* Medical Research Council Laboratory for Molecular Cell Biology and Department of Biochemistry, University College
London, London WC1E 6BT, United Kingdom; Department of Medicine, University of Pennsylvania, Philadelphia,
Pennsylvania 19104; § Laboratory for Molecular Pharmacology, Rigshospitalet 6321, DK-2100 Copenhagen, Denmark;
Division
of Molecular Sciences, Glaxo Research Institute, Research Triangle Park, North Carolina 27709; and ¶ Geneva Biomedical
Research Institute, GlaxoWellcome Research and Development SA, 1228-Plan-Les-Ouabes, Geneva, Switzerland
The chemokine receptor CXCR4 is required, together with CD4, for entry by some isolates of HIV-1, particularly those that emerge late in infection. The use of CXCR4 by these viruses likely has profound effects on viral host range and correlates with the evolution of immunodeficiency. Stromal cell-derived factor-1 (SDF-1), the ligand for CXCR4, can inhibit infection by CXCR4-dependent viruses. To understand the mechanism of this inhibition, we used a monoclonal antibody that is specific for CXCR4 to analyze the effects of phorbol esters and SDF-1 on surface expression of CXCR4. On human T cell lines SupT1 and BC7, CXCR4 undergoes slow constitutive internalization (1.0% of the cell surface pool/min). Addition of phorbol esters increased this endocytosis rate >6-fold and reduced cell surface CXCR4 expression by 60 to 90% over 120 min. CXCR4 was internalized through coated pits and coated vesicles and subsequently localized in endosomal compartments from where it could recycle to the cell surface after removal of the phorbol ester. SDF-1 also induced the rapid down modulation (half time ~5 min) of CXCR4. Using mink lung epithelial cells expressing CXCR4 and a COOH-terminal deletion mutant of CXCR4, we found that an intact cytoplasmic COOH-terminal domain was required for both PMA and ligand-induced CXCR4 endocytosis. However, experiments using inhibitors of protein kinase C indicated that SDF-1 and phorbol esters trigger down modulation through different cellular mechanisms.
SDF-1 inhibited HIV-1 infection of mink cells expressing CD4 and CXCR4. The inhibition of infection was less efficient for CXCR4 lacking the COOH-terminal domain, suggesting at least in part that SDF-1 inhibition of virus infection was mediated through ligand-induced internalization of CXCR4. Significantly, ligand induced internalization of CXCR4 but not CD4, suggesting that CXCR4 and CD4 do not normally physically interact on the cell surface. Together these studies indicate that endocytosis can regulate the cell-surface expression of CXCR4 and that SDF-1-mediated down regulation of cell-surface coreceptor expression contributes to chemokine-mediated inhibition of HIV infection.
SEVERAL members of the family of leukocyte chemokine receptors have been implicated in the fusion
and entry of human and simian immunodeficiency
viruses. Chemokine receptors are members of the superfamily of seven transmembrane domain, G protein-coupled
receptors that bind small peptides of the so-called CXC
( The precise role of chemokine receptors in virus entry is
unclear. The initial interaction of the viral envelope protein (Env) with CD4 is believed to induce conformational
changes in Env (19, 39, 57) that facilitate an interaction
with the chemokine receptor (62, 64) and assembly of a trimolecular complex of CD4, chemokine receptor, and Env
(36). The interaction of Env with both CD4 and CXCR4
appears to be crucial for the events that lead to viral fusion
and entry into the cell. Significantly, the CC chemokines,
macrophage inflammatory polypeptide (MIP)-1 We previously described a murine monoclonal antibody,
12G5, that is specific for CXCR4 (20). Among a panel of
CHO cell lines that stably expressed CXC (CXCR1, CXCR2,
and CXCR4) and CC (CCR1-5) receptors, 12G5 reacted
only with cells that expressed CXCR4. Subsequent studies
have mapped the 12G5-binding site to a conformational epitope that includes the second extracellular loop of CXCR4
(Hoxie, J.A., unpublished results). In this study, we have
used 12G5 to evaluate the effects of phorbol esters and
SDF-1 on CXCR4 expression and the extent to which surface levels of CXCR4 are regulated by endocytosis.
Reagents
All tissue culture reagents were from GIBCO BRL, Ltd. (Paisley, Scotland), and other chemicals were from Sigma Chemical Co. (Poole, England),
unless otherwise indicated. Tissue culture plastic was from Nunc (Roskilde,
Denmark), and radioactive reagents were from Amersham International
plc (Little Chalfont, England). Recombinant SDF-1 Cells
The CD4-positive human T cell line SupT1, and a CD4 CHO-K1 cells stably expressing human CXCR4, or either CXCR4 or
CCR4 tagged at the NH2 terminus with an epitope (YPYDVPYASLRS) from the influenza virus haemagglutinin (HA), have been described (20).
Mv-1-Lu-CD4 cells were transfected by electroporation with human
CXCR4 in the mammalian expression vector pTEJ8 (34), together with
pBABE-hygro (41). Clones were selected in medium containing 500 µg/
ml hygromycin and screened for CXCR4 expression by immunofluorescence using 12G5. Mv-1-Lu-CD4 cells expressing a CXCR4 Antibodies
The anti-CXCR4 mAb 12G5 (IgG2a) and the anti-CD4 mAb Q4120
(IgG1) were described previously (20, 29). FITC-conjugated L120 (anti-CD4) was purchased from Becton Dickinson UK Ltd. (Oxford, UK), and
rabbit antibodies against human LAMP1 were kindly provided by Dr.
Sven Carlsson (University of Umeå, Umeå, Sweden).
12G5 and Q4120 were 125I labeled using Bolton and Hunter reagent.
Briefly, Bolton and Hunter reagent (0.5 mCi at ~2,000 Ci/mmol) was dried
onto the sides of a 1.5-ml microcentrifuge tube. Antibody (~650 pmol) in
50 µl 0.1 M borate buffer, pH 8.5, was added, the tube vortexed, and the
reaction incubated at room temperature for 20 min. The reaction was
stopped by addition of 0.2 M glycine in borate buffer and the iodinated
protein separated from the reagents by gel filtration over an Econo-pac
10DG column (Bio Rad, Hemel Hempstead, UK) eluted with PBS containing 0.25% gelatine and 0.02% NaN3. Specific activities of 303 to 391 Ci/mmol were obtained for different iodinations. Radioiodinated proteins
were stored in small aliquots at Binding Assays
Antibody binding on live cells was carried out at 4°C. Adherent cells, usually in 16-mm wells, were incubated with radiolabeled antibody in binding
medium (BM: RPMI-1640 without bicarbonate, containing 0.2% BSA and
10 mM Hepes, and adjusted to pH 7.4, unless indicated otherwise) for 1 to
5 h at the indicated temperatures. Subsequently, the label was removed
and the cells washed 4 times with cold BM and twice with cold PBS. The
cells were then drained, collected in 400 µl 0.2 M NaOH, and each well
rinsed with 400 µl H2O. The cells and washings were transferred to tubes
for Phorbol Ester and SDF-1 Mediated Down Modulation
Cells were incubated in BM or in BM containing phorbol ester or SDF-1
as indicated in the text. For some experiments the cells were treated with
0.5 µM staurosporin or with 1 µM calphostin C (LC Laboratories Europe,
Alexis Corporation Ltd, Nottingham, UK) for 30 min before addition of
an equal volume of medium containing SDF-1 or phorbol ester. Cells
treated with calphostin C were incubated under a fluorescent strip light
for 3 min at room temperature before incubation at 37°C (9). After treatment, the cells were placed on ice and cooled by addition of 10 ml of ice-cold BM. T cells were then centrifuged (1,500 rpm for 5 min), washed
once in PBS, and fixed with PFA as described above. After quenching and
washing with BM the cells were labeled with 0.5 nM 125I-12G5 antibody
for 2 h at room temperature. Subsequently, the cells were washed twice in
BM and once in PBS, resuspended in 3 ml of PBS, layered onto a 1-ml
cushion of 5% BSA in PBS, centrifuged (1,800 rpm for 5 min), and the cell
pellets recovered for Endocytosis Assays
Endocytosis assays on adherent or suspension cells were performed essentially as described (48, 49). Suspension cells were harvested by centrifugation, washed twice, and resuspended at 5 x 106 cells/ml in 4°C BM containing 1 nM 125I-12G5. The cells were placed on a rotator and antibody
bound for 2 h at 4°C. Subsequently, the cells were washed twice in BM to
remove free antibody, and resuspended in 37°C BM with or without PMA.
