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MRL/lpr mice develop a spontaneous autoimmune disease that closely resembles human systemic lupus erythematosus (SLE) with DNA autoantibodies, hypergammaglobulinemia, immune complex glomerulonephritis, and systemic vasculitis. Little is known about the role of IL-3 in SLE. In order to study this we analyzed the expression of IL-3 in murine lupus and determined whether blockade of IL-3 with a monoclonal antibody or injection of recombinant IL-3 affects lupus nephritis in MRL/lpr mice. During disease progression IL-3 levels were increased in the plasma and in the supernatant of cultured splenocytes from MRL/lpr mice. Administration of IL-3 aggravated the disease with significantly higher renal activity scores, more renal fibrosis, and more glomerular leukocyte infiltration and IgG deposition. Blockade of IL-3 significantly improved acute and chronic kidney damage, reduced the glomerular infiltration of leukocytes and the glomerular deposition of IgG, and decreased the development of renal fibrosis. Furthermore, DNA autoantibody production, proteinuria, and serum creatinine levels were significantly lower in the anti-IL-3 group. Thus, IL-3 plays an important role in the pathogenesis of SLE and the progression of lupus nephritis. Hence, blockade of IL-3 may represent a new strategy for treatment of lupus nephritis.

Basophils are known to modulate the phenotype of CD4(+) T cells and to enhance T helper type 2 responses in vitro and in vivo. In this study, we demonstrate that murine basophils inhibit proliferation of CD4(+) T cells in autologous and allogeneic mixed lymphocyte reactions. The inhibition is independent of Fas and MHC class II, but dependent on activation of basophils with subsequent release of interleukin-4 (IL-4) and IL-6. The inhibitory effect of basophils on T-cell proliferation can be blocked with antibodies against IL-4 and IL-6 and is absent in IL-4/IL-6 double-deficient mice. In addition, we show that basophils and IL-4 have beneficial effects on disease activity in a murine model of acute graft-versus-host disease (GvHD). When basophils were depleted with the antibody MAR-1 before induction of GvHD, weight loss, GvHD score, mortality and plasma tumour necrosis factor levels were increased while injection of IL-4 improved GvHD. Basophil-depleted mice with GvHD also have increased numbers of CD4(+) T cells in the mesenteric lymph nodes. Our data show for the first time that basophils suppress autologous and allogeneic CD4(+) T-cell proliferation in an IL-4-dependent manner.

Depletion of B cells with the anti-CD20 antibody rituximab is an established therapy for rheumatoid arthritis. However, rituximab has only moderate efficacy, most likely due to insufficient depletion of B cells in lymphoid organs and expansion of pathogenic B cells. We found that an antibody against mouse CD79b profoundly blocks B-cell proliferation induced via the B-cell receptor, CD40, CD180, and chondroitin sulfate, but not via TLR4 or TLR9. Treatment with anti-CD79b also induces death in resting and activated B cells. B-cell inhibition is mediated by cross-linkage of CD79b, but independent of Fc-receptor engagement. In the model of collagen-induced arthritis, an antibody against mouse CD20 depletes B cells very efficiently but fails to suppress the humoral immune response against collagen and the development of arthritis. In contrast, the antibody against CD79b, and a deglycosylated variant of this antibody, almost completely inhibits the increase in anti-collagen antibodies and the development of arthritis. In mice with established arthritis only the fully glycosylated antibody against CD79b is effective. Our data show that targeting B cells via CD79b is much more effective than B-cell depletion with anti-CD20 antibodies for therapy of arthritis. These findings may have important implications for treatment of B-cell-mediated autoimmune diseases.

Cortical demyelination is a widely recognized hallmark of multiple sclerosis (MS) and correlate of disease progression and cognitive decline. The pathomechanisms initiating and driving gray matter damage are only incompletely understood. Here, we determined the infiltrating leukocyte subpopulations in 26 cortical demyelinated lesions of biopsied MS patients and assessed their contribution to cortical lesion formation in a newly developed mouse model. We find that conformation-specific anti-myelin antibodies contribute to cortical demyelination even in the absence of the classical complement pathway. T cells and natural killer cells are relevant for intracortical type 2 but dispensable for subpial type 3 lesions, whereas CCR2+ monocytes are required for both. Depleting CCR2+ monocytes in marmoset monkeys with experimental autoimmune encephalomyelitis using a novel humanized CCR2 targeting antibody translates into significantly less cortical demyelination and disease severity. We conclude that biologics depleting CCR2+ monocytes might be attractive candidates for preventing cortical lesion formation and ameliorating disease progression in MS. Electronic supplementary material The online version of this article (doi:10.1007/s00401-017-1706-x) contains supplementary material, which is available to authorized users.

