Chemokine receptor expression and functional effects of chemokines on B cells: implication in the pathogenesis of rheumatoid arthritis
© Nanki et al.; licensee BioMed Central Ltd. 2009
Received: 14 May 2009
Accepted: 5 October 2009
Published: 5 October 2009
Accumulation of B cells in the rheumatoid arthritis (RA) synovium has been reported, and it has been thought that these cells might contribute to the pathogenesis of RA by antigen presentation, autoantibody production, and/or inflammatory cytokine production. Chemokines could enhance the accumulation of B cells in the synovium. The aims of this study were to determine chemokine receptor expression by B cells both in the peripheral blood of normal donors and subjects with RA, and at the inflammatory site in RA, and the effects of chemokines on B cell activation.
Cell surface molecule expression was analyzed by flow cytometry. Cellular migration was assessed using chemotaxis chambers. Cellular proliferation was examined by 3H-thymidine incorporation. Tumor necrosis factor (TNF) production was assayed by enzyme-linked immunosorbent assay.
Significant numbers of peripheral blood B cells of healthy donors and subjects with RA expressed CC chemokine receptor (CCR)5 and CXCR3, and most B cells expressed CCR6, CCR7, CXCR4 and CXCR5. CCR5 expression was more frequent on CD27+ than CD27- peripheral blood B cells of healthy donors and RA. Synovial B cells more frequently expressed CCR5, but less often expressed CCR6, CCR7 and CXCR5 compared to peripheral blood in RA. Further functional analyses were performed on peripheral blood B cells from healthy donors. Migration of peripheral blood B cells, especially CD27+ B cells, was enhanced by CC chemokine ligand (CCL)20, CCL19, CCL21 and CXCL12. All four chemokines alone induced B cell proliferation; with CCL21 being the most effective. CCL21 also enhanced the proliferation of anti-immunoglobulin (Ig)M-stimulated B cells and blockade of CCR7 inhibited this effect. CCL20, CCL21 and CXCL12 enhanced TNF production by anti-IgM mAb-stimulated B cells. Finally, stimulation with CXCL12, but not CCL20, CCL19 and CCL21, enhanced inducible costimulator-ligand (ICOSL) expression by peripheral blood B cells of healthy donors and RA, but did not increase B cell-activating factor receptor or transmembrane activator and CAML-interactor.
The data suggest that CCR5, CCR6, CCR7, CXCR3, CXCR4 and CXCR5 may be important for the B cell migration into the synovium of RA patients, and also their local proliferation, cytokine production and ICOSL expression in the synovium.
Rheumatoid arthritis (RA) is characterized by chronic inflammation of multiple joints. As B cell depletion by treatment with rituximab, an anti-CD20 monoclonal antibody (mAb), is beneficial for RA patients [1, 2], B cells are considered to play important roles in the pathogenesis of RA. In this regard, the synovial tissue of RA patients shows abundant accumulation of inflammatory cells, including T cells, macrophages, dendritic cells and B cells [3–6]. Synovial B cells could present antigens to T cells. Importantly, rheumatoid factor-expressing B cells that are found within the synovium  can present any antigen in the context of an immune complex and, thereby, trigger T cells specific for a variety of foreign antigens . Notably, the severity of RA correlates with levels of rheumatoid factor . Furthermore, activated B cells produce inflammatory cytokines, such as TNF . Therefore, synovial B cells could contribute to the pathogenesis of RA by antigen presentation, autoantibody production, and inflammatory cytokine production. One of the mechanisms for accumulation of B cells in synovial tissues relates to the interaction with chemokines produced in the RA synovium and chemokine receptors expressed by the B cells .
