(Stroke. 2000;31:1715.)
© 2000 American Heart Association, Inc.
Original Contributions |
Lack of Interleukin-6 Expression Is Not Protective Against Focal Central Nervous System Ischemia
From the Oregon Stroke Center, Departments of Neurology (W.M.C., L.G.R., N.S.L., K.H.) and Cell Biology (J.K.H., M.S.-P., F.E.), Oregon Health Sciences University, Portland.
Correspondence to Wayne Clark, MD, Oregon Stroke Center, UHS 44, Oregon Health Sciences University, 3181 SW Sam Jackson Park Rd, Portland, OR 97201. E-mail clarkw{at}ohsu.edu
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Abstract |
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Background and Purpose—Interleukin-6 (IL-6) appears to be involved in the inflammatory response associated with central nervous system (CNS) ischemia. Although IL-6 levels increase after stroke, it is not known whether IL-6 directly influences CNS ischemic injury. In this study, we used a focal reversible stroke model to investigate whether mice lacking IL-6 were protected against acute ischemic injury.
Methods—We bred IL-6–deficient C57 black mice (I-129 IL-6 KO back-crossed with C57), including homozygous knockouts (IL-6 -/-), heterozygous littermates (IL-6 +/-), and normal littermates (IL-6 +/+). The status of all animals was confirmed by DNA sampling and polymerase chain reaction analysis. Reversible middle cerebral artery occlusion was produced by advancing a silicone-coated 8-0 filament into the internal carotid artery for 2 hours (experiment 1) or 45 minutes (experiment 2). At 24 hours, animals were evaluated on a 28-point clinical scale, blood and cerebrospinal fluid were obtained, and the brains were evaluated for infarct volume and IL-6 mRNA levels.
Results—In experiment 1 (severe ischemia), no differences
were seen in lesion size or neurological function between the groups:
lesion volume was IL-6 -/- (n=15), 57±13 mm3; IL-6
+/- (n=15), 58±23 mm3; and IL-6 +/+ (n=15),
58±18 mm3 (P=NS). ELISA testing
confirmed very low to absent levels of IL-6 in the serum and
cerebrospinal fluid of knockout animals. Brain mRNA levels of the other
proinflammatory cytokines, including tumor necrosis factor-
,
IL-1ß, and IL-1 receptor antagonist, were 50% lower in
IL-6–deficient ischemic animals than in normal animals. In
experiment 2 (mild ischemia), no differences were seen in
lesion size or neurological function between the groups: lesion volume
was IL-6 -/- (n=10), 16±8 mm3; IL-6 +/- (n=10),
14±4 mm3; and IL-6 +/+ (n=10), 19±12
mm3 (P=NS).
Conclusions—In this study, infarct size and neurological function at 24 hours were not different in animals deficient in IL-6 after transient CNS ischemia. This suggests that IL-6 does not have a direct influence on acute ischemic injury. Further study investigating the role of IL-6 on long-term recovery after stroke is in progress.
Key Words: inflammation • interleukins • stroke • mice, knockout
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferencesIntroduction |
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There is increasing evidence that the inflammatory response plays an important role in the potentiation of central nervous system (CNS) ischemic injury.1 Much of this inflammatory response appears to be mediated by interleukins (ILs), a multifunctional subclass of cytokines.2 The proinflammatory interleukins, including IL-1, tumor necrosis factor (TNF)-
, and IL-6, can influence the function and synthesis of other
cytokines by a complex cytokine network.2
These proinflammatory interleukins are produced by a variety of cells,
including microglial cells, astrocytes, and leukocytes, and appear to
directly modulate CNS cell apoptosis, differentiation, and
proliferation. Cytokines may also be involved in the activation
and recruitment of leukocytes into the CNS. IL-1ß, TNF-, and IL-6
have been shown to activate leukocytes and increase the
expression of adhesion receptors on leukocytes (CD-18), endothelial
cells, and astrocytes (intercellular adhesion molecule-1). Several
investigators have characterized the role of IL-1 and TNF- in
experimental CNS ischemia and have found a therapeutic benefit
of IL-1 receptor antagonist (IL-1RA)
treatment.3 4 5 6 7 The role of IL-6 in CNS ischemia has been studied less widely. Indirect evidence for the involvement of IL-6 in ischemic injury comes from clinical studies that found that cerebrospinal fluid (CSF) and plasma levels of IL-6 predict the functional recovery of the patient and correlate with the infarct size.8 9 10 11 12 IL-6 mRNA has also been shown to be upregulated after focal CNS ischemia.13 Other studies have suggested that IL-6 mRNA expression is highest in ischemia-reperfusion models.14 We recently used a focal CNS ischemia model to investigate the time course of IL-6 expression.15 The time course of the IL-6 response was similar to that of other cytokines, with peak plasma and CSF levels (6 hours) occurring before the levels peak in the brain (24 hours). CSF IL-6 levels had a stronger correlation with neurological function and infarct size than did plasma levels. Although all of these studies support the involvement of IL-6 in stroke, they did not determine whether modulation of IL-6 has any direct effect on CNS ischemia.
