Abstract

1. Introduction

Egress of leukocytes from the circulation into the extravascular space is a fundamental component of inflammation. Leukocyte recruitment from blood into tissues follows a well-established paradigm: (i) tethering and rolling on the vessel wall, (ii) firm arrest on the endothelium, (iii) spreading out and crawling in all directions on the vessel, and transendothelial migration to extravasate into tissue. This process is tightly controlled by adhesion molecules and chemoattractants including chemokines ⁽¹⁾.

Chemokines are chemotactic cytokines that regulate the migration of a wide variety of immune cells ⁽²⁾. In the context of the immune system, chemokines control the positioning of all immune cells and contribute to immune cell development, homeostasis and inflammation. More than 50 chemokines and 19 chemokine receptors have been identified in humans and mice (Table 1) ⁽²⁾. These small (8-10 kDa) secreted proteins are classified into four groups on the location of cysteine residues near the N terminus of their primary amino acid sequence: XC-chemokines contain a single N-terminal cysteine, CC-chemokines have two adjacent residues, CXC-chemokines have two cysteines separated by one other amino acid, and CX₃C-chemokines have three amino acids between the cysteine residues ⁽²⁾. Most chemokines are secreted into the extracellular space where they bind heparin-like glycosaminoglycans on the cell surface and are embedded in the extracellular matrix, forming transient or stable gradients. Chemokines and their gradients are detected by binding to specific chemokine receptors. Chemokine receptors are seven-transmembrane receptors and are expressed on the surface of immune cells. These receptors are G-protein coupled receptors that signal via Gi-type G proteins. There are approximately 20 classical chemokine receptors coupled to G proteins and regulate cell migration, whereas 5 atypical chemokine receptors that do not couple to G proteins and do not induce cell migration have been identified (Table 2) ⁽²⁾.

The chemokine system and other inflammatory mediators might play a central role in chronic inflammation such as autoimmune diseases. In fact, high levels of several chemokines are seen in the blood and disease tissues, such as the joints and blood vessels, in patients with rheumatoid arthritis (RA) ⁽³⁾ and vasculitis ⁽⁴⁾, and their levels have been associated with the disease severity and/or activity. Chemokines and their receptors likely contribute to the recruitment of immune cells into the affected tissue in RA and vasculitis and are also thought to contribute to the activation of leukocytes once in the tissue, resulting in integrin activation and the production of proteases and inflammatory mediators ⁽⁵⁾. Thus, the control of leukocyte entry into the tissue represents a major point at which new therapeutics, such as blockade of the chemokine system, could be developed to attenuate inflammation in RA and vasculitis.

In this review, we summarize the pathogenic roles of chemokines and their receptors in RA and vasculitis. Additionally, we provide an update on the clinical trials of chemokine- and chemokine-receptor-targeting drugs in arthritis and vasculitis and discuss their potential as therapeutic targets.

2. Rheumatoid Arthritis

Inflammatory arthritis, including rheumatoid arthritis (RA), is characterized by immune cell infiltration into joints, resulting in pannus formation with associated cartilage and bone destruction. This immune cell infiltrate drives RA pathogenesis and is composed of multiple cell types, including lymphocytes, macrophages, and neutrophils. Approximately 1 million people in Japan have RA. If not effectively treated, RA causes irreversible joint destruction resulting in significant disability, increased risk of cardiovascular disease, and markedly increased medical costs. Biological therapies, such as TNF inhibitors and IL-6 inhibitors, have revolutionized the treatment of RA. However, ~50% of RA patients still do not respond to these agents, and patients who do respond often have a residual disease and are at an increased risk of infection, such as pneumonia, associated with their use ⁽⁶⁾. Therefore, there remains an ongoing need to identify new therapeutic targets and treatment strategies for RA.

Animal models of inflammatory arthritis have provided valuable research tools for understanding the pathogenic mechanism of RA and for studying therapeutic targets. In fact, several animal models of inflammatory arthritis, such as the K/BxN arthritogenic serum transfer model of arthritis ⁽⁷⁾, the type II collagen-induced arthritis (CIA) ⁽⁸⁾ and the collagen antibody-induced arthritis (CAIA) ⁽⁹⁾, have a number of similarities to RA, including synovial hyperplasia, mononuclear cell infiltration and bone destruction. Therefore, these animal models have been used to define the role of chemokines and their receptors in inflammatory arthritis. In this section, we describe the functional role of chemokines and their receptors in animal models and in human RA and discuss therapies targeting these molecules.

