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Journal of Virology, August 2003, p. 8588-8592, Vol. 77, No. 15
0022-538X/03/$08.00+0 DOI: 10.1128/JVI.77.15.8588-8592.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
The Gammaherpesvirus Chemokine Binding Protein Binds to the N Terminus of CXCL8
Louise M. C. Webb,1 Ian Clark-Lewis,2 and Antonio Alcami1*
Department of Medicine and Department of Pathology, Division of Virology, University of Cambridge, Cambridge, United Kingdom,1
Biomedical Research Centre and Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, British Columbia, Canada2
Received 13 March 2003/
Accepted 7 May 2003

ABSTRACT
Viruses encode proteins that disrupt chemokine responses. The
murine gammaherpesvirus 68 gene
M3 encodes a chemokine binding
protein (vCKBP-3) which has no sequence similarity to chemokine
receptors but inhibits chemokine receptor binding and activity.
We have used a panel of CXCL8 analogs to identify the structural
requirements for CXCL8 to bind to vCKBP-3 in a scintillation
proximity assay. Our data suggest that vCKBP-3 acts by mimicking
the binding of chemokine receptors to CXCL8.

TEXT
Chemokines orchestrate leukocyte migration from blood to sites
of injury or infection. Their central importance is underscored
by the fact that many viruses (e.g., herpesvirus, poxvirus,
and retrovirus) subvert the chemokine system during infection
by encoding chemokine binding proteins (vCKBPs) or chemokine
and chemokine receptor homologues (
1,
17). During the course
of their evolution, viruses have finely tuned their immunomodulatory
proteins and offer us potential therapeutics for disease and
greater insight into chemokine biology.
Three types of vCKBPs have been identified: vCKBP-1 (encoded by myxoma virus) binds with low specificity to chemokines from all subfamilies (CC, CXC, C, and CX3C), vCKBP-2 (encoded by several poxviruses) binds with high affinity to only CC chemokines, and vCKBP-3 (unique to murine gammaherpesvirus 68) binds with high affinity to chemokines from all subfamilies (2, 11, 14, 18, 21, 22). Identification of the chemokine epitopes involved in vCKBP-1 and vCKBP-2 binding explain their different specificities. While vCKBP-1 binds weakly to the glycosaminoglycan (GAG) binding domain present in the majority of chemokines (14), vCKBP-2 specifically interacts with residues within the N-loop of certain CC chemokines (5, 20). Only the structural requirements for vCKBP-3 to bind to the CC chemokine, CCL2 (monocyte chemoattractant protein 1), have been determined (3). Like vCKBP-2, vCKBP-3 uses the N-loop of CCL2 for binding, with Tyr13 playing a key role. Both vCKBP-2 and vCKBP-3 bind to most CC chemokines. However, in contrast to vCKBP-2, vCKBP-3 is able to bind to some CXC chemokines with significant affinity (18, 22). It has been shown to bind CXCL1, CXCL8, CXCL10, and CXCL13 in a species-dependent manner (18, 22). It shows selectivity in the CXC chemokines in that it binds to many CXC chemokines with no significant affinity for vCKBP-3. As yet, the CXC epitopes used for vCKBP-3 binding remain unknown. vCKBP-3 can bind and inhibit CXCL8 (interleukin-8) (18, 22). Inactivation of the M3 gene (which encodes vCKBP-3 from murine gammaherpesvirus 68) has shown that vCKBP-3 is critical for establishing latency in splenic B cells and for induction of lethal meningitis (6, 23).
We have used chemically synthesized analogs of CXCL8 to map the vCKBP-3 binding site (Fig. 1). Chemokine analogs are powerful tools that have been extensively used to define key residues and domains required for chemokine activity (8-10, 12, 13, 15, 24). Analogs were synthesized with tertiary-butyloxycarbonyl chemistry and automated solid-phase methods (9). Purified recombinant vCKBP-3 protein containing a COOH-terminal six-histidine tag was produced in the baculovirus system and provided by Campbell Bunce and Mark Wilson (Xenova, Cambridge, United Kingdom) (18).
To investigate CXCL8 binding to vCKBP-3, we determined equilibrium
competition binding in a scintillation proximity assay with
