Dysregulation of CD4+CD25(high) T cells in the synovial fluid of patients with antibiotic-refractory Lyme arthritis.

There is increasing interest in the role of infection in triggering autoimmune diseases (1, 2). With infection, a pro-inflammatory response is induced to protect the host which includes the activation and expansion of innate and adaptive immune cells. However, this pro-inflammatory response must be properly down-regulated once the pathogen is controlled or eliminated to maintain tolerance and limit tissue pathology. In some individuals, these regulatory mechanisms do not work optimally, leading to pathogenic autoimmunity. Therefore, identifying quantitative and qualitative differences in immune cells between patients who can properly down-regulate their immune response after infection from those who cannot is critical to our understanding of infection-induced autoimmunity. Lyme arthritis, a late stage manifestation of infection with the tick-borne spirochete Borrelia burgdorferi (Bb) (3), provides a human model of infection that may lead to these two alternative outcomes (4). Most patients can be treated successfully with antibiotics, called antibiotic-responsive arthritis (5, 6). However, in a small percentage of patients, proliferative synovitis persists for months or years after ≥3 months of oral and IV antibiotics, called antibiotic-refractory arthritis (7). This outcome is postulated to result from persistent infection, retained spirochetal antigens, infection-induced autoimmunity, or a combination of these factors. In animal models, a small number of attenuated spirochetes may survive despite 1 month of antibiotic therapy (8), but in patients with antibiotic-refractory arthritis, culture and PCR results for Bb in synovial tissue have been uniformly negative after ≥3 months of antibiotics (9). Additionally, in MyD88−/− mice, which have a high pathogen load, spirochetal antigens are retained near cartilage surfaces after antibiotic therapy (10), but the relevance of this finding to human antibiotic-refractory arthritis is not yet clear. In the human disease, data supports the infection-induced autoimmunity model (7, 11, 12). For example, antibiotic-refractory arthritis is associated with specific HLA-DR alleles (particularly DRB1*0101 and 0401) (11), a risk factor commonly associated with autoimmune diseases. We postulate that these patients are unable to properly down-regulate their immune response with antibiotic therapy and apparent spirochetal killing leading to immune dysregulation and antibiotic-refractory arthritis. Previously, we showed that in patients with antibiotic-refractory arthritis, the percentage of CD4+FOXP3+ Treg cells in SF correlated inversely with the post-antibiotic duration of arthritis (13), implying that lower numbers of Treg led to slower arthritis resolution. Furthermore, suppression assays using cells from 2 patients with refractory arthritis showed that CD25-positive T cells (Treg) from PB and SF suppressed the proliferation of CD25-negative T cells (Teff) at a 1-to-1 ratio equally well, but CD25-negative T cells (Teff) from SF were more resistant to suppression than from PB. However, in this study, the expression of FOXP3 within various CD4+CD25 T cell subpopulations, the expression of activating or inhibitory T cell co-receptors, and the ability of these patients’ Treg cells to suppress cytokine secretion were not determined. In our current study, we compared the frequency, phenotype and function of immune cells in PB and SF from patients with antibiotic-responsive or antibiotic-refractory Lyme arthritis. Critical differences between the 2 patient groups were found in the CD4+CD25hi+ T cell population in SF. This cell population in the refractory group often had lower frequencies of Treg, higher expression of activation co-receptors, and less effective inhibition of pro-inflammatory responses, leading to immune dysregulation and persistent synovitis.