1
ROLE OF THE PLATELETS IN INFLAMMATION
AND IMMUNITY
Authors:
Radu Andrei Tomai
Cluj
Antoine Joubert
Nantes
Supervisor: Zsófia Mezei-Leprán, MD, PhD
associate professzor
University of Szeged, Faculty of Medicine
Department of Pathophysiology
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CONTENTS
1.INTRODUCTION
2.DEFENSE MECHANISM OF HUMAN BODY
2.1.First defense line
2.2.Second defense line
2.3. Third defense line
3.PLATELETS ROLE IN THE DEFENSIVE REACTIONS
3.1.Second defense line
3.2.Third defense line
4.ANTIPLATELET THERAPY EFFECT ON THE DISORDERS
5.SUMMARY
6.REFERENCES
7.DECLARATION
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ABBREVIATIONS:
ADP adenosine diphosphate
PAMP pathogen associated molecular
ALR AIM-2 like receptor
pattern
APC antigen presenting cell
PDGF platelet derived growth factor
CARD caspase activation and
PF4
recruitment domain
platelet factor 4
PRR pattern recognition receptor
COX cyclooxigenase
PSGL1p-selectin glycoprotein ligand 1
CpG Cytoidine-phosphate-Guanosine
RLR
EGF epithelial growth factor
GP
glycoprotein
ICAM2intercellular adhesion
molecule 2
IFN
interferon
Ig
immunoglobulin
like receptor
ROS reactive oxygen species
Rv
LBP lipopolysaccharide binding
LPS
resolvins
SAMP self associated molecular pattern
sensors
Siglec sialic acid recognizing Ig-like
ITIM immunoreceptor tyrosine-based
inhibitory motif
retinoid acid inducible gene 1
lectins
TGF transforming growth factor
TIR
toll interleukin receptor
protein
TLR toll like receptor
lipopolysaccharide
TPO thrombopoietin
LRR Leucine rich repeats
TSP
Lx
TXA2 thromboxane A2
lipoxins
Mac-1 macrophage-1 antigen
MHC major histocompatibility
complex
MyD88Myeloid differentiation primary
response gene 88
NADP Nicotinamide adenine
dinucleotide phosphate
NET neutrophil extracellular trap
NLR NOD like receptor
NO
nitric oxyde
NOD nucleotid-binding
oligomerization domain
receptors
thrombospondin
VWF/ vWF von Willebrand factor
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1.INTRODUCTION
Platelets are produced in the bone marrow and originate from megakaryocytes.
These megakaryocytes grow by endomitosis and are fragmented to produce the
platelets. They receive a positive signal from the thrombopoietin (TPO) which is
secreted by the liver. The platelets do not possess a nucleus and are the smallest cell-like
structures in the blood (2-3 μm). The platelets are made of two types of linings: the
exterior is rich in receptors (glycoproteins like GPIIbIIIa, GPIb but also TLRs, CD40,
P-selectin, P2Y12...) and the interior is composed of microtubules which form the inner
skeleton [Lam, 2015]. The cytoplasm consists of lysosomes, alpha and dense granules.
Lysosomal granules contain proteases and glycosidases. The alpha granules store
adhesion molecules, mitogenic factors, inflammatory mediators, growth factors, vWF,
fibrinogen and protease inhibitors [Blair, 2009; Lam, 2015]. The dense granules contain
ATP, ADP, serotonin and calcium ions. [Gale, 2011; Lam, 2015]
Fig 1.: Platelet activation
http://what-when-how.com/wp-content/uploads/2012/04/tmp4B46.jpg
GP: glycoprotein, vWF: von Willebrand factor, ADP: adenosine diphosphate, PDGF: platelet
derived growth factor, TSP: trombospondin
The platelets were first described in the primary haemostasis which includes
three steps. Adhesion which is mediated by its receptors and mainly the GPIb/Ia which
binds the vWF. This molecule may be released into the blood circulation from the
endothelial cells induced by high shear stress, hypoxia or cytokines. It induces a
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conformational change of the receptors (like GPIIb/IIIa and P2Y12) which leads to an
intracellular signalisation pathway. Activation of the platelet via the elevation of
intracellular calcium ion concentration leads to the releasing of granules. The GPIIb/IIIa
receptor of activated platelet binds to the other activated platelet through the fibrinogen
resulting aggregate formation (Fig. 1.) [ Gale, 2011; Ghoshal, 2014].