At the indicated times, duplicate 1-ml aliquots were removed, placed into
5 ml ice-cold BM, and the cells collected by centrifugation (1,500 rpm for 5 min). One aliquot for each pair was incubated for 5 min in cold BM adjusted to pH 2.0, to elute cell surface-bound antibody, and the other was
washed in BM. Subsequently, the cells were layered onto a 5% BSA cushion, centrifuged, and recovered for Adherent cells were seeded in 16-mm diameter wells in either 4- or 24-well plates and grown for 2 d to a final density of 1 to 2 x 105 cells/well.
The cells were cooled on ice, washed with BM, and incubated with 300 µl
BM containing 1 nM 125I-12G5 for 2 h on ice. Subsequently, the free antibody was washed away and the cells warmed by addition of 1 ml 37°C BM.
At the indicated times the cells were returned to 4°C, the media collected, and the cells washed with cold BM. For each time point at least four wells
were used. For half of the wells, the cells were collected directly in 400 µl
0.2 M NaOH and transferred to tubes for Immunofluorescence Microscopy
Method One.
T cells were immobilized on 13-mm poly-D-lysine-coated glass
coverslips, fixed and quenched as described above, and stained intact or
after treatment with 0.05% saponin for 10 min at room temperature. All
solutions were made in PBS. Antibodies were diluted in PBS containing
0.2% gelatine and, for permeabilized cells, 0.05% saponin. Cells were
incubated for 1 h with primary antibodies. Fluorescent second layer antibodies were all diluted 1:2,000 in 0.2% gelatine, and incubations were for 1 h.
Method Two.
T cells were washed twice in cold BM and labeled with 7 µg/ml (50 nM) 12G5 for 2 h at 4°C. Some samples were also labeled with
FITC-conjugated L120. The cells were washed twice in cold BM. One aliquot of cells was kept at 4°C, while the others were incubated at 37°C in
BM with or without PMA. At the indicated times, 1 ml of cell suspension
was transferred to cold BM, centrifuged, and washed in cold BM. The cells
were fixed in 3% PFA, washed twice in PBS, and free aldehyde groups
quenched using 50 mM NH4Cl. The cells were attached to poly-D-lysine
coated coverslips, washed in PBS containing 0.2% gelatine and 0.05% saponin for 15 min, and incubated for 30 min with anti-mouse-Biotin (Amersham Intl. plc) and, where indicated, antibodies against LAMP1. The
cells were washed again and stained with streptavidin conjugated to FITC
or Texas red, or Rhodamine-conjugated goat anti-rabbit antibodies (Pierce) as appropriate. Subsequently the coverslips were mounted in Moviol and examined using a microscope (Optiphot-2; Nikon, Melville, NY)
equipped with a laser scanner (MRC 1024; Bio Rad). The images were assembled in and printed directly from Adobe Photoshop.
Flow Cytometry
Expression of cell surface antigens was analyzed using a FACSCalibur®
flow cytometer (Becton Dickinson). Briefly, cells were centrifuged (2,000 RPM for 5 min), resuspended in ice cold PBS containing 0.1% BSA and
0.02% NaN3, and incubated with 50 nM 12G5 for 30 min at 4°C. Subsequently, the cells were incubated with FITC- or PE-conjugated F(ab) Electron Microscopy
T cells were collected by centrifugation (1,500 rpm for 5 min), washed
once in BM containing 1% BSA, and labeled in 50 nM 12G5 for 2 h at 4°C.
To determine nonspecific binding, some cells were incubated in medium
without the primary antibody. After three washes in BM/1% BSA, cells
were labeled with 10-nm protein A-gold particles (British Biocell International, Cardiff, UK) for an additional 2 h at 4°C. Cells were washed once
in BM/1% BSA and twice in BM containing 0.1% BSA and divided into
three aliquots. One sample was kept on ice. The other two were warmed
to 37°C for 2 min by resuspension in 37°C BM/0.1% BSA with or without PMA. Subsequently, the cells were returned to 4°C by dilution with 10 ml
of ice-cold BM/0.1% BSA, harvested by centrifugation, and fixed in 2.5%
glutaraldehyde in 50 mM sodium cacodylate buffer, pH 7.2, containing
50 mM KCl and 2.5 mM MgCl2. Alternatively, fresh cells were prefixed by
mixing cell suspensions in an equal volume of double-strength fixative
(4% PFA/0.4% glutaraldehyde in 0.1 M sodium phosphate buffer, pH
7.4). After 10 min at room temperature, the cells were collected and resuspended gently in 2% PFA/0.2% glutaraldehyde in 0.1 M sodium phosphate buffer, pH 7.4, for an additional 80 min. Cells were washed and
quenched overnight in PBS containing 1% BSA and 50 mM glycine before staining with 50 nM 12G5 for 2 h at room temperature, followed by
protein A-gold for 2 h. After postfixation in osmium tetroxide, cells were
stained in Kellenberger's uranyl acetate, dehydrated, and embedded in
Epon. Ultrathin sections were examined using a transmission electron microscope (EM 400; Phillips, Eindhoven, The Netherlands).
HIV-1 Infection
For HIV-1 infection, semi-confluent Mv-1-Lu-CD4/CXCR4 and -CD4/
CXCR4 12G5 Binding
To use 12G5 as a probe for CXCR4 trafficking we initially
characterized the binding properties of this antibody on human RD and SupT1 cells constitutively expressing CXCR4
and on CHO cells expressing recombinant human CXCR4
with or without an NH2-terminal HA tag. In preliminary
experiments we found that on cells labeled at 4°C and on
cells fixed with 3% PFA and labeled at room temperature or 37°C, antibody binding at ~Kd concentration was slow.
At 4°C it took 5 h to reach near saturation (t1/2 maximum
binding, 1 nM = 80 min), while at both room temperature
and at 37°C, binding was slightly faster (t1/2 maximum
binding = 55 min) but still required >5 h to reach steady
state (not shown). Similar amounts of antibody were
bound after 5 h at all temperatures, suggesting that the
12G5 epitope was not significantly affected by aldehyde fixation. No binding was seen on CHO cells expressing
HA-tagged CCR4 under the same conditions. Although
binding was slow, the dissociation rate for bound antibody
was also slow. On CHO-CXCR4-HA <12% of the bound
antibody dissociated from the cells over 2 h at 37°C (not
shown), while on BC7 cells, >80% remained cell bound after 1 h (see Fig. 3 A).
To determine the level of CXCR4 expression we incubated cells with increasing concentrations of 125I-12G5 for
5 h. Even at the highest 125I-12G5 concentration used,
binding on CXCR4-HA cells was not fully saturated (Fig.
1 A), probably because these cells express relatively high
levels of the CXCR4-HA protein. Scatchard analysis of
the data indicated ~1.3 x 106 antibody binding sites per
cell with a Kd of 4 nM (Fig. 1 B). Similar analysis of BC7
cells indicated 50,000-100,000 binding sites with a Kd of 2 to 4 nM (not shown). Analysis of the RD cell line indicated ~380,000 12G5 binding sites per cell with a Kd of 2.2 nM (Fig. 1 B). HeLa cells that are permissive for T cell line-adapted HIV-1 viruses, when transfected with CD4
(21), expressed <60,000 12G5 binding sites (not shown).
