Objective: To analyse the distribution of FOXP3+CD25+CD4+ regulatory T cells (Treg) in peripheral blood, synovial fluid and tissue of patients with rheumatic disease during relapse and following local treatment.
Methods: FOXP3 expression was assessed by flow cytometry, immunohistochemistry, immunofluorescence and real-time polymerase chain reaction (RT-PCR). The functional suppressive capacity of Treg was analyzed following co-culture with effector CD4+CD25- T cells through assessment of proliferation and cytokine secretion.
Results: We demonstrate that FOXP3 protein and mRNA expression in synovial fluid T cells was not confined solely to CD25bright T cells as seen in blood, but included CD25intermediate and even CD25neg T cells. Indeed, synovial fluid CD25high T cells showed similar suppressive capacity as CD25bright T cells indicating presence of functional Treg in T cells with lower intensity of CD25. In synovial tissue, FOXP3+ cells were present in low numbers within T cell infiltrates and decreased further following intraarticular glucocorticosteroid administration in parallel with the general reduction in inflammation.
Conclusions: Identification of synovial fluid FOXP3+ Treg with varying intensities of CD25 opens up possibilities for thorough characterization of this important T cell subset in the inflammatory compartment. However, only scarce synovial membrane expression of FOXP3 was found even in the absence of overt inflammation, suggesting that the synovial membrane is a site that would benefit therapeutically from Treg expansion.