Introduction
Relapses and intermittent remission phases characterize the disease course in rheumatic diseases and other chronic inflammatory conditions. This waxing and waning probably corresponds to a regulation of the disease, in which the immune system is believed to play a major role by balancing pro- and anti-inflammatory immune reactions.
CD25
+CD4
+ regulatory T cells constitute one cell population involved in maintaining this homeostatic balance, because a lack of cells with this phenotype has been demonstrated to be associated with autoimmune disease in animals. Early experiments showed that neonatal thymectomy of mice led to the development of various organ-specific autoimmune manifestations, such as gastritis, oophoritis, orchitis and thyroiditis [
1]. Later, it was demonstrated that CD25 identified these regulatory CD4
+ T cells [
2], and despite extensive research, so far no other cell surface marker has been found to be generally applicable for these cells. The transcription factor Foxp3 has been shown to be specific for these cells [
3‐
5]; however, its intracellular expression does not yet allow live sorting of cells for functional assays. In the peripheral immune system of naive mice and in any thymus, the expression of CTLA-4, GITR (glucocorticoid induced tumor necrosis factor [TNF] receptor) and CD62L are correlated with CD25
+CD4
+ regulatory T cells [
6‐
8]. However, because the expression of these molecules is also altered after T cell activation, they are not informative markers of CD25
+ regulatory T cells in antigen-experienced animals or humans [
9‐
11]. We instead subgroup the CD25
+ T cells according to the level of CD25 expression. This has been demonstrated to roughly divide activated (intermediate CD25 expression) from regulatory T cells (high CD25 expression) in the peripheral blood of healthy subjects [
12]. We have recently shown that patients with rheumatoid arthritis (RA) have an enrichment of CD25
brightCD4
+ T cells in their inflamed joints [
13]. Here, owing to the accumulation of activated cells in the target organ of the disease, the gate for inclusion of regulatory T cells is even more restricted than in peripheral blood (bright CD25 expression). Given the animal data in which cell transfer of regulatory T cells into animals can prevent the development of chronic inflammation, as reviewed in [
14], one would expect humans with an autoimmune disease to have smaller numbers of regulatory T cells, but surprisingly, despite counting only CD4
+ T cells with the highest level of CD25 expression, an enrichment of regulatory T cells was seen. Thus, the simple extrapolation of the animal data into future therapeutic strategies that aim at reconstituting this population does not seem to hold true in human organ-specific autoimmune diseases. Our results obtained from patients with RA [
13] therefore raised several questions: Do these cells exist in inflamed joints of rheumatic patients irrespective of diagnosis? Are they accumulated at the site of inflammation? Can these regulatory T cells from synovial fluid suppress cytokine production, which is an important feature of the chronically inflamed joint? Also, with a large patient cohort can we address the question of possible correlations of frequency of regulatory T cells and clinical parameters such as disease duration, severity of disease, and treatment?
To address these questions we decided to analyze adult patients with juvenile idiopathic arthritis (JIA) and patients with spondyloarthropathies, diseases in which peripheral joint inflammation occurs, and to compare them with results from patients with RA. Despite different clinical features of these disorders, the knee inflammation has similar characteristics: an infiltration of inflammatory cells; an increase in volume of synovial fluid; and a local production of proinflammatory cytokines. Interleukin-17 (IL-17) is a T cell-derived cytokine with similar proinflammatory properties to IL-1 and TNF in the inflamed joint [
15]. We recruited 192 patients with spondyloarthropathies, JIA or RA to study regulatory CD25
brightCD4
+ T cells in the inflamed joints. Our results clearly demonstrate an enrichment of CD25
brightCD4
+ regulatory T cells in the inflamed joints in comparison with peripheral blood. This cell population suppressed both cytokine production and proliferation of other T cells and can therefore be regarded as containing regulatory T cells. However, the frequency in the inflamed joint could not be associated with disease duration, disease severity or treatment.
Discussion
This study demonstrates that the fluid from inflamed joints of patients with PsA, SpA and JIA contains a population of CD25brightCD4+ T cells with a regulatory potential. These results indicate that the presence of regulatory T cells is not only a feature of an inflamed RA joint, either seropositive or seronegative, but more generally one of chronic rheumatic disease. We propose that these cells accumulate in the joints, because in parallel with the enrichment in the joint a decrease is observed in peripheral blood. In all three rheumatic diseases analyzed, the CD25brightCD4+ T cells suppressed not only proliferation but also cytokine production, indicating a potential role for regulatory T cells to influence the inflammatory processes in the joint. These features were apparent in the vast majority of the patients despite the different treatments they received, indicating that the anti-rheumatic drugs that patients receive do not affect the presence of CD25brightCD4+ T cells in the joint.
The frequency of synovial CD25
brightCD4
+ T cells was comparable between the different diseases. Also, a relative increase of this population in the joint in comparison with the circulation was observed in all patient groups. These are interesting findings with regard to the clinical differences between the diseases. First, the HLA associations differ: spondyloarthropathies are associated with class I HLA antigens, mainly HLA-B27, whereas RA is associated mostly with HLA-DR1 and HLA-DR4 of the MHC class II alleles. Thus, the local accumulation of regulatory T cells to inflamed joints does not seem to be dependent on specific MHC molecules despite the fact that the generation of regulatory T cells in the thymus seems to be antigen specific [
22]. Indeed, it has been demonstrated that the effector function, suppression, is not antigen specific in the periphery [
23].
