8 - Eosinophils

Question 1

What are eosinophils?

Answer 1

• Usually regarded as terminally differentiated cells, with anti-parasitic function, principally through degranulation
• Named for their reaction with acidophilic dyes

Question 2

Characteristics of eosinophils (life span, numbers etc.)

Answer 2

• Low numbers (<400 per mm3) compared with neutrophils
• Short presence (ca. 18 hour half-life) in circulation
• Move from blood into tissues (lifespan in weeks)
• Associated with lung pathologies

Question 3

What group are eosinophils part of?

Answer 3

• part of the polymorphic nuclear granulocyte group

Question 4

Features of mature eosinophils

Answer 4

Mature cells have low nos. of mitochondria, limited ER and Golgi

Question 5

How many granule proteins and what are they?

Answer 5

4 major granule proteins:
- Eosinophil peroxidase
- Major basic protein (MBP)*
- Eosinophil cationic proteins (ECP)*
- Eosinophil-derived neurotoxin (EDN)
+ cytokines, enzymes

• bactericidal

Question 6

Structure of eosinophils

Answer 6

• cell membrane: lipid mediator, chemoattractant and CD or MHC class II, FC receptors, adhesion receptors, interleukin 5 receptor
• primary granules & specific granules
• sombrero vesicles
• intracellular patter recognition receptor
• specific granules - major basic protein and eosinophil
• cationic proteins which are bactericidal

Question 7

Functions of eosinophil-derived mediators

Answer 7

• T cell activation & polarisation
• cell mediated immunity
• recruitment of dendritic & T cells
• humoral immunity
• maintenance of bone marrow plasma cells
• maintenance of secretory IgA in intestines
• tissue regelation & repair
• angiogenesis
• fibrosis
• adipose tissue - metabolic homeostasis
• steady state development in mammary gland and intestine
• cell:cell interactions
• host protection

Question 8

What activity have eosinophils recently been demonstrated to have?

Answer 8

Eosinophils recently demonstrated to have activity against respiratory viruses

Question 9

Where is the highest eosinophil residency?

Answer 9

In the gut and lungs

Question 10

What does eosinophil activity depend on?

Answer 10

Activity largely depends on granule release?

Question 11

What can tissue resident eosinophils mediate?

Answer 11

Tissue resident eosinophils now known to mediate a broad range of activities in local environment

Question 12

What do eosinophils in granules contribute to?

Answer 12

Eosinophils in granules contribute to host protecion
- some recent papers found more involvement of eosinophils in COVID pathology and outcomes

Question 13

Types of degranulation

Answer 13

• classic exocytotic degranulation
• cytolysis with release of cell-free granules
• piecemeal degranulation (PMD)

Question 14

Classic exocytotic degranulation

Answer 14

• Eosinophils adherent to the surface of a large multicellular parasite have been shown to undergo classic exocytotic degranulation, wherein intracellular granules fuse with the plasma membrane, creating a secretory pore through which the entire granule contents are released. - In compound exocytosis, granule–granule fusions occur within the cytoplasm, forming secretory channels that enable the wholesale degranulation of the combined content of multiple granules

Question 15

Piecemeal degranulation (PMD)

Answer 15

Piecemeal degranulation (PMD) differentially releases granule-derived proteins, including cytokines, as discrete packets

a. Granules within cells undergoing PMD exhibit varying degrees of ultrastructural alteration, including an apparent reorganization of electron-dense contents and the appearance of a membranous network of tubules within granules.

b. granule-derived proteins are differentially mobilized into small round vesicles and tubular structures, the latter termed eosinophil sombrero vesicles (EoSVs), that emerge from mobilized granules and seem to derive directly from the intragranular membrano-vesicular network of tubules.