2.1 Chemokine in RA

Numerous studies have demonstrated that multiple chemokines are highly expressed in serum, synovial fluid and synovial tissue of patients with RA, compared to healthy controls. For instance, the CXC-chemokines (CXCL5, CXCL8, CXCL9, CXCL10, CXCL12, CXCL13 and CXCL16), the CC-chemokines (CCL2, CCL3, CCL4, CCL5, CCL18, CCL19, CCL20, CCL21, CCL25), and the CX₃C-chemokine CX₃CL1 are increased in serum, synovial fluid, and synovial tissue of RA patients, compared with healthy controls ^{(10), (11), (12), (13), (14), (15)}. In the pathogenesis of human RA, CXC-chemokines promote neutrophils (CXCL1, CXCL2, CXCL5 and CXCL8) ⁽¹⁶⁾, the effector T cell (CXCL10) ⁽¹⁷⁾ and the B cell (CXCL13) ^{(18), (19)} recruitment into the joint. CC-chemokines induce monocytes (CCL2, CCL3, CCL4, CCL5 and CCL7) ^{(20), (21)}, T cells (CCL18, CCL19, CCL20, CCL21 and CCL25) ⁽²²⁾ and the B cell (CCL20) ⁽²³⁾ entry into the inflamed joint, whereas CX₃CL1 induces monocyte recruitment ⁽²⁴⁾.

In addition, CXCL8, CXCL12 and CXCL16 induce angiogenesis in synovial tissue in RA, and CXCL8, CXCL10 and CXCL13 are promising biomarkers of RA disease activity/severity ^{(11), (25)}. Synovial macrophages and fibroblast-like synoviocytes (FLSs) might be the main producers of many inflammatory chemokines in the joints of RA patients, whereas synovial endothelial cells mainly produce CX₃CL1 and some CC-chemokines in RA patients. However, the pathogenic roles of chemokines in human RA remain unclear.

These same chemokines are also elevated in the serum, synovial fluid and synovial tissue in animal models of inflammatory arthritis, compared with control animals ^{(20), (21), (22), (24), (26), (27), (28), (29), (30), (31), (32)}. Thus, animal models of arthritis are available to dissect the pathogenic roles of chemokines in inflammatory arthritis. Several studies using the animal models have indicated the development of arthritis in RA: (i) immune complexes stimulate synovial macrophages to generate CXCL1 that initiates neutrophil recruitment into the joints in the early phase of animal models of arthritis ⁽³³⁾; (ii) after neutrophils enter the inflamed tissue, they become activated and produce CXCL2, which contributes to the recruitment of additional neutrophils in a positive-feedback loop ^{(26), (33)}; (iii) neutrophils can also amplify the local response by secreting inflammatory cytokines, such as IL-1, that induce chemokine production by FLSs in the joint ⁽²⁶⁾; (iv) In later phases of the disease, FLSs and other cells might play important roles in recruiting T and B cells (Figure 1).

Open in a separate window

Figure 1.

Chemokines and chemokine receptors in rheumatoid arthritis.

①Neutrophil recruitment: Synovial macrophages generate CXCL1 and CXCL8, which promote neutrophil recruitment into the joint. Once neutrophils enter into the joint, they become activated and produce CXCL2, which amplifies neutrophil recruitment and stimulates additional chemokine production by macrophages. ②Monocyte recruitment: Synovial macrophages generate chemokines that promote monocyte recruitment through CCR2, CCR5, CCR9 and CX₃CR1. ③T cell recruitment: Synovial macrophages, endothelial cells (ECs) and fibroblast-like synovial cells (FLSs) generate chemokines that induce T cell trafficking through CXCR3, CXCR4, CXCR6, CCR5 and CXCR1. ④B cell recruitment; FLSs and dendritic cells (DCs) produce CXCL13 and CCL20 that promote B cell recruitment through CXCR5. ⑤Angiogenesis: CXCL8, CXCL12 and CXCL16 produced by synovial macrophage and FLSs induce angiogenesis through CXCR2, CXCR4, and CXCR6, respectively, expressed on ECs. Solid arrows depict cell movement in response to chemokines, and dashed arrows indicate chemokine secretion from cell. The red solid arrow indicates chemokine-inducing angiogenesis.

3. Vasculitis

Vasculitis is an autoimmune disease characterized by the presence of inflammatory leukocytes in vessel walls associated with destructive damage to vessel structures. The affected vessels vary in size, type, and location according to the type of vasculitic disease; (i) small vessels, e.g., ANCA-associated vasculitis (AAV); (ii) medium vessels, e.g., Kawasaki disease (KD); and (iii) large vessels, e.g., Takayasu disease (TD) and giant cell arteritis (GCA) ⁽⁵⁸⁾. Corticosteroids and immunosuppressive agents, such as cyclophosphamide, methotrexate and tacrolimus, are commonly used for the treatment of vasculitis. However, in some cases, the disease is refractory to these treatments, and immunosuppression often leads to significant clinical complications. Therefore, there is a need for new therapies for vasculitis that are safer and more effective than currently used treatments.

