Prevalence, clinical relevance and characterization of circulating cytotoxic CD4⁺CD28^- T cells in ankylosing spondylitis

The immunogenetic link between HLA-B27 and ankylosing spondylitis (AS) is the strongest reported association of an HLA class I molecule with a disease to date. HLA-B27 presents specific peptides to CD8⁺ T lymphocytes in the presence of β₂-microglobulin [1, 2], and much work has concentrated on characterizing B27 restricted cytotoxic T lymphocytes in spondyloarthropathy patients [3, 4]. CD8⁺ cytotoxic T lymphocytes identified by the CD28^- phenotype account for up to 78% of peripheral CD8⁺ T cells [5].

Several more recent studies have raised the possibility that HLA-B27 may be more than a restriction element of CD8⁺ T cells in AS, and that HLA-B27 may be recognized even by CD4⁺ T cells [6, 7]. In HLA-B27 transgenic rats CD4⁺ T cells were used to transfer AS disease [8, 9], and in autoimmune MRL/lpr mice CD4⁺ T cell depletion prevented the development of arthritis [10]. MHC class II molecules, which usually present antigenic structures to CD4⁺ T cells, are not necessarily required for development of AS, and MHC class II negative, B27⁺ transgenic mice may still develop spontaneous AS-like disease [11]. In humans, CD4⁺ T cells are more frequently present than CD8⁺ T cells in both the peripheral blood and in biopsies of sacroiliac joints [12, 13], and T cell clones with specificity for arthritogenic bacteria exhibited the CD4⁺ phenotype when derived from the synovial fluids of patients with Yersinia, Salmonella, and Chlamydia-induced reactive arthritis [14‐16]. However, bacteria and/or autoantigen specific CD8⁺ and HLA-B27 restricted T cells are also thought to contribute to the pathogenesis and regulation of the spondyloarthropathies [3].

In other autoimmune diseases, including rheumatoid arthritis, Wegener's granulomatosis, and multiple sclerosis, an unusual subset of proinflammatory, cytotoxic CD4⁺ T cells was described that is rare in healthy individuals [17‐20]. These T cells are clonally expanded and lack the important CD28 co-stimulatory molecule on their surface. This characteristic phenotype provides a means by which they may be distinguished from normal CD4⁺ T cells. Instead of the CD28-mediated co-stimulation, several alternate co-stimulatory pathways have been examined in these CD4⁺CD28^- T cells [21, 22]. Because CD4⁺CD28^- T cells share phenotypic as well as functional features with natural killer (NK) cells and express NK receptors on their surface [23‐25], these specific T cells should receive co-stimulatory signals by recognition of ubiquitous MHC class I molecules such as HLA-B27 [26].

The aim of the present study was to examine the prevalence and clinical relevance of CD4⁺CD28^- T cells in a cohort of AS patients in comparison with healthy control individuals, and to characterize NK cell features and functional properties of these unusual T cells with respect to HLA-B27 mediated mechanisms.

The present study shows that circulating CD3⁺CD4⁺CD28^-cells were expanded in the peripheral blood of AS patients but not in age-matched healthy control individuals. The percentages of CD4⁺CD28^- T cells were clearly lower than those of CD8⁺CD28^- T cells in AS patients (7.40 ± 6.6% and 41.1 ± 17.7%, respectively) [5]. Increased levels of CD4⁺CD28^- T cells have been described in patients with rheumatoid arthritis, Wegener's granulomatosis and multiple sclerosis [17‐20]. Although they also occur in unselected elderly individuals [34], the expansion of these cytotoxic and proinflammatory CD4⁺ T cells in AS disease was unexpected. AS is clearly associated with the MHC class I molecule HLA-B27, and not with MHC class II molecules. Until now, elevated percentages of CD4⁺CD28^- T cells have only been described in autoimmune diseases with established associations with specified MHC class II molecules. Our findings support a possible role of CD4⁺ T cells even in AS – a MHC class I associated disease – as suggested by animal studies [8‐10] and immunohistological studies of sacroiliac biopsies [13]. Further studies are needed to establish the role of IFN-γ in AS, but the rapid release of this T-helper-1 type cytokine by CD4⁺CD28^- T cells may be important in sustaining synovitis, which is comparable to its role in rheumatoid synovitis [35]. In addition, CD4⁺CD28^- T cells produce perforin, a membranolytic protein that is expressed in the cytoplasmic granules of cytotoxic T cells and NK cells, providing them with the ability to lyse target cells. Thus, CD4⁺CD28^- T cells are distinct from classic T helper cells in several aspects.

