Postischemic Inflammation in Acute Stroke

TLDR: This review describes the main phases of the inflammatory process occurring after arterial cerebral occlusion, with an emphasis on the role of single mediators.

Abstract: Cerebral ischemia is caused by arterial occlusion due to a thrombus or an embolus. Such occlusion induces multiple and concomitant pathophysiological processes that involve bioenergetic failure, acidosis, loss of cell homeostasis, excitotoxicity, and disruption of the blood-brain barrier. All of these mechanisms contribute to neuronal death, mainly via apoptosis or necrosis. The immune system is involved in this process in the early phases after brain injury, which contributes to potential enlargement of the infarct size and involves the penumbra area. Whereas inflammation and the immune system both exert deleterious effects, they also contribute to brain protection by stimulating a preconditioning status and to the concomitant repair of the injured parenchyma. This review describes the main phases of the inflammatory process occurring after arterial cerebral occlusion, with an emphasis on the role of single mediators.

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2017 Korean Neurological Association 1
Cerebral ischemia is caused by arterial occlusion due to a thrombus or an embolus. Such oc-
clusion induces multiple and concomitant pathophysiological processes that involve bioener-
getic failure, acidosis, loss of cell homeostasis, excitotoxicity, and disruption of the blood-brain
barrier. All of these mechanisms contribute to neuronal death, mainly via apoptosis or necro-
sis. e immune system is involved in this process in the early phases aer brain injury, which
contributes to potential enlargement of the infarct size and involves the penumbra area. Where-
as inammation and the immune system both exert deleterious eects, they also contribute
to brain protection by stimulating a preconditioning status and to the concomitant repair of
the injured parenchyma. is review describes the main phases of the inammatory process
occurring aer arterial cerebral occlusion, with an emphasis on the role of single mediators.
Key Words
zz
ischemic stroke, inammation, immune response.
Postischemic Inammation in Acute Stroke
INTRODUCTION
Stroke is a leading cause of disability in adults that has a heavy social burden worldwide.
is disease is the third highest cause of mortality, resulting in approximately six million
deaths annually.
1
Acute cerebral ischemia accounts for more than 80% of all strokes and is
due to brain arterial occlusion resulting from a thrombus or embolus. e pathophysiologi-
cal processes following ischemic stroke are complex, involving bioenergetic failure, acidosis,
loss of cell homeostasis, excitotoxicity, activation of neuronal and glial cells, and disruption
of the blood-brain barrier (BBB) with inltration of leukocytes.
2
ere is evidence that fac-
tors of the immune system are involved in all stages of acute cerebral ischemia (Fig. 1).
3
e
ischemic brain promotes a potent suppressive eect on lymphoid organs via the autonomic
nervous system, which increases the risk of the poststroke infections that are major deter-
minants of morbidity and mortality.
4
On the other hand, the innate immune system con-
tributes to subsequent repair of the damaged cerebral tissue.
5
In this review we describe the main phases of the inammatory processes during the ear-
ly postischemic period, with an emphasis on the role of single mediators.
INFLAMMATION, ENDOTHELIUM,
AND CLOT FORMATION
A growing amount of attention is being paid to the mechanisms of clot formation, particu-
larly in the eld of endovascular treatment of acute ischemic stroke. Although most of the
focus has been on intervention devices (with developments from rst- to second-generation
devices, thrombus aspiration, and balloon-occlusion guiding catheters), some research
groups have studied the physiopathological mechanisms of clot formation. e relation be-
tween inammation and clot formation has been described previously, and it indicates that
Simone Vidale
a
Arturo Consoli
b
Marco Arnaboldi
a
Domenico Consoli
c
a
Department of Neurology and Stroke
Unit, Sant’Anna Hospital, Como, Italy
b
Department of Interventional
Neurovascular Unit, Careggi University
Hospital, Florence, Italy
c
Department of Neurology,
G. Jazzolino Hospital, Vibo Valentia, Italy
pISSN 1738-6586 / eISSN 2005-5013 / J Clin Neurol 2017
;
13
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1-9 / https://doi.org/10.3988/jcn.2017.13.1.1
Received September 12, 2016
Revised
October 30, 2016
Accepted
October 31, 2016
Correspondence
Simone Vidale, MD
Department of Neurology and
Stroke Unit, Sant’Anna Hospital,
Via Napoleona 60, 22100 Como, Italy
Tel +39-0315859282
Fax +39-0315854989
E-mail simone.vidale@asst-lariana.it
cc
is is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Com-
mercial License
(
http
:
//creativecommons.org/licenses/by-nc/3.0
)
which permits unrestricted non-commercial
use, distribution, and reproduction in any medium, provided the original work is properly cited.
