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Fractalkine mediates T celldependent proliferation of synovial fibroblasts in rheumatoid arthritis.

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Vol. 56, No. 10, October 2007, pp 3215–3225
DOI 10.1002/art.22919
© 2007, American College of Rheumatology
Fractalkine Mediates T Cell–Dependent Proliferation of
Synovial Fibroblasts in Rheumatoid Arthritis
Hirokazu Sawai, Yong W. Park, Xiaowen He, Jörg J. Goronzy, and Cornelia M. Weyand
ation. TNF␣ amplified the expansion of FLS by enhancing their expression of CX3CR1 and FKN.
Conclusion. FKN–CX3CR1 receptor–ligand interactions regulate FLS growth and FLS-dependent T cell
function. FLS stimulate autocrine growth by releasing
FKN and triggering the activity of their own CX3CR1.
This growth-promotion loop is amplified by TNF␣ produced by CX3CR1-expressing T cells upon stimulation
by FKN-expressing FLS. These data assign a critical
role to FKN and its receptor in fibroblast proliferation
and pannus formation in RA.
Objective. In rheumatoid arthritis (RA), synovial
fibroblasts proliferate excessively, eventually eroding
bone and cartilage. The aim of this study was to examine
the mechanisms through which CD4 T cells, the dominant lymphocyte population in patients with rheumatoid synovitis, regulate synoviocyte proliferation.
Methods. Fibroblast-like synoviocyte (FLS) lines
were established from rheumatoid synovium. CD4 T
cells from patients with RA and age-matched control
subjects were cultured on FLS monolayers. FLS proliferation was quantified by cytometry, using carboxyfluorescein succinimidyl ester staining or microscopic enumeration of PKH26-stained FLS. Surface expression of
the fractalkine (FKN) receptor CX3CR1 was monitored
by fluorescence-activated cell sorting. The induction of
CX3CR1 and its ligand FKN in FLS was quantified by
real-time polymerase chain reaction.
Results. The proliferation of FLS was significantly increased in the presence of CD4 T cells from
patients with RA compared with control T cells.
CD4ⴙ,CD28– T cells were particularly effective in supporting FLS growth, inducing a 25-fold expansion compared with a 5-fold expansion induced by CD4ⴙ,CD28ⴙ
T cells. The growth-promoting activity of CD4ⴙ,CD28–
T cells was mediated through CX3CR1, a chemokine
receptor expressed on both T cells and FLS. AntiCX3CR1 antibodies inhibited T cell production of tumor
necrosis factor ␣ (TNF␣) and suppressed FLS prolifer-
Hyperplasia of the synovial layer is a principal
disease mechanism in rheumatoid arthritis (RA). The
expansion of synovial tissue generates pannus, a destructive tumorlike structure that penetrates into the cartilage
and subchondral bone, leading to erosion of these
structures. In the rheumatoid joint, the hyperplastic
membrane is composed of a specialized type of fibroblasts, fibroblast-like synoviocytes (FLS), which grow in
an anchorage-independent manner and are resistant to
apoptosis (1). FLS from the rheumatoid synovium have
long been recognized as a source of proinflammatory
cytokines and proteases, functioning as an amplifier of
inflammation and directly contributing to tissue damage
(2,3). FLS derived from patients with RA were shown to
attach to and invade normal human cartilage in a SCID
mouse model (4). It has been suggested that the invasive
behavior of FLS from patients with RA correlates with
the rate of joint destruction as the disease progresses (5).
FLS have also been implicated in regulating the fate of
tissue-invasive lymphocytes, placing them in a critical
position in the rheumatoid disease process. Specifically,
FLS have been described to provide signals to T cells,
altering their susceptibility to apoptosis and determining
their survival and migration pattern in the inflamed
lesions (6–8). Mutuality in the relationship between FLS
and T cells has been suggested by the demonstration that
activated T cells determine the functional profile of FLS
in a contact-dependent manner (9,10).
Supported in part by the NIH (grants R01-AR-42527, R01AI-44142, R01-AR-41974, and R01-AI-57266).
Hirokazu Sawai, MD, Yong W. Park, MD, Xiaowen He, PhD,
Jörg J. Goronzy, MD, PhD, Cornelia M. Weyand, MD, PhD: Emory
University School of Medicine, Atlanta, Georgia.
Drs. Sawai and Park contributed equally to this work.
Address correspondence and reprint requests to Cornelia M.
Weyand, MD, PhD, Lowance Center for Human Immunology, Emory
University School of Medicine, Room 1003 Woodruff Memorial
Research Building, 101 Woodruff Circle, Atlanta, GA 30322. E-mail:
Submitted for publication December 12, 2006; accepted in
revised form June 22, 2007.
CD4⫹ helper T cells are recognized as central
players in the pathogenesis of RA (11). Not only are
CD4 T cells the most prominent cell population in
synovial infiltrates, they also contribute to several disease pathways, such as the process of lymphoid neogenesis. Evidence has accumulated that dysfunctional CD4
T cells in patients with RA are not restricted to the joint.
