The initial stage of Wegener's Granulomatosis (WG) is often marked by symptoms of infection and it has been postulated that a bacterial infection could be the aetiologic factor of this disease. The objective of our work was to investigate T-cell-mediated immunity in WG by testing proliferative responses on bacterial antigens and particularly Staphylococcus aureus. We investigated the bulk proliferative response of peripheral blood lymphocytes (PBL) from patients with clinically active WG to gram-positive bacteria and purified proteinase 3 (PR-3), the major target antigen of c-ANCA. We generated S. aureus-specific PBL-derived T-cell lines and T-cell clones (TLC). In two WG patients 27 TLC were characterized in terms of reactivity to bacterial antigens/PR-3, phenotype, HLA class II restriction and pattern of cytokine secretion. Compared to coagulase-negative Staphylococci and beta-haemolytic Streptococci A, reactivity to S. aureus was significantly increased in all patients with WG. Using purified PR-3, we found a PBL proliferation in five out of 25 WG patients. The TLC were S. aureus-specific and did not cross-recognize Streptococci or coagulase-negative Staphylococci. The S. aureus-specific TLC were of the alphabeta-TCR+ CD4+ phenotype and HLA-DR-restricted. These TLC predominantly showed a Th2-type of cytokine secretion. Interestingly, seven of the S. aureus-reactive TLC also recognized the PR-3 antigen. From these data we conclude that Staphylococci-specific HLA-DR-restricted CD4+ T cells may play a key role in the initial triggering of immune responses in WG.