JCN Open Access
REVIEW

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Inammation in Acute Ischemic Stroke
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some cytokines (e.g., the RANTES)
6
are responsible for the
activation of a biochemical cascade or, indirectly, supports
the concept that infections/inflammation promote athero-
genesis and that some endothelial modifications that can
lead to a prothrombotic status.
7
Several research groups are
currently focusing on the clot structure, with many ndings
supporting the concept of an inammation-induced process.
In particular, fibrinogen is susceptible to oxidation, and
chronic exposure to oxidative stress supported by inamma-
tion may lead to prothrombotic alterations in brin forma-
tion and architecture. Previous studies concerning air pollu-
tion have shown that particulate matter contributes to
modulation of the brin structure.
8-11
Furthermore, some in-
teresting studies that have used electron microscopy to inves-
tigate the clot surface (in myocardial infarction) highlight the
less-investigated issue of the relation between the clot and
the endothelium.
12
EARLY POSTISCHEMIA TIME:
THE ISCHEMIC CASCADE
e ischemic cascade is represented by a complex series of
interlinked molecular and cellular mechanisms that contrib-
ute to ischemic cell death via necrosis or apoptosis (Fig. 2).
e primary insult aer arterial occlusion is hypoperfusion,
which dramatically reduces the availability of both glucose
and oxygen in the brain, with particular vulnerability to isch-
emic injury being evident in specific regions: the caudate
body, putamen, insular ribbon, paracentral lobule, precen-
tral, and middle and inferior frontal gyri.
13
This situation
contributes to bioenergetic failure by stopping or slowing ad-
enosine triphosphate (ATP) production.
14,15
A few minutes
after an arterial occlusion, an ionic imbalance occurs with
the abnormal inux of Na
+
and eux of K
+
, contributing to a
widespread anoxic depolarization in the membranes of neu-
rons and glial cells.
16
e increased inux of Na
+
into neu-
rons causes the osmotic transport of water into cells that
leads to cytotoxic edema, cell lysis, and necrosis. A recent
Fig. 1. Postischemic inflammation. Necrotic neurons release damage-associated molecular patterns (DAMPs), and these molecules activate mac-
rophages via pattern-recognition receptors and inflammasomes. The activated macrophages contribute to enhance the inflammatory process via
the release of proinflammatory cytokines and recruiting T-cells that contribute to maintain inflammation by interleukin (IL)-17. At several days af-
ter the acute injury, the cellular elements of the innate immune system change to an anti-inflammatory phenotype, contributing to inhibit the in-
flammation (dashed lines). In particular, anti-inflammatory cytokines (e.g., IL-10) are released. During this phase, the postischemic inflammation is
resolved by the clearance of debris as well as angiogenesis supported by the release of growth factors.
IGF: insulin-like growth factor, TGF, trans-
forming growth factor, TNF: tumor necrosis factor, VEGF: vascular endothelial growth factor.
Clearance of debris
DAMPs release
Necrotic neurons
TNF-a
IL-1b
IL-17
IL-23
Macrophage
activation
T-cells
Minutes/hours
Inflammation
Purines
VEGF
Angiogenesis
TGF-b
IGF-1
IL-20
Days/weeks
Anti-inflammatory
activation
Recovery Phase Acute Phase

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Vidale S et al.
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neuroimaging study using
23
Na MRI and quantitative histo-
chemical K
+
staining revealed heterogeneity in the rate of Na
concentration increase and in the K
+
distribution within the
ischemic core.
