I. Di Ceglie

FRI0049 Fc gamma receptor iv enhances bone erosion in experimental arthritis by promoting influx of pmns

Background FcγRs are involved in regulation of synovial activation and bone destruction during immune complex (IC)-mediated arthritis. The balance between activating FcγRs (FcγRI,III and IV) and inhibiting FcγRII determines synovial activation. Here we investigated the particular role of activating FcγRIV in bone erosion in IC-mediated antigen induced arthritis (AIA) by comparing FcγRI,II,III,IV-/- mice, FcγRI,II,III-/- mice and wild type controls (WT). Objectives To investigate the role of FcγRIV in bone erosion during experimental arthritis. Methods AIA was induced by injection of mBSA into knee joints of mice previously immunized with mBSA/CFA. Joint inflammation, bone destruction, number of TRAP+ osteoclasts and S100A8/A9 positive cells was determined using histology and immunohistochemistry. In vitro osteoclastogenesis was assessed using TRAP staining. Results Seven days after induction of AIA, we observed decreased inflammation and bone erosion in the knee joints of FcγRI,II,III,IV-/- mice compared to WT. The ability of bone marrow cells of FcγRI,II,III,IV-/- mice to differentiate into osteoclasts in vitro was comparable to the one of WT controls. Moreover, we observed comparable numbers of TRAP+ osteoclasts on the bone surface of FcγRI/II/III/IV-/- and WT arthritic mice, suggesting that the observed decrease in bone erosion is mainly caused by a reduced osteoclast activity, rather than decreased osteoclast number. However, in contrast to FcγRI/II/III/IV-/-, AIA induction in knee joints of FcγRI/II/III -/- resulted in increased bone erosion and inflammation compared to WT, highlighting the possible crucial role of FcγRIV in the pathology. Interestingly, the number of PMNs infiltrated in the knee joint of FcγRI/II/III-/- resulted increased, whereas it was decreased in the knee joints of FcγRI/II/III/IV-/- compared to their WT controls. This observation suggests that particularly FcγRIV is involved in regulating influx of PMNs. PMNs are potent producers of alarmins S100A8/A9 which are described to promote osteoclast activity. In line the number of S100A8/A9 positive cells in synovium was increased in FcγRI/II/III-/- while decreased in FcγRI/II/III/IV-/-, compared to their WT control. Conclusions FcγRIV promotes bone erosion in AIA by enhancing influx of PMNs within the synovium. PMNs are potent producers of S100A8/A9 which has been described to induce osteoclast activity. Disclosure of Interest None declared

Treatment of collagenase-induced osteoarthritis with a viral vector encoding TSG-6 results in ectopic bone formation

Objective Tumor necrosis factor-inducible gene 6 (TSG-6) has anti-inflammatory and chondroprotective effects in mouse models of inflammatory arthritis. Because cartilage damage and inflammation are also observed in osteoarthritis (OA), we determined the effect of viral overexpression of TSG-6 in experimental osteoarthritis. Methods Bone marrow-derived cells were differentiated to multinucleated osteoclasts in the presence of recombinant TSG-6 or after transduction with a lentiviral TSG-6 expression vector. Multi-nucleated osteoclasts were analyzed after tartrate resistant acid phosphatase staining and resorption activity was determined on dentin slices. Collagenase-induced osteoarthritis (CIOA) was induced in C57BL/6 mice after intra-articular injection of an adenoviral TSG-6 or control luciferase expression vector. Inflammation-related protease activity was measured using bioluminescent Prosense probes. After a second adenovirus injection, cartilage damage was assessed in histological sections stained with Safranin-O. Ectopic bone formation was scored in X-ray images of the affected knees. Results TSG-6 did not inhibit the formation of multi-nucleated osteoclasts, but caused a significant reduction in the resorption activity on dentin slices. Adenoviral TSG-6 gene therapy in CIOA could not reduce the cartilage damage compared to the luciferase control virus and no significant difference in inflammation-related protease activity was noted between the TSG-6 and control treated group. Instead, X-ray analysis and histological analysis revealed the presence of ectopic bone formation in the TSG-6 treated group. Conclusion Gene therapy based on the expression of TSG-6 could not provide cartilage protection in experimental osteoarthritis, but instead resulted in increased ectopic bone formation.

