Introduction
Rheumatoid arthritis (RA) is an autoimmune disease affecting about 1% of the world population and characterized by chronic inflammation of multiple joints, proliferation of the synovial cells, and destruction of the cartilage and bone of the affected joints. Genetic factors and environmental agents are assumed to be involved in the development of disease, but the precise etiopathogenesis has not been elucidated completely [
1,
2].
To elucidate the complex RA pathogenesis, various disease models of RA have been developed [
1,
3]. Collagen-induced arthritis (CIA), one of the well-established animal models of RA, can be induced in mice by immunization with type II collagen (IIC) [
4,
5]. Although the concept that humoral and cellular immunity to IIC is crucial for the development of CIA is widely accepted, multiple chemokines and cytokines are also important for the pathogenesis.
CXC chemokine receptor (CXCR) 4 is a chemokine receptor expressed in various cells of the immune system and the central nervous system [
6‐
8]. CXCR4 mediates migration of resting hematopoietic progenitors and B cells in response to its ligand, CXC ligand 12 (SDF-1), which is involved in various phenomena such as hematopoiesis and the development or survival of B cells [
7,
9]. Furthermore, the SDF-1-CXCR4 system is also suggested to be involved in the T-cell receptor (TCR) signaling [
10] or cell migration or both [
6].
Accumulating evidence suggests the involvement of the SDF-1-CXCR4 system in the pathogenesis of RA. CXCR4-expressing CD4
+CD45RO
+ T cells are abundantly detected in the synovial tissues of RA patients [
11]. T helper (Th) 1 cells, which are believed to be involved in the pathogenesis in part, are attracted by RA synovial fluid, and chemotaxis is interfered by anti-SDF-1 antibody
in vitro [
12]. SDF-1 is expressed at high levels in RA synovial tissues [
13]. Furthermore, it is suggested that CXCR4 is important for T-cell retention in RA synovial tissues [
11]. However, the importance of T cell-expressing CXCR4 in the development of RA and the functional role of CXCR4 in T cells still remain obscure, because many other cells, such as B cells and osteoclasts, also express CXCR4. Because null-knockout mice of
Cxcr4 and
Cxcl12 genes are embryonic lethal [
14‐
16], it was difficult to elucidate the roles of this molecule in different cell types in the pathogenesis of diseases.
In this study, we generated T cell-specific Cxcr4-deficient mice and showed that the incidence of CIA was significantly decreased in these mice. Moreover, we confirmed that T cells migrate toward SDF-1 in a CXCR4-dependent manner in vitro, and CXCR4-expressing T cells were enriched in the affected joints during the development of CIA, suggesting involvement of T cell-expressing CXCR4 in the development of RA.
Discussion
CIA is a model for autoimmune arthritis characterized by autoantibody production and massive infiltration of effector cells to joints [
29,
30]. However, immunization with native murine IIC shows a much lower incidence of arthritis in DBA/1J mice [
31], and immunization with heterologous IIC, such as chicken or bovine, shows a much higher incidence in CIA. A peptide from chicken IIC (aa. 256 to 270) is known as a major immunodominant epitope [
31,
32], and the difference between heterologous IIC and murine IIC, which is important in the development of arthritis, is the presence of a glutamic acid at position 266 in chicken IIC. The T-cell response to this epitope is suggested to break tolerance to induce responses toward homologous epitopes in murine IIC. Bovine IIC (256-270)-specific T cells also crossreact with mouse IIC [
32]. Interestingly, human IIC also shares this immunodominant epitope, because chicken IIC-reactive T-cell hybridoma responds to human IIC [
33]. Furthermore, anti-human IIC antibodies purified from RA patients' sera react with chick and bovine IIC [
34]. Thus, it is believed that chicken IIC-induced CIA is a good model for human RA.