Collagen-producing bone marrow–derived cells (fibrocytes) have been detected in animal models and patients with fibrotic diseases. In vitro data suggest that they develop from monocytes with the help of accessory cells and profibrotic soluble factors. Using a mouse model of renal fibrosis, unilateral ureteral obstruction, we found the number of circulating fibrocytes was not reduced when monocytes were depleted with a monoclonal antibody against CCR2 or when CCR2-/- mice with very low numbers of circulating or splenic monocytes were analyzed. The absence of CCR2, however, interfered with migration of fibrocytes into the kidney. The phenotype of splenic and renal fibrocytes was very similar and distinct from classical monocytes as fibrocytes expressed no CD115, medium levels of CCR2, and high levels of CD11b and Ly-6G. Using a depleting monoclonal antibody against Ly-6G or bone marrow chimeric mice expressing the diphtheria toxin receptor under the control of CD11b, we could efficiently deplete fibrocytes from the kidney. Depletion of fibrocytes or reduced migration of fibrocytes into the kidney resulted in lower renal expression of collagen-I. Thus, fibrocytes develop outside the kidney independent of infiltrating monocytes and rely on CCR2 for migration into target organs.

Little is known about the role of IL-3 in multiple sclerosis (MS) in humans and in experimental autoimmune encephalomyelitis (EAE). Using myelin oligodendrocyte glycoprotein (MOG) peptide-induced EAE, we show that CD4+ T cells are the main source of IL-3 and that cerebral IL-3 expression correlates with the influx of T cells into the brain. Blockade of IL-3 with monoclonal antibodies, analysis of IL-3 deficient mice, and adoptive transfer of leukocytes demonstrate that IL-3 plays an important role for development of clinical symptoms of EAE, for migration of leukocytes into the brain, and for cerebral expression of adhesion molecules and chemokines. In contrast, injection of recombinant IL-3 exacerbates EAE symptoms and cerebral inflammation. In patients with relapsing-remitting MS (RRMS), IL-3 expression by T cells is markedly upregulated during episodes of relapse. Our data indicate that IL-3 plays an important role in EAE and may represent a new target for treatment of MS.

The adipokine adiponectin is well known to affect the function of immune cells and upregulation of CCL2 by adiponectin in monocytes/macrophages has already been reported. In the current study the effect of adiponectin on CCL2, -3, -4, and -5 and their corresponding receptors CCR1, CCR2, and CCR5 has been analyzed. Adiponectin elevates mRNA and protein of the CC chemokines in primary human monocytes. Simultaneously the surface abundance of CCR2 and CCR5 is reduced while CCR1 is not affected. Downregulation of CCR2 by adiponectin is blocked by a CCR2 antagonist although expression of the CCL2 regulated genes CCR2 and TGF-beta 1 is not altered in the adiponectin-incubated monocytes. CCL2, -3, and -5 concentrations measured in supernatants of monocytes of normal-weight (NW), overweight (OW), and type 2 diabetic (T2D) patients positively correlate with BMI and are increased in obesity and T2D. In contrast CCL4 is similarly abundant in the supernatants of all of these monocytes. The degree of adiponectin-mediated induction of the chemokines CCL3, -4, and -5 negatively correlates with their basal levels and upregulation of CCL3 and CCL5 is significantly impaired in OW and T2D cells. Serum concentrations of these chemokines are almost equal in the three groups and do not correlate with the levels in monocyte supernatants. In conclusion these data demonstrate that adiponectin stimulates release of CCL2 to CCL5 in primary human monocytes, and induction in cells of overweight probands is partly impaired. Adiponectin also lowers surface abundance of CCR2 and CCR5 and downregulation of CCR2 seems to depend on autocrine/paracrine effects of CCL2.

