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The Journal of Neuroscience, February 15, 1999, 19(4):1517-1523
Interleukin-1 Mediates a Rapid Inflammatory Response After
Injection of Adenoviral Vectors into the Brain
Tammy
Cartmell1,
Tom
Southgate2,
Gwen S.
Rees3,
Maria G.
Castro1, 2,
Pedro R.
Lowenstein2, and
Giamal N.
Luheshi1
1 Division of Neuroscience, School of Biological
Sciences, and 2 Molecular Medicine Unit, Department of
Medicine, University of Manchester, Manchester M13 9PT, United Kingdom,
and 3 Division of Endocrinology, National Institute for
Biological Standards and Control, Herts EN6 3QG, United Kingdom
 |
ABSTRACT |
Adenovirus-mediated gene transfer into the brain is associated with
significant inflammation and activation of anti-vector and
anti-transgene immune responses that curtail the gene delivery of
adenoviruses and therapeutic efficacy. Elucidating the molecular mediators of inflammatory and immune responses to adenoviruses injected
into the brain should allow us to inhibit their inflammatory actions,
thereby reducing vector clearance and enhance adenoviral-mediated gene
transfer into the CNS. Cytokines are primary mediators of the immune
response and are released during inflammation. Here we report for the
first time that injection of replication-deficient adenovirus vectors
into the cerebral ventricles of rats causes a rapid increase in body
temperature. This fever response precedes any vector-encoded transgene
expression and occurs with vectors encoding no transgene, as well as
with vectors encoding a therapeutic transgene i.e., HSV1-thymidine
kinase. No fever is detected after infection of the striatum, an
important brain target in studies on neurodegeneration. After infection
of the brain ventricles, CSF levels of immunoreactive tumor
necrosis factor (TNF)- and interleukin (IL)-1 increase
significantly (up to 300-fold). In the hypothalamus, the locus of
thermoregulation in the brain, only IL-1 and IL-6 are significantly
elevated. A neutralizing TNF- antibody has no effect on
adenovirus-induced fever. However, pretreatment with either the IL-1
receptor antagonist or the cyclooxygenase inhibitor flurbiprofen
completely abolishes adenovirus-induced fever, suggesting that IL-1 and
prostaglandins are direct mediators of this response. These results are
the first to demonstrate that IL-1, but not TNF- , is the main
mediator of a very early inflammatory response to adenovirus in the brain.
Key words:
adenovirus; inflammation; cytokines; brain; rat; fever
 |
INTRODUCTION |
Despite their many advantages (Chen
et al., 1994 ; Eck et al., 1996 ; Lowenstein et al., 1996 ; Choi-Lundberg
et al., 1997 ; Geddes et al., 1997 ), adenoviral vectors induce innate
inflammatory and adaptive immune responses on delivery either to
peripheral organs such as the liver (Elkon et al., 1997 ; Lieber et al.,
1997 ; Worgall et al., 1997 ), respiratory tract (Devergne et al., 1991 ;
Ginsberg et al., 1991 ; McCoy et al., 1995 ), salivary glands (Adesanya
et al., 1996 ), or the CNS (Wood et al., 1996 ). After the
transduction of the liver or lungs, inflammatory and immune responses
lead to the complete elimination of first generation vector particles and transduced cells within 2-3 weeks (Elkon et al., 1997 ). In the
brain, however, low level persistence of biologically relevant amounts
of transgene expression can be sustained (Wood et al., 1996 ) despite
inflammatory, immune responses, and delayed-type hypersensitivity
phenomena that can be triggered through the peripheral readministration
of recombinant vectors (Byrnes et al., 1996a ). Nevertheless, the
mechanisms by which inflammatory and immune responses affect
adenovirus-encoded transgene expression and clear viral vectors from
the brain remain to be determined. This is reflected in contradictory
data regarding long-term transgene expression. Thus, although one study
reported a 4 months stable reversion of the diabetes insipidus
phenotype of Brattelboro rats infected with adenoviral vectors
expressing vasopressin (Geddes et al., 1997 ), a separate study failed
to detect any adenovirally mediated -galactosidase expression at 6 months after infection (Blomer et al., 1997 ).
