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Short Communication |
From the Alzheimer's Research Unit,*
Department of
Neurology, Massachusetts General Hospital, Harvard Medical School,
Charlestown, and LeukoSite Inc.,
Cambridge, Massachusetts
| Abstract |
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, RANTES, eotaxin, and MCP-3 (ligands
for CCR5 and/or CCR3) revealed the presence of MIP-1ß predominantly
in a subpopulation of reactive astrocytes, which were more
widespread in AD than control brains, and MIP-1
predominantly in neurons and weakly in some microglia in both AD and
controls. Many of the CCR3+ or CCR5+ reactive
microglia and MIP-1ß+ reactive astrocytes were found
associated with amyloid deposits. Immunoreactivity for eotaxin,
RANTES, and MCP-3 were not detected. Detection of these
ß-chemokine receptors on microglia and some of their ligands in
reactive astrocytes and neurons as well as microglia suggests a role
for this system in glial-glial and glial-neuronal interactions,
potentially influencing the progression of AD.
| Introduction |
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(CXC), ß (CC),
(C), and
(CX3C), have been described.1-3
Chemokine
receptors are correspondingly named as CXCR (1 to 4), CCR (1 to 8), CR,
and CX3CR. All of the receptors are members of the
seven transmembrane domain receptor superfamily. There have been a
number of reports showing the presence of some
chemokines4-9
and their receptors10-13
in the
central nervous system (CNS). Recently we have shown the presence of
one of the
class of chemokine receptors, interleukin (IL)-8
receptor B (IL-8RB or CXCR2) in hippocampal and cortical neurons in
normal brain and also in neuritic plaques of Alzheimer's disease (AD)
brains.10
Horuk et al11,12
reported in the CNS
the presence of Duffy antigen and similar findings for IL-8RB. Lavi et
al13
demonstrated the expression of CXCR4 in neurons as
well as microglia. ß-Chemokine receptors may also have a role in the CNS. CCR3 and CCR5 have drawn considerable attention as the recently found co-receptors for HIV entry into cells,14-16 including microglia,17 which are the primary target of HIV infection in the CNS. HIV-infected patients may develop a progressive dementia, with motor and behavioral impairment termed AIDS-dementia complex. This ß-chemokine system appeared to be activated and may contribute to this CNS pathology.18,19 We hypothesized that activation of this system may also occur in AD and may potentially be involved in the disease pathogenesis. Here we describe an immunohistochemical study of CCR3, CCR5, and their ligands in AD and control brains.
| Materials and Methods |
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One CCR3 monoclonal antibody (MAb) (7B11; IgG) and three CCR5 MAbs
(3A9, 5C7, and 2D7; all are IgG) were made at LeukoSite (Cambridge, MA)
by immunizing mice using human CCR3 and CCR5 transfectants,
respectively.20,21
For MIP-1ß detection, mouse MAbs
(1F12; IgG) from LeukoSite was used. For MIP-1
, a mouse MAb (11A3;
IgG)22
from LeukoSite and a mouse MAb (500-M74; IgG) from
Peprotech (Rocky Hill, NJ) were used. For eotaxin, mouse MAbs (6H9,
2G6, and 1F5; all IgG) from LeukoSite were used. For RANTES, mouse MAbs
from LeukoSite (5B9 and 9H9; both IgM),22
R&D Systems,
Minneapolis, MN (MAb-678; IgG) and Peprotech (500-M75; IgG) were used.
For MCP-3, the mouse MAb 6H5 (IgG) from LeukoSite was used. All
chemokine MAbs from LeukoSite were highly specific to each specified
chemokine as determined by ELISA and Western blotting, and most of them
were shown to be good reagents for immunohistochemical study on
paraffin or frozen sections. For amyloid-ß (Aß) detection, rabbit
polyclonal antibody R1282 (a kind gift of Dr. Selkoe) was used. For
glial acidic fibrillary protein (GFAP), a rabbit anti-GFAP MAb (G-9269,
Sigma Chemical Co., St. Louis, MO) was used. Antibodies were either
purified and used at a concentration of 5 µg/ml or as hybridoma
supernatants (1:3 diluted).
Preabsorption of the anti-MIP-1
antibody 11A3 was carried out
by incubating 5 µg/ml 11A3 with 10 µg/ml MIP-1
protein
(Peprotech 300-08) for at least 1 hour at room temperature before being
used for immunohistochemistry.
Tissue Preparation
Postmortem brain tissues, including temporal lobe (containing the hippocampal formation, parahippocampal gyrus, and adjacent temporal neocortex), visual cortex, caudate, putamen, and cerebellum were obtained from the Massachusetts AD Research Center brain bank, and the diagnosis of AD and control was made using Khachaturian criteria.23 All brains were processed within 24 (average, 12) hours of death. Fresh tissue blocks were immersion fixed in periodate-lysine-paraformaldehyde (PLP) and processed as previously described.10 Sections were cut with a sledge freezing microtome at 50 µm thickness.
