Alteration of the intestinal microbiome characterizes preclinical inflammatory arthritis in mice and its modulation attenuates established arthritis.

Perturbations of the intestinal microbiome have been observed in patients with new-onset and chronic autoimmune inflammatory arthritis. However, it is currently unknown whether these alterations precede the development of arthritis or are rather a consequence of disease. Modulation of intestinal microbiota by oral antibiotics or germ-free condition can prevent arthritis in mice. Yet, the therapeutic potential of modulation of the microbiota after the onset of arthritis is not well characterized. We here show that the intestinal microbial community undergoes marked changes in the preclinical phase of collagen induced arthritis (CIA). The abundance of the phylum Bacteroidetes, specifically families S24-7 and Bacteroidaceae was reduced, whereas Firmicutes and Proteobacteria, such as Ruminococcaceae, Lachnospiraceae and Desulfovibrinocaceae, were expanded during the immune-priming phase of arthritis. In addition, we found that the abundance of lamina propria Th17, but not Th1, cells is highly correlated with the severity of arthritis. Elimination of the intestinal microbiota during established arthritis specifically reduced intestinal Th17 cells and attenuated arthritis. These effects were associated with reduced serum amyloid A expression in ileum and synovial tissue. Our observations suggest that intestinal microbiota perturbations precede arthritis, and that modulation of the intestinal microbiota after the onset of arthritis may offer therapeutic opportunities.

Higher efficacy of anti-IL-6/IL-21 combination therapy compared to monotherapy in the induction phase of Th17-driven experimental arthritis.

Th17 cells and their cytokines are linked to the pathogenesis of rheumatoid arthritis, a chronic autoimmune disease characterized by joint inflammation. Th17 development is initiated by combined signaling of TGF-ß and IL-6 or IL-21, and can be reduced in the absence of either IL-6 or IL-21. The aim of this study was to assess whether combinatorial IL-6/IL-21 blockade would more potently inhibit Th17 development, and be more efficacious in treating arthritis than targeting either cytokine. We assessed in vitro Th17 differentiation efficacy in the absence of IL-6 and/or IL-21. To investigate in vivo effects of IL-6/IL-21 blockade on Th17 and arthritis development, antigen-induced arthritis (AIA) was induced in IL-6-/- x IL-21R-/- mice. The therapeutic potential of this combined blocking strategy was assessed by treating mice with collagen-induced arthritis (CIA) with anti-IL-6R antibodies and soluble (s)IL-21R.Fc. We demonstrated that combined IL-6/IL-21 blocking synergistically reduced in vitro Th17 differentiation. In mice with AIA, absence of IL-6 and IL-21 signaling more strongly reduced Th17 levels and resulted in stronger suppression of arthritis than the absence of either cytokine. Additionally, anti-IL-6/anti-IL-21 treatment of CIA mice during the arthritis induction phase reduced disease development more potent than IL-6 or IL-21 inhibition alone, as effective as anti-TNF treatment. Collectively, these results suggest dual IL-6/IL-21 inhibition may be a more efficacious therapeutic strategy compared to single cytokine blockade to suppress arthritis development.

Complementary action of granulocyte macrophage colony-stimulating factor and interleukin-17A induces interleukin-23, receptor activator of nuclear factor-κB ligand, and matrix metalloproteinases and drives bone and cartilage pathology in experimental arthritis: rationale for combination therapy in rheumatoid arthritis.

