Immunology of the gut

Question 1

Why do the majority of infectious agents invade the human body via mucosal surfaces?

Answer 1

Because their physiological function in gas exchange (lungs), food adsorption (gut), sensory activities (nose, mouth), mucosal surfaces are thin and permeable barriers to the interior of the body.

Question 2

The cells of the mucosal immune system are found where?

Answer 2

Within the lamina propria

Question 3

The lamina propria is separated from the lumen by …

Answer 3

A single layer of epithelium

Question 4

The most common epithelial cells of the intestinal epithelium are?

Answer 4

1. Enterocytes/colonocytes
2. Goblet cells
3. Paneth cells
4. Tuft cells
5. Enteroendocrine cells
6. M cells

Question 5

Enterocytes/colonocytes are connected by proteins called what?

Answer 5

Tight junctions, e.g. occludins and claudins

- These make the epithelial cells stick tightly together so nothing can get to the lamina propria

Question 6

Role of paneth cells?

Answer 6

Secrete antimicrobial peptides

Question 7

Role of goblet cells?

Answer 7

Produce mucins which form a mucus layer making it harder for microbes to come into contact with epithelial cells

Question 8

MALT can be divided into what?

Answer 8

NALT - lining of nose
BALT - URT
GALT

Question 9

The innate immune response is carried out by which cells?

Answer 9

1. Dendritic cells
2. Macrophages
3. Monocytes
4. Granulocytes (neutrophils, eosinophils, basophils)

Question 10

The adaptive immune response is carried out by which cells?

Answer 10

1. CD4+ T helper cells
2. CD8+ T cytotoxic cells
3. B cells

Question 11

Innate vs adaptive recognition of antigen?

Answer 11

Innate: Pattern recognition receptors e.g toll-like receptors/NODs/CARDs
- Recognise patterns/motifs e.g. peptidoglycan, LPS, dsRNA

Adaptive: T-cell receptor (TCR) and B-cell receptor (BCR)

• Recognise specific Ag
• T cells = recognise peptide/MHC complex
• B cells = recognise 3D structure

Question 12

CD4+ T helper cells come as effector subsets and regulatory subsets, what are examples of each?

Answer 12

1. Effector subsets = help us fight pathogens
   - Th1 = IFN-gamma
   - Th2 = IL4, IL5
   - Th17 - IL17
2. Regulatory subsets = regulate/dampen the effectors
   - Tr1 = IL10
   - Th3 = TGF-beta
   - CD25+ = IL10 and TGF-beta

Question 13

The Th1, Th2 and Th17 effector subsets are involved in which pathologies?

Answer 13

1. Th1 - IFN-gamma - chronic inflammation & autoimmunity (type 1 diabetes?)
2. Th2 - IL4/IL5 - Allergy asthma
3. Th17 - IL17 - Chronic inflammation, autoimmunity (RA, MS, psoriasis, IBD?)

Question 14

GALT is divided into organised tissues and scattered lymphoid tissues, what are examples of each?

Answer 14

1. Organised tissues
   - Peyer’s patches (in SI)
   - Isolated lymphoid follicles (in SI and LI)
   - Mesenteric lymph nodes
2. Scattered lymphoid cells
   - Lamina propria leukocytes
   - Intraepithelial lymphocytes

Question 15

What is the largest lymph node in the body?

Answer 15

Mesenteric lymph nodes - they drain the intestinal tract

Question 16

Where are peyer’s patches found?

Answer 16

Just beneath the epithelial layer

Question 17

Lamina propria leukocytes (LPL) are found where?

Answer 17

In the lamina propria

Question 18

Intraepithelial leukocytes (IEL) are found where?

Answer 18

In the epithelial layer, for every 10 epithelial cells you have 1 IEL

Question 19

What are IELs?

Answer 19

Specialised types of T cells mainly found in the gut

Question 20

What other cells are found within the lamina propria with the LPLs?

Answer 20

Mucosal immune system cells

• CD4+
• CD8+
• Dendritic cells
• Plasma cells
• Macrophages

Question 21

Why are immune system responses generated in the GALT distinct from those after immunisation or when an antigen enters the blood stream?

Answer 21

Because the GALT has its own contents of lymphoid cells, hormone, and other immunomodulatory factors

Question 22

Antigens present at the mucosal surfaces must be transported across an epithelial barrier before they can stimulate the mucosal immune system, how is this done?