Atherosclerosis is the most common cause of death worldwide, iducing stroke,
ischemic or hypertensive heart diseases (Fig. 2.).
Fig. 2. Death brats Worldwide 2012 (WHO) Combining IHD, Strokes and HHD.
http://baltimorepostexaminer.com/wp-content/uploads/Atherosclerosis-2012.png
IHD: ischemic heart disease, HHD: hypertensive heart disease,
WHO: World Health Organization
It is already known that platelets adhering to the damaged endothelial layer, play
a role in the complication of atherosclerosis. But in these days the role of the platelets is
examined in early periods, and in the progression of atherosclerosis. Atherosclerosis is a
chronic innate and adaptive immune-mediated inflammatory process. Earlier research
focuses only on the adaptive immunity, while Hovland et al. [2015] reported that innate
immunity also participates in the pathomechanism of atherosclerosis. More and more
scientific teams examine the role of the platelets in the inflammatory and innate or
adaptive immunological processes [Huo, 2004; Patzelt, 2015].
In this work we wish to summarize the latest scientific knowledge related to the
platelets’ role in inflammatory and immune processes.
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2.DEFENSE MECHANISM OF HUMAN BODY
The three lines of defense mechanism. First and second line is the innate or
natural immunity. Third is the adaptive or acquired immunity
2.1.First defense line
The first line of defense of the human body is a nonspecific resistance towards
the pathogens, displayed by the epithelial tissues and their microenvironment. The
tissues’ resistance towards the pathogen is manifested by physical, chemical and
cellular means. The entry of the microorganism is denied by the continuity of the
epithelia which does not allow the breach of most pathogens. The physical aspect of the
defense is supported by the flushing of the urinary tract, the movement of the cilia in the
airways and the peristaltism of the gastrointestinal tract [Male, 2006].
Chemically, the epithelial tissues provide an inhospitable environment for the
invading pathogens. The low values of pH in the stomach and vagina interfere with the
activity of the pathogen [Male, 2006]. Mucus is a viscous substance composed of
glycoproteins called mucins, secreted by the epithelia lining the airways,
gastrointestinal and genitourinary tracts, which limits the interaction of the pathogen
with the host’s epithelial cells [Janeway, 2001]. The sebum fatty acids present on the
skin surface have antibacterial properties exerted by enhancing the secretion of
microbicide peptides of the epithelial cells [Gallo, 2011].
Microbicide peptides are synthetized and secreted by epithelial cells and
commensals to kill the invading pathogens [Male, 2006]. The antimicrobial peptides
produced by the epithelial cells with important role in defense belong to the protein
families of defensins and cathelicidins. Defensins and cathelicidins acting on the
epithelial surfaces may also be produced by neutrophils [Tollin, 2003].
Cells involved in the protection of the epithelial tissues are the intraepithelial
lymphocytes belonging to the T lineage. The cells are αβ T cells and γδ T cells that
recognize microbial structures [Abbas, 2011].
2.2.Second defense line
The second line of defense in the human body is the innate immune system. It is
a nonspecific mechanism which encounters the pathogens that succeeded in breaching
the first line of defense. The mechanism is based on the activity of leukocytes and other
proteins present in the plasma of the blood. The characteristic of the innate immune
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system is that it does not require training, but can start countering the pathogen
immediately after encountering it.
