SOCS-3 negatively regulates innate and adaptive immune mechanisms in acute IL-1–dependent inflammatory arthritis

RA is an autoimmune disease characterized by sustained imbalance between pro- and antiinflammatory immune mechanisms. The SOCS proteins are negative regulators of cytokine signaling, but to date there has been little information on their function in disease. The generation of Socs3^–/Δvav mice, which lack SOCS-3 in the hematopoietic and endothelial cell compartment, allowed us to explore the role of endogenous SOCS-3 during acute inflammatory arthritis. Joint inflammation in Socs3^–/Δvav mice was particularly severe and was characterized by increased numbers of neutrophils in the inflamed synovium, bone marrow, peripheral blood, and spleen. These features were most likely due to increased production of and enhanced responsiveness to G-CSF and IL-6 during arthritis in these mice. Local osteoclast generation and bone destruction were also dramatically increased in the absence of SOCS-3, as was macrophage activation. Finally, SOCS-3 was found to negatively regulate CD4⁺ T lymphocyte activation, including production of the pleiotropic cytokine IL-17. The absence of SOCS-3 therefore had dramatic effects in this disease model, with a broader impact on cellular responses than SOCS-1 deficiency. These findings provide direct in vivo evidence that endogenous SOCS-3 is a critical negative regulator of multiple cell types orchestrating inflammatory joint disease.

RA is a complex autoimmune disease resulting from sustained imbalance between pro- and antiinflammatory mechanisms (1). IL-1 and TNF are key proinflammatory cytokines implicated in the pathogenesis of RA (2, 3). Targeted blockade of IL-1 and TNF is helpful in RA, but a significant number of patients fail to respond. Therapeutic manipulation of endogenous negative regulators of cytokine signaling might be a more effective therapeutic approach, as this may allow inhibition of multiple cytokines.

The SOCS proteins are key negative regulators of cytokine signaling that inhibit the JAK/STAT signal transduction pathway (4). Production of SOCS proteins may be induced by a wide range of stimuli, including LPS and TNF (5), IL-6 (6), and TGF-β (7). Mice lacking individual Socs genes develop different phenotypes. Mice deficient in SOCS-1 died within 2 weeks of age from fatty degeneration and necrosis of the liver and mononuclear cell infiltration into various organs (8). This pathology was dependent on IFN-γ, as mice lacking both SOCS-1 and IFN-γ (SOCS-1^–/–IFN-γ^–/–mice) survived to adulthood (9). The midgestational embryonic lethality in SOCS-3–deficient mice (10, 11) has been overcome by a conditional gene-targeting strategy using the cre recombinase (Cre) and loxP system (12). This allowed generation of viable mice following tissue-specific SOCS-3 deletion. Liver-specific deletion of SOCS-3 resulted in prolonged activation of STAT-3 in hepatocytes following IL-6 stimulation, indicating that SOCS-3 negatively regulates IL-6 responses in vivo (12). Interestingly, in the absence of SOCS-3, IL-6 also appeared to induce IFN-γ–responsive genes (12, 13), resulting in antiinflammatory effects (14, 15). These findings demonstrate that SOCS-3 may regulate both the quantity and the quality of biologic responses to IL-6. Deletion of SOCS-3 in hematopoietic and endothelial cells (Socs3^–/Δvav) resulted in dysregulated hematopoiesis characterized by peripheral blood neutrophilia, enhanced BM myelopoiesis and splenomegaly, and marked extramedullary hematopoiesis from the age of 17 weeks (16). Socs3^–/Δvav mice exhibited prolonged STAT-3 activation, increased frequency of clonogenic neutrophil progenitors, and enhanced survival and proliferation of mature granulocytes following in vitro stimulation with G-CSF. Systemic administration of G-CSF to Socs3^–/Δvav mice resulted in peripheral neutrophilia, splenomegaly, and neutrophil infiltrates in multiple tissues, indicating that SOCS-3 is also a key negative regulator of G-CSF signaling (16).

Intriguingly, SOCS-3 dysregulation may be specifically associated with joint disease. Mice with a mutation in the transmembrane receptor gp130 (Y757F mice), which ablates the SOCS-3 binding site, developed a spontaneous RA-like phenotype with advanced age (17). Mice expressing a knock-in mutation (gp130^ΔSTAT) that eliminated STAT-1 and -3 signaling developed a joint disease composed of both inflammatory and degenerative features (18). Impaired SOCS-3 induction following signaling through gp130^ΔSTAT was associated with overactivation of the SHP-2/ras/ERK pathway in these mice, which appeared to cause hyper-proliferation of synovial fibroblasts and chondrocytes (ref. 18 and our unpublished observations). SOCS-3 expression has been reported in synovial tissue from mice during experimental arthritis and in patients with RA (19). Intra-articular (IA) ankle injections of recombinant adenovirus expressing SOCS-3 reduced joint inflammation in experimental models of arthritis, a result associated with decreased STAT-3 phosphorylation and IL-6 production (19). However, the role of endogenous SOCS-3 in experimental arthritis has not been previously examined.

In RA, joint inflammation and damage results from interaction between hematopoietically derived immune cells and resident cells of joints such as synovial fibroblasts and chondrocytes (20). In order to evaluate the role of SOCS-3 in regulation of the cellular and molecular mediators implicated in inflammatory joint disease, we therefore induced inflammatory arthritis in mice that had undergone deletion of SOCS-3 in all hematopoietic and endothelial cells. This overcame the limitation of embryonic lethality following complete SOCS-3 deletion but also allowed us to examine the contribution of SOCS-3 in regulating cellular responses to joint inflammation.

