(gastro) gut immunology Flashcards
what is the surface area of the GI tract?
approx 200 square metres
explain why the GI tract is said to have a massive antigen load
resident microbiota = 10^14 bacteria
dietary antigens
exposure to pathogens
= collectively result in an a huuuge antigen load
what is microbiota?
mixture of microorganisms that make up a community in an environmental niche
what is the microbiome?
collective genomes of all the microbiota
differentiate between microbiota and microbiome
microbiota = mixture of microorganisms that make up a community in an environmental niche
microbiome = collective genomes of all the microbiota
explain why the GI tract is in a state of ‘restrained activation’
has to balance tolerance of food antigens and commensal bacteria with the active immune response against foreign pathogens
why is bacterial microbiota important?
for the immune homeostasis of the gut and the development of a healthy immune system
how much gut bacteria is present in our body?
approx 10^14 gut bacteria
what makes up the gut microbiota?
four major phyla of bacteria (Bacteroidetes, Firmicutes, Actinobacteria, Proteobacteria), also viruses & fungi
what does the gut microbiota provide?
provide traits we have not had to evolve on our own
how is bacterial growth stimulated by the host?
via ingested nutrients or secreted nutrients
how is bacterial lysis/elimination stimulated by the host?
via chemical digestive factors
OR
via peristaltic contractions and defecation
which host actions lead to an increase in cell numbers?
ingested nutrients or secreted nutrients
= bacterial growth
= increased bacterial cell numbers
which host actions lead to a decrease in cell numbers?
chemical, digestive factors + peristaltic contractions + defecation
= bacterial lysis and elimination
= decreased bacterial cell numbers
why are microbiota important to the host?
provide essential nutrients
metabolise indigested components
defence against colonisation by opportunistic pathogens,
contribute to intestinal architecture
what is the effect of ingested/secreted nutrients in the gut microbiota?
stimulate bacterial growth
= increased bacterial cell numbers
what is the effect of chemical digestive factors in the gut microbiota?
stimulate bacterial lysis
= decreased bacterial cell numbers
what is the effect of peristalsis, contractions and defecation in the gut microbiota?
stimulate bacterial elimination
= decreased bacterial cell numbers
what chemical, digestive factors are produced by the host in the stomach, and what is the impact of this?
hydrochloric acid (pH 1-4)
pepsin
gastric lipase
= chemical, digestive factors that stimulate bacterial lysis and therefore decrease bacterial cell numbers
(10^1)
what chemical, digestive factors are produced by the host in the liver and what is the impact of this?
bile acids
= stimulate bacterial growth and therefore result in increased bacterial cell numbers
(10^3)
what chemical, digestive factors are produced by the host in the pancreas, and what is the impact of this?
trypsin
amylase
carboxypeptidase
= secreted nutrients that stimulate bacterial growth and therefore increase bacterial cell number
(10^4)
what chemical, digestive factors are produced by the host in the small intestine, and what is the impact of this?
brush border enzymes
= increase bacterial content
(10^7)
what chemical, digestive factors are produced by the host in the colon, and what is the impact of this?
no host digestive factors
= reduced bacterial lysis
= bacterial cell numbers cannot remain decreased
= huuuge increase in colonic bacteria
(10^12)
define symbiosis
any relationship or interaction between two dissimilar organisms (not always to the mutual benefit of either species)
the specific kind of symbiosis depends on whether either or both organisms benefit from the relationship
define dysbiosis
reduction in microbial diversity
(a combination of the loss of beneficial bacteria + rise in pathobionts)
what are symbionts?
an organism living in symbiosis with another
(any kind of interaction between two dissimilar organisms)
what are commensals?
relationship between organisms where one organism benefits from the association while not harming the other
what are pathobionts?
any potentially pathological organism that, under normal circumstances, lives as a non-harming symbiont
(do have the potential to cause dysregulated inflammation and disease in specific environments)
what happens when pathobionts malfunction?
have the potential to cause dysregulated inflammation and disease
what are some possible causes of dysbiosis?
infection or inflammation
diet
xenobiotics
hygiene
genetics
which bacterial metabolites and toxins are produced as a result of dysbiosis?
