LIGHT is a recently identified member of the TNF superfamily and its receptors, herpesvirus entry mediator and lymphotoxin β receptor, are found in T cells and stromal cells. In this study, we demonstrate that LIGHT is selectively expressed on immature dendritic cells (DCs) generated from human PBMCs. In contrast, LIGHT is not detectable in DCs either freshly isolated from PBMCs or rendered mature in vitro by LPS treatment. Blockade of LIGHT by its soluble receptors, lymphotoxin β receptor-Ig or HVEM-Ig, inhibits the induction of DC-mediated primary allogeneic T cell response. Furthermore, engagement of LIGHT costimulates human T cell proliferation, amplifies the NF-κB signaling pathway, and preferentially induces the production of IFN-γ, but not IL-4, in the presence of an antigenic signal. Our results suggest that LIGHT is a costimulatory molecule involved in DC-mediated cellular immune responses.