We used a model of acute inflammatory arthritis that is dependent on IA antigen (methylated bovine serum albumin [mBSA]) and CD4⁺ T lymphocytes (21). IL-1 is a major mediator of RA (2, 22), and systemic injection of IL-1 in mice converts the transient CD4⁺ T lymphocyte–dependent inflammatory reaction to mBSA into a florid monoarthritis. Synovial macrophages, neutrophils, G-CSF, and IL-6 are also important mediators of pathology in this model (23, 24). While no one model of arthritis can completely reproduce the complexity of RA in humans, the strengths of this disease model are the high incidence of arthritis in injected knee joints; the rapid, reproducible disease kinetics; and the relatively well-defined molecular and cellular mediators. As such, this model provides insight into the earliest stages of the joint inflammation that occurs in RA.

We have previously used this model to examine the role of SOCS-1 (25), the p50 subunit of NF-κB (20), and TNF (26) in inflammatory arthritis. SOCS-1^–/–IFN-γ^–/– mice developed more severe arthritis compared with SOCS-1^+/+IFN-γ^–/–mice (25), and this was associated with an increased number of macrophages in the synovium and joint exudate, synovial granulomas, and lymphadenopathy of draining (popliteal) LNs. In contrast, there was only a small increase in synovial neutrophil numbers and osteoclast-mediated bone destruction in arthritic SOCS-1^–/–IFN-γ^–/– mice. SOCS-1 promoter activity was found in synovial macrophages, fibroblasts, and activated T lymphocytes but not synovial granulocytes, consistent with a SOCS-1–mediated response in these cells to cytokine stimulation during arthritis.

We now show that mBSA/IL-1–dependent arthritis was markedly exacerbated following SOCS-3 deletion in hematopoietic and endothelial cells. However, in contrast to SOCS-1 deficiency, the severe joint inflammation occurring in the absence of SOCS-3 was characterized by increased numbers of neutrophils in the inflamed synovium, peripheral blood, and spleen, most likely due to hyper-responsiveness to G-CSF and IL-6 but possibly due to IL-1 directly. SOCS-3 was also found to be an important negative regulator of CD4⁺ T lymphocyte and macrophage activation and osteoclast generation. These findings provide the first in vivo evidence to our knowledge that endogenous SOCS-3 is a critical negative regulator of the multiple cell types mediating inflammatory joint disease and suggest that SOCS-3 regulates an even wider range of cellular responses than does SOCS-1 during inflammatory arthritis.

Socs3^–/Δvav mice develop exacerbated inflammatory arthritis. To assess the role of endogenous SOCS-3 in the pathogenesis of inflammatory arthritis, we compared the severity of mBSA/IL-1–induced arthritis in WT and Socs3^+/Δvav mice as well as Socs3^–/Δvav mice, which have undergone deletion of SOCS-3 in all hematopoietic and endothelial cells. Knee joints were processed for histology on day 7 following mBSA injection, and sections were graded by an investigator blinded to the treatment groups. Socs3^–/Δvav mice developed markedly worse disease than age-, sex- and strain-matched WT and Socs3^+/Δvav controls (mean total histologic score, Socs3^–/Δvav, 20 ± 0.6; WT, 11.3 ± 0.7; Socs3^+/Δvav, 13.3 ± 0.6; P < 0.01, Socs3^–/Δvav versus WT and Socs3^+/Δvav; Figure 1, A, B, and G). Socs3^–/Δvav mice had dramatic synovial hyperplasia and aggressive pannus eroding into bone with increased inflammatory exudate. No synovial granulomas were seen in arthritic Socs3^–/Δvav mice, unlike those seen in arthritic SOCS-1^–/–IFN-γ^–/– mice (25). Cartilage destruction, including proteoglycan loss, was substantially increased in Socs3^–/Δvav mice (compare Figure 1E with Figure 1, D and F; also see Figure 1G). These findings indicate that endogenous SOCS-3 is an important negative regulator of mBSA/IL-1–dependent inflammatory arthritis.

Figure 1

SOCS-3 deficiency is associated with exacerbated mBSA/IL-1–induced arthritis. (A–F) Acute inflammatory arthritis was induced by IA injection of 200 μg mBSA into the knee joint followed by 3 daily subcutaneous injections of 250 ng rhIL-1. Mice were sacrificed on day 7, and knee joints were processed as described in Methods. Frontal H&E-stained sections through knee joints from arthritic WT (A) and Socs3^–/Δvav (B) mice. S, synovitis; F, femur; E, exudate; PN, pannus; P, patella. (C) Representative section from an arthritic SOCS-1^–/–IFN-γ^–/– mouse (25) using the same disease model is shown for comparison. Frontal Safranin O–stained sections through arthritic knee joints from a WT mouse (D) showing mild proteoglycan loss (asterisk indicates loss of red staining), while the Socs3^–/Δvav joint (E) had marked proteoglycan loss. (F) Arthritic knee joint from a SOCS-1^–/–IFN-γ^–/– mouse is shown for comparison. Magnification, ×200 (A–C); ×400 (D–F). (G) Joint sections were graded for 5 features of inflammatory arthritis, each on a scale of 0 (normal) to 5 (severe) by an investigator blinded to the experimental groups. Results shown are the mean ± SEM for 16–19 joints per group, pooled from 3 separate experiments. **P < 0.01 versus WT and Socs3^+/Δvav.