From the clinical perspective, the presence of CD4⁺CD28^- T cells in AS patients did not correlate with disease parameters such as time from onset of symptoms or disease duration, with serological parameters such as levels of C-reactive protein and blood cell counts, and with established clinical measurements such as scores for functional impairment (BASFI), disease status (BASMI) and general health (HAQ-S) [28‐30]. However, after grouping patients into those with minor, mean and major decrement in height (from onset of the disease), and those with minor, mean and major elevations in ESR, a correlation was found between these parameters and the peripheral levels of CD4⁺CD28^- T cells (Fig. 2a,2b). These findings support the clinical relevance of CD4⁺CD28^- T cell levels to disease status and chronic inflammation, as found in patients with rheumatoid arthritis and Wegener's granulomatosis [19, 36]. Interestingly, there was no correlation between CD3⁺CD4⁺CD28^- cells and established BASMI score, but again there was a correlation with our recently described modified metrology index [5]. Thus, the percentage of CD4⁺CD28^- T cells in AS patients appears to reflect the anatomical restrictions and status of disease, but not functional and quality of life restrictions. Other factors, including age, were not correlated with levels of CD4⁺CD28^- T cells in our cohorts of AS patients and healthy control indivuduals, although others had observed an accumulation of CD4⁺CD28^- T cells in unselected elderly patients [34]. This observation can be explained by the fact that more than 80% of all probands were younger than 60 years and all control individuals were pre-selected for a history not suspicious for an acute or chronic inflammatory disease. We suspect that increased levels of CD3⁺CD4⁺CD28^- T cells in the elderly may be a consequence of reduced apoptosis and persistence of these cells after an inflammatory disease over the years. Taken together, we believe that a specific metrology index is superior for describing disease status as an integral function of AS duration and activity.

A possible involvement of EBV in the pathogenesis of AS was recently suggested because different HLA-B27 subtypes are able to present the same EBV peptide [37]. In our study the levels of CD4⁺CD28^- T cells did not correlate with EBV IgG titres, suggesting a minor role of EBV in relation to the CD4⁺CD28^- T cells. Irrespective of EBV, CD4⁺CD28^- T cells were more expanded in CMV seropositive AS patients, as was described for healthy individuals and seropositive patients with RA [38].

Interestingly, HLA-B27 interacts with CD4⁺ T cells via several different modes of action. First, Boyle and coworkers [6] and other investigators [7] described human HLA-B27 specific CD4⁺ T cells that proliferated in response to B27-transfected cells without cross-linking of the T cell receptor. Most of the HLA-B27 mediated proliferative effect could be inhibited by CD4 specific monoclonal antibodies, suggesting a CD4 mediated recognition of the HLA-B27 molecule. In addition, CD4⁺ T cells can recognize ubiquitous MHC class I molecules by NK receptors on their cell surface [26]. Recent studies suggest a role of abnormal HLA-B27 molecules, especially HLA-B27 heavy chain homodimers, in the pathogenesis of spondylarthropathies [7, 31]. These aberrant forms of HLA-B27 can be recognized by immunomodulatory killer cell immunoglobulin receptors, such as KIR3DL1 and KIR3DL2, and the immunoglobulin-like transcript ILT-4. Indeed, the CD4⁺CD28^- T cells from AS patients that we tested expressed high levels of CD57 and NK receptors, but they lacked expression of CD7, thus sharing typical NK cell features [39]. The expression of NK receptors on CD4⁺CD28^- T cells in AS patients resembles that in rheumatoid arthritis patients, even though these two diseases are associated with different classes of MHC molecules. It is clear that not only CD4⁺CD28^- T cells from rheumatoid arthritis and melanoma patients but also those from AS patients show NK cell features and represent a hybrid lineage of NK T cells [40]. Activating NK receptors on the cell surface may recognize HLA-B27 [41]. Thus, an activating effect of HLA-B27 on CD4⁺CD28^- T cells may be expected, even in AS disease. As in rheumatoid arthritis, NK receptor mediated recognition of MHC class I molecules without the obligatory presence of specific peptides may be important, especially during a prolonged course of disease, independent of concurrent antigen presentation [42]. At least in part, CD4⁺CD28^- T cells appear to be under peptide-independent control of MHC class I molecules by signalling through activating NK receptors.

Cytotoxic, proinflammatory CD4⁺ T cells are enriched in the peripheral blood of AS patients. These unusual CD4⁺CD28^- T cells share phenotypic and functional properties of NK and T cells. NK receptor mediated recognition of HLA-B27 may be responsible for peptide independent activation of these CD4⁺ T cells in AS disease. Thus, the model of CD4⁺CD28^- NK T cells initiating and sustaining immune responses, and providing a link between the adaptive and the innate immune systems, may hold true for AS disease.