Roles of B cells in rheumatoid arthritis

B lymphocytes play several critical roles in the pathogenesis of rheumatoid arthritis. They are the source of the rheumatoid factors and anticitrullinated protein antibodies, which contribute to immune complex formation and complement activation in the joints. B cells are also very efficient antigen-presenting cells, and can contribute to T cell activation through expression of costimulatory molecules. B cells both respond to and produce the chemokines and cytokines that promote leukocyte infiltration into the joints, formation of ectopic lymphoid structures, angiogenesis, and synovial hyperplasia. The success of B cell depletion therapy in rheumatoid arthritis may depend on disruption of all these diverse functions.     

Inflammatory T cells rapidly induce differentiation of human bone marrow stromal cells into mature osteoblasts

Activated T cells secrete multiple osteoclastogenic cytokines which play a major role in the bone destruction associated with rheumatoid arthritis. While the role of T cells in osteoclastogenesis has received much attention recently, the effect of T cells on osteoblast formation and activity is poorly defined. In this study, we investigated the hypothesis that in chronic inflammation activated T cells contribute to enhanced bone turnover by promoting osteoblastic differentiation. We show that T cells produce soluble factors that induce alkaline phosphatase activity in bone marrow stromal cells and elevated expression of mRNA for Runx2 and osteocalcin. This data indicate that T cell derived factors have the capacity to stimulate the differentiation of bone marrow stromal cells into the osteoblast phenotype. RANKL mRNA was undetectable under any conditions in highly purified bone marrow stromal cells. In contrast, RANKL was constitutively expressed in primary osteoblasts and only moderately up-regulated by activated T cell conditioned medium. Interestingly, both bone marrow stromal cells and osteoblasts expressed mRNA for RANK, which was strongly up-regulated in both cell types by activated T cell conditioned medium. Although, mRNA for the RANKL decoy receptor, osteoprotegerin, was also up-regulated by activated T cell conditioned medium, it's inhibitory effects may be mitigated by a simultaneous rise in the osteoprotegerin competitor TNF-related apoptosis-inducing ligand. Based on our data we propose that during chronic inflammation, T cells regulate bone loss by a dual mechanism involving both direct stimulation of osteoclastogenesis, by production of osteoclastogenic cytokines, and indirectly by induction of osteoblast differentiation and up-regulation of bone turnover via coupling.     

Mesenchymal stem cells in arthritis: role of bone marrow microenvironment

Based on their capacity to suppress immune responses, multipotent mesenchymal stromal cells (MSCs) are intensively studied for regenerative medicine. Moreover, MSCs are potent immunomodulatory cells that occur through the secretion of soluble mediators including nitric oxide, transforming growth factor beta, and HLAG5. The MSCs, however, are also able to express inflammatory mediators such as prostaglandin E₂ or IL-6. MSCs in the bone marrow are in close contact with T cells and B cells, and they regulate immunological memory by organizing defined numbers of dedicated survival niches for plasma cells and memory T cells in the bone marrow. The role of MSCs in arthritis remains controversial - in some studies, murine allogeneic MSCs are able to decrease arthritis; in other studies, MSCs worsen the local inflammation. A recent paper in Arthritis Research and Therapy shows that bone marrow MSCs have decreased osteoblastic potential in rheumatoid arthritis, which may be related to chronic inflammation or to loss of expression of IL-1 receptor agonist. That article raises the importance of the bone marrow microenvironment for MSC biology.     

Interleukin-17: the missing link between T-cell accumulation and effector cell actions in rheumatoid arthritis?

The prominence of T cells and monocyte/macrophages in rheumatoid synovium suggests T cells may localize and amplify the effector functions of monocyte/macrophages in rheumatoid disease. However, while T cells are abundant in rheumatoid joints, classic T-cell derived cytokines are scarce, especially when compared to the levels of monokines IL-1 beta and TNF-alpha. For this reason, it has been speculated that monocyte/macrophages may act independently of T cells in rheumatoid disease and that the role of T cells may be more or less irrelevant to core disease mechanisms. The question of T-cell influence requires re-evaluation in light of the characterization of IL-17, a T-cell derived cytokine that is abundant in rheumatoid synovium and synovial fluid. IL-17 has a number of pro-inflammatory effects, both directly and through amplification of the effects of IL-1 beta and TNF-alpha. IL-17 is able to induce expression of pro-inflammatory cytokines and stimulate release of eicosanoids by monocytes and synoviocytes. Furthermore, IL-17 has been implicated in the pathogenesis of inflammatory bone and joint damage through induction of matrix metalloproteinases and osteoclasts, as well as inhibition of proteoglycan synthesis. In animal models of arthritis, intra-articular injection of IL-17 results in joint inflammation and damage. The recognition of IL-17 as a pro-inflammatory T cell derived cytokine, and its abundance within rheumatoid joints, provides the strongest candidate mechanism to date through which T cells can capture and localize macrophage effector functions in rheumatoid arthritis. As such, IL-17 warrants consideration for its potential as a therapeutic target in rheumatoid arthritis.     