c. As shown for eotaxin-elicited PMD of IL-4 in panel Cc, tubular EoSVs express lumen-oriented receptor chains that are bound by their cognate cytokine ligand, which indicates that a mechanism of receptor-mediated chaperoning may contribute to differential cytokine secretion. After emerging from granules, cytoplasmic EoSVs and small vesicles traverse the cytoplasm and fuse with the plasma membrane to release their granule-derived cargo

Question 16

Cytolysis with release of cell-free granules

Answer 16

• Eosinophils may also be induced to undergo a cytolytic cell death pathway characterized by dissolution of the nuclear and plasma membranes, extrusion of DNA nets and expulsion of intact granules that are observed individually and as clusters of cell-free extracellular granules within tissues.
• A portion of cell-free, extracellularly deposited eosinophil granules retain an intact trilaminar outer membrane, express outwardly oriented functional receptors on their outer membranes as shown in the right panel, and remain competent to undergo stimulus-dependent secretion within tissues.

Question 17

What is a classic parasite response?

Answer 17

Classic exocytotic degranulation
- primary parasitic defence-destroy from surface-dissolve from outside
- release of granule which fuses with plasma membrane (eosinophil sombrero vesicles)
- compound exocytosis - granules study together to form one big one which fuses with plasma membrane resulting in a large discharge of toxic factors

Question 18

Features of piecemeal degranulation

Answer 18

• formation of granule vesicles
• tubular vesicles - changes its appearance and fuses with plasma membrane (eosinophil sombrero vesicles)
• transports small packets of material from EoSV - less intense than classic exocytotic degranulation

Question 19

Which type of degranulation involves cell death?

Answer 19

Cytolysis with release of cell-free granules
- eosinophils undergoing nuclear and PM dissolution (granules undergoing cell death)
- granules caught in DNA strands - DNA strands make up a trap
- cluster free granules remain active in DNA strands so they can still kill

Question 20

What can be used in diagnosis?

Answer 20

Blood eosinophilia or hypereosinophilia

Question 21

What diseases are associated with eosinophilia?

Answer 21

Diseases such as asthma, hyper-eosinophilic syndrome, and eosinophilic granulomatous polyangiitis (a.k.a. Churg Strauss syndrome) associated with eosinophilia, therefore several therapies directly target eosinophils, resulting in eosinophil depletion

Question 22

Drug modes of action from eosinophilia

Answer 22

• Neutralization of eosinophil growth and of the activating factor IL-5 (e.g. mepolizumab and reslizumab)
• Induction of eosinophil depletion by antibody-dependent cellular cytotoxicity (e.g. the anti–IL-5 receptor drug benralizumab)

Question 23

Side effects of eosinopenia

Answer 23

Abnormally low eosinophil levels (eosinopenia) have shown no major side effects to date in patients undergoing such therapies

Question 24

Eosinophil depletion

Answer 24

Eosinophil depletion occurs naturally in response to triggers of acute inflammation, including sepsis, can be associated with poor outcomes in in critically ill patients

Question 25

What have eosinophils been shown to be capable of in mice

Answer 25

In mice, eosinophils capable of anti-viral activity against respiratory viruses

Question 26

Eosinophils and disease in humans

Answer 26

- Humans: eosinophil-associated lung pathology associated with certain viral infections (e.g. respiratory syncytial virus [RSV]) - Suggested that respiratory tract eosinophils may promote antiviral responses but responses could become dysregulated during allergic disease

Question 27

Eosinopenia is a complication of what?

Answer 27

Currently, eosinopenia is a known complication in some severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1) vaccination studies

Question 28

Eosinopenia and COVID

Answer 28

E- osinopenia reported in large proportion of Wuhan patients with SARS-CoV-2: * 53% of patients admitted to hospital had eosinopenia on day of admission * 81% of fatal COVID-19 cases had absolute eosinophil counts < normal range Pathophysiology for COVID-19 eosinopenia unclear but likely to be multifactorial, e.g. inhibiting egress from bone marrow or production of new cells, eosinophil apoptosis. No eosinophil enrichment into the pulmonary tissue has been observed in samples from COVID-19 patients at early stages or post-mortem Eosinopenia may be of use as prognostic indicator for more severe COVID?

