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R Regulation of the Cytokine/Chemokine Environment1



Departments of
* Immunology/Microbiology,
Orthopedic Surgery, and
Medicine, Section of Rheumatology, Rush-Presbyterian-St. Lukes Medical Center, Chicago, IL 60612
| Abstract |
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Rs are specialized cell surface receptors that coordinately
regulate immune responses. Although Fc
R expression is a prerequisite
for the development of several immune complex-mediated diseases, the
mechanism responsible for Fc
R-dependent regulation in autoimmunity
remains unclear. Therefore, we assessed Fc
R-dependent regulation of
inflammation in proteoglycan-induced arthritis (PGIA) using
Fc
R-/- mice. Fc
RIIb-/- mice developed
arthritis at an earlier time point and with a greater severity than
wild-type (WT) mice. In
-chain-/-
(Fc
RI-/- and Fc
RIII-/-) mice, no
clinical or histological evidence of inflammation was observed.
Exacerbation of arthritis in Fc
RIIb-/- mice correlated
with enhanced PG-specific Ab production, but did not significantly
affect PG-specific T cell priming. In
-chain-/- mice,
the absence of arthritis did not correlate with serum Ab responses, as
PG-specific Ab production was normal. Although PG-specific T cell
proliferation was diminished, spleen cells from
-chain-/- mice successfully adoptively transferred
arthritis into SCID mice. Our studies indicated that the mechanism
responsible for Fc
R regulation of PGIA development was at the level
of inflammatory cytokine and
-chemokine expression within the joint.
Fc
RIIb regulated the development of PGIA by controlling the
initiation of cytokine and chemokine expression within the joint before
the onset of arthritis, whereas the expression of Fc
RI and or
Fc
RIII controlled cytokine and chemokine expression late in the
development of PGIA during the onset of disease. These results suggest
that Fc
Rs are critical for the development of inflammation during
PGIA, possibly by maintaining or enhancing inflammatory cytokine and
-chemokine production. | Introduction |
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One of the main receptors responsible for immune complex activation of
cells is the Fc
R. The binding and cross-linking of immune complexes
to FcRs specific for IgG (Fc
Rs) on leukocytes triggers the
activation and regulation of a variety of cellular responses, including
Ab production, Ab-dependant cellular cytotoxicity, phagocytosis, and
release of inflammatory cytokines and chemokines (5, 6, 7, 8, 9).
There are three structurally distinct types of murine Fc
Rs: Fc
RI,
Fc
RIIb, and Fc
RIII (10). Fc
RI is a high affinity
receptor exclusively expressed on monocytes and macrophages (11, 12). Although Fc
RI can bind IgG-containing immune complexes,
it is the only Fc
R capable of binding monomeric IgG
(8). Fc
RIII is a low affinity receptor that is
predominantly expressed on lymphoid and myeloid cells
(10). Both Fc
RI and Fc
RIII are dual-chain receptors,
with the signal transduction unit of both receptors being the
-chain. The cytoplasmic domain of the
-chain contains an
immunotyrosine-based activation motif (13, 14), and
cross-linking either Fc
RI or Fc
RIII in vitro leads to activation
of cell functions such as inflammatory cytokine and
-chemokine
production (5, 6, 15). Fc
RIIb is the most widely
expressed Fc
R, expressed on virtually all hemopoietic cells except T
cells, NK cells, and RBCs. Fc
RIIb is a second low affinity receptor
and, like Fc
RIII, is only capable of binding IgG-containing immune
complexes (10). Fc
RIIb, unlike either Fc
RI or
Fc
RIII, is a single-chain receptor that contains an
immunotyrosine-based inhibition motif (16, 17, 18).
Cross-linking Fc
RIIb with either of the activation Fc
Rs, Fc
RI
or Fc
RIII, or the B cell receptor inhibits cell function
(19). By using Fc
R-/- mice,
these receptors have been demonstrated to play an important role in
controlling immune complex-mediated diseases such as acute anemia, the
Arthus reaction, glomerulonephritis, alveolitis, and arthritis
(4, 20, 21, 22, 23, 24, 25, 26, 27). Although Fc
Rs have been shown to regulate
the development of these immune complex-mediated diseases, the
mechanism responsible for Fc
R regulation remains unclear.
