Direct expansion of functional CD25+ CD4+ regulatory T cells by antigen-processing dendritic cells

An important pathway for immune tolerance is provided by thymic-derived CD25(+) CD4(+) T cells that suppress other CD25(-) autoimmune disease-inducing T cells. The antigen-presenting cell (APC) requirements for the control of CD25(+) CD4(+) suppressor T cells remain to be identified, hampering their study in experimental and clinical situations. CD25(+) CD4(+) T cells are classically anergic, unable to proliferate in response to mitogenic antibodies to the T cell receptor complex. We now find that CD25(+) CD4(+) T cells can proliferate in the absence of added cytokines in culture and in vivo when stimulated by antigen-loaded dendritic cells (DCs), especially mature DCs. With high doses of DCs in culture, CD25(+) CD4(+) and CD25(-) CD4(+) populations initially proliferate to a comparable extent. With current methods, one third of the antigen-reactive T cell receptor transgenic T cells enter into cycle for an average of three divisions in 3 d. The expansion of CD25(+) CD4(+) T cells stops by day 5, in the absence or presence of exogenous interleukin (IL)-2, whereas CD25(-) CD4(+) T cells continue to grow. CD25(+) CD4(+) T cell growth requires DC-T cell contact and is partially dependent upon the production of small amounts of IL-2 by the T cells and B7 costimulation by the DCs. After antigen-specific expansion, the CD25(+) CD4(+) T cells retain their known surface features and actively suppress CD25(-) CD4(+) T cell proliferation to splenic APCs. DCs also can expand CD25(+) CD4(+) T cells in the absence of specific antigen but in the presence of exogenous IL-2. In vivo, both steady state and mature antigen-processing DCs induce proliferation of adoptively transferred CD25(+) CD4(+) T cells. The capacity to expand CD25(+) CD4(+) T cells provides DCs with an additional mechanism to regulate autoimmunity and other immune responses.