Socs3^–/Δvav mice have exacerbated bone destruction, reduced basal trabecular bone volume, and increased osteoclast numbers during joint inflammation. Bone destruction was significantly worse in arthritic Socs3^–/Δvav mice compared with WT and Socs3^+/Δvav controls and was characterized by prominent osteoclasts (Figure 1G and Figure 2, A and B). When osteoclasts, which are not usually a prominent feature of mBSA/IL-1–induced arthritis, were identified by tartrate-resistant acid phosphatase (TRAP) staining, Socs3^–/Δvav mice had approximately 10-fold more osteoclasts present on the periosteal surfaces compared with controls (Figure 2C). This led us to investigate bones in naive Socs3^–/Δvav mice. Histomorphometric analysis showed that the percentage of trabecular bone covered by osteoclasts (osteoclast surface/bone surface) within the secondary spongiosa was tripled in naive male Socs3^–/Δvav mice (Figure 2D). The ratio of osteoblast surface to bone surface was doubled in naive Socs3^–/Δvav mice, indicating that enhanced basal bone resorption in Socs3^–/Δvav mice was associated with increased bone formation in vivo. These findings were associated with a 50% reduction in trabecular bone volume in adult male mice as assessed by histomorphometry (Figure 2, D–F). Similar effects were present but were less dramatic in female mice (data not shown), probably because trabecular bone volume is generally lower in female mice (27). These findings suggest that endogenous SOCS-3 is an important regulator of osteoclastogenesis both in the basal state and during inflammation.

Figure 2

SOCS-3 deficiency is associated with increased osteoclast numbers. (A and B) Frontal sections through an arthritic Socs3^–/Δvav knee joint showing H&E-stained synovitis (S) containing numerous granulocytes (boxed area) with several multinucleated osteoclasts (large arrows) eroding into bone (Bn; A) and TRAP-stained osteoclasts (small arrows; B). (C) TRAP⁺ cells on the periosteal surfaces of arthritic joints per high-power field (hpf) were counted by an investigator blinded to the experimental groups (mean ± SEM; n = 5 mice per group, pooled from 2 independent experiments). (D) Histomorphometry was performed by an investigator blinded to the experimental groups. OcS/BS, ratio of osteoclast surface to bone surface; ObS/Bs, ratio of osteoblast surface to bone surface; BV/TV, trabecular bone volume. Values are mean ± SEM; n = 8–10 male mice per group at 8–12 weeks of age. Similar findings were seen in female mice. (E and F) Representative images of von Kossa–stained tibiae of male 8-week-old WT (E) and Socs3^–/Δvav (F) mice showing differences in trabecular bone volume. White boxes show secondary spongiosa region used for histomorphometric measurements. Magnification, ×400 (A and B); ×100 (E and F). *P < 0.05 versus WT and Socs3^+/Δvav.

Increased infiltration of neutrophils into the synovium in arthritic Socs3^–/Δvav mice. To characterize the synovial inflammatory cell infiltrate, we prepared single-cell suspensions of synovial tissue by enzymatic digestion. Hematopoietic cells were identified by expression of CD45 and further characterized by staining with antibodies specific for CD11b and GR-1. The percentage of neutrophils, as defined by expression of both CD11b and GR-1 among the CD45-gated population of synovial cells, increased from approximately 20% in WT and Socs3^+/Δvav mice to 44% in Socs3^–/Δvav mice, with a corresponding fall in the percentage of CD11b⁺GR-1^– cells (macrophages) from over 60% in WT and Socs3^+/Δvav mice to 44% in Socs3^–/Δvav mice (Figure 3). There was no difference in the percentages of CD4⁺, CD8⁺, or B220⁺ cells in inflamed synovium from Socs3^–/Δvav mice compared with their respective controls (data not shown). These findings suggest that SOCS-3 particularly regulates the neutrophil contribution to acute inflammatory arthritis.

Figure 3

Accumulation of neutrophils (CD11b⁺ GR-1⁺ cells) in the synovium ofSocs3^–/Δvav mice during acute arthritis. Synovium was dissected from knee joints of arthritic mice on day 7 after arthritis induction. Pooled synovial cells from 5 mice per group were isolated by enzymatic digestion and stained for expression of CD45, CD11b, and GR-1. Analysis of CD11b and GR-1 expression was performed on CD45⁺ cells. Results shown are representative of 3 independent experiments. Percentages in top left and right boxes indicate the percentage of CD11b⁺GR-1^– and CD11b⁺GR-1⁺ cells, respectively.

Acute arthritis in Socs3^–/Δvav mice is accompanied by increased numbers of neutrophils in peripheral blood, spleen, and bone marrow. Given the elevated numbers of neutrophils in arthritic synovium of Socs3^–/Δvav mice, we investigated whether there were elevated neutrophil numbers in other tissue compartments. The peripheral blood counts of Socs3^–/Δvav and Socs3^+/Δvav mice without arthritis are normal (16). There was a 5-fold increase in neutrophil numbers in the peripheral blood of Socs3^–/Δvav mice following mBSA/IL-1–induced arthritis on day 7 compared with WT and Socs3^+/Δvav mice (Figure 4A). Arthritic Socs3^–/Δvav mice also developed peripheral blood thrombocytosis (mean platelet count, Socs3^–/Δvav, 2,088 ± 178 × 10⁹ cells/l; WT, 1,229 ± 286 × 10⁹ cells/l; n = 4 mice per group; P < 0.05). In contrast, there was no difference in other peripheral blood lineages in arthritic Socs3^–/Δvav mice compared with arthritic WT and Socs3+/Δvav mice (data not shown).

