Flashcards in L1. Diversity of Mast Cell Production Deck (35)
What is the distribution of mast cells?
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)
What structures are found near the mast cells? What is the significance of this?
Blood vessels, glands and nerves. These are often the site of action of the mast cell release products.
What are the external and internal triggers of mast cell activation?
External: UV/light, mechanical stress, drugs (vancomycin, morphine), bites and stings
Internal: Allergens (IgE binding), Osmotic stimuli, Neuropeptides, Activated complement
What is the triple response to histamine?
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.
What is the most important pathway for mast cell degranulation? Briefly describes this process
- 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.
What is meant by atopy? What genetic and non-genetic factors cause this?
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.
What is the allergy hypothesis? Are there any other hypotheses that explain atopy?
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.
Describe, in detail the signalling pathway following IgE: FcER1 binding
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.
What are the specific tyrosine kinases activated in this pathway and what are their consequences?
Syk and Lyn, they phosphorylate MAPK and Phospholipase C
What are the three major consequences of the FcER1 pathway?
1. Degranulation of the mast cell
2. Activation of the arachadonic acid pathway
3. Genetic transcription changes and events
Is mast cell degranulation a complete process? Is it cytotoxic? And what changes to the cell occur in degranulation?
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
What are the three waves of mast cell communication with the environment?
1. Immediate phase: degranulation
2. Rapid phase of arachadonic acid pathway activation
3. Gene transcription events
Describe the immediate wave of mast cell degranulation. What is released? And what is the time course?
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.
Describe the Rapid wave of mast cell activation. What is the main consequence and release products? What is the time course?
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.
Describe the slow wave of mast cell activation. What are the products, their actions and the time course of the response.
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.
- IL-4 (IgE activation)
- IL-5 (eosinophil recruitment)
- GM-CSF (eosinophil survival and macrophage recruitment)
It causes T cell angd eosinphil reactions to occur
Where does histamine act (on which receptors) and what are the consequences  of these reactions?
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.
Allergens are typically high molecular weight and often take time to cause a response. What is the importance of the histamine action to this?
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.
What are the cysteinyl leukotrienes? And how are they released?
These are the results of the 5-lipoxygenase on arachadonic acid. These are LTA4 and the metabolites and LTB4 and its metabolites.
What receptors are the cysteinyl leukotrienes active on? What does binding result in?
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).
What does the LTB4 leukotriene do?
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.
What do the leukotriene receptor antagonists aim to achieve? What is a major example?
They prevents the actions of the CTLs relieving congestion, vasodilation and inhibiting bronchospasms.
How do histamine and the cysteinyl luekotrienes work together to worsen anaphylactic shock?
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
Do the NSAIDs, like aspirin, have a net benefit in asthma or hayfever? Why or why not?
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.
What inhibits the calcium dependent phospholipase A2? What is the importance of this in allergy and asthma? Give an example
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.
How long to the effects of the third wave of mast cell activation last for?
These last for several hours to days after the trigger
What determines which gene expression changes occur in the late phase of mast cell activation?
The environmental conditions determine which genes are expressed in mast cell activation.
What are the two major consequences of the third phase of mast cell activation?
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.
What are some endogenous inhibitors of mast cell activation? 
PGE2, Adrenaline and Cortisol (an endogenous glucocorticoid)
What and/or where do antiallergic drugs mainly target? 
1. Inhibit mast cells
2. Inhibit activation
3. Inhibit mediator production
4. Inhibit mediator action
4. Prevention and desensitisation