Current Pharmaceutical Design, 2008, 14, 3549-3564 3549

1381-6128/08 $55.00+.00 © 2008 Bentham Science Publishers Ltd.

Inflammation as Therapeutic Objective in Stroke

Joaquín Jordán1,*, Tomás Segura

2, David Brea

3, Maria F. Galindo

4 and José Castillo

3

1Grupo de Neurofarmacología. Departamento de Ciencias Médicas. Facultad de Medicina. Universidad Castilla-La

Mancha. Centro Regional de Investigaciones Biomédicas. Spain; 2Servicio de Neurología. Complejo Hospitalario Uni-

versitario de Albacete. Albacete. Spain; 3Department of Neurology, Clinical Neuroscience Research Laboratory, Hospi-

tal Clínico Universitario de Santiago de Compostela, University of Santiago de Compostela. Spain and 4Unidad de Neu-

ropsicofarmacología Translacional. Complejo Hospitalario Universitario de Albacete. Albacete. Spain

Abstract: Ischemic stroke is the most frequent cause of persistent neurologic disability in modern Western societies. Al-

beit it is still not clear whether inflammation is merely an epiphenomenon or rather has a disease-promoting function, ac-

cumulating evidence implicates inflammation in many forms of acute neurodegenerative disorders including ischemia.

The immune cell influx during a neuropathological event is thought to be elicited by glial cells, especially microglia. This

article reviews the cellular and molecular pathways involved in stroke-induced inflammatory response in the CNS. We fo-

cused on how CNS innate immune cells including microglia and macrophages play integral roles in receiving and propa-

gating inflammatory signals, and how activated microglia secrete a wide range of factors. We present the relevance of the

expression of adhesion molecules after ischemia including selectin, immunoglobulin superfamily, integrins, and the role

of inflammatory mediators such as cytokines, chemokines and matrix metalloproteinases. Further, we explore the role of

transcription factors in inflammation, and the function of immunomodulation and innate and adaptive immunity in brain

ischemia, focusing on immunosupression therapies for acute stroke. Although several approaches for anti-inflammatory

treatment have proven effective in animal models, clinical trials of immune system modulation therapy after stroke have

not yet proved successful. There is still much to be done in order to translate interesting findings into therapies, but un-

doubtedly studying the cellular and molecular pathways may not only improve our understanding of inflammatory

mechanism but also serve as a basis for designing effective therapies.

1. INTRODUCTION

Stroke is the second leading cause of death and the bur-den of disease in high income countries [1]. Stroke occurs due to a loss of blood supply to part of the brain, initiating the ischemic cascade. As oxygen or glucose becomes de-pleted in ischemic brain tissue, the production of high energy phosphate compounds such as adenosine triphosphate (ATP) fails leading to failure of energy dependent processes (such as ion pumping) necessary for tissue cell survival. This sets off a series of interrelated events that result in cellular injury and death by necrosis. Dead cells by necrosis produce the release of all cytoplasmatic content into the extracellular space activating the corresponding inflammatory response.

The central nervous system (CNS) has for long been re-garded as an immune privileged organ, with the blood–brain barrier (BBB) tightly regulating the influx of immune cells and mediators from the vascular compartment to the brain parenchyma [2]. Inflammation is generally a beneficial re-sponse of an organism to infection but, when prolonged or inappropriate, it can be detrimental. Neuronal loss in acute (e.g. stroke and head injury) and chronic [e.g. multiple scle-rosis and Alzheimer's disease] CNS diseases has been asso-ciated with inflammatory processes systemically and in the

*Address correspondence to this author at the Grupo de Neurofarmacologia,

Departamento de Ciencias Médicas, Facultad de Medicina, Universidad

Castilla-La Mancha. Centro Regional de Investigaciones Biomédicas, Avda

Almansa, 14, 02006-Albacete, Spain; Tel: 34-967599200; Fax: 34-967-

599327; E-mail: Joaquin.jordan@uclm.es

brain. Brain inflammation is characterized by activation of microglia and astrocytes, expression of key inflammatory mediators, but limited invasion of circulating immune cells. Inflammation induces rapid expression of key inflammatory mediators -cytokines, chemokines and prostaglandins- which in turn up-regulate adhesion molecules, increase permeabil-ity of the BBB, facilitating invasion of peripheral immune cells, induce release of potentially toxic molecules and com-promise brain cells. Because the BBB is disrupted after stroke, the immune system comes into contact with CNS antigens, in both the brain and periphery [3].

In recent years, major advances in the study of the role of the immune system and inflammation in brain ischemia have been done. There are many evidences that inflammation and immune response play an important role in the outcome of ischemic stroke patients, and they have been associated to larger brain damage. However, on the other hand, these mechanisms might be necessary for the resolution of dead cells and the initiation of repairing mechanisms. In this work we review the inflammatory response to brain ischemia, the role of innate immunity, and the potential role of endogenous anti-inflammation and immune-modulation processes.

2. CELLULAR INFLAMMATORY RESPONSE

The CNS innate immune cells including microglia and macrophages play integral roles in receiving and propagating inflammatory signals. Microglia are a highly responsive population of cells with a well established role in regulating the immune surveillance of the nervous system [4,5]. Found

3550 Current Pharmaceutical Design, 2008, Vol. 14, No. 33 Jordán et al.

scattered throughout the CNS, microglia constitute 5–15% of the total brain cell population and form a meshed network able to detect and react to modifications of the local envi-ronment [6]. In the mature brain, microglia typically exist in a resting state characterized by ramified morphology, and monitor the brain environment. The CNS endogenous mi-croglia share many properties with macrophages, having developed from a common precursor cell [7-9]. Early in fetal development, monocyte-like cells infiltrate the CNS and develop into parenchymal microglia. In the adult, parenchy-mal microglia in contrast to perivascular “microglia” are not frequently repopulated by fresh monocytes [10].

Microglia are primarily involved in immune surveillance [6, 11], but when activated have macrophage-like capabili-ties including phagocytosis, inflammatory cytokine produc-tion, and antigen presentation [12]. Normally these neuroin-flammatory changes are transient with microglia returning to a resting state as the immune stimulus is resolved. Aging or neurological disease, however, may provide a brain envi-ronment where microglia are more “reactive or primed” to a peripheral immune challenge [13]. In response to certain cues such as brain injury or immunological stimuli, however, microglia are readily activated. Activated microglia secrete a wide range of factors, some of which can actively trigger apoptosis in neuronal cell cultures. At the same time, micro-glia are also reported to increase neuronal survival through the release of trophic and anti-inflammatory factors.

Features common to microglia and systemic macro-phages include the expression of innate immune receptors and the ability to phagocyte pathogens, cells or cellular de-bris [7, 14]. Microglia is activated within minutes of ische-mia onset and produces a plethora of inflammatory media-tors, which exacerbate tissue damage [15]. Furthermore, mi-croglia responses are segregated not only respect to time, but some extend also spatially. After focal cortical ischemia, there is a second involvement of the ipsilateral thalamus at-tributable to retrograde degeneration of thalamo-cortical pro-jection fibres [16]. The PET tracer

11C-PK11195 binds to the

peripheral benzodiazepine receptor, which is abundant on brain-derived activated microglia, and that has been em-ployed in experimental and clinical studies addressing ischaemia-related inflammation, has shown that microglial activation becomes significant after a few days of stroke and persists for over 30 days, with a peak around 2 weeks for stroke onset [17]. Notably, the spatial distribution of

11C-

PK11195 binding evolves to include not only the core but also the rescued penumbra where inflammation may be sec-ondary to selective neuronal damage [18].

