Human Rheumatoid Arthritis Synovial Tissue Research Articles

NGF facilitates ICAM-1-dependent monocyte adhesion and M1 macrophage polarization in rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic autoimmune inflammatory disorder in which monocytes adhering to synovial tissue differentiate into the pro-inflammatory M1 macrophage phenotype. Nerve growth factors (NGF) referred to as neurotrophins have been associated with inflammatory events; however, researchers have yet to elucidate the role of NGF in RA. Our examination of clinical tissue samples and analysis of data sourced from the Gene Expression Omnibus dataset unveiled elevated expression levels of M1 macrophage markers in human RA synovial tissue samples compared to normal tissue, with no such distinction observed for M2 markers. Furthermore, immunofluorescence data depicted increased expression levels of NGF and M1 macrophages in RA mice in contrast to normal mice. It appears that NGF stimulation facilitates macrophage polarization from the M0 to the M1 phenotype. It also appears that NGF promotes ICAM-1 production in human RA synovial fibroblasts, which enhances monocyte adhesion through the TrkA, MEK/ERK, and AP-1 signaling cascades. Our findings indicate NGF/TrkA axis as a novel target for the treatment of RA.

35 Deciphering complement system-dependent cellular pathways in human rheumatoid arthritis synovial tissues using large single-cell computational omics

35 Deciphering complement system-dependent cellular pathways in human rheumatoid arthritis synovial tissues using large single-cell computational omics

Nesfatin-1 Stimulates CCL2-dependent Monocyte Migration And M1 Macrophage Polarization: Implications For Rheumatoid Arthritis Therapy.

Rheumatoid arthritis (RA) is a prototypic inflammatory disease, characterized by the infiltration of proinflammatory cytokines into the joint synovium and the migration of mononuclear cells into inflammatory sites. The adipokine nesfatin-1 is linked to inflammatory events in various diseases, although its role in RA pathology is uncertain. Analysis of the Gene Expression Omnibus GSE55235 dataset revealed high levels of expression of the adipokine nesfatin-1 in human RA synovial tissue. Similarly, our human synovial tissue samples exhibited increasing levels of nesfatin-1 expression and Ccl2 mRNA expression. Nesfatin-1-induced stimulation of CCL2 expression and monocyte migration involved the MEK/ERK, p38, and NF-κB signaling pathways. Notably, nesfatin-1-induced increases in CCL2 expression favored M1 macrophage polarization, which increased the expression of proinflammatory cytokines IL-1β, IL-6, and TNF-α. Finally, nesfatin-1 shRNA ameliorated the severity of inflammatory disease and reduced levels of M1 macrophage expression in CIA mice. Our studies confirm that nesfatin-1 appears to be worth targeting in RA treatment.

METTL3 Promotes Activation and Inflammation of FLSs Through the NF-κB Signaling Pathway in Rheumatoid Arthritis.

Rheumatoid arthritis (RA), a common autoimmune disease, is extremely damaging to human health. Fibroblast-like synoviocytes (FLSs) have a vital role in the occurrence and development of RA. Methyltransferase-like 3 (METTL3), which is a crucial component of the N6-methyladenosine (m6A) methyltransferase complex, is involved in the progression of many diseases. In this study, we explored the role of METTL3 in the inflammatory response and proliferation, invasion, and migration of FLSs. We used human RA synovial tissues and the adjuvant-induced arthritis (AIA) animal model of RA. Experimental results revealed that METTL3 expression was significantly upregulated in human RA synovial tissues and in the rat AIA model. METTL3 knockdown suppressed interleukin (IL)-6, matrix metalloproteinase (MMP)-3, and MMP-9 levels in human RA-FLSs and rat AIA-FLSs. In contrast, they were increased by METTL3 overexpression. Additionally, we found that, in FLSs, METTL3 may activate the nuclear factor (NF)-κB signaling pathway. The experimental results showed that METTL3 may promote FLS activation and inflammatory response via the NF-κB signaling pathway.

ASIC1a induces synovial inflammation via the Ca2+/NFATc3/ RANTES pathway.

