Recruitment of CD16+ monocytes into synovial tissues is mediated by fractalkine and CX3CR1 in rheumatoid arthritis patients

Concentration and proteolysis of CX3CL1 may regulate the microglial response to CX3CL1

Fractalkine (FKN) is a membrane-bound chemokine that can be cleaved by proteases such as ADAM 10, ADAM 17, and cathepsin S to generate soluble fragments. Studies using different forms of the soluble FKN yield conflicting results in vivo. These observations prompted us to investigate the function and pharmacology of two commonly used isoforms of FKN, a human full-length soluble FKN (sFKN), and a human chemokine domain only FKN (cdFKN). Both are prevalent in the literature and are often assumed to be functionally equivalent. We observed that recombinant sFKN and cdFKN exhibit similar potencies in a cell-based cAMP assay, but binding affinity for CX3CR1 was modestly different. There was a 10-fold difference in potency between sFKN and cdFKN when assessing their ability to stimulate β-arrestin recruitment. Interestingly, high concentrations of FKN, regardless of cleavage variant, were ineffective at reducing pro-inflammatory microglial activation and may induce a pro-inflammatory response. This effect was observed in mouse and rat primary microglial cells as well as microglial cell lines. The inflammatory response was exacerbated in aged microglia, which is known to exhibit age-related inflammatory phenotypes. We observed the same effects in Cx3cr1-/- primary microglia and therefore speculate that an alternative FKN receptor may exist. Collectively, these data provide greater insights into the function and pharmacology of these common FKN reagents, which may clarify conflicting reports and urge greater caution in the selection of FKN peptides for use in in vitro and in vivo studies and the interpretation of results obtained using these differing peptides.

The monocyte-to-osteoclast transition in rheumatoid arthritis: Recent findings

Rheumatoid arthritis (RA) is an autoimmune disease characterized by joint inflammation leading to joint destruction and deformity. The crucial role of osteoclasts in the bone erosion in RA has been demonstrated. Deregulated osteoclastogenesis which is affected by environmental factors including the inflammatory state, as well as genetic and epigenetic factors, is one of hallmarks of RA pathogenesis. An enhanced-monocyte-to-osteoclast transition plays an important role in osteoclast upregulation in RA because under specific stimuli, circulating monocytes might migrate to a specific location in the bones and fuse with each other to become mature multinucleated osteoclasts. To understand the mechanism of bone damage in RA and to develop novel treatments targeting osteoclast upregulation, it is important to clarify our understanding of the monocyte-to-osteoclast transition in RA. Several potential targets which inhibit both inflammation and osteoclastogenesis, as well as regulators that affect the monocyte-to-osteoclast transition have been revealed by recent studies. Here, we review the factors affecting osteoclastogenesis in RA, summarize the anti-osteoclastogenic effects of current RA treatments, and identify promising therapeutic targets relating to both inflammation and osteoclastogenesis.

Serum Levels and in vitro CX3CL1 (Fractalkine), CXCL8, and IL-10 Synthesis in Phytohemaglutinin-Stimulated and Non-stimulated Peripheral Blood Mononuclear Cells in Subjects With Schizophrenia

INTRODUCTION

Although schizophrenia is a severe mental illness, whose etiology is still largely unknown, its pathogenesis may be associated with dysregulation of the immune mechanisms. The present study compares the levels of interleukin (IL)-10, interleukin-8 (CXCL8), and fractalkine (CX3CL1) between schizophrenia patients and healthy controls. It also assesses the ability of peripheral peripheral blood mononuclear cells (PBMCs) to produce these cytokines spontaneously and following mitogen-stimulation.

MATERIALS AND METHODS

A prospective study was performed of 60 adult schizophrenia patients and 32 controls. CXCL8, IL-10, and fractalkine concentrations were measured in serum and supernatants from cultured PBMCs. Anthropometric (BMI, WHR) and body composition measurements were taken using bioimpedance analysis (BIA) and dual-energy X-ray absorptiometry (DXA).

RESULTS AND CONCLUSION

The schizophrenia patients demonstrated significantly higher levels of serum CXCL8 (schizophrenia: 13.4 ± 15.7 pg/mL, control: 6.9 ± 4.2 pg/mL, p = 0.001) and lower level of serum fractalkine (schizophrenia: 22.8 ± 9.9 pg/mL, control: 45.4 ± 84.5 pg/mL, p = 0.041). Serum IL-10 levels did not significantly differ. No in vitro synthesis of fractalkine was observed. Neither unstimulated or PHA-stimulated CXCL8 secretion differed between the two groups (p >0.05). The patients not taking mood stabilizers (MS-) demonstrated significantly higher CXCL8 levels than those on mood stabilizers (MS+) (p = 0.03) and control (p < 0.001). In addition, the MS- sub-group demonstrated significantly lower serum fraktalkine than controls (p = 0.009). These effects could be described as pseudo-normalization of CXCL8 and fractalkine in schizophrenia patients taking mood stabilizers.

Monocytes, Macrophages, and Their Potential Niches in Synovial Joints - Therapeutic Targets in Post-Traumatic Osteoarthritis?