Second, subdividing patients for the presence of RF or erosions does not reveal any differences between the groups with regard to frequency of CD25brightCD4+ T cells. Our data therefore indicate that the enrichment of regulatory T cells in the joints is not correlated with disease severity.
Third, the different diseases analyzed here display differences with regard to the cellular assembly at the site of inflammation. Both immunohistochemical and flow cytometric analyses of T cells have shown a dominance of CD4
+ T cells in RA, whereas CD8
+ T cells are more frequent in inflamed joints of patients with PsA [
24]. Despite these differences, the frequency of CD25
brightCD4
+ T cells is similar in the inflamed joints analyzed in this study. However, regulatory T cells have the potential to suppress both CD4-driven and CD8-driven immune responses [
25] as well as innate immunity [
26], so this is perhaps not surprising. In summary, no significant differences with regard to the frequencies of CD25
brightCD4
+ T cells were found between the different rheumatic patients analyzed.
In this study, all patients had chronic disease; however, each joint was not necessarily continuously inflamed. The synovial fluid samples were, however, always taken during flares, which are the time points at which they can be obtained. In our longitudinal samples from patients with PsA, SpA and JIA, the frequency of CD25
brightCD4
+ T cells was found to be relatively stable over time. This parallels our recent observations in RA patients [
13]. This observation indicates that individual factors, as yet poorly understood, determine the frequency of CD25
brightCD4
+ T cells that can be reached in the inflamed site.
Our data support trafficking from peripheral blood to the site of inflammation, because a decreased frequency of peripheral blood CD25
brightCD4
+ T cells was observed in the patients in comparison with healthy controls. Again, this was true for the three groups of rheumatic diseases investigated so far, confirming the trend seen in our previous study with a limited number of RA patients [
13]. This finding suggests that the regulatory T cells home to inflammatory sites. Such a model is in line with the similar findings in rodents, in which a selective recruitment of CD25
+CD4
+ T cells to the skin of mice infected with
Leishmania major was demonstrated [
27]. Even though cell trafficking has not been addressed in our present study, it is tempting to speculate that the accumulation of CD25
brightCD4
+ T cells to the inflamed knee joints is due to selective recruitment. However, we cannot formally exclude the possibility that CD25
brightCD4
+ T cells with a regulatory function can also expand locally. Nevertheless, at least in patients with JIA it has been reported that T cells in synovial fluid are not in active cell cycle [
28].
This study is the first to demonstrate that CD25
brightCD4
+ T cells from patients with spondyloarthropathies, PsA and SpA, and JIA have a suppressive capacity on proliferation. We also showed that these cells can suppress cytokine production, which has not previously been reported for patients with rheumatic diseases. All cytokines that were produced by the responder T cells were suppressed by the regulators, irrespective of whether they were type 1 or type 2 derived cytokines. The suppression of cytokine production parallels the suppression of proliferation. This is in agreement with several other studies [
12,
29,
30]. However, the suppressive function
in vitro might not be as efficient
in vivo in the inflamed joints of the patients, where the inflammation is perpetuated despite the enrichment of regulatory T cells. In rheumatic diseases the innate immune system has clearly been shown to have a major role, but the regulatory T cells might not efficiently suppress TNF production by macrophages. However, this would be in contrast to the data by Maloy and colleagues [
26], who demonstrated that the immune suppressive properties of murine CD25
+CD4
+ T cells are not limited to T cells' responses but also include inhibition of immune pathology mediated by cells from the innate immune system. Another possible explanation for the lack of efficiency of regulatory T cells in the inflamed joint could be the presence of IL-6. It was recently shown that IL-6 produced by innate immune cells abrogated the suppression of CD25
+CD4
+ T cells [
31]. This cytokine is well represented in the inflamed joint, thus providing a possible way for the immune cells of the joint to avoid being regulated. In addition, low levels of IL-2 in the joint could influence the function of the regulatory T cells [
32]. Further studies are therefore needed to understand why the regulatory T cells do not efficiently halt the chronic inflammation in the rheumatic joint. It has been demonstrated that a dissociation between suppression
in vitro and
in vivo can indeed occur [
33].
The efficiency of suppression varied between patients. The reason for this remains speculative until CD25+CD4+ regulatory T cells can be uniquely identified, but it is likely that different degrees of 'contaminating' activated T cells also expressing CD25 account for at least some of the observed variability. This is a major difference from animal experiments, which are performed in naive animals with only a few activated T cells. In humans, owing to a large exposure of antigens, a large pool of activated memory cells is always present. Here we partly circumvent this problem by using CD25bright cells. The sorting gates include only CD4+ T cells expressing more CD25 than the activated CD8+ T cells; in this way the contamination of activated non-regulatory CD25+ T cells is reduced. In addition, the patients were receiving different combinations of treatments, but our previous study on RA patients also showed a high variability in the degree of suppression, despite their receiving the same treatments.