The purpose of the present study was to investigate whether modulation of IL-6 expression would influence acute CNS ischemic injury and neurological functional recovery in a focal CNS ischemia model in IL-6–deficient mice.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferencesIntroduction |
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All animal procedures were approved by the Oregon Health Sciences University Institutional Review Board and are in accordance with guidelines published by the National Institutes of Health for animal use. Because the inflammatory response appears to be involved predominantly in reperfusion injury, we chose a reversible focal CNS ischemic model: the filament middle cerebral artery occlusion (MCAO) model with reperfusion.16 17 In the first experiment, we used the MCAO model to investigate the effect of IL-6 expression on severe CNS ischemic injury with reperfusion. Two hours of MCAO was used, because this duration of ischemia has previously been found to produce a consistent large area of severe ischemic injury involving half of the hemisphere.17 In the second experiment, we chose a shorter duration of ischemia, 45 minutes, in an effort to investigate IL-6 effects in milder ischemic conditions. We have previously found that this duration of ischemia produces a stroke of milder ischemic injury involving <20% of the hemisphere.17
IL-6–Knockout Breeding
IL-6–knockout animals were bred as follows. IL-6–knockout
(-/-) male I-129 mice (B6I29-ILKO:TM1/J) were purchased from Jackson
Laboratory, Bar Harbor, Me. In these mice, transcriptional mRNA
for IL-6 is present; however, the mechanism for translation is
deleted at the second exon so that no IL-6 protein appears in CSF or
serum.18 This is a selective deletion such that the
translation of the other cytokines is not affected (data from
Jackson Laboratory). We have previously found that focal CNS
ischemia is not reliably produced in I-129 animals with the
MCAO model. We therefore back-crossed the I-129 IL-6–deficient animal
with C57 black mice (C57BL/SN), a strain that does produce reliable CNS
infarcts.
Homozygous knockout offspring of the I-129 and C57 pairing were back-crossed with normal C57 animals for a total of 6 generations. At each point, the knockout status of the animals was confirmed by tail DNA sampling with polymerase chain reaction analysis. At this point, we observed reliable CNS infarcts in the animals, and colony breeding was started with heterozygous pairs. This pairing allows for the production of homozygous IL-6–knockout animals (-/-) along with litter-matched heterozygous animals ( +/-) and normal controls (+/+). The IL-6–knockout mice exhibit no overt phenotypes compared with normal C57 mice.18 However, they tended to show decreased exploratory activity. This increase in fear-related behavior has been reported in IL-6–knockout animals.19
Experimental Design
In experiment 1 (severe ischemia), 45 male mice weighing
26 to 36 g were used. The 45 mice consisted of 15 animals each of
homozygous IL-6–knockouts (IL-6 -/-), heterozygous littermates (IL-6
+/-), and normal littermates (IL-6 +/+). All procedures and
evaluations were performed by investigators blinded to the
genotype of each animal. Animals were anesthetized by
inhalation of a nitrous oxide/oxygen/halothane (69%/30%/1%) mixture
administered by an inhalation mask during surgical preparation. Under a
dissection microscope, the right carotid bifurcation was exposed, and
the external carotid artery was coagulated distal to the bifurcation.
Cerebral blood flow over the MCA region was measured by laser
Doppler (Perimed: Periflow 5000). Silicone-coated 8-0 filament was
then inserted through the external cerebral artery stump and gently
advanced (9.0 to 11.5 mm) to occlude the origin of the middle
cerebral artery. Successful occlusion was confirmed by laser
Doppler reflecting <10% residual MCA flow. The proximal end of
the filament was cut to leave 1 to 1.5 mm protruding from the
external cerebral artery stump. The surgical wound was closed, and the
filament was left in place for 2 hours, during which the animal was
allowed to recover. The animal was then reanesthetized, and the
surgery area was reopened. The filament was gently withdrawn and the
incision closed. Successful reperfusion was confirmed by laser
Doppler reflecting 
90% of baseline value. Animals were allowed
to recover from anesthesia in a warm environment.
