To assess the induction, regulation, and the relative roles of cell surface tumor necrosis factor-related activation protein (TRAP; CD40 ligand) and the soluble form of TRAP (sTRAP) in the initial phase of T cell activation, primary CD4+ CD45RA+ (naive) T cells were co-cultured with mature Langerhans' cells (mLC) in the presence of superantigen. In this cell system, TRAP was very efficiently induced in T cells at both the mRNA and protein levels. After appearing on the cell surface, TRAP was rapidly down-regulated by a mechanism triggered through interaction of TRAP with CD40 on mLC. Co-culture of T cells with mLC led to the release of sTRAP, an 18-kDa protein capable of binding to CD40. Experimental data strongly suggest that sTRAP is not released by proteolytic cleavage of TRAP on the cell surface, but is generated in an intracellular compartment. Release of sTRAP and induction of TRAP cell surface expression were found to be regulated independently. In terms of function, sTRAP cannot compete with cell surface TRAP for ligation of CD40 on mLC, indicating that sTRAP release is not a mechanism for termination of the TRAP/CD40 interaction. However, sTRAP on its own rapidly down-regulates CD40 expression on mLC and has long-lasting anti-apoptotic effects on dendritic cells. Thus, we infer from our results obtained in vitro that primary activation of CD4+ T cells by dendritic cells in the lymphoid tissues leads to release of sTRAP, which may act on CD40+ bystander cells in a cytokine-like fashion.