Enterology Flashcards
What are distinctive features of the mucosal immune system? (7)
- Intimate interactions between epithelia & lymphoid tissues
- Discrete compartments of lymphoid tissue
- Specialized antigen-uptake mechanisms
- Activated/memory T-cells predominate even in absence of infection
- Non-specifically activated natural effector/regulatory T-cells present
- Active downregulation of immune responses
- Inhibitory macrophages and tolerance-inducing DCs
What are the specialized lymphoid tissues in the small intestine & colon?
Small intestine = Peyer’s patches
Colon = colonic patches & isolated lymphoid follicles
Why are lymphoid-like structures directly present in the mucosal surfaces of the intestine?
Lymph nodes are relatively far away -> presence of lymphoid structures in the intestines allows for rapid responses
What is the specialized antigen-uptake mechanism of Peyer’s patches?
M-cells
What is the function of M-cells?
Selective antigen-uptake in the intestine
What kind of T-cells predominates in the intestine? What is the advantage of this?
Memory cells -> primed phenotype, allows for rapid responses
Where are memory T-cells in the intestine located?
Lamina propria
How is tolerance to food antigens (mainly) maintained in the intestine?
Tregs
How are intestinal epithelial cells interconnected?
Tight junctions
Which three distinct areas can be distinguished in Peyer’s patches?
- Dome area containing DCs
- T-cell areas
- B-cell areas
Where are DCs located in the Peyer’s patches?
Dome area
True or false: Peyer’s patches and colonic patches have the same organization
False; Peyer’s patches have distinct tissue organization, whereas colonic patches are more loosely organized
Which cell types can be found in the intestinal lamina propria? (4)
- Memory T-cells
- Memory B-cells
- High amounts of macrophages and DCs
- Relatively low amounts of monocytes
True or false: the composition of immune cells in the lamina propria is the same across the whole digestive tract
False; the composition is site-dependent
What are IELs? Where are they mainly found?
Intra-epithelial lymphocytes; almost exclusively found in the small intestine, very little in the colon
What is the role of IELs?
Have a role in barrier function of the epithelium
How do antigens from the intestines reach lymph nodes?
Peyer’s patches and colonic patches contain lymph ducts that drain to mesenteric lymph nodes
What is the function of villi of intestinal epithelium?
Surface enlargement
What are the functions of microvilli of the intestinal epithelium? (2)
- Enlarge surface
- Prvent bacterial attachment
What do the villi of the duodenum look like?
High villi
What are Brunners glands? Where are they found, and what are their function?
Glands in the submucosa of the duodenum -> secrete alkaline solutions to neutralize stomach acid
What can be said about the folds of the jejunum as compaired to the duodenum? What happens to the folds more distally?
Higher folds, that lower distally
What happens to the crypts of the ileum, the more distal you get?
Crypts deepen towards the ileum
True or false: the ileum has plicae
False; the ileum barely has any folds
Where are Peyer’s patches mainly found?
Ileum
Why are Peyer’s patches mainly found in the ileum, and not more proximal?
Barely any bacteria in the duodenum/jejunum
Why does the colon not contain villi or folds?
No need to take up nutrients
Which cell type can be found at the bottom of colonic crypts?
Stem cells
Which cell types can be found in the intestinal epithelium? (4)
- Enterocytes
- Goblet cells
- Paneth cells
- Stem cells
What is the function of enterocytes?
Resorption of nutrients
What is the function of goblet cells?
Production of mucus
The colon contains [less/more] goblet cells than the small intestine
More
Where are goblet cells located in the small intestine?
Base of the villi
Where in the intestine can one find Paneth cells under physiological circumstances?
Small intestine
What is the function of Paneth cells?
Keeping crypts free of bacteria through the release of anti-microbial peptides
True or false: Paneth cells are the only intestinal cells producing AMPs
False; other epithelial cells also produce AMPs, but in far lower amounts
What happens when a Paneth cell is unable to clear bacteria?
Sensing of bacteria using TLRs & PRRs -> signals for assistance
In which instance can Paneth cells be found in the colon?
Paneth cell metaplasia -> occurs in severe colonic disease
How long does regeneration from crypt to villus take?