Arthritis can be induced in BALB/c mice immunized with human cartilage
proteoglycan (PG).3
Clinical and histologic studies of the diarthrodial joints of these
mice reveal that the inflammatory response that develops shares many
similarities to rheumatoid arthritis (28, 29). During the
development of PG-induced arthritis (PGIA), but before arthritis onset,
a systemic IgG Ab response develops that first recognizes human PG, but
later cross-reacts with native mouse PG (30, 31). These
Abs gain access to the joint and bind cartilage PGs, forming immune
complexes. Accumulation of PG-specific IgG in the cartilage is
characteristic of inflamed joints and is accompanied by a loss of
cartilage PGs (32). Synovium contains macrophages that
express Fc
Rs capable of binding immune complexes. Since both
Fc
R-bearing cells and IgG-containing immune complexes are present in
the joints of arthritic mice, it is possible that the interaction
between immune complexes and Fc
Rs drives the inflammatory response
associated with this arthritis model.
In this study we used two types of Fc
R-/-
mice,
-chain -/-
(Fc
RI-/- and
Fc
RIII-/-) and
Fc
RIIb-/- mice, to examine the involvement
of Fc
Rs in the development of inflammation in PGIA. The results
provide direct evidence that the inhibitory Fc
RIIb controls disease
severity, while the activating
-chain Fc
Rs are essential in the
development of arthritis. Moreover, the use of
Fc
R-/- mice demonstrates that
Fc
R-dependent activation/inhibition regulates inflammatory cytokine
and
-chemokine mRNA expression in the joint both before and after
the onset of arthritis. These results suggest that Fc
Rs are vital
for the development of arthritis by maintaining or propagating the
cytokine and chemokine responses associated with the development of
inflammation in the joint.
| Materials and Methods |
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Human cartilage tissue was obtained at the time of joint
replacement surgery. PG from adult cartilage was prepared as previously
described (28). Cartilage pieces were pulverized in liquid
nitrogen and then extracted with 4 M guanidinium chloride in 50 mM
sodium acetate, pH 5.8, containing protease inhibitors at 4°C. High
buoyant density PG monomers (aggrecan) were purified by dissociative
cesium chloride gradient centrifugation. PGs were sequentially digested
with endo-
-galactosidase and protease-free chondroitinase ABC
(Seikagaku America, Rockville, MD) overnight at 37°C and then further
purified on a Sephacryl S-200 column (Pharmacia Biotech, Uppsala,
Sweden). Murine PG was obtained from the cartilage of newborn mice and
prepared in a similar manner as human cartilage PG, except that murine
PG was not deglycosylated.
Induction and assessment of arthritis
Wild-type (WT), Fc
RIIb-/-, and
-chain-/- mice were on a BALB/c background.
All mice were bred at Taconic Farms (Germantown, NY) and used at >3 mo
age. Arthritis was induced by injecting female BALB/c mice with 150
µg/ml human PG (hPG) measured as protein in the presence of CFA as
previously described (28, 29). Mice were boosted at 3 and
6 wk with 100 µg/ml of hPG. The first booster was in IFA, and the
second booster was in CFA. Mice developed arthritis in the diarthrodial
joints
24 wk after the third immunization. Paws were scored for
erythema and swelling every third day after the second immunization to
assess arthritis onset and severity. Paws were scored on a scale from
one to four as follows: 0, normal; 1, mild erythema and swelling,
usually one or two toes; 2, moderate erythema and swelling of the paw;
3, more diffuse erythema and swelling of the paw; and 4, severe
erythema and swelling of the entire paw. Each paw was scored
individually; therefore, the cumulative score ranged from 016.
Histological studies were performed to determine the extent of joint
inflammation and damage. Hind paws were dissected, decalcified,
embedded in paraffin, and sectioned at 6 µm as previously described
(28). Sagittal sections were stained with H&E. Sections
were examined by a blinded histologist, and arthritic changes were
scored as normal (none), mild, moderate, or severe.
