GI immunity Flashcards
Levels of immune protection
Physical barriers
Chemical enzymatic defense - macrophages
Cell based immunity
How do DCs work
Dendritic cells encounter PAMPs - pathogen associated molecular patters, and pathogen-derived antigens at site of infection
- TLR stimulation via PAMPS in the DCs coupled with Ag uptake
- Antigen loaded/PAMP-experienced DC travel to lymph nodes
- Mature into professional antigen presenting cells
PAMPs provide, mature and presentable signal
What would happen without PAMPs activating DCs?
Antigens, even self- would be chronically on display by mature DCs that can initiate downstream immunity.
What components does mature DC have?
MHC molecule - presents the antigen
CD80 - signals adjacent T-helper cells
How are B cells activated - humoral immunity?
also APC
bind antigen via B-cell receptor
BCR: antigen complex internalized and presented to CD4+ T cells
- CD4+ T cells promote (via cytokines) B cell differentiation –> plasma cells/memory cells –> stimulate IgE production
What is the role of IgE
Sensitization of mast cells
What is a mast cell
- Contains granules - have cytotoxic and inflammatory factors
How is mast cell activated by IgE + allergies
- IgE produced by plasma cells bind to mast cells and trigger degranulation
- initiate anti-pathogen inflammatory cascades
- When allergy, IgE is against a harmless environmental antigen, mast cells will degranulate without pathogen
How a mast cell is activated, overview
- DC bind CD4+ T cells to deliver antigen-specific cytokine signalling to B cells
- signalling instruct B cell differentiation into IgE producing plasma cells
- IgE is against same antigen that DC presenting
Fc domain
Fragment crystallizable domain - of IgE binds to high-affinity receptors FceRI - displayed on mast cell surface
FceRI clustering and IgE + Mast cell
- IgE bound to FceRI converts the surface of the mast cell to an antigen-specific sensor
- Ligation of multivalent antigen via IgE bound to mast-cell FceRI - causes clustering of FceRI
- Clustering: induces stimulatory intracellular signalling events –> promote degranulation
- mutivalent antigen - scaffold, more effective
What do mast cells produce?
Variety of antimicrobial/inflam agents when crosslinking of surface-bound IgE
- Histamine - can damage host, toxic to microorganisms
- Histamine acts indirectly: promotes recruitment of additional leukocytes to site of immune insult
2 main products of degranulation
Toxic mediator
Lipid mediator
Toxic mediators
(Histamine
Heparin)
Toxic to parasites
Increase vascular permeability
Cause smooth muscle contraction
Anticoagulant
Lipid mediators
Prostaglandins -D2, E2
Leukotrienes - C4, D4, E4
- Smooth muscle contraction
- Chemotaxis of eosinophils, basophils and Th2 cells
- Increase vascular permeability
- Stimulate mucus secretion
- Bronchoconstriction
Pathology + Mast cells
- Swelling and fluid/mucus production – allergic disease
- REsult from increase blood flow into tissues from histmine - vascular permeability
- Beneficial responses when Ag harmful
- Unwanted when against harmless material
3 pathways were mast cell affects
GI tract
Eye, nasal, airways
Blood vessels
Mast cells in GI tract
Increase fluid secretion, and peristalsis
Result: Expulsion= diarrhea, vomiting
Mast cells: Eyes, nasal passages, airways
Decrease airway diameter, increase mucus
Congestion, blockage of airways - wheezing, coughing
Swelling, mucus secretion in nasal passages
Ocular itching
Sneezing
Mast cells + Blood vessels
Increase blood flow
Increase permeability
Increase fluid in tissues
Increase flor of lymph to nodes
Increase cells + proteins in tissue
Increase effector response in tissue
Hypotension
Anaphylactic shock
Allergies + mechanism
Adaptive immune response - mounted against harmless antigens
Mechanism - depends on IgE
IgE - recognizes harmless antigen
- Inflammatory immune response via specialized granulocytes == mast cells at dermis/epidermis
What can suppress degranulation?