Objective The chemokine receptor CCR2 is highly expressed on monocytes and considered a promising target for treatment of rheumatoid arthritis. However, blockade of CCR2 with a monoclonal antibody (mAb) during progression of collagen-induced arthritis results in a massive aggravation of the disease. In this study we investigated why CCR2 antibodies have proinflammatory effects, how these effects can be avoided, and whether CCR2+ monocytes are useful targets in the treatment of arthritis. Methods Arthritis was induced in DBA/1 mice by immunization with type II collagen. Mice were treated with mAb against CCR2 (MC-21), IgE, or isotype control antibodies at various time points. Activation of basophils and depletion of monocyte subsets were determined by fluorescence-activated cell sorter analysis and enzyme-linked immunosorbent assay. Results Crosslinkage of CCR2 activated basophils to release interleukin-6 (IL-6) and IL-4. In vivo, IL-6 release occurred only after exposure to high doses of MC-21, whereas application of low doses of the mAb circumvented the release of IL-6. Regardless of the dose level used, the antibody MC-21 efficiently depleted Gr-1+,CCR2+ monocytes from the synovial tissue, peripheral blood, and spleen of DBA/1 mice. Activation of basophils with high doses of MC-21 or with antibodies against IgE resulted in a marked aggravation of collagen-induced arthritis and an increased release of IL-6. In contrast, low-dose treatment with MC-21 in this therapeutic setting had no effect on IL-6 and led to marked improvement of arthritis. Conclusion These results show that depletion of CCR2+ monocytes may prove to be a therapeutic option in inflammatory arthritis, as long as the dose-dependent proinflammatory effects of CCR2 mAb are taken into account.

Fibrosis is a major component of chronic cardiac allograft rejection. Although several cell types are able to produce collagen, resident (donor-derived) fibroblasts are mainly responsible for excessive production of extracellular matrix proteins. It is currently unclear which cells regulate production of connective tissue elements in allograft fibrosis and how basophils, as potential producers of profibrotic cytokines, are involved this process. We studied this question in a fully MHC-mismatched model of heart transplantation with transient depletion of CD4(+) T cells to largely prevent acute rejection. The model is characterized by myocardial infiltration of leukocytes and development of interstitial fibrosis and allograft vasculopathy. Using depletion of basophils, IL-4-deficient recipients and IL-4 receptor-deficient grafts, we showed that basophils and IL-4 play crucial roles in activation of fibroblasts and development of fibrotic organ remodeling. In the absence of CD4(+) T cells, basophils are the predominant source of IL-4 in the graft and contribute to expansion of myofibroblasts, interstitial deposition of collagen and development of allograft vasculopathy. Our results indicated that basophils trigger the production of various connective tissue elements by myofibroblasts. Basophil-derived IL-4 may be an attractive target for treatment of chronic allograft rejection.

Binding of intact Ag is a hallmark of Ag-specific B cells. Apart from B cells, a small number of non-B cells can bind Ag with comparable efficacy as B cells and are found in the peripheral blood, spleen, and bone marrow of mice. This population has been observed for a long time and recently named “Ag-capturing cells.” Their identity remained enigmatic. In this study, we show that these cells are basophilic granulocytes. Their ability to capture Ags is dependent on surface IgE receptors and on Ag-specific plasma IgE molecules appearing after immunization. Several surface markers including surface bound IgE, IL-3R, CD45, CD16/32, and the chemokine receptor CCR2 were used to clearly identify these cells. Cross-linkage of surface Igs results in the release of large amounts of IL-4 and IL-6. The data identify basophils as Ag-capturing cells and support the concept of basophils as important regulators of humoral immune responses.

Glycosaminoglycans have anti-inflammatory properties and interact with a variety of soluble and membrane-bound molecules. Little is known about their effects on B cells and humoral immune responses. We show that CS but not dextran or other glycosaminoglycans induces a pronounced proliferation of B cells in vitro compared with TLR4 or TLR9 ligands. With the use of inhibitors and KO mice, we demonstrate that this proliferation is mediated by the tyrosine kinases BTK and Syk but independent of CD44. Antibodies against Ig-α or Ig-β completely block CS-induced B cell proliferation. Injection of CS in mice for 4–5 days expands B cells in the spleen and results in a marked increase of CD138+ cells in the spleen that is dependent on BTK but independent of CD4+ T cells. Long-term treatment with CS for 14 days also increases CD138+ cells in the bone marrow. When mice were immunized with APC or collagen and treated with CS for up to 14 days during primary or after secondary immune responses, antigen-specific humoral immune responses and antigen-specific CD138+ plasma cells in the bone marrow were reduced significantly. These data show that CD138+ cells, induced by treatment with CS, migrate into the bone marrow and may displace other antigen-specific plasma cells. Overall, CS is able to interfere markedly with primary and fully established humoral immune responses in mice.