Rationale: Synovial inflammation is one of the main pathological features of rheumatoid arthritis (RA) and is a key factor leading to the progression of RA. Understanding the regulatory mechanism of synovial inflammation is crucial for the treatment of RA. Acid-sensing ion channel 1a (ASIC1a) is an H+-gated cation channel that promotes the progression of RA, but the role of ASIC1a in synovial inflammation is unclear. This study aimed to investigate whether ASIC1a is involved in the synovial inflammation and explore the underlying mechanisms in vitro and in vivo.Methods: The expression of ASIC1a and nuclear factor of activated T cells (NFATs) were analyzed by Western blotting, immunofluorescence, and immunohistochemistry both in vitro and in vivo. The Ca2+ influx mediated by ASIC1a was detected by calcium imaging and flow cytometry. The role of ASIC1a in inflammation was studied in rats with adjuvant-induced arthritis (AA). Inflammatory cytokine profile was analyzed by protein chip in RA synovial fibroblasts (RASF) and verified by a magnetic multi-cytokine assay and ELISA. The NFATc3-regulated RANTES (Regulated upon activation, normal T cell expressed and secreted) gene transcription was investigated by ChIP-qPCR and dual-luciferase reporter assay.Results: The expression of ASIC1a was significantly increased in human RA synovial tissues and primary human RASF as well as in ankle synovium of AA rats. Activated ASIC1a mediated Ca2+ influx to increase [Ca2+]i in RASF. The activation/overexpression of ASIC1a in RASF up-regulated the expression of inflammatory cytokines RANTES, sTNF RI, MIP-1a, IL-8, sTNF RII, and ICAM-1 among which RANTES was increased most remarkably. In vivo, ASIC1a promoted inflammation, synovial hyperplasia, articular cartilage, and bone destruction, leading to the progression of AA. Furthermore, activation of ASIC1a upregulated the nuclear translocation of NFATc3, which bound to RANTES promoter and directly regulated gene transcription to enhance RANTES expression.Conclusion: ASIC1a induces synovial inflammation, which leads to the progression of RA. Our study reveals a novel RA inflammation regulatory mechanism and indicates that ASIC1a might be a potential therapeutic target for RA.

The activin-follistatin anti-inflammatory cycle is deregulated in synovial fibroblasts

BackgroundActivin A and follistatin exhibit immunomodulatory functions, thus affecting autoinflammatory processes as found in rheumatoid arthritis (RA). The impact of both proteins on the behavior of synovial fibroblasts (SF) in RA as well as in osteoarthritis (OA) is unknown.MethodsImmunohistochemical analyses of synovial tissue for expression of activin A and follistatin were performed. The influence of RASF overexpressing activin A on cartilage invasion in a SCID mouse model was examined. RASF and OASF were stimulated with either IL-1β or TNFα in combination with or solely with activin A, activin AB, or follistatin. Protein secretion was measured by ELISA and mRNA expression by RT-PCR. Smad signaling was confirmed by western blot.ResultsIn human RA synovial tissue, the number of activin A-positive cells as well as its extracellular presence was higher than in the OA synovium. Single cells within the tissue expressed follistatin in RA and OA synovial tissue. In the SCID mouse model, activin A overexpression reduced RASF invasion. In human RASF, activin A was induced by IL-1β and TNFα. Activin A slightly increased IL-6 release by unstimulated RASF, but decreased protein and mRNA levels of follistatin.ConclusionThe observed decrease of cartilage invasion by RASF overexpressing activin A in the SCID mouse model appears to be mediated by an interaction between activin/follistatin and other local cells indirectly affecting RASF because activin A displayed certain pro-inflammatory effects on RASF. Activin A even inhibits production and release of follistatin in RASF and therefore prevents itself from being blocked by its inhibitory binding protein follistatin in the local inflammatory joint environment.

Synovial GATA1 mediates rheumatoid arthritis progression via transcriptional activation of NOS2 signaling.

Transcriptional regulation of inducible nitric oxide synthase (iNOS) is critically involved in the pathogenesis and progression of rheumatoid arthritis (RA); however, the specific transcription factors that control this process remain largely unidentified. In the present study, it was discovered that expression of the key erythroid factor, globin transcription factor 1 (GATA1), is significantly greater in human RA synovial tissues than in osteoarthritis (OA) tissues. IL 6 was found to induce synovial GATA1 expression in a signal transducer and activator of transcription 3-dependent manner. Functionally, knockdown of GATA1 expression using specific small interfering RNA treatment was found to compromise immunoreaction-elicited expression of proinflammatory cytokines and thus impair invasiveness of the human fibroblast-like synovial cell line MH7A, whereas introduction of exogenous GATA1 was found to promote production of proinflammatory cytokines, leading to greater aggressiveness of MH7A cells. Mechanistically, GATA1 acts as the transcriptional coactivator of NOS2 (the gene encoding iNOS) transcription. Collectively, these data suggest that synovial GATA1 is an essential contributor to development and exacerbation of RA, presumably by inducing NOS2 transcription.