Rather, the entire T cell population is abnormal. Specifically, CD4⫹ T cells from patients with RA display
phenotypic and functional changes indicative of premature senescence (12,13). CD4⫹ T cells from patients
with RA have shortened telomeres, suggesting an intense proliferative history. The senescence program in
these cells is associated with loss of the costimulatory
molecule CD28 and de novo expression of numerous
immunoregulatory receptors, including molecules of the
killer cell immunoglobulin-like receptor (KIR) family
(14) and NKG2D (15). CD4⫹,CD28– T cells become
resistant to apoptosis (16) and overexpress the Th1
cytokines interferon-␥ (IFN␥) and tumor necrosis factor
␣ (TNF␣). Also, CD4⫹,CD28– T cells acquire cytolytic
activity and lyse target cells with efficiency similar to that
of professional killer cells (17).
With the shift in their receptor profile,
CD4⫹,CD28– T cells are obviously regulated by a new
set of environmental signals. Ligands for KIR and
NKG2D are HLA class I molecules. Thus,
CD4⫹,CD28– T cells no longer depend on professional
antigen-presenting cells for activation but rather become
responsive to a broad spectrum of cellular partners in
inflamed lesions. This principle is exemplified by recent
reports that CD4⫹,CD28– T cells in patients with RA
express CX3CR1 (18), a receptor binding the chemokine
fractalkine (FKN) and usually restricted to cytotoxic
CD8 T cells and natural killer cells. Synovial CD4 T cells
use CX3CR1 to costimulate T cell receptor–derived
signals and respond to cell-bound FKN with the enhanced release of cytokines. CX3CR1 is also expressed
on synovial cells other than T cells, such as macrophages,
dendritic cells, and synovial fibroblasts (18). The synovial microenvironment is rich in FKN (19), an unusual
chemokine that exists in both a membrane-integrated
and a soluble form. Its chemokine domain is displayed
on a long negatively charged mucin-rich stalk extending
from the cell surface. When cleaved from the cell
membrane, FKN yields a soluble form (sFKN) (19,20).
In RA synovium, the major source of FKN is FLS (18),
but synovial macrophages and endothelial cells may also
produce this chemokine. It is currently believed that
FKN mediates all of its biologic actions by binding
CX3CR1. Soluble FKN acts as a chemoattractant; the
membrane-bound form functions as an adhesion molecule (21).
It has been proposed that FKN promotes recruitment of monocytes and T cells into the rheumatoid
synovium (22–24). Recent work by our group suggests
that in RA, the role of FKN extends beyond chemoattraction and adhesion (18). FKN is abundantly expressed on cultured synovial fibroblasts and on hyperplastic synoviocytes in rheumatoid tissue. Among CD4⫹
T cells, only CD28⫺ T cells express the FKN receptor
CX3CR1. Such T cells strongly adhere to FLS in a
FKN/CX3CR1-dependent manner. More importantly,
FKN displayed on the surface of FLS cooperates with T
cell–activating signals and amplifies T cell proliferation,
IFN␥ production, and cytoplasmic granule expulsion.
In the present study, we investigated how
CD4⫹,CD28– T cells affect the proliferation of rheumatoid FLS. We found that interactions between
CX3CR1 and FKN not only regulate T cell function but,
equally importantly, affect fundamental functions of
synovial fibroblasts. Specifically, with CX3CR1 expressed on FLS and FKN secreted by FLS, the
CX3CR1–FKN receptor–ligand interaction functions as
an autocrine growth-promotion loop enhancing proliferative expansion of these fibroblasts. Expression of the
receptor and the ligand on FLS is regulated through the
T cell proinflammatory cytokine TNF␣, placing this
cytokine at a critical checkpoint for controlling synovial
hyperplasia. TNF␣ is provided from CD4⫹,CD28– T
cells that receive amplifying signals from FKNexpressing FLS, triggering the CX3CR1 receptor on T
cells. These data assign a dual regulatory role to FKN
and its receptor CX3CR1 in controlling synoviocyte
proliferation and T cell function in the synovial microenvironment.
Sample collection. The study cohort included 22 patients with RA (73% women, mean ⫾ SD age 51 ⫾ 15 years,
mean ⫾ SD disease duration 6.5 ⫾ 7 years) who met the
American College of Rheumatology (formerly, the American
Rheumatism Association) revised criteria for the classification
of RA (25). All patients were rheumatoid factor positive, 96%
had bony erosions and received disease-modifying therapy, and
32% had extraarticular manifestations. Individuals with no
family history of a chronic inflammatory disease served as
controls. All donors provided informed consent, and biologic
specimens were handled according to institutional review
board–approved protocols.
Cells and cell lines. CD4⫹ T cells were isolated from
the blood of patients and normal control subjects, using a
CD4⫹ T cell–enrichment cocktail (StemCell Technologies,
Vancouver, British Columbia, Canada). CD4⫹,CD28⫹ or
CD4⫹,CD28⫺ T cells were obtained by further incubating the
cells with biotinylated mouse anti-human CD28 monoclonal
antibody (ID Labs, London, Ontario, Canada) and
streptavidin-conjugated magnetic beads (Miltenyi Biotec,
Sunnyvale, CA), and separating the CD28⫹ and CD28⫺
subpopulations using magnetic separation columns (Miltenyi
Biotec) (18). Primary cultures of FLS were established using
synovial tissue specimens obtained from patients with RA (18).
