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Elevated synovial fluid levels of interleukin-6 and tumor necrosis factor associated with early experimental canine osteoarthritis.

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819
ELEVATED SYNOVIAL FLUID LEVELS OF
INTERLEUKIN-6 AND TUMOR NECROSIS FACTOR
ASSOCIATED WITH EARLY EXPERIMENTAL
CANINE OSTEOARTHRITIS
G. VENN, J. J. NIETFELD, A. J. DUITS, F. M. BRENNAN, E. ARNER, M. COVINGTON,
M. E. J. BILLINGHAM, and T. E. HARDINGHAM
Objective. To measure levels of cytokines, proteases, and glycosaminoglycans (GAG) in synovial fluid
(SF) From the knees of animals with experimental osteoarthritis (OA) and from their contralateral (control)
knees, and to compare and correlate these values with
each other as well as with measures of proteoglycan
synthesis in the corresponding articular cartilage. This
study will help to identify cytokines of potential importance in the early stages of the development of OA.
Methods. OA was induced in 12 mature animals
by sectioning the anterior cruciate ligament. After 3
months, SF From the operated and contralateral (control)
knee joints was assayed for interleukind (IL-6), tumor
necrosis Factor (TNF), IL-1, latent metalloproteinase, and
sulfated GAG. Proteoglycan synthesis in the corresponding articular cartilage was also measured.
Results. IL-6 levels in SF from the operated joint
compared with the control joint were significantly eleFrom the Biochemistry Division and Sunley Research Division, Kennedy Institute of Rheumatology, London, United Kingdom; University Hospital, Utrecht, The Netherlands; Experimental
Station, Dupont Merck, Wilmington, Delaware; and the Lilly Research Laboratories, Surrey, United Kingdom.
Dr Venn’s work was supported by the Medical Research
Council, United Kingdom.
G. Venn, PhD: Biochemistry Division, Kennedy Institute
of Rheumatology; J. J. Nietfeld, PhD: Department H04312, University Hospital; A. J. Duits, PhD: Department G04614, University
Hospital; F. M. Brennan, PhD: Sunley Research Division, Kennedy
Institute of Rheumatology ; E. Arner, PhD: Experimental Station,
Dupont Merck; M. Covington, PhD: Experimental Station, Dupont
Merck; M. E. J. Billingham, PhD: Lilly Research Laboratories;
T. E. Hardingham, PhD, DSc: Biochemistry Division, Kennedy
Institute of Rheumatology.
Address reprint requests to Gill Venn, PhD, Biochemistry
Division, Kennedy Institute of Rheumatology, 6 Bute Gardens,
Hammersmith, London W6 7DW, United Kingdom.
Submitted for publication July 9, 1992; accepted in revised
form January 6, 1993.
Arthritis and Rheumatism, Vol. 36, No. 6 (June 1993)
vated in 11 of 12 animals. TNF levels were also elevated
in 10 of 11 SF samples from operated joints, but to a
lesser extent than those of IL-6. IL-1 and IL-1 inhibitors
were undetectable in either the operated or control joint
SF. The GAG concentration was elevated in SF from
experimental OA joints. This elevation correlated with
that of TNF, but not IL-6. There was no significant
difference in the concentration of APMA-activatable
metalloproteinase. The rate of proteoglycan synthesis
was higher in the cartilage from the operated joint in 8
of 12 animals, and the mean rate of synthesis was
significantly higher than in the control joint. There was
a positive correlation between this increase in cartilage
proteoglycan synthesis (operated versus control) and the
increase in SF IL-6, but there was no correlation with
the levels of TNF or GAG.
Conclusion. This is the first study of SF levels of
cytokines in early experimental OA. Our results show
surprisingly high levels of IL-6 in operated joints, where
the cytokine could act directly on the chondrocytes,
and thus play a role in mediating their responses to
cartilage injury.
