


* Laboratory for Molecular Pharmacology, Department of Pharmacology, Panum Institute,
Department for Infectious Diseases, Rigshospitalet, and
7TM Pharma A/S, Copenhagen, Denmark.
Correspondence: Hans Rudolf Lüttichau, Laboratory for Molecular Pharmacology, Panum Institute 18/6, Blegdamsvej 3, DK-2200 Copenhagen, Denmark. E-mail: hrl{at}molpharm.dk
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Key Words: vMIP-I vMIP-II HHV8 MCV
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Chemokines are 70- to 80-amino-acid proteins with well-characterized three-dimensional structures usually stabilized by two disulfide bridges. They are involved in attracting and activating distinct leukocyte subsets [1 2 3 ]. The precise number of human chemokines has not been determined but is likely to be around 50. They are divided into four families on the basis of the pattern of the conserved cysteine residues located near their N termini and forming disulfide bridges with cysteines located further toward the C termini in the molecules. In the CC family, the two N-terminal cysteines are adjacent; in the CXC family the two residues are separated by a single amino acid; and in the CX3C family the separation is by three amino acids. The XC family, which as yet has one member, has only one cysteine near the N terminus. Chemokines exert their function through seven-transmembrane (7TM), G-protein-coupled receptors of which 11 are CC chemokine receptors (CCRs), 6 are CXC chemokine receptors (CXCRs), 1 is a CX3C chemokine receptor, and 1 is an XC chemokine receptor [3 ].
The lymphocytropic herpesviruses such as human herpesvirus (HHV) 6, HHV7, HHV8, and cytomegalovirus (CMV) have all been found to encode 7TM chemokine receptors [4 5 6 7 8 9 ]. Epstein-Barr virus does not encode a 7TM receptor; instead it has been found to up-regulate the encoding of human CCR (hCCR) 7 and EBI-II receptors [10 ]. A function has not been identified for most of these receptors except for the HHV8-encoded open reading frame 74, which may function as an oncogene due to its high constitutive activity [9 11 12 13 ] and the CMV-encoded US28, which might function as a chemokine scavenger [14 ] and might as well be involved in cellular transfer of the virus [15 ].
Most animal and human poxviruses have been found to encode a protein,
vCCI, with the ability to bind members from the CC chemokine family
[16
17
18
]. Furthermore, the murine
-herpesvirus 68 has
been found to encode another protein, M3, which binds several CC
chemokines [19
]. The function of these proteins could
very well be to block the inflammatory response from the host.
Finally, the large DNA viruses encode several chemokines. A number of these chemokines have been shown to act as agonists; others have been shown to act as antagonists, and others are still uncharacterized. The CMV-encoded chemokine vCXC1 has been shown to act as a CXCR2 agonist, and this function was suggested to aid in viral dissemination [20 ]. The HHV8-encoded chemokine vMIP-I has been found to act as a CCR8 agonist [21 22 ], and vMIP-III has been found to act as a CCR4 agonist [23 ]. These chemokines may help the virus to evade an antiviral T-helper (Th) 1 immune response by directing the response in a Th2 direction, because CCR4 and CCR8 are encoded on Th2 cells. We have shown that the HHV8-encoded vMIP-II functions as a broad-spectrum chemokine antagonist by blocking six receptors from all four chemokine receptor subfamilies [24 25 ]. Presumably the function of vMIP-II is to block the recruitment of leukocyte subsets necessary for an antiviral response. In contrast, we found MC148, the chemokine encoded by the skin-tropic molluscum contagiosum poxvirus (MCV), to be a highly selective CCR8 antagonist when it was tested on a panel of 16 categorized humane chemokine receptors [25 ]. The CXC chemokine vCXC2 encoded by CMV [26 ] and the CC chemokine U83 encoded by HHV6 [27 ] have so far not been characterized on individual cloned chemokine receptors.
The characterization of these virally encoded chemokine elements has provided tools to decipher the role of the chemokine elements encoded by the host. Thus, vCCI can, when used in animal models, show the role played by CC chemokines in the inflammatory response [16 , 28 , 29 ]. Furthermore, vMIP-II has been shown to reduce the inflammatory response in a rat model of glomerulonephritis [30 ].
MC148 is the only known high-affinity CCR8 antagonist. Therefore, this viral protein is a valuable tool in evaluating the role of the CCR8 receptor. However, when using animal models it is obviously important to characterize these proteins on specific animal chemokine receptors. Here we report the characterization of the virally encoded proteins vMIP-I, vMIP-II, and MC148 (Fig. 1 ) on the murine CCR (mCCR) 8 receptor, done in parallel with the hCCR8.
