Essential role of neutrophils in anti-type II collagen antibody and lipopolysaccharide-induced arthritis

Neutrophils in a mouse model of autoantibody-mediated arthritis: critical producers of Fc receptor gamma, the receptor for C5a, and lymphocyte function-associated antigen 1

OBJECTIVE

Neutrophils represent a prominent component of inflammatory joint effusions and are required for synovial inflammation in mouse models, but the mechanisms are poorly understood. In this study, we developed a system with which to test the importance of the production of specific factors by neutrophils in a mouse model of arthritis.

METHODS

Neutrophil-deficient Gfi-1(-/-) mice were administered sublethal doses of radiation and were then engrafted with donor bone marrow cells (BMCs), which resulted in the production of mature neutrophils within 2 weeks. By reconstituting with BMCs from mice lacking selected proinflammatory factors, we generated mice that specifically lacked these factors on their neutrophils. Arthritis was initiated by transfer of K/BxN serum to identify the role of defined neutrophil factors on the incidence and severity of arthritis.

RESULTS

Neutrophils lacking the signaling chain of stimulatory Fc receptors (FcRgamma(-/-)) were unable to elicit arthritis, but neutrophils lacking FcgammaRIII still did so. Neutrophils lacking the chemotactic or adhesion receptor C5a receptor (C5aR) or CD11a/lymphocyte function-associated antigen 1 (LFA-1) also failed to initiate arthritis but could enter joints in which inflammation had been initiated by wild-type neutrophils. Neutrophils unable to produce interleukin-1alpha (IL-1alpha) and IL-1beta (IL-1alpha/beta(-/-)) or leukotrienes (5-lipoxygenase [5-LOX(-/-)]) produced arthritis of intermediate severity. The inability of neutrophils to make tumor necrosis factor or to express receptors for tumor necrosis factor or IL-1 had no effect on arthritis.

CONCLUSION

A novel transfer system was developed to identify neutrophil production of FcRgamma, C5aR, and CD11a/LFA-1 as critical components of autoantibody-mediated arthritis. Neutrophil production of IL-1 and leukotriene B(4) likely contributes to inflammation but is not essential. Molecular requirements for neutrophil influx into joints become more permissive after inflammation is initiated.

Pharmacological targeting reveals distinct roles for CXCR2/CXCR1 and CCR2 in a mouse model of arthritis

Neutrophils and monocytes are abundantly represented in the synovial fluid and tissue in rheumatoid arthritis patients. We therefore explored the effects of small molecule chemokine receptor antagonists to block migration of these cells in anti-collagen antibody-induced arthritis. Targeting neutrophil migration with the CXCR2/CXCR1 antagonist SCH563705 led to a dose-dependent decrease in clinical disease scores and paw thickness measurements and clearly reduced inflammation and bone and cartilage degradation based on histopathology and paw cytokine analyses. In contrast, targeting monocyte migration with the CCR2 antagonist MK0812 had no effect on arthritis disease severity. The pharmacodynamic activities of both SCH563705 and MK0812 were verified by assessing their effects on the peripheral blood monocyte and neutrophil populations. SCH563705 selectively reduced the peripheral blood neutrophil frequency, and caused an elevation in the CXCR2 ligand CXCL1. MK0812 selectively reduced the peripheral blood monocyte frequency, and caused an elevation in the CCR2 ligand CCL2. The much greater impact of CXCR2/CXCR1 antagonism relative to CCR2 antagonism in this model of arthritis highlights the therapeutic potential for targeting CXCR2/CXCR1 in human arthritides.