INTRODUCTION: Type 17 T helper cells and interleukin (IL)-17 play important roles in the pathogenesis of human and murine arthritis. Although there is a clear link between IL-17 and granulocyte macrophage colony-stimulating factor (GM-CSF) in the inflammatory cascade, details about their interaction in arthritic synovial joints are unclear. In view of the introduction of GM-CSF and IL-17 inhibitors to the clinic, we studied how IL-17 and GM-CSF orchestrate the local production of inflammatory mediators during experimental arthritis. METHODS: To allow detection of additive, complementary or synergistic effects of IL-17 and GM-CSF, we used two opposing experimental approaches: treatment of arthritic mice with neutralising antibodies to IL-17 and GM-CSF and local overexpression of these cytokines in naive synovial joints. Mice were treated for 2 weeks with antibodies against IL-17 and/or GM-CSF after onset of collagen-induced arthritis. Naive mice were injected intraarticularly with adenoviral vectors for IL-17 and/or GM-CSF, resulting in local overexpression. Joint inflammation was monitored by macroscopic scoring, X-rays and histology. Joint washouts, synovial cell and lymph node cultures were analysed for cytokines, chemokines and inflammatory mediators by Luminex analysis, flow cytometry and quantitative polymerase chain reaction. RESULTS: Combined therapeutic anti-IL-17 and anti-GM-CSF ameliorated arthritis progression, and joint damage was dramatically reduced compared with treatment with anti-IL-17 or anti-GM-CSF alone. Anti-IL-17 specifically reduced synovial IL-23 transcription, whereas anti-GM-CSF reduced transcription of matrix metalloproteinases (MMPs) and receptor activator of nuclear factor κB ligand (RANKL). Overexpression of IL-17 or GM-CSF in naive knee joints elicited extensive inflammatory infiltrate, cartilage damage and bone destruction. Combined overexpression revealed additive and synergistic effects on the production of MMPs, RANKL and IL-23 in the synovium and led to complete destruction of the joint structure within 7 days. CONCLUSIONS: IL-17 and GM-CSF differentially mediate the inflammatory process in arthritic joints and show complementary and local additive effects. Combined blockade in arthritic mice reduced joint damage not only by direct inhibition of IL-17 and GM-CSF but also by indirect inhibition of IL-23 and RANKL. Our results provide a rationale for combination therapy in autoinflammatory conditions, especially for patients who do not fully respond to inhibition of the separate cytokines.

Periodontal pathogens directly promote autoimmune experimental arthritis by inducing a TLR2- and IL-1-driven Th17 response.

Increasing epidemiologic evidence supports a link between periodontitis and rheumatoid arthritis. The actual involvement of periodontitis in the pathogenesis of rheumatoid arthritis and the underlying mechanisms remain, however, poorly understood. We investigated the influence of concomitant periodontitis on clinical and histopathologic characteristics of T cell-mediated experimental arthritis and evaluated modulation of type II collagen (CII)-reactive Th cell phenotype as a potential mechanism. Repeated oral inoculations of periodontal pathogens Porphyromonas gingivalis and Prevotella nigrescens induced periodontitis in mice, as evidenced by alveolar bone resorption. Interestingly, concurrent periodontitis induced by both bacteria significantly aggravated the severity of collagen-induced arthritis. Exacerbation of arthritis was characterized by increased arthritic bone erosion, whereas cartilage damage remained unaffected. Both P. gingivalis and P. nigrescens skewed the CII-specific T cell response in lymph nodes draining arthritic joints toward the Th17 phenotype without affecting Th1. Importantly, the levels of IL-17 induced by periodontal pathogens in CII-specific T cells directly correlated with the intensity of arthritic bone erosion, suggesting relevance in pathology. Furthermore, IL-17 production was significantly correlated with periodontal disease-induced IL-6 in lymph node cell cultures. The effects of the two bacteria diverged in that P. nigrescens, in contrast to P. gingivalis, suppressed the joint-protective type 2 cytokines, including IL-4. Further in vitro studies showed that the Th17 induction strongly depended on TLR2 expression on APCs and was highly promoted by IL-1. Our data provide evidence of the involvement of periodontitis in the pathogenesis of T cell-driven arthritis through induction of Ag-specific Th17 response.

Toll-like receptor 2 controls acute immune complex-driven arthritis in mice by regulating the inhibitory Fcγ receptor IIB.

