This study explored the effect on endothelial cell (EC) activation of contact with T lymphocytes, which occurs during lymphocyte emigration into inflamed tissues. Addition of T cells to umbilical vein or dermal microvascular EC monolayers stimulated expression of EC E-selectin and VCAM-1. This response required direct cell:cell contact, but not T-cell activation. The capacity of resting CD4+ T cells to activate EC was restricted to the CD45RO+ subset and could be enhanced by 6 h prestimulation of T cells with PMA and ionomycin. The EC-stimulating capacity of resting or activated T cells was independent of CD40 ligand. Furthermore, inhibition of TNF-alpha/beta and IL-1alpha/beta, together with CD40 ligand, failed to inhibit EC activation by resting T cells and only inhibited the response to PMA- and ionomycin-activated T cells by 40 +/- 18%. Our data suggest that T-cell-EC interactions can lead to EC activation through a novel contact-dependent, but CD40 ligand-independent, mechanism.