The leukocytes activate the process of destroying an invading pathogen only
after they identify the entities they encounter as pathogens, or after they sense danger
associated signals. To do so, the cells possess a range of receptors which bind to
antigens specific to pathogens. Antigens specific to pathogens are lipopolysaccharides,
double-stranded RNA, unmethylated CG-rich oligonucleotides and other structures with
a different pattern than the host’s patterns.
2.2.1. Pattern Recognizing Receptors
The receptors that recognize these pathogen associated molecular patterns
(PAMP) are called Pattern Recognizing Receptors (PRR) [Abbas, 2011]. The PRRs are
receptors present in the cell membrane and in intracellular membranes as endoplasmic
reticulum. There are different classes of receptors which are PRRs. These are the toll
like receptors (TLRs), retinoid acid inducible gene 1 like receptors (RLRs), nucleotidbinding oligomerization domain receptors (NOD), NOD-like receptors (NLRs), AIM-2
like receptors (ALRs).
Fig. 3.: Role of the Toll like receptors [O'Neill, 2013]
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Toll like receptors share a common Leucin rich repeats (LRR) extracellular
domain and a TIR intracellular domain. They are subclassified into five subfamilies.
The TLR2 subfamily binds lipopolysaccharides, the TLR3 subfamily binds dsRNA, The
TLR4 subfamily binds lipopolysaccharides, TLR5 binds flagellin, TLR9 binds
unmethylated CG-rich oligonucleotides. The transduction of the signal is done via
MyD88 adaptor protein and leads to activation of nuclear factors and interferon
response factors in the nucleus, which cause the synthesis of cytokines to mediate the
immune response (Fig. 4.) [Takeda, 2003; O'Neill, 2013].
NOD like receptors are intracellular receptors that have a LRR domain for
PAMP recognition, a NACHT domain, and N-terminal caspase recruitment domain
(CARD) or pyrin or acidic or baculovirus inhibitor repeat domains. Binding of ligands
by the NLRs induces synthesis of inflammatory cytokines [Franchi, 2008].
RLRs sense viral RNA and induce antiviral and inflammatory response. Most of
the RLRs mediate the response via CARD domain [Loo, 2011].
2.2.2. Phagocytes
Phagocytes are the cells of the innate immune system that use phagocytosis to
neutralize pathogens. Phagocytes are derived from the myeloid lineage and are
neutrophils and monocytes/macrophages. The neutrophils and monocytes circulate in
the blood and migrate to the site of infection guided by cytokines. They pass between
the endothelial cells of the blood vessels by the process of extravasation and reach the
site with high concentration of cytokines. The phagocytes are activated by binding to
the PAMP with the PRRs, by complement system proteins and antibodies. The cells
phagocytose the pathogen into an endosome which fuses with a lysosome to form a
phagolysosome where the foreign organism is killed by reactive oxygen species (ROS),
nitric oxyde (NO) and lysosomal enzymes [Abbas, 2011].
2.2.3.Complement system
The complement system is a group of small plasma proteins which use a chain
proteolytic reaction to induce the destruction of a pathogen. The functions of the
complement system include the recruitment of leukocytes and perforating the
membrane of the invading organism leading to lysis. Complement proteins bind to the
membrane of the cells and if they are not inhibited by the complement control proteins
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expressed only by host cells, they decay spontaneously and trigger the proteolytic
cascade (Fig. x.) [Abbas, 2011; Hovland, 2015].
Fig. 4.: Activation and effects of complement system [Takeda, 2003]
2.2.4.Inflammation
Inflammation is a process of by which the body reacts to pathogens.