Recently it has been shown that patients with acute stroke had significantly better outcome with minocycline treatment compared with placebo [19], by involving minocycline anti-inflammatory properties, especially its ability to suppress activation of microglia, are likely to contribute to cytoprotec-tion in the CNS [20].

3. EXPRESSION OF ADHESION MOLECULES AF-TER ISCHEMIA

Inflammation after stroke involves leukocytes infiltration in brain parenchyma, specially neutrophils, that contribute to cerebral damage after ischemia [21] through reperfusion or

secondary injury mechanisms. Infiltration through the endo-thelium involves rolling, adhesion and transendothelial mi-gration of leukocytes. Therefore, adhesion molecules in leu-kocytes and endothelial cells are key molecules that contrib-ute to cerebral damage. Adhesion molecules that participate in this process are classified in three types; selectins, the superfamily of immunoglobulin and integrins.

3.1. Selectins

L-selectin, E-selectin and P-selectin are glycoproteins involved in the initial interaction between leucocytes and endothelial cells in the periphery of the infarct. They interact with P-selectin glycoprotein ligand (PSGL1) and other gly-cosylated ligand [22], to induce a transitory and reversible action that leads to other secondary cellular interactions me-diated by a different group of adhesins. As a consequence of the activation of adhesins, there is a recruitment of leuco-cytes, taking place their aggregation and adhesion to the vas-cular wall in a later stage. Such changes are responsible for the obstruction of the microvascularization and for the phe-nomenon of ‘no-reflux’. The conversion of the endothelium into a prothrombotic state, the production of free radicals and the rise in vascular permeability are other factors responsible for cellular damage mediated by inflammation.

At the individual level, P-selectin is found in platelets [23, 24] and endothelial cells, being its counter receptor, which contains the oligosaccharide sialyl Lewis X, present in leukocytes [25]. The existence of stores of P-selectin in the cytoplasm of endothelial cell [26] allows its mobilization to the cell surface within minutes from the activation of endo-thelial cells by thrombin, complement, and histamine [27].

Knowledge about the role of P-selectin has been pro-vided from both experimental models and clinical studies. From clinical studies it has been stated that surface expres-sion of P-selectin on platelets is related to clinical worsening after acute ischemic stroke, and a positive correlation be-tween P-selectin expression and NIHSS scores has been ob-served [28]. Studies on experimental models of cerebral ischemia have revealed that the lack of P-selectin or its blockage with monoclonal antibodies [29] or inhibitors [30, 31] was associated with improved neurological outcome and reduced cerebral infarct volumes.

The role of E-selectin and L-selectin in cerebral ischemia is not so clear. E-selectin can be found in endothelial cells and leukocytes, playing an important task in the develop-ment of inflammation in vivo [32]. E-selectin is also up-regulated in microvessels of animal models of ischemia, 2 hours after reperfusion [32,33]. L-selectin is found in endo-thelial cells and leukocytes mediating on leukocyte rolling, but its role in brain ischemia is less clear.

3.2. Anti-Selectin Treatment Possibilities

Studies in P-selectin knockout mice, and alternatively functional blockade of P-selectin with a monoclonal anti-body in wild-type mice, have shown decreased infarct vol-umes after transient permanent middle cerebral artery occlu-sion (MCAO) respect to controls. Furthermore, administra-tion of sCRsLex, an inhibitor of selectin-mediated platelet-leukocyte interactions, reduces infarct volumes in experi-mental models. However results varied from focal to global

Inflammation as a Therapy for Stroke Current Pharmaceutical Design, 2008, Vol. 14, No. 33 3551

experimental models of ischemia, since P-selectin blockade or deficiency was associated to better outcome and smaller infarcts in focal ischemia, while animals treated with anti-bodies against P-selectin showed reduced survival periods in models of gobal ischemia [34].

Knowledge about E-selectin and L-selectin is not such clear, therefore treatments against these selectins are less studied. Although it has been shown that E-selectin can in-duce immune tolerance and reduce injury upon intranasal administration to animals [35]. By the other hand, L-selectin treatments with L-selectin antibody has been shown ineffec-tiveness in animal models of stroke [36].

3.3. Immunoglobulin Superfamily

The immunoglobulin superfamily includes 5 members: intercellular adhesion molecule-1 (ICAM-1), intercellular adhesion molecule-2 (ICAM-2), vascular adhesion molecule-1 (VCAM-1), platelet–endothelial cell adhesion molecule-1 (PECAM-1), and the mucosal vascular addressing cell adhe-sion molecule 1 (MAdCAM-1). All of them are expressed on activated endothelial cells, mediating in the adhesion of leu-kocytes to endothelia, and creating stronger attachments than selectins [37].

ICAM1 is constitutively expressed, although its expres-sion in cerebral microvascular endothelial cells is increased by IL-1 , TNF- , lipopolysaccharide (LPS) [38], as ob-served in in vitro models of ischemia-like insults [39]. There are some other evidences that indicate its role in cerebral ischemia. Thus, increased expression of ICAM-1 has been found in areas of cerebral ischemia in rats submitted to MCAO [40]. Knock-out mice for ICAM-1 gene presented less cerebral damage than wild-type mice subjected to cere-bral ischemia [23]. The implication of ICAM-1 in cerebral ischemia has also been demonstrated in clinical studies, and a rise in serum concentrations of soluble adhesion molecules has been found after cerebral ischemia. Soluble ICAM-1 concentrations are elevated [41] and peak in patients within 24 hours of acute ischemic stroke, being these levels corre-lated with the infiltration of polimorphonuclear leukocytes [42]. Furthermore, ICAM-1 expression in brain microvessels is significantly increased in the cerebral infarcts of patients who die from ischemic stroke [43].

Localized ICAM-1 expression has been found in his-tological studies of normal human carotid bifurcation [44], a region of high-risk for the development of atherosclerotic plaque, while endothelial ICAM-1 expression is increased in symptomatic versus asymptomatic carotid plaque [45]. Pa-tients with carotid atherosclerosis who may be described as “prestroke stage” have raised concentrations of soluble (s)ICAM-1 [46]. This is in concordance with elevated solu-ble ICAM-1 levels that were found in patients with stroke risk factors [41].

Little is known about other intercellular adhesion mole-cules such as ICAM-2, VCAM-1, or PECAM-1. ICAM-2 may serve to stimulate the ligand binding of ICAM-1 to CD11b/CD18 [29]. Significantly, ICAM-2 is expressed not only by endothelial cells but also by resting and activated platelets [29]. In contrast to the other 2 integrin ligands, ICAM-1 and ICAM-3 (CD50), which are not found on plate-

lets, ICAM-2 may be involved in the mediation of leukocyte-platelet interactions in the microvessels during inflammation and thrombosis. The role of VCAM-1 in stroke is controver-sial. While some authors have described an increase in VCAM-1 mRNA after cerebral ischemia [29], others have failed to observe significant changes [47].

PECAM-1 is expressed at constant levels and plays addi-tional roles in attaching endothelial cells among them, and in negotiating with leukocytes [37]. These data support the role of cell adhesion molecules in the tight adhesion of leuko-cytes after cerebral ischemia, including the unique role of ICAM-2 in leukocyte-platelet interactions.

