Infection and immunology of the gut

Question 1

What is meant by restrained activation

Answer 1

tolerant to food antigens and commensal bacteria but reactive to pathogens

Question 2

Describe the importance of the microbiota

Answer 2

roles in nutrition, digestion, development- essential in our development.

Question 3

What should the G.I tract remain tolerant to

Answer 3

Food antigens

Commensal bacteria

Question 4

What should the G.I tract be immunoreactive to

Answer 4

Immunoreactivity

Pathogens

Question 5

Describe the G.I tract antigen load

Answer 5

§ There is a massive anti-gen load in the gut:
o 10^14 resident bacteria.
o Dietary antigens.
o Exposure to external pathogens.
§ The gut immune system remains in a state of restrained activation – tolerance vs. active immunity to antigens.
§ The Bacteroidetes are the most prevalent bacteria in the gut – especially in the colon (from terminal ileum).
o 4 major phyla: Bacteroidetes, Firmicutes, Actinobacteria and Proteobacteria.
Surface area of GI tract 400m2.

Question 6

Describe the epithelium of the G.I tract

Answer 6

The epithelium of the digestive tract is, by definition, an external environment, because it is possible for bacteria to reach there without needing to cross a membrane.
The gut has a huge internal surface area which facilitates its role of absorption
The enormous resident population of bacteria in our gut is ten times as many cells in our body, having 100 times as many genes as the human genome!

Question 7

What is essential for the development of a healthy immune system

Answer 7

Immune homeostasis of gut & development of healthy immune system requires presence of bacterial microbiota.

Question 8

Describe the presence of microbiota in different parts of the G.I tract

Answer 8

Loads in the mouth is because we put lots of dirty things into it, including food, fluid, cutlery, air etc.
Into the stomach, the low pH kills lots of bacterial populations (except H. pylori).
The number is kept low in the duodenum, jejunum and proximal ileum because of paneth cells and Peyer’s patches (discussed in detail later).
Beyond the ileocaecal valve the number of microorganisms increases markedly.

Question 9

List the immunological defects seen in germ-free mice

Answer 9

development of small intestine
development of mesenteric lymph nodes
CD8 and CD4 T cells
CD4 and CD25 T cells
production of secretory IgA
Expression of REG3 gamma and angiogenin 4 (Paneth cells)
levels of ATP (intestine)
Expression of MHC 2
Expression of TLR 9
Levels of IL-25

Question 10

Describe the enormity of our microbiota

Answer 10

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 11

How can the host increase the numbers of bacteria in the microbiota

Answer 11

Ingested and secreted nutrients encourage bacterial growth

However chemical digestive factors and peristaltic contractions and defecation leads to decreased cell numbers

Question 12

Describe the chemical protections of each part of the G.I tract

Answer 12

Stomach- HCL (pH 1.4) gastric lipase, pepsin
Liver- Bile acids
pancreas- trypsin, amylase, carboxypeptidase
S.I - brush border enzymes
Colon- no host digestive factors

Question 13

What is meant by dysbiosis

Answer 13

Altered microbiota composition

Question 14

Describe the symbiotic relationship we have with the microbiota

Answer 14

They take nutrients from us but provide us with benefits

Question 15

What can cause dysbiosis

Answer 15

Infection or inflammation
Diet
Xenobiotics 
Hygiene
genetics

Question 16

Describe the physical barriers of mucosal defence

Answer 16

a tight epithelial wall, glycocalyx, mucous and unstirred layer. Also, persitalsis to keep things moving along the GI tract.
Anatomical (epithelial barrier, peristalsis)

Question 17

Describe the chemical barriers of mucosa

Answer 17

Chemical (Enzymes, acidic pH)

bactericidal enzymes from paneth cells, and acid from stomach.

Question 18

Describe bacterial protection in mucosal defence

Answer 18

commensal bacteria maintain immune system priming and may attack foreign species.
Commensal bacteria: occupy “ecological niche”- provide competition for attachment to the epithelial wall

Question 19

Describe the role of immune system in mucosal defence

Answer 19

Mucosa-associated lymphoid tissue (MALT) rich in T cells & B cells, whose components can be further categorized into GALT (Gut-associated lymphoid tissue), BALT (Bronchus-associated lymphoid tissue) etc.