Immune response priming occurs efficiently after infection of the CSF,
but not the brain parenchyma, with replicating viruses (Stevenson et
al., 1997a ). This indicates that immune presentation is deficient in
the brain parenchyma, but not in the CSF, suggesting that the
elicitation of inflammatory or immune responses to viruses in the brain
may be brain region-specific (Stevenson et al., 1997b ). Thus, whether
inflammatory and immune responses to viral vectors are elicited in the
brain could depend on the anatomical area infected, the microbiological
purity of viral vectors used, or the surgical technique used during
virus delivery.
So far, the early molecular responses underlying the activation of
inflammatory and immune responses after the administration of
adenovirus vectors to target tissues are not well understood. In the
liver and lung, despite rapid macrophage-mediated clearing of a large
percentage of adenoviral genomes, a vast majority of hepatocytes become
transduced (Elkon et al., 1997 ; Worgall et al., 1997 ). Longer term
transgene expression, however, is further restricted by the cellular
arm of the immune system; thus, in immunodeficient mice lacking T and B
lymphocytes, transgene expression is prolonged (Zsengeller et al.,
1995 ). The same does not apply to adenoviral infection of the CNS,
where transgene expression is only moderately enhanced either in nude
rats, in the absence of T and B cells, or after treatment with
dexamethasone (Byrnes et al., 1996b ; Hermens and Verhaagen, 1997 ).
However, no information exists on the molecular basis responsible for
triggering adenovirus-induced inflammation and immune clearance in the brain.
In this study, we examined early inflammatory responses after the
injection of first generation (E1/E3 deleted) adenovirus vectors into
the CSF or brain parenchyma (striatum) and detected extremely rapid
increases in core body temperature and in the concentrations of
proinflammatory cytokines, tumor necrosis factor (TNF)- , interleukin
(IL)- , and IL-6 in the CNS. We obtained identical responses with
first generation adenoviral vectors encoding: (1) the marker enzyme
-galactosidase, (2) the therapeutic transgene thymidine kinase of
herpes simplex virus type 1 (HSV1-TK) under the control of the short
major immediate early human cytomegalovirus (sMIEhCMV) promoter
(RAd128), or (3) no transgene (RAd0).
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MATERIALS AND METHODS |
All animal procedures conformed with the requirements of the
British Home Office Animal Licensing Inspectorate.
Animals and injections. All experiments were
performed on adult male Sprague Dawley rats (Charles River
Laboratories) of 250-300 gm body weight. The animals were housed
individually in a temperature-controlled room (21 ± 2°C)
artificially lit from 8:00 A.M. to 8:00 P.M. and were provided with
food (pelleted rat chow; Beekay International) and water ad
libitum.
Core body temperature was monitored in free-moving animals by remote
radiotelemetry via small battery-operated temperature-sensitive radiotransmitters (Data Sciences International). Rats were anesthetized with halothane (3% in oxygen) and transmitters implanted into the
abdominal cavity. The animals were allowed to recover for 7 d
before experimentation. The output frequency (in Hertz) was monitored
by an antenna, mounted in a receiver board situated beneath the
individually caged animals, and converted to degrees centigrade (°C).
Central injections were administered via an indwelling guide cannula,
stereotaxically implanted into either the right lateral cerebral
ventricle (intracerebroventricular; bregma, 0.8 mm; lateral,
1.5 mm; ventral, 3.5 mm) or the striatum (bregma, +0.3 mm; lateral, 3.6 mm; ventral, 5.5 mm) (Paxinos and Watson, 1986 ), during the same
procedure as transmitter implantation. Intracerebroventricular and
striatal injections were administered at 10:00 A.M. in a volume of 2 µl to conscious, free-moving animals (n = 5-6 per treatment).
Adenovirus construction and purification. Construction and
characterization of RAd35 and the sMIEhCMV promoter, was described earlier (Wilkinson and Akrigg, 1992 ), and viruses were grown up and
purified as previously described (Shering et al., 1997 ; Morelli et al.,
1999 ). Briefly, the transfer vector pAL119/lacZ was
constructed from pXCX2 with the addition of a linker containing the
HindIII cloning site at the XbaI cleavage site.
lacZ was cloned under sMIEhCMV promoter control and
upstream of a polyadenylation signal on a HindIII expression
cassette cotransfected with pJM17 (Microbix Biosystems Inc., Toronto,
Canada) into HEK-293 cells by calcium phosphate precipitation.
Homologous recombination resulted in the recombinant adenovirus, RAd35.