A1.12 Local experimental osteoarthritis induces systemic changes in monocyte populations regulated by S100A8/A9

Inflammation is increasingly recognised to be involved in osteoarthritis (OA) pathology. In response to pro-inflammatory cytokines, monocytes can be recruited from the bone marrow (BM) where monocyte chemoattractant protein-1 (MCP-1) is a key molecule that drives monocyte efflux via binding with the C-C chemokine receptor type 2 (CCR2). In mice, two functionally distinct monocyte populations are described: pro-inflammatory Ly6C-high monocytes (CCR2high) and patrolling Ly6C-low monocytes (CCR2low). The objectives of our study are to investigate the systemic effects of locally induced OA on BM monocyte populations and their recruitment to the OA joint in collagenase induced OA (CiOA), and the role of the alarmins S100A8/A9 in that. CiOA was induced in C57BL/6 mice by unilateral-articular collagenase-injection and were sacrificed at day 7, 21 and 42, together with age-matched naive mice (n = 6/group), and synovial mRNA expression of several pro-inflammatory cytokines were measured. During CiOA, the absolute amount of cells in the BM per femur was measured and the mRNA expression of BM MCP-1 and CCR2 was determined. Cells from BM, blood and synovial tissue were isolated and analysed by FACS. Monocyte subsets were identified as (B220/CD90/CD49b/NK1.1/Ly6G)lowCD11bhigh(F4/80/MHCII/CD11c)low and further distinguished by their Ly6C expression. In addition, we investigated synovitis in S100A9-/-mice during CiOA. Synovial expression of IL-1β, IL-6, TNF-α, S100A8 and S100A9 was increased at day 7, but only expression of S100A8 and S100A9 remained high until day 21. Local induction of CiOA resulted in systemic effects within the BM showing decreased cell numbers at day 7 and 21 (15% and 14% respectively). Concurrently, the expression of MCP1 at day7 in BM was increased 3.3-fold, suggesting increased BM-efflux. Relative number of BM-Ly6C-high monocytes was increased at day 7 (164%), being reflected by increased CCR2 expression (2.8-fold). This suggests a specific regulation of BM Ly6C-high monocytosis and recruitment during CiOA. During the course of CiOA increased number of Ly6C-high monocytes were observed in the synovium: 398% at day 7 and 510% at day 42, compared to naïve mice. These effects may be caused by the sustained S100A8/A9 levels, we therefore investigated synovitis in S100A9-/- mice during CiOA. The number of cell layers and cell influx in the synovium of S100A9-/- mice was decreased compared to C57BL/6 mice Local induction of OA induces systemic release of BM-derived cells and increased Ly6C-high monocyte populations systemically and locally in the synovium, a process that may be regulated by the sustained release of S100A8/A9 from the synovium. Disclosure of Interest None declared.

OP0210 Apolipoprotein E Aggravates Inflammation and Joint Destruction in Murine Antigen-Induced Arthritis

