Infection and immunology of the gut Flashcards
What is meant by restrained activation
tolerant to food antigens and commensal bacteria but reactive to pathogens
Describe the importance of the microbiota
roles in nutrition, digestion, development- essential in our development.
What should the G.I tract remain tolerant to
Food antigens
Commensal bacteria
What should the G.I tract be immunoreactive to
Immunoreactivity
Pathogens
Describe the G.I tract antigen load
§ 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.
Describe the epithelium of the G.I tract
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!
What is essential for the development of a healthy immune system
Immune homeostasis of gut & development of healthy immune system requires presence of bacterial microbiota.
Describe the presence of microbiota in different parts of the G.I tract
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.
List the immunological defects seen in germ-free mice
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
Describe the enormity of our microbiota
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.
How can the host increase the numbers of bacteria in the microbiota
Ingested and secreted nutrients encourage bacterial growth
However chemical digestive factors and peristaltic contractions and defecation leads to decreased cell numbers
Describe the chemical protections of each part of the G.I tract
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
What is meant by dysbiosis
Altered microbiota composition
Describe the symbiotic relationship we have with the microbiota
They take nutrients from us but provide us with benefits
What can cause dysbiosis
Infection or inflammation Diet Xenobiotics Hygiene genetics
Describe the physical barriers of mucosal defence
a tight epithelial wall, glycocalyx, mucous and unstirred layer. Also, persitalsis to keep things moving along the GI tract.
Anatomical (epithelial barrier, peristalsis)
Describe the chemical barriers of mucosa
Chemical (Enzymes, acidic pH)
bactericidal enzymes from paneth cells, and acid from stomach.
Describe bacterial protection in mucosal defence
commensal bacteria maintain immune system priming and may attack foreign species.
Commensal bacteria: occupy “ecological niche”- provide competition for attachment to the epithelial wall
Describe the role of immune system in mucosal defence
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.
Describe the role of the epithelial barrier in mucosal defence
Mucus layer - Goblet cells Epithelial monolayer - Tight junctions Paneth Cells (small intestine) Bases of crypts of Lieberkühn. Secrete Antimicrobial peptides (defensins) and lysozyme.
What can the metabolites and toxins of bacteria cause
Systemic disease
Describe MALT
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)
What is the role of GALT
Responsible for both adaptive & innate immune responses through generation of lymphoid cells & Abs.
Also cell-mediated responses
Describe unorganised GALT
Intra-epithelial lymphocytes – Make up one-fifth of intestinal epithelium, e.g., T cells, NK cells
Lamina propria lymphocytes
Summarise the categories of GALT
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.
What are Peyer’s patches and where are they found
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.
Describe follicle associated epithelium
FAE - no goblet cells, no secretory IgA, lack microvilli
What is the function of Payer’s patches and what is essential in their development
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.
Describe the roles of M cells
Antigen uptake via M (microfold) cells within FAE.
M cells expressIgA receptors, facilitating transfer of IgA-bacteria complexinto the peyer’s patches.
Summarise the roles of M cells
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.
Describe the role of dendritic cells in antigen sampling
§ 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.
Describe the activation of B cells in the B cell adaptive response
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
Describe secretory IgA
Up to 90% of gut B-cells secrete IgA.
sIgA binds luminal antigen, thereby preventing its adhesion and consequent invasion (neutralisation)
Describe the structure and production of sIgA
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.
Describe a key fact of IgA
Although IgG is the most abundant circulating immunoglobulin in the body, IgA is the most abundant antibody in the body.
What is the difference between IgA1 and IGA2
1- Blood
2- secretions
How does the mucosal layer in the small intestine compare to that of the large
small- single layer- villi bigger
large- two layers of mucus (outer thicker)- smaller villi
Describe lymphocyte homing and circulation
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.
Describe leukocyte recruitment in the gut
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
Describe the mechanism of cholera infection
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 contactwith the epithelium releasescholera 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.
Describe the transmission and symptoms of cholera
Transmitted through faecal-oral route, and spreads through contaminated water & food.