Synovial joints are complex structures that enable normal locomotion. Following injury, they undergo a series of changes, including a prevalent inflammatory response. This increases the risk for development of osteoarthritis (OA), the most common joint disorder. In healthy joints, macrophages are the predominant immune cells. They regulate bone turnover, constantly scavenge debris from the joint cavity and, together with synovial fibroblasts, form a protective barrier. Macrophages thus work in concert with the non-hematopoietic stroma. In turn, the stroma provides a scaffold as well as molecular signals for macrophage survival and functional imprinting: “a macrophage niche”. These intricate cellular interactions are susceptible to perturbations like those induced by joint injury. With this review, we explore how the concepts of local tissue niches apply to synovial joints. We introduce the joint micro-anatomy and cellular players, and discuss their potential interactions in healthy joints, with an emphasis on molecular cues underlying their crosstalk and relevance to joint functionality. We then consider how these interactions are perturbed by joint injury and how they may contribute to OA pathogenesis. We conclude by discussing how understanding these changes might help identify novel therapeutic avenues with the potential of restoring joint function and reducing post-traumatic OA risk.

Efficacy and Safety of E6011, an Anti-Fractalkine Monoclonal Antibody, in Patients With Active Rheumatoid Arthritis With Inadequate Response to Methotrexate: Results of a Randomized, Double-Blind, Placebo-Controlled Phase II Study

Objective

To evaluate the efficacy and safety of E6011, a humanized IgG2 monoclonal antibody against human fractalkine (FKN), in a phase II, double‐blind, placebo‐controlled study in rheumatoid arthritis (RA) patients.

Methods

Patients with moderate‐to‐severe RA who had an inadequate response to methotrexate were randomly assigned to a placebo group or to E6011 100‐mg, 200‐mg, or 400/200‐mg groups at a 2:1:2:2 ratio. During the 24‐week period, patients received the study drug subcutaneously at weeks 0, 1, and 2 and then once every 2 weeks. The primary end point was the American College of Rheumatology 20% improvement criteria (ACR20) response rate at week 12.

Results

Study drugs were administered to 190 patients (placebo, n = 54; E6011 100 mg, n = 28; E6011 200 mg, n = 54; E6011 400/200 mg, n = 54), and 169 patients completed treatment. A significant difference from placebo was not found in ACR20 response rates at week 12 (37.0% [placebo], 39.3% [100 mg], 48.1% [200 mg], and 46.3% [400/200 mg], using nonresponder imputation). As a secondary end point, ACR20 response rate in the 200‐mg and 400/200‐mg groups attained statistical significance at week 24 (35.2% [placebo], 39.3% [100 mg], 53.7% [200 mg], and 57.4% [400/200 mg]). Subsequent exploratory subgroup analysis revealed greater efficacy of E6011, particularly in patients with a higher baseline proportion of CD16+ monocytes; ACR20 response rates in this patient subgroup at week 24 were 30.0% (placebo), 46.7% (100 mg), 57.7% (200 mg), and 69.6% (400/200 mg). E6011 administered for 24 weeks was well tolerated.

Conclusion

This is the first evidence that E6011, a novel cell trafficking inhibitor targeting the FKN–CX₃CR1 interaction, is modestly effective with 24 weeks of treatment in RA patients, although the primary end point was not met.

[Fractalkine inhibits lipopolysaccharide-induced M1 polarization of macrophages by activating Wnt/β-catenin signaling pathway]

OBJECTIVE

To explore the mechanism by which fractalkine (CX3CL1; FKN) inhibits lipopolysaccharide (LPS)-induced immunological response in RAW264.7 cells.

METHODS

A RAW264.7 cell model overexpressing FKN was established by transfection with the lentiviral vector CX3CL1. The effects of LPS, ICG-001 (a Wnt/β-catenin signaling pathway inhibitor), either alone or in combination, on M1 polarization of na?ve and FKN-overexpressing RAW264.7 cells were evaluated by detecting of intereukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) using ELISA. The protein expressions of the inflammatory factors (iNOS, TNF-α, and IL-6), FKN, Wnt-4, and β-catenin were detected by Western blotting. The subcellular localization of IL-6 in the cells was detected by immunofluorescence assay.

RESULTS

The RAW264.7 cell model of FKN overexpression was successfully established. In na?ve RAW264.7 cells, treatment with both ICG-001 and LPS, as compared with LPS alone, significant promoted TNF-α and IL-6 secretions, increased intracellular levels of TNF-α, IL-6 and iNOS (P < 0.05), and reduced intracellular FKN, Wnt-4 and β-catenin levels (P < 0.01). In FKN-overexpressing RAW264.7 cells, LPS treatment significantly reduced the secretion of TNF-α and IL-6 and intracellular levels of TNF-α, IL-6 and iNOS (P < 0.01), increased intracellular FKN, Wnt-4 and β-catenin protein contents (P < 0.01), and inhibited IL-6 localization in the cytoplasm; compared with LPS, the combined treatment with ICG-001 and LPS obviously enhanced IL-6 localization in the cytoplasm of the cells.

CONCLUSIONS

FKN overexpression suppresses LPS-induced M1 type polarization of RAW264.7 cells by activating Wnt/β-catenin signaling pathway.

Comparative Analysis of the Occurrence and Role of CX3CL1 (Fractalkine) and Its Receptor CX3CR1 in Hemophilic Arthropathy and Osteoarthritis

Objective

Hemophilic arthropathy is characterized by recurrent bleeding episodes in patients with hemophilia leading to irreversible joint degeneration. The involvement of CX3CL1 (fractalkine) and its receptor CX3CR1 was observed in the pathogenesis of numerous arthritis-associated diseases. Taking this into account, we have presented a study investigating the role of the CX3CL1/CX3XR1 axis in the course of hemophilic arthropathy, including the CX3CL1-dependent expression of CD56⁺, CD68⁺, and CD31⁺ cells along with evaluation of articular cartilage and synovial membrane morphology.