OBJECTIVE: Previous studies have demonstrated a protective role of Toll-like receptor 2 (TLR-2) and a proinflammatory function of TLR-4 during chronic T cell-driven arthritis. The involvement of TLRs in T cell-independent arthritic processes, however, remains unclear. This study was undertaken to determine the functional significance of TLR-2 and TLR-4 in T cell-independent immune complex-driven arthritis. METHODS: Serum-transfer arthritis was induced in wild-type and TLR-deficient mice by intraperitoneal injections of arthritogenic K/BxN mouse serum. Arthritis was assessed macroscopically and by histologic analysis. The influence of TLR-2 on macrophage cytokine profile, Fcγ receptor (FcγR) expression, and response to immune complexes was determined. RESULTS: While TLR-4, unexpectedly, did not play any significant role, TLR-2 deficiency accelerated the onset and markedly increased the severity of acute immune complex-driven arthritis in mice. TLR-2 deficiency resulted in a substantial increase in joint inflammation, bone erosion, and cartilage pathology, indicating a protective function of TLR-2 in passive FcγR-driven disease. Ex vivo study of the macrophage inflammatory phenotype revealed increased production of tumor necrosis factor α (TNFα) and interleukin-6 (IL-6) despite similar levels of IL-10, along with a significant increase in FcγR-specific response, in TLR-2-/- mouse macrophages early in the disease. Although distinct FcγR messenger RNA expression was not affected, cell surface protein expression of the inhibitory FcγRIIB in TLR-2-/- naive primary macrophages was specifically diminished, resulting in a higher proinflammatory response. Accordingly, TLR-2 stimulation specifically up-regulated FcγRIIB, but not the activating FcγR, on macrophages. CONCLUSION: TLR-2 regulates acute immune complex-driven arthritis by controlling macrophage FcγR response. Our findings indicate that the protective role of TLR-2 is extended beyond its previously described role in promoting Treg cells during T cell-mediated arthritis.

Destructive role of myeloid differentiation factor 88 and protective role of TRIF in interleukin-17-dependent arthritis in mice.

OBJECTIVE: Increasing evidence indicates the involvement of Toll-like receptors (TLRs) in the progression of arthritis; however, the contribution of the two signaling pathways used by TLRs, which are mediated by myeloid differentiation factor 88 (MyD88) and TRIF, remains unclear. The objective of this study was to investigate the specific roles of MyD88 and TRIF in chronic experimental arthritis and the accompanying adaptive immune responses. METHODS: Chronic arthritis was induced in wild-type, MyD88(-/-) , and Trif(lps2) (TRIF(-/-) ) mice by repetitive intraarticular injections of streptococcal cell wall (SCW) fragments. SCW-specific T cell and B cell responses, joint swelling, and histopathologic changes were analyzed during chronic arthritis. RESULTS: Both MyD88 and TRIF pathways contributed to antigen-specific T cell proliferation and antibody production, with the MyD88 pathway playing the dominant role. The severity of joint swelling and synovial inflammation, as well as the histopathologic damage to cartilage and bone, was strongly dependent on MyD88 signaling, whereas TRIF was redundant. MyD88 signaling was critical for the development of pathogenic T cell response (i.e., interleukin-17 [IL-17] production) in response to SCW antigen. Interestingly, when the T cell-dependent phase was prolonged, TRIF signaling appeared to down-regulate bone erosion, an effect accompanied by an inhibitory effect on IL-17 production. CONCLUSION: This study reveals a central role of MyD88 and a counterregulatory function of TRIF in T cell-driven arthritis. The findings provide a rationale for a pathway-specific interference in order to block the pathogenic features and to preserve or stimulate the beneficial aspects of TLR signaling.

Shift from toll-like receptor 2 (TLR-2) toward TLR-4 dependency in the erosive stage of chronic streptococcal cell wall arthritis coincident with TLR-4-mediated interleukin-17 production.

