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 after local treatment.
FOXP3 expression was assessed by flow cytometry, immunohistochemistry, immunofluorescence and real-time polymerase chain reaction (RT-PCR). The functional suppressive capacity of Treg was analysed after co-culture with effector CD4+CD25- T cells through assessment of proliferation and cytokine secretion.
It was shown that FOXP3 protein and mRNA expression in synovial fluid T cells was not confined solely to CD25(bright) T cells as seen in blood, but included CD25(intermediate) and even CD25(neg) T cells. Indeed, synovial fluid CD25(high) T cells showed similar suppressive capacity as CD25(bright) T cells, indicating the 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 after intra-articular glucocorticosteroid administration, in parallel with the general reduction in inflammation.
Identification of synovial fluid FOXP3+ Treg with varying intensities of CD25 opens up possibilities for thorough characterisation 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.