Inflammation consists of reactions of the blood vessels, leukocytes and other
components of the immune system. It begins with the recognition of pathogenic
markers inside the tissue. Cytokines are then produced to recruit leukocytes from the
circulation. In order for the migration of the white blood cells into the tissue to be
possible, endothelial cells lining the vessels adjacent to the infected tissue undergo
changes which lead to increasing the diameter and the permeability of the vessel. The
speed of blood flowing through the section is reduced and allows cells to adhere and
extravasate into the tissue. Recruited leukocytes become activated ,destroy the
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pathogen and phagocytose the dead cells. Then, the last phase of inflammation is
tissue repair. The clinical signs of inflammation are pain, erythema, swelling, heat and
loss of function. (Fig. 5.).
Fig. 5: Inflammatory process [Kumar, 2015]
2.3. Third defense line
There are two types of adaptive immune responses. A humoral one mediated via
immunoglobulins produced by the plasma cells (activated B lymphocytes). A cellular
one, mediated by the T lymphocytes. It is characterised by its specificity towards one
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antigen and by a long-lasting memory which makes it fast to respond to ulterior attacks
and very efficient. These cells originate from the bone marrow but while the B
lymphocytes stay until the latest stages of their differentiation, the T lymphocytes
quickly migrate to the thymus (Primary Lymphoid Organ). The thymocytes have the
capacity to express nearly all the proteins of the body and this limits the production of
self-reactive T lymphocytes.
The B cells go the lymph nodes at the end of their maturation process for the
negative selection. The T cells undergo a negative and a positive selection. At the end,
only a very small percentage has the capacity to be functional, the rest is either anergic
or destroyed. The B cells recognise the antigen with their B cell-receptor (BCR). The T
cells recognise the antigen when it is presented as a peptide on a MHC molecule present
at the surface of APCs.
These APCs are cells present in the peripheral tissues that have the possibility to
take up the pathogens and to present fragments of antigens on the surface (either with
MHC I or MHC II) to have an interaction with T/B cells. The clonal expansion happens
in the lymph node, in the germinal centre. This is also the place where the T and B
lymphocytes have interactions to further their maturation. The B cells are selected by T
helper lymphocytes.
There are two main types of T lymphocytes: T CD4+ helper (recognise the MHC II)
which help the macrophages to kill the pathogen they engulfed with Interferon-ɤ and the
T CD8+ cytotoxic (recognise the MHC I) which directly kill the infected cells, with
granzymes and perforine or Fas/FasL.
The B cells become long-life plasma cell (found in the bone marrow) to produce
immunoglobulins during an important period of time (sometimes for all our life); or
they become circulating memory B cells with a sentinel role, this allows a shortcut in
the adaptive activation pathway . The short lived B cells are the plasma cells which die
after a few days [Male, 2006; Abbas, 2011].
3.PLATELETS ROLE IN THE DEFENSIVE REACTIONS
3.1.Second defense line
Platelets circulate in the blood and participate mainly in the second line of
defense, with involvement in inflammation and immunity. The means by which
platelets sense their environment, allowing them react in a certain way are membrane
receptors. The receptors on the platelet’s surface have the main role of sensing signals
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of damaged endothelial cells, which leads to local microinflammation and recruitment
of molecules which repair the damage. Other roles of receptors are sensing PAMPs and
eliciting an adequate response to the pathogen.
3.2.1. The recognition of infectious organisms
The recognition of infectious organisms is mediated by PAMP receptors present
on the cell membrane of the platelets and also in membranes inside the cell. The main
PRRs are the toll like receptors (TLRs). TLR1, TLR2, TLR6, TLR4, TLR5 have been
identified on the cell membrane and TLR9, TLR3, TLR7 inside the cell. The TLRs are
receptors which upregulate the activity of the platelets [Cognasse, 2015; Lam, 2015;
Sample, 2010; Sample 2011].
TLR mRNA has been identified in megakaryocytes. The observation that the
Knock-out mice for the TLR4 gene have lower platelet count compared to the control
group of mice indicate that TLR4 may be involved in platelet production regulation.
The megakaryocytes also manifest changes in the transcripts for TLR1 and TLR6 when
stimulated by IFNγ. The increase in the transcripts for TLRs increases the antimicrobial activity of the platelets.