Stromal cell-derived factor-1-CXC chemokine receptor 4 interactions play a central role in CD4+ T cell accumulation in rheumatoid arthritis synovium

Rheumatoid arthritis (RA) is characterized by the accumulation of CD4(+) memory T cells in the inflamed synovium. To address the mechanism, we analyzed chemokine receptor expression and found that the frequency of CXC chemokine receptor (CXCR)4 expressing synovial tissue CD4(+) memory T cells was significantly elevated. CXCR4 expression could be enhanced by IL-15, whereas stromal cell-derived factor (SDF)-1, the ligand of CXCR4, was expressed in the RA synovium and could be increased by CD40 stimulation. SDF-1 stimulated migration of rheumatoid synovial T cells and also inhibited activation-induced apoptosis of T cells. These results indicate that SDF-1-CXCR4 interactions play important roles in CD4(+) memory T cell accumulation in the RA synovium, and emphasize the role of stromal cells in regulating rheumatoid inflammation.     

Synovial stromal cells from rheumatoid arthritis patients attract monocytes by producing MCP-1 and IL-8

Macrophages that accumulate in the synovium of rheumatoid arthritis patients play an important role in the pathogenesis of this inflammatory disease. However, the mechanism by which macrophages are attracted into the inflamed synovium and accumulate there has not been completely delineated. The results of this study show that rheumatoid arthritis synovial stromal cells produce the chemokines monocyte chemotactic protein-1 and IL-8, and these have the capacity to attract peripheral monocytes. These results suggest that one of the mechanisms by which macrophages accumulate in the inflamed synovium is by responding to the chemokines produced locally.     

Synoviocyte-derived CXCL12 is displayed on endothelium and induces angiogenesis in rheumatoid arthritis

CXCL12 (stromal cell-derived factor-1) is a potent CXC chemokine that is constitutively expressed by stromal resident cells. Although it is considered a homeostatic rather than an inflammatory chemokine, CXCL12 has been immunodetected in different inflammatory diseases, but also in normal tissues, ant its potential functions and regulation in inflammation are not well known. In this study, we examined the cellular sources of CXCL12 gene expression and the mechanism and effects of its interactions with endothelial cells in rheumatoid arthritis synovium. We show that CXCL12 mRNA was not overexpressed nor induced in cultured rheumatoid synoviocytes, but it specifically accumulated in the rheumatoid hyperplastic lining layer and endothelium. CXCL12 gene expression was restricted to fibroblast-like synoviocytes, whereas endothelial cells did not express CXCL12 mRNA, but displayed the protein on heparitinase-sensitive factors. CXCL12 colocalized with the angiogenesis marker alpha(v)beta(3) integrin in rheumatoid endothelium and induced angiogenesis in s.c. Matrigel plugs in mice. The angiogenic activity of rheumatoid synovial fluid in vivo was abrogated by specific immunodepletion of CXCL12. Our results indicate that synoviocyte-derived CXCL12 accumulates and it is immobilized on heparan sulfate molecules of endothelial cells, where it can promote angiogenesis and inflammatory cell infiltration, supporting a multifaceted function for this chemokine in the pathogenesis of rheumatoid arthritis.     