Animal models of experimentally induced joint
diseases allow the detailed investigation of the early
mechanisms of pathogenesis. This can be most important for chronic conditions such as osteoarthritis (OA),
in which the onset may be slow and may, after many
years, involve only preclinical changes.
One of the best characterized experimental
models of OA is the canine model described by Pond
and Nuki (l), in which the anterior cruciate ligament is
sectioned. The procedure involves minimal surgery on
fully mature animals. The extent of the response varies
between animals, but the typical changes resemble
VENN ET AL
those of early canine OA and, over 3-4 years, can
progress t o resemble advanced human OA (2,3).
Some of the earliest changes in the articular
cartilage of the operated joint are increased hydration
(4) and loss of superficial tensile strength (5); there is
also increased synthesis (6,7) and turnover (8) of
matrix components. These changes indicate a loosening of the collagen network and hyperactivity of the
chondrocytes. The mechanisms which bring about
these changes are unknown, but are likely to involve
mechanical effects together with biochemical effects of
the various cytokines and growth factors that may be
produced systemically or locally and may act o n the
cells of the joint tissues. Reports of cytokine levels
evaluated in human synovial fluids (SF) from patients
with different arthritides present conflicting data, presumably due to the extremely heterogenous nature of
these samples. The experimental model of canine OA
provides the opportunity t o investigate the presence of
cytokines during the early stages of the joint's response, and may thus allow us t o define more accurately the cytokines of potential importance in the
development of OA in humans.
This report is the first to describe a study of
interleukin-1 (IL-l), IL-6, and tumor necrosis factor
(TNF) levels in SF samples evaluated during early
experimental OA. Moreover, we present the results of
correlations between these values and levels of proteoglycan synthesis in the correspondingarticular cartilage.
MATERIALS AND METHODS
Induction of OA. Experimental canine OA was induced in mature female animals by resection of the anterior
cruciate ligament of the right stifle joint by stab incision. The
animals were allowed to exercise normally for 3 months and
were then killed by intravenous injection of sodium pentobarbital. The joints were immediately removed and transferred on ice to a laminar flow hood. Sterile phosphate
buffered saline (2 ml) was injected into the suprapatellar
pouch of the operated and contralateral knee joints and the
joints were articulated. The diluted SF were withdrawn from
the joints and stored at -20°C. Before assay the S F were
thawed and centrifuged (13,OOOg for 10 minutes).
After removal of the SF, the joints were opened and
the articular cartilage was removed from the tibia1 plateaux,
femoral condyles, and femoral patellar groove. In all cases
articular cartilage or S F from the left (contralateral) joint of
the same animal was used as a control.
Cartilage analysis. Pooled cartilage was diced into
1-mm2 pieces, and triplicate aliquots were transferred to
24-well tissue culture plates and incubated for 2 hours in 0.5
ml of Dulbecco's modified Eagle's medium (DMEM) containing 20 mM HEPES plus 50 &ml of gentamicin (serum-
free medium) to equilibrate. The medium was replaced with
1 ml of serum-free medium containing 20 pCi/ml of Na,35S0,
and incubation continued for 18 hours at 37°C. The labeling
medium was removed and the cartilage pieces extracted with
4M guanidine HCI, pH 5.8, containing 2% CHAPS and
protease inhibitors (for 48 hours at 4"C, with shaking). The
residue was digested for 18 hours at 65°C with 5 unitdm1 of
papain in 0.2M sodium acetate, pH 6.0. 3'S-labeled proteoglycans in the medium, extract, and digest were removed
from unincorporated isotope by Sephadex G25 chromatography (using PDlO columns; Pharmacia, Uppsala, Sweden)
and quantitated by scintillation counting (9).
The amount of sulfated GAG in the media, extract,
and digest was measured by 1,9-dimethyImethylene blue
binding assay (10). Proteoglycan synthesis was calculated as
the amount of '%-labeled proteoglycan per microgram of
GAG, according to the formula
Total 3'S-proteoglycan (labeled medium
Total pg GAG (all media
+
+
extract
extract
+
+
residue)
residue)
The results are expressed as the mean 2 SEM.