Neutrophils promote inflammatory angiogenesis via release of preformed VEGF in an in vivo corneal model

We investigated the role of neutrophilic cells (neutrophils) in inflammatory angiogenesis and explored the possible mechanisms involved. Corneal angiogenesis was induced in vivo with a 75% silver nitrate applicator. Depletion of neutrophils was accomplished by the intraperitoneal administration of RB6-8C5, a neutrophil-depleting antibody. Angiogenesis, neutrophil infiltration, and the localization of vascular endothelial growth factor (VEGF) were evaluated by biomicroscopic observations, histology, and immunohistochemistry in control and RB6-8C5 treatment groups. Protein levels of VEGF, macrophage inflammatory protein-1alpha (MIP-1alpha), macrophage inflammatory protein-2 (MIP-2), and tumor necrosis factor alpha in the cornea were determined by enzyme-linked immunosorbent assay. An in vitro model of neutrophil activation was also used to examine the ability of neutrophils to produce and release VEGF, MIP-1alpha, and MIP-2. At day 1 after injury, neutrophil infiltration in the cornea was highest, and VEGF was expressed in the infiltrating neutrophils. The enhanced protein levels of VEGF, MIP-1alpha, and MIP-2 correlated with the degree of neutrophil infiltration. Neutrophil depletion significantly inhibited corneal angiogenesis and reduced the protein levels of VEGF, MIP-1alpha, and MIP-2 in the cornea. Upon stimulation, isolated neutrophils released VEGF from preformed stores and MIP-1alpha and MIP-2 by de novo synthesis. Neutrophil depletion thus significantly impaired inflammatory angiogenesis, identifying neutrophils as an important player in inflammatory angiogenesis. Neutrophils may exercise their angiogenic function by releasing proangiogenic factors such as VEGF. Intervention measures targeting neutrophils may therefore help to deal with abnormal angiogenesis involved in chronic inflammatory diseases.

Neutrophil granule proteins tune monocytic cell function

Polymorphonuclear leukocytes (PMNs) release the contents of granules during their migration to inflammatory sites. On liberation from the first leukocyte to enter injured tissue, the granule proteins play a central role in the early inflammatory response. In particular, mononuclear phagocytes interact intimately with PMNs and their secretion products. PMN granule proteins enhance the adhesion of monocytes to the endothelium and stimulate subsequent extravasation of inflammatory monocytes. At the site of inflammation, PMN granule proteins activate macrophages to produce and release cytokines and to phagocytose IgG-opsonized bacteria. Furthermore, by direct cell-cell contacts, PMNs activate monocyte-derived dendritic cells, thereby enhancing antigen presentation. Efforts in this field might lead to the development of drugs for specific modulation of innate immune functions.

Alpha9 integrin and its ligands constitute critical joint microenvironments for development of autoimmune arthritis

Osteopontin is critically involved in rheumatoid arthritis; however, the molecular cross-talk between osteopontin and joint cell components that leads to the inflammatory joint destruction is largely unknown. We found that not only osteopontin but also tenascin-C and their common receptor, alpha(9) integrin, are expressed at arthritic joints. The local production of osteopontin and tenascin-C is mainly due to synovial fibroblasts and, to a lesser extent, synovial macrophages. Synovial fibroblasts and macrophages express alpha(9) integrin, and autocrine and paracrine interactions of alpha(9) integrin on synovial fibroblasts and macrophages and its ligands contribute differently to the production of proinflammatory cytokines and chemokines. alpha(9) integrin is also involved in the recruitment and accumulation of inflammatory cells. Inhibition of alpha(9) integrin function with an anti-alpha(9) integrin Ab significantly reduces the production of arthrogenic cytokines and chemokines and ameliorates ongoing arthritis. Thus, we identified alpha(9) integrin as a critical intrinsic regulator that controls the development of autoimmune arthritis.

Surface RANKL of Toll-like receptor 4-stimulated human neutrophils activates osteoclastic bone resorption