Question 29

Basophils and mast cells

Answer 29

Functionally and developmentally similar cells

Question 30

Characteristics of basophils

Answer 30

- Smallest leucocyte population, <1% of all white blood cells and least well-studied of the PMNs - Involved in anti-parasite defences, also associated with allergic responses due to high affinity IgE receptors - Only white blood cell to contain histamine - Recently identified as producing high concentrations of regulatory cytokines - Possess phagocytic ability, but not regarded as ‘professional’ - Life span in blood ca. 2-3 days

Question 31

What are the roles of the major effector mediators produced by basophils in allergic disease e.g., allergic asthma

Answer 31

- basophil produces histamine and IL-4 & IL-13 HISTAMINE: - increase vascular permeability - cause smooth muscle contraction IL-4, IL-13: - promotes Th2 lymphocyte differentiation - promotes IgE production

Question 32

What has a recent paper using single-cell RNA sequencing revealed?

Answer 32

They revealed a regulatory role of basophils in lung homeostasis - Lung analysed through-out development (>800 ligand-receptor interactions analysed) - Lung basophils distinct from peripheral blood basophils - Use of IL-33R knock-out mice, diphtheria-toxin depletion strategies and in vitro work confirmed regulatory role of lung basophils in alveolar macrophage development - AM directed towards M2 anti- inflammatory state - Interactions recorded between basophils and: fibroblasts, alveolar type Ii cells, innate lymphoid cells, MΦ Cohen et al 2018: Cell, 175, 1031-1044

Question 33

Characteristics of mast cells

Answer 33

- Progenitors circulate in blood before maturing in various tissues - Life span is tissues ca. 2-3 months - Most abundant in skin and mucosa, associated with allergic reactions - High content of secretory granules, contents released on cell degranulation - Not all activated mast cells degranulate - Activation can result in synthesis of lipid regulators and cytokines

Question 34

Anti-parasite activity: response to ticks

Answer 34

Cooperation between basophils and mast cells Primary engagement doesn't provoke a strong response - no basophil recruitment - activated dendritic cells - pick up fragments and carry antigens from tick bite into the lymph node - introduction of T cells - production of specific antibodies - recruitment of basophils - expression of Fc receptor of basophils - some cooperation of eosinophils

Question 35

Anti-parasite activity: response to helminths

Answer 35

Cooperation between basophils and mast cells - more tissue damage since internal - more signals occuring - hyperplasia of goblet cells - produce more mucus - release of cytokines - dendritic cells - activate basophils, mast cells - more release of different cytokines which drives epithelial cell turnover and hyperplasia - activation of macrophages into wound healing-activated fibroblasts leading to a build up of collagen - increased peristalsis - movement of smooth muscle cells - physically dislodge the parasite through movement

Question 36

Mast cells and basophils in allergic inflammation

Answer 36

- acquisition of allergen by dendritic cells - allergenic disruption of epithelial cells - driven by interaction in the lymph nodes, allergen picked up by dendritic cells in the lymph nodes - mast cells interact with IGE

Question 37

What interactions do eosinophils have?

Answer 37

Eosinophils now known to have high degree of interaction with both innate and adaptive cell types/functions

Question 38

What are associated with human disease?

Answer 38

Eosinophilia/eosinopenia associated with human disease conditions and suggested as diagnostic markers

Question 39

What is now known about basophils?

Answer 39

Basophils now believed to have regulatory role in lung homeostasis

Question 40

What responses are basophils and mast cells involved in?

Answer 40

Basophils and mast cells capable of cooperation during anti-parasite responses

Question 41

What cooperate in allergic reactions?

Answer 41

High level of involvement and cooperation between basophils and mast cells during allergic reactions