3.4. Immunoglobulin Superfamily Treatment Possibilities

A few studies have evaluated the possible treatment for ischemic stroke blocking receptors of the immunoglobulin superfamily.

Anti-ICAM1 antibody treatment has been shown to di-minish cerebral damage [32], neutrophil accumulation, apop-tosis and neurological deficits [49] in animal models of cere-bral ischemia. Furthermore, experimental studies performed on cultured endothelial cells have demonstrated an increase in the expression of ICAM-1 in cells subjected to ischemia [38].

Although ICAM-1 involvement in cerebral ischemia has been demonstrated in vivo, in vitro and in clinical studies, clinical trials with anti-ICAM-1 antibody have showed the existence of side effects and no improvement in outcome at 90 days, compared to control patients. The causes of these negative results could be that the antibody was obtained from mouse, and that included patients were not recanalized.

In a study of global cerebral ischemia in rats, leukotriene receptor antagonist improved neurological deficits and re-duced neuron death by inhibiting the ischemia-induced upregulation of VCAM-1 in the hippocampus of ischemic rats [50]. However, another study showed that treatment with VCAM-1 antibodies did not have any effect on stroke out-come suggesting that VCAM-1 may not play a significant role in ischemic brain injury [51].

3.5. Integrins

Integrins are another family of adhesion molecules that consist of heterodimeric membrane glycoproteins, with a common subunit and a variable subunit, that play a role in cell-cell and in cell-extracellular matrix interactions. There are three subfamilies of subunits, denoted 1–3. Members of the 1 subfamily bind collagen, laminin and fibronectin and are involved in the structure of the extrace- llular matrix. 2 integrins (CD18) are involved in leukocyte cell adhesion, and 3 integrins, also known as the cyto- adhesins, include the platelet glycoprotein IIb/IIIa ( IIb/ 3) and the vitronectin receptor ( v/ 3), which are factors in-volved in clot generation and stabilization. At the level of the basal lamina, integrins link endothelial cells to components of the extracellular matrix, such as laminin and collagen while in the brain, integrins join the endothelial cells, astro-cytes, and basal lamina that comprise the blood-brain barrier, being crucial for the maintenance of the integrity of the cere-bral microvasculature [52]. Leukocyte integrins are activated

3552 Current Pharmaceutical Design, 2008, Vol. 14, No. 33 Jordán et al.

by chemokines, cytokines, and other chemoattractants. In order to bind leukocytes to activated endothelium, integrins must be expressed on the cell surface, to be able to recognize endothelial cell adhesion molecules. Although leukocyte rolling is believed to be mediated primarily by P-selectin and E-selectin [27], the resulting firm cohesion to the vascular endothelium requires the expression of ICAM-1 in endothe-lial cells, and the interaction with the leukocyte integrin CD11b/CD18 [53]. In vitro studies have revealed that hy-poxia causes an increase of neutrophil CD11b expression compared to normoxia, and that this injury was protected by aprotinin because of the reduction of the upregulation of neutrophil CD11b [54].

3.6. Anti-Integrin Therapy as Treatment to Reduce

Ischemic Stroke Damage

Furthermore, in in vivo studies conducted on rats sub-jected to MCAO, administration of anti-CD11b or anti-CD18 monoclonal antibodies reduced infarct volumes and apopto-sis, and was associated to decreased accumulation of neutro-phils [55,56].

Despite the positive experimental data, when this ap-proach was performed in clinical studies, anti-integrin thera-pies with antibodies against CD11/CD18 in acute stroke pa-tients resulted negative, being these studies terminated pre-maturely due to a lack of effect on predetermined endpoints [57, 58].

4. INFLAMMATORY MEDIATORS

4.1. Cytokines

Most inflammatory reactions are mediated by cytokines which may potentiate ischemic brain injury. Cytokines are a group of small glycoproteins that play a significant role as activators of adhesion molecules. In the brain there are di- fferent cell types capable to secrete cytokines such as; mi-croglia, astrocytes, endothelial cells and neurons. In addition, it has been shown that peripherally derived cytokines are involved in brain inflammation. Thus, peripherally derived mononuclear phagocytes, T-lymphocytes, natural killer (NK) cells and PMN’s, produce and secrete cytokines and might contribute to inflammation of the CNS. Cytokines are up-regulated in the brain in response of a variety of stimulus including ischemia, being IL-1, interleukin-6 (IL-6), TNF- , interleukin-10 (IL-10) and TGF- , the most studied cytoki-nes related to inflammation in stroke.

Interleukine-1 (IL-1)

Members of the family of IL-1 family are expressed at low level or undetectable levels in healthy brain but their expression is rapidly up-regulated by ischemia. IL-1 has two isoforms, one called IL-1 and the second one named IL-1 . These two isoforms and its endogenous inhibitor, IL-1 recep-tor antagonist (IL-1ra) are the most studied ones in experi-mental stroke. IL-1 acts through two different receptor types (type I and II) [50, 60]. Type I receptor can be found on a variety of cell types and it binds to both IL-1 forms. On the contrary, type II receptor can be found on the cell surface of neutrophils, type B lymphocytes and macrophages and binds IL-1 with higher affinity [61].

Both IL-1 and IL- are synthesized as precursor proteins that lack a leader sequence. As a precursor IL-1 is fully active, whereas IL-1 is inactive and needs to be cleaved to mature IL-1 by the cysteine-aspartate protease caspase 1 [62].

IL-1 is produced in the CNS by various cell types such as; microglia, astrocytes, neurons and endothelium [63]. It has been shown that IL-1 mRNA is expressed following several brain injury types including kainite excitotoxicity [64] and LPS [65]. Furthermore, rising of mRNA within 15–30 minutes after ischemia has been demonstrated [66] which leads to an increase in protein a few hours later [67]. Twenty minutes after transient global cerebral ischemia in rats, IL-1beta mRNA and protein expression were increased not only during early reperfusion (1 h), but also at later times (6–24 h) indicating a biphasic expression [68].

There are studies that correlate an increase on the levels of IL-1 after ischemia and a worsening of the infarct sever-ity. However, evidences that IL-1 is neurotoxic are also con-siderable though controversial. IL-1 injected into a healthy brain does not cause any overt damage. Likewise, IL-1 added directly to pure neurons in culture does not cause death. Most IL-1 effects have been described in astrocytes. IL-1 promotes astrocytes proliferation and activation, which leads to astrogliosis [69].

IL-1 in the Treatment of Stroke

IL-1 is a potent pyrogen and increased body temperature exacerbates after experimental injury and worsen prognosis in acute stroke patients [70]. All studies have demonstrated an association between IL-1 upregulation and the infarct size increment, or a bad outcome, however they only show the existence of such association yet they did not comment any therapeutic possibilities using IL-1 as therapeutic tar-get. In this context other studies have introduced some con-troversy to this issue pointing to certain neuroprotective role of IL-1 [71, 72]. Thus, pretreatment of cultures of mouse primary cortical neurons with IL-1 or IL-1ß showed an at-tenuation of neurotoxicity induced by NMDA. Such neuro-protection mediated by IL-1 resulted inhibited when a neu-tralizing antibody to neuronal growth factor (NGF) was used [71]. Additionally, treatment of rat primary cortical neuron cultures with IL-1ß attenuated neuronal death induced by exposure to excytotoxic amino acids (glutamate, NMDA, AMPA, and kainate) [72]. The neuroprotective effects at-tributed to IL-1ß seem to be partially mediated by induction of NGF. In that study [72], IL-1ß was added to culture media both before (pretreatment) and after (post-treatment) expo-sure to excitatory amino acids. It was not determined whether pretreatment alone would have conferred greater neuroprotection nor if post-treatment alone would have ex-acerbated neurotoxicity. The common factor in both studies appears to be that the neuroprotection afforded by IL-1 de-pends on exposure of cultures to IL-1 prior to injury (pre-treatment). Other studies have demonstrated that intraven-tricular injection of recombinant IL-1B after MCAO in-creases the formation of brain edema, the volume of the size and the influx of neutrophils [73]. Those authors demon-strate that these phenomena happen when IL-1 is adminis-tered to rats [73], and observed that IL-1 deficient mice pre-sented smaller infarcts in comparison with wild type mice.

