Gut immunology Flashcards
What is the surface area of the GI tract?
200m^2
What does the antigen load of the GI tract consist of?
Resident microbiota of 10^14 bacteria
Dietary antigens
Exposure to pathogens
What does microbiota mean?
A mixture of microorganisms that make up a community within an anatomical niche
What does microbiome mean?
Collective genomes of all microbiota in all the different anatomical niches
Describe the state of “restrained activation” of the GI tract
Dual immunological role- tolerance (for food antigens, commensals) vs immunoreactivtity/ active immune response (against pathogens)
What is the presence of bacterial microbiota in the gut essential for?
Immune homeostasis of gut and developement 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
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 types of micro-organisms make up the gut microbiome?
Bacteria (bacterioidetes, firmicutes, actinobateria, proteobacteria), viruses, fungi
What kind of traits do gut microbiota provide for us?
How m any genes has the gut microbiota evolved compared to our own?
Provide traits we haven’t had to evolve on our own- genes in gut flora are 100x our own genome
What specific roles do gut microbiota have in maintaining immune homeostasis/ development in the gut?
Metabolise compounds that are indigestible 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 then does bacterial content and host digestive factors change as you go along the GI tract?
Stomach 10^1
HCl (pH approx. 1.4), pepsin, gastric lipase
Duodenum 10^3
Bile acids from liver
Jejunum 10^4
Pancreatic trypsin, amylase, carboxypeptidase)
Ileum 10^7
Small intestinal brush border enzymes
Colon 10^12
No host digestive factors
Define dysbiosis
Altered microbiota composition
Explain what the immunological equilibrium is
In the GI immunological equilibrium, on one side we have symbionts- humans and microbiota live with each other but not with benefit or harm to either
In the middle we have commensals- microorganisms that benefit from associating with host but have 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 are the causes of either equilibrium or dysbiosis (depending on the state?)
Infection or inflammation
Xenobiotics
Diet
Hygiene
Genetics
What substances does dysbiosis generate that negatively affect the rest of the body?
Bacterial metabolites and toxins
e.g.
TMAO (increases cholesterol deposition in artery walls to cause atherosclerosis)
4-EPS (associated with autism)
Short chain fatty acids (decreased numbers are associated with IBD and increased numbers are associated with neuropsychiatric disorders e.g. stress)
AHR ligands (associated with MS, rheumatoid arthritis, asthma)
What different effects does an increase in bacterial metabolites and toxins have on different body systems?
Brain- stress, autism, MS
Lung- asthma
Liver- NAFLD, NASH
Adipose tissue- obesity, metabolic disease
Intestine- IBD, coeliac disease
Systemic disease- type 1 diabetes, atherosclerosis, rheumatoid arthritis
What is the first line of defence the body has against pathogens?
Mucosal defence
What are the 3 layers of mucosal defence?
1st: physical barriers
2nd: commensal bacteria
3rd: immunological
What makes up the physical barriers of mucosal defence?
Anatomical barriers
-epithelial barrier
-peristalsis
Chemical barriers
- Enzymes
- Acidic pH
What makes up the epithelial component of the physical barriers of mucosal defence?
Mucus layer made up of goblet cells
Epithelial monolayer with tight junctions
Paneth cells
Where are Paneth cells found and what do they secrete?
Bases of crypts of Lieberkuhn of small intestine
Secrete antimicrobial peptides (defensins) and lysozyme
How do commensal bacteria provide mucosal defence?
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 in the body and in what form can you find MALT?
In submucosa below epithelium as a lymphoid mass containing lymphoid follicles
Oral cavity is packed full of this type of tissue- pharyngeal, palatine and lingual tonsils especially
What type of blood vessels surround lymphoid follicles of MALT and what does this facilitate?
HEV (high endothelial) postcapillary venules, allowing easy passage of lymphocytes
What forms the largest mass of lymphoid tissue in body?
GALT
What types of immune response is GALT responsible for?
Both adaptive and innate immune response through generations of lymphoid cells and antibodies
What are the two types of GALT
Non-organised
Organised
What cells make up non-organised GALT?
Intra-epthelial 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 the small intestine villus
Everything comes from stem cells
Intestinal epithelial cells- migrate to tip of villus
Intraepithelial lymphocytes are there between enterocytes
Goblet cells- secrete mucus
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
What are the organised GALTs?
- Peyer’s patches (small intestine)
- Caecal patches (large intestine)
- Isolated lymphoid follicles
-Mesenteric lymph nodes (encapsulated)
What do people mistake enlarged mesenteric lymph nodes as being?
Pathological even though they’re just reactive and are the largest lymph nodes you find in the body naturally
Where are Peyer’s patches found?
Submucosa of small intestine- mainly in distal ileum
What are Peyer’s patches (form)?
Aggregated lymphoid follicles covered with follicle associated epithelium (FAE)
What is FAE made up of?
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 Peyer’s Patches made up of?
organised collection of naive T and B cells covered with FAE cells
What does the development of Peyer’s patches requires?
Exposure to bacterial microbiota
How many Peyer’s patches do we have from last trimester foetus to teens?