Main symptoms – Severe dehydration and diarrhoea (watery).
Other symptoms - vomiting, nausea & abdominal pain.
Describe the diagnosis and treatment of cholera
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).
List the Other causes of infectious diarrhea (gastroenteritis)
Viral:
Rotavirus (children)
Norovirus ‘Winter vomiting bug’
protozoal parasitic:
Giardia lamblia
Entamoeba histolytica
Bacterial: Campylobacter jejuni Escherichia coli Salmonella Shigella Clostridium difficile
Describe rotaviruses
Description: RNA virus, replicates in enterocytes. 5 types A – E, type A most common in human infections
Describe the epidemiology of rotaviruses
Epidemiology: Most common cause of diarrhoea in infants and young children worldwide.
Describe the treatment and vaccination of rotaviruses
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.
What is the Norwalk virus a species of
Norovirus (genus) Norwalk virus (species)
Describe the norovirus
Description: RNA virus. Incubation period 24-48 hours
Describe how we diagnose norovirus
Diagnosis: Sample PCR.
Describe the epidemiology of norovirus
Epidemiology: Estimated 685 million cases per year
Describe the symptoms of norovirus
Symptoms: Acute gastroenteritis, recovery 1 – 3 days
Describe the transmission of norovirus
Transmission: Faecal-oral transmission. Individuals may shed infectious virus for up to 2 weeks.
Outbreaks often occur in closed communities.
What is the most common species of campylobacter
Most common species: Campylobacter jejuni, Campylobacter coli
Describe the transmission of campylobacter
Transmission: Undercooked meat (especially poultry), untreated water and unpasteurised milk. Low infective dose, a few bacteria
(<500) can cause illness.
Describe the epidemiology of campylobacter
Epidemiology: Estimated 280,000 cases per year in UK, 65,000 confirmed. Most common cause of food poisoning in the UK.
Describe the treatment of campylobacter
Treatment: Not usually required, azithromycin (macrolide) is standard antibiotic, resistance to fluoroquinolones is problematic
Describe E.coli
Diverse group of Gram negative intestinal bacteria, most harmless but 6 ”pathotypes” associated with diarrhea (diarrhoeagenic):
Describe Enterotoxigenic E. coli (ETEC
Cholera like toxin
Watery diarrhea
Describe Enteroinvasive E. coli (EIEC
Shigella like illness
Bloody diarrhea
Describe 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
List some other types of E.coli
Enteropathogenic E. coli (EPEC
Enteroaggregative E. coli (EAEC)
Diffusely adherent E. coli (DAEC)
Describe the management of C.Difficile
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
Describe the progression from a healthy state to a diseased state
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
What is Coeliac disease
An autoimmune disorder
Describe the mechanism of coeliac disease
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.
Outline the steps leading to coeliac disease
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
What are the symptoms of coeliac disease
Abdominal distension (bloating)
Diarrhoea
Sometimes dermititis herpetiformis
How do we diagnose coeliac disease
Ab blood tests -anti-gliadin.
Biopsy test of duodenum
Describe the treatment for Coeliac disease
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.
Describe the mechanisms of IBS
Visceral hypersensitivity
Triggered by diet / stress
Describe the symptoms of IBS
Recurrent abdominal pain
Abnormal bowel motility
Constipation and/or Diarrhoea
Describe the treatment of IBS
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
Describe the different irritable bowel diseases
Crohn’s and ulcerative colitis
Describe the mechanism of Crohn’s
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.
Describe the pathology of Crohn’s
Patches of inflammatory damage and healthy tissue
Cobblestone appearance
Describe the symptoms of Crohn’s
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
Describe the treatment of Chron’s
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
Describe the diagnosis of Chron’s and Ulcerative colitis
Ab blood tests
endoscopy
Barium X-Ray
Describe the mechanism of ulcerative colitis
Autoimmune disorder - T cells destroy the cells lining the walls of large intestine
Secondary cause - Diet and stress
Continuous inflammation
Describe the symptoms of ulcerative colitis
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
Describe the treatment of ulcerative colitis
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.