Background Rheumatoid arthritis (RA) is a chronic inflammatory disease characterized by severe bone destruction which has been associated with altered lipid metabolism. Apolipoprotein E (Apo E) is a lipoprotein crucial in lipid metabolism, mainly produced by macrophages. Moreover ApoE has been recently described as an important anti-inflammatory mediator regulating innate immunity and bone turnover. Objectives In the present study we investigated the role of Apo E in inflammation and bone destruction during antigen-induced arthritis (AIA). Methods Experimental arthritis (AIA) was induced by injection of 60 μg mBSA into the right knee joint of ApoE–/– and wild type (WT) control mice previously immunized with mBSA/CFA. Joint swelling was measured by uptake of 99mTechnecium (99mTc) and expressed as a ratio of the uptake in right (injected) and the left (non injected) knee joint. Humoral immunity (mBSA antibody titer) was measured by ELISA. Joint inflammation and bone erosion were measured by histological analysis using an arbitrary scale from 0 to 3. TRAP+ cells were determined using immunohistochemistry. Results ApoE–/– mice showed significantly less joint swelling at day 1, 3 and 7 after AIA induction compared to WT controls (21%, 17%, 18% lower, respectively). Serum mBSA antibody levels (total IgG, IgG1, IgG2a and IgG2b) are comparable between the two immunized mouse strains. At day 21, histology of the knee joints showed less infiltration of inflammatory cells (25% lower) and decreased bone erosion in the ApoE–/– mice compared to WT controls (25% lower from 1.5±0.2 to 1.1 ±0.1). In line with that, ApoE–/– mice showed a reduction of the number of osteoclasts present at the area of resorption within the arthritic knee joints (36% lower from 20±4 osteoclasts/section in WT mice to 12±5 in ApoE–/– mice), as measured by image analysis of TRAP staining. Conclusions ApoE aggravates bone destruction within the knee joints during AIA by increasing influx of inflammatory cells within the synovium and elevating the number of resorbing osteoclasts on the bone surface. Disclosure of Interest None declared

AB0092 Ablation of Fc Gamma Receptors Leads To A Decreased Bone Erosion in Experimental Arthritis by Inhibiting Synovial Inflammation and Osteoclast Activation

Background Rheumatoid Arthritis (RA) is characterized by bone destruction in joints caused by enhanced activity of osteoclasts. Ig-G containing immune complexes (ICs) correlate with disease severity and bone erosion and are recognized by Fc gamma receptors (FcγRs) on myeloid cells. In this study we investigated the role of FcγRs in osteoclast-mediated bone erosion comparing development of immune complex mediated antigen induced arthritis (AIA) between mice lacking all FcγRs (FcγRI, II, III, IV–/–) and their wild type controls. Objectives To investigate the role of FcγRs in bone erosion in experimental arthritis Methods AIA was induced by injection of 60 μg mBSA into the knee joint of FcγRI, II, III, IV–/– and wild type (WT) control mice previously immunized with mBSA/CFA. Joint inflammation and bone destruction was determined in total knee joints sections. mBSA antibody titers were measured using ELISA and T cell response monitored with a lymphocyte stimulation test. The percentage of osteoclast precursors in the bone marrow was defined through FACS analysis. Gene expression was measured using RT-PCR. Number of mature osteoclasts within the inflamed joints was defined through TRAP staining. Protein expression was measured with bead-based multiplex immunoassay luminex in the synovial wash-out or through immunohistochemistry on knee joint section. Results In FcγRI, II, III, IV–/– mice the development of bone erosion in knee joints was significantly reduced both at days 7 and 21 after induction of AIA (30% and 25% lower) when compared to WT controls. The immune response against mBSA (serum level of specific anti mBSA (total IgG, IgG1, IgG2a, IgG2b) and mBSA specific T-cell response) was comparable between the two strains. The percentage of osteoclast precursors within the bone marrow (CD11b low-neg/ Ly6C high, described to be upregulated during arthritis) was comparable between FcγRI, II, III, IV–/– and WT controls. At day 7 after AIA induction, the decrease in bone erosion in FcγRI, II, III, IV–/– was associated with a significantly decrease in the number of inflammatory cells present within the joint (infiltrate and exudate 29% and 27% lower respectively compared to WT control). Although less macrophages were detected within the synovium, no differences were found in the number of mature osteoclasts present at locations of bone erosion (32±13 osteoclasts/section in FcγRI, II, II, IV–/– versus 28±9 osteoclasts/section in WT), suggesting that mainly the activity of osteoclasts was affected in the KO animals. In line with this observation levels of macrophage derived factors like IL-1β and S100A8, known to strongly induce osteoclast activity, were reduced both at gene expression level and protein level in the synovium of FcγRI, II, III, IV–/– mice. Conclusions FcγRs promote bone erosion in AIA by enhancing influx of macrophages within the synovium and release of factors such as IL-1β and S100A8 able to stimulate osteoclast activity. Disclosure of Interest None declared