Question 20

Describe the role of the epithelial barrier in mucosal defence

Answer 20

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

Question 21

What can the metabolites and toxins of bacteria cause

Answer 21

Systemic disease

Question 22

Describe MALT

Answer 22

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
The oral cavity is rich in immunological tissue. (LINGUAL, PALATINE AND PHARYNGEAL TONSILS)

Question 23

What is the role of GALT

Answer 23

Responsible for both adaptive & innate immune responses through generation of lymphoid cells & Abs.
Also cell-mediated responses

Question 24

Describe unorganised GALT

Answer 24

Intra-epithelial lymphocytes – Make up one-fifth of intestinal epithelium, e.g., T cells, NK cells
Lamina propria lymphocytes

Question 25

Summarise the categories of GALT

Answer 25

GALT can be split into two categories depending on organisation. There are organised sites of lymphoid tissue such as Peyer's patches in the small intestine and lymphocytes in mesenteria lymph nodules (i.e. where the lymph from the villi drain). And, there are disorganised sites which include lymphocytes in the lamina propria (mainly IgA-secreting B-cells) and lymphocytes in the interstitial space below the basolateral membrane of the epithelium (called intra-epithelial cells). Additionally, although not technically GALT, there are Kuppfer cells in the liver which can phagocytose bacteria.

Question 26

What are Peyer's patches and where are they found

Answer 26

Peyer's patches consists of aggregated lymphoid follicles covered with follicle associated epithelium (FAE). They are found in the small intestine, having highest concentration in the distal ileum.

Question 27

Describe follicle associated epithelium

Answer 27

FAE - no goblet cells, no secretory IgA, lack microvilli

Question 28

What is the function of Payer's patches and what is essential in their development

Answer 28

They function as "immune sensors" since they are capable of monitoring local bacteria, and provide protection against pathogenic bacteria. Their development requires exposure to bacterial flora (i.e. they are barely present in animal models that are born and reared in sterile environments). Humans have about 50 by the last trimester of gestation, and hit the maximum of about 250 by their teenage years. Peyer's patches are rich in B cells, T cells, macrophages and dendritic cells.

Question 29

Describe the roles of M cells

Answer 29

Antigen uptake via M (microfold) cells within FAE. | M cells express IgA receptors, facilitating transfer of IgA-bacteria complex into the peyer's patches.

Question 30

Summarise the roles of M cells

Answer 30

FAE contains specialized enterocytes or M cells. The main function of M cells is to perform transcytosis of luminal bacteria, antigens and proteins. M cells express IgA receptors, facilitating transfer of IgA-bacteria compex into the peyer's patches. Antigen uptake is a combined effort by specialised M-cells and dendritic cells. These antigens are then presented to the lymphocytes for assessment and potential immunological response. Activated cells develop gut homing markers (see lymphocyte circulation) and migrate to mesenteric lymph nodes for proliferation.

Question 31

Describe the role of dendritic cells in antigen sampling

Answer 31

§ Dendritic cells sample antigen using dendrites. § DCs take up antigen and process, then present to: o Naïve T-cells in Peyer’s patch. o Transport to mesenteric lymph nodes.

Question 32

Describe the activation of B cells in the B cell adaptive response

Answer 32

Mature naïve B-cells express IgM in PPs Upon antigen presentation class switch to IgA T-cells & epithelial cells influence B cell maturation via cytokine production. B cells further mature to become IgA secreting plasma cells. Populate lamina propria

Question 33

Describe secretory IgA

Answer 33

Up to 90% of gut B-cells secrete IgA. | sIgA binds luminal antigen, thereby preventing its adhesion and consequent invasion (neutralisation)

Question 34

Describe the structure and production of sIgA

Answer 34

SIgA is a dimeric form of IgA produced by B cells in the lamina propria and transported across the enterocyte. In the plasma cell, two IgA molecules are bound together by a J-chain, and secreted into the interstitial space. The dimer binds to a special receptor on the external basolateral surface of enterocytes (polymeric immunoglobulin receptor; pIgR). This receptor becomes the secretory component and binds to the length of the IgA dimer, becoming SIgA. SIgA is then endocytosed into the epithelial cell and actively transported within a vesicle to the apical membrane, where it is exocytosed into the gut lumen. The secretory component (on top of its function of helping IgA move through the enterocyte) also protects the antibody dimer from enzymatic and acidic degradation.

