Lecture 30; Gut Microbiota and Immunity

Question 1

What is Gut microbiota?

Answer 1

Multicellular organisms exist as meta-organisms comprised of both the macroscopic host and its symbiotic commensal microbiota

Question 2

In recent times what is gut microbiota considered to be?

Answer 2

Potentially an organism itself and humans are a metaorganisms

Question 3

In humans to what extent are we outnumbered by microbes?

Answer 3

10;1

Question 4

Describe the normal gut flora;

Answer 4

‘Normal’ gut microbiota:
• Mostly anaerobic bacteria (100 – 1000-fold more than aerobic/facultative anaerobes)
• 10-fold more unique genes than host
• Most abundant phyla = Firmicutes (mostly G+ e.g. Bacillus, Clostridium) & Bacteriodetes (mostly G- e.g. bacteroides)
• Metabolic activity ≈ liver, can be considered as an organ
• Mutalistic association

Question 5

Describe whats happened to the view of microbiota in immunology?

Answer 5

• Paradigm shift to:
– Super-organism theory - not self/non-self – Mucosal tolerance to micro-organisms (and food)
– Immune system-microbiota alliance

Question 6

What can cause changes in the microbiota?

Answer 6

Antibiotics, dietary changes, recent elimination of key constituents (e.g. helminths)

= changes in structure and function

Question 7

What happens if there are Uncontrolled/misdirected immune response to the microbiota?

Answer 7

Allergies
Autoimmune disorders
Inflammatory disorders

Question 8

What tools are used to study the microbiota?

Answer 8

Next Generation Sequencing

Germ Free Mice

Question 9

What is special about a babies immune system?

Answer 9

It is skewed towards tolerance so that it can acquire commensual organisms

Question 10

Describe two factors that allow the acquisition and maintenance of microbiota in babies;

Answer 10

1) Tolerance
• Reduced production of inflammatory cytokines • B, T responses skewed towards regulation
2) Early exposure to colostrum/breast milk
Includes live microbes, metabolites, maternal IgA, immune cells, cytokines

Question 11

Whats a big factor that also determines microbiota at birth?

Answer 11

Delivery style

C setion vs natural (exposure)

As well as parental microbiota

Question 12

How does diet influence micriobiota?

Answer 12

Alterations in gut microbiome evident within 1 day of diet change

Increase in Bilophila wadsworthia on ‘animal-based diet’, supporting links between dietary fat, bile acids and outgrowth of microorganisms capable of triggering IBD

Question 13

Whats another major factor in determining gut microbiota?

Answer 13

Environment

Too clean or too dirty are both bad

Question 14

How does the microbiota influence the immune system?

Answer 14

1) Stimulation & maintenance of the ‘mucosal firewall’ (barrier between epi and food)
2) Development of mucosal associated lymphoid tissue (MALT)
3) Inhibition of pathogen growth
4) Dampening of mucosal immune responses
5) Regulation of mucosal immune responses (stimulate T regs, also helps contain inflam / infection)

Question 15

What maintains the commesals in the gut in a steady state?

Answer 15

The commensuals interact with surface receptors that results in IgA secretion that maintains them.

Localised response.

Question 16

In germ free mice what does not develop in the MALT?

Answer 16

Full maturation of Germinal Centre (GC) does not occur in Peyer’s Patches, MLN, spleen

No antimicrobial peptides

Thinner villi

No Tregs, Th17 in response to pathogens. (even though they could)

Question 17

How does gut microbiota prevent pathogen growth?

Answer 17

Colonisation Resistance

• Occupy same niche
• affect nutrient availability
• stimulate release of anti-bacterial products targeting related species

Question 18

What bacteria have been linked with healthy homeostatic immune responses?

Answer 18

Bacteroide fragalis

Clostridum

Both stimulate Treg responses, dampens any inflammatory immune responses

Question 19

What bacteria have been linked with disrupting homeostatic immune responses?

Answer 19

Segmented filamentous bacteria

Trigger Th17 = pro inflammatory

Question 20

List some bacteria and what T cell differentiation they drive;

Answer 20

Naive T cell;

CD4 -> TH17 (SFB)

CD4 -> TH1 (B fragallis)

CD4 -> Treg (clostridium)

Question 21

What is causing the ongoing immune development in the gut?

Answer 21

Ongoing exposure to PAMPs stimulates gut immune development:

Question 22

What does exposure to PAMPS do?

Answer 22

• Ongoing exposure to PAMPs stimulates gut immune development:
• RegIIIγ produced in response to MyD88 mediated signalling
• Exposure to TLR ligands alone, e.g. LPS, PGN, CpG can induce maturation of the GF gut mucosa
• Some ligands activate mucosal immunity/some dampen it

Question 23

Whats the major TLR in the gut?

Answer 23

TLR5 and it senses flagellin

Only on inner side of epithelial membrane, therefore senses when epithelium is breached

Question 24

What happens to immune function in the absence of the microbiota?

Answer 24

• TLR-MyD88 signalling pathway impaired – Important for immune homeostasis

TLR5 + Flagellin -> CD11b+ -IL23-> Innate lymphoid cells -> IL22 = Epithelial cell repair and REG3g production

This is in a steady State

REG3g is an antimicrobial peptide

Knockout of TLR5 leads to IBD in mice

Question 25

What gene was upregulated to systemic flu vaccine?

Answer 25

TLR5 is critical for developing an antibody response to the flue.

Question 26

Does the flu vaccine directly stimulate TLR5?

Answer 26

Vaccine preparation does not directly stimulate signalling via TLR5

Endogenous, host-derived signal, e.g. commensal bacteria in the gut, activating TLR5 pathway

i.e microbiota are essential for vaccine immunity

Question 27

What happened to the AB response to flu vaccine in germ free mice?

Answer 27

Majority of vaccine-induced IgG antibodies detected in first 7 days post vaccination are produced by plasmablasts or short-lived plasma cells in dLN, GC B cell responses unaffected (these are maintained by the presence of microbiota and TLR5)

Recall IgG responses also impaired without TLR5 signalling (due to lack of microbiota)

= Host microbiota necessary for early antibody response to ‘flu vaccine

Question 28

Whats the consequence of needing microbiota for vaccination?

Answer 28

“Predict that diet, nutrition, metabolic diseases, pre-existing gut-associated pathologies, and other compounding factors affecting the microbiota may in turn affect the capacity of current and future vaccines to
establish immunity”

Question 29

Overview of the micrbiota;

Answer 29

• Acquired early in life
• Crucial for maintaining gut immune homeostasis
• Indications that it may impact on systemic responses (e.g. vaccination)

Question 30

Describe the cycle of equilibrium and imbalance that can occur;

Answer 30

If the gut microflora are preturbed then;

• Leads to inflammation
• dysbiosis
• Restoration of metastable state with restraint of pathobionts and pathogens (colonisation resistance)
• Homeostasis

However if dysbiosis is maintained then it can result in chronic disease development i.e IBD (nasty bacteria that drive inflammatory sub cell sets)

Question 31

Can the gut affect other autoimmune diseases?

Answer 31

Gut microbiota affects extra-intestinal autoimmune disease

i.e too much segmented bacteria can causes MS

Athritis

Decreased fermicutes and increased bacteriodes = type 1 diabetes

Allergic inflammation susceptibility

(may need to go back and note down specific pathways)?

Question 32

Whats the therapeutic potential?

Answer 32

• Probiotics
• Small molecules/proteins to tweak the response
• Changing the balance from “good” to “bad”
• Altering drug up take by modifying the microbiome
• Diagnostic tool