Fresh tissue was digested with 500 ␮g/ml collagenase (Sigma,
St. Louis, MO), and single-cell suspensions were cultured in
Dulbecco’s modified Eagle’s medium (DMEM; BioWhittaker,
Walkersville, MD) supplemented with 10% fetal calf serum
(FCS; Summit Biotech, Fort Collins, CO), 2 mM L-glutamine,
100 units/ml penicillin, and 100 ␮g/ml streptomycin sulfate
(Life Technologies, Grand Island, NY). Nonadherent cells
were washed off after 3 days, and plastic-adherent cells were
collected using trypsin–EDTA (Sigma). FLS from the third to
eighth passages were used.
FLS coculture and proliferation assays. To examine T
cell–induced FLS proliferation, 2 ⫻ 103 FLS stained with
PKH26 (Sigma) were cocultured with 2 ⫻ 103 T cells in 200 ␮l
DMEM (1% FCS) on chamber slides (Nalge Nunc International, Rochester, NY). After 3–5 days, FLS proliferation was
evaluated by counting the number of fluorescent FLS per
visual field. Pilot experiments were performed to determine
the optimal time points when FLS numbers had increased
without fading of the stain. Alternatively, FLS were stained
with carboxyfluorescein succinimidyl ester (CFSE) at 1 ␮g/ml
and cocultured with T cells in 24-well plates. T cells were
harvested after 5–7 days to allow for sufficient cell divisions to
assess CFSE dilution by flow cytometry. The proliferation index was calculated as the ratio of the peak fluorescence intensities of nonproliferating and proliferating cells.
To examine the effects of growth factors on FLS
proliferation, 3 ⫻ 103 FLS were labeled with CFSE or PKH26
and cultured in DMEM (1% FCS) in the presence of sFKN
(R&D Systems, Minneapolis, MN) or TNF␣, at the concentrations indicated. After 72 hours, the proliferation of FLS was
evaluated by counting labeled FLS, using fluorescence microscopy. To examine the role of FKN–CX3CR1 interactions,
anti-CX3CR1 antibodies (Torrey Pines Biolab, La Jolla, CA)
or control immunoglobulin was added to the cultures at a
concentration of 10 ␮g/ml. In some experiments, either FLS or
T cells were preincubated with anti-CX3CR1 antibodies, and
excess antibodies were removed before the culture setup.
Analysis of CX3CR1, FKN, and TNF␣ expression. FLS
(0.5 ⫻ 106) were cocultured with 0.5 ⫻ 106 T cells in a 24-well
plate for 96 hours. Cells were collected and incubated with 1%
purified rabbit anti-human CX3CR1 (Torrey Pines Biolab) for
30 minutes at 4°C. After washing, cells were stained with
fluorescein isothiocyanate–linked anti-rabbit IgG for 30 minutes and analyzed by flow cytometry. To evaluate TNF␣ gene
expression induced by FLS and T cell interaction, 5 ⫻ 105 FLS
and 5 ⫻ 105 CD4⫹,CD28⫹ or CD4⫹,CD28⫺ T cells were
cocultured for 24 hours. TNF␣-induced FKN and CX3CR1
gene expression was assessed by culturing 5 ⫻ 105 FLS in
6-well plates in the presence of 5 ng/ml TNF␣ (R&D Systems).
Transcription of TNF␣, FKN, and CX3CR1 was
quantified by real-time polymerase chain reaction, utilizing the
Mx3000 system (Stratagene, La Jolla, CA). The following
number of specific transcript copies per 200,000 ␤-actin copies.
Statistical analysis. For each culture condition, the
number of FLS in 10 visual fields was counted, and the
Mann-Whitney rank sum test was used to compare absolute
numbers of proliferating FLS. FLS proliferation indices and
TNF␣, FKN, and CX3CR1 transcripts were compared by
paired t-test, using SigmaStat software (SPSS, Chicago, IL).
Enhanced FLS proliferation by CD4ⴙ T cells
from patients with RA. To examine whether CD4 T cells
regulate FLS proliferation, we cocultured PKH26labeled FLS lines established from rheumatoid synovial
tissue with CD4⫹ T cells isolated from the blood of
patients with RA. T cells from healthy age-matched
individuals served as controls. FLS growth was assessed
by fluorescence microscopy. To reduce the spontaneous
proliferation of FLS caused by FCS, the serum concentration in the culture medium was minimized to 1%.
CD4⫹ T cells from both healthy individuals and patients
with RA had a marked growth-promoting effect on
rheumatoid FLS. Within 5 days, FLS numbers more
than doubled in the presence of T cells. CD4 T cells
derived from patients with RA outperformed those from
control subjects in enhancing FLS outgrowth. As shown
in Figure 1, the number of FLS was ⬎50% higher in the
presence of RA-derived CD4 T cells compared with
control T cells (P ⫽ 0.0017). Thus, the spectrum of CD4
effector functions includes regulating fibroblast growth.