Methods

The study was carried out using cases (n = 20) of end-stage hemophilic arthropathy with a severe type of hemophilia A and control cases (n = 20) diagnosed with osteoarthritis. The biofluids including blood serum and synovial fluid were obtained intraoperatively for the evaluation of CX3CL1 using the ELISA test. Tissue specimens including articular cartilage and synovial membrane were similarly collected during surgery and stained immunohistologically using selected antibodies including anti-CX3CR1, anti-CD56, anti-CD68, and anti-CD31. Additionally, the analysis included the assessment of articular cartilage, synovial membrane, and blood vessel morphology.

Results

In our study, we have documented increased average concentration of CX3CL1 in the blood serum of the study group (7.16 ± 0.53 ng/ml) compared to the control group (5.85 ± 0.70 ng/ml) without statistically significant difference in synovial fluid concentration at the same time. We have observed an increased macrophage presence with more marked proliferation and fibrosis of the synovial membrane in the study group. Remaining results such as expression of CX3CR1 presence of NK cells and larger surface area of blood vessels within the synovial membrane were noted also without statistical significance.

Conclusions

This study has demonstrated collective CX3CL1/CX3CR1 axis involvement in hemophilic arthropathy pathogenesis introducing new interesting diagnostics and a therapeutic target.

Cathepsin S activation contributes to elevated CX3CL1 (fractalkine) levels in tears of a Sjögren's syndrome murine model

Autoimmune dacryoadenitis and altered lacrimal gland (LG) secretion are features of Sjögren's syndrome (SS). Activity of cathepsin S (CTSS), a cysteine protease, is significantly and specifically increased in SS patient tears. The soluble chemokine, CX3CL1 (fractalkine), is cleaved from membrane-bound CX3CL1 by proteases including CTSS. We show that CX3CL1 is significantly elevated by 2.5-fold in tears (p = 0.0116) and 1.4-fold in LG acinar cells (LGAC)(p = 0.0026) from male NOD mice, a model of autoimmune dacryoadenitis in SS, relative to BALB/c controls. Primary mouse LGAC and human corneal epithelial cells (HCE-T cells) exposed to interferon-gamma, a cytokine elevated in SS, showed up to 9.6-fold (p ≤ 0.0001) and 25-fold (p ≤ 0.0001) increases in CX3CL1 gene expression, and 1.9-fold (p = 0.0005) and 196-fold (p ≤ 0.0001) increases in CX3CL1 protein expression, respectively. Moreover, exposure of HCE-T cells to recombinant human CTSS at activity equivalent to that in SS patient tears increased cellular CX3CL1 gene and protein expression by 2.8-fold (p = 0.0021) and 5.1-fold (p ≤ 0.0001), while increasing CX3CL1 in culture medium by 5.8-fold (p ≤ 0.0001). Flow cytometry demonstrated a 4.5-fold increase in CX3CR1-expressing immune cells (p ≤ 0.0001), including increased T-cells and macrophages, in LG from NOD mice relative to BALB/c. CTSS-mediated induction/cleavage of CX3CL1 may contribute to ocular surface and LG inflammation in SS.

The effects of human immunoglobulin G on enhancing tissue protection and neurobehavioral recovery after traumatic cervical spinal cord injury are mediated through the neurovascular unit

Background

Spinal cord injury (SCI) is a condition with few effective treatment options. The blood-spinal cord barrier consists of pericytes, astrocytes, and endothelial cells, which are collectively termed the neurovascular unit. These cells support spinal cord homeostasis by expressing tight junction proteins. Physical trauma to the spinal cord disrupts the barrier, which leads to neuroinflammation by facilitating immune cell migration to the damaged site in a process involving immune cell adhesion. Immunosuppressive strategies, including methylprednisolone (MPSS), have been investigated to treat SCI. However, despite some success, MPSS has the potential to increase a patient’s susceptibility to wound infection and impaired wound healing. Hence, immunomodulation may be a more attractive approach than immunosuppression. Approved for modulating neuroinflammation in certain disorders, including Guillain-Barre syndrome, intravenous administration of human immunoglobulin G (hIgG) has shown promise in the setting of experimental SCI, though the optimal dose and mechanism of action remain undetermined.

Methods

Female adult Wistar rats were subjected to moderate-severe clip compression injury (35 g) at the C7-T1 level and randomized to receive a single intravenous (IV) bolus of hIgG (0.02, 0.2, 0.4, 1, 2 g/kg), MPSS (0.03 g/kg), or control buffer at 15 min post-SCI. At 24 h and 6 weeks post-SCI, molecular, histological, and neurobehavioral effects of hIgG were analyzed.

Results

At 24 h post-injury, human immunoglobulin G co-localized with spinal cord pericytes, astrocytes, and vessels. hIgG (2 g/kg) protected the spinal cord neurovasculature after SCI by increasing tight junction protein expression and reducing inflammatory enzyme expression. Improvements in vascular integrity were associated with changes in spinal cord inflammation. Interestingly, hIgG (2 g/kg) increased serum expression of inflammatory cytokines and co-localized (without decreasing protein expression) with spinal cord vascular cell adhesion molecule-1, a protein used by immune cells to enter into inflamed tissue. Acute molecular benefits of hIgG (2 g/kg) led to greater tissue preservation, functional blood flow, and neurobehavioral recovery at 6 weeks post-SCI. Importantly, the effects of hIgG (2 g/kg) were superior to control buffer and hIgG (0.4 g/kg), and comparable with MPSS (0.03 g/kg).

