Background
Rheumatoid arthritis (RA) is a frequent disease leading to joint destruction, deformity, and disability. Biologics have shown their efficacy in limiting joint destruction and have dramatically improved the outcome of RA-patients. However, some patients remain refractory or become nonresponder to these treatments, underlining the need in this context for new or complementary therapeutic strategies [
1-
3].
The pathogenesis of RA is characterized by chronic inflammation and synovial infiltration of immune cells [
4,
5]. It results from the combination of genetic susceptibility genes and environmental factors [
6]. Until now, most of the susceptibility genetic factors identified are involved in inflammatory response and autoimmunity [
7]. Furthermore, the majority of these genetic factors was also associated with other autoimmune diseases, which were not characterized by an arthritic phenotype, underlining the concept of a shared genetic background between autoimmune diseases [
8-
10].
Recently, a non-synonymous single nucleotide polymorphism (SNP) Gly307Ser (rs763361) in the
CD226 gene, which encodes the DNAX accessory molecule 1 (DNAM-1), has been associated with multiple autoimmune diseases including RA [
11-
18]. DNAM-1 is a 67 kDa type I membrane protein belonging to the immunoglobulin supergene family of receptors. It is constitutively expressed on the majority of CD4+ and CD8+ T cells, monocytes, natural killer cells, platelets and a subset of B cells. It is involved in the adhesion and co-stimulation of T cells in a Th1 pathway [
19]. Interestingly, there are accumulating evidences suggesting a key role of T cells in the pathogenesis of RA characterized by a marked shift toward Th1 and Th17 phenotypes [
20]. Furthermore, DNAM-1 was found to be significantly expressed on CD4+CD28- T cells from RA-patients and to be involved in co-stimulation of these cells [
21]. Therefore, it remains to determine whether
CD226 Gly307Ser (rs763361) contributes specifically to the expression of the arthritic phenotype in RA or does it just reflect a common genetic background between autoimmune diseases. For this purpose, we aimed to validate
in vivo this genetic susceptibility factor, using the collagen-induced arthritis (CIA) model, which is a widely used model for RA and has been important for understanding RA pathogenesis [
22-
24]. This might also reveal pathophysiological pathways leading to new potential therapeutic targets. We combined a targeted molecular approach with neutralizing anti-DNAM-1 monoclonal antibody (mAb) and a gene inactivation strategy using mice lacking DNAM-1 (dnam1−/−) in the CIA mouse model and demonstrated that inhibition of DNAM-1 does not have a direct influence on the development of inflammatory arthritis in mice.
Discussion
In the present study, we demonstrate that
CD226/DNAM-1 may be not a relevant target in RA, using an
in vivo approach in the CIA model. Despite several genetic studies revealing that
CD226 was a susceptibility genetic factor for RA in different ethnic groups [
11-
18], neither inactivation of DNAM-1 nor a molecular targeted strategy using a neutralizing mAb against DNAM-1 prevented from CIA. To assess rigorously the effect of invalidation of DNAM-1, we used four different validated parameters (clinical score, US score, histological assessment and collagen antibodies levels) [
34]. All these four parameters displayed the same results, demonstrating no significant effect of invalidation or neutralization of DNAM-1 in CIA. This effect was not due to an arthrogenic effect of the antibody on its own, since we did not observe any clinical arthritis in non-immunized control mice treated with this mAb in a previous project [
29].