Detection of serum Ab titers
Mice were bled from the orbital plexus, and isotype-specific serum anti-PG Abs were measured by ELISA. Plates (96-well Nunc-Immuno plates; Fisher Scientific, Pittsburgh, PA) were coated with 1.0 µg of chondroitinase ABC-digested hPG or 1.5 µg native mouse PG (mPG) in carbonate buffer (15 mM Na2CO3 and 35 mM NaHCO3, pH 9.6). Sera were serially diluted in 1x PBS in 0.5% Tween 20. Sera dilutions of 1/100, 1/500, and 1/2,500 were used for detection of mPG-specific Abs, while dilutions of 1/2,500, 1/12,500, and 1/62,500 were used to detect hPG-specific Abs. Isotypes were detected with isotype-specific Abs, rabbit anti-mouse IgG1-HRP and IgG2a-HRP (Zymed, San Francisco, CA), followed by the substrate orthophenylenediamine, and absorbance was measured on a ELISA reader at 490 nm. A standard curve of myeloma IgG1 and IgG2a proteins was titrated to determine the concentrations of IgG1 and IgG2a in the sera. IgG1 and IgG2a proteins were detected using the same isotype-specific Abs as those used to detect PG-specific Ab isotypes. Serum IgG2b and IgG3 titers were also assessed and were found to be nearly undetectable (data not shown).
T cell proliferation assay
Spleens were harvested 82 days after the initial immunization, and single-cell suspensions were prepared as previously described (33). T cells were purified by passage through nylon wool. T cells (1.25 x 106 cells/ml) were incubated with irradiated (2500 rad) spleen cells (1.25 x 106 cells/ml) from nonimmunized mice in a fixed volume of 200 µl in serum-free medium (HL-1 medium; Fisher), 100 µg/ml penicillin, 100 µg/ml streptomycin, and 2 mM L-glutamine in a 96-well plate. T cells were stimulated in the presence or the absence of PG (3.13 and 1.5 µg/ml). T cell cultures were incubated at 37°C in 5% CO2 for 5 days, the last 18 h pulsed with [3H]thymidine (0.5 µCi/well). The cells were harvested using a cell harvester (Tomtec, Orange, CT), and the amount of incorporated [3H]thymidine was measured using a scintillation counter (EG&G Wallac, Galesburg, MD).
Assessment of in vitro splenic cytokine production
Splenocytes (2 x 106 cells/ml) were
incubated in a 24-well Falcon plate (Fisher) in serum-free HL-1 medium.
IFN-
and TNF-
were measured from supernatants harvested on day 5
by ELISA using the OPT EIA mouse IFN-
set (BD PharMingen, San Diego,
CA) and the mouse TNF-
DuoSet ELISA development kit (R&D Systems,
Minneapolis, MN).
Adoptive transfer of PGIA into SICD mice
Splenocytes (5 x 107 cells/mouse)
from either arthritic WT mice or immunized
-chain-/- mice and 150 µg human PG
measured as protein were mixed in saline and injected i.p. into female
BALB/c SCID mice (National Cancer Institute, Frederick, MD). The SCID
mice were monitored for disease onset and severity every other day
after cell transfer.
RNase protection assay
One hind paw was homogenized with a Polytron homogenizer (KRI
Works, Cincinnati, OH) on ice. Homogenate was centrifuged to remove
large debris, and RNA was extracted with Tri-Reagent (Molecular
Research, Cincinnati, OH). RNase protection assay was preformed on 20
µg of RNA using the Riboquant Multiprobe RNase Protection Assay
System (BD PharMingen) according to the manufacturers directions. The
mCK-2b, mCK-3b, and mcK-5b templates were used to detect sets of
cytokines and chemokines (IL-12 p35, IL-12 p40, IL-10, IL-1
,
IL-1
, IL-1R antagonist (IL-1Ra), IL-18/IFN-
-inducing
factor, TNF-
, lymphotoxin
, TNF-
, IL-6, IFN-
,
IFN-
, TGF-
1, TGF-
2, TGF-
3, macrophage inflammatory factor,
lymphotactin, RANTES, eotaxin, macrophage inflammatory protein-1
(MIP-1
), MIP-1
, MIP-2, monocyte chemotactic protein-1 (MCP-1),
and T cell-activated gene-3) as well as housekeeping genes L32 and
GAPDH. Labeled ([
-32P]UTP) antisense RNA was
synthesized by in vitro transcription from a cDNA template provided in
the kit. Antisense RNA probe was purified by phenol/chloroform
extraction and ethanol precipitation and was hybridized with the mRNA
samples overnight at 56°C. RNase was used to digest ssRNA. Protected
dsRNA was purified by phenol-chloroform extraction and ethanol
precipitation. The samples were electrophoresed on a 5% denaturing
polyacrylamide gel. The gel was dried and exposed to a phosphorimager
screen. The radioactivity of the samples was measured and analyzed by
scanning densitometry on a STORM PhosphorImager (Molecular Dynamics,
Sunnyvale, CA). The level of cytokine and/or chemokine mRNA was
expressed as the ratio of each mRNA species to GAPDH. Increases in
inflammatory mediator mRNA expression were also reflected at the
protein level, as measured by ELISA (data not shown).