Several experimental models of vasculitis have been developed, including Polyoma virus infection-induced vasculitis, ANCA-induced vasculitis and Lactobacillus casei-induced vasculitis, but the ability to induce vasculitis in these models is limited depending on the genetic background of the mouse strain ⁽⁵⁹⁾. Injection of a Candida albicans water-soluble fraction (CAWS) into mice leads to inflammation of the aortic root and coronary arteries and can be used as a model for KD. CAWS-induced vasculitis can be induced in a variety of genetic backgrounds and is, therefore, more versatile compared to other animal models of studying the pathogenesis of vasculitis ^{(59), (60)}. In CAWS-induced vasculitis, multiple immune cells such as neutrophils, macrophages and T cells, infiltrated the inflamed cites. Additionally, neutrophils might be a key driver of inflammation in CAWS-induced vasculitis ⁽⁶⁰⁾.

3.1 Chemokines in vasculitis

Several chemokines are elevated in the plasma and serum of vasculitis patients and are correlated with disease activity (AAV: CCL17 ⁽⁶¹⁾, CCL18 ⁽⁶²⁾, CCL20 ⁽⁶³⁾, CXCL8-11 ⁽⁴⁾, CX₃CL1 ⁽⁶⁴⁾, and XCL1 ⁽⁶⁵⁾; TD: CCL2 ⁽⁶⁶⁾ and CCL5 ⁽⁶⁷⁾; GCA: CCL2 ⁽⁶⁷⁾, CXCL9-11 ⁽⁶⁸⁾ and CX₃CL1 ⁽⁶⁹⁾; and KD: CCL17 ⁽⁷⁰⁾ and CXCL9/10 ^{(70), (71)}). In addition, CCL2, CCL7, CXCL2-3 and CXCL9-10 are highly expressed in the aortic root and coronary arteries of the CAWS-induced vasculitis mouse model ^{(72), (73)}. Thus, the chemokine system also contributes to the development of vasculitis and RA.

In GCA, tissue-resident DCs in the adventitia of affected arteries may be a major chemokine producer in the early phase of disease onset, whereas vascular smooth muscle cells and inflammatory monocytes recruited into the artery generate chemokines during later phases (Figure 2) ⁽⁷⁴⁾. More recently, we have demonstrated that Dectin-2-mediated CCL2 produced by tissue-resident macrophages in the aortic root and coronary arteries may be the key initiator of vascular inflammation in the CAWS-induced vasculitis. Thus, tissue-resident immune cells, such as tissue-resident macrophages and DCs, might be the main source of chemokines igniting vascular inflammation in the development of GCA and KD. However, the main sources and pathogenic role of chemokine system in other forms of human vasculitis and animal models are still unknown.

Open in a separate window

Figure 2.

Chemokines and chemokine receptors in giant cell arteritis.

①Early phase: Tissue-resident dendritic cells (DCs) in adventitia produce CCL20 that promotes CCR6⁺ T cell infiltration. ②Late phase: Vascular smooth muscle cells in media produce CXCL9, CXCL10 and CXCL11, which attract CXCR3⁺ T cells. Vascular smooth muscle cells also produce CCL2 resulting in CCR2⁺ monocyte recruitment. Solid arrows depict cell movement in response to chemokines, and dashed arrows indicate chemokine secretion from a cell.

4. Conclusion

Many chemokines and their receptors have been implicated in leukocyte recruitment into inflamed tissue in RA and vasculitis. Thus, chemokines and their receptors represent promising targets for therapeutic intervention in the treatment of RA and vasculitis. In fact, inhibition of chemokines and their receptors ameliorates inflammation in animal models. However, clinical trials with chemokine and chemokine receptor blockers in human RA have failed to show clinical effects. In addition, the mechanism of action for many chemokines and their receptors in RA and vasculitis is still unknown. An important observation that requires further understanding is that different chemokines may be important at different stages of disease depending on RA and vasculitis. Recent advances in imaging technology have provided unprecedented views into immune cell function in live animals, providing an entirely new paradigm for immune cell function. In fact, our recent work using multiphoton intravital microscopy to study neutrophil entry into the joint in the KxB/N model demonstrated that the classical C5a receptor C5aR1 and the atypical C5a receptor C5aR2 were required for initial integrin-dependent neutrophil arrest on the joint endothelium but were not involved in inducing neutrophil extravasation ^{(16), (42)}. By contrast, the classical chemokine receptor CXCR2 and the atypical chemokine receptor ACKR1 were involved in neutrophil diapedesis into the joint space ^{(16), (42)}. In addition, other work has indicated that blockade of multiple chemokines might be more effective than single chemokine inhibition, because functions of some chemokines in immune cell trafficking overlap ⁽⁵⁷⁾. These studies point to a need to more fully dissect the functional roles of chemokines and their receptors in the pathogenesis of RA and vasculitis. A significant need remains for the treatment of RA and vasculitis. Recent clinical studies suggest that the development of more effective inhibitors of chemokines and their receptors has untapped therapeutic potential.

Article Information

References