metabolism of leukocytes and platelets [email protected]
TRANSCRIPT
Differentiation of the bone marrow stem cells
myeloidprogenitor
stem cell
lymphoid progenitor
macrophagedendritic cell
platelets
neutrophil eosinophil basophil
monocyte
maturelymphocytes
erythroblast
erythrocyte
plasmacell
megakaryocyte
A) Phagocytic cells:A) Phagocytic cells:
Neutrophils – most abundant Eosinophils Monocytes Macrophages – rise by differentiation of monocytes in tissues
Degradation of the ingested particle:
1) Activation of NADPH oxidase
2) Production of NO by nitric oxide synthase
3) Fusion of phagosome with lysosomes of the phagocytic cell that contain bactericidal substances and hydrolytic enzymes (often with acidic pHopt)
1) NADPH-oxidase
Protein complex of neutrophils, eosinophils, monocytes, macrophages
NADPH + 2 O2 → NADP+ + H+ + 2 O2•-
2 O2•- + 2 H+ → O2 + H2O2
H2O2 can damage bacteria directly or after conversion to OH• :
H2O2 + M+ → OH• + OH- + M2+ (M; metal)
superoxide anion
cytochrome b558
active NADPH-oxidase
Activation: by association of the components localized in cytosol with cytochrome b558 in the membrane; electrons from cytosolic NADPH are – via FAD and cytochrome – transferred to oxygen
plasma membrane
fusionwith
lysosomes
phagosome
Myeloperoxidase
Present in granules of neutrophils and monocytes, but not macrophages!
Significant portion of H2O2 (produced by dismutation of O2•- generated by
NADPH oxidase) is used by myeloperoxidase to oxidize Cl- to HClO
HClO is highly reactive, able to oxidize biomolecules; it also provides toxic chlorine gas:
HClO + H+ + Cl- → Cl2 + H2O
HClO also reacts with O2•- yielding OH•:
HClO + O2•- → O2 + OH• + Cl-
Chronic granulomatous disease
Caused by a deficiency of one of the NADPH oxidase subunits
Superoxide and the other reactive oxygen species are not produced
Severe infections that are very hard to treat – e.g.: Burkholdaria cepacea causes pneumonia Aspergillus causes intractable pneumonia, septicaemia; can lead
to death
Treatment: antibiotics, antifungal agents
2) Nitric oxide production Mainly by inducible nitric oxide synthase (iNOS) of macrophages which is
induced by cytokines (INF-γ, TNF) or bacterial lipopolysaccharide:
NO• can kill bacteria directly (e.g. by inhibition of the respiratory chain) or indirectly: by reaction with O2
•-, generating peroxynitrite ONOO- which attacks Fe-S proteins and essential –SH groups, inactivates enzymes…
Arg citrulline
NADPH oxidase is effective mainly in degradation of extracellular pathogens (Salmonella, Staphylococcus, Streptococcus pyogenes)…neutrophils
X
NO serves mainly to kill the intracellular parasites (Listeria, Brucella, Candida albicans)…macrophages
3) Granules (lysosomes) of neutrophils
Contain bactericidal substances and hydrolases that, after fusion with phagosome, destroy the engulfed particles: myeloperoxidase lysozyme – cleaves glycosidic bonds in peptidoglycan of the
bacterial (primarily G+) cell walls defensins – cationic peptides (Arg) with Mr of 3,5-6 kDa; interact with
anionic lipids of bacterial membrane and make pores in it; can also inhibit synthesis of DNA and proteins
hydrolases, e.g. elastase – serine protease: can damage bacteria and cleave virulence factors, but also cause harm to host tissues (cleaves the proteins of extracellular matrix, too)
Eosinophils
Main task: defence against multicellular parasites Display all the above-mentioned mechanisms with slight differences:
ROS production peroxidase of eosinophils – similar to myeloperoxidase, but prefers
Br- as a substrate (instead of Cl-), thus generating HBrO (instead of HClO)
basic protein of eosinophils disrupting the parasite cell membranes
B) Basophils and mast cellsB) Basophils and mast cells
Activated by antigens / allergens interacting with IgE bound to the surface IgE receptors of basophils (mast cells) Upon activation, content of their granules is released – substances that are harmful to parasite and induce reactions that should lead to its removal;
however, they can also be responsible for allergic symptoms: hydrolases histamine heparin
Synthesis of eicosanoids is activated; leukotrienes are potent bronchoconstrictors, stimulate chemotaxis and leukocyte activation
cytoplasmic granules
Histamine
Produced by histidine decarboxylation:
Causes vasodilation and bronchoconstriction helps to eliminate parasites (cough, peristalsis, enhanced production of mucus)
Atopy
IgE recognizing allergens (from pollen, food…) are produced and bind to IgE receptors of basophils (mast cells). Next exposure to the allergen can lead to release of histamine and heparin and synthesis of eicosanoids
Local symptoms occur: allergic rhinitis, asthma, conjunctivitis
