Dendritic cells (DCs) play a critical role as APCs in the induction of the primary immune response. Their capacity for Ag processing and presentation is tightly regulated, controlled by a terminal developmental sequence accompanied by striking changes in morphology, organization, and function. The maturation process, which converts DCs from cells adapted for Ag accumulation to cells adapted for T cell stimulation, remains poorly understood due in part to difficulties in the culture and manipulation of DCs of defined lineages. To address these issues, we have devised conditions for the culture of a single DC type, Langerhans cells (LCs), using CD34+ cells from G-CSF-mobilized patients. Homogenous populations of LCs, replete with abundant immunocytochemically demonstrable Birbeck granules, could be stably maintained as immature DCs for long periods in culture. Unlike other human DC preparations, the LCs remained fully differentiated after cytokine removal. Following exposure to TNF-alpha, LPS, or CD40 ligand, the LCs could be synchronously induced to mature. Depending on the agent used, distinct types of LCs emerged differing in their capacity for T cell stimulation, IL-12 production, intracellular localization of MHC products, and overall morphology. Most interestingly, the expression of different sets of Toll family receptors is induced or down-regulated according to the maturation stimulus provided. These results strongly suggest that different proinflammatory stimuli might drive distinct developmental events.