Immunology of the Gut Flashcards
What is the surface area of the GI tract?
200m^2
Size of tennis court
What does the gut have to deal with?
Massive antigen load
What comprises the massive antigen load of the gut?
Resident microbiota 1014 bacteria
Dietary antigens
Exposure to pathogens
What is the permanent state of the gut?
“restrained activation”
– Tolerance vs active immune response
– Dual immunological role.
What must the gut develop tolerance towards?
Food antigens
Commensal bacteria
What is Gnotobiology?
Gnotobiology
Take germ free animals and you selectively colonise them with selective bateria
Observe differences in these mice to conventioanlly housed mice
e.g. development of peyer’s patches, paneth cells
How many gut bacteria are there?
10^14 gut bacteria and 10^13 cells in body - most densely populated “ecosystem” on Earth
What are the 4 major phyla of bacteria?
Bacteroidetes, Firmicutes, Actinobacteria, Proteobacteria
Why is the gut microbiota important?
Provide traits we have not had to evolve on our own - Genes in gut flora 100 times our own genome
Give example of functions of the gut microbiota?
Provide essential nutrients that cannot be manufactures
Metabolise indigestible compounds
Defence against opportunistic pathogens
Contribute to architecture of the gut
What is the microbiome?
Collective genome of all microbiota
What can increase numbers of microbiota?
Ingested nutrients
Secreted nutrients
What can decrease the numbers of microbiota?
Chemical digestive factors leading to bacterial lysis
Peristalsis and defecation
What is the bacterial content of the stomach?
10^1
HCL, Pepsin and Gastic Lipase
What is the bacterial content of the duodenum?
10^3
Bile acids from liver
What is the bacterial content of the jejunum?
10^4
Trypsin, amylase, carboxypeptidase from pancreas
What is the bacterial content of the ileum?
10^7
Brush border enzymes
What is the bacterial content of the colon?
10^12
No host digestive factors
What is dysbiosis?
Altered microbota compsition
What are symbionts?
Organisms that live with the host without harm or benefit to host or organism
What are commnesals?
Benefits from host but does not benefit or harm host
What are pathobionts?
Symbionts that do not normally illicit an inflammatory response
But in certain environments have the potential to cause disregualted inflammation and disease
What causes dysbiosis?
Infection or inflammation
Xenobiotics
Hygiene
Genetics
What is TMAO?
Increased deposition of cholesterol in artery walls
What is 4-EPS?
Associated with Autism
What are SCFAs?
Short chain fatty acids
Decrease associated with IBS
Increase associated with stress
What are AHR ligands?
Associated with MS, rheumatoid arthritis and asthma
What is NAFLD?
alcoholic fatty liver disease
What is NASH?
alcoholic steatohepatitis
inflammation from fat
What comprises the bodies mucosal defence?
Anatomical Epithelial barrier Peristalsis Chemical Enzymes Acidic pH
What forms the epithelial barrier?
Mucus layer - Goblet cells
Epithelial monolayer - Tight junctions
Paneth Cells (small intestine)
What are the main features of paneth cells?
Bases of crypts of Lieberkühn.
Secrete antimicrobial peptides (defensins) & lysozyme
What are the different types of immunological tissue?
MALT (Mucosa Associated Lymphoid Tissue)
GALT (Gut Associated Lymphoid Tissue)
Where is MALT found?
Found in the submucosa below the epithelium, as lymphoid mass containing lymphoid follicles
What are they surrounded by?
Follicles are surrounded by HEV postcapillary venules, allowing easy passage of lymphocytes
Where is rich in MALT?
Oral cavity
Palatine and lingual tonsils
Pharyngeal tonsisl (Adenoids)
What is GALT responsible for?
both adaptive & innate immune responses
What comprises GALT?
Consists of B & T lymphocytes, macrophages, APC (dendritic cells), and specific epithelial & intra-epithelial lymphocyt
What are the two types of GALT?
Non-organised
Organised
What are the main features of non-organised GALT?
Intra-epithelial lymphocytes
Make up 1/5th of intestinal epithelium, e.g. T-cells, NK cells
Lamina propria lymphocytes
What are the main features of organised GALT?
Peyer’s patches (small intestine)
Caecal patches (large intestine)
Isolated lymphoid follicles
Mesenteric lymph nodes (encapsulated)
Describe the cells present in non-organised GALT
Stem cells produce enterocytes that rapidly migrate to apex
Apoptotic intra-epithelial cells
Goblet cells at base the migrate north and produce mucus
Paneth cells which produce anti-microbial peptides
Intra-epithelial lymphocytes
Majority of immunological cells found in lamina propia
What is the difference between small and large bowel GALT?
No villi just crypts
No paneth cells
Lots of goblet cells
What is the main type of organised GALT?
Peyer’s patches
Where are peyer’s patches found?
Found in submucosa small intestine – mainly distal ileum
What comprises peyer’s patches?
Aggregatedlymphoid follicles covered with follicle associated epithelium (FAE).
FAE - no goblet cells, no secretory IgA, no microvilli
How are peyer’s patches organised?
Sub-epithelial zone - mainly dendritic cells
B-cell follicles
Interfollicular T-cells
All move towards mesenteric lymph nodes
What does peyer’s patches development require?
Development requires exposure to bacterial microbiota
How many peyer’s patches are in the body?
50 in last trimester foetus, 250 by teens
How do peyer’s patches work?
Antigen uptake via M (microfold) cells within FAE
M cells expressIgA receptors, facilitating transfer of IgA-bacteria complexinto the Peyer’s patches.
What is the route for preventing bacterial invasion separate to M cells?