Question 35

Describe a key fact of IgA

Answer 35

Although IgG is the most abundant circulating immunoglobulin in the body, IgA is the most abundant antibody in the body.

Question 36

What is the difference between IgA1 and IGA2

Answer 36

1- Blood | 2- secretions

Question 37

How does the mucosal layer in the small intestine compare to that of the large

Answer 37

small- single layer- villi bigger | large- two layers of mucus (outer thicker)- smaller villi

Question 38

Describe lymphocyte homing and circulation

Answer 38

Mucosal lymphocytes in Peyer's patches, once stimulated by an antigen, migrate into the local mesenteric lymph nodes and drain into the lymphatic system. They reach the systemic circulation via the thoracic duct and spread throughout the body in the blood. These lymphocytes remain in the blood until activated by tissue-sepecific endothelial adhesion molecules at the site of inflammation, which permit transmigration of the lymphocytes into the gut mucosa. This is called lymphocyte homing and requires specialised post-capillary microvascular endothelial cells, such as the high endothelial venules (HEVs) of lymphoid tissue.

Question 39

Describe leukocyte recruitment in the gut

Answer 39

L-selectin, a carbohydrate-binding lectin, is constitutively expressed on the surface of lymphocytes and mediates the low adhesive interactions that enable leukocytes to roll in postcapillary venules and HEVs. In HEVs, L-selectin mediates lymphocyte rolling by its binding to mucosal addressin cell adhesion molecule-1 (MAdCAM-1). In addition to the HEVs of Peyer’s patches and mesenteric lymph nodes, MAdCAM-1 is constitutively expressed on the flattened endothelial cells localised in the lamina propria of the small and large intestine and enables lymphocyte recruitment in chronic gut inflammation a4b7 integrin binds to MAdCAM-1

Question 40

Describe the mechanism of cholera infection

Answer 40

Cholera is an acute bacterial disease caused by Vibrio cholerae serogroups O1 and O139 (cholera toxin). The bacteria reaches the small intestine where on making close contact with the epithelium releases cholera toxin. Cholera toxin on entering the epithelial cell, starts a series of biochemical reactions resulting in exit of ions such as Na+, K+, Cl- and water from the epithelial cell.

Question 41

Describe the transmission and symptoms of cholera

Answer 41

Transmitted through faecal-oral route, and spreads through contaminated water & food. Main symptoms – Severe dehydration and diarrhoea (watery). Other symptoms - vomiting, nausea & abdominal pain.

Question 42

Describe the diagnosis and treatment of cholera

Answer 42

Diagnosis: bacterial culture from stool sample on selective agar is the gold standard, rapid dipstick tests also available. 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. 95,000deaths/year (last indigenous UK case 1893: 2017 - 13 cases).

Question 43

List the Other causes of infectious diarrhea (gastroenteritis)

Answer 43

Viral: Rotavirus (children) Norovirus 'Winter vomiting bug' protozoal parasitic: Giardia lamblia Entamoeba histolytica ``` Bacterial: Campylobacter jejuni Escherichia coli Salmonella Shigella Clostridium difficile ```

Question 44

Describe rotaviruses

Answer 44

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

Question 45

Describe the epidemiology of rotaviruses

Answer 45

Epidemiology: Most common cause of diarrhoea in infants and young children worldwide.

Question 46

Describe the treatment and vaccination of rotaviruses

Answer 46

Treatment: oral rehydration therapy, but still cause about 200,000 deaths per year. Before vaccine, most individuals had an infection by age 5, repeated infections develop immunity. Vaccination: Live attenuated oral vaccine (Rotarix) against type A introduced in UK July 2013.