The virus was propagated on HEK-293 cells, purified on previously
prepared CsCl gradients (using a modified protocol with densities 1.45 and 1.33), dialyzed twice against a buffer of 10 mM Tris, 1 mM MgCl2, 135 mM NaCl, pH
7.5 and once against the same buffer plus 10% glycerol. The virus was
titrated by plaque assay on 293 cells, and the viral titer was
determined to be 6.55 × 1011 pfu/ml. Levels of
endotoxin were measured using the E-toxate assay (Sigma, St. Louis, MO)
and, in all virus preparations used, were below 0.3 endotoxin U/ml. The
characterization of RAd128 is described in Dewey et al. (1998) and RAd0
in David et al. (1997) .
Drugs. RAd35, RAd128, or RAd0 were diluted in sterile PBS
and administered at a dose of 1.31 × 108
pfu/rat (intracerebroventricular or striatum). Control animals were
injected with vehicle (sterile PBS) (intracerebroventricular or
striatum). The concentration of CsCl, as used in the viral purification
gradient, was dialyzed against the purification buffers and injected
intracerebroventricularly to control for the possible pyrogenic effect
of any remaining CsCl in the viral preparation. Recombinant human
IL-1ra (200 µg/rat; Peprotech, Rocky Hill, NJ) was administered
intracerebroventricularly in saline vehicle at 0 and 1 hr. The
cyclooxygenase inhibitor flurbiprofen (kindly provided by Dr. M. Dascombe, Manchester, UK), was dissolved in 1% sodium bicarbonate and
0.9% sterile saline, and administered intraperitoneally (1 mg/kg) 0.5 hr before intracerebroventricular treatment of adenovirus or vehicle.
Rabbit murine TNF- antiserum (kindly provided by Dr Steve Kunkel,
Ann Arbor, MI) was administered intracerebroventricularly 24 hr before
treatment with adenovirus or vehicle.
ELISA. At specific time points after injection of adenovirus
or vehicle (intracerebroventricular or striatum; n = 5 per treatment per time point) CSF was collected from the cisterna magna
of rats that had been anesthetized by halothane (3% in oxygen).
Thereafter, the animals were killed by cervical dislocation, and the
hypothalami and striati were removed. CSF samples containing any trace
of blood after centrifugation (10,500 rpm, 10 min, 4°C) were
discarded from all subsequent analyses. Brain samples were placed in
sterile PBS containing a protease inhibitor cocktail [0.2
mM 4-[2-aminoethyl]benzenesulfonylfluoride, HCl (AEBSF),
1 µg/ml aprotinin, 1 mM benzamidine, 1 mM
EDTA, 10 µg/ml leupeptin, and 10 µg/ml of pepstatin], homogenized,
centrifuged (10,500 rpm, 15 min, 4°C), and the supernatant removed
and stored at 70°C. All samples were assayed for immunoreactive
TNF- , IL-1 , and IL-6 using validated rat specific sandwich ELISAs
(Safieh-Garabedian et al., 1995 ; Rees et al., 1998 ). The assay
sensitivity for TNF- , IL-1 , and IL-6 in CSF was 3.8 pg/ml and in
brain tissue was 10 pg/ml.
Data analysis. All results are reported as mean values ± SEM. Body temperature data were analyzed for statistical
significance according to the integrated hyperthermic response,
calculated as the deviation from baseline over the 8 hr period after
administration of adenovirus (8 hr fever index in degrees
centigrade per hour). Differences between more than two groups were
determined by ANOVA followed by Tukey-Kramer multiple
comparisons post hoc test, and differences between two
groups were identified by an unpaired Student's t test.
Changes in cytokine levels were analyzed using unpaired Student's
t test. A two-tailed probability < 0.05 was considered
statistically significant.
 |
RESULTS |
Intracerebroventricular, but not intrastriatal, injections of
adenovirus vectors cause fever
Injection of 1.3 × 108 pfu of RAd35 into
the CSF resulted in a rapid increase in body temperature starting 1-2
hr after vector injection (Fig.
1A). At this time, no
transgene can be detected by either X-gal histochemistry or
immunocytochemistry (data not shown). Expression of -galactosidase
can be detected from 6 to 8 hr after infection (data not shown). The
rise in temperature after the adenoviral intracerebroventricular
injection was sustained, reaching a peak 7 hr after infection (vehicle
37.2 ± 0.2 vs RAd35 39.0 ± 0.1°C; t test;
p < 0.001; n = 6). When the same
amount of virus was injected into the striatum, no fever response was detected (Fig. 1B). Body temperature changes were
monitored for 48 hr after injection. The fever after
intracerebroventricular injection lasted for ~10 hr. By 24 hr after
infection, body temperature was back to normal and remained so for
another 24 hr (data not shown).