OBJECTIVE: Toll-like receptors (TLRs) may activate innate and adaptive immune responses in rheumatoid arthritis (RA) through recognition of microbial as well as endogenous ligands that have repeatedly been found in arthritic joints. The objective of this study was to investigate the involvement of TLR-2 and TLR-4 in the development of chronic destructive streptococcal cell wall (SCW)-induced arthritis, in which interleukin-1 (IL-1)/IL-17-dependent T cell-driven pathologic changes replace the macrophage-driven acute phase. METHODS: Chronic SCW arthritis was induced by 4 repeated intraarticular injections of SCW fragments in wild-type, TLR-2(-/-), and TLR-4(-/-) mice. Clinical, histopathologic, and immunologic parameters of arthritis were evaluated. RESULTS: The TLR-2 dependency of joint swelling during the acute phase was shifted to TLR-4 dependency during the chronic phase. Persistent joint inflammation in the latter phase of the model was significantly suppressed in TLR-4(-/-) mice. In the chronic phase, TLR-4 actively contributed to matrix metalloproteinase (MMP)-mediated cartilage destruction and to osteoclast formation, since the expression of the MMP-specific aggrecan neoepitope VDIPEN and the osteoclast marker cathepsin K was significantly reduced in TLR-4(-/-) mice. Furthermore, TLR-4(-/-) mice expressed less IL-1beta, tumor necrosis factor alpha, IL-6, and IL-23, cytokines that are implicated in IL-17 production. Accordingly, SCW-specific IL-17 production was found to be dependent on TLR-4 activation, since T cells from arthritic TLR-4(-/-) mice produced markedly less IL-17 upon SCW stimulation, whereas interferon-gamma production remained unaffected. CONCLUSION: These data indicate the involvement of TLR-4 in the chronicity and erosive character of arthritis coincident with the antigen-specific IL-17 response. The high position of TLR-4 in the hierarchy of erosive arthritis provides an interesting therapeutic target for RA.

T cell dependence of chronic destructive murine arthritis induced by repeated local activation of Toll-like receptor-driven pathways: crucial role of both interleukin-1beta and interleukin-17.

OBJECTIVE: The pathogenesis of rheumatoid arthritis is often linked to bacterial infections. The present study was undertaken to develop a mouse model of chronic destructive arthritis induced by repeated intraarticular (IA) exposure to bacterial cell wall fragments and to investigate the cytokine dependence of this model. METHODS: Mice that were deficient in various cytokines were injected IA with cell wall fragments of Streptococcus pyogenes on days 0, 7, 14, and 21. The development of chronic destructive arthritis was compared between groups of mice lacking different cytokines, to assess which cytokines were crucial for development of chronic destructive arthritis. RESULTS: Repeated exposure of a joint to S pyogenes cell wall fragments resulted in the development of chronic destructive arthritis. In mice deficient in recombination-activating gene 2, streptococcal cell wall (SCW)-directed T cell reactivity was found and chronic arthritis did not develop, implicating T cells in the generation of chronic SCW-induced arthritis. Interleukin-17 (IL-17) receptor-deficient mice showed a reduction of joint destruction in the chronic stage, implicating a detrimental role of the recently discovered IL-17-producing T helper cells (Th17 cells). IL-23 expression was apparent during the late stages of arthritis. Joint swelling was no longer dependent on tumor necrosis factor alpha (TNFalpha) after the last flare, and pronounced cartilage damage was found after 28 days in TNFalpha-deficient mice. In contrast, IL-1beta-deficient mice were fully protected against joint swelling and cartilage and bone destruction during the late stages of disease. CONCLUSION: These findings indicate that the TNFalpha dependence of arthritis is lost during the erosive stage, when Th17 cells become crucial. IL-1beta dependence remains strong, consistent with its pivotal role in the generation of Th17 cells.

Interleukin-17 acts independently of TNF-alpha under arthritic conditions.

The proinflammatory T cell cytokine IL-17 is a potent inducer of other cytokines such as IL-1 and TNF-alpha. The contribution of TNF in IL-17-induced joint inflammation is unclear. In this work we demonstrate using TNF-alpha-deficient mice that TNF-alpha is required in IL-17-induced joint pathology under naive conditions in vivo. However, overexpression of IL-17 aggravated K/BxN serum transfer arthritis to a similar degree in TNF-alpha-deficient mice and their wild-type counterparts, indicating that the TNF dependency of IL-17-induced pathology is lost under arthritic conditions. Also, during the course of the streptococcal cell wall-induced arthritis model, IL-17 was able to enhance inflammation and cartilage damage in the absence of TNF. Additional blocking of IL-1 during IL-17-enhanced streptococcal cell wall-induced arthritis did not reduce joint pathology in TNF-deficient mice, indicating that IL-1 is not responsible for this loss of TNF dependency. These data provide further understanding of the cytokine interplay during inflammation and demonstrate that, despite a strong TNF dependency under naive conditions, IL-17 acts independently of TNF under arthritic conditions.