Via dendritic cells
Open up tight junctions and send dendrites into epithelium to retrieve bacteria from lumen of gut
Bring back and transport to mesenteric lymph nose
They express tight junction proteins so maintain the tight-junction barrier
What is the B-cell adaptive response in peyer’s patches?
Mature naïve B-cells express IgM in Peyer’s Patches
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
Some enter lymphatic system
What happens to the immune cells that enter the lymphatics system?
Plasma cells migrate back to enterocytes
Taken up by epithelila cells
Secret IgA
What is the function of sIgA?
Up to 90% of gut B-cells secrete IgA
sIgA binds luminal antigen
→ preventing its adhesion and consequent invasion.
Summarise lymphocyte honing and circulation
Antigen presents in peyers patch
Activation
Enters mesenteric lymph node - lymphocyte proliferation
Go into circulation via thoracic duct (main duct for return to venous system)
Enters peripheral immune system
What comprises peripheral immune system?
Skin
Tonsils
BALT
How do lymphocytes return the peyer’s patches?
HEV express MAdCAM1 which is as specialised adhesion molecule
MAdCAM1 tethers lymphocytes and arrests them
Transports back to peyers patches
Why do enterocytes and goblet cells have such a short lifespan?
Enterocytes are first line of defense against GI pathogens & may be directly affected by toxic substances in diet.
Effects of agents which interfere with cell function, metabolic rate etc will be diminished.
Any lesions will be short-lived.
What is the mechanism of cholera infection?
Cholera -acute bacterial disease caused by Vibrio cholerae serogroups O1 & O139
Bacteria reaches small intestine → contactwith epithelium & releasescholera enterotoxin.
Gets internalised via retrograde endocytosis into enterocytes activating adenylase cyclase
This increases cAMP
Causes active secretion of salt and fluid via the cystic fibrosis transmembrane conductance regulator (CFTR)
Loss of salt, potassium, chloride, bicarbonate and water
How is cholera transmutted?
Transmitted through faecal-oral route
Spreads via contaminated water & food.
What are the symptoms of cholera?
Main symptoms
Severe dehydration & watery diarrhoea
Other symptoms
Vomiting, nausea & abdominal pain.
How is cholera diagnosed?
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 are the main features of the cholera vaccine?
Dukoral, oral, inactivated.
What are the viral causes of diarrhoea?
Rotavirus (children)
Norovirus “winter vomiting bug”
What are the bacterial causes of diarrhoea?
Campylobacter jejuni Escherichia coli Salmonella Shigella Clostridium difficile
What are the protozoal parasitic causes of diarrhoea?
Giardia lamblia
Entamoeba histolytica
What are rotaviruses?
RNA virus, replicates in enterocytes.
5 types A – E, type A most common in human infections.
What is the most common cause of diarrhoea in infants and young children?
Rotaviruses
What is the vaccine against rotaviruses?
Live attenuated oral vaccine (Rotarix) against type A introduced in UK July 2013.
What is the treatment for rotavirus?
Oral rehydration therapy
Still causes ~ 200,000 deaths/year.
Before vaccine, most individuals had an infection by age 5, repeated infections develop immunity.
Describe norovirus
RNA virus
Incubation period 24-48 hours
How is norovirus transmitted?
Faecal-oral transmission.
Individuals may shed infectious virus for up to 2 weeks
Outbreaks often occur in closed communities
What are the symptoms of norovirus?
Acute gastroenteritis, recovery 1 – 3 days
No specific treatment - just supportive
How is norovirus diagnosed?
Sample PCR
How many cases of norovirus are there?
Estimated 685 million cases per year.
What are the most common species of curved bacteria?
Campylobacter jejuni Campylobacter coli
How are campylobacter transmitted?
Undercooked meat (especially poultry), untreated water & unpasteurised milk Low infective dose, a few bacteria (<500) can cause illness
What is the treatment for campylobacter?
Not usually required
Azithromycin (macrolide) is standard antibiotic
Resistance to fluoroquinolones is problematic
Describe the epidemiology of campylobacter
Estimated 280,000 cases per year in UK, 65,000 confirmed
Commonest cause of food poisoning in the UK
What are the main features of E. coli?
Diverse group of Gram-negative intestinal bacteria
Most harmless
6 ”pathotypes” associated with diarrhoea (diarrhoeagenic):
What is the most problematic E. coli pathotype?
Enterohaemorrhagic or Shiga toxin-producing E. coli (EHEC/STEC)
E. coli O157 serogroup, Shigatoxin/verotoxin
5-10% get haemolytic uraemic syndrome: loss of kidney function
What are the features of Enterotoxigenic E. coli (ETEC)?
Cholera like toxin
Watery diarrhoea
What are the features of Enteroinvasive E. coli (EIEC)?
Shigella like illness
Bloody diarrhea
Why is C diff. called its name?
It is difficult to grow in a lab
What often causes c. diff infection?
Long term antibiotics
What is weird about C. diff?
Healthy microbiota can contain C. diff
When does C. diff become a problem?
Intermediate dysbiotic state caused by a exogenous disturbance e.g. antibiotics
C. diff starts colonising enetrocutes and you get an outgrowth in the distal gut , still not producing toxins
Pathogen induced disturbance creates supportive environment - then starts producing toxin
Causes inflammation of distal gut
What is the management in C. diff?
Isolate patient (very contagious)
Stop current antibiotics
Metronidazole, Vancomycin
Recurrence rate 15-35% after initial infection, increasingly difficult to treat.
Faecal Microbiota Transplantation (FMT) – 98% cure rate
What is weird about Metronidazole?
Can cause and treat C. diff