In alternative assays using unlabeled 12G5 to compete
125I-12G5 (1 nM), binding on CHO-CXCR4-HA cells was
saturated at 20 to 30 nM 12G5 with an IC50 of 3.3 nM.
Scatchard analysis of this data indicated a single class of
binding site with a Kd of 1 nM and ~106 binding sites per
cell (not shown).
Together the Kds for 12G5 binding on different cell lines
fell within the range 1-5 nM, and the iodinated antibody
and native protein had similar affinities for antigen. For subsequent biochemical experiments, 125I-12G5 was used at
1 nM, unless indicated otherwise, while higher concentrations of unlabeled mAb were used for morphological assays.
Down Modulation of Cell Surface CXCR4
Previous studies have indicated that phorbol esters can
modulate the cell surface expression of CD4 (31, 51) and
may also influence CXCR4 expression (2, 25, 26, 36). To
examine the effect of phorbol ester on CXCR4 expression
directly, we incubated SupT1 cells at 37°C in the presence
or absence of 100 ng/ml PMA. After 60 min the cells were
fixed and stained with 12G5 either intact or after permeabilization with saponin. In the absence of PMA, clear cell
surface CXCR4 staining was seen on intact SupT1 cells
(Fig. 2 A, A). When untreated cells were permeabilized
before incubation with antibody, much of the cell surface
staining was lost, suggesting that the 12G5 epitope on
CXCR4 was sensitive to detergent. Nevertheless, some internal punctate staining was observed (Fig. 2 A, B). After
phorbol ester treatment, the cell surface staining on intact
cells was significantly decreased (Fig. 2 A, C); concomitantly, the intracellular fluorescence increased markedly
(Fig. 2 A, D), suggesting that phorbol esters induced endocytosis of cell surface CXCR4.
To determine the extent, concentration dependence, and
kinetics of phorbol ester-induced down modulation, SupT1
and BC7 cells were incubated for 2 h in the absence or
presence of phorbol ester. The cells were then cooled, labeled at 4°C with 12G5 and goat anti-mouse-FITC, and
analyzed by FACS. Fig. 2 B indicates that the expression
of CXCR4 was reduced by 65 and 95% on the PMA-treated SupT1 and BC7 cells, respectively. Control experiments on SupT1 cells indicated that CD4 underwent similar down modulation in the presence of PMA, but that
MHC class 1 antigens did not (data not shown). Using a
similar assay we determined that the 100 nM (60 ng/ml)
PMA gave maximal CXCR4 down modulation on SupT1
(Fig. 2 C). Routinely, 100 ng/ml PMA was used for subsequent experiments. The kinetics of phorbol ester-induced
down modulation showed that cell surface CXCR4 levels
decreased with a half time of ~15 min for SupT1 cells (see
Fig. 8 A).
Endocytosis of CXCR4 in T Cells
The appearance of increased intracellular CXCR4 staining
after phorbol ester treatment (Fig. 2 A) suggested that
PMA-induced CXCR4 down modulation occurred through
endocytosis of CXCR4, rather than by mechanisms that altered the conformation of CXCR4 at the cell surface. To
measure endocytosis directly we used 125I-12G5 in assays
previously established for CD4 (48, 49, 51). BC7 cells were
labeled with 125I-12G5 for 2 h at 4°C (conditions that inhibit endocytosis), washed, and then warmed to 37°C in
the presence or absence of PMA. Antibody remaining at
the cell surface after the 37°C incubation was removed by
briefly incubating the cells in 4°C media adjusted to pH 2.0. Control experiments indicated that >95% of the cell surface mAb was eluted under these conditions (not shown).
The remaining acid-resistant, cell-associated activity was
intracellular.
In BC7 cells, bound 125I-12G5 was observed to undergo
slow (~1% of the cell surface pool/min) constitutive endocytosis (Fig. 3 A). Uptake reached steady state 30-60
min after warm up, when ~18% of the initial cell surface
pool of antibody was inside the cells. These rates were
similar to the bulk flow internalization of cytoplasmic domain-deleted forms of CD4 measured previously in T cell
lines (48) and suggest that CXCR4 undergoes slow ligand-independent internalization and recycling on these cells.
When cells were warmed in the presence of PMA, the rate
of uptake was increased by >6-fold and reached steady
state between 15 and 30 min, when ~80% of the cell-associated radioactivity was intracellular. The PMA-induced
increase in CXCR4 endocytosis was again similar to the
phorbol ester-induced endocytosis and down modulation
of CD4 (31, 32, 51) and suggests that the activation of protein kinase C can induce the exposure of one or more endocytosis signals in CXCR4. Very similar data were obtained with SupT1 cells (not shown).
Endocytosis of CXCR4 in Transfected CHO and
Mink Cells
We also examined the properties of human CXCR4 expressed in transfected cell lines. Initially we used stable
CHO cell lines but found that these cells show fast constitutive endocytosis of CXCR4 (~2.5% per min) in the absence of phorbol ester and ligand. In these cells the uptake
of radiolabeled antibody reached a peak after 30 min of
endocytosis, when 60% of the initial cell surface pool was
inside the cells (not shown). Furthermore, the uptake of
125I-12G5 was only slightly enhanced by PMA and cell surface CXCR4 expression was not significantly down modulated by PMA (data not shown).
As CHO cells did not reflect the trafficking of CXCR4
observed on T cell lines, we investigated alternative cells
to analyze CXCR4 trafficking. CXCR4-transfected Mv-1-Lu-CD4 cells, a nonpolarized mink epithelial cell line,
showed properties similar to those of T cells. When CXCR4
was stably expressed in these cells, we observed slow constitutive endocytosis of CXCR4 at ~1% of the cell surface
pool per minute. When phorbol ester was added to the medium, the rate of endocytosis increased 3-4-fold (Fig. 3 B).
At steady state, ~60% of the initial cell surface pool of 125I-12G5 was intracellular. In these cells we examined
whether a mutant CXCR4 construct containing an early stop
codon that prevents synthesis of the last 42 amino acids of
the 45 amino acid COOH-terminal cytoplasmic domain of
the molecule was able to undergo endocytosis. CXCR4 In addition, we analyzed the endocytosis of CD4 in Mv-1-Lu-CD4 cells expressing CXCR4 or CXCR4 Together these data indicate that the trafficking properties of CXCR4 can be variable in different cellular backgrounds. However, for CXCR4 trafficking, transfected mink
cells showed properties similar to those of T cells. In mink
cells, CXCR4 undergoes slow constitutive endocytosis and
recycling that is markedly enhanced by phorbol ester and
likely to involve one or more endocytosis signals associated with the COOH-terminal domain of the molecule.
Constitutive and Phorbol Ester-induced Endocytosis of
CXCR4 Occurs through Clathrin-coated Pits
To determine the route of CXCR4 endocytosis we examined the distribution of CXCR4 by immunoelectron microscopy. SupT1 and BC7 cells were labeled on ice with
12G5 and protein A-gold and either fixed directly or after
warming to 37°C for 2 min in the presence or absence of
PMA. The 2 min time point was selected as the time at
which we expected to see the highest numbers of gold particles undergoing endocytosis. Thin sections were examined in the electron microscope and the distribution of
gold particles on the cell surface scored.
As indicated in Table I, some background labeling was
seen on cells labeled with protein A-gold alone. However,
particle counts on cells incubated with 12G5 and protein
A-gold indicated that the specific labeling was at least sevenfold above background. On cells labeled with 12G5-protein A-gold and maintained at 4°C, 2-3% of the gold particles
were located over invaginations of the plasma membrane
with cytoplasmic coats characteristic of clathrin (Fig. 4). On
cells warmed to 37°C for 2 min, similar levels of labeling in
coated pits and coated vesicles were detected (1.4 and 2.0%
of particles for BC7 and SupT1 cells, respectively). After
warm up in media containing phorbol ester, the number of particles associated with coated pits and coated vesicles
was increased 3-4-fold (Table I). This increase was consistent with the phorbol ester-induced increase in the rate of
CXCR4 endocytosis determined biochemically (see above),
and suggested that the coated pit pathway is responsible
for the majority, if not all, of the constitutive and phorbol
ester-induced endocytosis of CXCR4 in these cells. Human lymphoid cells do not express Vip21/Caveolin (47), and
no indication of particle association with noncoated invaginations of the plasma membrane was seen. After warm
up, gold particles were also occasionally observed in larger
vesicular structures resembling endosomes (not shown).