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Figure 1.
A, A replication-deficient
recombinant adenovirus expressing -galactosidase (RAd35) injected
intracerebroventricularly (1.3 × 108 pfu in 2 µl) caused a significant increase in core body temperature that was
maximal 7 hr after injection (***p < 0.001 vs
vehicle; n = 6). Dotted line
indicates time of injection (0 hr, 10:00 A.M.). B,
Striatal injection of the same recombinant adenovirus vector resulted
in no significant change in core body temperature (vs vehicle;
n = 5) for the duration of the experiment (48 hr).
Dotted line indicates time of injection (0 hr).
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Importantly, identical results were obtained with other adenoviral
recombinants: (1) RAd128, a replication-deficient adenovirus expressing
the herpes simplex virus type 1 thymidine kinase (Dewey et al., 1998 ),
and (2) RAd0, a replication-deficient adenovirus encoding no transgene
(David et al., 1997 ). All vectors induced fever of similar magnitude
(Fig. 2). All viral preparations used in
this study were endotoxin-free, as defined by Cotten et al. (1994)
(<6 × 10 4 endotoxin units per dose of
adenovirus administered into the brain). Thus, because vectors were
purified on a double CsCl gradient-purified vector, and
intracerebroventricular injection of CsCl alone did not induce fever,
it is highly unlikely that the febrile response is caused by endotoxin
or any other contaminant. Furthermore, heat treatment of Rad0 (30 min
at 90°C) completely abolished adenovirus-induced fever (Fig. 2). This
procedure abolishes adenovirus infectivity but does not inactivate
endotoxin.

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Figure 2.
Temperature responses to intracerebroventricular
(2 µl) injection of 1.3 × 108 pfu/rat of
RAd35 (encoding the marker transgene -galactosidase), RAd0
(containing no transgene), or RAd128 (encoding HSV1-TK) were
significantly different from that of vehicle (ANOVA,
***p < 0.001). The concentration of CsCl, as used
in the viral purification gradient, was dialyzed against the
purification buffers and injected intracerebroventricularly to control
for the possible pyrogenic effect of any remaining CsCl in the viral
preparation. Neither the vehicle nor CsCl induced fever. Furthermore,
heat treatment of RAd0 (30 min at 90°C) completely eliminated the
fever response.
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Adenovirus vectors induce increases in TNF- , IL-1 , and IL-6
concentration in the brain
The proinflammatory cytokines, TNF- , IL-1 and IL-6, are
strong candidates for inducing fever in response to adenovirus
infection of the brain. TNF- can display pyrogenic effects (Stefferl
et al., 1996 ) and has been reported recently to mediate the clearing of
adenovirus from the liver after intravenous virus administration (Elkon
et al., 1997 ). Both IL-1 and IL-6 are potent pyrogens when administered
into the brain in many different experimental models both in rats and
mice (Kluger, 1991 ). We, therefore, measured the levels of these
cytokines in the CSF and hypothalamus after injection of adenovirus
into the lateral ventricle (Fig. 3).

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Figure 3.
A, Intracerebroventricular
injection of adenovirus elicited a marked increase in the levels of
TNF- (**p < 0.01) and IL-1
(#p < 0.05) in the CSF 1.5 hr after injection,
followed at the 3 hr time point by a reduction in the TNF- levels
(by 61%) and a continued increase in IL-1 levels
(#p < 0.05) when compared with vehicle-injected
controls. IL-6 increased significantly (**p < 0.01 vs vehicle) 3 hr after adenovirus injection. B,
Hypothalamic IL-1 increased significantly at the 1.5 hr time point
(#p < 0.05 vs vehicle). Both IL-1
(***p < 0.001) and IL-6 (***p < 0.001) levels were significantly elevated 3 hr after adenovirus
injection, compared with vehicle-injected controls.
