1B gut immunology Flashcards
What is the SA of GI tract?
200m^2
How big is the antigen load in GI tract and what does it consist of?
Massive antigen load consisting of:
- Resident microbiota 10^14 bacteria
- Dietary antigens
- Exposure to pathogens
What does microbiota mean?
A mixture of microorganisms that makes up a community within an anatomical niche
What does microbiome mean?
Collective genomes of all microbiota in all the different anatomical niches
Immunologically, what state is the GI tract in?
- State of ‘restrained activation’
- Balances tolerance vs active immune response
What is the GI tract tolerating?
- Food antigens
- Commensal bacteria
What does tolerance need immunoreactivity against?
Pathogens
Because of ‘restrained activation’, what is the presence of bacterial microbiota in the gut essential for?
Immune homeostasis of the gut and development of a healthy immune system
How do we study the effects of the microbiota on the immune system?
Gnotobiology
- We colonise germ free animals (e.g. mice) and give them a particular germ and compare them to normal house mice and look at microbiota
- e.g. below, if there’s an immunological defect in the development of the small intestine, then the Peyer’s patches in the germ free mice will be fewer and less cellular than house mice
How many gut bacteria vs cells do we have in the body?
10^14 gut bacteria and 10^13 cells → most densely populated ecosystem on Earth
What are the 4 major phyla of bacteria?
- Bacteroidetes
- Firmicutes
- Actinobacteria
- Proteobacteria
There are also viruses and fungi
What do gut bacteria provide for us?
Provide traits we haven’t had to evolve on our own- genes in gut flora are 100x our own genome
- Metabolise indigestible compounds to us
- Act as defence against colonisation by opportunistic pathogens
- Contribute to intestinal architecture
What host factors stimulate bacterial growth?
- Ingested nutrients
- Secreted nutrients
What host factors inhibit bacterial growth?
- Chemical digestive factors –> bacterial lysis
- Peristalsis, contractions, defecation –> bacterial elimination
How does bacterial content change as you go along the GI tract?
- Stomach has the least because of HCl and pepsin + gastric lipase
- Duodenum has bile acids and you have more bacteria
- By the time we get to colon there’s loads of bacteria because there’s not really any host digestive factors
Define dysbiosis
Altered microbiota composition
Explain what the immunological equilibrium is
- On one side of the GI immunological equilibrium, we have symbionts- microbiota and humans live with each other but not with benefit or harm to either, they just live
- In the middle there are commensals- microorganisms that benefits from associating with host but has no effect on the host
- On the other side we have pathobionts- symbionts that doesn’t normally elicit inflammatory response, but under specific conditions (usually environmental) it can cause dysregulated inflammatory disease
What happens when something goes wrong and pathobionts start replicating?
We get inflammation and disease
What causes either equilibrium or dysbiosis (depending on how it is)?
- Infection or inflammation
- Diet
- Xenobiotics
- Hygiene
- Genetics
How can dysbiosis negatively affect the rest of the body?
Through producing bacterial metabolites and toxins e.g.
- TMAO
- 4-EPS
- SCFAs
- AHR ligands
What is the first line of defence the body has against pathogens?
Mucosal defence
What are the three layers of mucosal defence?
- Physical barriers
- Commensal bacteria
- Immunological
What are the two types of physical barriers?
- Anatomical barriers
- Chemical barriers
What are some examples of anatomical barriers?
- Epithelial barrier
- Peristalsis
What is the epithelial barrier made up of?
- Mucus layer made by goblet cells
- Epithelial monolayer with tight junctions
- Paneth cells (small intestine)
Where are paneth cells and what do they do?
- Bases of crypts of Lieberkuhn
- Secrete antimicrobial peptides
What chemical barriers are there?
- Enzymes
- Acidic pH
What do commensal bacteria do?
Occupy an ecological niche and are an ecological barrier
What types of lymphoid tissue are there?
- MALT (Mucosa Associated Lymphoid Tissue)
- GALT (Gut Associated Lymphoid Tissue)
Where are MALT found?
- In submucosa below epithelium as a lymphoid mass containing lymphoid follicles
- Follicles are surrounded by HEV (high endothelial) postcapillary venules, allowing easy passage of lymphocytes
Which specific part of the body is packed full of lymphoid tissue?