A4.15 FC gamma receptors enhance development of bone erosion in experimental arthritis not by increasing the number of osteoclasts on the bone surface but by stimulating synovial inflammation and activation of osteoclasts

Background and objectives Rheumatoid arthritis is characterised by immune complex dependent chronic joint inflammation and severe cartilage and bone destruction. In earlier studies we showed that Fcg receptors (FcgRs) are crucial in regulating cartilage destruction during immune complex mediated antigen induced arthritis (AIA). Now we investigated the role of FcgRs in osteoclast-mediated bone erosion comparing development of AIA between mice lacking all FcgRs (FcgRI, II, III, IV -/-) and their wild type controls. Material and methods AIA was induced by injection of 60 µg mBSA into the knee joint of FcγI, II, III, IV-/- and wild type (WT) control mice previously immunised with mBSA/CFA. Joint inflammation and bone destruction was determined in total knee joints sections. mBSA antibody titers were measured using ELISA and T cells response monitored with a lymphocyte stimulation test. Osteoclast precursors was defined through FACS analysis. Gene expression was measured using RT-PCR. Bone marrow derived cells osteoclastogenesis was quantified with TRAP staining. Results FcgRI, II, III, IV -/- mice showed a significant decrease in development of bone erosion in knee joints both at 7 and 21 days after induction of AIA (30% and 25% lower) when compared to WT controls. The immune response against mBSA (serum level of specific anti mBSA total IgG, IgG1, IgG2b and mBSA specific T-cell response) was comparable between the two strains. The percentage of osteoclast precursor population within the bone marrow (CD11b low-neg/ Ly6C high, described to be upregulated during arthritis) was comparable between FcgRI, II, III, IV -/- and WT as was the ability of bone marrow cells to differentiate towards mature multinucleated osteoclasts upon stimulation with M-CSF and RANK-L. At day 7 after AIA induction, the decrease in bone erosion in FcgRI, II, III, IV-/- coincided with lowering of the number of inflammatory cells present within the joint (infiltrate and exudate 29% and 27% lower respectively compared to WT control). Interestingly no differences were found in the number of osteoclasts present at locations of bone erosion (measured by image analysis of TRAP staining) (32 +/ 13 osteoclasts/section in FcγRI/II/II/IV-/- versus 28 +/- 9 osteoclasts/section in WT) Furthermore gene expression of osteoclast activation factors (RANK-L, IL-1β, S100A8, Cathepsin K and TRAP) within the synovium were significantly lower in FcγRI/II/II/IV-/- (ddCt -2.7, -2.6, -3, -2.4, -3 respectively) suggesting that activation of osteoclasts may be driven by FcgRs. Conclusions FcgRs promote bone erosion in AIA not by increasing osteoclasts on the bone surface but by enhancing influx of inflammatory cells within the synovium and release of factors able to stimulate osteoclast activity.

A1.30 Locally induced experimental osteoarthritis results in a system bone marrow shift towards a pro-inflammatory monocyte subpopulation