Vascular endothelial growth factor signaling in VE-cadherin expression and tube-like formation by rheumatoid arthritic synovial fibroblast-like cells

An increase in the vasculature is one of most representative changes in the synovial tissue of joints in rheumatoid arthritis (RA) and is closely associated with disease progression. Although the vasculatures are believed to be a result of VE-cadherin-dependent angiogenesis and a possible therapeutic target of the disease, synovial fibroblastic cells express VE-cadherin and form tube-like structures, suggesting that vasculatures in RA synovium may not simply result from angiogenesis. This paper analyzes a mechanism of VE-cadherin expression by rheumatoid arthritic synovial fibroblast-like cells (RSFLs) and their involvement in the tube-like formation. A representative angiogenic factor, vascular endothelial growth factor (VEGF), and its binding to a predominant receptor (VEGFR2) activated VE-cadherin expression and the signaling pathways of ERK/MAPK and PI3K/AKT/mTOR. Treatment of RSFLs with signaling pathway inhibitors, VEGFR2 siRNA and a VEGF-antagonizing mimicking peptide inhibited VE-cadherin expression dose-dependently. VEGF-stimulated tube-like formation by RSFLs on Matrigel was hindered by the mimicking peptide and inhibitor treatment. This data demonstrates that RSFLs activated by VEGF binding of VEGFR2 express VE-cadherin and formed tube-like structure under the control of ERK/MAPK and PI3K/AKT/mTOR pathways suggesting that the inhibition suppresses vascular development in RA synovium.     

Inhibition of epidermal growth factor receptor tyrosine kinase ameliorates collagen-induced arthritis

Rheumatoid arthritis (RA) is an autoimmune synovitis characterized by the formation of pannus and the destruction of cartilage and bone in the synovial joints. Although immune cells, which infiltrate the pannus and promote inflammation, play a prominent role in the pathogenesis of RA, other cell types also contribute. Proliferation of synovial fibroblasts, for example, underlies the formation of the pannus, while proliferation of endothelial cells results in neovascularization, which supports the growth of the pannus by supplying it with nutrients and oxygen. The synovial fibroblasts also promote inflammation in the synovium by producing cytokines and chemokines. Finally, osteoclasts cause the destruction of bone. In this study, we show that erlotinib, an inhibitor of the tyrosine kinase epidermal growth factor receptor (EGFR), reduces the severity of established collagen-induced arthritis, a mouse model of RA, and that it does so by targeting synovial fibroblasts, endothelial cells, and osteoclasts. Erlotinib-induced attenuation of autoimmune arthritis was associated with a reduction in number of osteoclasts and blood vessels, and erlotinib inhibited the formation of murine osteoclasts and the proliferation of human endothelial cells in vitro. Erlotinib also inhibited the proliferation and cytokine production of human synovial fibroblasts in vitro. Moreover, EGFR was highly expressed and activated in the synovium of mice with collagen-induced arthritis and patients with RA. Taken together, these findings suggest that EGFR plays a central role in the pathogenesis of RA and that EGFR inhibition may provide benefits in the treatment of RA.     

Mast cells in inflammatory arthritis

Mast cells are present in limited numbers in normal human synovium, but in rheumatoid arthritis and other inflammatory joint diseases this population can expand to constitute 5% or more of all synovial cells. Recent investigations in a murine model have demonstrated that mast cells can have a critical role in the generation of inflammation within the joint. This finding highlights the results of more than 20 years of research indicating that mast cells are frequent participants in non-allergic immune responses as well as in allergy. Equipped with a diversity of surface receptors and effector capabilities, mast cells are sentinels of the immune system, detecting and delivering a first response to invading bacteria and other insults. Accumulating within inflamed tissues, mast cells produce cytokines and other mediators that may contribute vitally to ongoing inflammation. Here we review some of the non-allergic functions of mast cells and focus on the potential role of these cells in murine and human inflammatory arthritis.     

Mast cells in inflammatory arthritis

Mast cells are present in limited numbers in normal human synovium, but in rheumatoid arthritis and other inflammatory joint diseases this population can expand to constitute 5% or more of all synovial cells. Recent investigations in a murine model have demonstrated that mast cells can have a critical role in the generation of inflammation within the joint. This finding highlights the results of more than 20 years of research indicating that mast cells are frequent participants in non-allergic immune responses as well as in allergy. Equipped with a diversity of surface receptors and effector capabilities, mast cells are sentinels of the immune system, detecting and delivering a first response to invading bacteria and other insults. Accumulating within inflamed tissues, mast cells produce cytokines and other mediators that may contribute vitally to ongoing inflammation. Here we review some of the non-allergic functions of mast cells and focus on the potential role of these cells in murine and human inflammatory arthritis.     