Table I.
Cell Surface Distribution of 12G5-Protein
A-Gold-labelled CXCR4
We observed some clustering of gold particles when
cells were stained with 12G5 and protein A-gold, although
this was variable with cell type and protein A-gold preparation. Notably, the clustering was similar in cells labeled
on ice or warmed in the presence or absence of PMA, suggesting that it did not contribute to the enhanced localization in coated pits seen with PMA. Furthermore, when
BC7 cells were fixed in PFA/glutaraldehyde before staining with 12G5 and protein A-gold, only single scattered
gold particles were observed. Of these, 1.3% were found
in coated pits, a figure similar to that found on cells stained
on ice (see above).
Phorbol Ester Down Modulated CXCR4 Is Located in
Endosomes
To determine where the antibody, and hence CXCR4, was
located, we visualized 12G5 in SupT1 cells using indirect
immunofluorescence and confocal laser scanning microscopy. Cells were labeled with 12G5 in the cold and warmed
to 37°C in the presence or absence of 100 ng/ml PMA.
Subsequently the cells were returned to 4°C, fixed, permeabilized, and stained with anti-mouse Ig secondary reagents. For some experiments, cells were also labeled with
antibodies against CD4, or after fixation and permeabilization, with antibodies directed against the lysosomal membrane glycoprotein LAMP1.
Fig. 5 shows that when SupT1 cells were labeled at 4°C
with antibodies to CD4 (green) and CXCR4 (red), both labels were seen at the cell surface with overlapping, though
not completely colocalized, distributions. After warm up
to 37°C in the presence of PMA, the cell surface staining
for both labels was rapidly (within 5 min) reduced, and
both labels were relocated into intracellular organelles located in the peripheral cytoplasm and in a cluster on one
side of the nucleus. Frequently these organelles, which we
presumed to be early endosomes, were labeled for both
CXCR4 and CD4 (Fig. 5, indicated by the yellow/orange
color). After 30 min of warm up, the staining for both CD4
and CXCR4 remained overlapped, but by 120 min the degree of overlap appeared to decrease, suggesting that the
CD4 and CXCR4 molecules were segregated. The majority of vesicles containing 12G5 internalized for 60 min
were accessible to a pulse of FITC-dextran applied during
the final 15 min of 12G5 uptake, suggesting that CXCR4
was localized primarily to an endosome compartment (not
shown). In the absence of PMA, some punctate intracellular staining for 12G5 appeared, presumably as a consequence of the constitutive endocytosis of CXCR4, but the
cell surface labeling remained prominent (not shown). Little internalization of CD4 was observed in the absence of
PMA, in keeping with the low level of constitutive uptake
of this molecule in T cells (48, 50).
The location of intracellular CXCR4 was compared to
the distribution of LAMP1, an integral membrane glycoprotein of late endosomes and lysosomes. After 5 min of
warm up, both in the presence or absence (not shown) of
PMA, punctate 12G5 labeling (green) was again seen in the
periphery of the cell and in a more perinuclear location as
described above. This staining appeared distinct from that
of LAMP1 (red) and for the most part remained separate
through the course of the experiment. Some overlap appeared after 120 min of labeling, but it is unclear at present
whether this represents colocalization of CXCR4 and
LAMP1 or our inability to resolve spatially close but distinct LAMP1- and CXCR4-containing organelles.
In addition to the fluorescence staining, we also determined whether internalized 125I-12G5 was degraded. Analysis of TCA-soluble counts appearing in the medium indicated that <10% of the antibody initially bound to cells
was degraded after 2 h incubation at 37°C (not shown). Similar levels of TCA-soluble activity were released from
cells treated with PMA even though the intracellular level
of 125I-12G5 was increased 3-4-fold (Fig. 3). Together
these results indicated that both CD4 and CXCR4 were
internalized into endocytic organelles after phorbol ester
treatment, and that little of the internalized 12G5 antibody
was delivered to lysosomes within the time courses of
these experiments.
Internalized CXCR4 Recycles to the Cell Surface
To determine whether CXCR4 internalized in the presence of phorbol ester can recycle to the cell surface, we incubated SupT1 cells at 37°C in phorbol dibutyrate (PDB),
a phorbol ester that can be washed out of cells. The cells
were incubated with PDB for 60 min and then either left
with PDB for a further period, or washed and returned to
normal media for the indicated time periods. A second set
of cells was treated similarly, but cycloheximide was included in the medium. Fig. 6 shows that incubation in PDB down regulated 60% of CXCR4 from the cell surface, and
that CXCR4 expression remained low when the cells were
maintained in PDB for a further 90 min. However, when
the PDB-treated cells were returned to normal medium,
cell surface CXCR4 levels recovered to ~80% of the initial levels over the subsequent 90 min (Fig. 6). Treatment
with cycloheximide had no apparent effect on the reappearance of CXCR4 on the cell surface, suggesting that re-expression occurred through recycling rather than delivery
of newly synthesised CXCR4 to the plasma membrane.
This conclusion was supported by experiments in which
125I-12G5 internalized during incubation in medium containing PDB was observed to recycle to the cell surface
when the PDB was removed (not shown). Together these
experiments indicated that internalized CXCR4 can recycle to the cell surface.
SDF-1 Does Not Compete for 12G5 Binding on T Cells
The CXC chemokine SDF-1 was recently identified as the
native ligand for CXCR4 (7, 45). To determine whether
SDF-1 competes with 125I-12G5 for binding, SupT1 cells
were pre-incubated with SDF-1 either at 4°C or, after fixation, at room temperature and then incubated with 125I-12G5
in the presence of SDF-1. At SDF-1 concentrations up to 2 µg/ml (250 nM), there was a slight (<30%) reduction of
12G5 binding on fixed cells incubated at room temperature but <10% reduction on cells incubated at 4°C (Fig. 7
A). 12G5 can partially inhibit SDF-1-mediated chemotactic responses, SDF-1-induced modulation of intracellular
calcium, and SDF-1 binding to CXCR4 (8, 30). However,
on SupT1 cells at least, SDF-1 did not efficiently compete for 12G5 binding.
The preparations of SDF-1 used for these experiments
were tested for their abilities to elicit a Ca2+ flux in Fura-2-loaded SupT1 and CHO-CXCR4 cells. Both preparations of SDF-1 produced a rapid, transient increase in cytosolic free calcium, at 4 µg/ml (500 nM) as previously reported (7, 45; data not shown). We also examined the
ability of SDF-1 to inhibit HIV-1 infection of Mv-1-Lu-CD4
cells expressing either CXCR4 or CXCR4 SDF-1 Down Modulates CXCR4
For chemokine receptors CXCR1, CXCR2, and CCR1,
the presence of ligand initiates both signaling responses
and rapid internalization of the cell surface receptor (11,
61). To determine whether SDF-1 induced endocytosis of
its receptor, SupT1 cells were incubated in 500 nM SDF-1
for up to 60 min at 37°C. At the indicated times the cells
were transferred to ice, fixed, and labeled with 125I-12G5
to determine the level of cell surface CXCR4. Fig. 8 A shows that incubation in SDF-1 induced a rapid (50% in 5 min) down modulation of cell surface CXCR4, with only
20% of the initial cell surface levels remaining by 30 min
of treatment. Although, we found that SDF-1 did not significantly inhibit 12G5 binding (Fig. 7 A), we repeated
these experiments using the acid wash protocol (described
above) to remove the surface-bound SDF-1 and obtained
the same result (see below).
