8 - Eosinophils Flashcards
What are eosinophils?
- Usually regarded as terminally differentiated cells, with anti-parasitic function, principally through degranulation
- Named for their reaction with acidophilic dyes
Characteristics of eosinophils (life span, numbers etc.)
- 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
What group are eosinophils part of?
- part of the polymorphic nuclear granulocyte group
Features of mature eosinophils
Mature cells have low nos. of mitochondria, limited ER and Golgi
How many granule proteins and what are they?
4 major granule proteins:
- Eosinophil peroxidase
- Major basic protein (MBP)*
- Eosinophil cationic proteins (ECP)*
- Eosinophil-derived neurotoxin (EDN)
+ cytokines, enzymes
- bactericidal
Structure of eosinophils
- 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
Functions of eosinophil-derived mediators
- 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
What activity have eosinophils recently been demonstrated to have?
Eosinophils recently demonstrated to have activity against respiratory viruses
Where is the highest eosinophil residency?
In the gut and lungs
What does eosinophil activity depend on?
Activity largely depends on granule release?
What can tissue resident eosinophils mediate?
Tissue resident eosinophils now known to mediate a broad range of activities in local environment
What do eosinophils in granules contribute to?
Eosinophils in granules contribute to host protecion
- some recent papers found more involvement of eosinophils in COVID pathology and outcomes
Types of degranulation
- classic exocytotic degranulation
- cytolysis with release of cell-free granules
- piecemeal degranulation (PMD)
Classic exocytotic degranulation
- 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
Piecemeal degranulation (PMD)
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
Cytolysis with release of cell-free granules
- 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.
What is a classic parasite response?
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
Features of piecemeal degranulation
- 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
Which type of degranulation involves cell death?
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
What can be used in diagnosis?
Blood eosinophilia or hypereosinophilia
What diseases are associated with eosinophilia?
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
Drug modes of action from eosinophilia
- 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)
Side effects of eosinopenia
Abnormally low eosinophil levels (eosinopenia) have shown no major side effects to date in patients undergoing such therapies
Eosinophil depletion
Eosinophil depletion occurs naturally in response to triggers of acute inflammation, including sepsis, can be associated with poor outcomes in in critically ill patients
What have eosinophils been shown to be capable of in mice
In mice, eosinophils capable of anti-viral activity against respiratory viruses
Eosinophils and disease in humans
- 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
Eosinopenia is a complication of what?
Currently, eosinopenia is a known complication in some severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1) vaccination studies
Eosinopenia and COVID
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?
Basophils and mast cells
Functionally and developmentally similar cells
Characteristics of basophils
- 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
What are the roles of the major effector mediators produced by basophils in allergic disease e.g., allergic asthma
- 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
What has a recent paper using single-cell RNA sequencing revealed?
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
Characteristics of mast cells
- 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
Anti-parasite activity: response to ticks
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
Anti-parasite activity: response to helminths
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
Mast cells and basophils in allergic inflammation
- 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
What interactions do eosinophils have?
Eosinophils now known to have high degree of interaction with both innate and adaptive cell types/functions
What are associated with human disease?
Eosinophilia/eosinopenia associated with human disease conditions and suggested as diagnostic markers
What is now known about basophils?
Basophils now believed to have regulatory role in lung homeostasis
What responses are basophils and mast cells involved in?
Basophils and mast cells capable of cooperation during anti-parasite responses
What cooperate in allergic reactions?
High level of involvement and cooperation between basophils and mast cells during allergic reactions
