In chronic inflammatory conditions, mononuclear cells infiltrate the connective tissue attracted by fibroblast-secreted chemokines. The role of fibroblasts in sustaining the lymphocyte immune response upon cellular infiltration is so far unresolved. We here report that, upon prolonged stimulation with tumor necrosis factor-alpha, dermal fibroblasts enhance proliferation of activated T cells whereas unstimulated fibroblasts do not. T cell growth stimulation requires cell contact of tumor necrosis factor-alpha stimulated fibroblasts to T cells and is not due to soluble factors. Growth stimulation is substantially blocked by neutralizing antibodies to interleukin-15. Fluorescence-activated cell sorter analyses revealed that tumor necrosis factor alpha stimulated fibroblasts expose interleukin-15 in a membrane-bound form on the cell surface whereas nonstimulated fibroblasts and interferon-gamma treated fibroblasts do not. The amount of membrane interleukin-15 increases with the duration of tumor necrosis factor-alpha stimulation for at least 3 d. Unstimulated fibroblasts, however, accumulate interleukin-15 in the cytoplasm. No interleukin-15 could be detected in the culture supernatant. Immunohistochemical analyses confirmed membrane interleukin-15 on dermal fibroblasts in discoid lupus erythematosus skin lesions whereas no membrane interleukin-15 was found on the surface of fibroblasts in healthy skin. We conclude that dermal fibroblasts upon long-term tumor necrosis factor-alpha stimulation during chronic inflammation are involved via membrane-bound interleukin-15 in stimulating proliferation of accumulated, activated T cells.