Oral cavity- pharyngeal, palatine and lingual tonsils especially
What types of immune response is GALT responsible for?
Both adaptive and innate immune response through generations of lymphoid cells and antibodies
What two types of GALT are there?
- Non-organised
- Organised
What are non-organised GALT?
- Intra-epithelial lymphocytes (sit between enterocytes)
- Make up 1/5 of intestinal epithelium e.g. T cells, NK cells
- Lamina propria lymphocytes
Describe the important cells in this image of small intestine villus zoomed in
Everything comes from the stem cells
- Intestinal epithelial cells migrate up to tip of villus
- Mucous secreting goblet cells
- Paneth cells migrate to bottom of crypt (characterised by dense granules containing anti-microbial peptides)
- Lamina propria makes up middle of villus and contains immune cells e.g. T cells, macrophages, DCs
- Intraepithelial lymphocytes are there between enterocytes
What are organised GALT?
- Peyer’s patches (small intestine)
- Caecal patches (large intestine)
- Isolated lymphoid follicles
- Mesenteric lymph nodes (encapsulated)
Where are Peyer’s patches found?
Submucosa of small intestine- mainly in distal ileum
What are Peyer’s patches?
Aggregated lymphoid follicles covered with follicle associated epithelium (FAE)
What is FAE?
Made up of specialised enterocytes with the role of transferring things (like bacteria) to Peyer’s patches
What cells do FAE lack?
- No goblet cells
- No secretory IgA
- Lack microvilli
What cells are PP made up of?
Organised collection of naive T and B cells
What does the development of PP require?
Exposure to bacterial microbiota
How many Peyer’s patches do we have from last trimester foetus to teens?
- 50 in last trimester
- 200 in teens
How does antigen uptake occur in PP?
- Via M (microfold) cells within FAE
- M cells express IgA receptors, facilitating transfer of IgA-bacteria complex into the Peyer’s patches
What does this show?
Electron microscope images of M cells
- Left image shows dome shape of Peyer’s patch protruding between villi into lumen
- Middle image shows microvilli and M cells
- Right image shows M cells as epithelial cells with microfolds (instead of microvilli) through which bacteria get taken up
What is an alternative route of bacteria uptake than M cells?
- Transepithelial dendritic cells can open up tight junctions and send dendrites into lumen of GI tract and directly sample bacteria
- Brings them back then transports them to mesenteric lymph nodes
Describe the B cell adaptive response
- Mature naïve B-cells express IgM in PPs
- On antigen presentation class switches to IgA
- T-cells & epithelial cells influence B cell maturation via cytokine production
- B cells further mature to become IgA secreting plasma cells.
- Populate lamina propria
How does IgA get into the lumen?
- The IgA goes into a vesicle in the enterocyte
- Enzymatic cleavage occurs to produce secretory IgA (sIgA) which goes in the lumen
What does sIgA do?
Binds luminal antigen to prevent its adhesion and consequent invasion
Describe lymphocyte homing and circulation
1) Peyer’s patch presentation and activation occurs
2) Lymphocytes travel to mesenteric lymph nodes where lymphocyte proliferation occurs
3) Return to circulation through thoracic duct (main lymphatic vessel for return of lymph from GI tract to venous system)
4) It can now either go to peripheral immune system (skin, tonsils and BALT which is Bronchus Associated Lymphoid Tissue) or return to intestinal mucosa via vessels in lamina propria
How do lymphocytes move from blood vessels into lamina propria?
- HEV express MAdCAM1 (Mucosal Addressin Cell Adhesion Molecule 1; tissue specific)
- Lymphocytes express α4β7 integrin and they adhere to MAdCAM1
1) Lymphocytes roll along HEV wall
2) The rolling chemotactically activates T cell which means the α4β7 integrin and MAdCAM1 interact
3) The cell then gets pulled into lamina propria
Why do enterocytes have such a short life span?
- 36 hour life span of goblet cells and enterocytes
- This is because enterocytes are first line of defence against GI pathogens and may be directly affected by toxic substances in diet
- Effects of agents which interfere with cell function, metabolic rate etc will be diminished
- Lesions will be short lived
If escalator-like transit of enterocytes is interrupted through impaired production of new cells, what happens?