17
e reduced ATP production following Na/
K imbalance (due to Na/K ATPase) also contributes to re-
duce the reuptake of glutamate, which is the main excitatory
neurotransmitter.
18
is condition overstimulates the gluta-
mate receptors so as to inuence the Ca
2+
inux, producing a
series of nuclear and cytoplasmic events (excitotoxicity) that
lead to mitochondrial failure and apoptosis.
19,20
At the same
time, the Ca
2+
inux triggers the activation of catabolic en-
zymes with the production of arachidonic acid, and increases
the formation of reactive oxygen species (ROS), mainly in
neurons rather than astrocytes.
21
Previous studies found that mitochondrial failure could be
predicted from the K
+
concentration. Indeed, the mitochon-
drial ATP-dependent K
+
(mitoKATP) channel plays a critical
role in the neuroprotective action, contributing to the so-
called ischemic pre- and postconditioning states.
22
e open-
ing of mitoKATP channels attenuates the Ca
2+
overload and
inhibits the formation of free radicals and ROS that contrib-
ute to necrotic or apoptotic cell death.
23,24
e depolarization
of other neurons produces a further Ca
2+
influx and addi-
tional glutamate release, leading to local amplication of the
ischemic damage.
25
Contemporary with those processes, the
persistence of arterial occlusion contributes to a critical re-
duction of pO
2
and a concomitant increase in pCO
2
. In the
case of hypercapnia, the tissue pH could fall to around 6.6 or
lower if severe ischemia and tissue hypoxia occur; in the last
situation, anaerobic glycolysis leads to lactic acid accumula-
tion with signs of irreversible injury identifiable in the cell
morphology.
26
e acidosis state increases necrosis and cell
death via a mechanism called acidotoxicity and mediated by
Ca
2+
-permeable acid-sensing ion channels.
27-29
Other delete-
rious eects of acidosis inuence the synthesis and degrada-
tion of cellular constituents, the mitochondrial function, the
cell volume control, the postischemic ow, and the stimula-
tion of ROS production, all conditions that occur also in the
ischemic penumbra.
30
Acidosis and ROS contribute to trig-
ger a subsequent and concomitant phase represented by the
Fig. 2. Cerebral ischemic cascade. AMPA:
α
-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, BAD: Bcl-2-associated death promoter, BBB:
blood-brain barrier, COX: cyclo-oxigenase, IL: interleukin, NMDA: N-methyl-D-aspartate, TNF: tumor necrosis factor.
Arterial
occlusion
Hypoperfusion
Anaerobic glycolisis
Na/K-ATPase pump failure
Cell membrane depolarization
Excitotoxicity
glutamate release
Activation of
NMDA/AMPA
Activation
kinase and proteinases
NO synthase
activation
H2O
accumulation
Acidosis
Membrane
degradation
Cell adhesion
molecules
expression
Leukocyte
infiltration
Activation
COX2
BBB disruption
BAD, BAX
activation
Arachidonic acid
production
Cytotoxic
edema
Cell lysis
Production cytokines
(IL-6, TNF-a)
Apoptosis
Necrosis
NO
Mithocondrial
failure
Intracellular
Na, Ca
Free radicals
Oxidative stress
Inflammation

4 J Clin Neurol 2017
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activation of innate immunity and involving both resident
cells (microglia) and circulating cells.
INFLAMMATORY AND INNATE
IMMUNITY ACTIVATIONS
Inammation in the ischemic brain:
cell inltration and damage
Postischemic inammation begins in the vascular compart-
ment immediately aer arterial occlusion. e production of
ROS leads to an increase in the procoagulant state involving
the activation of complement, platelet, and endothelial
cells.
31,32
The increased activity of cyclooxygenase-2 in in-
ammatory cells and neurons may lead to tissue damage due
to excessive ROS production and toxic prostanoids.
33,34
ROS
contribute also to reduce the availability of NO, leading to
platelet aggregation and the adhesion of leukocytes, which
aggravate the ischemic damage.
35
ere is evidence of iNOS
(the inducible isoform of NO) being a critical effector and
amplier of tissue damage related to postischemic inamma-
tion (Table 1).