Endothelial cell phenotypes in the rheumatoid synovium: activated,angiogenic, apoptotic and leaky

Endothelial cells are active participants in chronic inflammatory diseases. These cells undergo phenotypic changes that can be characterised as activated, angiogenic, apoptotic and leaky. In the present review, these phenotypes are described in the context of human rheumatoid arthritis as the disease example. Endothelial cells become activated in rheumatoid arthritis pathophysiology, expressing adhesion molecules and presenting chemokines, leading to leukocyte migration from the blood into the tissue. Endothelial cell permeability increases, leading to oedema formation and swelling of the joints. These cells proliferate as part of the angiogenic response and there is also a net increase in the turnover of endothelial cells since the number of apoptotic endothelial cells increases. The endothelium expresses various cytokines, cytokine receptors and proteases that are involved in angiogenesis, proliferation and tissue degradation. Associated with these mechanisms is a change in the spectrum of genes expressed, some of which are relatively endothelial specific and others are widely expressed by other cells in the synovium. Better knowledge of molecular and functional changes occurring in endothelial cells during chronic inflammation may lead to the development of endothelium-targeted therapies for rheumatoid arthritis and other chronic inflammatory diseases.     

Mesenchymal stem cells: Emerging mechanisms of immunomodulation and therapy

Mesenchymal stem cells (MSCs) are a pleiotropic population of cells that are self-renewing and capable of differentiating into canonical cells of the mesenchyme, including adipocytes, chondrocytes, and osteocytes. They employ multi-faceted approaches to maintain bone marrow niche homeostasis and promote wound healing during injury. Biomedical research has long sought to exploit their pleiotropic properties as a basis for cell therapy for a variety of diseases and to facilitate hematopoietic stem cell establishment and stromal reconstruction in bone marrow transplantation. Early results demonstrated their usage as safe, and there was little host response to these cells. The discovery of their immunosuppressive functions ushered in a new interest in MSCs as a promising therapeutic tool to suppress inflammation and down-regulate pathogenic immune responses in graft-versus-host and autoimmune diseases such as multiple sclerosis, autoimmune diabetes, and rheumatoid arthritis. MSCs produce a large number of soluble and membrane-bound factors, some of which inhibit immune responses. However, the full range of MSC-mediated immune-modulation remains incompletely understood, as emerging reports also reveal that MSCs can adopt an immunogenic phenotype, stimulate immune cells, and yield seemingly contradictory results in experimental animal models of inflammatory disease. The present review describes the large body of literature that has been accumulated on the fascinating biology of MSCs and their complex effects on immune responses.     

RANK ligand, RANK, and OPG expression in type II collagen-induced arthritis mouse

Rheumatoid arthritis (RA) is a systemic disorder characterized by synovial inflammation and subsequent destruction and deformity of synovial joints. The articular lesions start with synovitis, focal erosion of unmineralized cartilage, and then culminate in the destruction of subarticular bone by pannus tissue. Periarticular osteopenia and systemic osteoporosis follow as late complications of RA. Osteoclasts, specialized cells that resorb bone, play a central role in developing these osteolytic lesions. To elucidate the mechanism of osteoclastogenesis and bone destruction in autoimmune arthritis, we investigated the expression of RANK ligand (RANKL), RANK, and osteoprotegerin (OPG) mRNA in a mouse type II collagen-induced arthritis (CIA) model by in situ hybridization. The results indicated that most of the TRAP-positive mono- and multinucleated cells in the inflamed and proliferating synovium and in the pannus were RANK-positive authentic osteoclasts and their precursors. In the inflamed synovium and pannus of the mouse CIA model, synovial fibroblastic cells around these RANK-positive cells were strongly positive for RANKL. Moreover, RANKL-positive osteoblasts on the endosteal bone surface, at a distance from the affected synovial joints, increased significantly in the mouse CIA model prior to periarticular osteopenia and systemic osteoporosis. These data indicated that the RANKL-RANK system plays an important role for osteoclastogenesis in both local and systemic osteolytic lesions in autoimmune arthritis, and can therefore be a good target for therapeutic intervention.     