Synovial fluid analyses. GAG was assayed as described above, except that the SF samples were digested
with Streptomyces hyaluronidase (10 unitslml, at 37"C, for 5
hours) prior to assay.
IL-6 was measured by the B9 assay (11). Briefly, B9
murine hybridoma cells (5,00O/well)were cultured in 96-well
flat-bottom microtiter plates in RPMI 1640 containing 5%
fetal calf serum (FCS), 100 units/ml penicillin, and 100 pg/ml
streptomycin. Unfiltered SF titrated in 2-fold dilutions or
recombinant human IL-6 standard (a gift from Dr. M. Helle,
Central Laboratory, Utrecht) was added, and the wells were
incubated for 64 hours. Cells were labeled with 18.5 kBq of
'H-thymidine for 7 hours, after which they were harvested
and counted.
B9 cells respond to IL-6 derived from several species
including canine IL-6 (12,13), but are not stimulated by
IL-la, IL-lp, IL-2, IL-3, IL-4, TNFa, interferon-y (IFNy),
or granulocyte-macrophage colony-stimulating factor (GMCSF) (11). Results are the mean of 2 replicates and are
expressed as picograms per milliliter of human 11-6 equivalents; the limit of detection in our assay was 10 pg/ml.
IL- 1 was measured by the lymphocyte-activation
factor assay, which is based on the ability of IL-1 to
stimulate proliferation of mouse thymocytes. C3H/HeJ
mouse thymocytes were isolated by passing through a wire
mesh screen, and washing 3 times with RPMI containing
10% FCS. Thymocytes were then resuspended in RPMI with
10% FCS and 1 pg/ml of the lectin phytohemagglutinin to
costimulate cell growth. Thymocytes were added to 96-well
plates containing various dilutions of SF samples or IL-1
standards (0.1-100 ng/ml) and incubated for 48 hours at 37"C,
in an atmosphere of 5% COz. 'H-thymidine (5 pCi/ml) was
added and the cells were incubated for an additional 24
hours. Cells were then harvested and counted for 'Hthymidine incorporation. The results reported here are the
mean of 4 replicates. To test for endogenous IL-1 inhibitors,
10 ng of recombinant human IL-1 was added to the diluted
SF samples prior to assay.
TNF was measured by cytotoxicity assay using the
IL-6 AND TNF IN EXPERIMENTAL OA
82 1
WEHI 164 clone 13 mouse fibrosarcoma cell line (14).
Briefly, WEHI cells were plated at 2 X lo4 cells/100 p1 into
96-well plates and cultured for 18 hours at 37°C in DMEM
containing 5% FCS. Actinomycin D (final concentration 1
pg/ml) was added to each well, followed by the addition of
either 100 pl of the TNFu standard (0.23-500 pg/ml) or
diluted SF, and the plates were incubated for 24 hours at
37°C. MTT (100 pl) was added to each well (final concentration 0.25 mg/ml). The pale yellow substrate is converted by
living cells to a dark blue formazan product. Four hours
later, the cells were lysed by the addition of 100 pl of 10%
sodium dodecyl sulfate in 0.01N HCI, and the plates were
incubated overnight, then read in an enzyme-linked immunosorbent assay reader, using a 540-nm filter. The results
reported are the mean of 3 replicates.
Stromelysin activity was measured according to the
method described by Okada et a1 (15). Unfiltered synovial
fluid (diluted 1:25 in 50 mMTris HC1, pH 7.5,0.1MNaCl, 10
mM CaCl,, 0.05% Brig, and 0.02% NaN,) was activated with
1.5 mM APMA and then incubated with 3H-labeled transferrin (4 hours at 37°C) to measure total (both active and pro)
metalloproteinase. Undigested substrate was precipated
with 3.3% trichloroacetic acid, and the supernatant was
counted for ,H-labeled digestion products. One unit of
activity is the amount which will digest 1 kg of ,H-transfemn
in 1 minute at 37°C. The results presented here are the mean
of 2 replicates.