Question 47

What is the Norwalk virus a species of

Answer 47

Norovirus (genus) Norwalk virus (species)

Question 48

Describe the norovirus

Answer 48

Description: RNA virus. Incubation period 24-48 hours

Question 49

Describe how we diagnose norovirus

Answer 49

Diagnosis: Sample PCR.

Question 50

Describe the epidemiology of norovirus

Answer 50

Epidemiology: Estimated 685 million cases per year

Question 51

Describe the symptoms of norovirus

Answer 51

Symptoms: Acute gastroenteritis, recovery 1 – 3 days

Question 52

Describe the transmission of norovirus

Answer 52

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

Question 53

What is the most common species of campylobacter

Answer 53

Most common species: Campylobacter jejuni, Campylobacter coli

Question 54

Describe the transmission of campylobacter

Answer 54

Transmission: Undercooked meat (especially poultry), untreated water and unpasteurised milk. Low infective dose, a few bacteria (<500) can cause illness.

Question 55

Describe the epidemiology of campylobacter

Answer 55

Epidemiology: Estimated 280,000 cases per year in UK, 65,000 confirmed. Most common cause of food poisoning in the UK.

Question 56

Describe the treatment of campylobacter

Answer 56

Treatment: Not usually required, azithromycin (macrolide) is standard antibiotic, resistance to fluoroquinolones is problematic

Question 57

Describe E.coli

Answer 57

Diverse group of Gram negative intestinal bacteria, most harmless but 6 ”pathotypes” associated with diarrhea (diarrhoeagenic):

Question 58

Describe Enterotoxigenic E. coli (ETEC

Answer 58

Cholera like toxin | Watery diarrhea

Question 59

Describe Enteroinvasive E. coli (EIEC

Answer 59

Shigella like illness | Bloody diarrhea

Question 60

Describe Enterohaemorrhagic or Shiga toxin-producing E. coli (EHEC/STEC)

Answer 60

E. coli O157 serogroup, Shigatoxin/verotoxin | 5-10% get haemolytic uraemic syndrome: loss of kidney function

Question 61

List some other types of E.coli

Answer 61

Enteropathogenic E. coli (EPEC Enteroaggregative E. coli (EAEC) Diffusely adherent E. coli (DAEC)

Question 62

Describe the management of C.Difficile

Answer 62

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

Question 63

Describe the progression from a healthy state to a diseased state

Answer 63

Healthy state ---- Intermediate dysbiotic state ( due to exogenous disturbance i.e antibiotics) Can recover OR progress to disease Pathogen induced disturbance (toxin production) creates a supportive state

Question 64

What is Coeliac disease

Answer 64

An autoimmune disorder

Question 65

Describe the mechanism of coeliac disease

Answer 65

Gliadin (33aa peptide component of gluten) is not broken down in the stomach, reaches the small intestine, and binds to the secretory IgA in the mucosal membrane. Gliadin-secretory IgA complex binds to the Transferrin receptor (TFR) and are transferred to the lamina propria. Enzyme tissue transglutaminase (tTG) cuts off amide group from the protein. Deamidated gliadin is phagocytosed by the macrophages, and presented by MHC II molecules. This leads to activation of immune system causing destruction of epithelial cells.

Question 66

Outline the steps leading to coeliac disease

Answer 66

Innate immune detection T- cell development T- cell and B-cell co-stimulation Cytokines, chemokines and their receptors ``` Cytokines released (TNF) Antibodies released (anti-tTG) Causing intestinal inflammation ```

Question 67

What are the symptoms of coeliac disease

Answer 67

Abdominal distension (bloating) Diarrhoea Sometimes dermititis herpetiformis

Question 68

How do we diagnose coeliac disease

Answer 68

Ab blood tests - anti-gliadin. | Biopsy test of duodenum

Question 69

Describe the treatment for Coeliac disease

Answer 69

Gluten-free diet (wheat, barley, rye exclusion) and medication. Can also get gluten-free foods on prescription. Factors affecting compliance are lifestyle, eating out, cross contamination at home and holidays.