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In the CSF (Fig. 3A) at 1.5 hr after infection, we detected
a 25-fold increase of TNF- (vehicle 443.5 ± 261.5 vs RAd35
10,923.6 ± 3623.1, p < 0.01) and a smaller,
fivefold increase of IL-1 (vehicle 74.8 ± 46.2 vs RAd35
296.2 ± 43.7; p < 0.05), whereas IL-6 levels
remained unchanged. At 3 hr after infection, TNF- levels had
decreased by 61% but were still 11 times higher than uninfected
controls; both IL-1 (vehicle 4.2 ± 4.2 vs RAd35 1412.0 ± 657.3; p < 0.05) and IL-6 (vehicle 24.4 ± 24.4 vs RAd35 9278.1 ± 2425.1; p < 0.01) levels had
increased by >300-fold compared with uninfected controls.
In the hypothalamus (Fig. 3B), however, IL-1 was the only
cytokine that was significantly elevated at the 1.5 hr time point (vehicle 390.6 ± 76.6 vs RAd35 1537.0 ± 779.4;
p < 0.05). By 3 hr after adenovirus-induced infection,
both IL-1 (vehicle 163.5 ± 98.1 vs RAd35 2533.6 ± 509.8;
p < 0.001) and IL-6 (vehicle 22.6 ± 16.3 vs
RAd35 322.3 ± 34.4; p < 0.001) levels were
significantly elevated in comparison to control animals. Importantly,
injection of RAd35 into the striatum induced a local (striatal)
increase in IL-1 (vehicle 416.60 ± 268.6 vs RAd35 1706.40 ± 509.0 pg/ml; p < 0.05) but did not induce increased
IL-1 in the hypothalamus (vehicle 113.6 ± 23.3 vs RAd35
89.5 ± 22.3 pg/ml). Similarly, intracerebroventricular injection
did not increase striatal IL-1 levels (data not shown).
IL-1 but not TNF- , is a necessary mediator of
adenovirus-induced fever
Having established that TNF- , IL-1 , and IL-6 are rapidly
released into the CSF in response to adenovirus administration, we
examined if these cytokines were necessary and/or sufficient to mediate
the adenovirus-induced fever. We first used a neutralizing antibody to
TNF- injected intracerebroventricularly 24 hr earlier. This
procedure significantly attenuated the temperature response to
intramuscular turpentine [the schedule of injection of TNF- antiserum is described in detail in Luheshi et al. (1997) ] (data not
shown). Intramuscular turpentine-induced fever has been previously shown to be mediated by brain TNF- (Luheshi et al., 1997 ). However, this TNF- antiserum had no effect on adenovirus-induced fever (Fig.
4A).

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Figure 4.
A, The temperature response to
intracerebroventricular injection of adenovirus (0 hr) after
pretreatment with vehicle or TNF- antiserum was only significantly
different (ANOVA; ***p < 0.001) from that of
vehicle or TNF- antiserum injected 24 hr earlier plus vehicle
injected at 0 hr. B, The increase in core temperature
elicited by adenovirus (i.c.v., 0 hr) was abolished by injection of
IL-1ra (200 µg/rat, i.c.v.; 0 and 1 hr). The temperature response to adenovirus was
significantly different from that of vehicle, IL-1ra, or adenovirus
plus IL-1ra (ANOVA; ***p < 0.001).
C, Intracerebroventricular injection of adenovirus (0 hr) elicited a marked and sustained increase in core temperature that
was totally abolished by intraperitoneal injection of flurbiprofen (1 mg/kg, 0.5 hr). Data for vehicle, flurbiprofen, or adenovirus plus
flurbiprofen were all significantly different from the temperature
response to adenovirus (ANOVA; ***p < 0.001).
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The involvement of IL-1 was then examined using the highly selective
endogenous IL-1 receptor antagonist (IL-1ra) (Eisenberg et al., 1990 ).
IL-1ra injected into the lateral ventricle at a dose previously shown
to inhibit IL-1-induced fever (data not shown) was coadministered with
RAd35 (Fig. 4B), and this completely abolished the
adenovirus-induced fever. It has been previously established that
IL-1-induced fever is prostaglandin-dependent (Elmquist et al., 1997 ).
Thus, we investigated the role of the cyclooxygenase pathway in the
fever response to adenovirus (Fig. 4C). Intraperitoneal
injection of flurbiprofen (1 mg/kg), a cyclooxygenase inhibitor, 30 min
before adenovirus injection completely inhibited the adenovirus-induced
fever. This regimen of flurbiprofen administration was previously shown
to inhibit intracerebroventricular IL-1-induced fever (data not shown).