Conclusions

hIgG (2 g/kg) is a promising therapeutic approach to mitigate secondary pathology in SCI through antagonizing immune cell infiltration at the level of the neurovascular unit.

Indispensable Role of CX3CR1+ Dendritic Cells in Regulation of Virus-Induced Neuroinflammation Through Rapid Development of Antiviral Immunity in Peripheral Lymphoid Tissues

A coordinated host immune response mediated via chemokine network plays a crucial role in boosting defense mechanisms against pathogenic infections. The speed of Ag presentation and delivery by CD11c⁺ dendritic cells (DCs) to cognate T cells in lymphoid tissues may decide the pathological severity of the infection. Here, we investigated the role of CX₃CR1 in the neuroinflammation induced by infection with Japanese encephalitis virus (JEV), a neurotrophic virus. Interestingly, CX₃CR1 deficiency strongly enhanced susceptibility to JEV only after peripheral inoculation via footpad. By contrast, both CX₃CR1^+/+ and CX₃CR1^-/- mice showed comparable susceptibility to JEV following inoculation via intranasal and intraperitoneal routes. CX₃CR1^-/- mice exhibited lethal neuroinflammation after JEV inoculation via footpad route, showing high mortality, morbidity, pro-inflammatory cytokine expression, and uncontrolled CNS-infiltration of peripheral leukocytes including Ly-6C^hi monocytes and Ly-6G^hi granulocytes. Furthermore, the absence of CX₃CR1⁺CD11c⁺ DCs appeared to enhance susceptibility of CX₃CR1^-/- mice to JE after peripheral JEV inoculation. CX₃CR1 ablation impaired the migration of CX₃CR1⁺CD11c⁺ DCs from JEV-inoculated sites to draining lymph nodes (dLNs), resulting in decreased NK cell activation and JEV-specific CD4⁺/CD8⁺ T-cell responses. However, CX₃CR1-competent mice showed rapid temporal expression of viral Ags in dLNs. Subsequently, JEV was rapidly cleared, with concomitant generation of antiviral NK cell activation and T-cell responses mediated by rapid migration of JEV Ag⁺CX₃CR1⁺CD11c⁺ DCs. Using biallelic functional CX₃CR1 expression system, the functional expression of CX₃CR1 on CD11c^hi DCs appeared to be essentially required for inducing rapid and effective responses of NK cell activation and Ag-specific CD4⁺ T cells in dLNs. Strikingly, adoptive transfer of CX₃CR1⁺CD11c⁺ DCs was found to completely restore the resistance of CX₃CR1^-/- recipients to JEV, as corroborated by the rapid delivery of JEV Ags in dLNs and attenuation of neuroinflammation in the CNS. Collectively, these results indicate that CX₃CR1⁺CD11c⁺ DCs play an important role in generating rapid and effective responses of antiviral NK cell activation and Ag-specific T cells after peripheral inoculation with the virus, thereby resulting in conferring resistance to viral infection by reducing the peripheral viral burden.

Differences of the Structure of Immune Regulatory Cell Populations between Cellular Material from Sonographically Detected Focal Thyroid Lesions and Peripheral Blood in Humans

Focal thyroid lesions are common ultrasound findings with the estimated prevalence up to 67% of the population. They form characteristically enveloped regions with individual encapsulated microenvironment that may involve the specific distribution of immune system compounds—especially antigen presenting cells (APC). We analyzed and compared the most potent APC—plasmacytoid and conventional dendritic cells (DCs) subpopulations and three monocyte subpopulations as well as other immune cells—in peripheral blood and local blood of thyroid gland obtained parallelly in patients with focal thyroid lesions using flow cytometry. The analysis revealed significant differences in the distribution of main subsets of assessed cells between peripheral blood and biopsy material. The results support the existence of local, organ-specific immune reaction control networks within thyroid nodules.

Shexiang-Wulong Pills Attenuate Rheumatoid Arthritis by Alleviating Inflammation in a Mouse Model of Collagen-Induced Arthritis

Shexiang-Wulong Pill (SWP) is derived from “Moschus Yuan,” first formulated during the Song Dynasty for the treatment of joint pain. The aim of this study was to evaluate the therapeutic effect of SWP in a mouse model of collagen-induced arthritis (CIA). Forty-five DBA/1 mice were randomly divided into control group, model group, and SWP-treated group. SWP was administered by oral gavage for 22 days after the booster immunization. The clinical arthritic scores and joint histopathology, including synovial hyperplasia and hypoxic regions, cartilage erosion, and bone destruction, were evaluated. Microcomputed tomography (micro-CT) was used to assess microstructural changes in the bone. Serum levels of TNF-a, IL-6, and IFN-γ were measured by enzyme-linked immunosorbent assay (ELISA). The results showed a statistically significant improvement in joint pathological changes in the SWP-treated group. Imaging assessment confirmed that SWP protected the bone tissue from CIA-induced erosion and increased the bone density. In addition, the serum levels of TNF-a, IL-6, and IFN-γ in SWP-treated mice were significantly lower than those in the model group (P<0.05). Taken together, Shexiang-Wulong Pill can effectively alleviate joint swelling in CIA mice, inhibit synovial tissue hyperplasia, reduce inflammatory cell infiltration, and delay bone destruction. These results indicate that Shexiang-Wulong Pills could be an efficient medication for the treatment of RA.