Statistical analysis
Fishers exact t test was used to determine statistical significance in the incidence of arthritis. The Mann-Whitney U test was used to compare nonparametric data for statistical significance. A value of p < 0.05 was considered significant.
| Results |
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R deficiency leads to either the absence or exacerbation of
arthritis
BALB/c mice immunized with hPG develop an Ab response to hPG,
which over time develops into a cross-reactive response to mPG
(30, 31). Abs specific for mPG appear before the
development of arthritis and correlate with Ab deposition in the joint
(in the form of immune complexes), the loss of cartilage PGs, and later
the development of arthritis (28, 34). To determine
whether Fc
Rs are involved in the development of inflammation, WT,
Fc
RIIb-/-, and
-chain-/- mice were immunized with hPG, and
the development of arthritis was monitored over time. Disease onset
occurred in Fc
RIIb-/- mice at an earlier
time point (50% on day 34) compared with WT mice (0% on day 34; Fig. 1
A). Not only did
Fc
RIIb-/- mice develop arthritis earlier
than WT mice, they also developed a more severe disease (13.6 ±
1.65) than WT mice (6.1 ± 3.88; Fig. 1
B). In contrast,
-chain-/- mice never displayed any signs of
inflammation (Fig. 1
, A and B, and Fig. 2
C). Over an extended period of time (162 days),
inflammatory symptoms were still absent in
-chain-/- mice (data not shown). These data
suggest that Fc
R expression is critical for controlling the
development of PGIA.
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-Chain-/- mice are protected from the
histopathological changes associated with arthritis
To determine the extent of the inflammation, we examined ankle
joint histology in WT, Fc
RIIb-/-, and
-chain-/- mice on day 82. In WT mice we
observed a histologic picture characteristic of acute arthritis.
Mononuclear and polymorphonuclear cell infiltration was abundant in the
tissues and joint spaces. There was edema of the synovial and
periarticular tissues accompanied by synovial hyperplasia. Cartilage
erosion and disintegrating chondrocytes were also seen in the remaining
layer of the articular surface (Fig. 2
A). In comparison,
Fc
RIIb-/- mice displayed histopathologic
signs of a more chronic disease. The ankle joints of
Fc
RIIb-/- mice exhibited less leukocyte
infiltration and synovial lining proliferation, with a complete
cartilage loss, severe bone erosion, abnormal remodeling of joint
structures, and osteophyte formation (Fig. 2
B). These
features correlated with the fact that arthritis was initiated in these
mice at an earlier time point than in WT mice (Fig. 1
). Conversely,
cellular infiltrate and/or histopathological signs of disease were
completely absent in
-chain-/- mice. In
these mice, the lack of paw erythema and swelling correlated with the
absence of cellular infiltration and joint destruction (Figs. 1
B and 2C).
Exacerbation of disease in Fc
RIIb-/- mice is
associated with early production of PG-specific Abs
It has been previously demonstrated that B cells are required for
the development of PGIA (34, 35). Fc
Rs regulate B cell
function by either directly suppressing Ab production through Fc
RIIb
or indirectly regulating Ab responses through
-chain Fc
Rs
(36). It is therefore possible that a deficiency in Fc
R
expression may augment B cell Ab production and possibly exacerbate or
accelerate the disease. To assess the role of Fc
Rs in the
development Ab responses in PGIA, we first analyzed PG-specific serum
Ab production in WT and Fc
RIIb-/- mice. Mice
were bled at 4 wk after the initial immunization (
12 wk before the
onset of arthritis in Fc
RIIb-/- mice) and
after the development of arthritis at 7 and 11.5 wk, and PG-specific
IgG1 and IgG2a Ab titers were assayed by ELISA. Correlating with the
rapid progression of disease in Fc
RIIb-/-
mice was a significant enhancement of the PG-specific serum Ab response
at wk 4 (Fig. 3
A). However, by
as early as wk 7, WT and Fc
RIIb-/- mice
produced the same amount of PG-specific Ab (Fig. 3
B).
|
-chain Fc
Rs play in the development of
Ab responses in PGIA, we analyzed PG-specific serum Ab production in WT
and
-chain-/- mice. Mice were bled at 7 wk
after the initial immunization (
12 wk before the onset of
arthritis in WT mice) and after the development of arthritis at 11.5
wk, and PG-specific IgG1 and IgG2a Ab titers were assayed by ELISA.
