L11.1 Mast cells and allergy

Question 1

Location of Mast cells

Answer 1

• Everywhere, but particularly at sites in contact with external env (lungs/skin/gut)
• Commonly found close to BV/N/glands

Question 2

Relation of mast cells with IgE

Answer 2

• ↑ IgE
• Associated with: hay fever, asthma…

Question 3

Stimuli for mast cells

Answer 3

• Antigens via IgE
• Complement fragments (result of innate immunity)
• Neuropeptides (mast cells found localised around neurons)
• Cytokines/chemokines
• Bacterial components
• Physical trauma

Question 4

Mast cell degranulation

Answer 4

• Degranulation occurs upon activation
  
  • Release anaphylactic agents
• Granules fuse with plasma membrane and release contents

Question 5

Degranulation via granule fusion

Answer 5

• Granules can fuse with other granules (already docked at plasma membrane) → prod large granule with common exit point
  
  • Compound degranulation (anaphylactic degranulation)

Question 6

Mediators released by mast cells: Granular

Answer 6

• Histamine
• Tryptase → used for activating precursor cytokines
• Preformed mediated (e.g. TNFα)

Question 7

De Mediators released by mast cells: De novo

Answer 7

• LTC4 → bronchoconstrictor
• PGD2 → bronchoconstrictor and regulatory roles

Question 8

Mediators released by mast cells: transcriptional regulation

Answer 8

• ↑ Cytokines/chemokines (Few hours later)

Question 9

Mediators released by mast cells

Answer 9

• Exosomes (Membrane enclosed mini granules)
  
  • Have variety of protein on surface that can transfer them to specific sites → have specific site actions
  • Contain mRNA and miRNA → alter transcriptional processes

Question 10

Features of the 3 subunits of FceR1

Answer 10

• α → binds IgE
  
  • Glycosylation of subunits → keeps α-subunits from bind with each other → prevent initiating allergen independent activation(pineapples)
• Β → spans membrane 4 times
  
  • Interacts with α
  • Attractant at the tail
  • Have motif to recruit kinases/adaptor molecules
• γ → Have motif at tail to signal downstream molecules

Question 11

ITAMS

Answer 11

• Motifs at β and γ tails = ITAMS (immunoreceptor tyrosine-based activation motifs - have common sequence)
• Motifs have tyrosine (Y) residues and are able to be phosphorylated (Y-P)

Question 12

Mechanism of antigen/IgE cross-linking

Answer 12

1. Antigens exposure → cross-links the FceR1 α R
2. Lyn kinase recruited → phosphorylates ITAMS of γ subunit
   
   • Lyn has Y-P motifs in the ITAMs
3. Syk brought in and is phosphorylated by Lyn → downstream signallings
4. Downstream pathways:
   
   • Ras
   • PLC
   • MAPK

Question 13

ITAM mediated signalling

Answer 13

• Commonly found in major immune regulating R
  
  • B-cell R/T-cell R
  • Also in virally encoded proteins

Question 14

Dampening IgG pathway

Answer 14

• Allows auto-regulation of immune pathways
• ITIM (immunoreceptor tyrosine-based inhibtory motifs)
  
  • e.g. IgG R (FcγR11b)
• Draw in phosphatases → remove phosphate groups → dampen ITAM signalling pathways
• IgG upregulated when you already have high AB against that antigen
  
  • When antigens bind → also binds FcγR11b → dampens downstream signalling for clonal proliferation

Question 15

Lipid Rafts/microdomains

Answer 15

• Sub-structures within cellular membrane
• Cross-linking of α R → might enable R to enter these microdomains to allow signalling (instead of recruiting Lyn)

Question 16

Histamine Receptors

Answer 16

• Mediates activity through H1,2,3,4
  
  • H4 selectively in immune cells
• All are GPCRs

Question 17

Triple response from histamine

Answer 17

• Redding - Vasodilatation at initial site
• Wheal - ↑ vascular permeability
• Flare - broader reddening from spreading of sensory neurons

Question 18

Anti-allergic therapies

Answer 18

• Selective H1 antagonist - preventing action of histamine on H1 R
• May also block mast cell actions
• Hayfever/dermatitis…
• NOT in colds/asthma

Question 19

Sedative H1 antagonist

Answer 19

• Chloropheniramine, promethazine
• Causes Drowsiness
• Crosses BBB therefore sedative

Question 20

Non-sedative H1 antagonists

Answer 20

• Terfenadine, astemizole
• Lack anti-Mus activity
• Does not cross BBB
• Withdrawn due to Ventricular arrhythmia

Question 21

Newer non-sedative H1 antagonists

Answer 21

• cetirizine, loratidine
• less cardiac adverse effects

Question 22

Omalizumab

Answer 22

• Humanised anti-IgE AB
  
  • Very specific binding to FceR1 α R (stabilises cell surface due to loss of IgE)
• Manage moderate/severe asthma
• Prevents initial antigen presentation (interacting with dendritic cells)

Question 23

Targeting mast cells: Disodium cromoglycate

Answer 23

• Prevents degranulation of mast cells
• Agonist at GPR35

Question 24

Targeting mast cells: Syk kinase inhibitors

Answer 24

• Impairs eisonophil levels in the airways

Question 25

Targeting mast cells: Immunotherapy

Answer 25

• Give small and increasing dose of substance allergic to → Desensitise IgE dependent activation pathway (develop a tolerance)
  
  • Skewing towards a IgG response → neutralise IgE response

Question 26

Problems with immunotherapy

Answer 26

• Adverse reaction → may exacerbate anaphylatic (allergic) rxn

Question 27

Rushed immunotherapy

Answer 27

• speedy densitisation → ↑allergen over short time
• Danger → suffer from ↑A.E
• Pre-treat with omlizumab → ↓IgE levels first → better compliance (dampen allergen specific effects of IgE)