Tissue-specific Role of CX3CR1 Expressing Immune Cells and Their Relationships with Human Disease

Chemokine (C-X3-C motif) ligand 1 (CX₃CL1, also known as fractalkine) and its receptor chemokine (C-X3-C motif) receptor 1 (CX₃CR1) are widely expressed in immune cells and non-immune cells throughout organisms. However, their expression is mostly cell type-specific in each tissue. CX₃CR1 expression can be found in monocytes, macrophages, dendritic cells, T cells, and natural killer (NK) cells. Interaction between CX₃CL1 and CX₃CR1 can mediate chemotaxis of immune cells according to concentration gradient of ligands. CX₃CR1 expressing immune cells have a main role in either pro-inflammatory or anti-inflammatory response depending on environmental condition. In a given tissue such as bone marrow, brain, lung, liver, gut, and cancer, CX₃CR1 expressing cells can maintain tissue homeostasis. Under pathologic conditions, however, CX₃CR1 expressing cells can play a critical role in disease pathogenesis. Here, we discuss recent progresses of CX₃CL1/CX₃CR1 in major tissues and their relationships with human diseases.

CX3CL1 promotes MMP-3 production via the CX3CR1, c-Raf, MEK, ERK, and NF-κB signaling pathway in osteoarthritis synovial fibroblasts

Background

Osteoarthritis (OA) is a degenerative joint disease that affects the cartilage, synovium, and subchondral bone and is the leading cause of disability in older populations. Specific diagnostic biomarkers are lacking; hence, treatment options for OA are limited. Synovial inflammation is very common in OA joints and has been associated with both OA’s symptoms and pathogenesis. Confirming the role of the synovium in OA pathogenesis is a promising strategy for mitigating the symptoms and progression of OA. CX3CL1 is the only member of the CX3C class of chemokines that combines the properties of chemoattractants and adhesion molecules. CX3CL1 levels in the synovium and serum were both discovered to be positively associated with OA pathogenesis. CX3CL1 and its receptor CX3CR1 belong to a family of G protein-coupled receptors. Matrix metalloproteinases (MMPs), which are responsible for matrix degradation, play a crucial role in OA progression. The relationship between CX3CL1 and MMPs in the pathophysiology of OA is still unclear.

Methods

CX3CL1-induced MMP-3 production was assessed with quantitative real-time PCR and ELISA. The mechanisms of action of CX3CL1 in different signaling pathways were studied using western blot analysis, quantitative real-time PCR and ELISA. Neutralization antibodies of integrin were achieved to block the CX3CR1 signaling pathway. Luciferase assays were used to study NF-κB promoter activity.

Results

We investigated the signaling pathway involved in CX3CL1-induced MMP-3 production in osteoarthritis synovial fibroblasts (OASFs). CX3CL1 was found to induce MMP-3 production in a concentration-dependent and time-dependent manner. Using pharmacological inhibitors and CX3CR1 small interfering RNA to block CX3CR1 revealed that the CX3CR1 receptor was involved in the CX3CL1-mediated upregulation of MMP-3. CX3CL1-mediated MMP-3 production was attenuated by c-Raf inhibitors (GW5074) and MEK/ERK inhibitors (PD98059 and U0126). The OASFs were stimulated using CX3CL1-activated p65 phosphorylation.

Conclusions

Our results demonstrate that CX3CL1 activates c-Raf, MEK, ERK, and NF-κB on the MMP-3 promoter through CX3CR1, thus contributing to cartilage destruction during OA.

Involvement of Monocyte Subsets in the Immunopathology of Giant Cell Arteritis

Monocytes/macrophages are critical in systemic and local inflammation in giant cell arteritis (GCA) and possibly in clinically overlapping polymyalgia rheumatica (PMR). Therefore, we aimed to understand the contribution of monocyte subsets and the CX3CR1-CX3CL1 and CCR2-CCL2 migratory pathways, to the pathology of GCA. Peripheral blood monocytes were enumerated in samples from newly-diagnosed, untreated GCA and PMR patients and after prednisone-induced remission. The distribution of classical (CD14^brightCD16^neg) and the more pro-inflammatory, intermediate (CD14^brightCD16+) and non-classical (CD14^dimCD16+) monocyte subsets was analysed by flow cytometry. The phenotype of macrophages in temporal artery biopsies (TABs) from GCA patients was studied by immunohistochemistry and immunofluorescence. A clear monocytosis was seen in newly diagnosed GCA and PMR patients caused by elevated numbers of classical monocytes. Prednisone treatment suppressed numbers of non-classical monocytes. Both chemokine CX3CL1 and CCL2 were highly expressed in the TAB. Most macrophages in the TAB of GCA patients expressed non-classical monocyte markers CD16 and CX3CR1 whereas co-localisation of CD16 with classical monocyte marker CCR2 was infrequent. In conclusion, we report an altered distribution of monocyte subsets in both GCA and PMR patients. The majority of macrophages in TABs of GCA patients were CD68 + CD16 + CX3CR1 + CCR2- and thereby resembled the phenotype of non-classical monocytes.