Inflammation as a Therapy for Stroke Current Pharmaceutical Design, 2008, Vol. 14, No. 33 3553

Furthermore, overexpression or treatment with IL-1ra re-duced the size of the infarcts and the severity of neurologic deficits [74, 75] while IL-1ra deficient mice exhibited a dra-matic increase in ischemic damage [76].

Interleukin 6 (IL-6)

IL-6 is a plethoric cytokine with several detrimental ef-fects which may contribute to early inflammatory injury in the brain. IL-6 is involved in the regulation of neuronal apoptosis [77]. IL-6 is up-regulated following cerebral ischemia [78]. Different studies suggest that IL-6 has detri-mental effects in cerebral ischemia. Thus, raised plasma con-centrations of IL-6 are a powerful predictor for early neuro-logical deterioration [79] and are associated with greater infarct volumes [80] and bad outcome [81]. Furthermore, as demonstrated by our group, the association between IL-6 and early neurological worsening, prevails without regard to the initial size, topography, or mechanism of the ischemic in-farction [81].

Tumor Necrosis Factor- (TNF- )

In the CNS, the pro-inflammatory cytokine TNF- is considered the principal mediator of neuroinflamattion that elicits a cascade of cellular events culminating in neuronal death. TNF- orchestrates a diverse array of functions in-volved in immune surveillance and defense, cellular homeo-stasis, and protection against certain neurological insults [82]. TNF- is upregulated in the brain after ischemia. Induc-tion of TNF- mRNA has been proved to happen in ischemic cortex after both permanent [83, 84] and transient MCAO [78] in rats. Barone et al., [85] have reported that following MCAO, the induction of TNF- was associated with exacer-bation of neurological deficits and infarct size. Analysis of the temporal profile of mRNA expression of cytokines in ischemic rats have revealed that the up-regulation of TNF- mRNA is proportional to IL-1 [86] and IL-6 [87] up-regula- tion. Initial increases are seen 1-3 h after ischemia onset [83], and have a two-phase pattern of expression with a second peak at 24-36 h [88, 89]. In clinical studies it has been shown that TNF- is upregulated in the brain tissue of patients with acute cerebral infarction [90], and appears sequentially in the infarction core and peri-infarct areas before it is expressed in the contralateral hemisphere and other remote brain areas [91]. Concentration of TNF- in cerebrospinal fluid (CSF) are increased in patients with acute ischemic stroke [79], including those with pronounced white matter lesions [92]. Serum concentrations of TNF- are also increased in most studies with acute ischemic stroke patients [79, 93] and raised TNF- concentrations in plasma of patients suffering from lacunar infarctions are associated with early neurologic deterioration and poor functional outcome [94].

TNF- in the Treatment of Stroke

There are a considerable number of studies about the neurotoxicity of TNF- but its role is still controversial [95]. Some of these studies support the deleterious effects of TNF-

in experimental stroke studies. Inhibition of TNF- reduces ischemic brain injury [96], while administration of recombi-nant TNF- protein after stroke onset worsens ischemic brain damage [85]. TNF- binding protein, an endogenously pro-duced inhibitor of TNF- signaling, is a soluble protein pro-duced by cleavage of the TNF- extracellular binding do-

main of its membrane-bound receptor [97]. The action of TNF- can be blocked using specific TNF- neutralizing antibody (TNF- ab) or TNF- binding protein, that binds TNF- and prevents it from interacting with its receptors. Administration of TNF- ab or TNF- binding protein [98] after cerebral ischemia has demonstrated to be beneficial [99]. TNF- deficient mice showed a clear reduction of the infarction, compared to wild-type mice, while infusion of TNF- worsen infarct volume in focal cerebral ischemia [85]. However, TNF- may also protect the brain under cer-tain circumstances. TNF- appears to be involved in the ischemic tolerance process [100], since TNF-receptor defi-cient mice present larger infarcts than wild-type ones [101].

The biological actions of TNF- are mediated through two distinct cell surface receptors, receptor 1 (TNFR1, p55) and TNF receptor 2 (TNFR2, p75), to which it exhibits fairly equal affinity. Most effects induced by TNF- are mediated by TNFR1, which contains a death domain (DD) that inter-acts directly with TNFR1 and may act as a bifurcation point for signaling related to cell death or cell survival. The differ-ential patterns of localization of TNF receptors on neuronal or glial cells, their expression profile and activational state on these cells, and the down-stream effectors that they acti-vate, are thought to play a critical role in determining if TNF- will have protective or cytotoxic role [102].

Ischemic preconditioning causes up-regulation of neu-ronal TNFR1, and intracerebral administration of TNFR1 antisense oligodeoxynucleotide, which causes a reduction in TNFR1 expression, inhibits the ischemic preconditioning-induced protective effect, suggesting that TNFR1 up-regulation is implicated in the phenomenon of ischemic tol-erance [101]. Finally preconditioning with TNF- also ap-pears to be neuroprotective in ischemic cerebral injury. In-tracisternal administration of TNF- significantly reduced infarct size and decreased microglial activation in a MCAO model of cerebral ischemia [103].

Interleukin 10 (IL-10)

Interleukin-10 is an anti-inflammatory cytokine, mainly secreted by lymphocytes and monocytes/macrophages, which acts by inhibiting IL-1 and TNF- , and by suppressing cytokine receptor expression and receptor activation as well. IL-10 is synthesized in the CNS and is up-regulated in ex-perimental stroke [104]. In acute ischemic stroke, elevated concentrations of IL-10 in CSF have also been found [105]. Clinical data have shown that subjects with reduced produc-tion levels of IL-10 have an increased risk of stroke, support-ing a protective role for this cytokine [93]. In addition, we have demonstrated that low plasma concentrations of IL-10 (<6 pg/ml) are associated with clinical worsening, independ-ently of hyperthermia, hyperglycemia or neurological condi-tion on admission [106].

IL-10 could be a Potential Anti-Inflammatory Therapy for Stroke

Since IL-10 has been shown as an anti-inflammatory cytokine, exogenous administration of this cytokine could be a possible therapeutic strategy to reduce brain damage after stroke. This strategy was showed to be a good approach in animal models. IL-10–deficient mice have an increased stroke lesion size after MCAO [107], and administration

3554 Current Pharmaceutical Design, 2008, Vol. 14, No. 33 Jordán et al.

[108] and gene transfer of IL- 10 [109] in models of cerebral ischemia seems to have beneficial effects independently of stroke subtype [110].