In particular, CD4 T cells from patients with RA provided a significant growth advantage compared with
Particular efficiency of CD4ⴙ,CD28ⴚ T cells in
promoting FLS growth. Our group previously reported
that the T cell pool from patients with RA is enriched for
presenescent and senescent CD4 T cells that have lost
expression of CD28 and gained alternative immunoregulatory receptors (26,27). CD4⫹,CD28⫺ T cells home to
both lymph nodes and CCL5-producing synovial lesions
(28) and utilize multiple nonconventional receptors to
costimulate T cell receptor–mediated signals (11,18). To
address the question of whether CD4⫹,CD28⫹ and
CD4⫹,CD28⫺ T cells differ in terms of their interaction
with FLS and differentially affect FLS proliferation,
both T cell subpopulations were isolated from the blood
Figure 1. Superiority of CD4⫹,CD28⫺ T cells in promoting
fibroblast-like synoviocyte (FLS) proliferation. A, PKH26-labeled FLS
were grown in the absence or presence of CD4⫹ T cells from a patient
with rheumatoid arthritis (RA) for 5 days. Fibroblast numbers were
assessed by fluorescence microscopy. B, The number of FLS per
low-power field (lpf) was obtained by counting 10 random lpfs of
triplicate cultures. Data are shown as box plots, where each box
represents the 25th to 75th percentiles, lines outside the boxes
represent the 10th and the 90th percentiles, and lines inside the boxes
represent the median. Results are representative of experiments
performed with CD4 T cells from 5 patients and 5 control subjects. C,
The proliferation of FLS was determined by flow cytometry and
carboxyfluorescein succinimidyl ester (CFSE) dilution. FLS were
stained with CFSE and cocultured for 7 days with CD4⫹,CD28⫹ or
CD4⫹,CD28⫺ T cells isolated from patients with RA. Shaded area
represents FLS control culture without CD4⫹ T cells; solid line
represents FLS plus CD4⫹,CD28⫺ T cells; broken line represents
FLS plus CD4⫹,CD28⫹ T cells. D, The proliferation indices were
determined, defined as the ratio of the CFSE peak fluorescence
intensities in FLS cultured with or without CD4 T cells. Bars show the
mean and SD results from triplicate cultures.
of patients with RA and tested for their growthpromoting capabilities. To quantify FLS proliferation,
the CFSE dilution of these cells was measured after 7
Figure 2. Role of CX3CR1 in T cell–induced growth of FLS. A, FLS
were labeled with PKH26 and cocultured with CD4⫹,CD28⫹ and
CD4⫹,CD28– T cells in the absence or presence of anti-CX3CR1
antibodies (Ab) for 5 days, and the number of FLS was evaluated by
fluorescence microscopy. B, CFSE-labeled FLS were grown in the
absence or presence of CD4⫹,CD28⫹ and CD4⫹,CD28⫺ T cells.
Anti-CX3CR1 antibodies were added at the initiation of culture, and
FLS proliferation was assessed by flow cytometry after 5 days. Proliferation indices were defined as described in Figure 1. C, FLS were
labeled with PKH26 and expanded in the absence or presence of CD4
T cells, and FLS growth was determined by fluorescence microscopy
after 5 days. Either T cells or FLS were pretreated with anti-CX3CR1
antibodies and intensively washed before being added to the cocultures. Blocking of CX3CR1 on CD4⫹,CD28⫺ T cells and FLS reduced
FLS proliferation; however, blocking of CX3CR1 was more effective
on CD4⫹,CD28⫺ T cells than on synoviocytes. Data in A and C are
shown as box plots, where each box represents the 25th to 75th
percentiles, lines outside the boxes represent the 10th and 90th
percentiles, and lines inside the boxes represent the median. Bars in B
show the mean and SD results from triplicate cultures. See Figure 1 for
other definitions.
Figure 3. Role of CX3CR1 in fractalkine (FKN) stimulation of FLS growth. CFSE-labeled FLS were
cultured for 72 hours in the absence or presence of soluble FKN (sFKN; 100 ng/ml) and anti-CX3CR1
antibodies (Ab) or isotype control antibodies. Control cultures were maintained in medium alone. A,
Fibroblast numbers were determined using fluorescence microscopy (original magnification ⫻ 200). B,
Cell numbers were counted on 10 randomly selected fields of triplicate cultures. Data are shown as box
plots, where each box represents the 25th to 75th percentiles, lines outside the boxes represent the 10th
and 90th percentiles, and lines inside the boxes represent the median. Results are representative of
experiments with 4 different FLS lines. See Figure 1 for other definitions.
days of T cell–FLS coculture. Proliferation indices demonstrated that FLS cocultured with CD4⫹,CD28⫹ T
cells divided ⬃2–3 times (average proliferation index of
5) within 7 days. In contrast, FLS cocultured with
CD4⫹,CD28⫺ T cells diluted the CFSE levels by almost
30-fold, indicating that they passed through 5 division
cycles (Figure 1C). Thus, senescent CD4⫹,CD28⫺ T
cells clearly outperformed their CD4⫹,CD28⫹ counterparts in enhancing fibroblast proliferation, providing an
explanation for the increased capacity of RA-derived T
cells to support FLS growth.