Inflammatory bone loss in septic and inflammatory conditions is due to increased activity of osteoclasts that requires receptor activator of NF-kappa B-ligand (RANKL). Neutrophils are the predominant infiltrating cells in these conditions. Although disease severity is linked to neutrophils, their role in evolution of bony lesions is not clear. We show that lipopolysaccharide (LPS), a toll-like receptor 4 ligand, up-regulated the expression of membrane RANKL in human blood neutrophils and murine air pouch-derived neutrophils. LPS-activated human and murine neutrophils, cocultured with human monocyte-derived osteoclasts and RAW 264.7 cells, respectively, stimulated bone resorption. Transfection of PLB-985 neutrophil-like cells with RANKL antisense RNA reduced osteoclastogenesis. Synovial fluid neutrophils of patients with exacerbation of rheumatoid arthritis strongly expressed RANKL and activated osteoclastogenesis in coculture systems. Osteoprotegerin, the RANKL decoy receptor, suppressed osteoclast activation by neutrophils from these different sources. Moreover, direct cell-cell contact between neutrophils and osteoclasts was visualized by confocal laser microscopy. Activation of neutrophil membrane-bound RANKL was linked to tyrosine phosphorylation of Src-homology domain-containing cytosolic phosphatase 1 with concomitant down-regulation of cytokine production. The demonstration of these novel functions of neutrophils highlights their potential role in osteoimmunology and in therapeutics of inflammatory bone disease.

Critical role of phospholipase Cgamma2 in integrin and Fc receptor-mediated neutrophil functions and the effector phase of autoimmune arthritis

β₂ integrins and Fcγ receptors are critically involved in neutrophil activation at the site of inflammation. Both receptor types trigger a receptor-proximal tyrosine phosphorylation cascade through Src family kinases and Syk, but further downstream signaling events are poorly understood. We show that phospholipase C (PLC) γ2 is phosphorylated downstream of Src family kinases and Syk during integrin or Fc receptor-mediated activation of neutrophils. PLCγ2^−/− neutrophils are completely defective in β₂ integrin or Fcγ receptor-mediated functional responses such as respiratory burst, degranulation, or cell spreading in vitro and show reduced adhesion/spreading in inflamed capillary venules in vivo. However, PLCγ2^−/− neutrophils respond normally to various other agonists, including chemokines, bacterial formyl peptides, Toll-like receptor ligands, or proinflammatory cytokines, and migrate normally both in vitro and in vivo. To confirm the in vivo relevance of these observations, the effect of the PLCγ2^−/− mutation was tested in the K/B×N serum transfer arthritis model, which is known to require β₂ integrins, Fcγ receptors, and neutrophils. PLCγ2 deficiency completely protected mice from clinical signs and histological features of arthritis as well as from arthritis-induced loss of articular function. These results identify PLCγ2 as a critical player of integrin and Fc receptor-mediated neutrophil functions and the neutrophil-mediated effector phase of autoimmune arthritis.

PGLYRP-2 and Nod2 are both required for peptidoglycan-induced arthritis and local inflammation

Peptidoglycan recognition proteins (PGRPs) are structurally conserved from insects to mammals. Insect PGRPs have diverse host-defense functions. Mammalian PGRPs PGLYRP-1, PGLYRP-3, and PGLYRP-4 have bactericidal activity, while PGLYRP-2 has amidase activity. To extend the known functions of mammalian PGRPs, we examined whether they have immunomodulating activities in peptidoglycan-induced arthritis in mice. We demonstrate that PGLYRP-2 and Nod2 are both required for arthritis in this model. The sequence of events in peptidoglycan-induced arthritis is activation of Nod2, local expression of PGLYRP-2, chemokine production, and recruitment of neutrophils into the limbs, which induces acute arthritis. Only PGLYRP-2 among the four mammalian PGRPs displays this proinflammatory function, and PGLYRP-1 is anti-inflammatory. Toll-like receptor 4 (TLR4) and MyD88 are required for maturation of neutrophils before peptidoglycan challenge. Our results demonstrate that PGRPs, Nod2, and TLR4, representing three different types of pattern-recognition molecules, play interdependent in vivo roles in local inflammation.

