CD25⁺CD4⁺ T cells are enriched in the synovial fluid in rheumatoid arthritis. Mononuclear cells were isolated from the peripheral blood (PB) or the synovial fluid (SF) of a patient with rheumatoid arthritis, stained with monoclonal antibodies against CD4 and CD25 and analyzed by flow cytometry. The numbers denote the frequency of cells in the gate as defined by the expression of CD4 and CD25.

Phenotype and function of CD25⁺CD4⁺ regulatory T cells from human peripheral blood. (a) CD25⁺CD4⁺ T cells are anergic. Purified CD25⁺ and CD25^-CD4⁺ T cells from the peripheral blood of a healthy individual were stimulated with a monoclonal antibody against CD3, and proliferation was assessed by incorporation of ³H-labeled thymidine into newly synthesized DNA after 96 hours of culture. (b) CD25⁺CD4⁺ T cells inhibit the proliferation of autologous peripheral blood mononuclear cells (PBMC). Human PBMC were stimulated with monoclonal antibodies against CD3 in the absence or presence of autologous purified CD25⁺ or CD25^-CD4⁺ T cells. Proliferation was assessed as described in (a). (c) The regulatory capacity of CD25⁺CD4⁺ T cells is inhibited by exogenous IL-2. Human PBMC were stimulated as in (b) in the presence of a non-mitogenic concentration of human IL-2. Proliferation was assessed as in (a). (d) Suppression by CD25⁺CD4⁺ T cells is contact-dependent and independent of regulatory cytokines. Human PBMC were stimulated with a monoclonal antibody against CD3 in the presence of autologous CD25⁺CD4⁺ T cells and neutralizing monoclonal antibodies against IL-10 (αIL-10) or IL-4 (αIL-4), or separated from CD25⁺CD4⁺ T cells by an insert ('transwell'). Proliferation was assessed as described in (a).