It was reported that the proportion of CXCR4-expressing memory T cells and the expression of the ligand, SDF-1, are increased in RA synovium, suggesting that the SDF-1-CXCR4 system is involved in the pathogenesis of RA [
11,
12,
35]. However, the precise functional roles of this molecule in T cells remains to be elucidated. In this study, we showed that the incidence, but not the severity, of CIA was significantly decreased in
Cxcr4
flox/flox
/
Lck-Cre mice, indicating that CXCR4 expression in T cells is important for the development of CIA. CXCR4 expression was enhanced in activated/memory T cells, and CXCR4-deficient T cells were defective in SDF-1-induced chemotaxis. Both CXCR4 expression in T cells and SDF-1 expression in the affected joints was augmented during the development of CIA, and CXCR4-expressing T cells were accumulated in the inflammatory sites where SDF-1 was strongly expressed. Importantly, on induction of CIA, the proportion of CXCR4-expressing T cells was increased in the inflammatory sites compared with that in the draining LNs, and infiltrated T cells were closely apposed with SDF-1-expressing cells. Although immunization with CFA without IIC enhanced CXCR4 expression in T cells, this could not induce arthritis, suggesting that CXCR4 expression in IIC-specific T cells is important for the induction of arthritis. Thus, the SDF-1-CXCR4 system in T cells is important to recruit memory T cells to the site of inflammation, rather than to the LNs. Because cytokines such as IL-15 and TGF-β can induce and/or sustain the expression of CXCR4 in T cells [
12,
36], cytokines from activated LN cells in IIC-immunized mice may enhance CXCR4 expression. Consistent with this notion, we showed that the antibody production and T-cell response against IIC were normal in
Cxcr4
flox/flox
/
Lck-Cre mice.
We suggest that CXCR4 expression in T cells plays an important role in mediating T-cell recruitment to inflamed sites. However, the importance of T cells, especially the recruitment of T cells to inflamed sites in the development of CIA, has been ignored for a long time. This is because adoptive transfer experiments suggest the importance of antibodies, but not T cells, in the development of arthritis [
29,
31]. CIA can be transferred by serum from arthritic mice, whereas T-cell transfer from arthritic mice to normal or lymphopenic mice cannot induce the development of arthritis [
29,
31]. However, it was reported that transfer of splenocytes from arthritic mice to SCID mice can induce arthritis [
37]. In this experiment, the incidence of arthritis was greatly decreased when either T cells or B cells were depleted from the splenocytes. These observations suggest that T cells are involved in the development of arthritis, although T cells alone cannot induce arthritis. Furthermore, the involvement of T cells is reported in a collagen-antibody-induced arthritis (CAIA) model, in which arthritis is induced by the administration of collagen-specific antibodies [
38]. This model has been considered to be independent of T cells, because T cell-deficient mice were susceptible to CAIA [
39]. However, injection of collagen-specific T cells to CAIA-induced mice prolonged the disease [
39], and T-cell suppression by CTLA-4 Ig treatment [
40] or CTLA-4 Ig and cyclosporine treatment [
41] in a later phase of CAIA suppressed the severity of arthritis, indicating that T cells play a role in CAIA as an amplifier. Accordingly, the following pathogenic processes are suggested. On production or administration of antibodies specific to IIC, these antibodies may activate mast cells by forming complexes with complements on cartilage IIC in joints, leading to the secretion of inflammatory cytokines and chemokines, including IL-1, similar to that reported in K/BxN arthritic mice [
42]. These inflammatory mediators may activate synovial cells to recruit effector T cells, which can secrete various inflammatory mediators on activation with endogenous IIC, resulting in the amplification of inflammation. Our results indicate that CXCR4-SDF-1 interaction plays an important role in recruiting effector T cells into inflamed sites.