Objective To study the role of the chemokine receptors CCR5 and CCR2 in patients with arthritis. Methods CCR5 expression on peripheral blood leukocytes was compared with the expression on leukocytes isolated from the synovial fluid of 20 patients with different rheumatic joint diseases. Three additional samples were studied for CCR2 expression. The expression of chemokine receptors on blood and synovial fluid leukocytes was determined by 3-color flow cytometry analysis. To test CCR5 receptor down-modulation from the cell surface, leukocytes were incubated in vitro with a RANTES (regulated on activation, normal T cell expressed and secreted) derivative, aminooxypentane (AOP)–RANTES. Patients were genotyped for the Δ32 CCR5 deletion by polymerase chain reaction. Results A high percentage of CCR5-expressing CD4+ and CD8+ T cells (74% and 81%, respectively), monocytes (51%), and natural killer cells (35%) was found in the synovial fluid of all patients, whereas in the peripheral blood, only a small percentage of these cells expressed CCR5 (13%, 32%, 7.8%, and 4%, respectively). Infiltration of CCR5-positive leukocytes was not reduced in CCR5-heterozygous patients. A similar, but less pronounced, distribution was observed for CCR2-positive T cells. In vitro, CCR5 was completely down-modulated on synovial fluid leukocytes by AOP-RANTES. Conclusion The predominance of CCR5-positive mononuclear cells in the synovial effusions of patients with arthritis suggests an important role for CCR5 in the process of joint inflammation, and identifies CCR5 as a possible new target for therapeutic intervention.

Staphylococcal enterotoxin D (SED) is a T cell superantigen which selectively targets alpha beta TCRs bearing particular V beta elements. A second function of SED relates to the preferential activation of a B cell subset characterized by a high frequency of rheumatoid factor (RF) producing B cells. To define the molecular basis of the SED-induced B cell repertoire shift, we have analyzed Ig heavy chain genes in B cell clones expanded after SED stimulation and compared them with B cell clones established in the presence of anti-CD3 stimulated helper cells. Gene segments of the VH3 family were most frequently utilized under both stimulation conditions (42% anti-CD3; 47% SED). Sequence analysis of VH3 gene segments demonstrated that the repertoire of VH3 elements in B cell clones from SED driven and anti-CD3 driven cultures were distinct (P = 0.01). RF activity was closely associated with the expression of selected VH3 elements. B cell clones stimulated with SED preferentially expressed VH3A10, whereas VH26 was the gene segment dominantly used in B cell clones expanded with anti-CD3 stimulated helper cells. The usage of JH and DH elements was indistinguishable in SED and anti-CD3 driven B cell clones, suggesting that SED targets VH3+ B cells through a VH-specific mechanism. Comparison of the closely related sequences of the SED responsive VH3A10 and the SED non-responsive VH26 element suggested a role of a sequence polymorphism in the CDR2 reminiscent of B cell reactivity to conventional antigens. In contrast to conventional antigens, SED can induce differentiation of a high frequency of naive B cells. Thus, this staphylococcal enterotoxin combines selective activation of T cells with selective activation of B cells and might be able to direct T cell help to RF producing B cells.

Objective Activation of basophils contributes to memory immune responses and results in exacerbation of collagen-induced arthritis (CIA). We undertook the present study to analyze the production and biologic effects of interleukin-3 (IL-3), a strong activator of basophils, in CIA. Methods Arthritis was induced by immunization with type II collagen. Mice were treated with blocking monoclonal antibodies against IL-3 or with recombinant IL-3. Clinical scoring, histologic analysis, fluorescence-activated cell sorter analysis, enzyme-linked immunosorbent assay, and cell culturing were performed to assess disease activity and IL-3 production. Results IL-3 was produced in large quantities by collagen-specific CD4+ T cells in the spleen and was present in the synovial tissue during onset of arthritis, but was down-regulated in paws with severe inflammation. Blockade of IL-3 during the time of arthritis onset resulted in profound improvement of the disease, with reductions in synovial leukocyte and cytokine levels, peripheral blood basophil levels, and anticollagen antibody titers. Blockade of IL-3 during the late phase of arthritis had no beneficial effect. Administration of recombinant IL-3 during onset of arthritis induced a marked exacerbation of the disease, with increased peripheral blood basophil and plasma IL-6 levels and increased titers of anticollagen antibody. In studies of the regulation of IL-3 expression in CD4+ T cells, IL-6 and IL-4 suppressed the release of IL-3 by activated CD4+ T cells, whereas lipopolysaccharide and CpG DNA up-regulated IL-3 secretion in activated CD4+ T cells by acting on costimulatory cells. Conclusion Taken together, the present results demonstrate for the first time that IL-3 has an important role in the early phase of CIA.