Interleukin-17 receptor deficiency results in impaired synovial expression of interleukin-1 and matrix metalloproteinases 3, 9, and 13 and prevents cartilage destruction during chronic reactivated streptococcal cell wall-induced arthritis.

OBJECTIVE: To examine the role of interleukin-17 receptor (IL-17R) signaling in cartilage destruction and its interrelationship with synovial IL-1 expression during chronic reactivated streptococcal cell wall (SCW)-induced arthritis. METHODS: SCW arthritis was repeatedly induced in wild-type (WT) and IL-17R-deficient (IL-17R-/-) mice. At different time points, joint inflammation was assessed by using calipers to measure joint swelling. On day 42, mice were killed, and knee joints were removed for histologic analysis. Quantitative polymerase chain reaction (PCR) analyses for different proinflammatory mediators and matrix metalloproteinases (MMPs) were performed on inflamed synovium from WT and IL-17R-/- mice after 5 repeated injections of SCW fragments. RESULTS: IL-17R signaling did not play a significant role in acute joint swelling induced by a single injection of SCW fragments directly into the joint. However, repeated local injections of SCW fragments into the knee joints of IL-17R-/- mice resulted in fewer infiltrating cells in the joint compared with WT mice. Moreover, histologic analysis on day 42 revealed a significant suppression of the degree of chondrocyte death and an absence of cartilage surface erosion in IL-17R-/- mice. Quantitative PCR analysis revealed impaired synovial expression of IL-1, IL-6, cyclooxygenase 2, stromelysin (MMP-3), gelatinase B (MMP-9), and collagenase 3 (MMP-13) in IL-17R-/- mice. CONCLUSION: These data show a critical role of IL-17R signaling in driving the synovial expression of proinflammatory and catabolic mediators, such as IL-1 and different MMPs, during progression from an acute, macrophage-driven joint inflammation to a chronic, cartilage-destructive, T cell-mediated synovitis. Prevention of IL-17R signaling warrants consideration as a therapeutic target in chronic destructive arthritis.

Blocking of interleukin-17 during reactivation of experimental arthritis prevents joint inflammation and bone erosion by decreasing RANKL and interleukin-1.

Rheumatoid arthritis is characterized by an intermittent course of disease with alternate periods of remission and relapse. T cells, and in particular the T-cell cytokine interleukin-17 (IL-17), are expected to be involved in arthritic flares. Here, we report that neutralizing endogenous IL-17 during reactivation of antigen-induced arthritis prevents joint inflammation and bone erosion. Synovial IL-17 mRNA expression was clearly up-regulated during primary arthritis and was further enhanced after antigen rechallenge. Neutralization of IL-17 significantly prevented joint swelling at day 1 of flare and significantly suppressed joint inflammation and cartilage proteoglycan depletion at day 4, as assessed by histology. Blocking IL-17 also clearly reduced bone erosions. Cathepsin K, a marker of osteoclast-like activity, and synovial RANKL mRNA expression were both suppressed. The degree of bone erosions strongly correlated with the severity of joint inflammation, suggesting that anti-IL-17 treatment reduced bone erosion by suppressing joint inflammation. Interestingly, blocking IL-17 suppressed synovial expression of both IL-1beta and tumor necrosis factor-alpha, whereas blocking IL-1 did not affect tumor necrosis factor-alpha levels. These data indicate that IL-17 is an important upstream mediator in joint pathology during flare-up of experimental arthritis.

