Gut Immunology

Question 1

What is the surface area of the GI tract?

Answer 1

200m2

Question 2

What makes up the antigen load that the GI Tract is exposed to ?

Answer 2

Resident microbiota 1014 bacteria
Dietary antigens
Exposure to pathogens

Question 3

What state describes the GI tract’s immunological activation?

Answer 3

State of “restrained activation”
– Tolerance vs active immune response
– Dual immunological role. Immune homeostasis of gut & development of healthy immune system requires presence of bacterial microbiota

Question 4

What makes up the Gut Microbiota?

Answer 4

1014 gut bacteria and 1013 cells in body - most densely populated “ecosystem” on Earth.
4 major phyla of bacteria (Bacteroidetes, Firmicutes, Actinobacteria, Proteobacteria), also viruses & fungi.
Provide traits we have not had to evolve on our own - Genes in gut flora 100 times our own genome.

Question 5

Which chemical digestive factors produced by the host affect bacteria?

Answer 5

Stomach - HCl pH-14, pepsin, gastric lipase.
Liver - bile acids.
Pancreas - trypsin, amylase, carboxypeptidase
Small intestine - brush border enzymes
Colon - No host digestive factors

Question 6

Define Symbionts, Commensals, Pathbionts.

Answer 6

Symbionts - no benefit or disbenefit.
Commensals - benefit from the host but do not cause harm.
Pathbionts - benefit host but have potenitial to cause disease

Question 7

What happens in an immunological dysequilibrium?

Answer 7

Increased pathobiont replication

Question 8

Which factors cause and influence Dysbiosis?

Answer 8

Infection
Diet
Xenobiotics
Hygiene
Genetics

Question 9

What are some examples of Bacteial metabolites and toxins?

Answer 9

TMAO, 4-EPS, SCFAs, bile acids, AHR ligands.

Question 10

What are the Mucosal Defence Barriers?

Answer 10

Physical barriers
Anatomical;
Epithelial barrier
Peristalsis
Chemical;
Enzymes
Acidic pH
Commensal bacteria

Question 11

What makes up the Epithelial barrier?

Answer 11

Mucus layer - Goblet cells
Epithelial monolayer - Tight junctions
Paneth Cells (small intestine)
Bases of crypts of Lieberkühn.
Secrete antimicrobial peptides (defensins) & lysozyme.

Question 12

What are the Immunological barriers?

Answer 12

MALT (Mucosa Associated Lymphoid Tissue)

GALT (Gut Associated Lymphoid Tissue)

Question 13

Describe the role of Mucosa Associated Lymphoid tissue MALT in immunological defence?

Answer 13

Found in the submucosa below the epithelium, as lymphoid mass containing lymphoid follicles
Follicles are surrounded by HEV postcapillary venules, allowing easy passage of lymphocytes

Question 14

Give an example of somewhere rich in immunological tissue?

Answer 14

Oral cavity

Question 15

Describe the role of Gut associated lymphoid tissue in immunological GULT defence?

Answer 15

Responsible for both adaptive and innate immune responses through generations of lymphoid cells & Antibodies. Two main types Organised & Non-organised.
Consists of B & T lymphocytes, macrophages, APC (dendritic cells), and specific epithelial & intra-epithelial lymphocytes

Question 16

Where would you find Non-organised GALT’s?

Answer 16

Intra-epithelial lymphocytes
Make up 1/5th of intestinal epithelium, e.g. T-cells, NK cells
Lamina propria lymphocytes

Question 17

Where would you find organized GALT’s?

Answer 17

Peyer’s patches (small intestine)
Caecal patches (large intestine)
Isolated lymphoid follicles
Mesenteric lymph nodes (encapsulated)

Question 18

What characterises paneth cells?

Answer 18

Presence of dense granules that contain antimicrobial peptides

Question 19

Where do you find the majority of the immuno-intestinal cells?

Answer 19

Lamina propria

Question 20

Describe the role of Peyer’s Patches in gut immunology?

Answer 20

Immune sensorFound in submucosa small intestine – mainly distal ileum
Aggregatedlymphoid follicles covered with follicle associated epithelium (FAE).
FAE - no goblet cells, no secretory IgA, no microvilliOrganised collection of naïve T cells & B-cells
Development requires exposure to bacterial microbiota
50 in last trimester foetus, 250 by teens

Question 21

How does Antigen uptake take place in Peyer’s patches?