Transforming Growth Factor- (TGF- )

Increased expression of TGF- mRNA has been shown in ischemic tissues 1-6 hours after ischemic insult in rodent models [111], remaining elevated up to 15 days [112]. Such expression may coincide with the influx of monocytes and macrophages and with microglial proliferation in injured tissues [113]. In this context, some experimental studies have tried to elucidate the potential neuroprotective or neurotoxic role of TGF- in ischemic stroke.

TGF- could be a Neuroprotectant Drug in Stroke

Overexpression of TGF- using an adenoviral vector resulted in mouse brain protection from ischemic stroke and in a reduction of the accompanying inflammatory response [114]. Another study has shown that TGF- protect cultured neurons from ischemia-like insults [115]. In addition, TGF- has demonstrated its neuroprotective role when administered before ischemic insult [116]. Other authors have reported both beneficial and insignificant effects when TGF- is ad-ministered after an ischemic insult [117]. It has been pointed out that results could be dependent on the TGF- administra-tion site since one study showed that TGF- reduced infarct volume when administered into the penumbra area of rats, 1 hour after MCAO, while its neuroprotective effect was ab-sent when injected in the core area of the infarct [118].

It has been proposed that TGF- could be neuroprote- ctant by blocking apoptosis, or that participates in the reco- very of ischemic stroke because its effect is visible in the penumbra area and it is present in the recovery phase of some CNS diseases [119].

4.2. Chemokines

Chemokines are a class of cytokines, generally small ones, which incite neutrophils and macrophages to migrate toward the source of the chemokine. They play important roles in cellular communication and inflammatory cell re-cruitment. Expression of chemokines after cerebral ischemia is thought to be deleterious by increasing leukocyte infiltra-tion [120]. In this context, levels of a variety of chemokines such as monocyte chemoattractant protein-1 (MCP-1), IL-8 and macrophage inflammatory protein-1 (MIP-1 ), have been found to increase in animal models of ischemia, and its inhibition or deficiency has been associated with reduced injury [121]. MCP-1 is a potent chemoattractant of mono-cytes and its expression induces an increment of monocytes infiltration in cerebral parenchyma after ischemia. A signifi-cant increase of MCP-1 levels in CSF was found in patients with acute ischemic stroke [122].

In addition to their chemotactic properties, it has been found that chemokines affect the permeability of the blood brain barrier (BBB) and thus, addition of MCP-1 enhanced 17-fold the permeability of the barrier in an in vitro model, suggesting their implication in the opening of the BBB in cerebral ischemia [123]. Finally it has been proposed that chemokines could have an important role in homing stem cells to injured regions and could also be involved in marrow derived stromal cell migration into ischemic brain [124].

On the other hand, chemokines are also signaling mole-cules that down regulate microglia activity. On such mole-cule is fractalkine (CX3CL1), which is primarily expressed by neurons and which has been shown before to inhibit se-cretion of pro-inflammatory cytokines by activated microglia [125]. After transient focal cerebral ischemia, fractalkine-deficient mice had a reduction in infarction size and lower mortality rate, when compared to wild-type littermates [126]. Fractalkine acts through its G-protein-coupled receptor CX3CR1 and may participate in the activation and chemoat-traction of microglia into the infarcted tissue. Fractalkine contributes to the control of leukocyte trafficking from blood vessels into the injured area. After ischemia, fractalkine im-munoreactivity was strongly increased in morphologically intact cortical neurons of the ischemic penumbra and its syn-thesis was also induced in endothelial cells of the infarcted area after ischemia. CX3CR1 expression was detected in the activated microglial cells of the ischemic tissue after ische-mia, and became strongly up-regulated in macrophages/ phagocytic microglia inside the infarted tissue after ischemia [127]. Indeed, Lavergne et al., proposed that the extra adhe-sion of monocytes observed in individuals carrying rare al-leles of CX3CR1 may favour mechanisms leading to stroke [128].

4.3. Cyclooxygenase (COX) and their Possible role in

Stroke Therapy

There are two isoforms of COX. COX-1 is constitutively expressed in many cells types, including microglia and leu-kocytes during brain injury [129]. COX-2 is constitutively expressed in excitatory neurons, whereas in many organs its expression is regulated by a variety of stimuli, such as in-flammatory mediators or mitogens [130].

It has been shown that COX-1 deficient mice have in-creased vulnerability to focal brain ischemia [131], although COX-1 inhibition increased the number of healthy neurons in hippocampus in transient global ischemia [132]. These discrepancies could be due to differences in the focal versus the global ischemic models.

COX-2, the rate-limiting enzyme of prostanoid synthesis, is associated with the production of free radicals and toxic prostanoids and is induced during inflammation and cerebral ischemia. COX-2 is up-regulated 12-24 hours after ischemia [133] and it is expressed in neurons and vascular cells in the border of the ischemic territory [134] and in other cerebral zones including remote regions from the infarct [135]. It has been proposed that COX-2 metabolites are deleterious in cerebral ischemia. Furthermore, COX-2 inhibitors treatment has been demonstrated to improve neurological outcome after cerebral ischemia [134,136], and COX-2 deficient mice have reduced injury after NMDA exposure [131], whereas COX-2 overexpression exacerbates brain injury [137].

4.4. Nitric Oxide (NO) and Nitric Oxide Synthase (NOS)

and their Potential as Therapeutic Targets

NO is an important signaling molecule involved in physiological processes such as neuronal communication, host defense, and regulation of the vascular tone [138]. This relatively stable gas readily diffuses into cells and cell mem-branes where it reacts with molecular targets. NO is synthe-

Inflammation as a Therapy for Stroke Current Pharmaceutical Design, 2008, Vol. 14, No. 33 3555

sized by nitric oxide synthetase (NOS). There are three known isoforms of NOS; 1) neuronal NOS (nNOS, NOS I), localized in particular groups of neurons; 2) inducible NOS (iNOS, NOS II), that is induced during pathological states, associated to inflammation, and 3) endothelial NOS (eNOS, NOS III), mainly found in endothelial cells, [139]. eNOS and nNOS are constitutively expressed and are regulated by in-tracellular calcium while iNOS is inducible, and it is not regulated by intracellular calcium.

NO may cause DNA damage in cerebral ischemia trough the formation of peroxynitrite [140], but its presence at nor-mal levels is also important. The beneficial or detrimental effects of this molecule will depend on where and when is expressed [141]. After induction of ischemia, vasodilator effect of NO produced by eNOS, is beneficial because in-duces vasodilatation and limits blood flow reduction [142]. However, when ischemia has been developed, NO produced by iNOS contribute to brain injury [141].

iNOS is expressed in the postischemic brain, reaching its peak level in infiltrating cells 48 hours from the onset. It has been demonstrated that its expression is detrimental and therefore its inhibition has yield in reduced infarct volumes and reduced neurological deficits [143, 144]. Furthermore, knock-out mice for iNOS gene have smaller infarcts than wild type mice when they are submitted to MCAO [143]. All these studies demonstrate the deleterious role of iNOS in cerebral ischemia, providing evidence that iNOS could be a therapeutic target for ischemic stroke.

4.5. Matrix Metalloproteinases (MMPs)

These proteases are a family of over 20 endopeptidases and during the development they play a central role in the modulation of extracellular matrix allowing neurite out-growth and cellular migration. MMPs are responsible for remodeling the extracellular matrix and that can degrade all its constituents. MMPs are secreted as pro-enzymes that need to be activated. Tissues contain inhibitors to their ac-tion, such as 2-macroglobulin and tissue inhibitors of me- talloproteinase.

