Lyme arthritis, which is caused by the tick-borne spirochete Borrelia burgdorferi, is characterized by intermittent or persistent inflammation in a few large joints, especially the knee, over a period of several years (1). In most patients, the joint infection can be treated successfully with oral antibiotics for 1–2 months or with intravenous (IV) antibiotics for 1 month, after which the arthritis resolves (2). However, in ∼10% of patients, particularly in those infected with highly inflammatory B burgdorferi strains (3), arthritis in 1 or both knees persists for >3 months after the patient has received oral antibiotic therapy, IV antibiotic therapy, or both for 2–3 months; this is called antibiotic-refractory Lyme arthritis (2).
In patients with antibiotic-refractory arthritis, polymerase chain reaction (PCR) results for B burgdorferi DNA in synovial fluid (SF) are usually negative after oral and IV antibiotic therapy (2), cellular and humoral immune responses to B burgdorferi antigens decline (4, 5), and breakthrough cases of active infection have rarely been observed during the post–antibiotic treatment period (2). Thus, synovitis in patients with antibiotic-refractory arthritis may persist even after total or nearly total eradication of spirochetes from the joint with antibiotic therapy.
The duration of antibiotic-refractory arthritis is variable. In a previous analysis of 67 patients, the median duration from the initiation of antibiotic treatment to the resolution of arthritis was 11 months (range 4– 44 months) (2). During the post–antibiotic treatment period, we usually treat patients with a nonsteroidal antiinflammatory drug (NSAID) and a disease-modifying antirheumatic drug (DMARD) (2). If patients have only a minimal-to-moderate response after treatment for 12– 18 months, we consider performing arthroscopic synovectomy.
Antibiotic-refractory Lyme arthritis shares certain pathogenetic themes with other forms of chronic inflammatory arthritis, particularly rheumatoid arthritis (RA). These include similar synovial histology (6, 7), HLA–DR associations with DRB1*0401, 0101, and other related alleles (8–10), a dominant Th1 response in SF and synovial tissue (11–15), and high SF levels of proinflammatory cytokines and chemokines (16–18), especially CXCL9 and CXCL10, which are strong chemoattractants for CD4+ and CD8+ T effector cells. We have postulated that antibiotic-refractory arthritis may result from infection-induced, tissue-specific autoimmunity within affected synovia (19).
The autoimmunity hypothesis has been reinforced recently by the development of a murine model (20). In this model, both the presence of the human HLA–DR4 transgene, which is associated with antibiotic- refractory arthritis, and lack of the CD28 coreceptor, which leads to dramatically reduced numbers of Treg cells (21), are necessary for persistent synovitis after antibiotic therapy. In mice that lack only the CD28 coreceptor, without the HLA–DR4 transgene, persistent synovitis does not develop after treatment (22). Similarly, in mice that lack the CD28 coreceptor and have the human HLA–DR11 transgene, which is associated with antibiotic-responsive arthritis, posttreatment synovitis does not develop (23). These outcomes in mice support the HLA–DR findings in patients with Lyme arthritis (8), but Treg cell numbers and function have not yet been examined in human Lyme arthritis.
In this study, we enumerated CD4+ T cell subsets, including Treg cells, in the peripheral blood (PB) and SF of 18 patients with antibiotic-responsive or antibiotic-refractory Lyme arthritis. In those with antibiotic-refractory arthritis, a higher percentage of Treg cells correlated with a shorter duration of time to the resolution of arthritis. However, as in the murine model, patients with antibiotic-refractory arthritis and lower numbers of Treg cells seemed unable to achieve resolution of synovial inflammation.
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We characterized the CD4+ T cell subsets in PB and SF from 18 patients with antibiotic-responsive arthritis or antibiotic-refractory Lyme arthritis. Even though the samples in the antibiotic-responsive group were obtained prior to or soon after the start of antibiotic therapy, and those in the antibiotic-refractory group were usually obtained near or soon after the beginning of the post–antibiotic treatment period, the most abundant CD4+ T cell subset in SF in both groups was IFNγ-positive Th1 cells. The prominence of IFNγ-positive Borrelia-specific Th1 cells has been noted in the PB of patients with erythema migrans early in the infection (28) and in the PB and SF of patients with Lyme arthritis (13–15). Borrelia-specific Th1 cell responses decline with spirochete killing prior to the resolution of arthritis (4). However, as shown previously (15) and again in this study, Th1 cell responses in these patients persist at high levels in the post–antibiotic treatment period, and the antigen specificity of these cells may include T cells that react with currently unidentified autoantigens.