Answer 22

1. Mechanism 1: M cells sit on top of Peyer’s patches are also found on top of isolated lymphoid follicles, they are involved in the taking up of and transporting of antigens to new cells in the lamina propria
2. Mechanism 2: They are taken up by macrophages which are extending processes in between epithelial cells which extend processes across the epithelial layer to capture antigens from the lumen of the gut
   - Macrophages are not good at migrating so they hand over the Ag to DCs in the lamina propria, these Ag-loaded DCs migrate via drainage lymphatics to the mesenteric lymph nodes where they activate T cells

Question 23

Which pathogens target M cells to gain access to the subepithelial space, into the body?

Answer 23

• Poliovirus
• Reovirus
• Some retroviruses: salmonella, shigella and yersinia

Question 24

How do M cells transport antigens to the lamina propria?

Answer 24

1. M cell takes up the antigen by endocytosis and phagocytosis
2. Antigen is transported across the M cell down towards the basolateral site of the epithelium, here you will have cells of the immune system such as DCs
3. The antigen is bound by DCs, which migrate to T cell areas within the Peyer’s patch and they can also migrate via drainage lymphatics to the mesenteric lymph nodes, where they activate T cells

Question 25

Give the different ways in which macrophages and dendritic cells are conditioned within our healthy intestines are conditioned

Answer 25

- In the lamina propria there are factors which make them anti-inflammatory: 1. Epithelial cells sense components of commensal bacteria and in turn release TSLP from epithelial cells which act on DCs 2. Vit A release retinoic acid which act on DCs 3. TGF-beta acts on macrophages and DCs 4. Bacterial products condition DCs, such as short chain fatty acids e.g. butyrate, acetate and proprionate. These are metabolites of bacteria

Question 26

In the healthy intestine, what do macrophages secrete?

Answer 26

Anti-inflammatory IL10

Question 27

In the healthy intestine, what do DCs secrete?

Answer 27

TGF-beta

Question 28

Due to the conditioning, which specific cell response do DCs favour?

Answer 28

Treg cell response (once they have migrated to mesenteric LNs with the Ag

Question 29

Explain how intestinal homeostasis is maintained and how we end up with inflammation

Answer 29

1. Intestinal homeostasis is maintained by a balance between effector T cells and regulatory T cells 2. Either too many effectors, so Tregs cant handle them, cause inflammation of the intestine OR a Treg defect so they cant balance the effector cells, causing an imbalance, leading to intestinal inflammation

Question 30

What is the dominant antibody class in the mucosal immune system?

Answer 30

IgA

Question 31

Blood vs mucosal IgA, whats the difference?

Answer 31

``` Blood = monomeric Mucosal = dimeric, linked by a J chain ```

Question 32

What are the functions of secretory IgA?

Answer 32

1. Binds to mucus layer coating the epithelial surface via carbohydrate determinants in secretory component, this binding allows: - Prevention of the adherence of microorganisms - Neutralisation of microbial toxins and enzymes 2. Limits access of pathogens to the mucosal surfaces, without risking inflammatory damage to the tissue as it has little capacity to active he complement pathway or act as an opsonin (cannot induce inflammation) 3. Important role in symbiotic relationship between individual and their commensal bacteria, helping to restrict these organisms to the gut lumen

Question 33

In IgA deficient patients, which antibody replaces its role?

Answer 33

Secretory IgM

Question 34

What are the important tasks of the mucosal immune system (MALT)?

Answer 34

1. Ignore harmless antigens | 2. Mount protective immune responses to pathogens

Question 35

Why are there normally no adverse responses against food antigens and commensal flora?

Answer 35

1. Food antigens - The default response to oral administration of a protein antigen is the development of specific peripheral unresponsiveness = oral tolerance - Antigen-specific effector T cells are turned off or deleted - Antigen-specific Treg cells are generated (anti-inflammatory) 2. Commensals - Induce IgA and Treg cells in the intestine - Ignored by the systemic immune systems - common antigens normally dont reach the rest of the body - only PPs, isolated lymphoid follicles and mesenteric LNs recognise them

Question 36

If under physiological conditions, conditioned DCs at mucosal surfaces are immature/non-activated and favour the induction of tolerance, how does the immune system mount an efficient response when exposed to pathogens?

Answer 36

In the presence of pathogens, DCs become fully activated, and induce CD4+ T cells to differentiate to effectors

Question 37

How does the immune system distinguish between harmless commensals and pathogenic bacteria?

Answer 37

1. The TLR4 is expressed at the base of crypts where you wont really see many commensals 2. TLR5 is expressed on the basolateral side of the intestinal epithelium (facing the lamina propria) this means that pathogens invading tissues can trigger TLR activation whereas commensals sitting out in the mucus layer wont have access to TLRs 3. Commensals have their own traits to avoid PRR activation, such as changes in flagellin sequence allows it to not bind as strongly to TLR5 (rendered hyporesponsive)