The concentration dependence of SDF-1-mediated
CXCR4 down modulation was determined by incubating
SupT1 cells in increasing dilutions of SDF-1 for 30 min.
The cells were then cooled on ice, fixed, and labeled with
125I-12G5. Fig. 8 B shows that maximum down modulation
was induced at SDF-1 concentrations >125 nM. Partial
down modulation was seen with lower concentrations.
Similar binding was seen whether or not the cells were
acid stripped before antibody labeling (Fig. 8 B).
To compare the down modulation induced by SDF-1
and phorbol ester, we treated SupT1 cells with 100 ng/ml
PDB in parallel to cells treated with SDF-1. As indicated
in Fig. 8 A, PDB induced down modulation of cell surface
CXCR4 expression, though the time course (50% in ~15
min) was slower than that seen with chemokine. Recently,
there has been interest in the notion that CXCR4 and CD4 associate on the cell surface and that the HIV-1 Env
is able to interact with complexes of these molecules (36).
In the phorbol ester experiments described above we observed that CD4 and CXCR4 were co-internalized into
common early endosomes (Fig. 5). To determine whether
SDF-1 could induce co-internalization of CD4, we induced
CXCR4 internalization with SDF-1 for 1 h at 37°C, and subsequently the cells were labeled either with 125I-12G5
or with an anti-CD4 monoclonal antibody 125I-Q4120. Fig.
8 C shows that although SDF-1 could down modulate CXCR4, it did not induce CD4 internalization.
SDF-1 also induced down modulation of CXCR4 expressed in mink cells (Fig. 9). Mv-1-Lu-CD4/CXCR4 cells
were treated with SDF-1 for periods up to 1 h. At the end
of the incubation time the cells were washed with acid medium and the cell surface CXCR4 levels measured using
125I-12G5. SDF-1 induced rapid down modulation of CXCR4
on these cells (Fig. 9) with a very similar time course to
that seen in T cells (Fig. 8 A). More than 50% of the cell
surface CXCR4 was removed within 10 min of addition of
SDF-1 and ~90% down modulated by 60 min. In contrast,
SDF-1 did not down modulate CXCR4
SDF-1 and Phorbol Ester-induced Down Modulation
of CXCR4 Involve Different Pathways
To determine whether similar mechanisms were involved
in ligand- and phorbol ester-mediated internalization of
CXCR4, we investigated the effect of PKC inhibitors on
down modulation. SupT1 cells were treated with either
staurosporin or calphostin C for 30 min at 37°C and then
challenged with SDF-1 or PDB for 30 min at 37°C. Subsequently, cell surface CXCR4 and CD4 levels were determined and compared to those of untreated cells and cells
treated with ligand or phorbol ester but not inhibitor. We
found that both staurosporin and calphostin C inhibited
PDB-induced down modulation of CXCR4 and CD4 (Fig.
10). However, neither inhibitor blocked SDF-1-induced
down modulation of CXCR4. These data suggest that the
phorbol ester-induced down modulation of CXCR4 and
CD4 involves the action of PKC. In contrast, SDF-1-mediated CXCR4 down modulation appears to be independent
of PKC activation.
The subfamily of 7TM G protein-coupled receptors for
chemokines has been implicated in the entry of human
and simian immunodeficiency viruses into cells (for review
see 40). In humans, the chemokine receptor family currently contains 13 members with known chemokine-binding activities. These include the receptors for the CC chemokines (CCR1-8), the receptors for the CXC chemokines
(CXCR1-4), and the Duffy antigen that binds both CC
and CXC chemokines (52, 54). In addition, a number of
other CCR and CXCR family members have been identified, both from cellular sources (44, 65) and in viral genomes (see for example 4), for which the ligand-binding specificities are less well characterized or unknown. Of the
well characterised receptors, CCR5 in conjunction with
CD4 appears to be the principal coreceptor for macrophage
tropic isolates of HIV-1 (40), whereas CXCR4 is used by
T cell tropic and T cell line-adapted strains of HIV-1 (7, 21)
and either in conjunction with, or independently of, CD4
by some strains of HIV-2 (20, 55). Other family members
may also mediate entry for particular isolates of HIV-1,
HIV-2, and SIV (40). At present, the exact role of the
chemokine receptors in viral entry remains unclear. However, it appears likely that in conjunction with CD4 they
facilitate conformational changes in the viral envelope glycoprotein that leads to fusion of the viral membrane with
the plasma membrane of the target cell (40).
Chemokine receptors are expressed widely on lymphoid
cells and have been implicated in the chemotactic recruitment of lymphocytes, neutrophils, and other leukocytes to
sites of inflammation (54). Significantly, the chemokine
ligands for CCR5 (MIP-1 We found that CXCR4 on T cell lines undergoes slow
constitutive endocytosis and recycling. The rate of internalization was ~1% of the cell surface pool per minute
and reached steady state in 30 to 60 min, when ~20% of
the initial surface pool was intracellular. EM analysis of
cells labeled with 12G5 and protein A-gold indicated that
~1-2% of cell surface CXCR4 was associated with coated
pits under these conditions. When SupT1 or BC7 cells were
treated with phorbol esters, the cell surface expression of
CXCR4 decreased. The possibility that the change in expression was due to phorbol ester-induced conformational
changes in the protein that disrupted the 12G5 binding site
was ruled out by the demonstration that CXCR4 with associated antibody was internalized. The biochemical experiments using 125I-12G5 indicated that phorbol esters
rapidly induced a sixfold increase in the rate of endocytosis of CXCR4 from the cell surface, resulting in >80% of
the cell-associated radioactivity being located in intracellular compartments within 30 min. Furthermore, electron microscopy of immunogold-labeled PMA-treated cells indicated that the increased endocytosis occurred through
enhanced interaction of CXCR4 with coated pits. The intracellular accumulation of CXCR4 may occur through increased endocytosis; in addition, inhibition of recycling
may also contribute to down modulation. We have been unable to measure the rates of recycling in the continued
presence of ligand or phorbol ester directly and cannot at
present rule out effects on recycling. However, modelling
calculations (51) and the observation that down modulation does not go to completion suggest that the increased
rate of endocytosis is primarily responsible for the observed down modulation.
We previously demonstrated that CD4 endocytosis is
also modulated by phorbol esters (32, 51). CD4 internalization is regulated through its association with the Src
family kinase p56lck and by the presence of an endocytosis
signal in the cytoplasmic domain of CD4. This signal is dependent on phosphorylation of critical serine residues (38)
and facilitates CD4 association with coated pits leading to
endocytosis (51). The CD4 endocytosis signal involves
serine 408 and a pair of leucine residues at positions 413 and 414 (58, 59; Pitcher, C., and M. Marsh, unpublished results). This motif (SQIKRLL in CD4, where S lies within a
PKC phosphorylation site) is representative of a group of
regulated endocytosis and trafficking signals that are active when the serine is phosphorylated but inactive when it
is not (see for example 15, 16). A similar motif (SSLKIL;
Ile can replace Leu in these signals) is present in the
COOH-terminal cytoplasmic domain of CXCR4. Whether
this sequence is involved in phorbol ester-induced endocytosis and down modulation of CXCR4 remains to be established. However, initial experiments indicate that CCR5,
which lacks this motif, is not down modulated by phorbol
esters (Hoxie, J.A., and M. Marsh, unpublished results). In
addition, in keeping with results from the IL-8 receptor CXCR2 (53), our experiments indicate that the COOH
terminus is required for rapid phorbol ester-induced CXCR4
endocytosis.
