Langerhans cells (LC) represent the dominant antigen-presenting cells (APC) in the epidermis and thus play an important role in cutaneous immune responses to approaching pathogens. These responses are mediated by several costimulatory molecules after antigenic challenge. OX40 ligand (OX40L), a member of TNF superfamily, is expressed on several APC such as splenic dendritic cells (DC) and activated B cells. This molecule has been reported to provide potent costimulation in APC-T cell interactions upon binding to its cognate receptor, OX40, on activated T cells. Little is known, however, regarding OX40L expression and function on LC. In the present study, we report the expression of both OX40L and OX40 on differentiated LC derived from draining lymph nodes in the FITC-sensitized mice. During contact hypersensitivity responses, OX40L-deficient mice demonstrated a significant reduction in both hapten-induced ear swelling and hapten-specific T cell responses despite intact migratory responses. Conversely, these responses were markedly increased in two different OX40L-transgenic strains with variations in OX40L overexpression. In the LC-induced MLR, OX40L-deficient and OX40L-overexpressing LC were capable of reducing and elevating the responses of allogeneic CD4+ T cells, respectively. Thus the requirement of OX40L during the antigen presentation function of LC in T cell priming is here demonstrated.