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Figure 1. Alignment of known ligands for the mCCR8 and the hCCR8 receptors.
vMIP-I, vMIP-II, MC148, I-309, and TCA-3 were aligned using ClustalW
1.8 software. Identical amino acids are shown in white on black,
whereas similar amino acids are shown in white on gray. Asterisks
indicate Cys residues.
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Cloning of mCCR8
A 5.5-kb SacI fragment containing the genomic mCCR8
was provided by Sergio Lira (Schering-Plough, Kenilworth, NJ). The
mCCR8 gene was amplified by PCR and inserted into the pTEJ8 vector.
Start and end primers were designed from a GenBank sequence (accession
no. NM007720). Nucleotide sequence analysis was performed on an ABI 310
sequence system (Perkin-Elmer). The sequence of mCCR8 was identical to
that of the GenBank sequence.
Stable cell lines
mCCR8 was transfected into Chinese hamster ovary (CHO) cells
using the GenePorter transfection reagent (Gene Therapy Systems, San
Diego, CA) according to the suppliers instructions to establish pool
clones, which subsequently were tested in calcium mobilization assays
with TCA-3. mCCR8 and hCCR8 were transfected into the murine pre-B-cell
line L1.2 by electroporation, and stable transfectants were obtained
after limiting dilution and chemical selection with the selection agent
geneticin ("G418") and subsequent functional selection by testing
the clones for calcium response to TCA-3 and I-309, respectively. In
addition an L1.2 cell line expressing hCCR8 established at ICOS
(Seattle, WA) was also used.
Binding
Whole-cell binding (2.0 x105 cells per well)
was performed at 4°C for 3 h in 0.5 mL of 25 mM HEPES buffer
containing 1 mM CaCl2 and 5 mM MgCl2 at pH 7.2,
supplemented with 0.5% bovine serum albumin on transiently transfected
COS-7 cells. The incubation was stopped by washing four times with 0.5
mL of ice-cold binding buffer made 0.5 mM in NaCl. Cell-associated
radioactivity was determined after extraction of the cells with 8 M
urea in 3 M acetic acid supplemented with 1% Nonidet P-40. Nonspecific
binding, determined in the presence of the relevant chemokine peptide
(0.1 µM), was subtracted. 125I-labeled MC148 was prepared
in house by Bolton-Hunter iodination prior to high-pressure liquid
chromatography purification.
Calcium mobilization experiments
L1.2 cells stably transfected with mCCR8 and hCCR8 and CHO cells
stably transfected with mCCR8 were loaded with Fura-2AM (Molecular
Probes, Eugene, OR) in RPMI 1640 with 1% fetal calf serum for 2030
min and washed in the same buffer. Aliquots comprised 106
cells. Each aliquot was pelleted and resuspended in 500 µL of
phosphate-buffered saline plus 1% fetal calf serum with 10 mM EGTA.
Fluorescence was measured on a Jobin Yvon FlouroMax-2 (Jobin Yvon Spex,
Edison, NJ) as the ratio of emission at 490 nm when excited at 340 nm
and 380 nm.
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Figure 2. Homologous competition binding experiments with recombinant MC148 for
hCCR8 and mCCR8. Homologous MC148 binding curves are shown for hCCR8
( ) and mCCR8 (). n = 3.
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In calcium mobilization assays, the mCCR8-expressing L1.2 cell line was found to respond to the endogenous murine ligand TCA-3, as well as I-309 and vMIP-I, with potencies for all three chemokines in the subnanomolar range (Fig. 3 ). In contrast, the hCCR8-expressing cell line was found to respond to the endogenous human ligand I-309 as well as vMIP-I with potencies in the subnanomolar range, but this cell line barely showed a response to submicromolar concentrations of TCA-3 (Fig. 3) .
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Figure 3. Dose-response curves of calcium mobilization by the agonists I-309,
TCA-3, and vMIP-I for L1.2 cells stably transfected with hCCR8 and
mCCR8. I-309, TCA-3, or vMIP-I at concentrations of 10-7,
10-8, 10-9, and 10-10 M or
vehicle was added to the cells at 50 s. One representative assay
out of two is shown for each agonist.