Chemokines are classified into C, CC, CXC, and CX3C subclasses based on the conserved cysteine motifs , and are involved in cellular migration, activation of adhesion molecules, cellular proliferation, cytokine production and regulation of apoptosis [12, 13]. Chemokines contribute to homeostatic migration as well as entry into acute and chronic inflammatory sites. Expression of chemokines and chemokine receptors in the RA synovial tissue has been extensively analyzed, and chemokines are thought to be potential therapeutic targets [14, 15]. However, the role of chemokines specifically on B cells in RA has not been completely delineated.
In this study, we examined chemokine receptor expression by peripheral blood in both normal donors and subjects with RA, and also synovial B cells from subjects with RA, and determined the functional effects of chemokines on B cells.
Materials and methods
Peripheral blood samples were obtained from healthy donors and subjects with RA after obtaining informed consent. RA was diagnosed according to the criteria of the American College of Rheumatology . Synovial tissues were obtained at the time of total knee joint replacement from RA patients. Signed consent forms were obtained prior to the operation. The study protocol was approved in advance by the Ethics Committee of the Tokyo Medical and Dental University.
Chemokine receptor expression
Peripheral blood mononuclear cells (PBMCs) were isolated by ficoll-hypaque (Immuno-Biological Laboratories, Gunma, Japan) gradient centrifugation. The synovial tissue was minced and incubated with 0.3 mg/ml collagenase (Sigma, St. Louis, MO, USA) for one hour at 37°C in Dulbecco's Modified Eagle Medium (DMEM) (Sigma, St. Louis, MO, USA). Partially digested pieces of the tissue were pressed through a metal screen to obtain single cell suspensions. The following mAbs were used for FACS analysis: phycoerythrin (PE) Cy5-conjugated anti-CD19 mAb (J4.119; Beckman Coulter, San Jose, CA, USA), fluorescein isothiocyanate (FITC)-conjugated anti-CD27 (M-T271; Ancell, Bayport, MN, USA) mAb, PE-conjugated anti-CC chemokine receptor (CCR)5 (2D7; BD Bioscience, San Jose, CA, USA), -CCR6 (53103; R&D Systems, Minneapolis, MN, USA), -CCR7 (150503; R&D Systems, Minneapolis, MN, USA), -CXCR3 (49801; R&D Systems, Minneapolis, MN, USA), -CXCR4 (12G5; R&D Systems, Minneapolis, MN, USA) and -CXCR5 (51505.111; R&D Systems, Minneapolis, MN, USA) mAbs, and isotype-matched control mAbs. PBMCs or synovial tissue cells were incubated with the mAbs for 20 minutes, and then rinsed with PBS-3% fetal calf serum (FCS; Sigma, St. Louis, MO, USA). More than 5000 stained cells were analyzed with a FACSCalibur (BD Bioscience, San Jose, CA, USA).
Cell migration was assessed in 24-well chemotaxis chambers (6.5 mm diameter, 5 μm pore polycarbonate transwell culture insert; Costar, Cambridge, MA, USA). ECV304 cells (2 × 105) were cultured in the chemotaxis chambers for 48 to 72 hours in medium 199 (Sigma, St. Louis, MO, USA) with 10% FCS. The migration medium (Roswell Park Memorial Institute (RPMI)1640 medium (Sigma, St. Louis, MO, USA):medium 199 = 1:1, 0.5% BSA) supplemented where indicated with various concentrations of chemokines (CC chemokine ligand (CCL)20, CCL19, CCL21, and CXCL12: PeproTech, Rocky Hill, NJ, USA) was added to the lower wells. ECV304 coated chemotaxis chambers were placed in each well, and 5 × 105 PBMCs suspended in migration medium were added to the upper wells. After two hours of incubation, the membrane was removed, and migrated cells were stained with PE Cy5-conjugated anti-CD19 mAb (J4.119) and FITC-conjugated anti-CD27 mAb (M-T271). The cells were counted by FACSCalibur.