Until now, genetic approaches have revealed that
CD226 Gly307Ser (rs763361) polymorphism was significantly associated with several different autoimmune diseases and the risk of multiple autoimmune diseases [
11-
18,
36-
38]. There is increasing evidence suggesting that autoimmune diseases share a genetic background and that autoimmune phenotypes represent pleiotropic outcomes of nonspecific disease genes [
9,
10,
39]. Several susceptibility genes for autoimmune diseases have been identified without confirmation of their involvement in the development of the pathogenic phenotype. Therefore, it is crucial to proceed to their functional validation
in vivo. So far, the contribution of
CD226/DNAM-1 to the phenotype of several autoimmune diseases has been demonstrated
in vivo in some murine models [
29,
38,
40]. In a murine model of multiple sclerosis, i.e. experimental autoimmune encephalomyelitis, anti-DNAM-1 treatment delayed the onset and reduced the severity of the disease [
40]. In a mouse model of acute graft-versus-host disease (GVHD), invalidation of DNAM-1 and an anti-DNAM-1 mAb were both associated with a milder GVHD and prolonged survival [
38]. Recently, our group demonstrated that inhibition of DNAM-1 significantly ameliorated dermal fibrosis, in a murine model of systemic sclerosis [
29]. In all these murine models, the blocking of DNAM-1 was associated with decreased infiltration of T cells, suggesting that it could reflect a general effect of DNAM-1 on inflammation rather that an effect linked to a specific genetic susceptibility for the autoimmune disease considered. Here we show that blocking
CD226/DNAM-1 has no effect in another T cell-mediated inflammatory disease, despite a decrease in infiltrating T cells, consistent with what was observed in the mouse models of dermal fibrosis [
29]. This suggests that other cells are involved in the physiopathology of CIA. This also highlights that DNAM-1 may be implicated in the development of a specific pathogenic phenotype, such as dermal fibrosis, encephalomyelitis and GVHD, but non RA.
In this report, we confirm that mice on a C57BL/6 background can develop clinical signs of arthritis with a high incidence around 100%, using a modified procedure. Incidence of arthritis in our study was much higher than in previous reports (60-70%) [
27,
28,
41]. Several differences may account for this discrepancy: first we used a concentration of CII equal to 4 mg/ml instead of 2mg/ml, secondly we performed immunization only on male mice and thirdly male were younger than in previous studies. This slightly modified procedure (previously used by our team (unpublished results)) could increase the incidence of arthritis in C57/BL6 mice, but should be evaluated in further studies. This is of interest, since most of the knockout mice are on a C57/BL6 background. Nonetheless, the severity of clinical arthritis was lower in C57/BL6 mice than in DBA/1 mice, which is consistent with a previous study [
27]. In this report, we also confirmed that arthritis develops later in mice on a C57/BL6 background (around 40th day) as compared to DBA/1 mice (on the 20th day) [
27,
28,
41]. However, we observed a lower incidence and severity at histological assessment with low inflammatory scores in C57/BL6 mice than in DBA/1 mice.
US was proved to be a valuable method for evaluating arthritic lesions in mice on a DBA/1 background [
34]. Here we confirm these results and suggest that US could also be used in other background, such as C57/BL6 with preliminary results, revealing high concordance with clinical analysis. Moreover, power Doppler allows visualizing and quantifying joint vascularization during the course of the disease. Therefore, the ability of US in investigating arthritic lesions in C57/BL6 mice should be evaluated in further studies.
Our study should be interpreted within its limitations. First, we could not exclude an immunization of our mice against this antibody. However, there were a significant reduction of infiltrating T cells, especially those expressing DNAM-1, following treatment with anti-DNAM-1 mAb suggesting that the Ab was biologically effective on these mice, despite no effect on the arthritic phenotype was detected. Moreover, we did not observe any bone and cartilage erosions unlike previous publications and induction of arthritis on the C57/BL6 background was challenging [
27,
28,
41]. However, there was evidence of induction of arthritis with clinical signs of arthritis, inflammatory infiltrates at histology and production of CII antibodies. Nevertheless, we can suggest using other models, such as antigen-induced arthritis using mBSA or K/BxN serum-induced arthritis, to confirm our results.
In addition, we could not assess the correlation between clinical and US assessments for all joints, since some joints were assessed only clinically (i.e. wrists and fingers), whereas knees only were evaluated by US alone. However, our results demonstrate a high correlation between clinical and US evaluations for all joints investigated. Moreover, US is an operator-dependent method and here the assessment was only performed by one examinator. Nevertheless, this was performed by a trained investigator, who developed this strategy [
34].
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Competing interests
All the authors declare to have no competing interests.
Authors’ contributions
Study conception and design: ME, G.C, M.C, G.B, Y.A, J.A. Acquisition of data: M.E, G.C, C.M, F.L, G.R. Analysis and interpretation of data: M.E, G.C, C.M, G.R, Y.A, J.A. All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be published.