Allergic contact dermatitis is a T cell-mediated immune response , which in its relapsing chronic form is of high socioeconomic impact. The phosphoglycoprotein osteopontin (OPN) has chemotactic and Th1 cytokine functions and in various models is essential for robust T cell-mediated immunity. Here we demonstrate that OPN is abundantly expressed by both effector T cells and keratinocytes in allergic contact dermatitis lesions. T cells from nickel-allergic donors secrete high levels of OPN following antigen-specific stimulation. OPN may substitute for missing IFN-␥ secretion in T effector cells because low IFN-␥-producing T cell clones secrete high levels of OPN , and OPN down-modulates their interleukin-4 expression. Furthermore , interferon-␥ from T effector cells augments OPN in allergic contact dermatitis by inducing OPN in keratinocytes , which in turn polarizes dendritic cells and attracts inflammatory cells. In the murine contact hypersensitivity (CHS) model for allergic contact dermatitis , OPN is strongly induced in antigen-specific proliferating T cells , and OPN null mice display a reduced chronic CHS inflammatory response due to a decreased influx of effector T cells. Importantly, because of its function for chronic allergic contact dermatitis , OPN may well be a therapeutic target , because anti-OPN antibody treatment in part suppresses established chronic CHS.