-Chain-/- and WT mice produced equivalent
levels of PG-specific Ab both before and after the onset of arthritis
(Fig. 3
-chain Fc
Rs do not appear to regulate Ab production during
the development of arthritis, Fc
RIIb does exert control over Ab
production at a point before the onset of arthritis, possibly during
the initiation of inflammation. Alterations in disease outcome are not associated with a change in T cell function
It has been previously shown that PG-specific T cells are critical
for the development of arthritis (34, 37). Since the
Fc
R
-chain is shared by the TCR, it may play an important role in
TCR signaling (38, 39). In addition, Fc
R-mediated
uptake of Ag is an efficient mechanism for Ag presentation
(40). Consequently, it is possible that the Fc
Rs
directly and/or indirectly control T cell priming, and thus a
deficiency in Fc
R expression may alter disease outcome in a T
cell-dependent manner. Therefore, we examined T cell proliferation in
response to PG in WT, Fc
RIIb-/-, and
-chain-/- mice. T cells isolated from
PG-immunized Fc
RIIb-/- mice proliferate to a
similar extent as T cells isolated from WT mice, indicating that
Fc
RIIb is not involved in T cell priming (Fig. 4
A). By comparison,
proliferation of
-chain-/- T cells was
significantly reduced compared with that of WT T cells (Fig. 4
A), suggesting the possibility that
-chain-/- mice do not succumb to disease
because T cells were not sufficiently primed.
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-chain-/- T cells were
sufficiently primed to induce arthritis, we used an adoptive transfer
model in which splenocytes from PG-immunized mice were transferred into
SCID mice. The SCID mice were then immunized with hPG to activate the
primed T cells. If
-chain-/- T cells were
fully capable of inducing arthritis, then adoptively transferring
PG-immunized
-chain-/- splenocytes into SCID
mice should induce disease (Fig. 4
-chain-/- mice and hPG into
SCID mice and monitored the onset and severity of disease. SCID mice
that received splenocytes from
-chain-/-
mice developed disease with similar kinetics and severity as SCID mice
that received WT splenocytes. By 35 days after the initial transfer,
100% of SCID mice receiving either WT or
-chain-/- splenocytes developed arthritis
with a similar severity (10.1 ± 2.36 to 8.0 ± 3.08,
respectively; Fig. 4
-chain-/- mice were primed to PG and fully
capable of inducing arthritis despite the reduced level of PG-specific
T cell proliferation.
Disrupting the Fc
R/immune complex interaction alters
inflammatory cytokine and
-chemokine mRNA expression during the
effector phase of inflammation
It has been previously demonstrated that immune complex
stimulation through cross-linking of Fc
RIII causes an increase in
inflammatory cytokine and
-chemokine production in vitro (5, 6). Since immune complexes and Fc
R-bearing cells are present
in the joints of arthritic mice, we assessed whether Fc
R-dependent
signaling controls the expression of cytokines and chemokines during
the development of inflammation. To this end, we quantitated cytokine
and chemokine mRNA transcripts before erythema and swelling of the paws
and once mice overtly displayed signs of arthritis (Figs. 5
and 6
). Levels of inflammatory cytokine
mRNA (TNF-
, IFN-
, IL-6, IL-1
), IL-1Ra, and
-chemokine mRNA
(MIP-1
, MIP-2, MCP-1) expression were significantly increased in
Fc
RIIb-/- mice during both the
pre-arthritic, initiation phase (3.5 wk after the initial immunization;
Fig. 5
, A and C) and the arthritic, effector
phase (5 wk after the initial immunization; Fig. 5
, B and
D) compared with levels expressed in WT mice at these same
time points. Considering that both the initiation and effector phases
occur in Fc
RIIb-/- mice before the
initiation phase in WT mice (7.5 wk after the initial immunization), it
is not surprising that expression of cytokine and chemokine mRNA is
elevated only in the Fc
RIIb-/- mice.