Induction of cartilage damage by overexpression of T cell interleukin-17A in experimental arthritis in mice deficient in interleukin-1.

OBJECTIVE: To examine the capacity of T cell interleukin-17A (IL-17A; referred to hereinafter as IL-17) to induce cartilage damage during experimental arthritis in the absence of IL-1. METHODS: Local IL-17 gene transfer was performed in the knee joint of IL-1-deficient mice and wild-type controls during streptococcal cell wall (SCW)-induced arthritis. Knee joints were isolated at various time points for histologic analysis of cartilage proteoglycan (PG) depletion. Expression of messenger RNA for inducible nitric oxide synthase, matrix metalloproteinases (MMPs) 3, 9, and 13, and ADAMTS-4 was determined by quantitative polymerase chain reaction analysis. VDIPEN staining was analyzed to study MMP-mediated cartilage damage. In addition, systemic anti-IL-1alpha/beta antibody treatment was performed in mice immunized with type II collagen and injected locally with an adenoviral vector expressing IL-17 or with control adenovirus. Knee joints were isolated and analyzed for cartilage PG depletion, chondrocyte death, and cartilage surface erosion. RESULTS: During SCW-induced arthritis, local T cell IL-17 gene transfer turned this acute, macrophage-driven joint inflammation into a severe, chronic arthritis accompanied by aggravated cartilage damage. Of high interest, the IL-1 dependency of cartilage PG depletion was fully abrogated when IL-17 was locally overexpressed in the joint. Moreover, local IL-17 gene transfer increased MMP expression without the need for IL-1, although IL-1 remained essential for part of the cartilage VDIPEN expression. Furthermore, when IL-17 was overexpressed in the knee joints of mice with collagen-induced arthritis, anti-IL-1 treatment did not reduce the degree of chondrocyte death or cartilage surface erosion. CONCLUSION: These data show the capacity of IL-17 to replace the catabolic function of IL-1 in cartilage damage during experimental arthritis.

Interleukin-18 promotes joint inflammation and induces interleukin-1-driven cartilage destruction.

Interleukin (IL)-18 is a member of the IL-1 family of proteins that exerts proinflammatory effects and is a pivotal cytokine for the development of Th1 responses. The goal of the present study was to investigate whether IL-18 induces joint inflammation and joint destruction directly or via induction of other cytokines such as IL-1 and tumor necrosis factor (TNF). To this end we performed both in vitro and in vivo kinetic studies. For in vivo IL-18 exposure studies C57BL/6, TNF-deficient, and IL-1-deficient mice were injected intra-articularly with 1.10(7) pfu mIL-18 adenovirus followed by histopathological examination. Local overexpression of IL-18 resulted in pronounced joint inflammation and cartilage proteoglycan loss in control mice. Of high interest, IL-18 gene transfer in IL-1-deficient mice did not show cartilage damage, although joint inflammation was similar to that in wild-type animals. Overexpression of IL-18 in TNF-deficient mice showed that TNF was partly involved in IL-18-induced joint swelling and influx of inflammatory cells, but cartilage proteoglycan loss occurred independent of TNF. In vitro cartilage degradation by IL-18 was found after a 72-hour culture period. Blocking of IL-1 with IL-1Ra or an ICE-inhibitor resulted in complete protection against IL-18-mediated cartilage degradation. The present study demonstrated that IL-18 induces joint inflammation independently of IL-1. In addition, we showed that IL-1beta generation, because of IL-18 exposure, was essential for marked cartilage degradation both in vitro and in vivo. These findings implicate that IL-18, in contrast to TNF, contributes through separate pathways to joint inflammation and cartilage destruction.

Treatment with a neutralizing anti-murine interleukin-17 antibody after the onset of collagen-induced arthritis reduces joint inflammation, cartilage destruction, and bone erosion.