TMAO
4-EPS
SCFA
bile acids
AHR ligands
what is the effect of the bacterial metabolite TMAO?
increase cholesterol deposition in the artery wall
= can lead to atherosclerosis
what is the effect of the bacterial metabolite SCFA?
short-chain fatty acids decrease and can cause IBD
if increase can cause neuropsychiatric disorders
what is the effect of the bacterial metabolite AHR ligands?
present in multiple sclerosis, rheumatoid arthiritis, asthma
which conditions can occur as a result of dysbiosis?
what are the physical barriers of mucosal defense?
anatomical = epithelial barrier, peristalsis
chemical = enzymes, acidic pH
what are the anatomical physical barriers?
epithelial barrier, peristalsis
what are the chemical physical barriers?
enzymes, acidic pH
what are the immunological barriers of mucosal defense?
MALT (mucosa associated lymphoid tissue)
GALT (gut associated lymphoid tissue)
summarise the barriers for mucosal defense in the body
explain how the epithelium acts as a barrier
has a mucus layer (secreted by goblet cells) = to trap pathogens
has tight junctions (within the epithelial monolayer) = prevent barrier intrusion by pathogens into the epithelium
how is the epithelium of the small intestine specialised for mucosal defense?
contains Paneth cells in the bases of the crypts of Lieberkuhn
= secrete antimicrobial peptides (defensins) and lysozyme
(and mucus layer + tight junction of the epithelial monolayer)
what happens if the physical barriers for mucosal defense fail?
rely on the immunological barriers (MALT, GALT)
what is MALT?
mucosa-associated lymphoid tissue
what is GALT?
gut-associated lymphoid tissue
where are Paneth cells found?
in the bases of the crypts of Lieberkuhn of the small intestine
why are Paneth cells important?
secrete antimicrobial peptides (defensins) & lysozyme
where is MALT found?
found in the submucosa below the epithelium
how is MALT structured?
as lymphoid mass containing lymphoid follicles
follicles are surrounded by HEV postcapillary venules, allowing easy passage of lymphocytes
how are MALT follicles arranged?
surrounded by HEV postcapillary venules, allowing easy passage of lymphocytes
what are HEV postcapillary venules?
high endothelial venules
(postcapillary venules)
how are HEV postcapillary venules structured?
plump endothelial cells as opposed to the usual thinner endothelial cells found in regular venules
= enable lymphocytes circulating in the blood to directly enter a lymph node (by crossing through the HEV)
why are HEV postcapillary venules important?
enable lymphocytes circulating in the blood to directly enter a lymph node (by crossing through the HEV)
which cavity is rich in MALT?
oral cavity (rich in immunological tissue)
what is GALT responsible for?
responsible for both adaptive & innate immune responses
what does GALT consist of?
consists of B & T lymphocytes, macrophages, APC (dendritic cells), and specific epithelial & intra-epithelial lymphocytes
what are the two types of GALT?
non-organised GALT
organised GALT
what is non-organised GALT?
intraepithelial lymphocytes (make up 1//5th of the intestinal epithelium = T cells, NK cells)
lamina propria lymphocytes
what are intra-epithelial lymphocytes?
make up 1//5th of the intestinal epithelium = T cells, NK cells
what is organised GALT?
Peyer’s patches (small intestine)
caecal patches (large intestine)
isolated lymphoid follicles
mesenteric lymph nodes (encapsulated)
how does GALT compare to other lymphoid tissue?
largest part of lymphoid tissue in the body
what is the effect of the bacterial metabolite 4-EPS?
associated w autism
where are intra-epithelium lymphocytes found?
slotted in between the enterocytes of the small intestine
(approx 1/5th of the intestinal epithelium)
= include T cells and NK cells
what are the most common cell types to develop from the stem cells in the crypts of Lieberkuhn?
intestinal epithelial cells (enterocytes)
migrate to the tip of the villi
what types of cells are produced by the stem cells in the crypts?
absorptive enterocytes
mucus-secreteing goblet cells
where do Paneth cells migrate to?
to the bottom of the crypt
what is the characteristic feature of Paneth cells?
contain antimicrobial peptides (defensins) = dense granules
what makes up the central part of the villus?
lamina propria
= containing the majority of the intestinal cells (T cells, dendritic cells, macrophages)