To investigate the role of the CD40-CD154 interaction in rheumatoid arthritis (RA), we analysed the expression of CD154 on CD3+ and CD4+ T cells in synovial fluid (SF) from patients with RA and in peripheral blood (PB) from patients and normal controls. As interleukin (IL)-15 is a potent activator of synovial T cells we wanted to study whether IL-15 also regulated the expression of CD154 on these T cells. Freshly isolated synovial T cells did not express significant levels of CD154, as evaluated using flow cytometry, whereas the expression of CD86 and human leucocyte antigen (HLA)-DR was significantly elevated on SF T cells when compared with PB T cells from patients or controls. Synovial T cells could up-regulate their CD154 expression following activation with phorbol 12-myristate 13-acetate (PMA) + ionomycin or anti-CD3 + anti-CD28 monoclonal antibodies (mAbs), but the maximal level of expression remained lower than in control T cells. IL-15 significantly increased the expression of CD154 on SF and PB T cells from patients, whereas IL-2 had minimal effects. Furthermore, IL-15 induced extensive proliferation in SF T cells. Our results show that SF T cells up-regulate the expression of CD154 in the presence of IL-15, a cytokine present in the synovium of patients with RA. These results further emphasize the role of IL-15 in the pathogenesis of RA.