In experiment 2 (mild ischemia), 30 male mice weighing 26 to 36 g were used. The 30 mice consisted of 10 homozygous IL-6–knockouts (IL-6 -/-), 10 heterozygous littermates (IL-6 +/-), and 10 normal littermates (IL-6 +/+). All procedures were the same as in experiment 1 except that the animals were subjected to only 45 minutes of MCA ischemia before the suture was removed.
Sample Collection
At 24 hours after reperfusion, the animals were rated on a
28-point neurological scale for focal deficits by 2 investigators (see
Table 1
).17 At the
time of death, the animal was anesthetized, and CSF was
obtained via suboccipital puncture with a 27-gauge needle. CSF was
collected and immediately placed in dry ice. The still
anesthetized animal was then decapitated, and 
1 mL of blood
was collected. The blood was allowed to clot for 2 hours before being
centrifuged for 20 minutes at 2000g. Approximately
300 µL of serum was collected and frozen on dry ice. In experiment 1,
the brain was quickly removed and embedded in a warm bed of agarose on
a Stoelting tissue slicer, by which a 1-mm coronal section of tissue
(fourth section from the rostral end of the brain) was removed from the
center of the brain for histological analysis.
The rostral and caudal sections of the brain were then divided by left
and right hemispheres and placed in tubes in powdered dry ice for mRNA
analysis. All samples except the 1-mm brain section for
histology were stored at -80°C until ready for use. In experiment 2,
the entire brain was sliced into six 1-mm sections for lesion volume
assessment.
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Histology
In experiment 1, the 1-mm tissue section was fixed in 10%
formalin for 24 hours and then placed in 15% sucrose. Once the section
sank to the bottom of the well plate, the 15% sucrose solution was
replaced by 30% sucrose containing 0.05% sodium azide and stored at
4°C until sectioning occurred. The 1-mm section was mounted on a
freezing microtome and was embedded in OCT Tissue-Tek (No. 4583, Miles
Inc). Frozen tissue was cut into 50-µm sections. Approximately 10
good sections were obtained, and 2 of these were mounted on
chrome-albumin-jelly slides. Slides were dried at room
temperature overnight and then baked in a 37°C oven for a minimum of
6 hours. Slides were then immersed in a 50%/50% chloroform/ethanol
37°C bath for a minimum of 2 hours to delipidize the tissue. Sections
were stained with Luxal fast blue (ICN Biomedicals Inc) and
counterstained with cresyl violet acetate (Eastman Kodak Co) to
differentiate between the area of ischemia and unaffected
tissue.
In experiment 2, the six 1-mm sections were placed into 2% 2,3,5-triphenyltetrazolium chloride (TTC) in 0.9% PBS and stained at 37°C for 30 minutes.20 After staining, the TTC was replaced with 10% phosphate-buffered formalin. Areas not stained red with TTC were considered lesions and were used to calculate lesion areas.
Image Analysis
The NIH Image Analysis program in conjunction with a
1200–dots per inch flatbed scanner was used to measure the area
of ischemia and total hemisphere area (ipsilateral and
contralateral). Slides were placed on a scanner bed, and an image of
each section was captured and saved for later analysis. Two
investigators independently imaged the scanned sections, and the areas
were averaged. In experiment 1, estimated volumes, in cubic
millimeters, were calculated by use of the 0.5-mm slice thickness and
the sum of the 2 measured areas, multiplied by a predetermined factor
previously calculated to estimate total lesion volume using a center
slice area (R2=0.96 with
actual total infarct volume).15 This was done to estimate
the lesion volume in the tissue used for mRNA analysis. In
experiment 2, volumes were determined by use of slice thickness and the
measured areas of the lesion and ipsilateral hemisphere.
ELISA
To confirm that IL-6 was not being produced in plasma and CSF in
knockout animals, a sandwich enzyme immunoassay technique was applied
by use of a Quantikine mouse IL-6 kit from R & D Systems (experiment 1
only). All serum samples were diluted 1:2 according to kit instructions
and run on 96-well plates with 2 wells per sample (50 µL/well). The
CSF samples (6 µL) were diluted to 50 µL. The ELISA was performed
with 2 polyvinyl 96-well plates coated with polyclonal antibody
specific for mouse IL-6 according to the manufacturer’s
specifications. The well plates were then read on a Molecular Devices
kinetic microplate reader with Softmax software at a wavelength of 450
nm and correction wavelength of 540 nm.