Enhanced FLS growth by CX3CR1 on both T
cells and FLS. One of the characteristic features of
CD4⫹,CD28⫺ T cells is the spontaneous expression of
CX3CR1, the receptor for FKN (18). When partnering
with FKN-expressing FLS, CD4⫹,CD28⫺ T cells utilize
this chemokine receptor to costimulate cytokine production, granule expulsion, and survival. To investigate
whether CX3CR1–FKN interactions are involved in
regulating FLS growth, antibodies specific to CX3CR1
were added to the T cell–FLS cocultures. As shown in
Figure 2A, blocking FKN from binding to its receptor
essentially did not affect the growth-promoting activity
of CD4⫹,CD28⫹ T cells but markedly reduced FLS
growth in the cocultures containing CD4⫹,CD28⫺ T
cells (P ⫽ 0.001). Essentially, FLS growth rates in the
presence of both T cell subpopulations were very similar
if anti-CX 3 CR1 was added. The importance of
CX3CR1–FKN interactions in mediating T cell–induced
FLS proliferation was confirmed using the CFSE dilution as readout (Figure 2B). The addition of antiCX 3 CR1 antibodies reversed the superiority of
CD4⫹,CD28⫺ T cells in driving FLS outgrowth.
Although the blocking experiments established
the relevance of CX3CR1 in regulating the outcome of T
cell–FLS interactions, they did not address on which of
the cellular partners FKN and its receptor were expressed. Previous studies have shown that FKN is found
mainly in FLS (18). Besides expressing CD4⫹,CD28⫺ T
cells, FLS also express CX3CR1. We therefore performed blocking experiments in which either T cells or
FLS were preincubated with anti-CX3CR1 antibodies
and washed extensively before being added to the coculture system (Figure 2C). These experiments confirmed
the competence of CD4⫹,CD28⫺ T cells in enhancing
FLS proliferation and the role of CX3CR1 blockade in
disrupting this effect. Preincubation of CD4⫹,CD28⫹ T
cells with antibodies did not have any effect, which is
consistent with the observation that these cells do not
express CX3CR1. Blocking CX3CR1 on CD4⫹,CD28⫺
T cells had the most significant impact on impairing FLS
growth (P ⫽ 0.001). The consequences of blocking
CX3CR1 on FLS were smaller but significant (P ⫽
0.001) and were seen irrespective of whether FLS were
cocultured with CD4⫹,CD28⫹ or CD4⫹,CD28⫺ T
Responsiveness of FLS to the growth factor FKN
determined by FLS expression of CX3CR1. The role of
CX3CR1 in regulating the proliferation behavior of FLS
raised the question of whether FKN is a growth factor
for these specialized fibroblasts. To address that question, early-passage rheumatoid FLS were expanded in
the absence or presence of sFKN. CFSE-labeled FLS
were cultured for 3 days with 100 ng/ml sFKN. As shown
in Figure 3, the expansion rate more than doubled,
establishing that the chemokine FKN enhances multiplication of FLS. The growth-promoting effect of sFKN
was completely blocked when anti-CX3CR1 was added
with sFKN. Even in the absence of exogenous sFKN,
anti-CX3CR1 antibodies lowered the rate of FLS proliferation, indicating the effect of constitutive FKN secretion in an autocrine growth-promoting loop. These
results confirmed a critical role of the FKN–CX3CR1
pathway in regulating synovial fibroblast growth.
Role of TNF␣ in T cell–induced FLS proliferation. The results of experiments described in Figures 1
and 2 demonstrated that T cells modulate the cell cycle
behavior of FLS. The mechanism involved CX3CR1,
which raised the possibility that T cells interfered with
the FKN–CX3CR1 pathway. Our group previously reported that CX3CR1 stimulation on CD4⫹,CD28⫺ T
cells augments IFN␥ and TNF␣ production as well as
granule expulsion (18). To determine whether these T
cell cytokines influence the growth behavior of FLS,
early-passage FLS were exposed to increasing doses of
TNF␣ and IFN␥, and their proliferative responses were
quantified. IFN␥ failed to stimulate FLS growth, and
high doses inhibited FLS proliferation (data not shown).