To examine the domains of CXCR4 involved in trafficking requires an appropriate cell line for transfection. In
our initial experiments we used CHO-K1 cells stably expressing CXCR4 and HA-tagged CXCR4 molecules. However, we found that CXCR4 expressed in these cells underwent relatively fast constitutive endocytosis and that CXCR4 cell surface levels and endocytic rates were not
significantly modified by phorbol ester. We have previously found that the constitutive endocytosis of CD4 stably expressed in CHO cells is faster than that seen in other
cell types (M. Marsh, unpublished results). As an alternative to CHO cells we analyzed the trafficking of CXCR4
expressed in mink lung cells Mv-1-Lu (these cells are used in our laboratory as an indicator cell line for HIV-2 infection studies as they lack 7TM coreceptors for HIV-2). In
these cells the constitutive phorbol ester and ligand-induced
trafficking of CXCR4 showed properties very similar to
those observed in T cells. However, it is perhaps important
to stress that the anomalous results we observed with CHO
cells indicate that the analysis of the trafficking properties
of these receptors in transfected cells should be approached
with caution.
The CXC chemokine SDF-1 has been identified as a
ligand for CXCR4. For a number of 7TM proteins, binding
of ligand induces rapid internalization of the receptor
through both clathrin-dependent and clathrin-independent pathways (11, 23, 24, 27, 33, 56, 61, 63, 66), though in
some cases ligand binding does not induce internalization
(35, 46). As shown here SDF-1 can induce rapid down
modulation of CXCR4 in both transformed T cell lines
and transfected mink cells. Furthermore, this down modulation does not require activation of PKC. The mechanism
through which CXCR4 endocytosis occurs is unclear. Experiments with the Although there have been many studies on the trafficking of other 7TM G protein-coupled receptors, there is
currently little data on the trafficking of the CXC and CC
chemokine receptors. Here we have shown that CXCR4 is
able to undergo efficient endocytosis after treatment of
cells with phorbol esters, and that the receptor is also
down modulated by SDF-1. In mice, SDF-1 is required for B cell lymphopoiesis, bone-marrow myelopoiesis, and for
correct development of the heart (43). CXCR4 is known
to be expressed on various lymphocytes (8) and other leukocytes (40), and SDF-1 is a potent chemokine for CD34+ve
hematopoietic progenitor cells (1). In addition, the protein is also expressed on endothelial (Hoxie, J.A., and L.F. Brass, unpublished observations) and neuronal cells (30), though
it is unclear whether it has similar functions on these cells.
For T cells at least, it is perhaps significant that CXCR4
appears to respond to similar modulatory signals as CD4.
CD4 down modulation can be induced by antigen and
other stimuli and involves the activation of PKC (38). Comodulation of CXCR4 together with CD4 may play a role
in regulating the activity of CD4 positive T cells. By immunofluorescence we observed that CD4 and CXCR4 were
initially co-internalized into the same endosomal organelles.
However, we have no indication that the two molecules interact. The different constitutive endocytosis rates of CD4
and CXCR4 expressed on SupT1 cells (48), the absence of
comodulation of CXCR4 SDF-1 inhibits the entry of some T cell line-adapted
HIV viruses into cells (7, 45). Although there is currently
little data, initial studies have suggested that antagonists of
chemokine receptors that block virus entry do not induce
chemokine receptor internalization (60, 61), indicating that
the chemokines may be able to interfere with Env binding.
However, our findings, and similar data reported by Amara et al. (2), indicate that SDF-1 can induce rapid endocytosis of CXCR4, and that this chemokine is a more effective
inhibitor of HIV-1 infection on cells expressing endocytosis-competent CXCR4 than on cells expressing CXCR4
) and CC (
) families of inflammatory chemokines (for
review see 42, 52, 54). Initially, the CXC chemokine receptor CXCR4 (previously termed LESTR, HUMSTER, and
Fusin [21, 37]) was identified as a coreceptor, together
with CD4, for the entry of T cell line-adapted human immunodeficiency virus (HIV)1-1 viruses (6, 21). Subsequently, the CC chemokine receptor CCR5 was found to
be required for the entry of macrophage tropic viruses (10,
14, 18). Other chemokine receptors (CCR3, CCR2b, and
CCR1) have been implicated in the entry of dual (10, 17) and neurotropic viruses (28), while CXCR4, CCR3, and an
orphan receptor VT28 can mediate the entry of CD4-independent strains of HIV-2 (20, 55; for an extensive review
of HIV coreceptor usage see 40). The use of particular
chemokine receptors by HIV-1 may have important biological consequences not only for the viral host range, but
also for pathogenesis, since viruses isolated in the initial
stages of infection primarily use CCR5, while those isolated from patients with advanced immunodeficiency may
use CXCR4 in addition to, or in place of, CCR5 (13).
, MIP1
, and RANTES (regulated on activation normal T cell expressed and secreted) can inhibit the entry of macrophage
tropic HIV-1 isolates into CCR5-positive target cells (12)
and stromal cell-derived factor (SDF)-1, the ligand for
CXCR4, can inhibit infection of at least some T cell line-adapted viruses (7, 45). The mechanism through which
these agents inhibit infection is unclear. The chemokines could inhibit viral entry by blocking the interaction of the
Env with the chemokine receptor (62, 64). Alternatively,
as observed with other G protein-coupled receptors (33,
56, 61, 63), the ligand may induce internalization, thereby
preventing assembly of the fusion complex.
Materials and Methods
was purified from E.
coli. This SDF contained an additional NH2-terminal methionine. However, the protein was biologically active as demonstrated by (a) Ca2+ flux
assays on Fura-2-loaded SupT1 cells and CHO-CXCR4 cells; (b) potent
activity (10-100 pM) in a CXCR4-transfected melanophore assay; (c) inhibition of HIV-1 entry, and (d) ligand-induced receptor down modulation
(see text). In addition, chemically synthesised SDF-1 (7, 45) was kindly
provided by Dr. Ian Clark-Lewis (University of British Columbia, Vancouver, Canada).
ve derivative of
SupT1 called BC7 (20), were maintained in RPMI-1640 containing 10%
FCS, 2 mM glutamine, 100 U/ml penicillin, and 0.1 mg/ml streptomycin (PenStrep). CHO-K1 were maintained in DMEM F12 containing 10% FCS, glutamine, and PenStrep as above. The rhabdomyosarcoma cell line
RD was obtained from P. Clapham (Institute of Cancer Research, London, UK) and maintained in DME containing 5% FCS, glutamine, and
PenStrep as above. Mv-1-Lu mink cells stably expressing human CD4
(Mv-1-Lu-CD4) were obtained from the Medical Research Council AIDS
Reagents Programme (NIBSC, Potters Bar, UK) and maintained in DME
containing 10% FCS, glutamine, and PenStrep as above and 1 mg/ml
G418.
Cyt protein
lacking 42 amino acids from the COOH-terminal cytoplasmic domain
were generated using a human CXCR4 construct in which the threonine
311 codon was replaced with a stop codon by site-directed mutagenesis.
20°C and were stable for up to 4 mo.
counting. Fixed cells were used for binding analysis at ambient temperature (20-22°C) or 37°C. For these experiments, the cells were washed
in PBS and fixed in 3% paraformaldehyde (PFA) in PBS for 10 min at
room temperature. Subsequently, the cells were washed 4 times with PBS
and free aldehyde groups quenched with 50 mM NH4Cl in PBS. The cells
were again washed with PBS and then incubated with labeled antibody as
above. Protein concentrations were determined using bicinchoninic acid
(Pierce, Chester, UK).
counting. For adherent cells 12G5 binding was as
described above.
counting as described above.
counting (total cell-associated
activity). To determine the intracellular activity, the remaining wells were
rinsed twice with 0.5 ml of 4°C BM adjusted to pH 2.0, and then incubated
twice for 3 min with 1 ml of the same medium to remove cell-surface antibody. The cells were harvested in NaOH as above. The proportion of internalized activity for each time point was determined by dividing the
acid-resistant activity by the total cell-associated activity, and endocytic rates were calculated by analysis of the data from the first 5 min of warm up.
2
goat anti-mouse IgG (Tago Laboratories, Burlingame, CA).
