Figure 1.

CD4⁺ T_EM cells constitutively express CD40L and promote DC maturation in vitro through CD40L–CD40 interaction. (a) Surface expression of CD40L (black lines) in CD4⁺ T naive (CD44^lowCD62L⁺CD127⁺), T_CM (CD44^highCD62L⁺CD127⁺), and T_EM (CD44^highCD62L⁻CD127⁺) cells. Gray lines represent control staining with isotype-matched control antibodies. (b) Kinetics of CD40L surface expression on CD4⁺ T cell subsets sorted as in panel a and stimulated in vitro with anti-CD3 and PdBU. Data are the means ± SD of three separate experiments. Student's t test on values at time 0: T_EM versus T_CM, P = 0.041; T_EM versus T naive, P = 0.023; T_CM versus T naive, NS. Unstimulated T cells cultured for the same time had a level of CD40L expression comparable to time 0 (not depicted). (c) Expression of CD40, CD86, and MHC class II (black lines) on DCs that had been cultured for 12 h with the indicated CD4⁺ T cell subsets or stimulated with LPS. Gray lines represent staining of untreated immature DCs. Results are representative of four independent experiments. (d) Expression of CD86 (black lines) in wild-type DCs (top) or CD40^−/− DCs (bottom) cultured for 12 h with live or fixed CD4⁺ T_EM cells or with LPS as control. Gray lines represent staining of untreated immature DCs. Results are representative of three independent experiments.