Human activation inducer molecule (AIM/CD69), a dimeric glycoprotein of 33 and 27 kDa, is the earliest inducible cell surface antigen expressed during lymphocyte activation, which has been also involved in lymphocyte proliferation. Although AIM is absent from peripheral blood resting lymphocytes, it is expressed by in vivo activated lymphocytes infiltrating sites of chronic inflammation in several pathologies, as well as by lymphocytes after in vitro activation with different stimuli. We have investigated the possibility that tumor necrosis factor-α (TNF-α) gene expression and protein secretion could be induced in peripheral blood T cells through the AIM/CD69 molecule. Anti-AIM monoclonal antibodies (mAb) were able to induce TNF-α secretion in T cells when protein kinase C (PKC) was simultaneously activated by treatment with phorbol esters. TNF-α secretion was detected at 24 h and peaked at day 3 upon T lymphocyte activation with anti-AIM mAb. Immunoprecipitation studies with an anti-TNF-α mAb from surface iodinated T cells activated through AIM, demonstrated that TNF-α first appeared as a cell surface molecular form of 26 kDa, which is subsequently released to the extracellular medium as the 17-kDa molecular form of TNF-α. AIM stimulation dramatically increased TNF-α mRNA levels, and this mRNA induction and subsequent TNF-α secretion were virtually abrogated by the immunosuppressive drug cyclosporin A. Taken together these results indicate that AIM constitutes a novel molecular pathway in T lymphocytes for induction of TNF-α, and suggest a relevant pathologic role for AIM+ lymphocytes located at sites of tissue injury in the pathogenesis of different chronic inflammatory diseases.