OBJECTIVE: Interleukin-17 (IL-17) is a proinflammatory cytokine that is expressed in the synovium of rheumatoid arthritis (RA) patients. This T cell cytokine is implicated in the initiation phase of arthritis. However, the role of IL-17 during the effector phase of arthritis has still not been identified; this was the objective of the present study. METHODS: Mice with collagen-induced arthritis (CIA) were treated with polyclonal rabbit anti-murine IL-17 (anti-IL-17) antibody-positive serum or normal rabbit serum after the first signs of arthritis. In addition, during a later stage of CIA mice were selected and treated with anti-IL-17 antibody or control serum. Arthritis was monitored visually, and joint pathology was examined radiologically and histologically. Systemic IL-6 levels were measured by enzyme-linked immunosorbent assay, and local synovial IL-1 and receptor activator of NF-kappaB ligand (RANKL) expression was analyzed using specific immunohistochemistry. RESULTS: Treatment with a neutralizing anti-IL-17 antibody after the onset of CIA significantly reduced the severity of CIA. Radiographic analysis revealed marked suppression of joint damage in the knee and ankle joints. Histologic analysis confirmed the suppression of joint inflammation and showed prevention of cartilage and bone destruction after anti-IL-17 antibody therapy. Systemic IL-6 levels were significantly reduced after anti-IL-17 antibody treatment. Moreover, fewer IL-1beta-positive and RANKL-positive cells were detected in the synovium after treatment with neutralizing IL-17. Interestingly, initiation of anti-IL-17 antibody therapy during a later stage of CIA, using mice with higher clinical arthritis scores, still significantly slowed the progression of the disease. CONCLUSION: IL-17 plays a role in early stages of arthritis, but also later during disease progression. Systemic IL-6 was reduced and fewer synovial IL-1-positive and RANKL-positive cells were detected after neutralizing endogenous IL-17 treatment, suggesting both IL-1-dependent and IL-1-independent mechanisms of action. Our data strongly indicate that IL-17 neutralization could provide an additional therapeutic strategy for RA, particularly in situations in which elevated IL-17 may attenuate the response to anti-tumor necrosis factor/anti-IL-1 therapy.

IL-17 promotes bone erosion in murine collagen-induced arthritis through loss of the receptor activator of NF-kappa B ligand/osteoprotegerin balance.

IL-17 is a T cell-derived proinflammatory cytokine in experimental arthritis and is a stimulator of osteoclastogenesis in vitro. In this study, we report the effects of IL-17 overexpression (AdIL-17) in the knee joint of type II collagen-immunized mice on bone erosion and synovial receptor activator of NF-kappa B ligand (RANKL)/receptor activator of NF-kappa B/osteoprotegerin (OPG) expression. Local IL-17 promoted osteoclastic bone destruction, which was accompanied with marked tartrate-resistant acid phosphatase activity at sites of bone erosion in cortical, subchondral, and trabecular bone. Accelerated expression of RANKL and its receptor, receptor activator of NF-kappa B, was found in the synovial infiltrate and at sites of focal bone erosion, using specific immunohistochemistry. Interestingly, AdIL-17 not only enhanced RANKL expression but also strongly up-regulated the RANKL/OPG ratio in the synovium. Comparison of arthritic mice from the AdIL-17 collagen-induced arthritis group with full-blown collagen-arthritic mice having similar clinical scores for joint inflammation revealed lower RANKL/OPG ratio and tartrate-resistant acid phosphatase activity in the latter group. Interestingly, systemic OPG treatment prevented joint damage induced by local AdIL-17 gene transfer in type II collagen-immunized mice. These findings suggest T cell IL-17 to be an important inducer of RANKL expression leading to loss of the RANKL/OPG balance, stimulating osteoclastogenesis and bone erosion in arthritis.

Association of interleukin-18 expression with enhanced levels of both interleukin-1beta and tumor necrosis factor alpha in knee synovial tissue of patients with rheumatoid arthritis.