Tertiary Lymphoid Structures in Rheumatoid Arthritis: NF-κB–Inducing Kinase–Positive Endothelial Cells as Central Players

Tertiary lymphoid structures (TLSs) in chronic inflammation, including rheumatoid arthritis (RA) synovial tissue (ST), often contain high endothelial venules and follicular dendritic cells (FDCs). Endothelial cell (EC)-specific lymphotoxin β (LTβ) receptor signaling is critical for the formation of lymph nodes and high endothelial venules. FDCs arise from perivascular platelet-derived growth factor receptor β(+) precursor cells (preFDCs) that require specific group 3 innate lymphoid cells (ILC3s) and LTβ for their expansion. Previously, we showed that RA ST contains ECs that express NF-κB-inducing kinase (NIK), which is pivotal in LTβ-induced noncanonical NF-κB signaling. We studied the relation between NIK(+) ECs, (pre)FDCs, and ILC3s with respect to TLSs in RA ST. TLS(+) tissues exhibited a significantly increased expression of genes involved in noncanonical NF-κB signaling, including NIK, and immunohistochemical analysis revealed that NIK was almost exclusively expressed by ECs. ILC3s were present in human RA ST in very low numbers, but not differentially in TLS(+) tissues. In contrast, TLS(+) tissues contained significantly more NIK(+) ECs and perivascular platelet-derived growth factor receptor β(+) preFDCs, which correlated significantly with the quantity of FDCs. We established a strong link between NIK(+) ECs, (pre)FDCs, and the presence of TLSs, indicating that NIK(+) ECs may not only be important orchestrators of lymph node development but also contribute to the formation of TLSs in chronic inflammation.

5-Lipoxygenase inhibitors attenuate TNF-α-induced inflammation in human synovial fibroblasts.

The lipoxygenase isoform of 5-lipoxygenase (5-LOX) is reported to be overexpressed in human rheumatoid arthritis synovial tissue and involved in the progress of inflammatory arthritis. However, the detailed mechanism of how 5-lipoxygenase regulates the inflammatory response in arthritis synovial tissue is still unclear. The aim of this study was to investigate the involvement of lipoxygenase pathways in TNF-α-induced production of cytokines and chemokines. Human synovial fibroblasts from rheumatoid patients were used in this study. 5-LOX inhibitors and shRNA were used to examine the involvement of 5-LOX in TNF-α-induced cytokines and chemokines expression. The signaling pathways were examined by Western Blotting or immunofluorescence staining. The effect of 5-LOX inhibitor on TNF-α-induced chemokine expression and paw edema was also explored in vivo in C57BL/6 mice. Treatment with 5-LOX inhibitors significantly decreased TNF-α-induced pro-inflammatory mediators including interleukin-6 (IL-6) and monocyte chemo-attractant protein-1 (MCP-1) in human synovial fibroblasts. Knockdown of 5-LOX using shRNA exerted similar inhibitory effects. The abrogation of NF-κB activation was involved in the antagonizing effects of these inhibitors. Furthermore, 5-LOX inhibitor decreased TNF-α-induced up-regulation of serum MCP-1 level and paw edema in mouse model. Our results provide the evidence that the administration of 5-LOX inhibitors is able to ameliorate TNF-α-induced cytokine/chemokine release and paw edema, indicating that 5-LOX inhibitors may be developed for therapeutic treatment of inflammatory arthritis.

The discoidin domain receptor 2/annexin A2/matrix metalloproteinase 13 loop promotes joint destruction in arthritis through promoting migration and invasion of fibroblast-like synoviocytes.