36
e oxidative stress and inammatory medi-
ators affect the permeability of the BBB, impairing the so-
called neurovascular unit that includes endothelial cells,
astrocytes, and neurons [involving matrix metalloproteinases
(MMPs) or ROS], and allowing the extravasation of proteins
and endothelial cells along with the activation of macro-
phages and mast cells via the ischemia and reperfusion mech-
anisms.
37
This extravasation is also supported by a break-
down of the BBB−secondary to the pericyte death−leading
to a long-lasting decrease in capillary blood ow.
38
A few minutes aer arterial occlusion and the associated in-
tra- and extracellular modications, the acute local damage is
detected also by pattern-recognition receptors (PRR) (includ-
ing Toll-like receptors) that respond to microbial structures
(pathogen-associated molecular patterns) and host-derived
danger signals (damage-associated molecular patterns).
39-41
ese molecules can be released by stressed cells, such as dur-
ing the ischemic cascade. ere is new evidence that the PRR
of neurons and glial cells can play a fundamental role in acti-
vating intracellular signaling pathways so as to enhance the
proinammatory expression of dierent genes (Fig. 3).
42,43
is
mechanism activates the immune system elements, resulting
in mast cells releasing vasoactive mediators (e.g., histamine),
proteases, and tumor necrosis factor (TNF), while macro-
phages release proinammatory cytokines.
44
Adhesion-molecule P- and E-selectins (Table 2) and inter-
Fig. 3. DAMP receptors and signaling pathways. Cells detect DAMPs via DAMP receptors in two ways: (1) activation of a type of pattern-recogni-
tion receptor [Toll-like receptor (TLR)] and (2) activation of inflammasomes. The first mechanism involves proinflammatory factors being released
by the nuclear gene expression mediated by transcriptional mediators activated by TLR. The second mechanism involves the activation of cas-
pase-1 leading to the clivation of the proinflammatory cytokines IL-1 and IL-18, converting them into their activated forms. DAMP: damage-asso-
ciated molecular pattern, IL: interleukin, NLRP: nod-like receptor pyrin.
DAMPs
TLR
Transcriptional mediators
Pro-inflammatory
gene expression
Pro-caspase 1
Nucleus
Pro-IL-1
Pro-IL-18
IL-1
IL-18
NLRP and 3
Caspase 1
Inflammasome

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Vidale S et al.
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cellular adhesion molecule-1 are expressed on the membrane
aer their intracellular translocation and with the rapid gen-
eration of proinammatory signals.
45,46
e adhesion recep-
tors mediate interactions between adhesion molecules and
integrins, contributing to an initial rolling mechanism of leu-
kocytes and leading to adhesion to the endothelium and to a
subsequent transmigration into the brain parenchyma (leu-
kocyte infiltration). Following ischemia, these cells rapidly
release proinammatory mediators into the area, which pro-
motes ischemic injury via dierent pathways: 1) the cerebral
no-reow phenomenon by impeding the ow of red blood
cells, 2) increased production of ROS and proteases at the
endothelium surface, 3) platelet aggregation by leukotriene,
prostaglandin, or eicosanoid production due to activation of
leukocyte phospholipases, and 4) deleterious activity of pro-
inammatory cytokines in the penumbra area. In particular,
during the acute phase of brain ischemia, microglia and
macrophages release interleukin (IL)-1b, IL-6, and IL-18
from the transcriptional intracellular pathways activated by
nucleotides from injured cells.
47
ese cytokines contribute
to leukocyte inltration in the damaged tissue, and they acti-
vate the presentation of antigens between dendritic cells and
T-cells.
48
T-cells lead to tissue damage by innate immunity,
through interferon-gamma and ROS. T-cells activated by IL-
23 released from microglia and macrophages produce IL-17,
which worsens the acute ischemic cerebral injury.
49
is un-
balanced interplay between the immune and sympathetic
nervous systems contributes to an early down-regulation of
systemic cellular immune responses that leads to a functional
deactivating of monocytes, T-helper cells, and invariant nat-
ural-killer T-cells.