Answer 21

Antigen uptake via M (microfold) cells within FAE

M cells expressIgA receptors, facilitating transfer of IgA-bacteria complexinto the Peyer’s patches.

Question 22

What is the role of Trans-epithelial Dendritic cells?

Answer 22

Antigen sampling
Open tight junction proteins and send their dendrites into GI lumen to sample bacteria, then send them to Mesenteric Lymph node.

Question 23

Explain the adaptive response?

Answer 23

Pathogens taken up by M cells, excreted into pocket formed on the inner surface of enterocytes. Pocket contains Antigen presenting cells, these engulf the antigens and present them via MHC2 expression, dendritic cells then migrate to the peyers patch where the APC, t cells, bcells all form an organised lymphoid molecule. some antigen presenting cells escape via lymph nodes that develop in the Mesenteric lymph node.

Question 24

Describe the adaptive response?

Answer 24

Mature naïve B-cells express IgM in Peyer’s Patches
On antigen presentation class switches to IgAT-cells & epithelial cells influence B cell maturation via cytokine production
B cells further mature to become IgA secreting plasma cells.
Populate lamina propria

Question 25

How is Secretory IgA produced?

Answer 25

The plasma cell produces IgA which is taken into the enterocyte via a vesicle where there is enzymatic cleavage, Producing secretory IgA which is secreted into the lumen.

Question 26

Which cells secrete IgA

Answer 26

Up to 90% of gut B-cells secrete IgA sIgA binds luminal antigen → preventing its adhesion and consequent invasion.

Question 27

How does Lymphocyte

Answer 27

Lymphocytes travel to Mesenteric lymph nodes for Lymphocyte proliferation following Activation & presentation in Peyer's Patch, return to circulation via thoracic duct. It can then either enter toncils, skin, Brochus associated lymphoid tissue BALT. It can also return to intestinal mucosa via Lamina propria.

Question 28

What is important for chemotaxis of lymphocytes in the High Endothelial venule?

Answer 28

𝝰4β7 Integrin/MAdCAM-1 Adhesion & Gut Homing

Question 29

Why is rapid turnover of Enterocytes important?

Answer 29

Enterocytes are first line of defense against GI pathogens & may be directly affected by toxic substances in diet. Effects of agents which interfere with cell function, metabolic rate etc will be diminished. Any lesions will be short-lived.

Question 30

What is the mechanism of action of Cholera infection?

Answer 30

Cholera -acute bacterial disease caused by Vibrio cholerae serogroups O1 & O139 Bacteria reaches small intestine → contact with epithelium & releases cholera enterotoxin.

Question 31

Why do you get diarrhoea in cholera?

Answer 31

Cholera endotoxin gets internalised via retrograde endocytosis, once inside you get increased adenylate cyclase activity. Increased cAMP, causes secretion of salts and water follows it. Activation of CFTR, allows salt secretion.

Question 32

What are the main symptoms of Cholera infection?

Answer 32

``` Transmitted through faecal-oral route Spreads via contaminated water & food. Main symptoms Severe dehydration & watery diarrhoea Other symptoms Vomiting, nausea & abdominal pain. ```

Question 33

How do you diagnose Cholera infection?

Answer 33

: bacterial culture from stool sample on selective agar is the gold standard, rapid dipstick tests also available.

Question 34

How do you treat Cholera?

Answer 34

Treatment: oral-rehydration is the main management ; up to 80% of cases can be successfully treated. Vaccine: Dukoral, oral, inactivated. Globally 1.3 - 4 million cases, avg. 95,000 deaths/year (last indigenous UK case 1893: 2017 - 13 cases).

Question 35

What are some other causes of Infectious diarrhoea - Gastroenteritis?

Answer 35

``` Viral - Rotavirus, Norovirus Bacterial - Campylobacter jejuni Escherichia coli Salmonella Shigella Clostridium difficile Protozoa Parasitic - Giardia lamblia, Entamoeba histolytica ```

Question 36

Describe Rotaviruses

Answer 36

RNA virus, replicates in enterocytes. | 5 types A – E, type A most common in human infections.

Question 37

What is the most common cause of diarrhoea in infants and young children worldwide?