MMP-2 (gelatinase A) and MMP-9 (gelatinase B) have been implicated in cerebral ischemia [145]. Elevated MMP-9 levels were found in brain tissue and in serum from patients with acute ischemic stroke [146], and had been claimed to be responsible for the rupture of the BBB, leading to the devel-opment of vasogenic edema and facilitation of hemorrhagic transformation of infarctions [147, 148].

The relationship between inflammatory response and expression of the MMPs is being revealed nowadays. Both IL-6 and TNF- are cytokines capable of expressing MMP-9. The gene promoter region of MMP-9 contains a union place for the activated protein 1 (AP-1) and for the NF- B that respond to a large number of inflammatory stimulants. The immediate response genes (c-fos and c-jun) form the heterodimer AP-1 that activates the MMP-9 gene [149].

MMPs as Possible Therapeutic Agents or Targets

Inhibition of MMPs in experimental models of ischemia have been shown to reduce infarct size and brain edema [150]. MMP-9 deficient mice had smaller infarcts than wild-type controls subjected to ischemia [151]. Since transplanted

mice with bone marrow obtained from MMP-9 deficient mice had smaller injury than mice transplanted with cells containing MMP-9, it is considered that such deleterious effects are due to MMP-9, derived from peripheral inflam-matory cells.

However despite its deleterious effects, it is thought that MMP has potential beneficial effects in ischemic stroke, since its elevation in later phases of cerebral ischemia seems to be related to plasticity and recovery. Since, although an early increase in MMPs has been associated with the BBB breakdown and aggravation of ischemic injury, delayed ex-pression MMPs in the peri-infarct cortex has been associated with neurovascular remodeling and stroke recovery [152]. MMP-9 is associated with several growth factors such as vascular endothelial growth factor (VEGF), which is in-volved in angiogenesis. Furthermore, increased ischemic injury was observed at 14 days after MCAO, in rats treated with MMP inhibitors [152].

5. ROLE OF TRANSCRIPTION FACTORS IN IN-FLAMATION

Largely, inflammation promotes an orchestral response involving the rapid upregulation and activation of a variety of genes. Cerebral ischemia induces massive changes in gene expression. Recent studies have shown that transcription factors like P53, peroxisome proliferator-activated receptors (PPARs), interferon regulatory factor (IRF)-1, signal trans-ducer and activator of transcription (STAT)-3 and nuclear factor (NF)-kappaB promote inflammatory gene expression and thus precipitates severe neuronal damage.

The p53 tumor suppressor gene is a sequence-specific transcription factor that activates the expression of genes engaged in promoting growth arrest or cell death in response to multiple forms of cellular stress [153]. In the mature ner- vous system, numerous studies indicate that p53 plays a key role in neuronal death following certain number of insults including ischemia. PPARs are ligand-activated transcription factors of the nuclear hormone receptor superfamily. The 3 PPAR isoforms (alpha, delta/beta and gamma) are known to control many physiological functions including glucose ab-sorption, lipid balance, and cell growth and differentiation. Of interest, PPAR-gamma activation was recently shown to mitigate the inflammation associated with chronic and acute neurological insults. After focal ischemia, PPAR-gamma expression was observed to be increased in the brain, espe-cially in the peri-infarct area. Nuclear factor kappa-B (NFkB) is one of the most important transcription factors playing a pivotal role in mediating inflammatory responses to a variety of signals, including inflammatory cytokines, bacterial and viral products, oxidative stress, hypoxia-reoxygenation and irradiation.

As the transcription of inflammatory genes is the first step of any inflammatory cascade, therapies that target the pro-inflammatory transcriptional events will potentially curtail inflammation at the very beginning of the signalling process.

5.1. Transcription Factors as Neuroprotectant Targets in Stroke

Transcription factors are being studied as molecular tar-gets for therapeutic repair, since they intricately regulate a

3556 Current Pharmaceutical Design, 2008, Vol. 14, No. 33 Jordán et al.

variety of genes that modulate cellular functions. Transcrip-tional activation can be viewed as a double-edged sword since individual transcription factors can induce either neu-roprotective or neurotoxic genes. p53-deficient mice show reduced neuronal death after ischemia [154] and the an-tisense knockdown of p53 resulted in a significant increase in neuronal survival after ischemia [155]. Moreover, inhibi-tion of NF B signaling pathway has been associated with alteration in activity-dependent synaptic plasticity, suggest-ing that that the NF B signaling pathways are actively in-volved in modulation of important neuropsychological func-tions such as synaptic remodeling and plasticity [156].

PPAR-alpha and the PPAR-gamma agonists protect against stroke and that this beneficial outcome is associated with improved endothelial relaxation, reduced oxidative stress and decreased VCAM1 and ICAM1 expression. In adult rodents, pre-treatment with rosiglitazone or pioglita-zone one day prior to ischemia resulted in decreased micro-glial activation and macrophage infiltration, as well as de-creased expression of proinflammatory COX2, iNOS and IL-1 mRNA in the ischemic hemisphere [157]. Rosiglitazone pre-treatment in both rats and mice also significantly de-creased the infarct volume following focal ischemia and this effect was completely reversed when a specific PPAR-gamma antagonist, GW9662 was administered prior to thia-zolidinediones treatment [157]. Finally, mild hypothermia, a small decreases in brain temperature to 30-34 degrees, which inhibits inflammation after experimental stroke and brain inflammation, and results in reduced inflammatory cell infil-trate [158].

6. IMMUNOMODULATION IN STROKE

Since inflammation has an important role at all stages of the disease process and it is produced by the immune system, immune system could be a therapeutic target in the treatment of cerebrovascular disease. Secondary brain damage origi-nated by inflammation is influenced by innate and adaptive immune responses. Innate immunity represents a fast but relatively blunt inflammatory and toxic response to invading microorganisms but also interacts with several modified en-dogenous antigens. Adaptive immunity is much more spe-cific than innate immunity but may take several days or even weeks to be fully mobilized. It involves a stochastic rea- rrangement process in immunoblasts, leading to generation of a large number of T and B cell receptors and immuno- globulins, which can recognize foreign antigens.

After stroke organism suffers a series of changes some of them produce inflammation and other of them try to contain the inflammatory response. This response is known as im-munomodulation. Evidences that immunomodulation takes part in stroke is the high rate of infections in stroke patients. This process should be studied and its knowledge could help to establish new immunomodulatory therapies. In this part we review the role of the immune system and the immuno-modulation process in stroke.

7. INNATE INMUNITY IN STROKE

Innate immunity represents the first line of defense against pathogens and does not require prior exposure to foreign antigens to be activated. Macrophages, neutrophils,

dendritic cells, NK cells, are the cell types that constitute the innate immune system, which in the CNS includes microglia and perivascular macrophages. On the other hand, the induc-tion of adaptive immunity requires some signals provided by the innate immune system to facilitate the expansion of anti-gen specific T and B lymphocytes, which are important for the production of antibodies and for the formation of long-living memory cells. Unlike adaptive immunity, in which a huge number of potential antigens can be recognized by T and B cells, in the innate immune system, cells must recog-nize their cognate antigens by using a predetermined subset of germline encoded receptors. Because of their limited ex-pression of receptors, cells of the innate immune system may not be able to recognize every possible antigen. Thus, these cells focus on a few highly conserved structures, expressed by large groups of microorganisms. The conserved structural patterns are called pathogen-associated molecular patterns (PAMPs), calling pattern recognition receptors (PRRs) to the receptors of the innate immune system that recognize these structures.