Statistical analysis. The Wilcoxon signed rank test
was used to calculate the statistical significance of differences. This test takes into consideration the weight of the
differences between paired values. A P value less than 0.05
(2-tailedtest) was considered significant.
(16-19). IL-6 activity was abundant in the operated
joint fluids, and in some, it was particularly high (Table
1). Eleven of the 12 animals had higher IL-6 content in
S F from the operated joint than in that from the
contralateral joint; only 2 of the 12 control S F samples
contained amounts that were significantly above the
assay detection limit (>lo pg/ml). Overall, there was a
highly significant difference in IL-6 levels in the operated versus the control S F samples ( P = 0.012).
TNF bioactivity was detected in all S F from
operated joints as well as in 8 of the 11 control joint
fluids (Table 1). Seven of the 11 OA S F samples
showed more than a 3.5-fold increase in TNF, and 3
showed a 1.5-2.0-fold increase. Only 1 animal (animal
6) had a decrease; this was the animal which also
showed a decrease in IL-6 levels in control versus
operated joints. Again considering the operated and
control joint fluids as a paired population, the concentration of TNF was significantly increased in the SF
samples from operated joints (P = 0.033).
IL-1 bioactivity was assayed in S F samples
from 7 animals. No detectable activity in either the
control or the operated joint fluids was demonstrated.
In control experiments, we established that the assay
detected IL-1 present in canine synovium conditioned
medium and quantitated the recombinant human IL- 1
that had been added to the synovial fluid samples. The
results show that neither operated nor control joint SF
contains IL-1 activity nor do they contain significant
amounts of bioactive inhibitors of IL-1. These findings
thus contrast with the findings of TNF levels in the SF,
particularly in those obtained from operated joints.
Synovial fluid samples from 8 animals were
assayed for metalloproteinase following APMA activation, but similar concentrations were found in operated and control joints (2.13 0.14 units/ml versus 2.2
+- 0.14 unitdml, mean & SEM). It could be argued that
since the volume in the operated joint is considerably
larger than that in the control joint, the estimated total
content in the joint could be used for comparison. On
this basis, the difference in cytokine content between
operated and control joint fluids is even more marked.
However, even allowing for this difference in recovered volumes, the content of APMA-activatable
metalloproteinase in the operated joint was only 1.8
times higher than that in the control joint.
The levels of glycosaminoglycans in synovial
fluid reflect the turnover and release of proteoglycans
in articular cartilage. The findings of the present study
are similar to those previously described in this model
of experimental OA (8,20,21), in which the turnover of
RESULTS
The articular cartilage was taken from operated
and contralateral (control) knee joints of 12 animals, 3
months after section of the anterior cruciate ligament.
The rate of proteoglycan synthesis in the freshly
explanted cartilage was determined by measuring the
incorporation of 35S-sulfate into proteoglycan (Table
1). As reported in previous studies, the extent of the
response varied between animals, but there was significant stimulation of synthesis in 8 of the 12 animals
and 4 showed little change. Comparing results from
operated and control joint cartilage as a paired population, proteoglycan biosynthesis was significantly
stimulated in the experimental OA cartilage ( P =
0.012).
The synovial fluid samples taken from these
joints were assayed for TNFa, IL-6, and total GAG
content (Table 1). S F samples 1-7 were also assayed for
IL-1 and samples 1-8 for activated metalloproteinase.
IL-6 bioactivity was detected by stimulation of
B9 cells, which are known to respond to canine IL-6
(12,13) as well as IL-6 from other animal species
*
822
VENN ET AL
Table 1. Analyses of synovial fluid and articular cartilage from the knees of animals with experimental OA*
Synovial fluid
Neutral
metalloproteinase,
unitdm1 (mean)
Articular cartilage
35S-proteoglycan
synthesis, dpm/pg of
GAG (mean f SEM)
Animal
source
Vol.
