6.6 - Immunology of the gut Flashcards
What is the surface area of the 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
- dual immunological role
What does the GI tract need tolerance against? (2)
- food antigens
- commensal bacteria
What does the GI tract need immunoreactivity against?
Pathogens
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
- colonise germ-free mice and give them a particular germ and compare them to normal house mice and look at the microbiota
- e.g. immunological defect in the development of the small intestine –> Peyer’s patches in germ-free mice will be fewer and less cellular than house mice
- e.g. immunological defect in expression of angiotensin 4 –> reduced Paneth cells
How many gut bacteria vs cells do we have in the body?
10^14 gut bacteria and 10^13 cells in the body –> most densely populated ecosystem on earth
What are the four major phyla of bacteria (4), and what else is also present in the body (2)?
- Bacteroidetes
- Firmicutes
- Actinobacteria
- Proteobacteria
- viruses
- fungi
What do the gut microbiota provide for us? (4)
- provide traits we have not had to evolve on our own - genes in gut flora are 100x our own genome
- metabolise indigestible compounds
- defence against colonisation by opportunistic pathogens
- essential nutrients
What host factors stimulate bacterial growth? (2)
- ingested nutrients
- secreted nutrients
What host factors inhibit bacterial growth? (2)
- 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 due to HCl and pepsin + gastric lipase
- duodenum has more as it has bile acids
- by the time we get to the colon, there is a lot of bacteria because there are no host digestive factors
What is dysbiosis?
Altered microbiota composition, lack of homeostatic immunological equilibrium
Explain what the immunological equilibrium is.
- one side = symbionts - microbiota and humans live with each other but not with benefit or harm to either, they just live (regulation)
- middle = commensals - microorganisms that benefit from associating with host, but has no effect on host
- other side = pathobionts - symbionts that usually do not elicit inflammatory response, but under specific conditions can cause dysregulated inflammatory disease (inflammation)
What happens when something goes wrong and pathobionts begin replicating?
Inflammation and disease
What influences GI equilibrium/dysbiosis? (5)
- infection or inflammation
- diet
- xenobiotics
- hygiene
- genetics
How can dysbiosis negatively affect the rest of the body? (5)
Through producing metabolites and toxins e.g:
- TMAO - increases cholesterol deposition in artery walls to cause atherosclerosis
- 4-EPS - associated with autism
- SCFAs (short chain fatty acids) - decreased numbers associated with IBD, increased numbers associated with neuropsychiatric disorders
- AHR ligands - associated with MS, rheumatoid arthritis, asthma
- bile acids
What is the first line of defence the body has against pathogens?
Mucosal defence
What are the three layers of mucosal defence?
- 1st - physical barriers
- 2nd - commensal bacteria - occupy an ecological niche and are an ecological barrier
- 3rd - immunological (lymphoid tissue) - MALT and GALT
What are the two types of physical barriers (mucosal defence)?
- anatomical - epithelial barrier, peristalsis
- chemical - enzymes, acidic pH
What is the epithelial barrier made up of?
- mucus layer - goblet cells
- epithelial monolayer with tight junctions
- Paneth cells (small intestine)
What are Paneth cells and what do they do?
- bases of crypts of Lieberkuhn
- secrete antimicrobial peptides (defensins) and lysozyme
What types of lymphoid tissue are there? (2)
- MALT (mucosa associated lymphoid tissue)
- GALT (gut associated lymphoid tissue)
Where is MALT found?
- found in the submucosa below the epithelium, as a lymphoid mass containing lymphoid follicles
- follicles are surrounded by HEV (high endothelial venule) postcapillary venules, allowing easy passage of lymphocytes
Which specific part of the body is packed full of MALT?
Oral cavity is rich in immunological tissue - pharyngeal, palatine and lingual tonsils especially
How big is GALT?
Largest mass of lymphoid tissue in body
What types of immune response is GALT responsible for?
Responsible for both adaptive and innate immune responses through generations of lymphoid cells and antibodies
What are the two types of GALT?
- non-organised
- organised
What are non-organised GALT? (2)
- 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 the small intestine villus.
- intestinal epithelial cells migrate up to tip of villus
- mucus-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
- intra-epithelial lymphocytes are there between enterocytes