Background and objectives A significant role for inflammation during osteoarthritis (OA) is increasingly recognised, which involves the recruitment of immune cells, including monocytes, towards the inflamed synovium. In mice two functionally distinct monocyte populations are described; Ly6C-high monocytes, which express CCR2 and are considered pro-inflammatory and Ly6C-low monocytes, which express CX3CR1 and are suggested to be involved in repair processes. These monocytes arise from the bone marrow (BM) where monocyte chemoattractant protein-1 (MCP-1) is a key molecule that drives the monocyte efflux. As second tissue from which monocytes may originate is the spleen. The aim of this study is to investigate systemic effects of locally induced OA on BM and splenic monocyte subpopulations and the recruitment of these monocytes to the OA joint synovium in collagenase induced osteoarthritis (CiOA). Materials and methods CiOA was locally induced in C57Bl/6 mice by injection of collagenase in the right knee joint. Seven and 42 days after induction, mice (n = 6) were sacrificed, together with age-matched naive C57Bl/6 mice. Cells from BM, spleen, blood and knee synovial tissue were isolated and analysed by FACS. Ly6C-high monocytes were identified as (B220/CD90/CD49b/NK1.1/Ly6G)lowCD11bhigh(F4/80/MHCII/CD11c)lowLy6Chigh and Ly6C-low monocytes as (B220/CD90/CD49b/NK1.1/Ly6G)lowCD11bhigh(F4/80/MHCII/CD11c)lowLy6Clow. BM expression of MCP-1, CCR2 and CX3CR1 mRNA was determined at day 7 and 42 by q-PCR. Results In naive synovium few monocytes were present (1012 ± 925 Ly6C-high and 621 ± 488 Ly6C-low monocytes), which is likely an artefact of residual blood. At day 7 after CiOA induction, the number of Ly6C-high and -low monocytes in the OA synovium was 420% and 300% increased, respectively, compared to naive synovium. In blood, monocyte subpopulations were not changed, but in BM, the number of Ly6C-high monocytes was 160% increased, while Ly6C-low monocytes were decreased by 170%. Furthermore, expression of MCP-1 and CCR2 was increased by 3.2 and 2.8 times, while CX3CR1 expression remained unchanged. Even at day 42 increased levels of both monocyte subpopulations were observed in the OA synovium (Ly6C-high; 280% and Ly6C-low; 220%). In the BM, no change in both monocyte subpopulations was observed anymore as well as no change in expression of MCP-1, CCR2 and CX3CR. In spleen no changes in Ly6C-high or -low monocytes were observed throughout the course of CiOA, indicating no role for splenic monocytes in the recruitment of monocytes towards the OA synovium. Conclusions These data indicate that compared to naïve synovium, increased numbers of both Ly6C-high and -low monocytes are present in the OA synovium throughout the course of CiOA, but that a systemic effect on the BM monocyte subpopulations and their efflux is only observed in the early phase of OA. In the BM a clear skew towards a pro-inflammatory monocyte subset is visible, indicating that locally induced OA may also be systemically regulated.

THU0053 FC Gamma Receptors Enhance Bone Erosion in Experimental Arthritis by Stimulating Synovial Inflammation and Activation of Osteoclasts