OBJECTIVE: To examine the expression patterns of interkeukin-18 (IL-18) in synovial biopsy tissue of patients with rheumatoid arthritis (RA), and to determine whether expression of this primary cytokine is related to the expression of other cytokines and adhesion molecules and related to the degree of joint inflammation. METHODS: Biopsy specimens of knee synovial tissue either without synovitis (n = 6) or with moderate or severe synovitis (n = 11 and n = 12, respectively) were obtained from 29 patients with active RA. Paraffin-embedded, snap-frozen sections were used for immunohistochemical detection of IL-18, tumor necrosis factor alpha (TNFalpha), IL-1beta, IL-12, and IL-17. Furthermore, adhesion molecules, such as intercellular adhesion molecule 1, vascular cell adhesion molecule 1, and E-selectin, and cell markers CD3, CD14, and CD68 were stained. RESULTS: IL-18 staining was detectable in 80% of the RA patients, in both the lining and sublining of the knee synovial tissue. IL-18 expression in the synovial tissue was strongly correlated with the expression of IL-1beta (in the sublining r = 0.72, in the lining r = 0.71; both P < 0.0001) and TNFalpha (in the sublining r = 0.59, P < 0.0007, and in the lining r = 0.68, P < 0.0001). In addition, IL-18 expression in the sublining correlated with macrophage infiltration (r = 0.64, P < 0.0007) and microscopic inflammation scores (r = 0.78, P < 0.0001), and with the acute-phase reaction as measured by the erythrocyte sedimentation rate (r = 0.61, P < 0.0004). Interestingly, RA synovial tissue that coexpressed IL-18 and IL-12 demonstrated enhanced levels of the Th1-associated cytokine IL-17. CONCLUSION: Our results show that expression of IL-18 is associated with that of IL-1beta and TNFalpha and with local inflammation in the synovial tissue of patients with RA. In addition, synovial IL-18 expression correlates with the acute-phase response. These data indicate that IL-18 is a primary proinflammatory cytokine in RA that drives the local production of IL-1beta and TNFalpha.

Increase in expression of receptor activator of nuclear factor kappaB at sites of bone erosion correlates with progression of inflammation in evolving collagen-induced arthritis.

OBJECTIVE: The receptor activator of nuclear factor kappaB (RANK)/RANK ligand (RANKL) pathway is critical in osteoclastogenesis and bone resorption and has been implicated in the process of focal bone erosion in arthritis. This study was undertaken to identify in vivo the hitherto-unknown origin and localization of RANK-expressing osteoclast precursor cells at sites of bone erosion in arthritis. METHODS: DBA-1 mice were immunized with bovine type II collagen/Freund's complete adjuvant and were given an intraperitoneal booster injection of type II collagen on day 21. Arthritis was monitored visually, and joint pathology was examined histologically. RANK and RANKL expression were analyzed using specific immunohistochemistry, and tartrate-resistant acid phosphatase (TRAP) staining was performed. In addition, TRAP and cathepsin K messenger RNA expression were analyzed by in situ hybridization. RESULTS: A marked increase in the number of cells expressing RANK correlated with the progression of synovial inflammation and clinical disease severity in evolving collagen-induced arthritis (CIA). Interestingly, RANK expression demonstrated a gradient pattern with increased numbers of RANK-positive cells within the synovial infiltrate in areas closer to periosteum and cortical bone. Cells expressing RANK included cells in synovial tissue, bone lining cells on the surface of trabecular bone at sites of erosion, and cells in periosteal areas adjacent to synovial inflammation. In areas where RANK-positive cells were abundant, TRAP-positive, multinucleated osteoclast-like cells were also present at sites of focal bone erosion, suggesting differentiation of synovially derived RANK-positive osteoclast precursor cells into osteoclasts. In addition, TRAP- and cathepsin K-double-positive osteoclast-like cells were detected on the synovial side of cortical bone at sites of early and advanced cortical bone erosion. Sites of RANK expression also correlated well with sites of RANKL expression, and there was a close correlation of the temporal expression of the receptor-ligand pair. CONCLUSION: Cells expressing RANK increased in abundance with the progression of arthritis in evolving CIA, and sites of RANK-expressing cells correlated with sites of TRAP-positive, multinucleated osteoclast-like cells as well as with sites of RANKL expression. These data support the hypothesis that the RANK/RANKL pathway plays an important role in the process of bone erosion in CIA.