However, when we compared cytokine and chemokine mRNA expression levels
during the arthritic, effector phase for WT and
Fc
RIIb-/- mice (11.5 and 5 wk after the
initial immunization, respectively), we observed similar levels of
expression in both mice (Fig. 5
, B and D, and
Fig. 6
, B and D). These data indicate that
Fc
RIIb-/- mice express similar levels of
cytokines and chemokines as WT mice, but express them at an earlier
time point. These results suggest that increased expression of cytokine
and chemokine mRNA transcripts correlates with the early development of
disease in Fc
RIIb-/- mice.
|
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-chain-/- mice, we observed that TNF-
,
IFN-
, IL-6, IL-1Ra, MIP-1
, MIP-1
, MIP-2, and MCP-1 mRNA
transcripts were expressed at equivalent levels to those obtained in WT
mice during the pre-arthritic, initiation phase (7.5 wk after the
initial immunization; Fig. 6
-chain-/- mice during the arthritic,
effector phase for WT mice (11.5 wk after the initial immunization),
-chain-/- mice expressed significantly lower
amounts of inflammatory cytokines and
-chemokines than WT mice (Fig. 6
-chain Fc
Rs are not responsible for the cytokine
and chemokine responses observed during the initiation phase of this
disease. However,
-chain Fc
R expression is necessary for the
maintenance or enhancement of the expression of cytokines and
chemokines during the arthritic, effector phase.
Cellular infiltrate was absent in
-chain-/-
mice during the arthritic, effector phase of inflammation (Fig. 2
C); therefore, the reduction in inflammatory cytokine and
-chemokine mRNA expression could be attributed to the absence of
infiltrating cells. However, a decrease in the expression of
inflammatory mediator transcripts could also be due to an inability of
-chain-/- mice to respond to immune complex
stimulation. Consequently, we examined ex vivo production of TNF-
and IFN-
from spleens dissected from WT and
-chain-/- mice to evaluate the overall
functionality of
-chain-/- cells in terms of
cytokine production. When we assessed production of these inflammatory
cytokines 11.5 wk after the initial immunization, we found that
splenocytes isolated from
-chain-/- mice
produced significantly less TNF-
and IFN-
than those from WT mice
(Fig. 6
E), suggesting that
-chain-/- cells are dysfunctional in
cytokine production. Taken together, these results suggest that
-chain Fc
R expression controls inflammatory cytokine production,
and that the reduction in
-chain Fc
R-dependent cytokine and
chemokine production might contribute to the suppression of disease in
-chain-/- mice.
| Discussion |
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Rs
(Fc
RI and Fc
RIII) and the inhibitory Fc
R (Fc
RIIb) regulates
the inflammatory process that determines the susceptibility to PGIA. We
show that Fc
Rs regulate the inflammatory response in PGIA by
controlling cytokine and chemokine expression in the joint. Our first
observation is that
-chain-/-
(Fc
RI-/- and
Fc
RIII-/-) mice are completely protected
from arthritis, demonstrating that Fc
RI and/or Fc
RIII are
required for development of the disease. In contrast,
Fc
RIIb-/- mice develop augmented disease
that begins at an earlier time and is more severe than that in WT mice,
indicating that Fc
RIIb is essential for controlling disease
severity.
Protection from PGIA in
-chain-/- mice
confirms similar observations in collagen-induced arthritis (CIA),
immune-complex-mediated arthritis (ICA), and the K/BxN model. However,
there are unique differences between the PGIA model and these other
models of arthritis. For example, in PGIA both joint swelling and
histological signs of inflammation are completely absent in
-chain-/- animals, even 162 days after
immunization. Contrary to PGIA, in CIA a mild form of arthritis
develops in some
-chain-/- mice
(25). Since anti-collagen type II Abs are a
prerequisite for the development of CIA, other factors, such as
complement, may contribute to a greater extent in the development of
inflammation in CIA (41, 42). The requirement for Fc
RI
or Fc
RIII differs between murine models of arthritis as well. In the
ICA model, reduced cartilage damage is observed in
Fc
RI-/- mice, but not in
Fc
RIII-/- mice, whereas in the K/BxN model
a significant reduction in ankle swelling is only observed in
Fc
RIII-/- mice (43, 44). This
disparity in the requirement for
-chain Fc
Rs suggests that
different mechanisms are involved in the development of disease in
these arthritis models.