Discoidin domain receptor 2 (DDR-2)/matrix metalloproteinase (MMP) signaling is an important pathway involved in cartilage destruction in rheumatoid arthritis (RA). However, the molecular mechanisms of this pathway have not been clearly identified. This study was undertaken to screen key molecules involved in this pathway and evaluate their biologic functions in synovium invasion of RA. DDR-2-interacting proteins were examined in vitro by immunoprecipitation and mass spectrometry, and annexin A2 was acquired. The effects of annexin A2 on fibroblast-like synoviocyte (FLS) migration were evaluated using a Transwell invasion assay and an Erasion trace test. In Ddr2(-/-) mice with collagen-induced arthritis (CIA), hematoxylin and eosin (H&E) staining, immunohistochemical analysis, and Western blot analysis were used to assess expression of DDR-2, annexin A2, and MMP-13, as well as synovial hyperplasia. Rats with CIA were treated with lentivirus annexin A2 small interfering RNA (siRNA), and annexin A2 siRNA effects on joint damage were analyzed based upon arthritis index scores and results of micro-computed tomography and H&E staining. The differences between annexin A2 expression in clinical samples from RA and osteoarthritis patients were compared using Western blotting. Annexin 2 was identified for the first time as a DDR-2 binding protein. It may be phosphorylated by phospho-DDR-2, leading to MMP-13 secretion. The annexin A2 phosphorylation level and MMP-13 expression level were decreased and collagen-induced joint damage greatly reduced in Ddr2(-/-) mice. Joint damage in rats with CIA was significantly ameliorated when annexin A2 was down-regulated. Annexin A2 expression and phosphorylation were elevated in human RA synovial tissue. Annexin A2 is a key molecule in the DDR-2/annexin A2/MMP-13 loop, the activation of which contributes to joint destruction in RA, mainly through promoting invasion of FLS. Annexin A2 might therefore become a novel clinical target for RA treatment.

PILRα Negatively Regulates Mouse Inflammatory Arthritis

Paired Ig-like type 2 receptor (PILR)α inhibitory receptor and its counterpart PILRβ activating receptor are coexpressed on myeloid cells. In this article, we report that PILRα, but not PILRβ, is elevated in human rheumatoid arthritis synovial tissue and correlates with inflammatory cell infiltration. Pilrα(-/-) mice produce more pathogenic cytokines during inflammation and are prone to enhanced autoimmune arthritis. Correspondingly, engaging PILRα with anti-PILRα mAb ameliorates inflammation in mouse arthritis models and suppresses the production of proinflammatory cytokines. Our studies suggest that PILRα mediates an important inhibitory pathway that can dampen inflammatory responses.

FRI0343 Acidification Enhances Acid Sensing Protein and Matrix Metalloproteinase Expression in Human Rheumatoid Fibroblast like Synoviocytes

BackgroundIn rheumatoid arthritis (RA), fibroblast-like synoviocytes (RAFLS) develop an aggressive phenotype, promoting cartilage damage and bone destruction through the secretion of proteolytic enzymes and cytokines. We have recently shown that...

Btk inhibition suppresses agonist-induced human macrophage activation and inflammatory gene expression in RA synovial tissue explants

ObjectivesBruton's tyrosine kinase (Btk) is required for B lymphocyte and myeloid cell contributions to pathology in murine models of arthritis. Here, we examined the potential contributions of synovial Btk expression...

A1.78 Ectopic lymphoid neogenesis in rheumatoid arthritis: a potential role for NIK expressing endothelial cells as orchestrators of tertiary lymphoid structures

Background and ObjectivesTertiary lymphoid structures (TLS) can be observed in rheumatoid arthritis (RA) synovial tissue, and often contain high endothelial venules (HEV) and follicular dendritic cells (FDC). Endothelial cell (EC)-specific...

T cell lessons from the rheumatoid arthritis synovium SCID mouse model: CD3‐rich synovium lacks response to CTLA‐4ig but is successfully treated by interleukin‐17 neutralization

To provide an intermediate step between classic arthritis models and clinical trials, the rheumatoid arthritis (RA) synovium SCID mouse model is a valuable tool for use during preclinical research. We undertook this study to investigate the validity of this humanized mouse model using anti-tumor necrosis factor (anti-TNF) and anti-interleukin-1 (anti-IL-1) treatment and to investigate the direct effect of T cells- and B cell-related therapies on the transplanted RA synovial tissue. CB17/SCID mice were engrafted with human RA synovial tissue and systemically treated with anti-TNF, anti-IL-1, anti-IL-17, CTLA-4Ig, anti-CD20, or isotype control antibodies. Validation of the model with anti-TNF treatment significantly reduced serum cytokine levels and decreased histologic inflammation, whereas anti-IL-1 therapy did not show any effect on the RA synovial grafts. In mice engrafted with B cell-rich synovial tissue, anti-CD20 treatment showed clear therapeutic effects. Surprisingly, CTLA-4Ig treatment did not show any effects in this transplantation model, despite prescreening of the synovial tissue for the presence of CD3+ T cells and the costimulatory molecules CD80 and CD86. In contrast, great therapeutic potential was observed for anti-IL-17 treatment, but only when CD3+ T cells were abundantly present in the RA synovial tissue. This human RA synovium SCID mouse model enabled us to show that CTLA-4Ig lacks direct effects on T cell activation processes in the synovial tissue. Further evidence was obtained that IL-17 might indeed be an interesting therapeutic target in RA patients with CD3-rich synovial tissue. Further characterization of the RA patients' individual synovial profiles is of great importance for achieving tailored therapy.