TLR4 is the main TLR present on the surface of the platelets. Its binding to LPS
is assisted by proteins such as MD2, lipopolysaccharide binding protein (LBP) and
CD14. The TLR4 activation leads to increased expression of Tissue Factor and to
activating the transducing pathway leading to cytokine secretion. The transducing
pathway includes the following proteins TRIF, MyD88, TBK-1, IRAK-1, JNKs, MAPk,
TRAF3, TRAF6, IRF-3, IKK-i, IkB-α, and NF-kB. Stimulation of TRAF6 protein leads
to production of IF1-β. Expression of TLR4 on the platelet’s surface is increased on
activated platelets. TLR4 stimulation by LPS induces regulating of cytokynes. Some are
upregulated, such as sCD40L and PF4 proinflammatory cytokines, some remain the
same: sCD62P, IL-8, EGF,TGF and some are downregulated: PDGF-AB.
TLR2 is a PRR which binds lipopeptides, lipoteichoic acids and peptidoglycans,
induces hemostasis and inflammation. TLR2 forms dimers with TLR1 or TLR6 to bind
triacylated lipopeptides of gram-negative bacteria, respectively diacylated lipopeptides
of gram-positive bacteria. Stimulation of TLR2-TLR6 complex with diacylated agonist
leads to inhibition of platelet activation which indicates that in the case of a gram
positive bacterial infection, platelet activation is downregulated by TLR2 blocking.
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Stimulation of TLR2-TLR1 complex by triacylated agonist Pam3CSK4 induces
signaling, platelet adhesion to collagen, and ROS production. Pam3CSK4 stimulation
increases intracellular Ca2+ concentration, release of ATP and synthesis of TXA2.
TLR9 is an intracellular PRR which binds unmethylated cytidine-phosphateguanosine (CpG)DNA motifs present in prokaryotic organisms, viruses and parasites
and carboxy-alkyl-pyrolle, an oxidative stress product. Upon binding the oxidative
stress product, the platelet initiates aggregation and degranulation. Expression of TLR9
is increased by CpG motifs and by thrombin.
Sialic acid recognizing Ig-like lectins (Siglec) receptors are proteins belonging
to the Ig-like family, which bind self molecules to modulate the immune response to a
pathogen. The modulation is done via the intracellular immunoreceptor tyrosine-based
inhibitory motif (ITIM). Siglec are constitutively bound to sialic acid by cis saline
bonds. When the sialyated glycans are bound to PAMP structures, or when the cis bond
sites are cleaved by sialidase, the trans bond is formed. The regulation of the immune
response is due to the alternative forms of the bond as follows: the cis-ligation
downregulates the response while the trans-ligation upregulates the response. In
platelets, Siglec-7 functions as an apoptosis inducing receptor when bound to its ligand
ganglioside G2. This functions as a mechanism of negative regulation of inflammatory
responses [Cognasse, 2015].
3.2.2. The platelets-leukocytes interactions
One of the main implications of platelets in immunity is their ability to interact
with and influence leukocytes. The platelets-leukocytes interactions are divided into two
classes. The adhesive mechanisms and the soluble mechanisms of interaction.
The adhesive mechanisms consist of interaction of P-selectin of platelets with pselectin glycoprotein ligand 1(PSGL-1) of leukocytes mediating rolling and diapedesis
(Fig.6.); GPIbα of platelets with Mac-1 integrin promoting inflammation and
thrombosis; ICAM-2 of platelets with Mac-1 mediating adhesion and migration. The
soluble mechanisms include PF4 cytokine which activate neutrophils, fibrinogen which
binds to Mac-1 mediating adhesion [Lam, 2015].
The soluble mechanisms are means by which platelets also respond to viral
infections. Activated platelets release α-granules which contain kinocidins and antiviral
molecules. PF4 (CXCL4) has been observed to inhibit HIV infection of T lymphocytes.