There are several explanations for why inhibition or exacerbation of
disease is dependent on Fc
R expression. In PGIA, systemic immunity
to the immunizing Ag develops before any signs of disease (30, 31, 45). Therefore, Fc
Rs may contribute to systemic immunity
by regulating B cell or T cell priming. With regard to Fc
R control
of B cell priming, evidence suggests that
-chain Fc
Rs may alter B
cell Ab production (36, 46). However, a deficiency in
-chain expression does not affect the secretion of PG-specific Abs,
indicating that Ab production in this model is independent of the
-chain Fc
R (Fig. 3
). In contrast, Fc
RIIb directly suppresses
Ab production (36). Our data are consistent with this
hypothesis, since PG-specific Ab levels in
Fc
RIIb-/- mice are elevated early in disease
concomitant with the early onset of arthritis. This concept is also
supported by the observation in the K/BxN model that Abs from
arthritic mice can transfer disease into WT and
Fc
RIIb-/- mice with similar kinetics and
severity (47). These data imply that by circumventing the
development of an Ab response, one bypasses the requirement for
Fc
RIIb regulation of that response. Taken together, these data
indicate that Fc
RIIb controls the development of arthritis by
regulating Ab production.
With regard to Fc
R control of T cell priming, T cell priming might
be altered in
-chain-/- mice because the
-chain is shared with the CD3 complex and/or because efficient Ag
presentation occurs through Fc
R uptake of Ag (38, 39, 47, 48). Despite a decrease in overall PG-specific T cell
proliferation in cells isolated from PG-immunized
-chain-/- mice, splenocytes from
-chain-/- mice adoptively transferred into
SCID mice induce arthritis with similar kinetics and severity as WT
splenocytes. These results demonstrate that T cells and B cells in
immunized
-chain-/- and WT mice are primed
to a similar extent. In addition, these experiments separate the
initial development of autoreactive B cell and T cells from downstream
effector functions mediated through stimulatory Fc
Rs.
Our second important observation in this study is that coordinate
expression of Fc
Rs regulates disease severity by controlling
cytokine and chemokine activation. Although Fc
RIIb and the
-chain
Fc
Rs control the stimulation and inhibition of cytokines and
chemokines, these receptors control function at different points in the
development of PGIA. Development of arthritis in PGIA can be separated
into an initiation phase and an effector phase. During the initiation
phase, clinical and histologic signs of arthritis are absent; however,
systemic PG-specific T cell and B cell responses are detected. Here we
show that cytokine and chemokine transcripts are also expressed in the
joint before the onset of arthritis (Fig. 6
, A and
C) (30, 31). During the effector phase of PGIA,
once erythema and swelling are observed, the expression of cytokine and
chemokine transcripts is maintained and/or further elevated, followed
by joint destruction (Fig. 6
, B and D)
(33). The release of these inflammatory mediators can be
mimicked in vitro by cross-linking Fc
Rs on monocytes (5, 6). Since Fc
R-bearing macrophages are present in the synovial
membrane, it is possible that the activation of cytokines and
chemokines are Fc
R dependent.
In WT mice, cytokine and chemokine expression in the joint remained
stable at early time points after the initial immunization (3.5 and 5
wk) compared with that in naive mice. However, by 7.5 wk mRNA
transcript expression was enhanced (Fig. 5
and Fig. 6
, A and
C), and this time point corresponds to the initiation phase
in WT mice. Conversely, in Fc
RIIb-/- mice,
activation of cytokines and chemokines is enhanced as early as 3.5 wk
(Fig. 5
, A and C), demonstrating that the
initiation phase for Fc
RIIb-/- mice begins
much earlier than that for WT mice. These results correlate early,
elevated levels of cytokine and chemokine expression with accelerated
disease development in Fc
RIIb-deficient mice.
In contrast to the expression patterns observed in
FcgRIIb-/- and WT mice, equivalent levels of
inflammatory cytokines and
-chemokines are expressed in the joints
of
-chain-/- and WT mice during the
initiation phase of the disease (Fig. 6
, A and
C). Although initiation of inflammatory mediator expression
appears to be
-chain Fc
R independent, it is possible that
cytokine and chemokine expression is driven by Ab deposition in the
joint. In support of this hypothesis, in
Fc
RIIb-/- mice, enhanced levels of
Abs coincide with the activation of inflammatory mediators. There
are several possible explanations of how Abs might stimulate
cytokine and chemokine release during the initiation phase
independently of
-chain Fc
R stimulation. One
possibility is the recent finding of a low level of expression of
Fc
RI due to the continual expression of Fc
RI
-chain in
-chain-/- mice. However,
-chain-/- macrophages are unable to
phagocytose or produce cytokines in response to Fc
R stimulation
(46). Therefore, the Fc
RI that is expressed in the
absence of
-chain expression is unlikely to be functional. A second
explanation is that immune complexes may trigger the activation of the
complement system and the generation of pro-inflammatory mediators. In
support of this concept, in CIA and the K/BxN model, complement plays
a critical role in the development of inflammation (41, 43, 49). In ICA, Ab and complement deposition in the joint is
observed before the onset of arthritis (50). In addition,
complement components, such as active C1q, C3a, and C5a, have the
ability to stimulate production of inflammatory mediators, such as
IL-1
, IL-6, IL-8, TNF-
, MIP-1
, MIP-1
, MIP-2, and MCP-1
(51, 52, 53, 54, 55, 56). Together, these studies suggest that Ab
deposition, followed by complement activation, may activate
inflammation.