Objectives. We have characterized the expression and the function of the chemokine receptor CCR7 on fibroblast-like synoviocytes (FLS) of patients with RA and OA and on dermal fibroblasts. Methods. FLS were obtained after enzymatic digestion of synovial tissue (ST) of patients with RA and OA undergoing knee replacement surgery and taken into culture for chemokine receptor analysis by RT–PCR, flow cytometry and functional tests. Immunofluorescence for CCR7, fibroblast and T-cell markers was performed on ST of RA and OA patients. To study the response of FLS to CCR7 ligands and other chemokines, migration assays were performed in modified Boyden chambers. After stimulation of FLS with CCR7 ligands, the secretion of VEGF was evaluated by ELISA and Luminex. Results. CCR7 is expressed on FLS of patients with RA and OA, but not on dermal fibroblasts. FLS migrated in response to the CCR7 ligands, CCL19 and CCL21. Stimulation of FLS with CCL19 resulted in a significantly increased secretion of VEGF of RA- and OA-FLS. Conclusion. Apart from the migration of FLS in response to CCL19 and CCL21, it was shown that activation of the CCR7 receptor on FLS results in an enhanced VEGF secretion. A considerable expression of CCR7 ligands in proximity to perivascular infiltrates has previously been described in inflamed synovial tissue of RA patients. Stimulation of FLS via CCR7 could thereby contribute to angiogenesis in the synovial tissue.

Objectives. Leucocyte infiltration is the hallmark of vasculitis, chemokines being mainly responsible for leucocyte migration into inflamed tissues. The objective was to evaluate the local expression of chemokines and chemokine receptors in biopsies of patients with giant cell arteritis (GCA) compared with arteries from patients with polymyalgia rheumatica (PMR). We studied the expression of CCR5, CXCR3 and that of the Duffy antigen/receptor of chemokine (DARC), a chemokine internalizing receptor (interceptor), in parallel to the expression of the CCR5 ligand RANTES/CCL5. Methods. Paraffin-embedded tissue sections from six patients with GCA and five patients with PMR were available for immunohistological analysis of chemokine receptor expression. RANTES/CCL5 mRNA was detected in tissue sections by in situ hybridization. Results. In patients with biopsy-proven giant cell arteritis, CCR5 and CXCR3 were highly expressed by infiltrating leucocytes in involved tissue sections. Predominant clustering of CCR5 + and CXCR3 + leucocytes was found in the adventitia and was co-localized with the expression of CCL5/RANTES mRNA. Interestingly, we found marked expression of DARC on adventitial high endothelial venules in vasculitis lesions of patients with GCA, while in arteries from patients with PMR DARC was only expressed on a low number of vessels with flat lining endothelium. Conclusions. The co-localization of infiltrating CCR5 + and CXCR3 + leucocytes together with CCL5/RANTES and DARC in vasculitis lesions suggests a role for these chemokine receptors in leucocyte infiltration, possibly supported by DARC-mediated vascular presentation of chemokines.

The sequestration of chemokines on the surface of microvascular endothelium is an early event in the selective recruitment of leukocytes. The sequestration and presentation of chemokines must be tightly controlled to confine the extravasation of leukocytes and to prevent uncontrolled inflammation. We investigated whether soluble molecules released under physiological conditions could control chemokine immobilization on cell surfaces and function as regulatory chemokine binding molecules. We determined that human serum contains a molecule that suppresses RANTES (CCL5) binding to endothelial cells, PBMC and CHO cells. Using platelet-rich and platelet-free plasma, serum from patients with thrombocytopenia, and purified platelets, we identified platelets as the source of the chemokine-binding molecule and further identified it as chondroitin sulfate A. In contrast to platelet-derived fully-sulfated chondroitin sulfate A, low-sulfated chondroitin sulfate A present in plasma was almost inactive. Under physiological flow conditions chondroitin sulfate A was found to block RANTES-mediated firm adhesion of monocytes to endothelial cells. It also prevented RANTES-mediated influx of calcium in CCR5-transfected CHO cells while internalization of CCR5 was only marginally reduced. Taken together, chondroitin sulfate A released from platelets appears to act as an important regulatory molecule for cellular responses to chemokines.