Citrullination of fibrinogen: generation of neoepitopes and enhancement of immunostimulatory properties

Rheumatoid arthritis (RA) affects approximately 0.5% of the world population, yet the mechanisms underlying the development and progression of RA remain poorly understood. We are investigating the role of citrullinated fibrinogen as a pathogenic antigen in RA. Using arthritis antigen arrays we demonstrate that citrullinated fibrinogen is one of the earliest targets of the autoantibody response in RA, with autoantibodies against citrullinated fibrinogen appearing up to 10 years prior to the development of clinical arthritis. We further demonstrate that approximately 50% of CCP+ RA patients possess circulating immune complexes containing citrullinated fibrinogen, and that citrullinated fibrinogen containing immune complexes are deposited in human RA synovial tissues. To determine whether citrullinated fibrinogen can induce inflammatory arthritis in mice, we immunized mice with citrullinated fibrinogen and demonstrated that an inflammatory arthritis results and that both T cells and serum can transfer arthritis to naive mice. Fibrinogen is an endogenous ligand for the innate immune receptor TLR4, and to determine whether citrullination might alter the ability of fibrinogen to bind TLR4 we performed in vitro macrophage stimulation assays with native and citrullinated fibrinogen. We found that citrullinated fibrinogen was ten-fold more potent than native fibrinogen at stimulating macrophage TNF release. Further, macrophage derived from mice deficient for TLR4 or MyD88 did not produce TNF in response to citrullinated fibrinogen. Thus, our results demonstrate a novel mechanism by which anti-citrullinated protein antibodies (ACPA) specifically targeting citrullinated fibrinogen may directly stimulate macrophage TNF production, via co-ligation of TLR4 and Fc-gamma-R. Our findings demonstrate a role for citrullination both in creating neoantigens targeted by the adaptive immune response in RA as well as by increasing the potency of fibrinogen as an endogenous innate immune ligand. These results provide insights into the mechanisms by which anti-citrulline autoimmunity, and specifically the citrullination of fibrinogen, may contribute to both the onset and propagation of inflammation in RA.

Angiogenesis in rheumatoid arthritis: the role of fut 1

In rheumatoid arthritis (RA), targeting the vasculature may be beneficial to control the disease. Endothelial cells lining blood vessels are involved in a variety of functions in inflammation, including recruitment of leukocytes and cellular adhesion, antigen presentation, coagulation, cytokine production and angiogenesis. Angiogenesis, the growth of new vessels, is important for the proliferation of the rheumatoid synovial tissue pannus where these vessels also serve as a conduit for cells entering the inflamed synovium from the blood. We have shown before that the endothelial adhesion molecule E-selectin, in soluble form, mediates angiogenesis via its endothelial receptor sialyl Lewisx on adjacent endothelium [1]. We have used human RA synovial tissues to produce an antibody detecting related molecules, Lewisy/H-5-2, which are mainly known as blood group antigens but are also found on endothelium in select organs such as skin, lymph node and synovium, but not most other endothelium. This antigen is rapidly upregulated on endothelium in vitro in response to stimuli such as tumor necrosis factor-alpha, that is present in the RA joint. Additionally, this antigen is upregulated on RA vs. normal synovial endothelial cells, and in soluble form is upregulated in RA synovial fluid vs. osteoarthritic synovial fluid. In soluble form, Lewisy/H-5-2 mediates angiogenesis, cell adhesion via intercellular adhesion molecule-1, and monocyte recruitment. Fucosyl transferases (fut1 and fut2) are enzymes that control the synthesis of Lewisy/H-5-2. We have examined fut1 deficient mice to determine if fucosylation is important in angiogenesis and arthritis. Fut1 gene deficient mouse endothelial cells did not form endothelial sprouts on Matrigel in vitro to the same extent as wild type mouse endothelial cells. Moreover, the fut1 gene deficient mice were resistant to the development of angiogenesis in the Matrigel plug and sponge granuloma angiogenesis models in vivo. In terms of arthritis development, the Lewisy/H-5-2 gene deficient mice were resistant to development of K/BxN arthritis. Moreover, the harvested joints of these mice had decreased monocyte chemoattractant protein-1/CCL2 and interleukin-1 compared to wild type littermates, indicating that some inflammatory mediators were downregulated when fut1 was absent. These experiments suggest that futs may be important in the development of angiogenesis and inflammatory arthritis and that they may serve as novel targets in RA therapy.