The family of the Toll-like receptors (TLR) is the best characterized kind of PRRs in mammalian species. Although exact gene numbers may differ from one specie to another, it is likely that most mammals have 10 to 15 TLRs. TLRs de-tect multiple PAMPS [159]. TLR4 detects lipopolysaccha-ride (LPS), TLR2 detects bacterial lipoproteins and lipo-teichoic acids, TLR5 detects flagellin, TLR9 detects un-methylated CpG DNA of bacteria and viruses, TLR3 detects double stranded RNA, and TLR7 detects single-stranded viral RNA [160].

TLRs also detect endogenous ligands that might signal other dangerous conditions, such as the presence of degrada-tion products from macromolecules, products from prote-olytic cascades, intracellular components of broken cells and products from genes that are activated by inflammation, such as hyaluronan, fibrinogen, fibronectin, HSP60, HSP70, and others [159-164].

TLR signalling pathway has been studied above all in TLR2 and TLR4 and it has been demonstrated that is very similar to IL-1 receptor pathway. In Fig. (2) is showed a schematic representation of TLR pathway and the principal effects of its activation. TLR pathway is activated when en-dogenous or exogenous ligands interact with their receptor. This receptor has an adaptor protein named as MyD88. MyD88 interact with the kinase IRAK-1 (IL-1R associated kinase) and IRAK-2 that activates another adaptor protein, TRAF6 (TNF Receptor Activated Factor-6). This adaptor protein activates NF- B because it allows I B degradation. Therefore, NF- B is translocated to the nucleus and activates the transcription of several genes such as IL-1, TNF- , IL-6, iNOS, COX-2, ICAM-1, VCAM-1… This process starts inflammatory response [165].

TLRs have been detected in CNS, such as TLR2, TLR3, TLR4 and TLR9. All of them are expressed by microglia, astrocytes express TLR2, TLR3 and TLR4 and neurons ex-press TLR2 and TLR4 [166]. Furthermore other cells from immune system express TLRs in the CNS, such as mono-cytes, neutrophils, basophils, eosinophils, NK cells… Mono-cytes are important cells in the immune system that express TLRs and contribute to generate adaptive immune response.

Inflammation as a Therapy for Stroke Current Pharmaceutical Design, 2008, Vol. 14, No. 33 3557

Monocytes can differentiate to dendritic cells (DC) and these cells promote T cells differentiation to Th1 or Th2 [160] (Fig. 3).

So far there are only few studies that examine the role of TLRs in cerebral ischemia, and all of them suggest that TLRs are involved in the enhancement of cell damage fo- llowing ischemia and that their absence is related to lower infarction volumes in experimental models of ischemia [166-168]. Furthermore psychological stress causes an inflamma-tory response in the brain and is able to exacerbate brain damage caused by experimental stroke, and this process has been showed to be TLR4 dependent [169].

As TLR has been involved in cerebral brain damage in stroke, TLR inhibition or blocking could be a potential the- rapy to reduce ischemic brain damage. Furthermore, TLRs are a family of receptors whose activation occurs at the star- ting of inflammatory response and so their blocking could eliminate all inflammatory response.

In addition, apart from being involved in the cellular damage, it seems that TLRs could also be involved in me-

chanisms of tissue regeneration. This point has been demon-strated in studies of liver regeneration after hepatectomy, epithelium regeneration, and in spinal cord cell proliferation [170]. In this context, TLR activation promotes TNF- ex-pression. TNF- can activate two different receptors; TNFR-1 (Tumor Necrosis Factor Receptor-1) that is related with neurotoxic mechanisms and TNFR-2 (Tumor Necrosis Fac-tor Receptor-2), that is expressed by neuroblasts and whose activation promote progenitor cells to proliferate. So, it is possible that TLR activation can be involved in CNS repair-ing after cerebral ischemia. Furthermore, TLR activation promotes IL-1 expression, too. IL-1 can activate IL-1R in astrocytes and these could respond expressing neutrophins and growth factors that stimulate neuroblasts proliferation (Fig. 2). In summary, TLR activation can trigger neuroblasts proliferation in different pathways and so TLR activation could be involved in neurogenesis process.

8. ADAPTIVE INMUNITY IN STROKE

The inflammatory response of CNS after stroke is fo- llowed by at least one or two phases of T cell infiltration.

Fig. (1). Adhesion molecules expression and function in blood vessels when stroke occurs.

Fig. (2). Schematic representation of the inflammatory mediators that participates in stroke and the cells that express them.

3558 Current Pharmaceutical Design, 2008, Vol. 14, No. 33 Jordán et al.

Surprisingly, whether T cells play a beneficial or detrimental role in these processes is still controversial. T-cells are dif-ferentiated in two different subsets, T-helper1 (Th1) and T-helper2 (Th2). Th1 cells secrete proinflammatory cytokines, including interleukin-2 (IL-2), IL-12, interferon-gamma, and tumor necrosis factor-alpha, that may play a key role in the pathogenesis of stroke, whereas CD4

+ Th2 cells may play a

protective role through anti-inflammatory cytokines such as IL-4, IL-5, IL-10, and IL-13 (Fig. 3). Therefore, Th1 cells promote inflammation and the following secondary brain damage and Th2 cells promote anti-inflammatory responses reducing secondary brain injury. However, inflammation could be necessary to remove cell debris from necrotic tissue that dies after ischemia. Therefore, Th1-Th2 balance could be the clue of a regulated inflammatory response. It has been demonstrated that stroke induces a Th1/Th2 shift in mice. This phenomenon could be a neuroprotective response of the brain to reduce inflammatory response [171].

T-cells should be considered as therapeutic targets for ischemic stroke. However, because infection is a leading cause of mortality in the postacute phase of ischemic stroke, and considering the anti-inflammatory role of Th2 cells, treat-ment targeting T-cells should be carefully designed to reduce deleterious and enhance protective actions of T-cells [172].

9. STROKE INDUCES IMMUNODEPRESSION

As it has been already exposed, innate and adaptive im-munity play an important role in the outcome following cerebral ischemia. Regarding to this, we must consider the existence of an important balance between inflammation and immunodepression. The unbalance of this system involves an increase of secondary damage to the tissue caused by in-flammatory damage or by infection.

Eighty-five percent of all stroke patients have relevant complications, being infection the most frequent one, to such an extent that infection is the leading cause of death in pa-tients suffering from stroke. To support this, it has been shown that after cerebral ischemia, mice develop sponta- neous pneumonia and septicemia due to apoptotic loss of lymphocytes, shift of Th cells to Th2 cytokine production, and lost of monocytes [173]. There are some evidences, such as infection rate, showing that stroke induces immuno- depression, however the mechanisms and the signaling that down-regulate immune responses after ischemia remains unclear. It has been proposed that pro-inflammatory cytoki-nes produced by damaged brain tissue can directly lead to hypothalamic-pituitary axis and CNS activation, resulting in immune dysfunction [174].