Temporal lobes from at least 12 age-compatible healthy controls and 18 AD cases, ages from 21 to 104 years, were examined unless indicated otherwise. Sections of visual cortex, putamen, caudate, and cerebellum from several cases were also studied.
Immunohistochemistry
Immunohistochemistry was carried out on free-floating sections using a previously described method.10 Horseradish peroxidase (HRP)-conjugated goat anti-mouse Ig (Jackson ImmunoResearch Laboratories, West Grove, PA) at 1:200 dilution was used as the secondary antibody. 3',3'-Diaminobenzidine (DAB) peroxidase substrate kit (Vector Laboratories, Burlingame, CA) was used as chromogen. Video images were captured on a Bioquant (Nashville, TN) image analysis system.
Confocal Microscopic Analysis
Double immunofluorescent staining was carried out essentially as previously described.10 Bodipy- or Cy3-conjugated anti-mouse or anti-rabbit IgG (1:200 diluted in 1.5% normal goat serum) were used as the secondary antibodies. Confocal images of fluorescent immunostained sections were obtained on a BioRad MRC 1024 confocal microscope with a krypton/argon laser (BioRad, Richmond, CA). The excitation and the emission filters were 488 nm and 522 nm for Bodipy-fluorescein and 568 nm and 605 nm for Cy3.
| Results |
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Using well characterized CCR3 (7B11) and CCR5 (3A9) antibodies,
CCR3 and CCR5 expression was found on microglia of all tested regions
in both control and AD brains (Figure 1)
.
Staining by two other CCR5 antibodies, 2D7 and 5C7, gave a similar
staining pattern. The intensities of CCR3 and CCR5 staining were very
similar. In control brains, most of the CCR3+ or
CCR5+ microglia appeared to be in a resting state. However,
in AD, the expression of CCR3 and CCR5 were increased on many reactive
microglia (Figure 1)
, and clusters of CCR3+ or
CCR5+ reactive microglia were frequently found scattered
throughout hippocampal formation and neocortex. Double immunostaining
in AD revealed that many CCR3+ or CCR5+
reactive microglia were associated with amyloid deposits (Figure 2, ad)
, although many were also found
in apparently plaque-free areas. Case to case variations were
attributed in part to postmortem interval (PMI), with weaker staining
in longer PMI cases.
|
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, and RANTES.2
As both CCR3 and CCR5 were found to be present on microglia in both AD
and non-AD brains, we further examined whether some of their ligands
are also expressed in the brain. Immunohistochemical study for
MIP-1ß, MIP-1
, RANTES, eotaxin, and MCP-3 were then carried out. Elevated MIP-1ß Expression in Reactive Astrocytes
MIP-1ß expression was found in a subpopulation of astrocytes in
all brain tissues tested. Although some apparently resting astrocytes
were weakly positive for MIP-1ß, most of the strongly stained
astrocytes appeared to be in a reactive state as judged by morphology.
In control brains, weak MIP-1ß staining appeared in a subpopulation
of resting astrocytes (Figure 3, a and b)
, with some reactive astrocytes mainly seen in subpial areas. In
contrast, in AD brains, more widespread and much stronger MIP-1ß
staining of astrocyte was observed (Figure 3, c and d)
. Generally, in
AD brains, the astrocyte staining appeared to be more abundant in
the hippocampal formation and the entorhinal cortex than in the
neocortex. MIP-1ß+ astrocytes were frequently seen
associated with amyloid deposits (Figure 2, e and f)
.
|
There was rather large case to case variation in the abundance and intensity of MIP-1ß staining of astrocytes, probably due to technical factors, including postmortem interval, as well as differences potentially attributable to different stages of the disease process. In some cases, cortical neurons were also weakly stained for MIP-1ß.
Detection of MIP-1
in Neurons and Microglia
MIP-1
immunoreactivity (detected by MAb 11A3) was found in the
gray matter in both AD and control brains. A diffuse pattern of
neuronal staining was observed. Neurons and neuropil of dentate gyrus,
CA regions, and cortex were all positive (Figure 3, e and f)
. The
clarity of neuronal staining differed greatly among cases. Some CA
region neurons and cortical neurons showed increased expression of
MIP-1
. Weak microglia staining mainly in the white matter was also
observed, and stronger microglia staining was seen in some AD brains. A
similar pattern of staining was also observed using another MIP-1
MAb from a different source (Peprotech). Specificity of the antibody
11A3 was tested by inhibition assay. Its immunoreactivity was
completely abolished by preabsorption with MIP-1
protein (data not
shown), supporting the specificity of its immunostaining pattern.