Figure 4

Acute inflammatory arthritis inSocs3^–/Δvav mice is accompanied by peripheral blood neutrophilia, splenomegaly, and increased granulopoiesis. (A) Peripheral blood from arthritic mice at day 7 was analyzed on an automatic cell analyzer. Mean ± SEM cell counts pooled from 2 independent experiments are shown (n = 7–10 mice per group). (B) Mean ± SEM spleen weights of arthritic mice from 2 independent experiments are shown (n = 4–7 spleens per group). (C and D) Histologic sections through H&E-stained spleens from arthritic WT (C) and Socs3^–/Δvav (D) mice aged 8–12 weeks showing loss of delineation between white and red pulp in the Socs3^–/Δvav spleen due to infiltrating granulocytes. Magnification, ×100. (E) Spleen cells were stained with FITC- or PE-conjugated antibodies to CD4, CD8, B220, CD11b, or GR-1. (F) BM cells from arthritic mice were stained with FITC- or PE-conjugated antibodies to GR-1 and CD11b. Percentages above boxed regions (demonstrating CD11b⁺GR-1⁺ area) indicate the percentage of neutrophils. *P < 0.05 versus WT and Socs3^+/Δvav.

Socs3^–/Δvav mice also developed splenomegaly following mBSA/IL-1–induced arthritis (Figure 4B). Spleens from Socs3^–/Δvav mice had markedly disordered architecture with loss of delineation between white and red pulp due to infiltration of inflammatory cells (Figure 4, C and D). To determine the phenotype of inflammatory cells in the spleen following arthritis induction, single-cell suspensions of splenic tissue were stained for flow cytometry. The percentage of neutrophils (CD11b⁺GR-1⁺ cells) increased from less than 10% of cells in spleens from arthritic WT and Socs3^+/Δvav mice to over 40% in Socs3^–/Δvav mice (Figure 4E). The percentages of CD4⁺, CD8⁺, and B220⁺ cells were reduced due to the increased proportion of CD11b⁺GR-1⁺ cells in spleens of arthritic Socs3^–/Δvav mice. There was no difference in the percentage of monocytes/macrophages (CD11b⁺GR-1^– cells) in the spleens of Socs3^–/Δvav mice compared with control mice.

Given the increased numbers of neutrophils in peripheral blood and spleen, we next examined the BM. The percentage of neutrophils (CD11b⁺GR-1⁺ cells) in BM increased from approximately 50% of cells from arthritic WT and Socs3^+/Δvav mice to 69% in Socs3^–/Δvav mice (Figure 4F). While naive Socs3^–/Δvav mice older than 17 weeks develop splenomegaly due to neutrophil accumulation and increased BM granulopoiesis (16), this does not occur in younger mice. We were careful to use Socs3^–/Δvav mice aged 8–12 weeks to avoid this age-related phenotype.

Deletion of SOCS-3 therefore appears to result in articular and extra-articular effects in this disease model. In contrast to arthritis occurring in the absence of SOCS-1, exacerbated acute arthritis in Socs3^–/Δvav mice was characterized by markedly increased numbers of neutrophils in the synovium, peripheral blood, spleen and BM.

Elevated serum levels of G-CSF and IL-6 in arthritic Socs3^–/Δvav mice. SOCS-3 regulates G-CSF (16) and IL-6 (12) signaling in vivo, and both these cytokines are implicated in mBSA/IL-1–induced arthritis (23, 24). While G-CSF is the main regulator of granulocyte production (28–30), IL-6 may also be involved (31, 32). Given the increased numbers of neutrophils in the synovium, blood, spleen, and BM of arthritic Socs3^–/Δvav mice, we hypothesized that systemic levels of G-CSF and IL-6 might be elevated. Arthritic Socs3^–/Δvav mice had serum levels of G-CSF and IL-6 elevated 2- and 20-fold, respectively (Table 1).

Table 1

Acute arthritis in Socs3^–/Δvav mice is accompanied by elevated serum levels of G-CSF and IL-6

Arthritic Socs3^–/Δvav mice develop enlarged draining LNs. Since mBSA/IL-1–induced arthritis is dependent on T lymphocytes, we examined the phenotype of cells within draining LNs on day 7 following arthritis induction. Because of the small size of individual popliteal LNs, results shown represent average LN size calculated from pooled LN cells. In 2 independent experiments, there were up to 3-fold more cells in the popliteal LNs of arthritic Socs3^–/Δvav mice compared with WT controls (Figure 5A). There was a 4-fold increase in absolute number of B lymphocytes (B220⁺ cells) and a 2-fold increase in CD4⁺ and CD8⁺ T lymphocytes in popliteal LNs from arthritic Socs3^–/Δvav mice (Figure 5B).

Figure 5

Acute arthritis in Socs3^–/Δvav mice is accompanied by enlarged draining LN. (A) Cell counts of popliteal LN harvested from arthritic mice on day 7 following arthritis induction, pooled from 2 independent experiments. (B) Single-cell suspensions of popliteal LN tissue were stained with FITC- or PE-conjugated antibodies to CD4, CD8, B220, CD11b, and GR-1.

Socs3^–/Δvav LN cells hyperproliferate and produce more IL-17 in vitro. One possible explanation for the observed hypercellularity of draining LN in arthritic Socs3^–/Δvav mice (Figure 5A) is that these LN cells are hyperresponsive to cytokines produced following IA antigen injection. In order to test this, draining (popliteal) and nondraining (axillary) LN cells from arthritic mice were cultured in the presence of mBSA for 72 hours, and cellular proliferation was assessed by [³H]thymidine incorporation. While popliteal or axillary LN cells from WT mice showed low-level proliferation in vitro in response to mBSA, as previously reported (21), popliteal and axillary LN cells from Socs3^–/Δvav mice proliferated strongly following mBSA stimulation (Figure 6, A and B). Overall, proliferation of popliteal LN cells from Socs3^–/Δvav mice was increased up to 100-fold compared with cells from WT and Socs3^+/Δvav mice. Cells from Socs3^–/Δvav axillary LNs, which do not directly drain the knee, proliferated up to 20-fold more than did WT or Socs3^+/Δvav cells in response to mBSA. In addition, popliteal and axillary LN cells from Socs3^–/Δvav mice showed enhanced proliferation to the nonspecific mitogen Concanavalin A (Con A) and in the absence of exogenous antigen, possibly due to increased sensitivity of these activated cells to autocrine cytokine stimulation.