Ribonuclease Protection Assay
To determine whether the absence of IL-6 expression would
influence the expression of other proinflammatory cytokines
known to be important in CNS ischemia, we measured brain levels
of IL-1ß, TNF-, and IL-1RA using ribonuclease protection assay
(experiment 1 only). The total RNA was prepared from frozen brain
tissue samples by use of Tri-Reagent (Molecular Research Center, Inc).
Left and right hemispheres from each subject were
homogenized in 5.5 mL of Tri-Reagent with an IKA
Labortechnik homogenizer. Total RNA was extracted in
chloroform, ethanol-precipitated, and frozen at -80°C. All samples
were then read on a spectrophotometer (260 and 280 nm) to determine
concentration and purity. These samples were then brought to the same
concentration, and pools of RNA were made from group and condition (eg,
stroke IL-6 -/-, control IL-6 -/-, etc).
A RiboQuant Multi-Probe RNAase Protection Assay kit (Pharmingen)
containing a total of 17 cytokine probes, including IL-1RA,
IL-1ß, and TNF-, was used to analyze mRNA. Templates of
distinct sizes were used to generate a
[32P]-labeled antisense RNA probe set and
hybridized in excess to target RNA. Total RNA (10 µg) was hybridized
(56°C, 18 hours) with the labeled probe set, followed by RNAse
treatment and analyses of protected bands on a denaturing 5%
polyacrylamide gel. Gels were subjected to PhosphorImage
analysis, and bands were quantified by densitometry with
the NIH Image Analysis program, allowing a comparison of each
cytokine band to a known housekeeping band (L32). This method
produces quantifiable mRNA unit values that can be compared with each
other within an experiment. However, the units cannot be expressed in
any standardized external value.
Statistics
An ANOVA was used to assess the statistical significance of
differences in lesion volume and cytokine values between
groups. Corrected unpaired t tests were used to assess
individual between-group differences. Between-group differences in
neurological function were assessed by nonparametric testing.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferencesIntroduction |
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Experiment 1 (2 Hours of Ischemia)
A total of 45 animals were evaluated, 15 in each genotype group. There was no difference in temperature between the groups at any time point. All animals had successful MCAO confirmed by laser Doppler: baseline blood flow in the MCA was IL-6 -/-, 387±131 perfusion units (pu); IL-6 +/-, 380±133 pu; and IL-6 +/+, 396±140 pu (P=NS). All postocclusion (5 minute) values were <10%: IL-6 -/-, 24±11 pu; IL-6 +/-, 26±13 pu; and IL-6 +/+, 23±14 pu (P=NS). Overall, all ischemic groups had consistent large infarcts with very small SDs for this model. The results of the infarct area (center slice), calculated volume analyses, and functional outcome are summarized in Table 2
. There was a correlation of 0.841
between calculated infarct volume and functional outcome. Calculated
lesion volume at 24 hours was IL-6 -/-, 57±13
mm3 (50±12% of hemisphere); IL-6 +/-,
58±23 mm3 (48±18%); and IL-6 +/+,
58±18 mm3 (52±14%) (P=NS).
Neurological function score [median (range)] was IL-6 -/-, 14.5 (9
to 25.5); IL-6 +/-, 14.5 (11.5 to 20.5); IL-6 +/+, 15 (9 to 26.5)
(P=NS). Serum IL-6 levels (ELISA) at 24 hours were IL-6
-/-, 2±3 pg/mL; IL-6 +/-, 314±119* pg/mL; IL-6 +/+, 337±131*
pg/mL. CSF IL-6 levels at 24 hours were IL-6 -/-, 4±12 pg/mL; IL-6
+/-, 36±40* pg/mL; IL-6 +/+, 67±69* pg/mL (*P<0.05
compared with IL-6 -/-). Ribonuclease protection assay was performed
to determine how the lack of IL-6 influenced the expression of IL-1ß,
TNF-, and IL-1RA mRNA levels in the brain. The results for the 3
cytokine brain mRNA levels are shown in the
Figure for each of the genotype
groups. Each data point represents pooled data from the 15
animals. Across genotype groups, levels of each of the 3
cytokines were higher in the ischemic hemispheres than
in controls irrespective of genotype: IL-1ß,
P=0.009; IL-1RA, P<0.001; TNF-,
P=0.008. Within the transgenic groups for all of the
cytokines, the mRNA levels in the ischemic
IL-6–knockout animals were significantly lower (50%) than in
normal-type ischemic animals (P=0.03 overall ANOVA;
P=0.001 IL-6 -/- versus IL-6 +/+); see Figure.
