Exposure to p-phenylenediamine (pPD), a primary intermediate in hair dye formulations, is often associated with the development of allergic contact dermatitis. Such reactions involve activation of the subject's immune system. The aim of these studies was to explore the relationship between pPD oxidation and functional maturation of human monocyte-derived dendritic cells in vitro. Dendritic cells were incubated with pPD and Bandrowski's base (BB) for 16 h, and expression of the costimulatory receptors CD40, CD80, CD83, CD86, and major histocompatibility complex class II intracellular glutathione levels and cell viability were measured. In certain experiments, glutathione (1 mM) was added to culture medium. Liquid chromatography-mass spectrometry (LC-MS) analysis and exhaustive solvent extraction were used to monitor the rate of [(14)C]pPD oxidation and the extent of pPD binding to cellular and serum protein, respectively. Proliferation of allogeneic lymphocytes was determined by incorporation of [(3)H]thymidine. Exposure of dendritic cells to pPD (5-50 microM), but not BB, was associated with an increase in CD40 and MHC class II expression and proliferation of allogeneic lymphocytes. Dendritic cell activation with pPD was not associated with apoptotic or necrotic cell death or depletion of glutathione. Neither pPD nor BB altered dendritic cell expression of CD80, CD83, or CD86. LC-MS analysis revealed pPD was rapidly oxidized in cell culture media to BB. Addition of glutathione inhibited BB formation but did not prevent covalent binding of pPD to dendritic cell protein or dendritic cell activation. Collectively, these studies show that pPD, but not BB, selectively activates human dendritic cells in vitro.