In contrast, the percentage of Th17 cells was low in each patient group, although in several patients the percentages were as high as 10–25%. Th17 cells were originally identified in the SF of a patient with Lyme arthritis (29), and neutrophil-activating protein A of B burgdorferi induced T cell lines derived from the SF of patients with Lyme arthritis to secrete IL-17 in culture (30). Thus, these cells may also play a role in control of the spirochete in Lyme arthritis, at least in some patients. However, high levels of IFNγ, as seen in the SF of patients with Lyme arthritis (18), may often serve as a negative regulator of Th17 cell differentiation (31).
In both patient groups, the percentage of IL-4–positive Th2 cells was usually low in both PB and SF. However, compared with patients with antibiotic-responsive arthritis, those with antibiotic-refractory arthritis had significantly higher levels of Th2 cells in SF, and there was a trend toward higher percentages of Th2 cells and faster resolution of arthritis. Thus, Th2 cells may have a role in the resolution of the postinfectious phase of antibiotic-refractory arthritis. Although Th1 cell and Th2 cell responses are more polarized in mice than in humans, the switch from Th1 cell responses to Th2 cell responses in B burgdorferi–infected BALB/c mice is accompanied by arthritis resolution (32).
In this study, the major difference in CD4+ T cell subsets was the higher median percentage of FoxP3-positive Treg cells in the SF of patients with antibiotic-refractory arthritis than in those with antibiotic-responsive arthritis. However, the Treg cell percentage in SF varied greatly, from 3% to 22%. Moreover, among patients with antibiotic-refractory arthritis, there appeared to be 2 subgroups. One consisted of patients with a higher percentage of Treg cells in whom arthritis resolved within several months after the completion of antibiotic treatment, either because of the natural history of the illness or hydroxychloroquine therapy, or both. The second subgroup had persistently lower percentages of Treg cells in the post–antibiotic treatment period, as in the murine model (20), less response to DMARDs, and longer courses of arthritis, which often ended with synovectomy. In patients with antibiotic-responsive arthritis, it was not possible to determine whether the percentages of Treg cells increased prior to arthritis resolution, but in these patients, synovial inflammation was clearly down-regulated along with spirochete killing.
Although the number of patients in whom enough cells were available for functional assays was small, Treg cell suppression seemed similar in the various patient groups. In patients with antibiotic-responsive arthritis or antibiotic-refractory arthritis of short or long duration, Treg cells from SF repressed T effector cells similarly in B burgdorferi–specific assays. In addition, in 1 patient each with antibiotic-refractory arthritis of short or long duration, nonspecific crossover suppression assays showed that Treg cells from either PB or SF were equally effective in suppressing T effector cells, although the level of suppression exerted on T effector cells from SF (50%) was lower than that exerted on those from PB (80%). The difference between Treg cell suppression of SF and PB may be explained simply by differences in cell composition in the 2 compartments, because SF presumably contained more activated T cells than PB, and activated cells are relatively more resistant to Treg cells than are naive T cells (33, 34). However, it is also possible that T effector cells were functionally altered by the local inflammatory environment in the joint (35). Although these factors need to be determined in larger numbers of patients, current observations suggest that patients with antibiotic-refractory arthritis are more likely to have functional differences in T effector cells than intrinsic defects in Treg cells.
As in antibiotic-refractory Lyme arthritis, a dominant Th1 response (13–15) and increased numbers of Treg cells have been observed in SF in several types of chronic inflammatory arthritis, including RA (35–41) and juvenile idiopathic arthritis (42). Moreover, as was seen here, DMARD therapy increases the numbers of Treg cells in patients with RA (41, 43, 44). In one study of untreated patients with RA, Treg cells from PB appeared to be defective due to their inability to suppress T effector cell inflammatory cytokine production (43). However, in another study in which most patients were treated with methotrexate, Treg cells from PB were as functional as cells from normal donors (35), as seen here. Furthermore, Treg cells from SF had enhanced suppressive abilities, but this enhanced ability was offset by heightened T effector cell function. Thus, even in RA, the relative contribution of T effector cells or Treg cells to persistent synovial inflammation is not yet clear and may not be the same in all patients.
In antibiotic-refractory Lyme arthritis, the correlation between higher percentages of Treg cells and faster resolution of arthritis suggests that the determination of Treg cell numbers in the post–antibiotic treatment period may be a useful prognostic marker. It will be important to learn prospectively whether a high percentage of Treg cells, as determined at the completion of antibiotic treatment, predicts a favorable course, whereas a low percentage indicates a longer and more difficult course. Such information may be valuable when making treatment decisions.
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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. Dr. Steere had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study conception and design. Shen, Shin, Strle, Glickstein, Steere.
Acquisition of data. Shen, Shin, McHugh, Steere.
Analysis and interpretation of data. Shen, Li, Glickstein, Drouin, Steere.