T cell lessons from the RA synovium SCID mouse model: synovial tissue rich in CD3 T cells lacks response to CTLA4-Ig, but is successfully treated with anti-IL-17 antibodies

BackgroundTarget discovery and drug development is a very time consuming and expensive process, which is partly due to the fact that preclinical findings from animal models cannot always be translated...

REDUCTION OF INTERLEUKIN-1.BETA. EXPRESSION IN RHEUMATOID ARTHRITIS BY LINEAR POLARIZED NEAR INFRARED LIGHT IRRADIATION

Background: Rheumatoid arthritis (RA) is a systemic autoimmune disorder that involves inflammation and pain of joints. Low-level light therapy (LLLT) is being evaluated for treating RA. However, the molecular basis mechanism underlying the effectiveness of linear polarized near infrared light irradiation (LPIL) is unclear. It has been reported that interleukin 1β (IL-1β) plays a key role in the progression of RA.Aim: The objective of this study was to determine whether LPIL (Super Lizer™; SL) decrease gene expression and production IL-1β in human RA synovial cells, MH7A, and RA joint tissue of collagen-induced-rheumatoid arthritis (CIA) rats.Materials and Methods: IL-1β challenged MH7A and CIA rat joints were irradiated with SL. Total RNA was isolated from MH7A, and gene expression profiles were analyzed using DNA microarray.The mRNA levels were confirmed by reverse transcription-polymerase chain reaction (RT-PCR) and real-time PCR. The production of IL-1β in rat knee joints was analyzed by immunohistochemistry.Results: DNA microarray analysis showed that IL-1β increased IL-1β gene expression in MH7A, and SL significantly reduced its IL-1β mRNA level. The reduction of IL-1β mRNA level by SL was successfully confirmed by reverse RT-PCR and real-time PCR. SL reduced the swelling of CIA rat knee joints, and the immunohistochemical study demonstrated that a strong IL-1β staining in synovial membrane tissue of CIA rat joint, and the immuno-staining was significantly reduced by SL.Conclusion: Since autocrine IL-1β production has been identified to be an important proinflammatory cytokine in the pathogenesis of RA, the reduction of IL-1β expression in RA synovial cells is one of mechanisms in reduction of the inflammation in RA by SL.

Synovial tissues concentrate secreted APRIL

IntroductionA proliferation-inducing ligand (APRIL) from the TNF family, owing to its role in the generation and survival of plasma cells (PCs), is currently targeted for rheumatoid arthritis (RA) treatment. However, little is known about APRIL expression in RA lesions, hampering our understanding of the way APRIL may modulate this autoimmune disease.MethodsWe performed immunological staining of human normal, non-RA and RA synovial tissues with a pair of antibodies specifically recognizing APRIL-producing cells and secreted APRIL.ResultsWe detected significant amounts of secreted APRIL in normal synovium mostly concentrated around blood vessels and at the lining layer, but no cells producing APRIL. Meanwhile, we observed that blood neutrophils constitutively secrete APRIL, indicating that blood APRIL may diffuse into the synovium via its fenestrated vessels. Synovium from non-RA and RA patients retained similarly secreted APRIL, but in this case APRIL-producing cells, including neutrophils and macrophages, were present in the tissue. Notably, PCs - when present in RA synovium - accumulated in areas of APRIL retention, spreading from blood vessels towards the lining layer.ConclusionsPCs accumulate in synovial zones rich in secreted APRIL, consistent with a pro-survival role of APRIL for PCs in RA. The concentration of APRIL by normal synovium indicates that this tissue may constitute a proper environment for PCs even before RA onset.