The roles of IL-17A in inflammatory immune responses and host defense against pathogens

T-helper 17 (Th17) cells are a newly discovered CD4(+) helper T-cell subset that produces interleukin-17A (IL-17A) and IL-17F. IL-17A plays important roles in allergic responses such as delayed-type hypersensitivity, contact hypersensitivity, and allergic airway inflammation. IL-17A promotes inflammation by inducing various proinflammatory cytokines and chemokines, recruiting neutrophils, enhancing antibody production, and activating T cells. IL-17A expression is also augmented in autoimmune diseases such as multiple sclerosis and rheumatoid arthritis. Using mouse models of these diseases, we found that IL-17A plays a central role in their development. IL-6 is required for the development of Th17 cells and tumor necrosis factor functions downstream of IL-17A during the effector phase. IL-1 is important both for developing Th17 cells and eliciting inflammation. Th17 cells, like Th1 and Th2 cells, are involved in host defense against infections, but the contribution of these Th subsets to defense mechanisms differs among pathogens. The roles of IL-17F remain largely unknown. In this review, we introduce how IL-17A/IL-17F are involved in inflammatory immune responses and host defense mechanisms and discuss their relationship with other cytokines in the development of inflammatory and infectious diseases.

Inflammatory response in patients with active and inactive osteoarthritis

In the present study, we have investigated comparatively the inflammatory response of patients with active and inactive osteoarthritis. The sera from 31 healthy individuals, 37 patients with active OA, and 19 patients with inactive OA were assayed for TNF-alpha, IL-6, sRANKL, RANTES, and MRP8 using ELISA in order to evaluate their potential as markers of disease activity. Also, the spontaneous and LSP-induced release of TNF-alpha and IL-6 by peripheral blood neutrophils was determined. The activation of OA is associated with elevated TNF-alpha, IL-6, and RANTES serum levels while sRANKL and MRP8 appeared to be increased in both active and inactive OA. The neutrophil spontaneous and up-regulated by LPS cytokine release can contribute to the exacerbation of OA.

Rapid appearance of resolvin precursors in inflammatory exudates: novel mechanisms in resolution

Resolution of inflammation is essential. Although supplementation of omega-3 fatty acids is widely used, their availability at sites of inflammation is not known. To this end, a multidisciplinary approach was taken to determine the relationship of circulating omega-3 to inflammatory exudates and the generation of resolution signals. In this study, we monitored resolvin precursors in evolving exudates, which initially paralleled increases in edema and infiltrating neutrophils. We also prepared novel microfluidic chambers to capture neutrophils from a drop of blood within minutes that permitted single-cell monitoring. In these, docosahexaenoic acid-derived resolvin D1 rapidly stopped neutrophil migration, whereas precursor docosahexaenoic acid did not. In second organ injury via ischemia-reperfusion, resolvin metabolically stable analogues were potent organ protectors reducing neutrophils. Together, these results indicate that circulating omega-3 fatty acids rapidly appear in inflammatory sites that require conversion to resolvins that control excessive neutrophil infiltration, protect organs, and foster resolution.

The chemokine receptors CXCR1/CXCR2 modulate antigen-induced arthritis by regulating adhesion of neutrophils to the synovial microvasculature

OBJECTIVE

The chemokine receptors CXCR1 and CXCR2 play a role in mediating neutrophil recruitment and neutrophil-dependent injury in several models of inflammation. We undertook this study to investigate the role of these receptors in mediating neutrophil adhesion, subsequent migration, and neutrophil-dependent hypernociception in a murine model of monarticular antigen-induced arthritis (AIA).

METHODS

AIA was induced by administration of antigen into the knee joint of previously immunized mice. Intravital microscopy studies were performed to assess leukocyte rolling and adhesion. Mechanical hypernociception was investigated using an electronic pressure meter. Neutrophil accumulation in the tissue was measured by counting neutrophils in the synovial cavity and assaying myeloperoxidase activity. Levels of tumor necrosis factor alpha (TNFalpha) and the chemokines CXCL1 and CXCL2 were quantified by enzyme-linked immunosorbent assay. Histologic analysis was performed to evaluate the severity of arthritis and leukocyte infiltration.

RESULTS

Antigen challenge in immunized mice induced production of TNFalpha, CXCL1, and CXCL2 and also resulted in neutrophil recruitment, leukocyte rolling and adhesion, and hypernociception. Treatment with reparixin or DF2162 (allosteric inhibitors of CXCR1/CXCR2) decreased neutrophil recruitment, an effect that was associated with marked inhibition of neutrophil adhesion. Drug treatment also inhibited TNFalpha production, hypernociception, and the overall severity of the disease in the tissue.