Other antiviral molecules are CCL3 and CCL5. Platelets recruit dendritic cells to the
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site of inflammation and their modulation of leukocyte functions is enhanced by the
CD40L binding to the CD40 on leukocytes [Assinger, 2014; Sample, 2011].
Fig. 6: [Chiara C,2010]
The formation of neutrophil extracellular traps (NETs)
This is another important interaction between platelets and leukocytes. NETs are
formed by the DNA and histones of neutrophils and are used to attract and bind
pathogens. CCl5-CXCL4 dimer, TLR4, Mac-1 are involved in formation of NETs (Fig.
7.) [Sample, 2010; Sample, 2011].
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Fig.7.: Neutrophil extracellular trap (Andrews, 2014)
Platelets can activate the complement system by releasing kinases to
phosphorylate C3 and C3b units. They can also interact with complement system
through platelet activation factor (PAF) which enhances phagocytosis of complement
bound erythrocytes and by classical pathway. At the same time, complement system can
also activate the platelets [Lam, 2015].
Platelet’s involvement in inflammation is mainly mediated by P-selectin which
mediates aggregation of platelets and adhesion and rolling of leukocytes. The activation
of leukocytes succeeds adhesion and is due to chemokines present on the endothelial
cell surface [Sample, 2009].
3.3. Third defense line
The platelets express the CD40 ligand on their outer membrane. The CD40
receptor is an important co-receptor to the activation of lymphocytes and so the platelets
help in this matter. This is a direct mediation of these cells.
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Platelets are the main source of sCD40L. This molecule is responsible for a lot
of interactions with the adaptive immune cells:
– helps the maturation of B cells in the germinal centre even if there are few T
CD4+
– induce the immunoglobulin class-switching [Semple, 2011]
– induce the secretion of IgGs from the B lymphocytes
– increase the production of T lymphocytes via the action of CD40L on the
germinal centre
– help the T CD8+ functions of cytotoxicity with GPIIIa [Assinger, 2014] and
their survival [Blumberg, 2010] via an intense activation of APCs: the
dendritic cells. [Elzey, 2012]
According to Elzey and colleagues, CD40L is a major molecule in that is does
positively modulate many immune processes such as « antigen presenting cell (APC)
activation, lytic activity, cross presentation and memory formation. » Its absence has
important consequences in the protection of the organism. For example, genetically
modified CD40L-/- mice had low counts of T CD8+, and transfusions of functional
platelets resolved the problem for a limited period of time.
There is also an indirect mediation which is achieved through the use of
cytokines such as CXCL4 and CCL5. They orient the differentiation of T cells and
make them produce more pro-inflammatory cytokines.
The platelets are yet another way to activate the adaptive immune response.
But the CD40L is not the only molecule to be used on the cells of the adaptive
immunity. Serotonin releasing has been shown to prevent the penetration of T CD8+
cells into the intralobular region and as a result, to prevent the liver from possible acute
damages. We can see that platelets are not simple anucleated cells: they can have effect
both ways according to the situation.
4.ANTIPLATELET THERAPY EFFECT ON THE DISORDERS
The scientific publications we have overviewed provide clear evidence that
platelets have a role in the innate as well as in the adaptive immune-mediated
inflammatory processes. These processes have a vital role in maintaining the
physiological conditions and homeostasis. In those cases when the trigger factor – the
inducer – is not eliminated, the delicate balance gets disturbed resulting in the cascadelike activation of the platelets and the related cells and molecules, and as a consequence,
17
pathophysiological processes are induced. A typical example is the development of
atherosclerosis, which is considered the epidemic disease of the 21st century, its
progression and the manifestation of its complications.