Figure 6

Socs3^–/Δvav T lymphocytes have markedly enhanced antigen-specific proliferation and IL-17 production in vitro. (A and B) Popliteal (A) and axillary (B) LN cells from arthritic mice were cultured with mBSA for 72 hours, and cellular proliferation was assessed by [³H]thymidine incorporation. Data are representative of 2 independent experiments. (C) Supernatants were harvested from T lymphocyte cultures after 72 hours of stimulation with mBSA and assayed for IL-17 by ELISA. IL-17 was undetectable in WT supernatants following mBSA stimulation. *P < 0.05 versus WT; **P < 0.01 versus WT and Socs3^+/Δvav.

Osteoclasts and bone destruction were prominent features of the inflammatory arthritis seen in Socs3^–/Δvav but not WT mice (Figure 2, A–C). IL-17 is produced by T lymphocytes and has been shown to promote osteoclastogenesis (33). Given the increased antigen-specific mitogenic response of draining LN cells, we hypothesized that greater IL-17 production by T lymphocytes might contribute to enhanced osteoclastogenesis. Following mBSA stimulation, Socs3^–/Δvav T lymphocytes produced markedly elevated levels of IL-17 (Figure 6C). In comparison, IL-17 was undetectable in supernatants from WT cultures. There was no difference between Socs3^–/Δvav and WT T lymphocyte production of IL-17 following Con A stimulation (Figure 6C), demonstrating that this was an antigen-specific effect.

SOCS-3 deficiency therefore results in hyperactive T lymphocytes and enhanced antigen-specific proliferation. Strong proliferative responses from axillary LNs were also noted, probably following trafficking of activated T lymphocytes. These findings suggest that SOCS-3 is an important regulator of T lymphocyte expansion following antigenic stimulation. Activated T lymphocyte production of IL-17 may also promote local osteoclastogenesis and thereby contribute to bone destruction.

Socs3^–/Δvav CD4⁺ T lymphocytes are hyperproliferative in vitro. Purified CD4⁺ T lymphocytes from naive Socs3^–/Δvav and WT mice were cultured with immobilized anti-CD3 to evaluate whether the absence of SOCS-3 resulted in increased proliferation following T cell receptor stimulation. Socs3^–/Δvav CD4⁺ T lymphocytes had up to 3-fold increases in proliferation and IL-2 production compared with WT cells following stimulation with anti-CD3 (Figure 7, A and B).

Figure 7

NaiveSocs3^–/Δvav CD4⁺ T lymphocytes are hyperproliferative in vitro. Purified splenic CD4⁺ T lymphocytes from naive WT and Socs3^–/Δvav mice were cultured in the presence of immobilized anti-CD3, rhIL-1, rhG-CSF, and rhIL-6 with sIL-6R, and cellular proliferation was assessed by [³H]thymidine incorporation. Supernatants were harvested at 24 hours and assayed for IL-2 by ELISA. Naive Socs3^–/Δvav CD4⁺ T lymphocytes had increased proliferation (A) and IL-2 production (B) compared with WT cells following anti-CD3 stimulation. Data are representative of 2 independent experiments. (C) Naive Socs3^–/Δvav CD4⁺ T lymphocytes had increased proliferation in the absence of exogenous cytokine that was further enhanced following addition of each of the cytokines. Data are representative of 3 independent experiments. *P < 0.05 versus WT.

mBSA/IL-1–induced arthritis is dependent on CD4⁺ T lymphocytes (21) and is driven by exogenous IL-1. However, G-CSF (23) and IL-6 (24) are important proinflammatory mediators in this model. Cells lacking SOCS-3 are hyperresponsive to these cytokines (12, 16), and T lymphocytes express receptors for both G-CSF (34) and IL-6 (35). We therefore investigated whether CD4⁺ T lymphocytes from naive Socs3^–/Δvav and WT mice were hyperresponsive to stimulation with IL-1, IL-6, or G-CSF. Purified CD4⁺ T lymphocytes (>90% purity) were cultured in the presence of IL-1, IL-6 (with soluble IL-6 receptor; sIL-6R), and G-CSF. Socs3^–/Δvav CD4⁺ T lymphocytes had a 2-fold greater mitogenic response in the absence of exogenous cytokine (Figure 7C). Following 24 hours of stimulation with these cytokines, naive Socs3^–/Δvav CD4⁺ T lymphocytes had up to 4-fold greater proliferation compared with WT CD4⁺ T lymphocytes (Figure 7C).

SOCS-3 therefore appears to be involved in regulation of naive and activated CD4⁺ T lymphocyte proliferative responses following T cell receptor–mediated activation. SOCS-3 may also regulate in vivo T lymphocyte proliferative responses following bystander stimulation with proinflammatory cytokines such as IL-1, G-CSF, and IL-6.

Hyperresponsiveness of Socs3^–/Δvav macrophages to IL-1. During inflammatory joint disease, macrophages are an important source of proinflammatory cytokines such as TNF and IL-1 (36), IL-6 (37), and G-CSF (38). Synovial macrophages are required for mBSA/IL-1–induced arthritis (24). Given that our model is dependent on exogenous IL-1 administration, we investigated whether Socs3^–/Δvav macrophages have enhanced responsiveness to IL-1 stimulation. Socs3^–/Δvav BM-derived macrophages had markedly greater IL-6 production compared with WT and Socs3^+/Δvav macrophages following IL-1 stimulation (Figure 8A).