CONCLUSION

Blockade of CXCR1/CXCR2 receptors inhibits neutrophil recruitment by inhibiting the adhesion of neutrophils to synovial microvessels. As a consequence, there is decreased local cytokine production and reduced hypernociception, as well as ameloriation of overall disease in the tissue. These studies suggest a potential therapeutic role for the modulation of CXCR1/CXCR2 receptor signaling in the treatment of arthritis.

The role of collagen antibodies in mediating arthritis

This review examines evidence that rheumatoid arthritis (RA) depends on autoimmunity to articular collagen, and mechanisms whereby autoantibodies to type II collagen contribute to disease development. Three major autoantigenic reactants have been identified in RA; the corresponding autoantibodies are rheumatoid factor (RF), antibodies to citrullinated peptide antigens (ACPA), citrullinated peptides (anti-CCP), and anti-type II collagen (anti-CII). Both RF and ACPA are well-validated and predictive markers of severe erosive RA, but cannot be linked to pathogenesis. By contrast, in various animal species immunized with CII there occurs an erosive inflammatory arthritis resembling that seen in human RA, together with antibodies to CII with an epitope specificity similar to that in RA. We discuss the well-known role of immune complexes in the induction of inflammation within the joint, and present recent data showing, additionally, that antibodies to CII cause direct damage to cartilage in vitro. The close resemblances between human RA and collagen-induced arthritis in animals suggest that autoimmunity, and particularly autoantibodies to CII, are important for both the initiation and perpetuation of RA in a dual manner: as contributors to the inflammation associated with immune complex deposition, and as agents with direct degradative effects on cartilage integrity and its repair.

Fibrin(ogen) exacerbates inflammatory joint disease through a mechanism linked to the integrin alphaMbeta2 binding motif

Fibrin deposition within joints is a prominent feature of arthritis, but the precise contribution of fibrin(ogen) to inflammatory events that cause debilitating joint damage remains unknown. To determine the importance of fibrin(ogen) in arthritis, gene-targeted mice either deficient in fibrinogen (Fib-) or expressing mutant forms of fibrinogen, lacking the leukocyte receptor integrin alphaMbeta2 binding motif (Fibgamma390-396A) or the alphaIIbbeta3 platelet integrin-binding motif (FibgammaDelta5), were challenged with collagen-induced arthritis (CIA). Fib- mice exhibited fewer affected joints and reduced disease severity relative to controls. Similarly, diminished arthritis was observed in Fibgamma390-396A mice, which retain full clotting function. In contrast, arthritis in FibgammaDelta5 mice was indistinguishable from that of controls. Fibrin(ogen) was not essential for leukocyte trafficking to joints, but appeared to be involved in leukocyte activation events. Fib- and Fibgamma390-396A mice with CIA displayed reduced local expression of TNF-alpha, IL-1beta, and IL-6, which suggests that alphaMbeta2-mediated leukocyte engagement of fibrin is mechanistically upstream of the production of proinflammatory mediators. Supporting this hypothesis, arthritic disease driven by exuberant TNF-alpha expression was not impeded by fibrinogen deficiency. Thus, fibrin(ogen) is an important, but context-dependent, determinant of arthritis, and one mechanism linking fibrin(ogen) to joint disease is coupled to alphaMbeta2-mediated inflammatory processes.

