T cells differentiated in the presence of DCs preactivated by SLAM and CD40 signals produce reduced amounts of IFN-γ. We analyzed whether SLAM-mediated inhibition of DC IL-12 production is reflected in the ability of these cells to promote Th1 differentiation. DCs were activated with CD40L-L cells or with SLAM/CD40L-L cells for 12 hours, then purified by cell sorting for the CD11c⁺ population. Purified, allogeneic CD4⁺ naive T cells (A) or autologous CD4⁺ naive T cells (B) were cultured with these preactivated DCs for 4 days, and then the T cells were activated with anti-CD3 mAb for 24 hours. IFN-γ concentrations were measured in the supernatants by ELISA. To confirm the role of SLAM-mediated inhibition of IL-12 production in Th1 polarization, human recombinant IL-12p70 was exogenously added to cultures of naive CD4⁺ T cells mixed with autologous CD40L or CD40L/SLAM-activated DCs (C). T-cell priming/activation was analyzed as described for panel B. In the supernatants of cultures obtained from experiments described for panel B the concentration of secreted IL-13 was also determined (D). Representative results of 3 independent experiments are shown. Error bars represent the standard deviation of triplicate cultures.