Activation of clonal T cells by DC during cocultures is associated with upregulation of IL-2R- (CD25) expression. Purified CD3⁻CD4⁺ cells from patient 1 were cultured alone or with mature autologous irradiated DC in the absence or in the presence of CTLA4-Ig (10 μg/mL), blocking MoAb against CD2 (10 μg/mL) or corresponding isotypic controls at a DC:T cell ratio of 1:30. Cells were harvested after 5 days and stained with fluoro-conjugated MoAbs against CD3, CD4, and CD25. Flow cytometric analysis of surface expression of CD25 is shown on at least 10,000 viable cells after gating on CD3⁻CD4⁺lymphocytes. Data from one of two experiments with similar results for each condition are shown.