Inflammatory Cell Migration in Rheumatoid Arthritis: A Comprehensive Review

Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disease that primarily affects the joints. Self-reactive B and T lymphocytes cooperate to promote antibody responses against self proteins and are major drivers of disease. T lymphocytes also promote RA independently of B lymphocytes mainly through the production of key inflammatory cytokines, such as IL-17, that promote pathology. While the innate signals that initiate self-reactive adaptive immune responses are poorly understood, the disease is predominantly caused by inflammatory cellular infiltration and accumulation in articular tissues, and by bone erosions driven by bone-resorbing osteoclasts. Osteoclasts are giant multinucleated cells formed by the fusion of multiple myeloid cells that require short-range signals, such as the cytokines MCSF and RANKL, for undergoing differentiation. The recruitment and positioning of osteoclast precursors to sites of osteoclast differentiation by chemoattractants is an important point of control for osteoclastogenesis and bone resorption. Recently, the GPCR EBI2 and its oxysterol ligand 7a, 25 dihydroxycholesterol, were identified as important regulators of osteoclast precursor positioning in proximity to bone surfaces and of osteoclast differentiation under homeostasis. In chronic inflammatory diseases like RA, osteoclast differentiation is also driven by inflammatory cytokines such as TNFa and IL-1, and can occur independently of RANKL. Finally, there is growing evidence that the chemotactic signals guiding osteoclast precursors to inflamed articular sites contribute to disease and are of great interest. Furthering our understanding of the complex osteoimmune cell interactions should provide new avenues of therapeutic intervention for RA.

CCN1/CYR61-mediated meticulous patrolling by Ly6Clow monocytes fuels vascular inflammation

Inflammation is characterized by the recruitment of leukocytes from the bloodstream. The rapid arrival of neutrophils is followed by a wave of inflammatory lymphocyte antigen 6 complex (Ly6C)-positive monocytes. In contrast Ly6C(low) monocytes survey the endothelium in the steady state, but their role in inflammation is still unclear. Here, using confocal intravital microscopy, we show that upon Toll-like receptor 7/8 (TLR7/8)-mediated inflammation of mesenteric veins, platelet activation drives the rapid mobilization of Ly6C(low) monocytes to the luminal side of the endothelium. After repeatedly interacting with platelets, Ly6C(low) monocytes commit to a meticulous patrolling of the endothelial wall and orchestrate the subsequent arrival and extravasation of neutrophils through the production of proinflammatory cytokines and chemokines. At a molecular level, we show that cysteine-rich protein 61 (CYR61)/CYR61 connective tissue growth factor nephroblastoma overexpressed 1 (CCN1) protein is released by activated platelets and enables the recruitment of Ly6C(low) monocytes upon vascular inflammation. In addition endothelium-bound CCN1 sustains the adequate patrolling of Ly6C(low) monocytes both in the steady state and under inflammatory conditions. Blocking CCN1 or platelets with specific antibodies impaired the early arrival of Ly6C(low) monocytes and abolished the recruitment of neutrophils. These results refine the leukocyte recruitment cascade model by introducing endothelium-bound CCN1 as an inflammation mediator and by demonstrating a role for platelets and patrolling Ly6C(low) monocytes in acute vascular inflammation.

Therapeutic intervention of inflammatory/immune diseases by inhibition of the fractalkine (CX3CL1)-CX3CR1 pathway

Inflammatory and immune responses are generated locally by the selective invasion and accumulation of the immune cells into the lesion site. The infiltration process of the immune cells into the tissue from the blood through the vascular endothelial cells is closely regulated by a number of chemotactic factors and cell adhesion molecules.

Fractalkine (FKN)/CX3CL1 is a membrane-bound chemokine possessing a chemokine/mucin hybrid structure and a transmembrane domain and has a dual function as an adhesion molecule and a chemoattractant. FKN is mainly expressed on activated endothelial cells, activated fibroblasts, and osteoblasts. Its receptor, CX3CR1, is expressed on cytotoxic effector lymphocytes, monocytes/macrophages, and osteoclasts. To date, a lot of key functional aspects of the FKN-CX3CR1 axis has been identified: (1) the rapid capture and firm adhesion of immune cells to vascular endothelial cells, (2) chemotaxis, (3) the enhancement of the transmigration to other chemokines, (4) the crawling behavior of the monocytes that patrol on vascular endothelial cells, (5) the retention of monocytes as the accessory cells of the inflamed endothelium to recruit inflammatory cells, and (6) the survival of the macrophage.

In this review, we will focus on the pathological role of FKN in rheumatoid arthritis (RA) and the physiological role of FKN on osteoclast differentiation. Furthermore, we will discuss the therapeutic potential of anti-FKN mAb for RA patients and its distinct mode of action from other cytokine inhibitors.

The story of CD4+ CD28- T cells revisited: solved or still ongoing?

CD4⁺CD28⁻ T cells are a unique type of proinflammatory T cells characterised by blockade of costimulatory CD28 receptor expression at the transcriptional level, which is still reversible by IL-12. In healthy individuals older than 65 years, these cells may accumulate to up to 50% of total CD4⁺ T lymphocytes as in many immune-mediated diseases, immunodeficiency, and specific infectious diseases. Here we focus on CD4⁺CD28⁻ T cells in chronic immune-mediated diseases, summarizing various phenotypic and functional characteristics, which vary depending on the underlying disease, disease activity, and concurrent treatment. CD4⁺CD28⁻ T cells present as effector/memory cells with increased replicative history and oligoclonality but reduced apoptosis. As an alternative costimulatory signal instead of CD28, not only natural killer cell receptors and Toll-like receptors, but also CD47, CTLA-4, OX40, and 4-1BB have to be considered. The proinflammatory and cytotoxic capacities of these cells indicate an involvement in progression and maintenance of chronic immune-mediated disease. So far it has been shown that treatment with TNF-α blockers, abatacept, statins, and polyclonal antilymphocyte globulins (ATG) mediates reduction of the CD4⁺CD28⁻ T cell level. The clinical relevance of targeting CD4⁺CD28⁻ T cells as a therapeutic option has not been examined so far.

