Treg mouse models and a feeding and vaccination program.
- IT was found that DCs are important in activating Tregs, and Tregs provide protection from immunity.
- Treg numbers expand on the induction of mucosal tolerance
How do Dendritic cells and Tregs work together at mucosal surfaces?
They combine to regulate tolerance and prevent inflammation.
