Gut immunity and microbiome

Question 1

What factors allow healthy growth of microbiota from conception?

Answer 1

Conception - adequate nutrition, good hygiene, antibiotic use
Birth - Normal vaginal delivery, term delivery, exclusive breastfeeding, high human milk oligosaccharides (HMO) in breastmilk
Weaning - adequate nutrition, adequate hygiene

Question 2

How does commensal bacteria contribute to health?

Answer 2

-Outcompete pathogenic species
- Degrade toxins into harmless components
- Critical for development of the GI immune system and establishment of tolerance
- Produce vitamins
- Break down non-digestible carbohydrates into smaller molecules

Question 3

What effect does the microbiota have with a diet low in animal fat and protein but high in plant fibre?

Answer 3

Indigestible but fermentable polysaccharides are broken down into short chain fatty acids (SCFAs)
Further broken down and causes L cells to release PYY and GLP1 hormones
These decrease blood glucose, increase energy expenditure and increase glucose stimulated insulin secretion

Question 4

What is the role of SCFAs?

Answer 4

Bacteria ferment non-digestible carbohydrates into SCFAs - acetate, propionate and butyrate
High levels of SCFAs -> lower risk of obesity and insulin resistance

Question 5

What is acetate and its role?

Answer 5

Substrate for synthesis of cholesterol and long chain fatty acids (LCFAs) in the liver
Modulates the activity of immune cells

Question 6

What is propionate and its role?

Answer 6

Substrate for gluconeogenesis in the liver
Influences the secretion of appetite regulating hormones

Question 7

What is butyrate and its role?

Answer 7

Energy source for colonic epithelial cells
Promotes gut barrier integrity

Question 8

What relation does gut microbiota and obesity have?

Answer 8

When on a low fat/low carbohydrate diet, bacteriodetes abundance increases in correlation with reduction in body weight

Question 9

What is the importance of a healthy gut microbiota?

Answer 9

Healthy gut microbiota:
- Decreased gut permeability
- Decreased endotoxaemia
- Decreased pro-inflammatory cytokines
- Increased insulin sensitivity
- Increased beneficial molecules

Question 10

How can a healthy gut microbiota be maintained?

Answer 10

Healthy diet and lifestyle, prebiotics, probiotics

Question 11

What can occur with a dysbiotic gut microbiota?

Answer 11

• Increased gut permeability
• Increased endotoxaemia
• Increased pro-inflammatory cytokines
• Increased insulin resistance
• Increased metabolic syndrome
  Can lead to metabolic disease

Question 12

What factors may cause a dysbiotic gut microbiota?

Answer 12

High fat and sugar diet, stress and antibiotics

Question 13

What is the immunological defences in the gut?

Answer 13

• Commensal microbiota localised mainly to the outer mucus layer
• Inner mucus layer is sterile and full of antimicrobial peptide
• Abundant IgA in mucus which mainly binds with binds commensals
• Paneth cells make defensins in the small intestines, defensins expressed more broadly

Question 14

PRRs in the gut
Types
Location
Function

Answer 14

TLRs and NOD2
NOD2 respond to peptidoglycan component MDP (component of bacterial cell wall)
TLR5 is only located on basolateral surface of gut epithelial cells
Gut regulatory DC express PRR
Activation coupled to regulatory cytokine production
TLR triggering enhances epithelial barrier and enhances AMP and mucus secretion

Question 15

How do DC detect antigens?

Answer 15

Antigens (Ag) delivered via M cells via transcytosis of ab-ag conjugates by directly sampling from gut lumen and via uptake of apoptotic cells

Question 16

Where are T cells found in the gut, and which T cells are normal in epithelial defence?

Answer 16

T cells found in Peyer’s patches and make up IELs
Th17 part of normal epithelial defence - aberrant Th17 linked to immune-mediated inflam diease

Question 17

How are regulatory DC conditioned in the gut?

Answer 17

Colonic epithelium release retinoic acid (RA), TGF-B and TSLP
These condition CD103+ DC to become regulatory DCs.
Regulatory DCs produce immunosuppressive factors e.g. RA and TGF-B that favour Treg production
DC imprint gut homing on Treg - express CCR9 and a4B7 integrin
Macrophages are also skewed to regulatory phenotype

Question 18

How do Treg suppress T cells in the gut?

Answer 18

Produce inhibitory cytokines e.g. IL-10, TGF-B will suppress effector T cells
Express CD25 - component of IL-2 receptor - reduce IL-2 levels required for Teff proliferation.
Release perforin, granzyme B and galectin-1 to kill Teff
Expresses CTLA-4 which has higher affinity for CD86 and CD80 on DC than CD28 on Teff

Question 19

How does T reg aid healing in gut?

Answer 19

Release AREG - in family of EGF - for epithelial repair

Question 20

What are the two pathways that B cells are switched into IgA producers?

Answer 20

T-dependent pathway:
- ?TGF-B activates the??T cell to become TFH with CD40L binds to CD40 on B cells which are switched to produce IgA
T-independent pathway:
- DC produce BAFF and APRIL to stimulate B1 B cells to produce IgA