Cyt cells were trypsinized, washed, and 2 x 104 cells seeded in
each well of a 96-well plate in 200 µl DME with 10% FCS and cultured for
2 d before infection. The cultures were treated with SDF-1 or RANTES at
twice the indicated concentration for 30 min at 37°C. An equal volume of
HIVIIIB diluted from a stock virus (103 focus forming units) was added and
the cells cultured for 14 h before the virus and chemokine were removed.
The cells were cultured for an additional 2 d and then fixed in methanol/
acetone. They were then stained with the anti-HIV-1 Gag mAbs E67.1
and 38:96K for 1 h and subsequently with a goat anti-mouse IgG conjugated to
galactosidase (Seralab, Crawley Down, UK) for 1 h.
Galactosidase activity was detected by incubation with 5-bromo-4-chloro-3-indolyl-
-D-galactopyranoside (X-Gal), and foci of blue cells were viewed by
light microscopy and counted.
Results
Fig. 3.
Endocytosis of CXCR4 in BC7 and Mv-1-Lu cells. BC7
cells in suspension (A) or confluent cultures of Mv-1-Lu-CD4/
CXCR4 cells (B) were labeled at 0 to 4°C with 125I-12G5, washed,
and warmed to allow endocytosis of the ligand. The amount of internalized antibody was determined by acid washing as described
in Materials and Methods. The plots show the acid-resistant activity as a proportion of the total cell-associated counts for cells
warmed in the absence () or presence of PMA (
). In A the total cell-associated activity is shown for the course of the experiment (
). B (
and
) shows the endocytosis kinetics of CXCR4
Cyt in the absence and presence of PMA, respectively.
All data points show means and standard deviations for triplicate samples of representative experiments.
[View Larger Version of this Image (16K GIF file)]
Fig. 1.
Binding properties of 12G5 on CXCR4-expressing
cells. (A) Concentration dependence of 12G5 binding. CHO cells
expressing CXCR4-HA () or CCR4-HA (
) and RD cells (
)
were incubated with increasing concentrations of 125I-12G5 (up to
10 nM) for 5 h at 4°C. Aliquots of the unbound label (free) were
taken for counting and the cells washed and harvested. The protein per well was determined and used to calculate the amount of
antibody bound per 106 cells. The binding recorded on CCR4-HA
cells was taken as background and was deducted from the other
cell lines to generate the binding data used in the Scatchard analysis illustrated in B. (B) Scatchard analysis of 12G5 binding. The
bound and free 12G5 activities derived from the experiment illustrated in A were used for Scatchard-type analysis of 12G5 binding
to native CXCR4 expressed on RD cells (
) and CXCR4-HA expressed on CHO cells (
).
[View Larger Version of this Image (15K GIF file)]
Fig. 2.
Phorbol ester-induced down modulation of CXCR4.
(A) Immunofluorescence analysis of CXCR4 down modulation
on SupT1 cells. Cells were incubated in medium with (A, C and
D) or without (A, A and B) 100 ng/ml PMA for 60 min at 37°C.
The cells were then fixed and stained with 12G5 either intact (A,
A and C) or after permeabilization (A, B and D) with saponin.
(B) SupT1 and BC7 cells were incubated in medium with () or
without (
) 100 ng/ml PMA for 120 min at 37°C. Subsequently
the cells were fixed and stained first with 12G5 and then with a
FITC-conjugated anti-mouse reagent. The stained cells were
analysed by FACScan® and the mean fluorescence intensity determined for each sample. (C) The dose dependence for PMA-
induced down modulation of CXCR4 was determined on SupT1
cells. Cells were incubated in medium containing the indicated
concentration of PMA for 60 min at 37°C. The cells were then
fixed, stained as described for B, and analyzed by FACScan®.
Down modulation was calculated from the mean fluorescence intensity for each sample and compared to cells stained without primary antibody. Bar, 25 µm.
[View Larger Versions of these Images (39 + 14 + 63K GIF file)]
Fig. 8.
SDF-1- and PDB-induced down modulation of CXCR4 on SupT1 cells. (A) SupT1 cells were incubated in medium (), medium containing 100 ng/ml PDB (
), or medium containing 500 nM SDF-1 (
) for up to 60 min at 37°C. At the indicated time points,
aliquots of cells were placed on ice, washed with cold binding medium, fixed, and incubated with 0.5 nM 125I-12G5 for 2 h at room temperature. The values indicate the means and standard deviations for triplicate samples from a representative experiment. (B) SupT1
cells were incubated in twofold dilutions of SDF-1 for 30 min at 37°C. The cells were then cooled to 4°C and either fixed and labeled as
in A with 125I-12G5 for 2 h (
), or briefly incubated in low pH medium before fixation and labeling (
). (C) SupT1 cells were incubated with (
) or without (
) 125 nM SDF-1 for 60 min at 37°C. The cells were then cooled to 4°C, fixed, and labeled as in A with 125I-12G5 or 0.3 nM 125I-Q4120 to detect cell surface CXCR4 and CD4, respectively.
[View Larger Versions of these Images (14 + 39 + 28K GIF file)]
Cyt
was expressed on the cell surface at levels similar to
CXCR4 but, in contrast to the full length constructs,
CXCR4
Cyt was internalized slowly (Fig. 3 B) at rates
and to levels comparable to the basal endocytosis of CXCR4.
Cyt. In the
absence of phorbol ester, CD4 was internalized slowly
(~1% per min) on both CXCR4- and CXCR4
Cyt-expressing cells, as reported for other transfected cells (48, 49).
Addition of phorbol ester induced rapid endocytosis of
CD4 on both CXCR4 and CXCR4
Cyt cells (data not
shown), indicating that both were capable of responding to
phorbol ester and mediating endocytosis. Thus the lack of
phorbol ester-induced endocytosis of CXCR4
Cyt was
due to the loss of the CXCR4 COOH-terminal domain and not defects in the ability of these cells to mediate endocytosis.
Fig. 4.
EM immunolocalization of CXCR4 on the surface of
SupT1 and BC7 cells. SupT1 (A and B) and BC7 (C and D) cells
were labeled at 4°C with 12G5 followed by protein A-gold
(PAG10) and either fixed directly (A) or warmed to 37°C for 2 min in medium containing PMA (100 ng/ml; B and C). Alternatively, cells were fixed in 2% paraformaldehyde/0.2% glutaraldehyde before labeling with 12G5 and PAG10 (D). Bars, 50 nm.
[View Larger Version of this Image (154K GIF file)]
Fig. 5.
Immunofluorescence analysis of CXCR4 and CD4 internalization on SupT1 cells. Top figures (CD4-green/CXCR4-red):
SupT1 cells were labeled with 12G5 and FITC-conjugated L120 (anti-CD4) at 4°C before warming to 37°C in the medium containing 100 ng/ml PMA. At the indicated times the cells were cooled, fixed, permeabilized, and stained with a biotin-conjugated isotype-specific
anti-mouse reagent to detect 12G5, followed by streptavidin-Texas red. Lower figures (CXCR4-green/LAMP1-red): SupT1 cells were
initially labeled with 12G5 alone. After fixation and permeabilization the cells were stained with anti-LAMP1 and Rhodamine-conjugated anti-rabbit antibodies and with a biotin-conjugated anti-mouse reagent and streptavidin-FITC to detect 12G5.
[View Larger Version of this Image (14K GIF file)]
Fig. 6.
Recycling of internalized CXCR4. SupT1 cells were either left untreated or incubated in medium containing 100 ng/ml
PDB for 60 min at 37°C. Aliquots of the treated cells were placed
on ice or left in PDB for a further 90 min (+90). The remaining
cells were washed three times with media to remove the PDB and
incubated in fresh 37°C medium for the indicated time periods
(
). Subsequently, all cells were cooled to 4°C and incubated
with 125I-12G5 to determine cell surface CXCR4 levels. In parallel, a duplicate set of cells was treated in the same way with 100 µg/ml cycloheximide present throughout (
). The data show the
means and standard deviations from triplicate samples.