(ml)
IL-6, pg/ml
(mean f SD)
TNF, pg/ml
(mean t SEM)
GAG,
pg/ml
(mean)
I
Control knee
OA knee
0.05
4.0
<10
1,380 f 360
<1
88.2 t 11.1
105
69
NA
2.49
94.8 f 8.77
155.8 t 14.3
2
Control knee
OA knee
1.o
3.5
< 10
160 f 10
<1
784 f 161
62
268
2.11
1.75
114.6
247.7
3
Control knee
OA knee
1.2
3.0
11
998 t 36
73.6 t 17.7
119.1 t 25.7
99.7
159.7
2.07
2.39
44.2 t 1.8
95.5 t 4.9
4
Control knee
OA knee
0.9
I .9
1,333 f 583
6,912 t 256
55.8 t 8.2
202 f 60.9
90
209
2.07
1.75
77.6 t 2.8
101.8 2 1.7
5
Control knee
OA knee
0.55
0.5
< 10
184 f 0
<I
1,242 2 269
68
200
1.89
2.24
66.9 f 2.2
94.5 f 3.1
6
Control knee
OA knee
0.5
1 .o
1,088 t 128
704 t 114
411 t 84.6
67.1 f 25
243
65
2.97
2.80
51.9 t 1.2
50.7 f 0.7
7
Control knee
OA knee
0.9
2.1
< 10
349 t 40
23.3 t 13.1
86.2 f 16.6
124
195
2.44
1.86
76.6
98.4
Control knee
OA knee
0.9
2.0
52.8
172.8 t 34
286.8 t 61
101.2
75.5
1.84
1.78
180.2 t 15.3
169.1 f 32.8
Control knee
OA knee
0.25
2.0
317
14.4
281.2
64
187
NA
NA
209.6 f 18.7
206.2 f 20.3
10
Control knee
OA knee
0.3
2.0
<10
94 f 18
NA
NA
35
112
NA
NA
104.7 t 12.5
111.5 f 15.6
11
Control knee
OA knee
0.35
3.5
< 10
52,000 f 1,000
77.8 f 20.2
131.1 f 30.7
230
270
NA
NA
75.9
212.8
f
f
6.4
7.1
12
Control knee
OA knee
0.45
4.0
<10
5,500 f 2,176
50.9 f 4.3
280.6 f 29.7
52
106.6
NA
NA
118.3
313.5
?
f
6.5
17.5
SF or cartilage
8
9
< 10
f
3.2
< 10
f
38
f
L
1.8
69
f
f
f
f
7.3
23.6
3.6
7.1
* Osteoarthritis (OA) was induced in the right stifle joint of adult female animals as described in Materials and Methods. The left stifle joint
served as a control. Animals were killed after 3 months, and synovial fluid (SF) and cartilage were obtained. SF was examined for levels of
interleukin-6 (IL-6), 1L-I, tumor necrosis factor (TNF), glycosaminoglycan (GAG), and neutral metalloproteinase; articular cartilage was
examined for proteoglycan synthesis as described in Materials and Methods. NA = not available.
cartilage proteoglycans is typically increased. Thus, 9
of 12 animals had a GAG concentration that was
1.2-4.3 times higher in the operated than the control
SF. Although the concentrations in the operated SF
from the other 3 animals were lower than in the
controls, the GAG concentrations in the operated joint
fluids compared with the contralateral (control) joint
fluids were significantly elevated ( P = 0.05).
Tests for correlation among SF levels of IL-6,
TNF, and sulfated GAG, and with stimulation of
proteoglycan biosynthesis in cartilage, showed 2 that
were significant (Table 2). The increase in IL-6 was
strongly positively correlated with the level of stimu-
lation of proteoglycan synthesis in the same joint (r =
0.81) (Figure 1). There was no correlation of the
increase in IL-6 with the other parameters examined.
The increased TNF in S F correlated with the increased GAG in the same S F samples (r = 0.72)
(Figure 2).