Although immunodepression has been related with se- condary brain damage, it could also be a neuroprotective mechanism to reduce brain damage. Therefore, depletion of circulating T-cell populations and suppression of IFN , mechanisms of immunodepression in stroke, might counter-act the inflammatory brain after stroke. Indeed, stroke sup-press autoaggressive Th1 responses. Furthermore, brain epi-topes that usually are inaccessible to immune system are exposed after the disruption of the BBB, in stroke. That may induce to the immune system to attack the CNS. Therefore,

immunodepression could be a neuroprotective mechanism to avoid autoimmunity [174].

10. IMMUNE SYSTEM MODULATION

There are two major ways to manipulate immune system: immunosuppression with different drugs and immunization. Immunization can either be active, in which immune re-sponse is induced through exposure to an antigen, or passive, in which preformed antibodies are administered directly.

10.1 Immunosupression Therapies for Acute Stroke

Since inflammation is deleterious in acute stroke, immu-nosupression could be beneficial for acute ischemic stroke patients. The immunosuppressive drugs available today in-clude; anti-inflammatory corticosteroids, cytotoxic drugs such as azothioprine and cyclosphosphamide, fungal and bacterial derivates inhibiting T-cell activation such as cy-closporine A and rapamycin and non-conventional immuno-suppressive drugs whose immunosupressor role has been recently known. In this context statins and Granulocyte Col-ony Stimulating Factor (G-CSF) could be promising immu-nomodulators therapies.

G-CSF as Immunomodulator

There is compelling evidence that G-CSF exerts im-munoregulatory effects in adaptive immunity. G-CSF en-hances the total lymphocyte count in both bone marrow and peripheral blood and increases CD3

+, CD4

+ and CD8

+ cells

as well as CD3-CD16

+CD56

+ NK cells, while the increase in

CD4+ and CD8

+ T cells results from CD45RO

+ memory T

cells and from cells expressing the CD38 activation marker 2. G-CSF not only alters T cell numbers, but also T cell functions through the shift of T cell subsets in both bone marrow and peripheral blood. G-CSF polarizes T cell differ-entiation from Th1 to Th2 cells and induces Th2 responses with the production of IL-4 and IL-10 [175-177] accompa-nied by a decrease in production of IFN- and IL-2 [178], thereby suppressing T cell proliferative responses to alloge-neic stimulation [177]. In addition, G-CSF increased the production of TGF- [179], and decreased the production of TNF- . G-CSF treatment elevates a CD4

+CD25

+ T cell sub-

set that constitute functional regulatory T cells [180]. Since these cells act secreting anti-inflammatory cytokines they have a role as immunomodulators.

Furthermore, G-CSF treatment induces tolerogenic dendritic cells in the peripheral blood. These cells are poor stimulators of Th1 cells but they are good stimulators of Th2 cells which are the T cells that express anti-inflammatory mediators [181].

Statins could have a Role as Immunomodulators

There are some evidences indicating that statins have some beneficial effects independent of cholesterol reduction. Statins inhibit recruitment and activation of immune-competent cells, such as macrophages, and they inhibit the IFN- -induced expression of class II major histocompatibil-ity complexes (MHCII) on antigen-presenting cells. MHCII are required for antigen presentation and T-cell activation via the T-cell receptor (TCR). TCR activation may trigger both proliferation and differentiation of T cells and influences

Inflammation as a Therapy for Stroke Current Pharmaceutical Design, 2008, Vol. 14, No. 33 3559

their effector function, such as the release of cytokines. Cy-tokines released by activated T cells induce further T-cell proliferation and differentiation, APC activation and B-cell antibody production. The ability of statins to downregulate expression of MHCII may lead to decrase Th1 differentiation and activation in vivo and thus inhibit the release of pro-inflammatory cytokines. This suggest that statins may be useful to reduce inflammatory response after stroke [182].

10.2. Immunization and Induction of Tolerance

Pre-exposition of the brain to a short ischemic event can result in subsequent resistance to severe ischemic injury [183]. This phenomenon has been named as preconditioning, and it has been described to be present in several organs, especially heart and brain. Preconditioning proportions ischemic tolerance and it has been observed in humans in clinical practice. Indeed, less severe strokes have been de-scribed in patients with prior ipsilateral transient ischemic attacks (TIA) within a short period of time [184].

Although nature of induced protection and the underlying molecular mechanisms of preconditioning remain poorly understood, several mechanisms for the induction and main-tenance of tolerance in the brain have been postulated [185]. Several works have tried to solve this question, pointing some of them to different mechanisms that involve in some extent inflammatory mediators such as NF- B [186] and their activators signals like proinflammatory cytokines, neu-rotrophic factors and neurotransmitters [187].

Numerous studies have also demonstrated that precondi-tioning of the brain with a variety of potentially harmful stimuli, induces a profound tolerance to a subsequent epi-sode of ischemia. A brief ischemic insult can also induce ischemic tolerance in the spinal cord [188] and other organs, such as the heart [189], liver [190] or kidneys [191]. The exact molecular mechanisms underlying delayed ischemic tolerance are not well understood, but requirements for de novo protein synthesis [192], activation of the proinflamma-tory transcription factor NF- B [186], and induction of in-flammatory cytokines such as TNF, IL-1, IL-6 and IL-8 have been demonstrated.

Since administration of lipopolysacharide (LPS) can in-duce ischemic tolerance [193], Karikó et al. [194] have de-veloped an hypothetic model to explain this phenomenon. They hypothesized that tolerance is dependent on inhibition of TLR and cytokine signaling pathways, suppressing in this way the inflammatory response to ischemia. When an ischemic event takes place, the ischemic cascade involves TLR activation and cytokine expression, activating inflam-mation, among other mechanisms. Furthermore, TLR and cytokine signaling, trigger other pathways to increase ex-pression of signaling inhibitors, decoy receptors and anti-inflammatory cytokines. These pathways induce immune suppression and when an ischemic event occurs, inflamma-tory inhibitors are present, reducing inflammatory response and the subsequent secondary cell death.

Alternatively, brain preconditioning could be the conse-quence of immunomodulation strategies. Following stroke, novel CNS antigens are exposed to the immune system that could increase cellular damage. Therefore, strategies to re-

duce this response have been proposed. Animals can acquire tolerance to a variety of vascular antigens including myelin-related proteins and E-selectin, before MCAO surgery is performed. These animals experience smaller infarcts than animals tolerized to an irrelevant antigen [35, 195].

Ischemic tolerance is an endogenous mechanism that evolves to protect the organism from ischemia, being conse-quently a good example of immunomodulation.

11. CONCLUSIONS

Cerebral ischemia triggers a very important inflammatory response, which has been associated to an increase in brain damage and poor outcome in stroke patients. Therefore, anti-inflammatory therapies should be considered to reduce brain damage. However, inflammation is necessary to initiate the processes of neovascularization and regeneration. Therefore, “contained” inflammatory response might be necessary and beneficial after stroke. It is possible that optimal treatment of inflammation in stroke include an inhibition in the acute phase and activation in later phases to promote neovasculari-zation, plasticity and regeneration.

Furthermore, the latest advances in the immunomodula-tion field, together with the advances on the knowledge of the ischemic tolerance phenomenon and the role of innate immunity in such phenomenon could open a possibility for the application of immunomodulatory therapies prior to a stroke insult to prepare the organism to can stand better the inflammatory response when stroke occurs.

In summary, inflammation has been associated to an in-crease in brain damage in stroke patients. Furthermore, in-flammation is necessary to activate repairing mechanisms. Therefore, it is necessary a strict control of inflammatory response after stroke to reduce brain damage without inhibi-tion of the repairing mechanisms.

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