4.1.PREVENTION
4.1.1.Role of the physical activity
Sport is in a domain in which platelet functions are investigated. Although
results are still conflicting we can observe general information. Platelet aggregation is
inhibited for several days by regular exercise as opposed to acute exercise This is a
promising way of reducing or preventing cardiovascular incidents such as
atherosclerosis and infarcts of the myocardium [Rauramaa,1986; Goette, 2004].
4.1.2.Role of the diet
It has been shown that a correct diet could diminish the occurrence of
cardiovascular events. There is much to discover in this way, to prevent the accidents by
soft ways such as diets. For example, the people living near the Mediterranean sea have
less of these events, 25% according to a certain study. [Trichopoulo, 2003]
The vitamin E has not been systematically measured as inhibiting platelets,
unlike vitamin C even if both are antioxidants. Vitamin C has direct effects on O2-, it
supresses it and thus impede the platelet activation [Violi, 2010].
The polyphenols are molecular compounds commonly found in red wine and are
particularly present in Mediterranean diet food such as olive oil, nuts, fruits and
vegetables. The people following this diet have been described as having less
cardiovascular incidents. It has now been proved that these polyphenols act on platelet
recruitment and adhesion functions. The polyphenols inhibit the NADPH oxidase (that
is why we can say they have an antioxidant effect) which produces O2- and this, in turn
increases the quantity of NO produced. Normally O2- destroys NO to limit its antiaggregating action. NO down-regulates the number of GPIIb/IIIa on the platelet surface
and thus perturbs the interaction of the platelets between themselves [Pignatelli, 2006].
This mechanism is involved in the prevention of atherosclerosis.
The initiation of the acute inflammatory response is characterized by
vasodilation, increased permeability, activation of endothelial cells, platelets and
leucocytes. These processes are mediated by released endogenous chemical mediators.
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Polyunsaturated fatty acids (eicosapentaenoic acid, docosahexaenoic acid) play
important role in the regulation of cell (granulocytes, monocytes, macrophages,
platelets, endothelial cells) function, activity during the inflammatory process. Lipid
mediators such as prostaglandins and leukotrienes, and cytokines and chemokines,
regulate the well-coordinated initial events of acute inflammation (Fig. 8. and Fig.9.)
[Freire, 2013, Spite, 2010].
Fig.8.: Lipid mediators [Freire, 2013].
AT-PD1, aspirin triggered protectin D1; DHA, docosahexaenoic acid; EPA, eicosapentaenoic
acid; HpETE, hydroperoxyeicosatetraenoic acid; LT, leukotriene; LX, lipoxin; PG,
prostaglandin; PGH2, prostaglandin H2; PLA2, phospholipase A2; PUFA, polyunsaturated fatty
acid; Rv, resolvin; TX, thromboxane.
Arachidonic acid released from membrane phospholipids by phospholipase A2 can be
further metabolized by COX-1 and COX-2 to generate prostanoids, including
prostaglandins and thromboxanes. Lipoxygenas enzymes produce leukotrienes and
hydroxy acids. Lipoxins are generated during transcellular biosynthesis, which requires
two cell types involving distinct lipoxygenases. Lipid mediator class-switches yield proresolution lipid mediators such as lipoxin A4 and the eicosapentaenoic acid-derived
resolvins (i.e. resolvin E1 and resolvin E2) and docosahexaenoic acid-derived lipid
mediators, including D-series resolvins, protectins and maresins (Fig. 8.) [Freire, 2013;
Spite, 2010].
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Fig. 9. Anti-inflammatory and pro-resolving actions of lipid mediators (Spite, 2010)
The vasodilator prostaglandins play important role in the the initiation of the
acute inflammatory response (vasodilation, elevation of permeability, granulocytes
activation). After killing the invading microbes, granulocytes undergo apoptosis and
must be cleared by macrophages to allow for tissue homeostasis to be restored. During
the time course of the acute inflammatory response, endogenous lipid mediators, such
as the lipoxins (LX), resolvins (Rv) and protectins are generated and act locally to stop
further vascular permeability and granulocytes chemotaxis, and promote the formation
of anti-adhesive and anti-thrombotic mediators, NO and prostacyclin (PGI2). These
novel lipid mediators also stimulate phagocytosis and clearance of apoptotic
granulocytes and microorganisms (Fig. 9.) [Spite, 2010].