Figure 8

Socs3^–/Δvav macrophages are hyperresponsive to IL-1, and IL-1 induces SOCS-3 directly in WT macrophages. (A) BM macrophages were cultured in the presence of rhIL-1 for 24 hours. Supernatants were harvested and assayed for IL-6 by ELISA. (B) BM macrophages from naive WT, G-CSF^–/–, and IL-6^–/– mice were cultured with rhIL-1 (100 ng/ml), and expression of SOCS-3 mRNA was determined by real-time PCR. (C) BM macrophages from WT mice were cultured with rhIL-1 in the presence of saturating concentrations of soluble G-CSFR–Fc (sG-CSFR–Fc; 5 μg/ml) and an IL-6 (5 μg/ml) neutralizing antibody, and expression of SOCS-3 mRNA was determined by real-time PCR. (D) BM macrophages from naive WT mice were stimulated with rhIL-1 (100 ng/ml) or recombinant mouse IL-6 (100 ng/ml) for the indicated times, and the total cellular levels of SOCS-3 were analyzed by immunoprecipitation and Western blot. Similar results were obtained in 2 independent experiments.

IL-1 directly induces SOCS-3 expression in macrophages. As Socs3^–/Δvav macrophages were hyperresponsive following IL-1 stimulation, we investigated whether IL-1 induces SOCS-3 directly in macrophages or indirectly through the production of G-CSF and/or IL-6. To help distinguish between these possibilities, BM macrophages from naive WT, G-CSF^–/–, and IL-6^–/– mice were cultured for up to 4 hours in the presence of IL-1, and SOCS-3 expression was determined by real-time PCR. Socs3 mRNA was rapidly induced in WT macrophages in response to IL-1, with levels peaking at 30 minutes (Figure 8B). This pattern of induction was seen in WT, G-CSF^–/–, and IL-6^–/– macrophages, suggesting that SOCS-3 induction was independent of either G-CSF or IL-6. The pattern of Socs3 mRNA induction in response to IL-1 remained unchanged following inhibition of both G-CSF and IL-6 bioactivity (Figure 8C), excluding the possibility that SOCS-3 was indirectly induced by either of these cytokines following IL-1 stimulation. To ensure that SOCS-3 induction was due to IL-1 and not contaminating bacterial LPS (5), IL-1 was inactivated by boiling. Stimulation with boiled IL-1 reduced SOCS-3 induction in WT macrophages to background levels (data not shown). Taken together, these findings show direct induction of SOCS-3 in macrophages by IL-1 rather than indirect induction of SOCS-3 via G-CSF or IL-6 production. However, IL-1 was a less potent inducer of SOCS-3 than was I-6 (Figure 8D).

To our knowledge, ours is the first study investigating the role of endogenous SOCS-3 in an in vivo model of human disease. Deletion of SOCS-3 in the hematopoietic and endothelial cell compartment was associated with particularly severe IL-1 dependent inflammatory arthritis, characterized by a prominent neutrophil synovial infiltrate and increased osteoclast generation and bone destruction. In naive Socs3^–/Δvav mice, trabecular bone volume was decreased and was associated with increased osteoclastic activity. IL-1 appeared to directly induce SOCS-3 expression in WT macrophages. SOCS-3–deficient macrophages overproduced IL-6 following IL-1 stimulation. Arthritis was accompanied by lymphadenopathy in the absence of SOCS-3, with markedly enhanced antigen-specific and mitogenic T lymphocyte responses. Extra-articular features of disease included peripheral neutrophilia, splenomegaly, enhanced granulopoiesis, and elevated systemic levels of G-CSF and IL-6.

Deficiency of SOCS-3 resulted in a qualitatively different inflammatory arthritis to SOCS-1 deficiency in the same disease model (25). There were several distinct features of acute arthritis in Socs3^–/Δvav mice. The most striking difference was that arthritic Socs3^–/Δvav mice had greater numbers of neutrophils in the synovium, peripheral blood, spleen, and BM, probably due to both neutrophil hyperresponsiveness to G-CSF (16) and elevated systemic levels of G-CSF. When stimulated with G-CSF in vitro, SOCS-3–deficient granulocyte lineage cells had enhanced survival and proliferation (16), and naive Socs3^–/Δvav mice receiving exogenous G-CSF developed neutrophilia, progenitor cell mobilization, and splenomegaly (16). Increased numbers of neutrophils entering the joint and undergoing local activation may contribute to joint damage by producing elevated levels of mediators such as oxygen radicals and MMPs (39). Neutrophils can also produce proinflammatory cytokines such as IL-1 (40) and oncostatin M (41). We have recently shown that G-CSF is a critical mediator of inflammatory arthritis (23). A major reason for the severe joint disease in the absence of SOCS-3 may therefore be hypersensitivity of neutrophils and BM progenitors to G-CSF–induced effects. In contrast, when mBSA/IL-1 arthritis was induced in SOCS-1^–/–IFN-γ^–/–mice, joint disease was characterized by a monocyte/macrophage synovial infiltrate with prominent granuloma formation but without peripheral neutrophilia or splenomegaly (25). While SOCS-1 promoter activity was found in synovial macrophages, lymphocytes, and fibroblasts during arthritis, it was not detected in synovial granulocytes (25). The predominant cell types at the cartilage-pannus junction in established RA are macrophages and synovial fibroblasts (42). However, neutrophils can also be found at this site, as well as in the joint exudate (39, 43, 44). These findings suggest that SOCS-3 and SOCS-1 have nonredundant functions in regulating myeloid lineage cells and that SOCS-1 may be more important for macrophage responses, whereas SOCS-3 plays a critical role in regulation of neutrophil responses during inflammation.