Answer 37

Rotavirus

Question 38

How do you treat Rotavirus?

Answer 38

Oral rehydration therapy Still causes ~ 200,000 deaths/year. Before vaccine, most individuals had an infection by age 5, repeated infections develop immunity.Live attenuated oral vaccine (Rotarix) against type A introduced in UK July 2013.

Question 39

Describe Norovirus

Answer 39

RNA virus | Incubation period 24-48 hours

Question 40

How is Norovirus transmitted?

Answer 40

Faecal-oral transmission. Individuals may shed infectious virus for up to 2 weeks Outbreaks often occur in closed communities

Question 41

What are the symptoms of Norovirus?

Answer 41

Acute gastroenteritis, recovery 1 – 3 days

Question 42

How do you diagnose Norovirus?

Answer 42

PCR, no treatment necessary

Question 43

How is Campylobacter transmitted?

Answer 43

``` Undercooked meat (especially poultry), untreated water & unpasteurised milk Low infective dose, a few bacteria (<500) can cause illness ```

Question 44

What are the most common species of Campylobacter?

Answer 44

Campylobacter jejuni, Campylobacter coli

Question 45

What is the treatment for Campylobacter?

Answer 45

Not usually required Azithromycin (macrolide) is standard antibiotic Resistance to fluoroquinolones is problematic

Question 46

Is Campylobacter still common?

Answer 46

Estimated 280,000 cases per year in UK, 65,000 confirmed | Commonest cause of food poisoning in the UK

Question 47

What are the Pathotypes of E. coli

Answer 47

Gram negative intestinal bacteria. Enterotoxigenic E. coli (ETEC) Cholera like toxin Watery diarrhoea Enterohaemorrhagic or Shiga toxin-producing E. coli (EHEC/STEC) E. coli O157 serogroup, Shigatoxin/verotoxin 5-10% get haemolytic uraemic syndrome: loss of kidney function Enteroinvasive E. coli (EIEC) Shigella like illness Bloody diarrhea Enteropathogenic E. coli (EPEC) Enteroaggregative E. coli (EAEC) Diffusely adherent E. coli (DAEC)

Question 48

How do you get C Diff?

Answer 48

C Diff exists normally in the gut, however when there is a dysbiosis, usually caused by antibiotic treatmen, then you can develop a diseased state where there is a supportive environment for toxin production

Question 49

How do you manage C Diff?

Answer 49

Isolate patient (very contagious) Stop current antibiotics Metronidazole (can give you it, but it also causes it), Vancomycin Recurrence rate 15-35% after initial infection, increasingly difficult to treat. Faecal Microbiota Transplantation (FMT) – 98% cure rate

Question 50

How do you define the severity of C Diff infection?

Answer 50

Non-severe infection WCC<15, Creat <150 Severe infection WCC>15, Creat >150 Fulminant colitis Hypotension or shock, ileus, toxic megacolon

Question 51

What is the first line treatment for fulminant colitis with toxis megacolon?

Answer 51

Medical therapy with antibiotics and supportive management

Question 52

What are indications for surgery following C Diff infection?

Answer 52

Colonic perforation Necrosis or full-thickness ischaemia Intra-abdominal hypertension or abdominal compartment syndrome Clinical signs of peritonitis or worsening abdominal exam despite adequate medical therapy End-organ failure

Question 53

What is Pseudomembranous colitis?

Answer 53

Most often associated with C. difficile infection Manifestation of severe colonic disease Characteristic yellow-white plaques that form pseudomembranes on the mucosa Confirmed on endoscopy +/- bio

Question 54

What are the management options for ulcerative colitis?

Answer 54

Steroids 5 ASA Immune suppressants Azathioprine Methotreaxate Biologic therapy Others –diet, FMT, antibiotics, probiotics, novel agents

Question 55

How do you define severity of Ulcerative colitis?

Answer 55

Different scopes including clinical disease activity index, Montreal classification and Trulov & Witt scores. Mild 4 x BMs/day, no systemic toxicity, normal ESR/CRP, mild symptoms. Moderate > 4x BMs/day, mild anaemia, mild symptoms, minimal systemic toxicity, nutrition maintained and no weight loss. Severe > 6 BMs/day, severe symptoms, systemic toxicity, significant anaemia, increased ESR/CRP and weight loss.