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Experiment 2 (45 Minutes of Ischemia)
A total of 30 animals were evaluated at 24 hours, 10 in each
genotype group. There was no difference between groups
at any time point. All animals had successful MCAO confirmed by laser
Doppler: baseline blood flow in the MCA was IL-6 -/-, 378±160
pu; IL-6 +/-, 381±130 pu; IL-6 +/+, 403±160 pu (P=NS).
All postocclusion (5 minute) values were <10%: IL-6 -/-, 26±13 pu;
IL-6 +/-, 27±11 pu; IL-6 +/+, 25±13 pu (P=NS). In this
experiment, the laser Doppler was left in place, and values were
obtained 10 minutes after the suture was removed to confirm successful
reperfusion: IL-6 -/-, 366±240 pu; IL-6 +/-, 381±136 pu; IL-6 +/+,
415±174 pu (P=NS). The results of the volume
analyses and functional outcomes are summarized in Table 2
. We have also included infarct area (center slice) and what
the corresponding calculated infarct volume would be to allow
comparisons to the 2-hour group results. All 45-minute groups had
consistent small to medium-size infarcts, although the
variability was higher than in the 2-hour-ischemia group.
Lesion volume at 24 hours was IL-6 -/-, 17±10
mm3 (16±8%); IL-6 +/-, 15±4
mm3 (14±4%); IL-6 +/+, 21±14
mm3 (19±12%) (P=NS). Neurological
function score [median (range)] was IL-6 -/-, 9 (6 to 12.5); IL-6
+/-, 10 (5 to 12.5); IL-6 +/+, 9 (6.5 to 12.5) (P=NS).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferencesIntroduction |
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This study found that after stroke, the neurological function and CNS injury seen in animals deficient in IL-6 was no different from those in littermate controls. This was true both in animals with severe ischemia and in groups subjected to milder ischemia. These results suggest that IL-6 does not have a direct influence on acute CNS injury related to ischemia. All animals in the present study were evaluated at 24 hours. As such, these results represent early ischemic injury events, including cerebral edema, and do not address whether IL-6 has a role in later recovery from stroke. Further study involving evaluations at later time periods (1 month) are needed to answer this question.
This lack of IL-6 effect on acute stroke injury and function is somewhat surprising, given the strong association between clinical stroke recovery, infarct size, and risk of recurrent stroke.3 4 5 6 7 However, all of these previous clinical studies reported on correlations between IL-6 and various stroke outcomes and did not attempt to directly modulate IL-6 levels. It is possible that in the acute phase, IL-6 is merely a marker of the extent of underlying injury.
Using ELISA testing, we confirmed the absence of IL-6 in the serum and
CSF of IL-6 -/- animals (probably zero, given test sensitivity and
variability). The increased levels of IL-6 in serum and CSF at 24 hours
in the heterozygous and normal animals are similar to those seen in our
previous experimental studies.15 21 In these studies, peak
levels of IL-6 after MCA occlusion actually occur at 6 hours
(1200 pg/mL), with the 24-hour value being 25% of the peak. It
appears that both the normal and heterozygous animals had similar IL-6
release, suggesting that the heterozygous animals had sufficient IL-6
mRNA translation for normal cytokine production. A
potential weakness of this study is that because we back-crossed our
animals for only 6 generations, there may have been an uneven
representation of C57/I-129 genetic backgrounds between
animals. However, our goal was to produce a group of animals that would
have consistent infarcts when subjected to MCAO, even if they
were not genetically pure. This goal appears to have been achieved,
given the consistent infarct sizes and small SDs (for this
model) in the 2-hour ischemic groups.