~36 hours
How do intestinal microbiota contribute to host physiology? (4)
- Bacteria facilitate digestion & absorption of nutrients
- Bacteria-derived signals are needed for normal intestinal physiology
- Commensal bacteria limit pathogen colonization
- Host provides a protected & nutrient-rich environment
Which two systems in the intestine are dependent on the presence of signals from intestinal microbiota?
- Epithelium
- Immune system development
Which constituent parts form the intestinal epithelial barrier, which limits bacterial penetration of host tissue? (5)
- Microvillar extension -> prevent bacterial attachment
- Epithelial tight junctions
- Mucinous secretions by goblet cells
- Epithelial transcytosis of IgA
- Antimicrobial peptides
True or false: epithelial tight junctions in the intestine always remain tightly closed
False; they can be opened in a controlled way, but without breaking the barrier
What is the main component of mucus?
Polysaccharides
The [small intestine/colon] has a thicker mucus layer
Colon has a thicker mucus layer
What does a lack of mucus-production lead to?
Lack of bacterium-free zone above the epithelium -> bacterial colonization of crypts -> colitis
What is the daily prodyuction of IgA in the intestine? How much % of the total immunoglobulin production is this?
3-5 grams of IgA/day, 75% of total production
What is the role of IgA in intestinal barrier functions?
Allows for selective colonization by bacteria, whilst preventing others from attaching
What triggers secretion of AMPs by Paneth cells?
TLR activation
What is the effect of a deletion of Paneth cells?
Disturbances of commensal bacteria, leading to more drainage of commensals to mesenteric lymph nodes
How do M-cells selectively take up antigens?
PRRs recognize relevant antigens
Which three mechanisms does the intestine have for antigen-uptake across the epithelium?
- Passive diffusion
- M-cells
- Macrophage uptake
How do macrophages take up antigens from the intestine?
Break tight junctions between epithelial cells and extend cytoplasm into the lumen to sample it
What is the effect of increased bacterial load on the number of macrophage protrusions in the intestinal epithelium?
Amount of macrophage protrusions increases
What are the epithelial barrier & antigen sampling characteristics of the small intestine? (4)
- Presence of M-cells
- Thin mucus layer
- Presence of Paneth cells
- Peyer’s patches
What are the epithelial barrier & antigen sampling characteristics of the colon? (4)
- Absence of M-cells
- Thick mucus layer
- Absence of Paneth cells
- Isolated lympoid follicles
How does a cell in the intestine determine whether to respond to PRR activation or not?
Specific combinations of PRR activation produces specific cocktails of inflammatory mediators, dictating responses
What are the two functions of NF-κB in the intestine?
- Production of inflammatory mediators upon activation of PRRs
- Intestinal epithelial homeostasis
By which factor is NF-κB inhibited?
IκB
How is IκB inactivated, allowing for the activation of NF-κB?
Phosphorylation of IκB allows it to be ubiquinated -> IκB broken down in proteasome, releasing NF-κB
What is the function of NF-κB in intestinal epithelial cells?
Necessary for epithelial cell survival
What is the effect of loss of NF-κB in intestinal epithelial cells?
Disrupted epithelial barrier function -> bacteria leak in -> macrophage activation -> chronic intestinal inflammation
What is the effect of NF-κB in macrophages? (3)
- Survival
- Pro-inflammatory factor expression
- Host defence
Where are most PRRs in the intestine located? Why there?
Basolateral side -> allows for sensing of bacterial tissue invasion, whilst not getting activated by intraluminal presence of bacteria
In which areas of the intestine are TLRs located on the apical side of epithelium?
- Close to Peyer’s patches
- Colon
True or false: small intestine & colon express the same TLRs
False; they express different TLRs because they have a different microbiological profile, requiring different receptor signatures
What is the effect of repetitive activation of PRRs by the same pathogen?
Desensitization of epithelium, leading to hyporesponsiveness for that pathogen
What are the homeostatic functions of resident macrophages in the intestine? (4)
- Elimination of invading commensal bacteria
- Maintenance of Tregs
- Phagocytosis of senescent and apoptotic epithelial cells
- Transfer of antigens to migratory DCs