Question 70

Describe the mechanisms of IBS

Answer 70

Visceral hypersensitivity | Triggered by diet / stress

Question 71

Describe the symptoms of IBS

Answer 71

Recurrent abdominal pain Abnormal bowel motility Constipation and/or Diarrhoea

Question 72

Describe the treatment of IBS

Answer 72

Diet modification - Avoiding certain foods such as apples, beans, cauliflowers. Treatment of constipation - soluble fiber, stool softeners and osmotic laxatives. Treatment of spasms and pain - anti-diarrheals, anti-muscarinic. Management of stress, anxiety, depression

Question 73

Describe the different irritable bowel diseases

Answer 73

Crohn's and ulcerative colitis

Question 74

Describe the mechanism of Crohn's

Answer 74

Immune-related disorder - triggered by pathogens such as mycobacterium paratuberculosis, pseudomonas, and listeria. Unregulated immune response causing the destruction of cells in the GI tract. Gene mutations such as frame-shift mutation in NOD2 gene are thought to be responsible for development of this disease.

Question 75

Describe the pathology of Crohn's

Answer 75

Patches of inflammatory damage and healthy tissue | Cobblestone appearance

Question 76

Describe the symptoms of Crohn's

Answer 76

Pain in affected area, most commonly in right lower quadrant. Diarrhea and blood in stool. Disease of distal ileum and colon weight loss, fatigue, anaemia

Question 77

Describe the treatment of Chron's

Answer 77

Use of medications such as anti-inflammatory drugs and antibiotics. For severe symptoms, immunosuppressants such as corticosteroids can be prescribed. Surgical removal of affected tissue, but it does not cures the disease. Liquid diet: nutritionally complete liquid feeds if other medical therapies contraindicated. Adjunctive therapy with corticosteroids and other treatment. Consider type of feed, route, compliance, monitoring, assessment, refeeding syndrome risk. Food reintroduction needs dietician supervision and nutritional supplementation. Try to identify trigger/problem foods. Crohn’s: liquid diet, low fibre/low residue, food reintroduction Managing strictures: decrease obstruction, pain and gas production. Need to change diet so that food can get through the stricture; fibre can block the intestines. Avoid fibrous parts of fruit and vegetables (skins, seeds, woody stalks), whole- grains, nuts, seeds, gristle and skin from meat and fish, bones etc. May require dif- ferent consistency e.g. liquidising Food intolerance tests are inappropriate and can result in food exclusion and nutritionally inadequate diets, no evidence for intolerance in Crohn’s Nutritional support oral when possible, enteral tube feed or parenteral nutrition can be used to top up oral feeding post-surgery (ileus, bowel obstruction, enterocutane- ous fistula or short bowel Diet in remission: if stricturing disease a low fibre, balanced diet, otherwise if non- stricturing a healthy balanced diet

Question 78

Describe the diagnosis of Chron's and Ulcerative colitis

Answer 78

Ab blood tests endoscopy Barium X-Ray

Question 79

Describe the mechanism of ulcerative colitis

Answer 79

Autoimmune disorder - T cells destroy the cells lining the walls of large intestine Secondary cause - Diet and stress Continuous inflammation

Question 80

Describe the symptoms of ulcerative colitis

Answer 80

Pain in left lower quadrant due to ulcers along the inner surface of large intestine, including the colon and rectum. Severe and frequent diarrhea (sometimes blood in the stool). Anaemia weight loss and fatigue disease of the colon only

Question 81

Describe the treatment of ulcerative colitis

Answer 81

Anti-inflammatory drugs such as sulfasalazine and mesalamine For severe cases, Immunosuppressant drugs such as corticosteroids, azathioprine, cyclosporine might be prescribed. Colectomy - surgical removal of colon cures the disease( curative). Dietary manipulation to minimise exacerbation of diarrhoea. Pre/probiotics: to treat and prevent pouchitis (when colon is removed leaving a pouch and becomes inflamed), helps remission of ulcerative colitis. Prebiotics may cause ab- dominal pain, bloating diarrhoea and flatulence Diarrhoea: drink fluid, nutritious drinks, replace salt. Eating soluble fibre helps the gut absorb more water from stool. Avoid gas producing foods, high fibre or whole- grain cereals, alcohol (worsens dehydration), caffeine and personal triggers.