In contrast, TNF␣ had excellent growth factor function
for FLS (Figure 4B). Particularly at TNF␣ doses of 1–5
ng/ml, FLS responded with more than doubling of cell
numbers by day 3. Higher doses of TNF␣ were not as
effective; 100 ng/ml TNF␣ could no longer boost FLS
To examine the regulation of TNF␣ production
in the T cell–FLS coculture system, TNF␣ messenger
RNA was quantified in cocultures containing either
CD4⫹,CD28⫹ or CD4⫹,CD28⫺ T cells. The allogeneic
stimulation provided in the system was sufficient to
induce TNF␣ transcription (Figure 4A), and TNF␣
levels were clearly higher in the cultures with
CD4⫹,CD28⫺ T cells. The excess production of TNF␣
was blocked by adding anti-CX3CR1 antibodies at the
initiation of the T cell–FLS coculture. These data confirmed our prior reports that CD4⫹,CD28⫺ T cells are
an excellent source of TNF␣ and utilize CX3CR1 to
costimulate the production of this cytokine.
To examine whether TNF␣-driven FLS proliferation was dependent on CX3CR1, TNF␣ stimulation
was performed in the absence or presence of antiCX3CR1 antibodies (Figure 4C). TNF␣ induced robust
enhancement of FLS proliferation at doses of 1 ng/ml
and 5 ng/ml. The addition of anti-CX3CR1 antibodies
abrogated this enhancement. The results of these experiments established that TNF␣-driven proliferation of
FLS depends on signaling through CX3CR1.
TNF␣-regulated CX3CR1 expression and FKN
production by FLS. Possible mechanisms through which
TNF␣ could modulate the growth pattern of FLS include enhanced CX3CR1 expression as well as regulation of FKN production. Stimulation of early-passage
FLS with TNF␣ at a dose of 5 ng/ml promptly induced
up-regulation of FKN. The spontaneous production of
Figure 4. Relationship between the CX3CR1–fractalkine (FKN) axis and tumor necrosis factor ␣
(TNF␣). A, CD4⫹,CD28⫹ or CD4⫹,CD28⫺ T cells were cultured with FLS for 24 hours. TNF␣
sequences were quantified by real-time polymerase chain reaction (PCR) and adjusted for ␤-actin
copies. B, PKH26-labeled FLS were expanded in the presence of increasing doses of TNF␣, and
densities of FLS were assessed after 72 hours. C, FLS proliferation was induced by TNF␣ in the
absence or presence of anti-CX3CR1 antibody (Ab). FLS density was measured after 3 days, by
fluorescence microscopy. D, FLS were seeded at a density of 5 ⫻ 105 cells and stimulated with 5
ng/ml TNF␣ for 24 hours. Total RNA was extracted, and FKN- and CX3CR1-specific transcripts
were quantified by real-time PCR. Bars in A and D show the mean and SD results from triplicate
cultures. Data in B and C are shown as box plots, where each box represents the 25th to 75th
percentiles, lines outside the boxes represent the 10th and 90th percentiles, and lines inside the
boxes represent the median. See Figure 1 for other definitions.
FKN was minimal, but exposure to TNF␣ resulted in
robust expression of FKN sequences (Figure 4D).
CX3CR1 transcripts were detectable at low levels in the
absence of stimulation. Within 24 hours, TNF␣ in-
creased CX3CR1 transcript production by more than
3-fold (Figure 4D).
To test whether this mechanism is functional in T
cell–FLS interactions, we used fluorescence-activated
were explored by measuring the proliferation response of T
cell–exposed FLS to FKN. For these experiments, FLS
were cocultured with the 2 T cell subpopulations for 3 days.
The T cells were removed, and sFKN was added. FLS that
were exposed to CD4⫹,CD28⫺ T cells reacted with an
enhanced proliferative response to FKN (Figure 5B). In
contrast, FLS cultured in the presence of CD4⫹,CD28⫹
T cells showed only a modest enhancement of cell replication. These data support the concept that T cells determine
FLS growth behavior by regulating the expression of
growth factor receptors.
This study demonstrates that CD4 T cells regulate proliferation of synovial fibroblasts, which is a
Figure 5. Role of CD4 T cells in up-regulating the expression of
CX3CR1 on FLS and sensitization of FLS toward the growth-inducing
effect of fractalkine (FKN). A, CX3CR1 expression on FLS was
analyzed by fluorescence-activated cell sorting after coculture with
CD4 T cells for 96 hours. Shaded area represents FLS cultured with
medium only; bold line represents FLS cultured with CD4⫹,CD28⫺ T
cells; broken line represents FLS cultured with CD4⫹,CD28⫹ T cells;
black line represents isotype control antibody. Results are shown as
histograms of mean fluorescence intensities and are representative of
5 experiments. B, CD4⫹,CD28⫹ (solid line) or CD4⫹,CD28⫺ (broken line) T cells were cocultured with FLS and washed away after 72
hours. Soluble FKN (sFKN) was added at doses of 50 ng/ml and 100
ng/ml. After an additional 96 hours, the number of FLS per lpf was
obtained by counting 10 randomly selected fields of triplicate cultures.
Results are shown as the mean ⫾ SD. See Figure 1 for other
cell sorting to monitor the surface expression of CX3CR1
in FLS cocultured with either CD28⫹ or CD28⫺ CD4
T cells. FLS grown in the absence of either type of
T cells expressed CX3CR1 on their surface. The mean
fluorescence intensity of CX3CR1 staining doubled in the
presence of CD4⫹,CD28⫹ T cells and increased almost 4-fold when FLS were exposed to CD4⫹,CD28⫺
T cells (Figure 5A). These experiments demonstrated
that T cells regulate the expression of growth receptors
and chemokine receptors on fibroblasts.