Although it was reported that CXCR4 signaling enhances T-cell activation and proliferation in a synergistic manner with TCR signaling [
10,
27,
28,
43], the proliferative response of LN cells from IIC-immunized
Cxcr4
flox/flox
/
Lck-Cre mice was normal on stimulation with IIC, and the cytokine production from these cells was also normal. We also did not detect any effects of SDF-1 on the proliferative response of IIC-immunized T cells to IIC stimulation. This apparent discrepancy might be caused by the difference of the experimental conditions; anti-CD3 antibodies were used to stimulate T cells in the previous reports [
10,
27,
28,
43], whereas whole LN cells, including antigen-presenting cells, were activated with natural antigens in our experiments. Weak costimulatory activity of CXCR4 may be masked by the strong costimulatory signaling from the antigen-presenting cells.
Recently, it was reported that CXCR4 is involved in the β-selection during thymic T-cell development in T cell-specific
Cxcr4-deficient mice [
44]. In contrast to this report, no significant abnormality was detected in the T-cell population and cell number in lymph nodes and the spleen between
Cxcr4
flox/flox
/
Lck-Cre mice and
Cxcr4
+/+
/
Lck-Cre mice (Additional file
2d and
2e), although in the thymus, the T-cell number was decreased without affecting the CD4
+ or CD8
+ cell content in
Cxcr4
flox/flox
/
Lck-Cre mice. We think this difference from the previous report was caused by the difference of the
Lck-Cre transgenic mouse line we used. We found that, in our
Cxcr4
flox/flox
/
Lck-Cre mouse thymus, CXCR4 expression still remained in the DN3 stage (Additional file
2c), suggesting that this remaining CXCR4 rescued potential defects in the thymic β-selection.
Cxcr4-deficient mice developed arthritis with very low incidence (two of 11), but the severity of arthritis was comparable to that of control mice (Figure
1b-d). These data suggest that CXCR4 is required for the initiation of inflammation by recruiting IIC-specific T cells into inflammatory sites. However, IIC-specific effector T cells would migrate into joints by the action of other chemokines or even by chance, without involvement of CXCR4, although the frequency may be low. Then these T cells are activated with IIC in the joints and produce various cytokines and chemokines, which recruit or activate other inflammatory cells, such as neutrophils. Therefore, once inflammation starts, other chemokines and cytokines may play more important roles than CXCR4, or other chemokines may substitute for the role of CXCR4 in the later stages of inflammation, resulting in the development of arthritis in
Cxcr4
flox/flox
/
Lck-Cre mice. In support for this contention, we showed that most of the inflammatory cells infiltrated in inflammatory sites were neutrophils, and only a part of them were CD3
+ T cells (Figure
5). Furthermore, we showed that CCR6, which is important for the development of arthritis in SKG mice by enhancing the migration of Th17 cells, was expressed normally in
Cxcr4-deficient T cells (Figure
3e). The expression of CD62L, which is important for T-cell migration to peripheral tissues, was also normal (Figure
3e).
On induction of CIA, the expression of SDF-1 was augmented. Regarding SDF-1, it was reported that inflammatory cytokines, such as IL-17 [
45] and IL-18 [
46], can induce production of this molecule by fibroblast-like synoviocytes and stromal cells in the RA synovium. Conversely, CXCR4 is induced by TGF-β and IL-15 [
36]. Thus, the cytokine production associated with the immune response against IIC may activate the expression of these molecules, resulting in trapping of activated T cells and recruitment of new T cells to the inflammatory sites. Because SDF-1-CXCR4 system is also active in B cells to activate and to migrate [
47], in osteoclasts to produce MMP-9 [
48], in chondrocytes to produce MMP-3 [
49], in endothelium to induce angiogenesis [
50], and in Th1 cells to migrate into synovial tissues [
35], this system may play very important roles in a multiple ways in the development of RA. Therefore, these findings highlight the SDF-1-CXCR4 system as a good target for the treatment of RA.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
SC mainly contributed throughout this work in collaboration with KS, NF, and SS. Immunohistochemical analysis was operated in collaboration with KBK and AI. BC generated Cxcr4
flox/flox
/Lck-Cre mice. YI organized and supervised the project and edited the draft manuscript. All authors read and approved the final manuscript.