These data demonstrate that both IL-1 and prostaglandins are necessary
mediators of the febrile response to RAd35. It is possible that IL-6,
which is known to be increased in response to IL-1 (Klir et al., 1994 ),
contributes to the fever response detected (Kluger, 1991 ). This
possibility was not explored further in the present study.
 |
DISCUSSION |
The inflammatory responses detected occurred very early after
vector administration. They were independent of the particular vector
used and whether the virus encoded or expressed a therapeutic marker or
no transgene. Furthermore, heat treatment of adenoviral vectors at a
temperature that abolishes viral infectivity without affecting
endotoxin integrity completely abolished adenovirus-induced fever.
Importantly, given that transgene expression can only be detected by
histochemical or immunocytochemical methods at 6-12 hr after
infection, the responses observed do not depend on the encoded
transgene but on the virion particle itself. Similar observations on
the proinflammatory potential of nonreplicating and inactivated adenovirus vectors have been previously shown to occur after the administration of such viruses to the respiratory tract (Ginsberg et
al., 1991 ; McCoy et al., 1995 ).
The initial site of action through which the adenovirus acts to trigger
the fever response must lie within the ventricles themselves. The lack
of histochemically detectable -galactosidase activity in the
hypothalamus after infection of the CSF demonstrates that RAd35 does
not reach this brain area after intracerebroventricular injection.
These data confirm that the ependymal cell layer prevents the virus
itself from entering the brain from the CSF, a finding also reported by
other authors (Bajoccki et al., 1993 ). Thus, initially, adenovirus must
interact with one of several potential target cells accessible within
the CSF (e.g., the ependymal cell layer, the choroid plexus, or immune
cells present within the CSF) to induce the CSF increase of TNF- ,
IL-1 , and IL-6. The concomitant increase of hypothalamic IL-1 and
IL-6 could be caused by either a diffusion of these cytokines from the
ventricle or secondary to an intraventricular proinflammatory signal
that is transferred to the hypothalamus and eventually leads to the
intrahypothalamic increase of interleukins.
It is most likely that in our experiments IL-1 is acting within the
hypothalamus, its recognized main site of action as a pyrogen (Kluger,
1991 ), to induce fever in response to the intraventricular adenovirus
injection. Although intracerebroventricular IL-1ra was very effective
in blocking adenovirus-induced fever, we believe IL-1ra inhibits the
intrahypothalamic pyrogenic action of IL-1 by diffusing from the CSF to
the hypothalamus. Whether IL-1 and IL-6 diffuse from the ventricle to
the hypothalamus or are produced within the hypothalamus itself in
response to a proinflammatory signal originating within the ventricles
remains to be determined. Prostaglandins are also involved in
adenovirus-induced fever. If produced within the ventricle, they could
diffuse to the hypothalamus to convey the proinflammatory signal.
The striatum has not been shown to have a direct role in
thermoregulation. Anatomically, the striatum is not directly
interconnected with the thermoregulatory centers in the hypothalamus,
and thus, the intrastriatal increase in IL-1 is unlikely to reach
the hypothalamus in high enough concentrations to induce fever. Thus,
our results predict different inflammatory responses to adenovirus
injection into different brain areas.
The adenovirus-induced increase in brain IL-1 causes fever. TNF- is
not involved in this response, and the increase in IL-6 is most likely
induced by IL-1 (Klir et al., 1994 ). The increase in prostaglandins is
downstream of IL-1, and prostaglandins have not been shown to increase
IL-1 secretion or production on their own (Elmquist et al., 1997 ). The
increase in CSF and hypothalamic IL-1 at the 1.5 hr time point was
comparable, whereas at the 3 hr time point the increase was
substantially larger in the CSF compared with the hypothalamus.
Furthermore, IL-1 synthesis within the hypothalamus itself has been
demonstrated previously (Tringali et al., 1996 ), and its receptors are
also present therein (Takao et al., 1990 ; Yabuuchi et al., 1994 ).