If the allergen enters bloodstream, it can cause a massive degranulation of basophils (mast cells) increase in vascular permeability, decrease in blood pressure pulmonary oedema, ischemia… anaphylactic shock
Treatment: antihistamines – block histamine receptors
C) LymphocytesC) Lymphocytes
Have specific receptors recognizing one particular antigen: B cell receptors (BCR) and T cell receptors (TCR), respectively
BRC is a membrane-bound immunoglobulin, TCR is very similar to Ig
B cells (after proliferation and differentiation into plasma cells) secrete large amounts of antibodies (soluble immunoglobulins)
Soluble immunoglobulins
2 heavy chains (H) interconnected by disulfide bonds
2 light chains (L), each connected to one of the H chains (by disulfide bond)
H chain: 4-5 domains, 50-75 kDaL chain: 2 domains, 25 kDa
N-terminal domains of H- and L-chains are variable (VH resp. VL), the others are constant (CH resp. CL), i.e. the same in one type of Ig
Variable domains of H a L chains form
the antigen-binding site
VH
VL
CH1
CL
CH2
CH3
Fc
Fab
Types of immunoglobulins (Ig)
There are 2 isotypes of L: κ, λ There are 5 isotypes of H:
, γ, δ, ε, μ According to these isotypes of H,
5 types of immunoglobulins can be distinguished: IgA (2 subtypes) IgG (4 subtypes) IgD IgE IgM
IgM can form pentamer, IgA can form dimer or trimer
155 kDa
900 kDa(similar: IgD, IgE)
D) PlateletsD) Platelets No nucleus many of their metabolites come from megakaryocytes Form blood clots, act as vasoconstrictors Participate in defence against infections, e.g.: they suppress the growth
of Plasmodium falciparum (infectious agent that causes malaria) Generate O2
•- and H2O2 that may synergize with pro-aggregatory stimuli
Contain thromboxan A synthase that catalyzes conversion of prosta-glandin H2 to thromboxan A2:
TXA2 – promotes platelet aggre-gation and vasoconstriction
Platelets also release two very important factors that can influence not only platelets but also other cell types:
Platelet-Activating Factor (PAF) Platelet-Derived Growth Factor (PDGF)
Platelet-Activating Factor
Mainly juxtacrine and paracrine signalling via GPCR Promotes platelet aggregation Induces activation of leukocytes, adhesion, chemotaxis, cytokine
production, causes vasodilation and bronchoconstriction Mediates interplay between thrombotic and inflammatory cascades BUT: it is also suspected of contributing to allergy, anaphylactic shock… It is produced also by endothelial cells, monocytes, granulocytes…
phospholipid
Platelet-Derived Growth Factor
Dimeric protein, 3 isoforms Receptors: tyrosine kinases – expressed on fibroblasts, glia, smooth
muscle cells, leukocytes…. Effects:
proliferation chemotaxis cytoskeletal rearrangements differentiation of certain types of cells (e.g. in CNS) participates in wound healing, capillary formation, embryonic and
postnatal development! BUT: probably also plays a role in pathogenesis (some tumours)
CytokinesCytokines
Proteins secreted by leukocytes and other cells (but there are also membrane cytokines) that influence (via receptors) the cells of the immune system
Cytokine signalling: autocrine – a cytokine influences the same cell that produces it paracrine – a cytokine influences the nearby cells endocrine – a cytokine influences distant cells (after transport by the
bloodstream)
Types of cytokines
Interleukins – e.g. IL-6: produced by macrophages, neutrophils, stimu-lates lymphocytes, secretion of Ig, synthesis of acute phase reactants
Chemokines – induce chemotaxis
Interferons – e.g. INF-: produced by lymphocytes, monocytes, and macrophages, participates in antiviral defense (induces synthesis of enzymes that block viral replication)
Transforming growth factors – e.g. TGF-β: produced by T-lymphocytes, macrophages, and platelets, displays anti-inflammatory effects
Tumor necrosis factors – e.g. TNF-β: able to induce apoptosis
Leukocyte infiltration into tissues= diapedesis (extravasation):
Leukocytes are slowed down by the interaction of their mucins with selectines on the surface of endothelial cells (EC)
Cytokines on the surface of EC interact with the receptors of leukocytes A strong adhesion mediated by the interaction of integrins with molecules on the
surface of EC → migration of leukocytes into the tissue directed by cytokins released by inflammatory cells or EC
Taken from:Halliwell, Gutteridge,
Oxford University Press, 1999
RegulationRegulation
Many functions of leukocytes are regulated by monomeric GTP-binding proteins, e.g. Rac, Rho: activation of NADHP oxidase chemotaxis phagocytosis fusion of phagosome with granules
Rho and Rac are able to modulate the assembly of actin filaments, which plays a role in the processes listed above
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