Although increased IL-6 levels appear to be associated with CNS ischemia, there are conflicting data regarding whether this increased IL-6 may be exerting neuroprotective or neurotoxic actions. Although IL-6 appears to promote neuronal survival and inhibit NMDA toxicity in vitro,22 higher concentrations of IL-6 have been shown to be toxic in neuronal cultures, and transgenic mice overexpressing IL-6 have shown early neurodegenerative changes.23 24 IL-6 also appears to contribute to the reactive gliosis that appears after ischemia.25
It does appear that IL-6 is involved in modulating the acute expression
of other proinflammatory cytokines in the brain after
ischemia. The finding that the expression of IL-1ß, IL-1RA,
and TNF- was less in ischemic knockout animals suggests that
IL-6 may enhance the expression of these cytokines via positive
feedback mechanisms. This theory is supported by the finding that
IL-6–deficient mice have an impaired acute-phase response to infection
or trauma.18 The increased expression of IL-1ß, TNF-,
and IL-1RA mRNA at 24 hours in focal ischemia is
consistent with our previous studies and
others.2 8 10 26 We previously found that peak brain
cytokine mRNA levels in the mouse MCAO model occur at 24
hours.15 21 Although this is the first study to directly
investigate the effect of IL-6 in stroke, the modulation of other
proinflammatory cytokines has produced changes in CNS
ischemic injury. Injection of IL-1ß into the lateral
ventricle during transient ischemia produced an increase in the
infarct size, whereas injection of an anti–IL-1 monoclonal antibody
produced a decrease in postischemic infarct
size.27 Treatment with IL-1RA injected directly into the
brain also decreased focal experimental CNS ischemic injury,
associated cerebral edema, and neutrophil infiltration into the
ischemic areas.28 29 Finally, Relton et
al30 found that in a rat MCAO model,
peripherally administered IL-1RA significantly reduced
infarct size and associated cerebral edema. No transgenic studies have
been reported with IL-1. Studies involving TNF- have found that
local injection of TNF- into the brain greatly increased leukocyte
infiltration to this area, suggesting that TNF- may potentiate CNS
reperfusion injury.2 Other studies have found that TNF-
promotes neuronal cell death in cultures.31 Although these
studies suggest that TNF- is detrimental during CNS
ischemia, a conflicting view has recently been reported. Using
TNF receptor knockout (TNFR-KO) mice, Bruce et al32 found
that animals without TNF- had larger infarcts (MCAO) and lower
neuronal survival than control animals.
No previous studies have been reported in IL-6–knockout animals with CNS ischemia. However, IL-6–knockouts have been used in other disease models. Alonzi et al33 used an IL-6–knockout mouse in an autoimmune arthritis model and found that animals lacking IL-6 were protected against all arthritis-related damage, including the presence of inflammatory cells. In another study, knockout IL-6 animals were found to have a decreased inflammatory response to LPS.34 These results, along with our findings of decreased expression of proinflammatory cytokines in the brain of IL-6–knockouts, suggest that IL-6–deficient animals do have a decreased inflammatory response. In our study, however, this decreased inflammatory response was not associated with a reduction in neurological ischemic injury.
Conclusions
This study found that the response to either severe or moderate
CNS ischemic injury was the same in animals lacking IL-6 as in
matched controls. At 24 hours after reversible focal ischemia,
both the extent of CNS injury and functional outcome were similar in
all groups. These results suggest that strategies targeting inhibition
of IL-6 during stroke are unlikely to prove beneficial in the
short-term recovery phase. Further studies investigating the role of
IL-6 in long-term recovery after stroke are in progress.
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Acknowledgments |
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This study was funded by an American Heart Association Established Investigator grant. The authors wish to thank Valerie Roska for her assistance with the manuscript.
Received September 2, 1999; revision received April 27, 2000; accepted April 27, 2000.
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Editorial Comment
Department of Neurology University of Virginia Health System Charlottesville, Virginia
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferencesIntroduction |
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Inflammation has long been recognized to be a component process in the complex series of events that accompany acute infarction of the brain. A substantial body of evidence has accrued which suggests that activation of various cytokines occurs as a result of acute stroke, but the problem has been in discerning whether cytokine activation is a part of the problem, exacerbating ischemic damage, or part of the solution, helping to clean up the mess.
In the accompanying article, Clark and colleagues hypothesize that if the proinflammatory cytokine IL-6 were exacerbating ischemic brain injury, then "knockout" animals genetically incapable of producing IL-6 should have smaller infarcts when subjected to the same ischemic injury as IL-6–producing controls. Their data convincingly show that in either mild or severe focal ischemic injury, there was no significant difference in infarct size at 24 hours regardless of IL-6 status, thereby exonerating IL-6 as a "bad actor" in the early infarct scenario. Whether there may be longer-term influences is the subject of ongoing research.
These observations suggest that blockade of IL-6 activity may not be a useful therapeutic target for acute ischemic stroke. The jury is still out on the other elements of the inflammatory cascade, but perhaps this study may serve as a model to guide future research on which factors are of greatest importance in this complicated, multifaceted problem.
Received September 2, 1999; revision received April 27, 2000; accepted April 27, 2000.
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