Co-engagement of FceRI and inhibitory receptors (CD33) –> supress degranulation
if drug has TNP + CD33 ligand
when both FceRI + CD33 activated in mast cell - degran inhibited
Histamine biosynthesis
Histidine (electrically charged side chains) –> decarboxylated into histamine - beta-aminoethylimidazole + CO2
Important!! enzyme: Histidine decarboxylase
Histamine receptors - how many
4 major subtypes - H1-H4
H1 - airway
H2 - GI
H1 receptor, signalling?
G1/G11 –> PLC stimulation
H1: expression
CNS neurons
smooth muscle cells - vascular, resp, GI
CVS
neutrophils, eosinophils, monocytes
macrophages
DCs
T B cells
Endothelial cells
Epithelial cells
H1: drugs
Chlorpheniramine
Diphenhydramine
hydroxyzine
cetirizine
desloratadine
fexofenadine
levocetirizine
loratidine…
H1 clinical use:
Clinical rhinitis, allergic conjunctivitis
Urticaria
CNS disease
Allergic and non-allergic disorders
H2 receptors : signalling
Gs family –> AC –> cAMP
H2 expression
Gastric parietal cells
Smooth muscle
CNS
CVS
neutrophils, eosinophils, monocytes
macrophages
DCs
T B cells
endothelial cells
epithelial cells
H2: antihistamines
Cimetidine
Ranitidine
famotidine
nizatidine
H2: clinical use
Peptic ulcer disease
Gastroesophageal disease
Histamine release when allergy, location
- Histamine - released by mast cells
- Binds to H1 receptors on immune/endothelial cells
- Leads to local inflammation
- Effect adjacent to mucus membranes
- Systemic histamine is still possible
- Anaphylactic shock if not treated
Strategies to counter histamine release
- Drugs can act by decreasing mast cell ability to release histamine
- or target downstream effects mediated by histamine
- most first-line drugs target receptors recognizing histamine
2 main treatments of allergies
Mediator action
Chronic inflammatory reactions
Mediator action: mechanism, approach
Mechanism: 1.Inhibit effects of mediators on specific receptors
2. Inhibit synthesis of specific mediators
Approach:
Antihistamines, beta-agonists, leukotriene receptor blockers, lipoxygenase inhibitors
Chronic inflammatory reactions: Mechanism of drugs, approach
Mechanism: General anti-inflammatory effects
Specific approach: Corticosteroids
H1 Antihistamines
Act via antagonism or inverse agonism of histamine receptors
- Act on H1 receptor - spec for allergy
- Two drugs against H1 receptors
2 gens of antihistamines
1st gen:contains alkyl amine, side effects
2nd: Piperidine, piperazine motifs, less side effects
Adverse effects of H1 antihistamines
- Ligate (inhibitory) muscarinic, alpha-adrenergic, dopaminergic, serat. receptors - CNS side effects
- Leads to side effects
- drowziness, tachycardia, pupil dilation, reduced mucous prod
- risk of abuse
- 2nd gen - less able to cross BBB, more selective for H1 rec
H1 antihistamines - mechanism
- Inverse agonists
- H1 in consitiutively active state (balance between active and inactive state)
- Antihistamines stabilize inactive state of H1 receptor
- Prevents ligation by histamine, and constitutive signalling
Gq/11 –> GTP –> PLC –> constriction
What is H2 for?
GI pathologies
Expression on parietal cell in stomach is clinically relevant
Stomach physiology
- Gastric pits - multiple cell types
- Parietal cells - produce gastric acid
- Vasculature adjacent to gastric pits (leukocyte transport there)
H2 receptor as a target
- H2 expressed on cells within GI tract
- In stomach, stimulation H2 receptors - increases gastric acid secretion
- Histamine released by mast cell
- but also produced by entero-chromaffin-like cells in the stomach
Mast cell activation leads to…
smooth muscle contraction, vomiting, fluid outflow - diarrhea.