SummaryChemokine receptors play a crucial role in the recruitment of leucocyte subsets into inflamed tissue. Using FACS analysis we have studied the surface expression of different CC- and CXC-chemokine receptors on synovial fluid (SF) and peripheral blood leucocytes from 20 patients with various forms of arthritis. In the SF the majority T cells stained positive for CCR5 (93%) and CCR2 (57%), compared to the peripheral blood (36% and 25%). In addition, most of the T cells expressed CXCR4 in both compartments, with a somewhat higher percentage in the SF (90%) versus peripheral blood (83%). To date little information is available on chemokine receptor expression on monocytes in arthritis. We report a marked increase of CCR5+ monocytes in the SF (87%) compared to the peripheral blood (22%). In contrast, the frequency of CXCR1+, CXCR2+, CXCR4+ and CCR1+ monocytes was considerably lower in the SF than in the peripheral blood. Moreover, we report the expression CXCR4 on neutrophils in the SF. Approximately 60% of neutrophils stained positive for CXCR4 in the SF, while in the peripheral blood the number of CXCR4+ neutrophils was low (24%). Surface expression of CXCR1 and CXCR2 was significantly reduced on SF neutrophils (53% and 68%) compared to the peripheral blood. Chemokine receptors are differentially expressed on leucocyte subsets in arthritis. The identification of their pattern of expression might help to identify suitable targets for therapeutic intervention.

RANTES (regulated on activation normal T cell expressed) has been found at elevated levels in biological fluids from patients with a wide range of allergic and autoimmune diseases and is able to attract several subtypes of leukocytes including eosinophils and monocytes into inflamed tissue. Amino-terminal modifications of RANTES produce receptor antagonists which are candidates for blocking this cellular recruitment. Met-RANTES has been shown to modulate inflammation in vivo, while AOP-RANTES is a potent inhibitor of R5 human immunodeficiency virus type 1 (HIV-1) strains and has been shown to down-modulate CCR5 and prevent recycling of the receptor. We have studied the effect of AOP-RANTES in eosinophil activation and have found that it is able to efficiently elicit eosinophil effector functions through CCR3, as measured by the release of reactive oxygen species and calcium mobilization, whereas Met-RANTES is inactive in these assays. AOP-RANTES is found to inhibit CCR3-mediated HIV-1 infection with moderate potency, in contrast to its potent inhibition of CCR5-mediated HIV-1 infection. Furthermore, we have investigated the abilities of these modified proteins to down-modulate CCR1 and CCR3 from the surface of monocytes and eosinophils. We show here that AOP-RANTES is much less effective than RANTES in down-modulation of CCR1. Surprisingly, recycling of CCR1 was minimal after incubation with RANTES while there was complete recycling with AOP-RANTES. In the case of CCR3, no significant difference was found between RANTES and AOP-RANTES in down-modulation and recycling. It therefore appears that trafficking of RANTES receptors follows different patterns, which opens up potential new targets for therapeutic intervention.

Chemokines play an important role in the recruitment of leukocytes and have recently been shown to also attract regulatory T cells. Using blocking mAbs, we analyzed the role of the chemokine receptor CCR2 during initiation and progression of collagen-induced arthritis in mice. Blockade of CCR2 from days 0 to 15 markedly improved clinical signs of arthritis and histological scores measuring leukocyte infiltration, synovial hyperplasia, and bone and cartilage erosion. CCR2 blockade during disease initiation significantly reduced plasma titers of collagen Abs in vivo. In vitro CCR2 blockade also interfered with collagen-specific activation and proliferation of T cells. Surprisingly, CCR2 blockade from days 21 to 36 markedly aggravated clinical and histological signs of arthritis and increased the humoral immune response against collagen. We show that CCR2 is expressed on regulatory T cells. Purified CCR2+ T cells are fully anergic toward polyclonal and collagen-specific activation and potently suppress activation of other T and B cells. The subpopulation of CCR2+ CD25+ regulatory T cells increases ∼5-fold in the progression phase, while CCR2 expression on other leukocyte populations remains unchanged. These findings identify CCR2+ T cells as regulatory T cells and indicate that CCR2 also plays an important role in down-modulating an inflammatory response.