Several reports have linked SOCS-3 with T lymphocyte activation. SOCS-3 negatively regulated IL-2 signaling (45, 46) and bound the phosphatase calcineurin, thereby inhibiting the key transcription factor nuclear factor of activated T cells (47). Binding of SOCS-3 to the PI3K-binding site in CD28 reduced CD28-mediated IL-2 production, thereby regulating T helper lymphocyte proliferation (48). Antigenic stimulation of naive T helper lymphocytes downregulated SOCS-3 expression but upregulated SOCS-1 and -2 and cytokine-inducible src homology 2–containing protein (46), suggesting that SOCS proteins may be differentially regulated by T lymphocyte activation. As the mBSA/IL-1 model of acute arthritis is dependent on CD4⁺ T lymphocytes (21) and increased numbers of T lymphocytes were found in draining LNs, we examined T lymphocyte proliferative responses in LN of arthritic mice. Both draining and nondraining LN cells from arthritic Socs3^–/Δvav mice exhibited marked antigen-specific T lymphocyte proliferation. In contrast, while T lymphocytes from draining LNs of arthritic SOCS-1^–/–IFN-γ^–/– mice had increased antigen-specific proliferative responses, T lymphocytes from nondraining LNs did not (25). These findings suggest that SOCS-3 is a critical regulator of antigen-specific T lymphocyte activation and perhaps trafficking. Consistent with these in vivo findings, Socs3^–/Δvav CD4⁺ T lymphocytes were hyperproliferative following stimulation with anti-CD3 antibody in vitro with enhanced IL-2 production.

As naive T lymphocytes express receptors for both G-CSF (34) and IL-6 (35), we also investigated the effect of IL-1, G-CSF and IL-6 on naive Socs3^–/Δvav CD4⁺ T lymphocytes. Socs3^–/Δvav CD4⁺ T lymphocytes were hyperproliferative following stimulation with each of these cytokines. This suggests that in the absence of SOCS-3, IL-1, IL-6, and G-CSF are able to directly activate bystander CD4⁺ T lymphocytes. These findings demonstrate that SOCS-3 is a major regulator of CD4⁺ T lymphocyte activation in response to T cell receptor engagement and cytokine receptor signaling. Adaptive immune responses are influenced by regulatory T lymphocyte activity (49). Given the role of IL-6 in overcoming regulatory T lymphocyte-mediated suppression (50), it is possible that SOCS-3 may modulate regulatory T activity by regulating systemic levels of, and responsiveness to, IL-6.

Enhanced biologic responses to IL-6 signaling (12) may contribute to acute inflammatory arthritis through other mechanisms. IL-6 promotes joint destruction through modulation of the balance between MMPs and tissue inhibitor of metalloproteinases at sites of joint inflammation (51, 52). While G-CSF is the major regulator of neutrophil production, IL-6 may also contribute to enhanced BM granulopoiesis (32) and peripheral neutrophilia (31). Of interest, the extramedullary hematopoiesis (16) and splenomegaly noted in our arthritic mutant mice were also found in IL-6–sIL-6R double transgenic mice (53). While neutrophilia was the most marked peripheral blood abnormality seen in Socs3^–/Δvav mice, thrombocytosis was also noted, possibly due to enhanced IL-6 signaling (54). Given that IL-6 is a B lymphocyte mitogen (55), our finding of increased B lymphocyte accumulation in draining LNs of arthritic Socs3^–/Δvav mice may also be a consequence of enhanced B lymphocyte responsiveness to IL-6 signaling and elevated IL-6 levels. Of note, there was no increase in proliferation when purified splenic Socs3^–/Δvav B lymphocytes were cultured in the presence of anti-IgM, LPS, or CD40 ligand (data not shown). However, while IL-6–induced B lymphocyte expansion and cytokine production in draining LNs could contribute to increased disease in the absence of SOCS-3, the generation of mBSA/IL-1–induced arthritis is independent of B lymphocytes (21).

Osteoclast generation and bone destruction are not typically pronounced in mBSA/IL-1–induced arthritis in WT mice. Osteoclast recruitment in the inflamed joint was not increased in arthritic SOCS-1^–/–IFN-γ^–/– mice (25). In contrast, the marked inflammatory arthritis seen in Socs3^–/Δvav mice was associated with prominent osteoclast generation and bone destruction. There was comparatively more bone than cartilage destruction during mBSA/IL-1–induced arthritis in Socs3^–/Δvav mice. This may be because chondrocytes in Socs3^–/Δvav mice still express SOCS-3 whereas osteoclasts, which are derived from hematopoietic precursors (56), lack SOCS-3 following Cre-mediated deletion. T lymphocyte–derived IL-17 appears to be important in the pathogenesis of bone destruction in RA (57, 58) by upregulating osteoblast expression of receptor activator of NF-κB ligand relative to levels of the soluble decoy receptor osteoprotegerin, thereby promoting osteoclastogenesis (59). We found that draining LN T lymphocyte production of IL-17 was increased following antigen-specific proliferation, providing a potential mechanism by which T lymphocyte activation may have promoted local osteoclastogenesis and bone destruction in Socs3^–/Δvav mice. IL-17 has also been reported to upregulate G-CSF production by fibroblasts and endothelial cells in vitro (60, 61). Transgenic overexpression of G-CSF resulted in increased numbers of BM neutrophils and marked osteoporosis due to osteoclast-mediated bone resorption (62). Another possible mechanism for the enhanced osteoclastogenesis observed in arthritic Socs3^–/Δvav mice may therefore be a combination of increased T lymphocyte production of IL-17, elevated systemic levels of G-CSF, and enhanced responsiveness to G-CSF signaling.