Flashplates (Perkin Elmer Life Sciences) (
7). His-tagged vCKBP-3
(40 ng/ml) was incubated with 200 pM
125I-CXCL8 (wild type [WT])
and a dose response of cold CXCL8 analog in a total volume of
100 µl in 0.1% bovine serum albumin-phosphate-buffered
saline. Background
125I-CXCL8 binding was measured without vCKBP-3
and was similar to binding in the presence of excess unlabeled
CXCL8. Data were analyzed by subtracting the background binding
for each assay. Affinities were calculated with the LIGAND program
(
16). Scatchard analysis of a saturation curve was used to first
calculate the affinity of CXCL8 for vCKBP-3 (7.4 pM) (data not
shown). The differences between the CXCL8 affinities for vCKBP-3
obtained with saturation and competition assays (23.5 pM) (Table
1) are due to different chemical sources of CXCL8 and assays
used.
We first used CXCL8 analogs with progressive deletions in the
N terminus to compete with
125I-CXCL8 for binding to vCKBP-3
in scintillation proximity assays. Removal of the first six
residues of CXCL8 decreased its ability to compete for vCKBP-3
binding (Fig.
2A and B; Table
1). This region contains the ELR
motif which binds to the CXCL8 receptors (
8). The N-loop of
CXCL8 (residues 8 to 16) is also important for receptor interactions.
Within the N-loop, Ile
10 is crucial for receptor binding and
activation (
9). The only analog within the N-loop to show a
significant drop in vCKBP-3 affinity was I10A (Fig.
2C and Table
1). Leu
49 packs into the bulge in the N-loop and provides optimal
conformation and binding for CXCR1 (
10). We also found that
this analog had a reduced affinity for vCKBP-3 (Fig.
2C and
Table
1).
The similar tertiary structure of chemokines has enabled the
construction of hybrid analogs to identify the minimal CXCL8
structure required for activity (
8). We used hybrids of CXCL8
with CXCL10 (interferon-inducible protein 10) (Fig.
1) to confirm
the role of the N-loop for vCKBP-3 binding. CXCL10 has no neutrophil
binding or activating properties and has a significantly lower
affinity for vCKBP-3 than CXCL8 (Table
1; Fig.
2D) (
8). CXCL10-CXCL8H1
differs from CXCL10-CXCL8H7 by lacking the N-loop of CXCL8 (residues
4 to 15 and 24 to 26) (Fig.
1). We found that CXCL10-CXCL8H7
competed with
125I-CXCL8 for vCKBP-3 as effectively as CXCL8,
whereas CXCL10-CXCL8H1 behaved like CXCL10 (Fig.
2D).
We next assessed the importance of the C-terminal alpha helix of CXCL8 for vCKBP-3 binding. This region does not directly participate in receptor binding but is critical for GAG binding and provides a platform for the receptor binding domains (Fig. 1) (24). We used analogs in which the C terminus was progressively truncated. Removal of the last three or six residues of CXCL8 resulted in a slight decrease in affinity (Fig. 3A; Table 1). This is in contrast to vCKBP-1, which is unable to bind the analog of CXCL8 containing residues 1 to 66, providing a structural explanation for its ability to prevent the chemokine-GAG interaction (14). There was a significant loss in affinity for vCKBP-3 when the last 14 residues of CXCL8 were removed (analog containing residues 1 to 58) (Fig. 3B; Table 1). To determine whether the C terminus of CXCL8 interacts directly with vCKBP-3 or is simply required for optimal conformation of CXCL8, we used hybrid analogs of CXCL8 with either CXCL12 (stroma-derived factor 1
) or CXCL4 (platelet factor 4) (Fig. 1). Neither CXCL12 nor CXCL4 could bind vCKBP-3 (Table 1; Fig. 3C and D) (18). As shown in Fig. 3C, CXCL12-CXCL8H1 was unable to bind vCKBP-3, showing that the C-terminal region of CXCL8 (residues 18 to 72) is insufficient for vCKBP-3 binding. The hybrid CXCL4-CXCL8H1 bound vCKBP-3 with high affinity (Table 1 and Fig. 3D), showing that while the C terminus of CXCL8 is not important for vCKBP-3 binding, the N terminus is critical. Unlike CXCL10-CXCL8H1, CXCL4-CXCL8H1 contains the N-loop of CXCL8 and binds to vCKBP-3 with high affinity. This experiment not only shows that the C terminus of CXCL8 is dispensable for vCKBP-3 binding but also restates the importance of the N-loop of CXCL8 for vCKBP-3 binding. In agreement with this, the hybrid CXCL10-CXCL8H7 (which contains the N-loop of CXCL8 and the C terminus of CXCL10) behaved like CXCL8 (Fig. 2D). We also examined the role of the C-terminal alpha helix of CXCL8 alone in vCKBP-3 binding. This analog was unable to bind vCKBP-3 (Fig. 3E).