Background Rheumatoid arthritis is characterized by immune complex dependent chronic joint inflammation and severe cartilage and bone destruction. In earlier studies we showed that Fcγ receptors (FcγRs) are crucial in regulating cartilage destruction during immune complex mediated antigen induced arthritis (AIA)(1,2). In this study we investigated the role of FcγRs in osteoclast-mediated bone erosion comparing development of AIA between mice lacking all FcγRs (FcγRI,II,III,IV -/-) and their wild type controls. Objectives To investigate the role of FcγRs in bone erosion in AIA. Methods AIA was induced by injection of 60 μg mBSA into the knee joint of FcγRI,II,III,IV-/- and wild type (WT) control mice previously immunized with mBSA/CFA. Joint inflammation and bone destruction were determined in total knee joints sections. mBSA antibody titers were measured using ELISA and T cell response was monitored with a lymphocyte stimulation test. The percentage of osteoclast precursors in the bone marrow was defined through FACS analysis. Gene expression was measured using RT-PCR. Mature osteoclasts within the knee joint were visualized with TRAP staining and counted in the locations of bone erosion in the patella, femur and tibia. Results In FcγRI,II,III,IV -/- mice the development of bone erosion in knee joints was significantly reduced both at days 7 and 21 after induction of AIA (30% and 25% lower) when compared to WT controls. The immune response against mBSA (serum level of specific anti mBSA (total IgG, IgG1, IgG2b) and mBSA specific T-cell response) was comparable between the two strains. The percentage of osteoclast precursor population within the bone marrow (CD11b low-neg/ Ly6C high, described to be upregulated during arthritis) was comparable between FcγRI,II,III,IV -/- and WT controls. Moreover FcγRI,II,III,IV -/- and WT bone marrow cells showed the same ability to differentiate into osteoclasts upon stimulation with M-CSF and RANK-L in vitro. At day 7 after AIA induction, the decrease in bone erosion in FcγRI,II,III,IV -/- was associated with a significantly decrease in the number of inflammatory cells present within the joint (infiltrate and exudate 29% and 27% lower respectively compared to WT control). Interestingly no differences were present in the number of mature osteoclasts present at locations of bone erosion within the knee join (32±13 osteoclasts/ total knee joint section in FcγRI,II,II,IV-/- versus 28±9 osteoclasts/ total knee joint section in WT). Gene expression of osteoclast activation factors (RANK-L, IL-1β, S100A8) within the synovium was however significantly lower in FcγRI,II,III,IV-/- (ddCt -2.7, -2.6, -3, respectively) suggesting that FcγRs may drive osteoclast activation. Conclusions FcγRs promote bone erosion in AIA not by increasing the number of osteoclasts present on the bone surface but by enhancing influx and activation of inflammatory cells within the synovium thereby releasing factors able to stimulate osteoclast activity. References van Lent PL et al., Am J Pathol. 2001 van Lent PL et al., Arthritis Rheum., 2010 Disclosure of Interest None declared

THU0044 Simultaneous Inhibition of JAK and Syk Kinases Ameliorates Chronic, Severe and Destructive Arthritis in Mice

Background Current drugs for rheumatoid arthritis, including the conventional disease-modifying antirheumatic drugs (e.g. methotrexate) and the more recent biologic agents (e.g. TNF inhibitors), have several drawbacks, such as limited clinical efficacy and side effects. In this context, small molecule inhibitors of protein kinases are emerging since they can suppress multiple intracellular signals simultaneously with important roles in the functioning of the immune system. Recent inhibitors of the tyrosine kinases JAK and SYK have demonstrated clinical efficacy in immune disorders. Objectives We compared the efficacy and safety of inhibiting JAK and SYK individually versus simultaneously, and with the corticosteroid prednisolone and the anti-TNF drug etanercept. We are currently investigating the effect of these small molecules on relevant pathogenic cells. Methods We used a well-established murine model of chronic (non self-remitting) and destructive arthritis induced by G6PI and both preventive and curative protocols, in which treatment either started the day of immunization or when the disease was submaximal, were assayed. Mice were treated with a selective pan-JAK inhibitor (tofacitinib), a selective SYK inhibitor (PRT2607) or a combination of both, and clinical, immunoinflammatory and histopathological responses were evaluated and compared to prednisolone and etanercept. Moreover, immunotoxicity was evaluated in mice treated for a month under the G6PI arthritis model and TDAR test, as well as in standard in vitro toxicity assays. Results Dual JAK/SYK inhibition was able to completely prevent the development of arthritis. In a curative protocol, dual inhibition ameliorated up to 90% clinical score, over 90% joint inflammation and 75-85% bone/cartilage erosion, as well as diminished over 90% RANKL and Th1/Th17 cytokine expression. In contrast, single inhibition of JAK or SYK, or treatment with etanercept or prednisolone were less effective, i.e. decreased clinical score by 58, 19, 28 and up to 70%, respectively. Interestingly, dual inhibition was fast-acting, led to disease remission in half of the animals and, in some cases, treatment withdrawal was accompanied by long-lasting amelioration. While daily dual treatment decreased lymphoid organ cellularity and T/B cell counts, these levels were never below those in naïve or prednisolone-treated mice. Red blood cell counts were intact. Dual inhibition decreased regulatory T and NK cell counts to the same extent as single JAK inhibition, without compromising cytotoxic activity. Preliminary work suggests that dual inhibition reduces the inflammatory cytokine cascade and the function of joint cells, such as the activity of osteoclasts to degrade bone. Conclusions Concurrent JAK and SYK inhibition results in higher efficacy than single kinase inhibition in preventing and treating chronic and severe arthritis without additional immunotoxicity. Thus, therapeutic approaches that block multiple immune signals, such as dual JAK/SYK inhibition, represent a reasonable therapeutic strategy for rheumatoid arthritis, in particular in patients with inadequate response to current treatments. Our data supports the multiplicity of events leading to this heterogeneous and complex disease. Disclosure of Interest None declared

