L1. Diversity of Mast Cell Production

Question 1

What is the distribution of mast cells?

Answer 1

Mast cells are distributed everywhere in the body, in higher proportions in mucosal areas that are in close contact with the external environment (skin, RT and the gut)

Question 2

What structures are found near the mast cells? What is the significance of this?

Answer 2

Blood vessels, glands and nerves. These are often the site of action of the mast cell release products.

Question 3

What are the external and internal triggers of mast cell activation?

Answer 3

External: UV/light, mechanical stress, drugs (vancomycin, morphine), bites and stings
Internal: Allergens (IgE binding), Osmotic stimuli, Neuropeptides, Activated complement

Question 4

What is the triple response to histamine?

Answer 4

1. Redness: due to vasodilation allowing more blood flow into the region.
2. Oedema: due to the effects of histamine and CLT on vascular permeability.
3. Flare: the spread through sensory nerves (anti-dromic) to result in flare and increased sensation.

Question 5

What is the most important pathway for mast cell degranulation? Briefly describes this process

Answer 5

• This is allergen binding to already sensitised IgE (specific). - These IgE antibodies (bound to the specific allergen) has an Fc portion which binds with very high affinity to the FcER1 membrane bound receptors on mast cells. These too are highly specific receptors.
• Binding of these IgE to the FcER1 receptor leads to clustering of internal domains and subsequent intracellular signalling.

Question 6

What is meant by atopy? What genetic and non-genetic factors cause this?

Answer 6

Atopy is a genetic disposition to produce high numbers of IgE antibodies, often to triggers (antigens) that are not normally antigenic - called allergens. This is an inherited trait. Non-genetic factors include sensitisation in early childhood, especially with LRTIs like Respiratory Syncytial Virus and Rhinovirus which skew the immune response towards a Th2 (eosinophilic) response.

Question 7

What is the allergy hypothesis? Are there any other hypotheses that explain atopy?

Answer 7

The allergy hypothesis (hygiene hypothesis) states that the slow decrease in infectious diseases has lead to an increase in autoimmune disease and allergic diseases. Some believe that atopy may be a result of Treg cell deficiency or dysfunction. This is a current mode of research towards a sensitisation therapy.

Question 8

Describe, in detail the signalling pathway following IgE: FcER1 binding

Answer 8

IgE binds to the FcER1 leading to receptor cross linking. This cross links internal aspects of the receptor called the IMMUNORECEPTOR TYROSINE-BASED ACTIVATION MOTIFS (ITAMs) which recruit and activate kinase activity. These kinases (Syk and Lyn) phosphorylate Mitogen Activated Protein Kinase (MAPK) and Phospholipase C which both act to eventually activate Protein Kinase A (PKA) and increase intracellular calcium concentrations. Leading to degranulation, the arachadonic acid pathway stimulation and gene transcription events.

Question 9

What are the specific tyrosine kinases activated in this pathway and what are their consequences?

Answer 9

Syk and Lyn, they phosphorylate MAPK and Phospholipase C

Question 10

What are the three major consequences of the FcER1 pathway?

Answer 10

1. Degranulation of the mast cell
2. Activation of the arachadonic acid pathway
3. Genetic transcription changes and events

Question 11

Is mast cell degranulation a complete process? Is it cytotoxic? And what changes to the cell occur in degranulation?

Answer 11

Mast cell activation is usually partial and so is degranulation. It takes a very strong trigger to cause complete degranulation of a mast cell. Normal mast cell activation leads to the preformed granules to budd to the surface forming a continuous pore (cts with the cytoplasm) through which granules release their contents. After degranulation the mast cell reforms its granules. Not cytotoxic

Question 12

What are the three waves of mast cell communication with the environment?

Answer 12

1. Immediate phase: degranulation
2. Rapid phase of arachadonic acid pathway activation
3. Gene transcription events

Question 13

Describe the immediate wave of mast cell degranulation. What is released? And what is the time course?

Answer 13

The immediate wave occurs within 30-40 seconds of mast cell activation. It is the release of PREFORMED mediators from the granules containing: Histamine, TNF alpha, Tryptase and Heparin.

Question 14

Describe the Rapid wave of mast cell activation. What is the main consequence and release products? What is the time course?

Answer 14

The rapid phase occurs within 10-30 minutes of activation and is the activation of phospholipase A2 which mobilises arachadonic acid from the phospholipid membranes. This action leads to the production of the prostaglandins and the cysteinyl leukotrienes.

Question 15

Describe the slow wave of mast cell activation. What are the products, their actions and the time course of the response.

Answer 15

The slow wave occurs after about 2-4 hours and is the activation and expression of various cytokines that cause the TH2 phenotype, promoting the allergic condition.
These include:
- IL-4 (IgE activation)
- IL-5 (eosinophil recruitment)
- GM-CSF (eosinophil survival and macrophage recruitment)
It causes T cell angd eosinphil reactions to occur

Question 16

Where does histamine act (on which receptors) and what are the consequences [9] of these reactions?

Answer 16

Histamine acts on 4 receptors: H1-H4. H1 and H2 are the most common and known about receptors.
H1: vasodilation, increased vascular permeability (oedema), pain and itch (through sensory nerves), bronchospasms, increased mucous secretion, increased alertness (CNS located receptors) and gastric smooth muscle contraction
H2: gastric acid secretion (cholic) and positive inotropic and chronotropic effects on the receptors located in the heart.