Finally, we examined the effect of vCKBP-3 on chemotaxis induced
by either WT or C-terminally truncated CXCL8 (residues 1 to
58). Using the values obtained in our competition assays for
the affinity of vCKBP-3 for WT CXCL8 and CXCL8 containing residues
1 to 58, we were able to predict how much vCKBP-3 would be required
to inhibit chemotaxis. We estimated that 69 µM vCKBP-3
would be required to inhibit 50% of the chemotaxis induced by
30 ng of CXCL8/ml and 46.8 mM vCKBP-3 would be required to inhibit
50% of the chemotaxis induced by 900 ng of CXCL8/ml, assuming
a 1:1 binding of vCKBP-3 to CXCL8. WT CXCL8 (30 ng/ml) or CXCL8
containing residues 1 to 58 (900 ng/ml) was preincubated with
various doses of vCKBP-3 in a neutrophil chemotaxis assay. The
higher dose of CXCL8 (residues 1 to 58) was used to obtain a
similar induction of chemotaxis as WT CXCL8 since it is 30 times
less potent than WT CXCL8 (
9). vCKBP-3 inhibited chemotaxis
induced by both WT CXCL8 and CXCL8 containing residues 1 to
58, showing that binding to the N terminus is sufficient to
block CXCL8 activity (Fig.
4). The higher concentration of vCKBP-3
required for inhibition of CXCL8 (residues 1 to 58) reflects
the larger amount of chemokine present. The functional 50% inhibitory
concentration of vCKBP-3 for blocking WT CXCL8 and residues
1 to 58 of CXCL8 were 66.7 µM and 68.7 mM, respectively,
calculated with the Prism Program (GraphPad Prim). This showed
that the observed inhibition of chemotaxis correlated well with
our in vitro competition assays.
Our data show that it is the N terminus of CXCL8 that binds
to vCKBP-3, with the greatest contribution coming from the N-loop.
This is in contrast to vCKBP-2, which uses only the N-loop of
CC chemokines for binding, since removal of the N terminus of
CCL2 actually increases vCKBP-2 affinity. This may explain the
broader specificity of vCKBP-3. Unlike vCKBP-2, vCKBP-3 binds
to some (but not all) CXC chemokines (
18,
22). The crystal structure
of vCKBP-3 bound to CCL2 has been determined and shows that,
like CXCL8, the N-loop of CCL2 contributes most to vCKBP-3 binding
(
3). The N-loop is the secondary receptor binding site and is
thought to confer specificity. Within the N-loop, Ile
10 in CXCL8
is important for both receptor and vCKBP-3 binding. This residue
aligns in the same position as Tyr
13 within CCL2, which is important
for the interaction of CCL2 with vCKBP-3 and vCKBP-2 (
3,
20).
The observation that similar binding epitopes are used for recognition
of vCKBP-3 and chemokine receptors provides a structural basis
for the ability of vCKBP-3 to occlude the binding of chemokines
to their host receptors.
In conclusion, we have defined the key residues of a CXC chemokine, CXCL8, that interact with vCKBP-3. This provides valuable information on the recognition of chemokines by viral proteins and will allow the rational design of chemokine inhibitors.

ACKNOWLEDGMENTS
We acknowledge the support of the Wellcome Trust (grant 051087/Z/97/Z).
A. A. is a Wellcome Trust Senior Research Fellow.
We thank Campbell Bunce and Mark Wilson for providing recombinant vCKBP-3.

FOOTNOTES
* Corresponding author. Mailing address: Department of Medicine, University of Cambridge, Addenbrooke's Hospital, Level 5, Box 157, Hills Rd., Cambridge CB2 2QQ, United Kingdom. Phone: 44 1223 763403. Fax: 44 1223 330158. E-mail:
aa258{at}mole.bio.cam.ac.uk.

This paper is dedicated to the memory of Ian Clark-Lewis (1955 to 2002). 

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Journal of Virology, August 2003, p. 8588-8592, Vol. 77, No. 15
0022-538X/03/$08.00+0 DOI: 10.1128/JVI.77.15.8588-8592.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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