Critical but overlapping role of FcgammaRIII and FcgammaRIV in activation of murine neutrophils by immobilized immune complexes

Immune complex-induced activation of neutrophils through cell surface FcRs plays a central role in the pathogenesis of autoimmune inflammatory diseases. These diseases are often modeled using genetically modified mice. However, in contrast to the number of studies on human cells, the identity of FcRs involved in immune complex activation of murine neutrophils is at present unknown. Furthermore, little is known about the cellular functions mediated by the recently identified murine FcgammaRIV. In this study, we tested the identity of FcRs involved in the activation of neutrophils by plate-bound immune complexes, using various knockout mouse strains, function-blocking mAbs, or the combination of both approaches. Activation of murine neutrophils by immobilized IgG immune complexes was abrogated in FcR gamma-chain-deficient cells, but not by the single or combined deficiency of the gamma-chain-associated FcgammaRI and FcgammaRIII, or by blocking Abs against either FcgammaRIII or FcgammaRIV alone. However, treatment of FcgammaRIII-deficient neutrophils with FcgammaRIV-blocking Abs or simultaneous blocking of FcgammaRIII and FcgammaRIV in wild-type cells completely inhibited the immune complex-induced cellular responses. In parallel studies, activation of human neutrophils by immobilized immune complexes was abrogated by blocking Abs against either FcgammaRIIA or FcgammaRIIIB alone. Taken together, neutrophil activation by immobilized immune complexes requires the murine FcgammaRIII/FcgammaRIV or the human FcgammaRIIA/FcgammaRIIIB molecules. Although both of the two human receptors are required for this response, the two murine receptors play overlapping, redundant roles. These results promote our understanding of autoimmune diseases and identify an IgG-dependent cellular function of FcgammaRIV.

Distinct in vivo roles of CD80 and CD86 in the effector T-cell responses inducing antigen-induced arthritis

CD80 and CD86 play a critical role in the initiation of T-cell responses. However, their role in the in vivo effector CD4+ T-cell responses has been less extensively investigated. The current studies have examined the functional relevance of CD80 and CD86 in the effector CD4+ T-cell responses inducing antigen-induced arthritis. Arthritis was induced in C57BL/6 mice by sensitization to methylated bovine serum albumin (mBSA) on day 0, booster immunization (day 7) and intra-articular injection of mBSA (day 21). Control or anti-CD80 and/or anti-CD86 monoclonal antibodies were administered from day 21 to day 28. Arthritis severity and immune responses were assessed on day 28. The development of arthritis was significantly suppressed by inhibition of CD80 or CD86. Blockade of both CD80 and CD86 caused a trend towards reduced disease severity compared to control antibody-treated mice. Neutralization of CD80 attenuated accumulation of CD4+ T cells in joints and enhanced splenocyte production and circulating levels of interleukin-4. Inhibition of CD86 or both CD80 and CD86 reduced T-cell accumulation in joints without affecting T helper type 1/type 2 (Th1/Th2) differentiation or antibody levels. Blockade of CD86, and not CD80, significantly suppressed splenocyte interleukin-17 (IL-17) production. These results provide further in vivo evidence that CD80 and CD86 play important pathogenic roles in effector T-cell responses. CD80 exacerbates arthritis by downregulating systemic levels of IL-4 and increasing T-cell accumulation in joints without affecting IL-17 production. CD86 enhances disease severity by upregulating IL-17 production and increasing the accumulation of effector T cells in joints without affecting Th1/Th2 development.

Neutrophil serine proteases fine-tune the inflammatory response

Neutrophil serine proteases are granule-associated enzymes known mainly for their function in the intracellular killing of pathogens. Their extracellular release upon neutrophil activation is traditionally regarded as the primary reason for tissue damage at the sites of inflammation. However, studies over the past several years indicate that neutrophil serine proteases may also be key regulators of the inflammatory response. Neutrophil serine proteases specifically process and release chemokines, cytokines, and growth factors, thus modulating their biological activity. In addition, neutrophil serine proteases activate and shed specific cell surface receptors, which can ultimately prolong or terminate cytokine-induced responses. Moreover, it has been proposed that these proteases can impact cell viability through their caspase-like activity and initiate the adaptive immune response by directly activating lymphocytes. In summary, these studies point to neutrophil serine proteases as versatile mediators that fine-tune the local immune response and identify them as potential targets for therapeutic interventions.