Osteoclasts have been reported to express SOCS-3 mRNA (7), and overexpression of SOCS-3 in osteoclasts appeared to promote osteoclastogenesis, possibly via suppression of the inhibitory effect of IFN-β (7). However, targeting Cre-mediated SOCS-3 deletion to monocytes/macrophages using the LysM promoter (LysM-Cre:Socs3^fl/fl mice) appeared to have no effect on bone mass and bone density in vivo, at least when compared with Socs3^fl/fl mice (63). In contrast, we found that nonarthritic Socs3^–/Δvav mice had enhanced basal osteoclast generation in vivo compared with WT mice. Our findings suggest that endogenous SOCS-3 therefore normally inhibits basal osteoclastogenesis. As osteoblasts are mesenchymally derived (64), Socs3^–/Δvav osteoblasts should express SOCS-3 normally. The enhanced bone formation observed in nonarthritic Socs3^–/Δvav mice is therefore probably a response to increased osteoclastic activity. Dysregulated “coupling” between bone destruction and bone formation most likely explains the 50% reduction in trabecular bone volume seen in naive Socs3^–/Δvav mice. These findings indicate that endogenous SOCS-3 regulates osteoclastogenesis and bone turnover.

Macrophages are important cellular mediators in the pathogenesis of inflammatory arthritis due to abundant production of proinflammatory cytokines (36, 65). mBSA/IL-1–induced arthritis requires synovial macrophages (24). We found enhanced macrophage IL-6 production following IL-1 stimulation in the absence of SOCS-3. Hence activated macrophages in the synovium or possibly in the hematopoietic tissues are most likely responsible for the elevated systemic levels of IL-6 observed. IL-6 has been shown to be a potent inducer of macrophage and osteoclast differentiation (66, 67). SOCS-3 is therefore important in the regulation of macrophage responses, and dysregulated synovial macrophage function may contribute to enhanced inflammatory arthritis.

It is unclear from previous literature whether SOCS-3 is directly induced by IL-1 in a physiologically relevant time frame (68, 69). We found that within 30 minutes of stimulation with IL-1, SOCS-3 mRNA was detected in WT macrophages. Production of SOCS-3 protein in response to IL-1 peaked at 2 hours. This pattern of SOCS-3 induction persisted in G-CSF^–/– and IL-6^–/– macrophages and in WT macrophages following neutralization of both G-CSF and IL-6 bioactivity. We were careful to exclude any effect on SOCS-3 expression from contaminating LPS, as this is a potent inducer of SOCS-3 via Toll-like receptor activation (5, 70). These findings strongly suggest that IL-1 directly induces SOCS-3 in macrophages and raise the possibility that SOCS-3 may negatively regulate IL-1 signaling. However, we found that IL-1 was a less potent inducer of SOCS-3 in macrophages than IL-6.

Deletion of SOCS-3 resulted in markedly exacerbated IL-1–dependent arthritis and increased systemic G-CSF and IL-6. In vitro, IL-6 induced expression of IFN-γ–responsive genes in both hepatocytes (12) and macrophages (13) lacking SOCS-3, and it was hypothesized this might result in immunosuppressive effects (14). IL-6 also attenuated LPS-induced TNF production in macrophages lacking SOCS-3 in vitro, and LysM-Cre:Socs3^fl/flmice were protected against LPS-induced endotoxic shock in vivo, raising the possibility that in the absence of SOCS-3, IL-6 may induce an IL-10–like antiinflammatory effect (15). Whilst we have previously shown that IFN-γ is dispensable for peak disease severity in mBSA/IL-1–induced arthritis in WT mice (25), this does not exclude the possibility that IL-6–induced IFN-γ could have antiinflammatory effects in the absence of SOCS-3 (14). However, our findings clearly show that in vivo, the absence of SOCS-3 is profoundly proinflammatory in this model of acute arthritis.

Our data implicate SOCS-3 as a critical negative regulator of IL-1–dependent acute inflammatory arthritis. SOCS-3 appeared to regulate responses in a wider range of hematopoietic cell types than SOCS-1. Unlike SOCS-1, SOCS-3 may regulate the G-CSF–driven neutrophil response in acute arthritis, thereby contributing to exacerbated disease. Macrophage cytokine production and osteoclast generation were markedly enhanced in the absence of SOCS-3. Enhanced T lymphocyte activation in the absence of SOCS-3 may drive disease and also promote osteoclastogenesis by upregulating IL-17 production. SOCS-3 therefore appears to be a potent negative regulator of multiple proinflammatory pathways in joint disease. Our findings raise the possibility that pharmacologic manipulation of SOCS-3 may have a role in the treatment of inflammatory arthritis and other systemic inflammatory diseases.

This work was supported by the Reid Charitable Trusts, Arthritis Australia, the National Health and Medical Research Council of Australia (program grant 257500 and project grant 305523), the National Institutes of Health (grant CA22556) and AMRAD Operations Pty. Ltd. We thank A. Dunn (Ludwig Institute for Cancer Research, Parkville, Victoria, Australia) for provision of G-CSF–/– mice; S. Mihajlovic and E. Tsui for histology; and J. Merryfull, J. Morrow, and G. Panoschi for animal husbandry.

Address correspondence to: Ian P. Wicks, Reid Rheumatology Laboratory, Division of Autoimmunity and Transplantation, Walter and Eliza Hall Institute, 1G Royal Parade, Parkville, Victoria 3050, Australia. Phone: 61-3-9345-2466; Fax: 61-3-9347-0852; E-mail: wicks@wehi.edu.au .

Nonstandard abbreviations used: Con A, Concanavalin A; IA, intra-articular; mBSA, methylated bovine serum albumin; -R; receptor; rh-, recombinant human; s-, soluble; TRAP, tartrate-resistant acid phosphatase.

Conflict of interest: The authors have declared that no conflict of interest exists.

Reference information: J. Clin. Invest.116:1571–1581 (2006). doi:10.1172/JCI25660.

Paul J. Egan and Ben A. Croker contributed equally to this work.

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