The application of a diet rich in eicosapentaenoic and docosahexaenoic acid
helps the production of lipid mediators, such as resolvins, which have anti-inflammatory
effect and they facilitate resolution as well.
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4.1.3.Application of drugs
The administration of low-dose aspirin has several beneficial effects. By
inhibiting platelet COX1 it reduces the synthesis of thromboxane (which is inducing
thrombocyte activation), while in this concentration it does not hinder the production of
endothelial prostacyclin (which suppresses the platelet aggregation). In addition, it
acetylates endothelial COX2 resulting in a compound with dual effect, aspirin-triggered
lipoxin, which is on the one hand anti-inflammatory, on the other hand facilitates the
resolution of tissue structure and tissue function.
Fig. 10.: Synthesis of 15 epi-LXA4 [Freire, 2013]
Nevertheless, we must not forget that all medicines have side effects, in this case
it may induce gastrointestinal bleeding – depending on the sensitivity of the individual.
Some people may show resistance to aspirin, while in case of others it may provoke
allergic or asthmatic attacks.
4.2.THERAPY
There are now many drugs on the market that inhibit the platelets, through
different mechanisms.
21
Fig. 11.: Antiplatelet agents (Franchi, 2015)
5-HT2A: serotonin receptor 2A; COX: cyclooxygenase; GP: glycoprotein; PAR: proteaseactivated receptor; TP: thromboxane prostanoid receptor; TXS: thromboxane A2 synthase;
vWF: von Willebrand factor.
The antiplatelet therapies are based on blockade of the thromboxane synthetize
and/or COX1 inhibitors, and on inhibition of platelet receptors (thromboxane, thrombin,
ADP, and serotonin receptor 2A) (Fig.11.) [Franchi, 2015].
Current guidelines recommend triple antithrombotic therapy consisting of anticoagulant
and dual antiplatelet therapy, with different mechanism (COX1 inhibitor, and receptor
blocker) [Valgimigli, 2011].
22
5.SUMMARY
The platelets’ study is important because they are involved in atherosclerosis,
which is a major cause of death worldwide. Atherosclerosis is an immune condition
involving both innate and adaptive responses. The scientific publications we have
overviewed provide clear evidence that platelets have a role in the innate as well as in
the adaptive immune-mediated inflammatory processes. These processes have a vital
role in mantaining the physiological conditions and homeostasis. In those cases when
the trigger factor – the inducer- is not eliminated, the delicate balance gets disturbed
resulting in the cascade-like activation of the platelets and the related cells and
molecules, and as a consequence, pathophysiological processes are induced. A typical
example is the development of atherosclerosis, which is considered the epidemic
disease of the 21st century, its progression and the manifestation of its complications.
The prevention and management of the excessive, unregulated and uncontrolled
activation of the platelets is an important task in order to maintain the physiological
conditions.
23
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7.DECLARATION
I …………….
, student of the …………………………….. am aware of my
responsibility of the penal law, declare and certify with my signature that my thesis
entitled ……………………………………………………………….. is entirely the
result of my own work.
I have faithfully and accurately cited all my sources, including books, journals,
handouts and unpublished manuscripts, as well as any other media, such as the Internet,
letters or significant personal communication. I understand that
- literal citing without using quotation marks and marking the references
- citing the contents of a work without marking the references
- using someone else’s work which was published as my own thoughts are
counted as plagiarism.
I declare that I’ve understood the concept of plagiarism and I acknowledge that my
thesis will be rejected in case of plagiarism.
Szeged,…………..year …………month……………...day
………………………………………. Signature of thesis writer