- 50 in last trimester
- 250 in teens
How does antigen uptake occur in Peyer’s patches?
- Via M (microfold) cells within FAE
- M cells express IgA receptors, facilitating transfer of IgA-bacteria complex into the Peyer’s patches
Describe the following 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 after M cells take up pathogens
They are excreted into a pocket formed in inner surface of enterocyte
Pocket contains APCs like dendritic cells that engulf antigen and presents them through MHC II
DC (or APC) migrates to Peyer’s Patch where the T and B cells are
Mature naive B cells express IgM in PPs and on antigen presentation they switch class to IgA (up to 90% of gut B cells secrete IgA)
Some APCs escape into lymph to go to activate B and T cells in mesenteric lymph nodes
Some of these activated cells return to gut tissue (effector sites) to produce Abs
T cells and epithelial cells overall basically influence B cell maturation via cytokine production
IgA secreting B cells then populate lamina propria
How do these IgA get from lamina propria 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
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
What is the lifespan of goblet cells and enterocytes?
36 hours
Why do enterocytes have such a short life span?
- 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 infection of cholera?
It is an acute bacterial disease caused by Vibrio cholerae serogroups O1 and O139
Bacteria reach small intestine, get in contact with epithelium and start producing 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?
Through faecal-oral route via contaminated water and food
Symptoms of cholera?
Main symptoms- Watery diarrhoea, severe dehydration
Other symptoms- Nausea, vomiting, abdominal pain
How do we diagnose cholera?
Bacterial culture from stool sample on selective agar is gold standard
Rapid dipstick tests
How is cholera treated?
Oral-rehydration is the main management- up to 80% of cases can be successfully treated
Cholera vaccine?
Dukoral- oral, inactivated
What are viral causes of infectious diarrhoea (gastroenteritis)?
Rotavirus (children)
Norovirus (winter vomiting bug)
What pathogen is the most common cause of diarrhoea in infants and young children worldwide
Rotavirus
Describe rotaviruses
What types are there and which is most common in human infections
- RNA virus, replicate in enterocytes
- 5 types A-E → type A most common in human infections
How is rotavirus treated?
- Oral rehydration therapy
- Still causes 200k deaths a year
Rotavirus vaccine?
Live attenuated oral vaccine (Rotarix) against type A introduced in 2013- very successful
Before vaccine, most people had an infection by age 5 and repeated infections develop immunity
Describe noroviruses
Incubation period?
- RNA virus
- Incubation period is 24-48 hours
Transmission of norovirus
- Faecal-oral transmission
- Individuals may shed infectious virus for up to 2 weeks
- Outbreaks often occur in closed communities
Symptoms of norovirus infection
Acute gastroenteritis, recovery 1-3 days
How is a norovirus infection diagnosed?
Sample PCR
What protozoan parasitic infections can cause infectious diarrhoea (gastroenteritis)?
Giardia lamblia
Entamoeba histolytica
What bacterial infections can cause infectious diarrhoea (gastroenteritis)?
Salmonella
Shigella
Cholera
Campylobacter jejuni/ coli aka curved bacteria
C. difficile
Escherichia coli
How is Campylobacter jejuni/ coli aka curved bacteria transmitted?
- Undercooked meat (esp poultry), untreated water and unpasteurised milk
- Low infective dose, a few bacteria (<500) can cause illness
How is Campylobacter jejuni/ coli aka curved bacteria treated?
- Not usually required
- Azithromycin (macrolide) is standard Ab
- Resistance to fluoroquinolones is problematic
What is the commonest cause of food poisoning in the UK?
Campylobacter jejuni/ coli aka curved bacteria
Describe the E. coli bacterium
- Diverse group of gram -ve intestinal bacteria
- Most harmless
How many types of E. coli are associated with diarrhoea?
6 pathotypes
ETEC- Enterotoxigenic E. coli
EIEC- Enteroinvasive E. coli
EHEC/ STEC- Enterohaemorrhagic or Shiga toxin-producing E. coli
EPEC- Enteropathogenic E. coli
EAEC- Enteroaggressive E. coli
DAEC- Diffusely adherent E. coli
What does enterotoxigenic E. coli cause?
- Cholera like toxin
- Watery diarrhoea
What does enteroinvasive E. coli cause?
- Shigella like illness
- Bloody diarrhoea
What does enterohaemorrhagic E. coli cause?
- Causes the most problems
- E. coli O157 serogroup, shigatoxin/verotoxin
- 5-10% get haemolytic uraemic syndrome- loss of kidney function
Clostridium difficile- how does it cause problems?
- It usually lives normally in GI tract without causing problems
- If there’s a dysbiosis (usually due to antibiotics) you go into a intermediate dysbiotic state where there is more C diff and more inflammation tolerant commensals and less disturbance-sensitive commensals
- There’s a chance still for things to go back to normal
- If not, you get pathogen induced disturbance (toxin production) and inflammation derived metabolites
How do we manage Clostridium difficile infections?
- 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 infections?
- 15-35% after initial infection
- Increasingly difficult to treat if recurrence occurs
How do we cure Clostridium difficile infections?
Faecal Microbiota Transplantation (FMT) → 98% cure rate