Surface APRIL Is Elevated on Myeloid Cells and Is Associated with Disease Activity in Patients with Rheumatoid Arthritis

OBJECTIVE

To assess surface APRIL (a proliferation-inducing ligand; CD256) expression by circulating myeloid cells in rheumatoid arthritis (RA) and to determine its relationship to disease activity.

METHODS

Peripheral blood mononuclear cells (PBMC) and plasma were obtained from patients with RA and healthy donors. PBMC were stained for flow cytometry to detect surface APRIL and blood cell markers to identify circulating myeloid cell subsets. Based on CD14 and CD16 phenotypes, monocyte subsets described as classical (CD14+CD16-), intermediate (CD14+CD16+), and nonclassical (CD14loCD16+) were identified. Levels of surface APRIL expression were measured by flow cytometry and median fluorescence intensity was used for comparisons. Levels of soluble APRIL in the plasma were determined by ELISA. Disease activity was measured by the Disease Activity Score in 28 joints.

RESULTS

In patients with RA, total myeloid cells showed expression of surface APRIL that correlated with disease activity and with plasma APRIL levels observed in these patients. In healthy donors, classical monocytes were composed of > 80% of circulating monocytes. However, in patients with RA, the intermediate and nonclassical subsets were elevated and made up the majority of circulating monocytes. In contrast to healthy donors, where high levels of surface APRIL were only observed in nonclassical monocytes, patients with RA showed high levels of surface APRIL expression by all circulating monocyte subsets.

CONCLUSION

Surface APRIL is elevated in circulating myeloid cells in patients with RA where it is highly correlated with disease activity. Patients with RA also showed skewing of monocytes toward subsets associated with secretion of tumor necrosis factor-α and/or interleukin 1β.

Differential expression of the fractalkine chemokine receptor (CX3CR1) in human monocytes during differentiation

Circulating monocytes (Mos) may continuously repopulate macrophage (MAC) or dendritic cell (DC) populations to maintain homeostasis. MACs and DCs are specialized cells that play different and complementary immunological functions. Accordingly, they present distinct migratory properties. Specifically, whereas MACs largely remain in tissues, DCs are capable of migrating from peripheral tissues to lymphoid organs. The aim of this work was to analyze the expression of the fractalkine receptor (CX3CR1) during the monocytic differentiation process. Freshly isolated Mos express high levels of both CX3CR1 mRNA and protein. During the Mo differentiation process, CX3CR1 is downregulated in both DCs and MACs. However, MACs showed significantly higher CX3CR1 expression levels than did DC. We also observed an antagonistic CX3CR1 regulation by interferon (IFN)-γ and interleukin (IL)-4 during MAC activation through the classical and alternative MAC pathways, respectively. IFN-γ inhibited the loss of CX3CR1, but IL-4 induced it. Additionally, we demonstrated an association between CX3CR1 expression and apoptosis prevention by soluble fractalkine (sCX3CL1) in Mos, DCs and MACs. This is the first report demonstrating sequential and differential CX3CR1 modulation during Mo differentiation. Most importantly, we demonstrated a functional link between CX3CR1 expression and cell survival in the presence of sCX3CL1.

The chemokine CX3CL1 (fractalkine) and its receptor CX3CR1: occurrence and potential role in osteoarthritis

Chemokines are molecules able to induce chemotaxis of monocytes, neutrophils, eosinophils, lymphocytes and fibroblasts. The complex chemokine acts in many physiological and pathological phenomena, including those occurring in the articular cartilage. To date, chemokine CX3CL1 (fractalkine) is the only member of the CX3C class of chemokines with well-documented roles in endothelial cells. CX3CL1 is a unique chemokine that combines properties of chemoattractant and adhesion molecule. The main roles of CX3CL1 include promotion of leukocyte binding and adhesion as well as activation of the target cells. The soluble chemokine domain of CX3CL1 is chemotactic for T cells and monocytes. CX3CL1 acts via its receptor, CX3CR1, which belongs to a family of G protein-coupled receptors. Stimulation of CX3CR1 activates both CX3CL1-dependent and integrin-dependent migrations of cells with synergistically augmented adhesion. Genetic polymorphisms of CX3CR1 may significantly modify the biological roles of CX3CL1, especially in pathologic conditions. Osteoarthritis (OA) is the most common joint disease, affecting approximately 7–8 % of the general population. Development of OA is largely driven by low-grade local background inflammation involving chemokines. The importance of CX3CL1/CX3CR1 signalling in the pathophysiology of OA is still under investigation. This paper, based on a review of the literature, updates and summarises the current knowledge about CX3CL1/CX3CR1 in OA and indicates possible interactions with a potential for therapeutic targeting.