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Figure 4. Cross-desensitization assays in L1.2 cells stably transfected with
hCCR8 and mCCR8. In each experiment a supramaximal dose of agonist was
added at 50 s followed by a supramaximal dose of another agonist
at 250 s. The hCCR8/L1.2-transfected cells were stimulated with
10-8 M I-309 and 10-8 M vMIP-I . TCA-3 was
not used for the hCCR8/L1.2-transfected cells because it did not induce
a maximal response. The mCCR8/L1.2-transfected cells were stimulated
with 10-7 M I-309, 10-8 M TCA-3, and
10-9 M vMIP-I. One of two representative experiments is
shown for the hCCR8/L12-transfected cells and one of three for the
mCCR8/L1.2-transfected cells.
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Figure 5. Effect of recombinant MC148 and vMIP-II on calcium mobilization on L1.2
cells stably transfected with hCCR8 and mCCR8. MC148 (10-7
M), vMIP-II (10-7 M), or vehicle was added to the cells at
50 s followed by a submaximal dose of 10-9 M I-309 or
10-9 M TCA-3 at 150 s. The height of the response
curve with the endogenous ligand was measured, and the heights in the
experiments with MC148 and vMIP-II were expressed as percents of the
height in the experiment with the vehicle. A representative example of
the results from each experiment is shown at the left, whereas the box
diagram to the right shows the average inhibition + SE
as indicated. Asterisks indicate inhibition, for P <
0.05.
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vMIP-I, an agonist for hCCR8 and mCCR8
Earlier reports have shown vMIP-I to be a selective CCR8 agonist
[21
22
]. In this study, we showed that vMIP-I also acts
as an agonist on the mCCR8 with potencies in the subnanomolar range
comparable with potencies found for the hCCR8.
vMIP-II an antagonist for hCCR8 and mCCR8
We also found that vMIP-II inhibits calcium mobilization through
the hCCR8 and mCCR8 receptors expressed in L1.2 cells. This result is
in agreement with findings that vMIP-II inhibited calcium mobilization
in hCCR8-transfected human embryonic kidney 293 cells and activated
human T cells and that vMIP-II blocked I-309-induced chemotaxis of
hCCR8-transfected L1.2 cells [22
25
]. In contrast,
another study found that vMIP-II acts as an agonist on
hCCR8-transfected Jurkatt cells [32
]. Besides targeting
CCR8, vMIP-II has been found to block responses mediated through CCR1,
CCR2, CCR5, CXCR4, XCR1, and CX3CR1 as well as to inhibit responses
through CCR3, CCR4, and CXCR3 [24
25
]. The action of
vMIP-II on CCR3 could be dependent on the cellular setting, because vMIP-II has been shown to induce chemotaxis on eosinophils
[33
].
MC148, a selective antagonist for the hCCR8
Previously, we found that MC148 when probed against a panel of 16
individually cloned receptors targeted only CCR8, on which it acted as
a high-affinity antagonist [25
]. With these
characteristics, MC148 could be a useful tool to determine the function
of CCR8 in the immune system. Here we were surprised to find that MC148
was unable to bind and block signals through the mCCR8, although it
bound and blocked responses through the hCCR8 with high affinity. Thus,
it appears that murine disease models should not be chosen, if one
wants to use MC148 to decipher the role played by CCR8 in the immune
system.
What role does CCR8 play in our immune system?
Questions about the role CCR8 plays in our immune system can be
answered only indirectly because no studies have reported the use of
CCR8 antagonists in animal models or the phenotype of animals deleted
for the CCR8 gene. Functional data for I-309 as well as receptor
expression data show that CCR8 is found on monocytes [34
,
35
], Th2 cells [36
, 37
],
natural killer (NK) cells [38
], and thymocytes
[31
, 39
, 40
]. Furthermore,
I-309 is produced when T cells [41
], monocytes
[42
], human mast cells [43
], and
endothelial cells [44
] are activated. Thus it seems that
cells of different origin can produce I-309 when activated, which
attracts cells such as NK cells and monocytes that initiate an immune
response. A prominent example to support this notion has been provided
by the poxvirus MCV. It is tempting to ascribe the absence of an
inflammatory cell infiltrate in MCV lesions [45
46
47
] to
the MCV-encoded CCR8 antagonist MC148, although MCV also encodes other
immunomodulating proteins [48
].
In addition CCR8 has a unique role for thymocytes, as I-309 has been found to protect thymocyte cell lines from dexamethasone-induced apoptosis [49 ]. It could be speculated that the function of vMIP-I is to protect HHV8-infected lymphocytes from apoptosis.
It will be interesting to see whether MC148 can function as an immunosuppressive agent in nonmurine animal disease models.
Received November 17, 2000; accepted March 6, 2001.
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