Antigen diffuses into vessels, elicits further allergic reactions
H+ in the stomach
Parietal cell - epithelial cells in stomach
- Export H+ via H+/K+ ATPase pump to decrease pH
- Ligation of H2 receptor - histamine –> stimulates cAMP –> AC –> PKA
- result in increase H+/K+ ATPase
- H+ are produced by intracellular carbonic anhydrase
Intracellular carbonic anhydrase
CO2 + H2O –> H2CO3 –> HCO3- + H+
Stomach pH
1
Peptic ulcers
Breaks in the inner lining of stomach
Causes:
Excessive acid production - pH too low
Chronic NSAID use
H pylori infection
Weak bases
- Hyperacidity - weak bases used
- Antacids - OTC
- direct neutralized by donating anions like OH or carbonate HCO3-
H2 receptor antagonists
Not inverse agonists
- Decrease gastric secretion from parietal cells
- Some are inverse though
- H2 receptor antagonists largely replaced by PPI
Example H2 antagonist drugs
Cimetidine - was one of first blockbuster drugs
- Smith Kline, French in 70s
- Was one of first treatment available for patients with peptic ulcers
- Has alkyne group
Proton pump inhibitors
- PPI - replaced H2 receptor antagonists in clinic
- Direct blockage of H+/K+ ATPase pump on parietal cells
- Prevents acid secretion from source
- better than H2: because not only histamine can promote secretion of gastric acid
PPI main structure
Conserved benzimidazole-sulfoxide functionalities are key to activity
PPI are prodrugs
Require acid to:
- activating a benzimidazolium ion form
- Intramolecular cyclization - results in spiro - intermediate
- Intermediate collapses –> rearrangement of ring system
- Result: disulfide bond formation with available cysteine from the proton pump
PPI –> covalent inhibitors
- Cyclic sulfenamide produce – associates with H/K ATPase, on parietal cells –> disulfide linkage
- Covalent adduct - inhibiting ability of pump to export protons
PPI mechanism advantages:
- Prodrugs only activated in acidic env.
- H-K ATPase - expression restricted to parietal cells
Adverse effects of PPI
- reduce stomach acid pH
- if suppresed chronically
- Hypochloryhydria = pH too high
- Hypergastrinaemia and hyperplasia relation:
increase exposure of parietal cells to hormones (gastrin) –> produced in stomach - less negative feedback - Polyps - cancerous
H. pylori + peptic ulcers
Stomach - infected H. pylori
- promote ulceration can lead to gastric cancer
- inflammatory cascades by immune system – gastritis
- destroys mucus layer by neutrophils, epithelium
How bacterium survive low pH env?
- Sophisticated survival –> uses urea– in stomach
- creates ammonia to buffer local environment
- Urease enzyme in H. pylori–> produced ammonia
- Basic, quenches HCl
- Lead to stomach epithelium and ulceration –> destroyed
Reaction with urease
urea + water –> (urease) –> carbamic + ammonia –> (water) –> carbonic acid + 2 ammonia (decarboxylation)
Detection of H. pylori and treatment.
- Use 13C isotopic assay
- Test via breath test
- PCR test on stool sample usually
- PPI not clear role –> synergy between PPI + antibiotics
- PPI can contributed to H-pylori mediated pathology –> promote basification of stomach
- bacteria migration
Inflammatory bowel disease
- inflammation of digestive tract
- Crohn’s disease: - affect large and small intestines no clearly defined cause
symptoms: Chronic diarrhea, abdominal pain, fever, weight loss - Ulcerative colitis - restricted to large intestine, similar symptoms to Crohn’s.
IBD: Leukocytes and inflammation
- Loss of mucus layer in intestinal tract of IBD patients increases epithelial permeability
- Enables gut microbiome to interact with leukocytes in lamina propria –> immune reaction
- Leukocytes - macrophages – produce inflammatory cytokines when contacting microbe-derived material
due to TLR stimulation - Inflammatory cytokines increase leukocyte recruitment to gut, exacerbates inflammation
Ways of treating IBD
- Mesalazine - 5 ASA, first line
unclear mechanism - Sulfasalazine - metabolized to 5 ASA
- Corticosteroids - second line, more aggressive treatment
Methotrexate, biologicals mAb (anti-TNFa), surgery in extreme cases
New mode of IBD treatment
Fecal microbiota transplantation
- Altered comp of gut microbiota –> cause of IBD, maybe
- Transplantation - of still from healthy donor
- efficacy in patients with C. difficile infections
- Additional research required for efficacy approval