[View Larger Version of this Image (42K GIF file)]
Fig. 7.
12G5 binding to CXCR4 in the presence of SDF-1. (A)
SupT1 cells were washed, fixed (3% PFA, 15 min), or cooled on
ice and incubated in twofold dilutions of SDF-1 for 2 h at room
temperature (
) or 3 h at 4°C (
), respectively. The cells incubated at 4°C were then fixed, and all the cells were labeled with
0.5 nM 125I-12G5 for 2 h at room temperature. Subsequently, the
cells were washed and the amount of bound antibody determined. Antibody bound in the presence of SDF-1 is expressed as
a percentage of antibody bound in the absence of ligand and represents the means and SD for triplicate samples. (B) SDF-1 inhibition of HIV-1 infection. Mv-1-Lu-CD4/CXCR4 (
) and Mv-1-Lu-CD4/CXCR4
Cyt (
) were cultured in 96-well plates. The
cells were incubated with twice the indicated concentration of
SDF-1 (GlaxoWellcome) for 30 min before the addition of HIV-1IIIB. After 14 h the virus and chemokines were removed and the
cells incubated for a further 2 d. Finally, the cells were fixed and
stained for infected cell foci and each focus scored as a single infection event. The number of focus-forming units per 100 µl of virus innoculum is plotted on the y axis. The bars show the means
of two wells; the error bars are one standard deviation.
[View Larger Version of this Image (34K GIF file)]
Cyt. These cells
can be infected with HIV-1 when induced to express both
CD4 and an appropriate chemokine coreceptor (21). Before infection the cells were treated for 30 min with SDF-1
as indicated (Fig. 7 B) and then challenged with infectious
HIV-1IIIB. The cells were cocultured with virus overnight,
washed, and then incubated for a further 2 d. Subsequently,
the cells were fixed and stained for HIV-1 Gag. Fig. 7 B
shows that HIV-1 infection of Mv-1-Lu-CD4/CXCR4 cells,
as indicated by the numbers of stained foci, was virtually completely blocked in the presence of 500 nM SDF-1. Less
effective inhibition was seen with lower SDF-1 concentrations. The
chemokine Rantes, which does not bind CXCR4,
had no effect on infection. The CXCR4
Cyt molecule
was also able to support HIV-1 infection in these cells (Fig.
7 B). However, infection of Mv-1-Lu-CD4/CXCR4
Cyt cells was only inhibited ~60% at 500 nM SDF-1 (Fig. 7 B).
Cyt expressed in
Mv-1-Lu cells (Fig. 9), indicating that the COOH-terminal cytoplasmic domain was crucial for ligand and phorbol ester-induced down modulation.
Fig. 9.
The COOH-terminal cytoplasmic domain is required for SDF-1
down regulation of CXCR4. Mv-1-Lu-CD4/CXCR4 and Mv-1-Lu-CD4/
CXCR4Cyt were incubated in BM
(
) or BM containing 500 nM SDF-1
(
) at 37°C. At the indicated times the
cells were cooled on ice, washed with
BM, and then incubated in BM adjusted to pH 2.0 (acid medium) for 10 min. The cells were then returned to
BM (pH 7.4) at 4°C and cell surface
CXCR4 determined using 0.5 nM 125I-12G5. Each point shows the mean and
SD of triplicate samples from a representative experiment.
[View Larger Version of this Image (13K GIF file)]
Fig. 10.
Effect of PKC inhibitors on
SDF-1- and phorbol ester-induced down
modulation of CXCR4 and CD4. SupT1
cells were washed twice by centrifugation and resuspended in 6 ml of BM,
alone or with 0.5 µM staurosporin or 1 µM calphostin C, and incubated for 30 min at 37°C. For each condition 6 x 1 ml
of cell suspension (~3.25 x 106 cells/
ml) were diluted 1:1 with BM or BM
containing 200 ng/ml PDB or 250 nM
SDF-1 at 37°C for 30 min. The cells
were then rapidly cooled on ice by dilution with 10 ml of cold PBS, centrifuged, and washed once in cold PBS.
They were then fixed for 15 min in 3%
PFA, washed, and quenched in 50 mM
NH4Cl and labeled with 0.5 nM 125I-12G5 (for CXCR4) or 0.3 nM 125I-Q4120 (for CD4) for 2 h at room temperature. The cell-associated activity was determined as described in Materials and Methods. Binding medium alone (), 100 ng/ml PDB (
), and 125 nM SDF-1 (
).
[View Larger Version of this Image (22K GIF file)]
Discussion
, MIP-1
, and Rantes), CCR3
(Eotaxin), and CXCR4 (SDF-1), as well as some receptor
antagonists, can inhibit infection of cells by HIV viruses that use these receptors (3, 60). Studies with other 7TM proteins, and with IL-8 receptors (CXCR1 and 2) in particular, have indicated that ligand binding can induce rapid
internalization and down modulation of the receptors
from the cell surface (11, 22, 56, 61). Therefore, chemokines could exert their antiviral effects by sterically blocking binding of viral Env or by inducing internalization of
the chemokine receptor. To understand the cellular mechanisms that regulate the surface expression of CXCR4,
and thus its ability to function as an HIV coreceptor, we
used the CXCR4-specific monoclonal antibody 12G5 (20)
to evaluate CXCR4 endocytosis in response to phorbol ester and its natural ligand, SDF-1.
2 adrenergic receptor suggest that
ligand binding stimulates phosphorylation of the receptor
by
adrenergic receptor-kinases (G protein-coupled receptor kinases) and interaction with
arrestins that can
act as adaptors for clathrin-coated pits (27, 66). It is at
present unclear whether similar mechanisms operate for
CXCR4. However, our results indicate that phorbol esters
and SDF-1 operate through different intracellular signaling pathways to induce CXCR4 internalization. The mechanism of phorbol ester-induced CXCR4 internalization
may well involve PKC-mediated phosphorylation of a
COOH-terminal domain signal similar to the endocytosis
signal in CD4. Although the COOH-terminal is also required for SDF-1-induced down modulation, PKC activation is not required for ligand-induced down modulation. It remains to be determined whether CXCR4 contains
multiple independent endocytosis signals or whether different signaling pathways can activate the same endocytosis signal.
Cyt with CD4 on phorbol ester-treated Mv-1-Lu cells, and the selective SDF-1-induced down modulation of CXCR4 but not CD4, suggest these
two proteins do not normally form stable associations. Thus
the comodulation of CD4 and full length CXCR4 seen after phorbol ester treatment most likely occurs as a consequence of the two molecules containing similar trafficking
signals. Moreover, the complex of CD4, CXCR4, and HIV-1
gp120 that has been proposed as an intermediate in viral fusion (36, 62, 64) is likely to be induced by the presence of
the viral gp120 protein.
Cyt, suggest that endocytosis may make a significant contribution to chemokine protection. The potential to down
regulate the cell surface expression of the coreceptor molecules by ligand-dependent or -independent means may
provide novel strategies for limiting HIV infection.
Received for publication 16 May 1997 and in revised form 24 July 1997.
Address all correspondence to Mark Marsh, Medical Research Council, Laboratory for Molecular Cell Biology and Department of Biochemistry, University College London, Gower Street, London WC1E 6BT, UK. Tel.: (44) 171-380-7807. Fax: (44) 171-380-7805. E-mail: m.marsh{at}ucl.ac.ukWe are grateful to colleagues who have contributed reagents, ideas, and discussions to this work. In particular, Dr. Ron Barrett and colleagues (Affymax, Inc.) for providing transfected CHO cells and M.-J. Bijlmakers for critically reading the manuscript.
HA, hemagglutinin; HIV, human immunodeficiency virus; MIP, macrophage inflammatory peptide; PDB, phorbol dibutyrate; SDF, stromal cell-derived factor.
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