Giant cell vasculitis is an arteritis that predominantly affects medium- and large-sized arteries. Genetic risk factors and clonal expansion of selected CD4+ T cell specificities in the vascular lesions support the model that giant cell arteritis is a T-cell-driven disease. Interferon (IFN)-gamma production in the tissue is intimately associated with the formation of the inflammatory infiltrates. Antigens inducing stimulation of T cells are unknown. To provide indirect evidence for the type and the tissue localization of the antigen, we examined CD4+ T cells in the lesions that secrete IFN-gamma. Temporal artery specimens from patients with giant cell arteritis were analyzed bu two-color immunohistochemistry applying antibodies to T cell markers. IFN-gamma, the interleukin-2 receptor alpha-chain (CD25) and talin, a cytoskeletal protein that is reorganized in T cells interacting with antigen-presenting cells. Proliferating cells in the lesions were identified through the expression of the Ki-67 nuclear antigen. More than 90% of tissue-infiltrating IFN-gamma-producing cells were CD4+ CD45RO+. They represented a minute subset (2 to 4%) of tissue-infiltrating T cells. IFN-gamma+ T cells aggregated in the adventitial layer of the inflamed artery where they were either diffusely distributed or arranged in clusters. The majority of IFN-gamma-secreting T cells expressed CD25. IFN-gamma+ T cells included a fraction of cells that had reorganized the cytoskeletal protein talin, indicating an interaction of the T cell receptor and an antigen-presenting cell. A subset of IFN-gamma-expressing T cells was undergoing proliferation in the tissue. IFN-gamma-producing T cells in vasculitic lesions of giant cell arteritis express several markers that identify them as T cells that have recently been stimulated through their antigen-specific receptor. These putatively disease-relevant T cells represent only a very minor fraction of tissue-infiltrating cells. Their preferential accumulation in the adventitia is most compatible with the model that they contact the relevant antigen primarily in this particular region of the artery. Their regulatory function appears to extend into the inner media and intima where pathological changes in giant cell arteritis are most pronounced.