50
Previous studies have demonstrated the predictive role of
certain acute immune and stress biomarkers on clinical out-
comes: copeptin and mid-regional proatrial natriuretic pep-
tide.
51,52
Even if the immune system is activated with recruit-
ing elements in the focal ischemic area, specific injured
cerebral sites contribute to dierent down-regulating respons-
es being exhibited by the autonomous nervous system. In
particular, involvement of the right frontoparietal cortex, in-
sula, or brainstem could lead to increased cerebral inamma-
tion and concomitant systemic immunosuppression.
53
This
process is characterized by the increased apoptosis of lym-
phocytes, suppression of peripheral cytokine release, and -
Table 2. Mediators of initial postischemic inflammation
Mediator Type Producing cell
P-selectin Adhesion molecule EC, PLT
IL-1b Cytokine MG, PVM, MC
IL-1a Cytokine PLT
TNF-a Cytokine MC
CCL5 Chemokine
CXCL4 Chemokine
CXCL7 Chemokine PLT
CX3CL1 Chemokine Neurons
Elastase Protease
MMP8 Protease
MMP9 Protease
MT6-MMP Protease Leuk
Clotting factors Protease Plasma
Complement Protease Plasma, EC, neuron
Prostanoids Small molecule EC, PLT, MG, neurons
Leukotrienes Small molecule EC, PLT, MG, neurons
ATP Small molecule Plasma, neurons
Free radicals Small molecule
EC, PLT, Leuk, PVM, MG,
neurons
Modified from Iadecola et al. Nat Med 2011;17:796-808, with permis-
sion of Springer Nature.
45
ATP: adenosine triphosphate, CCL: chemokine ligand, CX: d-chemokine
ligand, EC: endothelial cell, IL: interleukin, Leuk: leukocytes, MC: mast
cells, MG: microglia, MMP: matrix metalloproteinase, PLT: platelets,
PVM: perivascular macrophages, TNF: tumor necrosis factor.
Table 1. Mediators of amplification of postischemic inflammation
Mediator Type Producing cell
ICAM1 Adhesion molecule EC, Leuk, PVM, MG, AG
VCAM-1 Adhesion molecule EC, Leuk, PVM, MG, AG
P-selectin Adhesion molecule EC, Leuk, PVM, MG, AG
E-selectin Adhesion molecule EC, Leuk, PVM, MG, AG
Mac-1 Adhesion molecule EC, Leuk, PVM, MG, AG
VLA-1 Adhesion molecule EC, Leuk, PVM, MG, AG
IL-1 Cytokine EC, neurons, PVM, MG, AG
IL-6 Cytokine EC, neurons, PVM, MG, AG
IL-10 Cytokine EC, neurons, PVM, MG, AG
IL-17 Cytokine EC, neurons, PVM, MG, AG
IL-20 Cytokine EC, neurons, PVM, MG, AG
TNF Cytokine EC, neurons, PVM, MG, AG
CCL2 Chemokine EC, neurons, PVM, MG, AG
CCL3 Chemokine EC, neurons, PVM, MG, AG
CCL5 Chemokine EC, neurons, PVM, MG, AG
CXCL2/3 Chemokine EC, neurons, PVM, MG, AG
CXCL8 Chemokine EC, neurons, PVM, MG, AG
MMP2 Protease Circ, EC, AG, neurons
MMP9 Protease Circ, EC, AG, neurons
Complement Protease Circ, EC, AG, neurons
iNOS Other mediator MG, Leuk, EC
COX-2 Other mediator Neurons, MG, Leuk, EC
Modified from Iadecola et al. Nat Med 2011;17:796-808, with permis-
sion of Springer Nature.
45
AG: astroglia, CCL: chemokine ligand, Circ: plasma, COX: cyclo-oxige-
nase, CX: d- chemokine ligand, EC: endothelial cell, ICAM: intercellular
adhesion molecule, IL: interleukin, iNOS: inducible isoform of NO, Mac:
macrophage antigen, MG: microglia, MMP: matrix metalloproteinase,
PVM: perivascular macrophages, TNF: tumor necrosis factor, VCAM:
vascular cell adhesion molecule, VLA: very late antigen.