FRI0346 Activatory FC Gamma Receptor IV Plays A Crucial Role in Cartilage and Bone Erosion during the Chronic Phase of Antigen Induced Arthritis

Background Rheumatoid arthritis is characterized by immune complexes dependent chronic joint inflammation and severe cartilage and bone destruction. Earlier we found that in the absence of activatory FcγRI and III, joint destruction during the early phase of murine antigen-induced arthritis (AIA) was protected (1). Recent studies show a crucial role of activatory FcγRIV in the K/BxN serum transfer arthritis model (2) but the role of this receptor in chronic phase of arthritis has not yet been elucidated. Objectives To study the role of activatory FcγRIV in joint pathology in acute and chronic phase of AIA. Methods AIA was induced by injection of mBSA into the knee joint of FcγRI,II,III–/–, FcγI,II,III,IV–/– and wildtype (WT) control mice previously immunized with mBSA/CFA. Histology of total knee joints was taken at day 7 and 21 after arthritis induction. Joint inflammation and bone destruction was scored using an arbitrary scale (0-3). Cartilage damage was measured as proteoglycan (PG) depletion and matrix erosion. mBSA antibody titers were determined using ELISA. Results Both KO strains showed significantly higher antibody titers against mBSA when compared to immunized control mice. In the early phase of AIA (day 7), joint inflammation was significantly higher in FcγRI,II,III–/– (infiltrate 47% and exudate 107% higher) when compared to WT controls. In FcγRI,II,III,IV–/– however, although high antibody titers were present, inflammation was significantly lower (infiltrate 30% and exudate 46% lower) than in WT controls. Early cartilage was protected in both strains reflected by significantly lower PG depletion (34% and 20% lower respectively) when compared to their WT controls. During the chronic phase at day 21, joint inflammation in FcγI/II/III–/– was still significantly higher (infiltrate 450% and exudate 170% higher) when compared to WT controls. Protection of cartilage damage seen in early AIA was lost and PG depletion significantly increased in FcγRI,II,III–/– by 260%. Bone erosion also increased by 150%. These results suggest that FcγR I and III regulate destruction in the early phase whereas FcγRIV may be more important in chronic stages of arthritis. In line with this we found that at day 21 joint inflammation in FcγRI,II,III,IV–/– was much lower when compared with FcγI/II/III–/– and remained at the same level as their WT controls, implying an active role of FcγRIV in inflammation during the chronic phase. The amount of PG depletion in FcγRI,II,III,IV–/– was comparable to that observed in WT. In contrast erosion of cartilage matrix but also of bone were found to be significantly lower when compared to WT controls (76% and 34% lower respectively). Conclusions Activatory FcγRIV is crucial in mediating severe cartilage and bone destruction during the chronic phase of antigen-induced arthritis probably by regulating joint inflammation and stimulating myeloid cells by binding immune complexes. References van Lent P et al., Arthritis Rheum. 2006. Nimmerjahn F et al., Proc. Natl. Acad Sci. U. S. A 2010. Disclosure of Interest None declared DOI 10.1136/annrheumdis-2014-eular.3200