DISCUSSION
We have shown that 3 months after induction of
experimental canine OA, synovial fluid from the operated joint contains unexpectedly high IL-6 activity.
The B9 murine cell assay we used to detect IL-6
IL-6 AND TNF IN EXPERIMENTAL OA
Table 2. Correlation of levels of cytokines, GAG, and PG synthesis in the knees of animals with experimental OA*
Correlation
coefficient
823
91
A
(r)
~
IL-6 versus PG synthesis
IL-6 versus TNF
IL-6 versus GAG
TNF versus PG synthesis
TNF versus GAG
GAG versus PG synthesis
~
0.81
0.11
0.02
0.12
0.72
0.11
* Osteoarthritis (OA) was induced in the right stifle joint of adult
female animals as described in Materials and Methods. The left stifle
joint served as a control. After 3 months, animals were killed, and
the synovial fluid levels of glycosaminoglycan (GAG), interleukin-6
(IL-6), and tumor necrosis factor (TNF), and the level of proteoglycan (PG) synthesis by articular cartilage were measured (see Materials and Methods). The ratios of OA SF to control S F levels were
determined. and these values were used to test for correlation.
activity is routinely used for determining human IL-6
activity and responds to IL-6 derived from other
species (12,13,1619). In the absence of antibodies to
canine IL-6, the possibility that other factors contribute to the proliferation of B9 cells cannot be ruled out;
however, the activity is most likely to be due to IL-6.
On testing a number of human and murine cytokines,
alone and in combination (IL-la, IL-I@,IL-2, IL-3,
IL-4, TNFa, IFNy, and GM-CSF), Helle et a1 (11)
found that only murine IL-4 showed any activity
toward B9 cells, and that IL-4 was only 0.02% as
active as IL-6.
There are many possible sources of the IL-6
-
z
3
1
L
0
z
0
1
.n
c'
c
-U
x
o
a
-+-r
7 - T
.1
1-
1
l
r
n
v 7
-1
---iirn-
10
100
1000
IL-6 (experimental OA/control)
rrrrnl
10000
Figure 1. Elevation of synovial fluid levels of interleukin-6 (IL-6)
(ratio of operated to control) correlates with elevation of proteoglycan synthesis (ratio of operated to control) in joints of animals with
experimental osteoarthritis (OA) compared with contralateral (control) joints.
.1
1
10
100
1000
TNF (experimental OAlcontroI)
10000
Figure 2. Elevation of synovial fluid levels of tumor necrosis factor
(TNF) (ratio of operated to control) correlates with elevation of
glycosaminoglycan (GAG) (ratio of operated to control) in joints of
animals with experimental osteoarthritis (OA) compared with contralateral (control) joints.
present in synovial fluid. It is reported to be produced
constitutively by both synoviocytes (22) and chondrocytes (23), as well as by T cells and B cells (24). Its
production by synoviocytes and chondrocytes is
markedly up-regulated by cytokines, such as IL-1 and
TNF (22,23,25), and by transforming growth factor P
(TGFP) (23). IL-6 synthesis by peripheral blood mononuclear cells can also be induced by TNFa (26) and
TGFP (27). Production of IL-6 by the higher proportion of activated mononuclear cells in the rheumatoid
joint may partly account for the generally higher levels
of IL-6 found in the synovial fluid of patients with
rheumatoid arthritis (RA) compared with those with
osteoarthritis (22,25,28,29).
Because the IL-6 gene promotor is activated by
IL-1 and TNF (30), we also assayed for bioactive IL-1
and TNF in the synovial fluid samples. Although TNF
was detected in almost all the SF samples, there was
no detectable IL-1. To investigate the presence of IL-1
inhibitors in the SF samples (31,32), the activity of
exogenously added IL- 1 was determined. There was
no inhibition and we therefore concluded that the SF
contained neither active IL-1 nor an excess of IL-1
inhibitors, unless they were of a type that could be
completely inactivated by the freeze-thawing procedures we adopted (31).