The functional consequences of this T cell activity
Figure 6. Fractalkine (FKN)–CX3CR1 interactions in rheumatoid
synovitis. CX3CR1 on CD4⫹,CD28⫺ T cells recognizes FKN expressed on the surface of fibroblast-like synoviocytes (FLS) and
costimulates production of tumor necrosis factor ␣ (TNF␣). FLS also
have CX3CR1 on the surface and secrete FKN and utilize this
receptor–ligand pair for autocrine growth stimulation. TNF␣ can
amplify this autocrine loop by inducing CX3CR1 expression and
enhancing FKN production. TCR ⫽ T cell receptor. Color figure can
be viewed in the online issue, which is available at http://www.
critical disease mechanism in the rheumatoid joint.
Underlying molecular pathways involve FKN and its
receptor CX3CR1, a receptor–ligand pair formerly implicated in chemotaxis and cell adhesion. Crosslinking of
CX3CR1 on CD4 T cells amplifies the production of
TNF␣. Interestingly, FLS also express CX3CR1 and, by
releasing the ligand FKN, sustain an autocrine growthpromotion loop. Both components of this autocrine loop
are enhanced by TNF␣, which connects T cell responses
and FLS proliferation (Figure 6). CX3CR1 expression is
a feature of a unique T cell subpopulation, senescent
CD4⫹,CD28⫺ T cells, which accumulate in patients
with RA and mediate many of the proinflammatory
processes. Implicating CX3CR1 and FKN in 2 interconnected pathways of T cell–induced FLS proliferation
identifies these molecules as pluripotent mediators and
potential therapeutic targets in rheumatoid synovitis.
CD4⫹,CD28⫺ T cells are end-differentiated
with short telomeres and sluggish cell replication, identifying them as senescent cells (12,13). Besides loss of
the costimulatory molecule CD28, they are functionally
characterized by the de novo expression of a series of
novel regulatory receptors (11,14,18). Equipped with an
unusual set of receptors and signal transduction pathways (28), CD4⫹,CD28⫺ T cells are capable of communicating and interacting with cellular partners that are
primarily not antigen-presenting cells but are mesenchymal cells localized at sites of tissue inflammation (29).
This rule certainly holds for FLS, a cell population that
builds the stroma of the synovial membrane and is
meant to provide structural support and nutrition to the
bony and cartilaginous elements of the joint. Our group
recently demonstrated that CD4⫹,CD28⫺ T cells, as
opposed to their CD28⫹ counterparts, express and use
the chemokine receptor CX3CR1 to communicate with
FLS (18). Those studies emphasized the role of FLSexpressed FKN in costimulating T cell activation, including CD69 induction, cytoplasmic granule expulsion, and
IFN␥ release. The current study extends the role of this
costimulatory signal beyond cytokine release by activated CD4⫹,CD28⫺ T cells. Instead, it becomes obvious that a CX3CR1-mediated signal enhances the ability
of CD4⫹,CD28⫺ T cells to intensify the hyperproliferative response of synovial fibroblasts. Enhanced FLS
replication is mediated by TNF␣, extending the list of
disease-relevant functions of TNF␣ in RA by yet another process.
An intriguing aspect of the current study was the
finding that CX3CR1 was not restricted to T cells but
was also expressed on FLS. Sequential antibodyblocking experiments clarified that CX3CR1 on both T
cells and FLS was functionally relevant for fibroblast
proliferation, although the impact of blocking CX3CR1
on the T cells was clearly more significant than hindering
access to CX3CR1 on FLS. Blocking CX3CR1 on T cells
markedly reduced TNF␣ production, confirming earlier
data that TNF␣ release by CD4⫹,CD28⫺ T cells is
modulated by a CX3CR1-derived signal (18). TNF␣,
when dosed optimally, had a strong proliferative effect
on FLS.
The concentration of sFKN in the joint fluid of
patients with RA is elevated (23,24). Blaschke and
coworkers (30) reported that sFKN induces matrix metalloproteinase 2 production in synovial fibroblasts in
vitro, suggesting that sFKN has broad proinflammatory
activities in the rheumatoid joint. The relevance of FKN
as a fibroblast growth factor was confirmed in experiments testing the growth-promoting effect of exogenous
FKN. A proinflammatory role for this chemokine has
been supported by gene expression experiments in rat
adjuvant-induced arthritis (22). FKN and its receptor
CX3CR1 were abundantly expressed on day 18, a time of
intense inflammation in the rat joint. Soluble FKN is
produced by proteolytic cleavage of membrane-bound
FKN, and it has been suspected that its major functions
are chemotactic and proadhesive (20). Accordingly, inhibition of FKN can ameliorate murine collageninduced arthritis (31). Treatment with anti-FKN monoclonal antibodies inhibited migration of adoptively
transferred cells into the inflamed synovium. FKN is a
unique member of the chemokine gene family, with a
single transmembrane region and a short intracellular C
terminus. We did consider the possibility that CX3CR1
expressed on the surface of T cells promotes FLS cell
cycling by signaling through membrane-integrated FKN.