The levels of cytokines and chemokines expressed in the joints of
Fc
RIIb-/- and WT mice during the initiation
phase are sustained or enhanced in Fc
RIIb-/-
and WT mice during the effector phase (5 and 11.5 wk, respectively)
(Fig. 5
, B and D, and Fig. 6
, B and
D). On the other hand,
-chain-/-
mice at 11.5 wk express reduced inflammatory cytokine and
-chemokine
transcripts compared with arthritic WT mice (Fig. 6
, B and
D). These data correlate the expression of inflammatory
mediators with
-chain Fc
R expression as well as arthritis, and
they also demonstrate that
-chain Fc
Rs are necessary for the
maintenance or enhancement of the cytokine and chemokine responses that
began during the initiation phase. It is possible that the increases in
cytokine and chemokine expression observed in arthritic WT mice are due
to the influx of inflammatory leukocytes. Therefore, the absence of
infiltrating cells in
-chain-/- mice could
lead to a reduction in inflammatory mediator expression. Our ex vivo
studies also demonstrate an inability of
-chain-/- splenocytes to produce similar
levels of inflammatory cytokines as WT cells. These results suggest
that
-chain-/- cells infiltrating the joint
would not be activated by immune complex cross-linking of the Fc
Rs,
and thereby would not be able to maintain or enhance the production of
inflammatory mediators that commenced during the initiation phase. In
further support of this hypothesis, in ICA studies infiltration without
enhancement of inflammatory mediator expression is observed in the
joints of
-chain-/- mice
(50).
Taken together, these studies suggest a possible cooperative mechanism
of inflammation between the complement system and Fc
Rs, in that both
are necessary for full induction of arthritis. We propose a model of
arthritis that begins with systemic T cell and B cell responses,
followed by Ab deposition and the formation of immune complexes in the
joints. Subsequent complement deposition in the joint initiates
inflammatory cytokine and
-chemokine expression that is observed
before the development of arthritis. The chemotaxtic properties of
complement plus
-chemokines stimulate the influx of primed B and T
cells, macrophages, and neutrophils into the joint. Infiltrating
Fc
RI/Fc
RIII-bearing cells bind to immune complexes
that are either within the synovial fluid or bound to the joint itself,
thereby further activating the expression of cytokines and chemokines.
The release of these newly expressed inflammatory mediators serves to
maintain or amplify the response that was initiated by the activation
of complement.
Our studies demonstrate that Fc
Rs play an important and vital
role in the development of joint inflammation. The possible role of
Fc
Rs in PGIA is to control both inflammatory cytokine and
-chemokine production. Further studies are necessary to conclusively
determine whether complement or immune complex/Fc
R
interactions drive inflammatory cytokine and
-chemokine
production.
| Acknowledgments |
|---|
| Footnotes |
|---|
2 Address correspondence and reprint requests to Dr. Charles D. Kaplan, Department of Immunology/Microbiology, Rush-Presbyterian-St. Lukes Medical Center, 1653 West Congress Parkway, Chicago, IL 60612. E-mail address: charles_kaplan{at}rush.edu ![]()
3 Abbreviations used in this paper: PG, proteoglycan; CIA, collagen-induced arthritis;
-chain-/-, Fc
RI-/- and Fc
RIII-/-; ICA, immune-complex-mediated arthritis; PGIA, proteoglycan-induced arthritis; hPG, human PG; mPG, native mouse PG; IL-1Ra, IL-1R antagonist; MCP, monocyte chemotactic protein; MIP, macrophage inflammatory protein; WT, wild type. ![]()
Received for publication July 3, 2002. Accepted for publication September 11, 2002.
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