Bioactive TNF was detected in SF from both
the experimental OA and the control joints we examined. TNF has been demonstrated in human synovial
fluid (33-36), usually at higher levels in inflammatory
VENN ET AL
824
than in noninflammatory disease states, although
Westacott et a1 (36) reported finding higher levels in
normal human SF than in either RA or OA SF. In the
present study, the TNF concentration was elevated in
SF from the operated compared with the contralateral
joint. Some control joint fluids also contained TNF,
and it is of interest that in the one animal with a higher
level of TNF in the control than in the operatedjoint
fluid, a higher content of IL-6 was also detected in the
same sample.
TNFa has been shown to act on chondrocytes
to increase the release of degraded proteoglycan from
immature cartilage (37). This function may have contributed to our finding of a correlation between increased TNF and GAG in SF from the operated joints
(Figure 2), as well as the increased release of proteoglycans from the cartilage of operated joints, which we
and others have previously reported (8,20,21).
The other major effect of TNFa on articular
cartilage is inhibition of proteoglycan synthesis (3739); however, in this canine model, it clearly does not
exert an overriding inhibitory effect. This suggests the
presence of other, as-yet-unidentified, cartilage- or
synovial fluidderived factors that overcome the action of TNF and stimulate proteoglycan synthesis.
There were no differences in the concentrations
of activatable neutral metalloproteinase in SF from
control and experimental OA joints. We have also
assayed conditioned medium from explanted cartilage
derived from experimental OA and control joints and
have not detected any stromelysin or collagenase
production (Murphy G, Venn G: unpublished observations). In contrast to IL-1 and TNF, IL-6 has been
shown to induce expression of the specific inhibitor of
metalloproteinase (tissue inhibitor of metalloproteinase, or TIMP) and not the enzymes themselves (4042). Such an effect may account for the lack of
difference in metalloproteinase activity in our studies,
and therefore suggests that these are not the major
proteinases responsible for increased cartilage matrix
turnover in this OA model. This is in contrast to an
inflammatory model of arthritis (type I1 collageninduced arthritis), in which metalloproteinase expression is reported to be increased in joint tissues (43).
It is interesting that there is strong correlation
between the concentration of IL-6 in joint fluids and
the stimulation of cartilage proteoglycan biosynthesis
in joints with experimental OA (Figure 1). Increased
proteoglycan synthesis in cartilage is not an effect
associated with IL-6, but it can be induced by various
growth factors. We have evidence that proteoglycans
from experimental OA cartilage exhibit quantitative
and qualitative changes in structure that are similar to
those elicited by treatment with TGFP (44). Since
TGFP is also reported to stimulate IL-6 synthesis in
chondrocytes (23) and since it may be released during
the bone and cartilage remodeling that occurs in this
OA model, it could play a role in IL-6 induction. This
will require further investigation.
The contribution of IL-6 to the onset and progression of inflammatory and noninflammatory arthritis is presently unclear. IL-6 is a pleiotropic cytokine
with many known immunologic roles, such as T cell
activation, proliferation, and differentiation as well as
induction of the acute-phase response (24), but these
systemic roles are not readily identified with any
known responses in experimental OA. In this study
the presence of high levels of IL-6 in fluid from
operated joints but not from the contralateral control
joints is quite marked and shows it to be local and not
systemic in origin. The effects of IL-6 on the metabolism of extracellular matrix molecules and its production by connective tissue cells are much less well
defined than its roles in immunologic disorders (24).
Chondrocytes do, however, proliferate and undergo
phenotypic changes in response to OA (45). What
initiates this is not known, but it could involve IL-6,
which can enhance proliferation of many types of cells
including isolated chondrocytes (23). The high levels
of IL-6 we have detected in this model may therefore
act on chondrocytes to influence their proliferation
and their responses to cartilage injury.
1. Pond MJ, Nuki G: Experimentally-induced osteoarthri-
2.
3.
4.
5.
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