Importantly, another virus, certain strains of vaccinia, express a
soluble IL-1 receptor that binds IL-1 but not IL-1 . Deletion of
this gene induces fever, showing that vaccinia-induced fever is
mediated by IL-1 (Alcami and Smith, 1996 ). This suggests that
several viruses can induce fever via IL-1. Furthermore, even if the
direct intrahypothalamic injection of IL-6 can induce fever on its own
(Klir et al., 1993 ), we believe that IL-6 is not the final mediator of
adenovirus-induced fever, because at the 1.5 hr time point, only
hypothalamic IL-1 is significantly elevated. Nevertheless, the
putative role of IL-6 in adenovirus-induced fever remains to be examined.
We favor the hypothesis that hypothalamic IL-1 and IL-6 are produced
endogenously and are not diffusing from the ventricle and that
hypothalamic IL-1 is responsible for the adenovirus-mediated fever
response. If cytokines could diffuse freely from the CSF to the
hypothalamus, we would have expected TNF- levels to increase within
the hypothalamus. However, despite the large increase of intraventricular TNF- , we did not detect any intrahypothalamic increase of this cytokine. An in situ hybridization study of
IL-1 mRNA in the hypothalamus in response to intracerebroventricular adenovirus injection could clarify whether the hypothalamic increase in
IL-1 is effectively caused by locally enhanced IL-1 synthesis.
The role of TNF- in response to adenovirus injection into the brain
remains to be assessed. We have demonstrated that despite an increase
in the CSF levels of TNF- , there is no direct evidence to suggest
that this cytokine is involved in mediating the febrile response to
adenovirus, by, for example acting as a trigger for IL-1 release, as
has been suggested in other models of inflammation (Stefferl et al.,
1996 ). Importantly, Klir et al. (1993) report that the direct injection
of TNF- into the anterior hypothalamus did not increase body
temperature. In addition, recent studies in TNF- knock-out mice
suggest TNF- has an anti-inflammatory role in the brain (Liu et al.,
1998 ).
Previous studies have shown that the production of another
proinflammatory cytokine, IL-8, is rapidly increased in HeLa cells after adenovirus infection (Bruder and Kovesdi, 1997 ).
Intracerebroventricular injection of IL-8 can induce fever in rats,
however, unlike IL-1 and adenovirus-induced fever, IL-8-induced fever
in the rat is prostaglandin-independent (Zampronio et al., 1994 ). Given
that flurbiprofen efficiently blocked adenovirus-induced fever, this response is unlikely to be mediated by induction of CSF IL-8.
This is the first study to report the extremely rapid increase in brain
concentrations of proinflammatory cytokines in response to injection of
replication-deficient adenovirus vectors. Previous studies have
investigated the immune responses to adenovirus vectors at later stages
after infection, thus leaving the early inflammatory changes poorly
characterized. These early inflammatory changes are likely to be
extremely important in the brain, where most of the initial response
eliminating viral particles is likely to be performed by cells of the
innate immune system. The rapid cytokine and fever response to
adenovirus, independently of whether or not these express a transgene,
suggests that either the viral particle by itself, or through direct
interactions with the plasma membrane of target cells (Bruder and
Kovesdi, 1997 ), rapidly stimulates inflammatory cytokine secretion. If
the adenoviral particle per se triggers a fever response, we would
predict that new generation "gutless" adenoviral vectors would do
so also. Our work demonstrates that administration of
replication-deficient adenoviruses into the CSF, but not the striatum,
induces an extremely rapid cytokine release and a febrile response that
is mediated by IL-1 but is independent of TNF- . The role of IL-6
remains to be determined.
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FOOTNOTES |
Received Oct. 20, 1998; revised Dec. 1, 1998; accepted Dec. 4, 1998.
This work was supported by The Wellcome Trust, Medical Research
Council, Biotechnology and Biological Sciences Research Council, The
Parkinson's Disease Society, the European Community Concerted Action
Programme (BIOMED 1, Cytokines in the Brain PL96, and BIOMED 2, Suicide
Gene Therapy PL95), and the University of Manchester. We thank Mrs.
Anthea Hughes for technical assistance and Ms. Tricia Maleniak for
preparation of viral stocks and endotoxin assays. T.C. is a Wellcome
Trust International Postdoctoral Travel Fellow, T.S. a predoctoral
Action Research Fellow, and P.R.L. a Research Fellow of The Lister
Institute for Preventive Medicine.
Correspondence should be addressed to Professor P. R. Lowenstein,
Room 1.302 Stopford Building, Molecular Medicine Unit, Department of
Medicine, University of Manchester, Oxford Road, Manchester M13 9PT, UK.
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