Peripheral blood CD19+ B cells were purified by magnetic-activated cell sorting microbead-coupled mAb and magnetic cell separation columns (Miltenyi Biotec, Auburn, CA, USA). Purity of CD19+ B cells was determined by flow cytometry, and was more than 95%. To block CCR7, B cells were incubated with 5 μg/ml anti-CCR7 mAb (150503; R&D Systems, Minneapolis, MN, USA) or control mAb for 30 minutes. Then, the 5 × 105 B cells were incubated in 96-well with the indicated chemokines with or without pre-coated anti-IgM mAb (2 μg UHB; SouthernBiotech, Birmingham, AL, USA) in RPMI1640 with 10% FCS at 37°C for 48 hours. 3H-thymidine (1 μCi; Amersham Biosciences, Little Chalfont, Buckinghamshire, UK) was added and the B cells were incubated for 24 hours. Afterward, the incorporated radioactivity was quantified. After the 72-hour incubation, viabilities of the cells, determined by trypan blue exclusion, were 87.3% and 80.3% without and with anti-IgM stimulation, respectively.
Purified 5 × 105 peripheral blood B cells were stimulated with the indicated chemokines with or without coating of wells with anti-IgM mAb (2 μg UHB) in 96-well in RPMI1640 with 10% FCS at 37°C for 24 hours. Afterward, the concentration of TNF in the culture supernatant was assayed using an ultra sensitive ELISA kit (BioSource International, Camarillo, CA).
Cell surface molecule expression
PBMCs were cultured with the indicated chemokine in RPMI1640+10% FCS for 24 hrs. Afterward, the cells were stained with PE Cy5-conjugated anti-CD19 mAb (J4.119) and FITC-conjugated anti-inducible costimulator-ligand (ICOSL) mAb (MIH12; eBioscience, San Diego, CA, USA), PE-conjugated anti-B cell-activating factor receptor (BAFF-R; 8A7; eBioscience, San Diego, CA, USA), -transmembrane activator and CAML-interactor (TACI) mAb (11H3; eBioscience, San Diego, CA, USA), or isotype-matched control mAb. The stained cells were analyzed with a FACSCaliber.
Paired t test was used to compare paired samples of CD27- and CD27+ peripheral blood B cells, and peripheral blood and synovial B cells from the same subjects for chemokine receptor expression and migration. Differences in migration, fold increase of proliferation and TNF production were examined for statistical significance using the unpaired t test. All data were expressed as mean ± standard error of the mean (SEM). A P value less than 0.05 denoted the presence of a statistically significant difference.
Chemokine receptor expression by B cells
Next, we analyzed the chemokine receptor expression by CD27- B cells and CD27+ B cells from peripheral blood and synovial tissue of subjects with RA. The frequency of CD27-expressing peripheral blood B cells was not significantly different between subjects with RA and healthy donors (data not shown). The proportion of the chemokine receptor expression of RA peripheral blood B cells was similar to that of healthy donors without any statistically significant differences. As with healthy donors, CCR5 expression by RA peripheral blood CD27+ B cells was more frequent than that of CD27- B cells, and CXCR4 expression by CD27+ B cells was less frequent than that of CD27- B cells. In addition, the proportions of CCR6, CCR7 and CXCR5 expression were significantly less by CD27+ compared with CD27- B cells in subjects with RA.
Cell surface molecule expression
In this study, we showed that significant numbers of peripheral blood and RA synovial B cells express CCR5, CCR6, CCR7, CXCR3, CXCR4, and CXCR5. The ligand chemokines, CCL3, CCL4 and CCL5 for CCR5, CCL20 for CCR6, CCL19 and CCL21 for CCR7, CXCL9, CXCL10 and CXCL11 for CXCR3, CXCL12 for CXCR4, and CXCL13 for CXCR5 has been reported to be expressed in the RA synovium [22–29]. Therefore, interactions between the chemokines and the chemokine receptors might contribute to B cell migration into the synovial tissue in patients with RA.