However, the soluble form of FKN effectively enhanced
FLS proliferation. When cocultured with CD4⫹,CD28⫺
T cells, FLS were rendered more susceptible to the
growth-inducing action of sFKN, even after the removal
of T cells.
These data strongly support the concept that T
cell–mediated FLS proliferation reflects a change in the
responsiveness of FLS to the growth factor. This notion
was strengthened by experiments demonstrating a
TNF␣-driven enhancement in the expression of the
FKN receptor CX3CR1 on FLS. In essence, selected T
cells appear to regulate the threshold of FLS growth
responses in the rheumatoid joint. The facts that FLS
themselves supply the growth factor and that production
of the growth factor is subject to T cell regulation add
new twists to the T cell–FLS relationship. The interactions are contact dependent, as far as stimulation of
CX3CR1 on T cells is involved (18). T cells reside in the
sublining and are therefore distal from FLS in the
synovial lining. However, cadherin 11–positive cells are
also found in the sublining layer in inflamed synovium
and likely represent FLS migrating to the synovial lining
(32). In addition, T cells may secrete soluble mediators
that up-regulate the FKN–CX3CR1 pathway in synovial
lining cells.
The current study did not examine how CX3CR1
interferes with cell cycle regulation in FLS. Reports by
Lucas et al (33) have indicated that aortic smooth
muscle cells, another type of specialized mesenchymal
cells, utilize the CX3CR1–FKN pathway for proliferation control. In vitro, the soluble form of FKN promotes
aortic smooth muscle cell proliferation through NF-␬B
signaling (34). The possibility remains that membraneintegrated FKN is also capable of enhancing FLS and
smooth muscle cell replication. In the tissue site, FLS
may receive growth-promoting signals from neighboring
fibroblasts as well as through the secreted form of the
ligand. FLS have long been recognized as displaying
almost autonomous features in terms of their proliferative response, a characteristic that has encouraged discussions on whether these cells are tumorlike (35,36).
The autocrine growth loop described here, fueled by
FKN, may well contribute to the hyperplastic reaction
pattern of mesenchymal cells in sites of inflamed tissue.
In that sense, intimal hyperplasia driven by vascular
smooth muscle cells and synovial hyperplasia driven by
FLS may share pathways of dysregulation. Vascular
smooth muscle cells are now emerging as important
partners of T cells in inflamed atherosclerotic plaques,
not only because they are the subjects of T cell effector
functions but also because they create synaptic communication platforms with tissue-infiltrating T cells (37,38).
An unresolved issue of the current study is the
question of how CD4⫹,CD28⫺ T cells are activated
when interacting with FLS. Possibly, CD4⫹,CD28⫺ T
cells recognize alloantigens on FLS lines. However,
CD4⫹,CD28⫺ T cells are generally oligoclonal (39) and
have been shown to be defective in alloreactive responses (40). In contrast, CD4⫹,CD28⫺ T cells respond
vigorously in autologous mixed-lymphocyte reactions
and may indeed be autoreactive (26). Evidence has been
provided that synovial T cells are easier to stimulate,
irrespective of their antigen specificity (41). This may
point toward a fundamental abnormality in the response
threshold of T cells in RA, an abnormality that may be
a functional consequence of T cell senescence.
Rheumatoid FLS play an important role in the
pathogenesis of RA. They demonstrate pseudotumor-
like overgrowth and resist apoptosis (42–44). The
present study demonstrated the pivotal roles the
CX3CR1 pathway plays in senescent CD4⫹,CD28⫺ T
cell and FLS interaction and in the T cell–induced
overgrowth of rheumatoid FLS. Activation of the
CX3CR1 pathway on CD4⫹,CD28⫺ T cells through T
cell–FLS interactions leads to activation of T cells,
enhanced TNF␣ production, and up-regulated FKN and
CX3CR1 expression on FLS (Figure 6). The CX3CR1
pathway in FLS is directly linked to enhanced FLS
proliferation and can be activated by both membranebound FKN and sFKN, which are present in high
concentrations in rheumatoid joints (Figure 6). Because
CX3CR1 stimulation is a common pathway used by T
cells, exogenous TNF␣, and sFKN to stimulate FLS
proliferation, FKN receptor blocking might serve as an
effective therapeutic intervention and reduce hyperplasia of the synovium and joint inflammation in RA.
We thank Dr. Sergey Pryshchep for help with preparing the figures and Tamela Yeargin for editorial support.
Dr. Weyand had full access to all of the data in the study and
takes responsibility for the integrity of the data and the accuracy of the
data analysis.
Study design. Goronzy, Weyand.
Acquisition of data. Sawai, Park, He.
Analysis and interpretation of data. Goronzy, Weyand.
Manuscript preparation. Goronzy, Weyand.
Statistical analysis. Sawai, Park.
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