Severe intestinal dysfunction
What is the mechanism of a cholera infection?
- It’s an acute bacterial disease caused by Vibrio cholerae serogroups O1 and O139
- Bacteria reach small intestine and get into contact with epithelium to release cholera enterotoxin
- The enterotoxin gets internalised by retrograde endocytosis
- This increases adenylate cyclase activity which leads to increased cAMP levels
- This increases active secretion of salts (Na+, K+, Cl-, HCO3-) via CFTR (cystic fibrosis transmembrane conductance regulators)
- Water follows and leads to diarrhoea
How is cholera transmitted?
Transmitted through faecal-oral route via contaminated water and food
What are the main symptoms of cholera?
- Severe dehydration
- Watery diarrhoea
Other symptoms:
- Vomiting
- Nausea
- Abdominal pain
How do we diagnose cholera?
- Bacterial culture from stool sample on selective agar is the gold standard
- Rapid dipstick tests also available
How is cholera treated?
Oral-rehydration is the main management- up to 80% of cases can be successfully treated
What vaccine is given for cholera?
Dukoral, oral, inactivated
What viral causes of infectious diarrhoea (gastroenteritis) are there?
- Rotavirus (children)
- Norovirus (winter vomiting bug)
What is the rotavirus?
- RNA virus, replicate in enterocytes
- 5 types A-E → type A most common in human infections
What is the epidemiology of the rotavirus?
Most common cause of diarrhoea in infants and young children worldwide
What treatment is given to rotavirus induced diarrhoea?
- Oral rehydration therapy
- Still causes 200k deaths a year
- Before vaccine, most people had an infection by age 5 and repeated infections develop immunity
What vaccine is given for the rotavirus?
Live attenuated oral vaccine (Rotarix) against type A introduced in 2013- very successful
What is the norovirus?
- RNA virus
- Incubation period is 24-48 hours
How is the norovirus transmitted?
- Faecal-oral transmission
- Individuals may shed infectious virus for up to 2 weeks
- Outbreaks often occur in closed communities
What symptoms occur for a norovirus infectin?
Acute gastroenteritis, recovery 1-3 days
How is a diagnosis of norovirus infection made?
Simple PCR
What protozoal parasitic causes of infectious diarrhoea (gastroenteritis) are there?
- Giardia lamblia
- Entamoeba histolytica
What bacterial causes of infectious diarrhoea (gastroenteritis) are there?
- Campylobacter jejuni/coli aka curved bacteria
- Escherichia coli
- Salmonella
- Shigella
- Clostridium difficile
What is the transmission for campylobacter jejuni/coli (curved bacteria)?
- Undercooked meat (esp poultry), untreated water and unpasteurised milk
- Low infective dose, a few bacteria (<500) can cause illness
What is the treatment for curved bacteria infection?
- Not usually required
- Azithromycin (macrolide) is standard Ab
- Resistance to fluoroquinolones is problematic
Describe Escherichia coli
- Diverse group of gram -ve intestinal bacteria
- Most harmless
How many types of E.coli are associated with diarrhoea?
6 pathotypes:
- Enterotoxigenic E.coli (ETEC)
- Enteroinvasive E.coli (EIEC)
- Enterohaemorrhagic/Shiga toxin-producing E.coli (EHEC/STEC)
- Enteropathogenic E.coli (EPEC)
- Enteroaggressive E.coli (EAEC)
- Diffusely adherent E.coli (DAEC)
What does ETEC cause?
- Cholera like toxin
- Watery diarrhoea
What does EIEC cause?
- Shigella like illness
- Bloody doarrhoea
What does EHEC/STEC cause?
- Causes the most problems
- E. coli O157 serogroup, shigatoxin/verotoxin
- 5-10% get haemolytic uraemic syndrome- loss of kidney function
How do we manage clostridium difficile problems?
- Isolate patient (v contagious)
- Stop current antibiotics
- Give vancomycin or metronidazole (which is odd because met. can cause c diff as well as treating it)
What are recurrence rates like for clostridium difficile problems?
- 15-35% after initial infection
- Increasingly difficult to treat if recurrence occurs
How do we cure clostridium difficile problems?
Faecal Microbiota Transplantation (FMT) → 98% cure rate