Question 17

Allergens are typically high molecular weight and often take time to cause a response. What is the importance of the histamine action to this?

Answer 17

They take longer to get through membranes to cause a response. The action of histamine (especially in vasodilation and increased vascular permeability) allows for the allergens to get through and cause a longer response.

Question 18

What are the cysteinyl leukotrienes? And how are they released?

Answer 18

These are the results of the 5-lipoxygenase on arachadonic acid. These are LTA4 and the metabolites and LTB4 and its metabolites.

Question 19

What receptors are the cysteinyl leukotrienes active on? What does binding result in?

Answer 19

The CLTs act on the CysLT1R in the airways, vasculature and other areas to cause smooth muscle cell constriction (bronchospasms and gut constriction) and increases in mucous production (important to hayfever, asthma and nasal congestion).

Question 20

What does the LTB4 leukotriene do?

Answer 20

It does not act on the CysLT1R and therefore does not lead to constriction or increased mucous production. Instead it has a role in inflammation, recruiting leukocytes and infiltrates.

Question 21

What do the leukotriene receptor antagonists aim to achieve? What is a major example?

Answer 21

They prevents the actions of the CTLs relieving congestion, vasodilation and inhibiting bronchospasms.
Eg. Montelukast

Question 22

How do histamine and the cysteinyl luekotrienes work together to worsen anaphylactic shock?

Answer 22

The histamines increase the leakiness of the vessels leading to an increased oedema and a decrease in blood volume. The CTLs also act to do this and the compounding action can lead to hypovoleamia and subsequent shock. H2 receptors on the heart also cause increase in CO which delivers more blood out to the interstitium

Question 23

Do the NSAIDs, like aspirin, have a net benefit in asthma or hayfever? Why or why not?

Answer 23

No. They inhibit the COX pathway, which may actually increase the numbers of leukotrienes (the balance shifts) by increasing the substrate for the 5-lipoxygenase pathway. This leads to an exacerbation of the atopic response.

Question 24

What inhibits the calcium dependent phospholipase A2? What is the importance of this in allergy and asthma? Give an example

Answer 24

Calcium dependent phospholipase A2 is inhibited by glucocortiocoids. A major example being Annexin-1. These are important in inhibiting both the COX and 5-lipoxygenase pathways leading decreases in inflammation. These are used as preventor medications.

Question 25

How long to the effects of the third wave of mast cell activation last for?

Answer 25

These last for several hours to days after the trigger

Question 26

What determines which gene expression changes occur in the late phase of mast cell activation?

Answer 26

The environmental conditions determine which genes are expressed in mast cell activation.

Question 27

What are the two major consequences of the third phase of mast cell activation?

Answer 27

1. Attraction of T cells and eosinophils setting up the TH2 immune response
2. Eventual cellular infiltration of the TH2 response and the structural changes of airway remodelling and fibrosis.

Question 28

What are some endogenous inhibitors of mast cell activation? [3]

Answer 28

PGE2, Adrenaline and Cortisol (an endogenous glucocorticoid)

Question 29

What and/or where do antiallergic drugs mainly target? [5]

Answer 29

1. Inhibit mast cells
2. Inhibit activation
3. Inhibit mediator production
4. Inhibit mediator action
5. Prevention and desensitisation

Question 30

What pharmacological interventions affect mast cell activation? How do they work?

Answer 30

Disodium ccromoglycate/ nedocromil sodium
These are well tolerated drugs that do not absorb further past the mucosa, inhaled or topical that somehow (unclear mechanism) reduce mast cell degranulaion and cause annexin-1 release.

Question 31

What is a major example of a drug that inhibits mast cell activation? How does it achieve this? What are its advantages and disadvantages?

Answer 31

Omalizumab: a humanised murine antibody against the alpha subunit of the IgE binding region.
It renders the mast cell unresponsive over time and leads to the eventual decrease of IgE and of the FcER1 receptors. However they are expensive and must be administered quite frequently subcutaneously.

Question 32

What are some methods to inhibit the mediator actions? Such as treatment to atopic dermatitis, motion sickness, angioedema and anaphylaxis.

Answer 32

Adjuncts to steroids and adrenaline. As well as muscarinic antagonists (for motion sickness).

Question 33

What are the three generations of anti-histamines? Give examples for each, their positives and negative effects. What generation(s) are currently in use?

Answer 33

1. Sedative: chlopheniramine, promethazine: good for allergy, but bad for lifestyle
2. Non-sedative: terfenadine, these did not enter the CNS so didn’t affect weakness but they caused an effect on cardiomyoctye channels and rare ventricular arrhythmia so were withdrawn.
3. Newer Non-selective: cetrizine and loratidine: reduced CNS and reduced cardiac effects.
   Bot the 1 and 3 generations are used

Question 34

Describe the cysteinyl leukotriene receptor antagonists. How are they used?

Answer 34

These are Montelukast and Zafilukast which are used prophylactically causing some bronchodilation. These are used in combination with glucosteroids and the beta 2 agonists.

Question 35

What are some of the major symptoms and signs of anaphylaxis? What drugs are used in anaphylaxis?

Answer 35

Burning and itching, warmth and flushing, tachycardia and palpitations, hypotension, hives, colic and nausea, gastric acid hyper-secretion, bronchospasm and eventual vascular collapse.
The drugs used are adrenaline, hydrocortisone (a very strong glucocorticoid) and anti-histamines