Anti-type II collagen antibody accelerates arthritisvia CXCR2-expressing cells in IL-1 receptor antagonist-deficient mice

Arthritis can be induced in mice by the injection of anti-type II collagen (anti-CII) Ab and LPS. To elucidate the role of IL-1 receptor antagonist (IL-1ra) in Ab-induced arthritis, WT and IL-1ra(-/-) mice were administered anti-CII Ab and LPS. These IL-1ra(-/-) mice developed severe arthritis even at low doses of anti-CII Ab and LPS, while WT mice did not. The cells that invaded the arthritic joints were mainly Gr-1(+) neutrophils, and the number of the cells in the joints remained high over 4 weeks in the IL-1ra(-/-) mice. KC, a ligand for CXCR2, is found in higher levels in the arthritic paws of IL-1ra(-/-) mice compared with the WT, and most of the cells that infiltrated into the joints of the IL-1ra(-/-) mice were CXCR2-expressing neutrophils. Administration of anti-CXCR2 Ab completely inhibited arthritis development. The anti-CXCR2 Ab decreased the number of neutrophils in the blood and also inhibited the migration of neutrophils to KC. These results suggested that the high susceptibility of IL-1ra(-/-) mice to anti-CII Ab-induced arthritis was due to the higher expression of chemotactic factors like KC and the sustained infiltration of CXCR2-expressing neutrophils into the joints.

Chemical genetics define the roles of p38alpha and p38beta in acute and chronic inflammation

The p38 MAP kinase signal transduction pathway is an important regulator of proinflammatory cytokine production and inflammation. Defining the roles of the various p38 family members, specifically p38alpha and p38beta, in these processes has been difficult. Here we use a chemical genetics approach using knock-in mice in which either p38alpha or p38beta kinase has been rendered resistant to the effects of specific inhibitors along with p38beta knock-out mice to dissect the biological function of these specific kinase isoforms. Mice harboring a T106M mutation in p38alpha are resistant to pharmacological inhibition of LPS-induced TNF production and collagen antibody-induced arthritis, indicating that p38beta activity is not required for acute or chronic inflammatory responses. LPS-induced TNF production, however, is still completely sensitive to p38 inhibitors in mice with a T106M point mutation in p38beta. Similarly, p38beta knock-out mice respond normally to inflammatory stimuli. These results demonstrate conclusively that specific inhibition of the p38alpha isoform is necessary and sufficient for anti-inflammatory efficacy in vivo.

Interferon-gamma protects against the development of structural damage in experimental arthritis by regulating polymorphonuclear neutrophil influx into diseased joints

OBJECTIVE

Local interaction between soluble mediators within the inflamed synovium is a key factor that governs the pathologic outcome of inflammatory arthritides. Our aim was to investigate the interplay between the Th1 lymphokine interferon-gamma (IFNgamma) and pivotal cytokines that drive rheumatoid arthritis (RA) pathology (interleukin-1beta [IL-1beta] and tumor necrosis factor alpha [TNFalpha]) in modulating inflammation and arthritis in vitro and in vivo.

METHODS

Monarticular antigen-induced arthritis (AIA) was initiated in IFNgamma-deficient (IFNgamma(-/-)) mice and age-matched wild-type (IFNgamma(+/+)) mice. Joint swelling was measured and histologic analysis was performed in order to assess changes in both inflammatory and degenerative parameters in vivo. In vitro, the influence of IFNgamma in regulating IL-1beta- and TNFalpha-driven CXCL8 and CCL2 production was quantified by enzyme-linked immunosorbent assay.

RESULTS

In murine AIA, both inflammatory and degenerative arthritis parameters were significantly exacerbated in the absence of IFNgamma. IFNgamma appeared to be a crucial factor in regulating CXCR2+ neutrophil influx in the joint. In in vitro studies using RA fibroblast-like synoviocytes, IFNgamma modulated both IL-1beta- and TNFalpha-driven chemokine synthesis, resulting in the down-regulation of CXCL8 production.

CONCLUSION

IFNgamma exerts antiinflammatory, chondroprotective, and antiosteoclastogenic effects in murine AIA through a mechanism that involves the regulation of chemokine synthesis and local neutrophil recruitment. These studies suggest a potential therapeutic role of modulating IFNgamma signaling in the treatment of inflammatory arthritides.