Blockade of chemokine receptor CXCR3 inhibits T cell recruitment to inflamed joints and decreases the severity of adjuvant arthritis

T lymphocytes expressing the chemokine receptors, CCR2, CCR5, CXCR3, and CXCR6 are increased in inflamed tissues in rheumatoid arthritis. The role of CXCR3 in autoimmune arthritis induced in Lewis rats was investigated. CXCR3+ T cells migrated 2- to 3-fold more than CXCR3- T cells to inflamed joints in arthritic animals. CXCR3-expressing in vivo Ag-activated T lymphoblasts and in vitro-activated lymph node cells from arthritic animals were strongly recruited to the arthritic joints, and treatment with anti-CXCR3 mAb significantly inhibited this T cell recruitment by 40-60%. Immune T cells from the spleen and lymph nodes of actively immunized arthritic donors adoptively transferred arthritis to naive rats. Treatment with anti-CXCR3 mAb delayed the onset of arthritis and significantly reduced the severity of joint inflammation with a >50% decrease in the clinical arthritis score. Blockade of CXCR3 also significantly reduced the weight loss in the arthritic animals and inhibited neutrophil accumulation in the joints by 50-60%. There was a marked reduction in the leukocyte infiltration of the synovium in the presence of CXCR3 blockade and a decrease in the loss of articular cartilage of the joints. In conclusion, CXCR3 on T cells has an essential role in T cell recruitment to inflamed joints and the development of joint inflammation in adjuvant arthritis.     

Arthritis: where are the T cells?

T-helper (Th) lymphocytes contribute to arthritis pathogenesis by helping B cells to produce antibodies, by producing cytokines that activate effector cells involved in the destruction of cartilage and bone, and by contributing to osteoclast differentiation. There are murine models of arthritis, most notably collagen- and proteoglycan-induced arthritis, in which arthritis depends on T-cell recognition of antigens that are expressed in the joints. In spite of this, we still do not know the antigens recognised by arthritogenic Th cells in humans. Moreover, current evidence for Th cells exerting arthritogenic effector functions within the joints is only indirect.     

Marrow stromal cells and osteoclast precursors differentially contribute to TNF-alpha-induced osteoclastogenesis in vivo

The marrow stromal cell is the principal source of the key osteoclastogenic cytokine receptor activator of NF-kappaB (RANK) ligand (RANKL). To individualize the role of marrow stromal cells in varying states of TNF-alpha-driven osteoclast formation in vivo, we generated chimeric mice in which wild-type (WT) marrow, immunodepleted of T cells and stromal cells, is transplanted into lethally irradiated mice deleted of both the p55 and p75 TNFR. As control, similarly treated WT marrow was transplanted into WT mice. Each group was administered increasing doses of TNF-alpha. Exposure to high-dose cytokine ex vivo induces exuberant osteoclastogenesis irrespective of in vivo TNF-alpha treatment or whether the recipient animals possess TNF-alpha-responsive stromal cells. In contrast, the osteoclastogenic capacity of marrow treated with lower-dose TNF-alpha requires priming by TNFR-bearing stromal cells in vivo. Importantly, the osteoclastogenic contribution of cytokine responsive stromal cells in vivo diminishes as the dose of TNF-alpha increases. In keeping with this conclusion, mice with severe inflammatory arthritis develop profound osteoclastogenesis and bone erosion independent of stromal cell expression of TNFR. The direct induction of osteoclast recruitment by TNF-alpha is characterized by enhanced RANK expression and sensitization of precursor cells to RANKL. Thus, osteolysis attending relatively modest elevations in ambient TNF-alpha depends upon responsive stromal cells. Alternatively, in states of severe periarticular inflammation, TNF-alpha may fully exert its bone erosive effects by directly promoting the differentiation of osteoclast precursors independent of cytokine-responsive stromal cells and T lymphocytes.     

What is MRI bone oedema in rheumatoid arthritis and why does it matter?

MRI bone oedema occurs in various forms of inflammatory and non-inflammatory arthritis and probably represents a cellular infiltrate within bone. It is common in early rheumatoid arthritis and is associated with erosive progression and poor functional outcome. Histopathological studies suggest that a cellular infiltrate comprising lymphocytes and osteoclasts may be detected in subchondral bone and could mediate the development of erosions from the marrow towards the joint surface. There is emerging evidence from animal models that such an infiltrate corresponds with MRI bone oedema, pointing towards the bone marrow as a site for important pathology driving joint damage in rheumatoid arthritis.