In the RA synovium, the proportion of memory CD27+ B cells was increased compared with peripheral blood of RA patients. The results also showed that CCR5 was expressed more frequently by peripheral blood CD27+ B cells compared with CD27-, and the proportion of synovial B cells expressing CCR5 was increased compared with peripheral blood. These results suggest that interaction between CCR5 and the ligand chemokines could contribute to the accumulation of CD27+ B cells in the synovium. Alternatively, because the migration of CD27+ B cells to all the chemokines analyzed was greater than that of CD27- B cells, the increased proportion of CD27+ B cells in the synovium might be related to their higher chemotactic activity. In contrast, the expression of CCR6, CCR7 and CXCR5 was downregulated by the synovial B cells. As most peripheral blood B cells express these chemokine receptors, it is not likely that the chemokine receptor-negative B cells selectively migrated into the synovium. Rather chemokine receptor expression might be downregulated after ligation of the corresponding ligand chemokine. Alternatively, stimulation with cytokines or adhesion molecules may downregulate chemokine receptor expression in the synovium.
The present study showed that stimulation with chemokine regulates peripheral blood B cell proliferation. Previous studies showed the presence of germinal center-like structures in the RA synovium , somatic hypermutation of the Ig variable region genes, B cell clonal expansion, and a skewed Ig repertoire in the synovium [31, 32]. Collectively, these results suggest that synovial B cells might be antigenically stimulated at the inflammatory site. Based on such B cell stimulation in the synovium, the interaction between chemokines and chemokine receptors, especially CCL21 and CCR7, might also contribute to B cell proliferation. There is an evidence that follicular dendritic cells in the RA synovium produce CXCL13, a ligand for CXCR5 . Interaction with the expressed CXCL13 and CXCR5 on B cells might contribute to the formation of the germinal center-like structures in the synovium.
Stimulation with CCL20, CCL21 and CXCL12 enhanced TNF production by anti-IgM mAb-stimulated peripheral blood B cells suggesting that chemokine stimulation in the RA synovium might also increase TNF production by synovial B cells. It is widely known that TNF plays important roles in the pathogenesis of RA and blockade of this cytokine is an effective therapy for RA . Moreover, CXCL12 upregulated ICOSL expression on peripheral blood B cells. ICOSL could interact with inducible costimulator (ICOS), which is expressed by activated T cells . We showed previously that ICOS expression was upregulated on RA synovial T cells . Thus, upregulated ICOSL on CXCL12-stimulated B cells could augment T cell stimulation in the synovium. Taken together, interaction between chemokine and chemokine receptor might play roles not only on B cell migration into the synovium, but also B cell activation in the synovium. In this regard, we reported previously that CXCL12 enhanced cellular proliferation and expression of cytokines and activation markers by peripheral blood T cells [36, 37], and that CCL2, CCL5 and CXCL12 upregulated the expression of cytokines and chemokines by fibroblast-like synoviocytes from RA . Thus, chemokine stimulation in the RA synovial tissue could play an important role on the chronic immune activation found in this tissue.
CCR5, CCR6, CCR7, CXCR3, CXCR4, and CXCR5 might be important for B cell migration into the synovium of RA. Chemokines are suggested to contribute to B cell migration as well as their proliferation, cytokine production and ICOSL expression in the RA synovium.
B cell-activating factor receptor
bovine serum albumin
CC chemokine ligand
CC chemokine receptor
Dulbecco's Modified Eagle Medium
enzyme-linked immunosorbent assay
fetal calf serum
peripheral blood mononuclear cells
Roswell Park Memorial Institute
standard error of the mean
transmembrane activator and CAML-interactor
tumor necrosis factor.
We thank Fumiko Inoue and Aya Sato for the excellent technical support. This work was supported in part by grants-in-aid from the Ministry of Health, Labor and Welfare, and the Ministry of Education, Science, Sports and Culture, Japan, and the Japanese Ministry of Education, Global Center of Excellence (GCOE) Program, International Research Center for Molecular Science in Tooth and Bone Diseases.
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