Tumor necrosis factor alpha activates release of B lymphocyte stimulator by neutrophils infiltrating the rheumatoid joint

OBJECTIVE

The tumor necrosis factor (TNF) family member B lymphocyte stimulator (BLyS) is an important regulator of B cell-dependent autoimmunity. Similar to other TNF family members, it is generally expressed as a transmembrane protein and cleaved from the surface to release its active soluble form. This study was undertaken to investigate the expression of BLyS and regulation of BLyS release from the surface of neutrophils infiltrating the rheumatoid joint.

METHODS

BLyS expression was studied in neutrophils from the synovial fluid and peripheral blood of patients with rheumatoid arthritis (RA) and healthy controls, by flow cytometry, Western blotting, and immunofluorescence analyses. Peripheral blood neutrophils cultured with 50% RA synovial fluid were study for membrane expression of BLyS. Neutrophils were exposed to a range of proinflammatory cytokines to study the mechanisms of surface loss of BLyS.

RESULTS

Expression of BLyS was detected on the surface of peripheral blood neutrophils from both RA patients and healthy controls, whereas BLyS expression on synovial fluid neutrophils was very low. Constitutive expression of BLyS was observed in neutrophils, both on the cell membrane and in intracellular stores; however, BLyS release from each of these sites was found to be regulated independently. Of the various cytokine stimuli, only TNFalpha triggered release of BLyS from the neutrophil membrane. This process led to release of physiologically relevant quantities of soluble BLyS, which was dependent on the presence of the pro-protein convertase furin. In contrast, stimulation of neutrophils with granulocyte colony-stimulating factor induced BLyS release from the intracellular stores. Incubation of peripheral blood neutrophils with RA synovial fluid led to TNFalpha-dependent shedding of BLyS from the cell surface.

CONCLUSION

These findings indicate that as neutrophils enter the site of inflammation, they release surface-expressed BLyS in a TNFalpha-dependent manner, and thus may contribute to local stimulation of autoimmune B cell responses.

Administration of anti-type II collagen antibody sustains footpad swelling of mice caused by a delayed-type hypersensitivity reaction and induces severe arthritis

Delayed-type hypersensitivity (DTH) is an immune reaction induced by antigen. In the mice footpads at which DTH is elicited, transient swellings which usually peaks at 24-48 h after the antigen challenge are observed. We found that the footpad swellings of mice are sustained for at least 7 days after the antigen challenge if the mice were injected with anti-type II collagen monoclonal antibody (anti-CII MoAb) before the antigen challenge. A histological section of the swelled hindpaw revealed that severe joint inflammation and bone destruction was induced. These features were not observed in the footpads of the DTH-induced mice. Analysis of the inflammatory reaction induced by both the DTH and the anti-CII MoAb injection, here named as DTH arthritis, revealed the following: (1) DTH arthritis is elicited in an antigen-specific manner; and (2) the development of DTH arthritis is mediated by antigen-specific T cells, especially CD4+ T cells.

The complex role of Fcγ receptors in the pathology of arthritis

Autoantibodies of the IgG class and the immune complexes they form are central players in the pathology of rheumatoid arthritis (RA). Receptors for the Fc part of IgG, FcgammaR constitute one of the main effector mechanisms through which IgG immune complexes exert their action. The different members of the FcgammaR family exhibit extensive structural homology leading to redundancy in ligand specificity and signal transduction. Moreover, the initiation of effector mechanisms by IgG immune complexes can also be mediated by the complement system. This strong redundancy and high degree of complexity hampers a direct in vivo analysis of antibody effector pathways. Over the last decade, mice deficient for different combinations of FcgammaR have been generated by gene targeting. These knockout mice provide excellent tools to define the specific contribution of the different FcgammaR to IgG effector pathways in well-established in vivo mouse models for arthritis. This review will discuss the results of the studies that analyze the role of the different members of the FcgammaR family in murine arthritis models and their implications for our understanding of the human disease.