CD16 (FcRgammaIII) as a potential marker of osteoclast precursors in psoriatic arthritis

Introduction

Psoriatic arthritis (PsA) is a chronic inflammatory arthritis characterized by bone erosion mediated by osteoclasts (OC). Our previous studies showed an elevated frequency of OC precursors (OCP) in PsA patients. Here, we examined if OC arise from CD16-positive monocytes in PsA.

Methods

Peripheral blood mononuclear cells (PBMC) or monocytes were isolated from human peripheral blood and sorted based on CD16 expression. Sorted cells were cultured alone or with bone wafers in the presence of receptor activator of nuclear factor kappa-B ligand (RANKL) and macrophage colony-stimulating factor (M-CSF). Enumeration and bone erosion activity of OC were examined after culture. The effects of tumor necrosis factor-alpha (TNFα), OC-promoting (M-CSF plus RANKL), and dendritic cell (DC)-promoting (GM-CSF plus interleukin (IL)-4) cytokines on CD16 surface expression were examined by flow cytometry.

Results

PsA and psoriasis (Ps) subjects had a higher percentage of circulating inflammatory CD14+CD16+ cells than healthy controls (HC). Exposure of cells to OC-promoting, but not DC-promoting media, was associated with CD16 up-regulation. PBMC of Ps and PsA had a higher frequency of cells expressing intermediate levels of CD16. OC were mainly derived from CD16+ cells in PsA. Increased CD16 expression was associated with a higher bone erosion activity in PsA.

Conclusions

An increased frequency of circulating CD14+CD16+ cells was noted in PsA compared to controls, and intermediate levels of CD16 may suggest a transitional state of OCP during osteoclastogenesis. Intriguingly, TNFα blocked CD16 expression on a subset of CD14+ monocytes. Collectively, our data suggest that CD16 has the potential to serve as an OCP marker in inflammatory arthritis.

CX3CL1 up-regulation is associated with recruitment of CX3CR1+ mononuclear phagocytes and T lymphocytes in the lungs during cigarette smoke-induced emphysema

CX3CR1 is expressed on monocytes, dendritic cells, macrophages, subsets of T lymphocytes, and natural killer cells and functions in diverse capacities such as leukocyte adhesion, migration, and cell survival on ligand binding. Expression of the CX3CL1 gene, whose expression product is the sole ligand for CX3CR1, is up-regulated in human lungs with chronic cigarette smoke-induced obstructive lung disease. At present, it is unknown whether CX3CL1 up-regulation is associated with the recruitment and accumulation of immune cells that express CX3CR1. We show that mice chronically exposed to cigarette smoke up-regulate CX3CL1 gene expression, which is associated with an influx of CX3CR1+ cells in the lungs. The increase in CX3CR1+ cells is primarily comprised of macrophages and T lymphocytes and is associated with the development of emphysema. In alveolar macrophages, cigarette smoke exposure increased the expression of both CX3CR1 and CX3CL1 genes. The inducibility of CX3CR1 expression was not solely dependent on a chronic stimulus because lipopolysaccharide up-regulated CX3CR1 in RAW264.7 cells in vitro and in mononuclear phagocytes in vivo. Our findings suggest a mechanism by which macrophages amplify and promote CX3CR1+ cell accumulation within the lungs during both acute and chronic inflammatory stress. We suggest that one function of the CX3CR1-CX3CL1 pathway is to recruit and sustain divergent immune cell populations implicated in the pathogenesis of cigarette smoke-induced emphysema.

17beta-Estradiol utilizes the estrogen receptor to regulate CD16 expression in monocytes

Estrogen can significantly influence CD16 expression and alter monocytic cytokine release upon CD16 receptor activation. However, the function of the estrogen receptor (ER) alpha and beta in this response is unclear. To test whether estrogen binds ERalpha and/or ERbeta to affect CD16 expression, monocytic cells were treated with and without physiological levels of 17beta-estradiol and various doses of the ERalpha and ERbeta antagonist fulvestrant followed by measurement of CD16 transcript levels. To determine how estrogen induced changes in TNF-alpha and IL-1beta release due to CD16 activation we quantitated the amount of cytokines after treatment with estrogen, fulvestrant and antibodies that specifically bind and activate the CD16 receptor. Interaction of ERalpha and the CD16 promoter was then determined by chromatin immunoprecipitation. Furthermore, specific promoter elements utilized by estrogen to control CD16 expression were mutated and expression from a luciferase reporter quantitated after transfection. Using the luciferase reporter construct containing a wild type CD16 promoter, the role of ERalpha and ERbeta in the estrogen response was tested by treating transfected monocytes with an ERalpha specific agonist or an ERbeta specific agonist and measuring expression. Our results show that CD16 transcript levels significantly decreased in monocytic cells due to estrogen and that the observed decrease in message was blocked by the antagonist fulvestrant. Estrogen reduced CD16 expression and decreased TNF-alpha and IL-1beta release upon CD16 activation but the administration of fulvestrant blocked this decrease. ERalpha was found to interact with a region 5' of the CD16 gene in the presence of estrogen, and site-directed mutational analysis of this region indicated the necessity for an estrogen response element in modulating estrogen effects on CD16 expression. Moreover, both an ERalpha and an ERbeta agonist reduced expression of the CD16 reporter construct suggesting both receptors can play a role in CD16 regulation. In conclusion, CD16 expression can be altered by the activity of ERalpha or ERbeta and our results also show that ERalpha can associate with a region within the CD16 promoter that is important in production of transcript.