Evaluation of Eotaxin, RANTES, and MCP-3 Expression
Five cases that showed the strongest MIP-1ß and MIP-1
staining were tested for the other known ligands, eotaxin, RANTES, and
MCP-3 expression. Despite using antibodies from several sources, we
were unable to detect immunoreactivity for these ligands in brain.
| Discussion |
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There are several known ligands for these receptors. Ligands for CCR3
include eotaxin, RANTES, MCP-3, and MCP-4; ligands for CCR5 include
MIP-1ß, MIP-1
, and RANTES.1,2
Some of those ligands
have previously been reported in the CNS. In rodent models of spinal
cord injury, MIP-1ß and MIP-1
mRNA were found to be up-regulated
within the first hour after injury,31
and resident CNS
cells were the main source for these chemokine mRNAs.
Immunohistochemical staining has revealed a diffuse expression of both
RANTES and MIP-1ß in necrotic tissue after brain trauma and of
MIP-1ß on reactive astrocytes near the lesion site.32
In
rodent experimental autoimmune encephalomyelitis (EAE) models,
expression of RANTES by astrocytes and microglia has also been
observed.33,34
The capacity of the human CNS to express
some ß-chemokines under pathophysiological conditions has also been
demonstrated. Northern blot data have suggested the constitutive
presence of MIP-1
in the brain.35
Elevated MIP-1
and
MIP-1ß have been detected by reverse transcriptase polymerase chain
reaction, and reverse transcriptase in situ polymerase chain
reaction in brain tissue from patients with AIDS dementia compared with
infected patients without dementia18,19
and cells
expressing these chemokine mRNAs were identified morphologically as
astrocytes and microglia. In vitro experiments have also
shown that MIP-1
, MIP-1ß, and RANTES can be released by mixed
brain cell cultures by tumor necrosis factor-
treatment.36
There has been no report so far on the
expression of eotaxin, MCP-3, or MCP-4 in CNS cells either in rodents
or in primates.
Our current study detects two of the ß-chemokines, MIP-1
(also a
ligand for CCR1 in addition to CCR5) and MIP-1ß (a ligand only for
CCR5), on specific populations of cells in the human brain. MIP-1
appears to be constitutively expressed at a low level by neurons and
microglia, whereas MIP-1ß is predominantly expressed by a
subpopulation of reactive astrocytes. These results are largely in
accord with the data on their mRNA expression.18,19
Although the MIP-1
immunoreactivity does not apparently differ
between AD and control cases, the amount of its expression remains to
be quantified. The observation of more abundant MIP-1ß-positive
astrocytes in AD brains and their association with amyloid deposits
suggests an involvement in plaque-associated responses. We speculate
that these chemokines could contribute to the recruitment of microglia
to Aß deposits. Interestingly, MIP-1ß and MIP-1
are the major
ß-chemokines found selectively induced from monocytes after active
HIV-1 infection and have been found elevated in brains of AIDS dementia
patients compared with those HIV-1-infected patients without
dementia.18,19
The potentially detrimental role of these
chemokines to neuronal functions requires further investigation.
It is known that cytokines are difficult to detect by immunohistochemistry in human brain, so that the failure to detect RANTES, eotaxin, and MCP-3 in our study may reflect a sensitivity issue and does not rule out their potential presence in brain nor their up-regulation in AD. Antibodies for the other CCR3 ligand, MCP-4, are not currently available.
The presence of both ß-chemokines and their receptors in the control
brains suggest a physiological role for these molecules, which are
important in cell-cell communication in the periphery. Although it is
unclear what effect these ß-chemokines have on neuronal functions,
the elevated expression of CCR3/5 on reactive microglia and MIP-1ß in
reactive astrocytes of AD brains and their association with AD
pathology suggest their involvement in AD pathogenesis. Taken together
with earlier studies demonstrating
-chemokine receptors CXCR2 and
CXCR4 on neurons, the presence of the ß-chemokine receptors CCR3 and
CCR5 on microglia and CCR5 ligands in glia as well as in neurons
implies a new role for this class of molecules in mediating glial-glial
and glial-neuronal interactions.
| Acknowledgements |
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| Footnotes |
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Supported by NIH grants AG05134 and AG08487.
Accepted for publication April 24, 1998.
| References |
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and MIP-1ß in rat experimental autoimmune encephalomyelitis by in situ hybridization. J Neuroimmunol 1997, 77:17-26[Medline]
/LD78 in the human brain. Neuroreport 1997, 8:1215-1218[Medline]
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