Antineutrophil cytoplasmic autoantibodies (ANCA) have been described in sera of patients with several forms of systemic vasculitis, including Wegener's granulomatosis and microscopic polyarteritis. The two main targets of ANCA in vasculitis are proteinase 3 (PR3) and myeloperoxidase (MPO). ANCA are capable of activating neutrophils primed by tumor necrosis factor-α (TNF-α) in vitro, which may be relevant for the induction of the vascular inflammation observed in vivo. Recently, it has been suggested that engagement of Fcγ receptor IIa (FcγRlla) on the neutrophils is involved in the activation by ANCA. In the present study, we show that activation of the neutrophil respiratory burst by anti-PR3 and anti-MPO is strongly enhanced after TNF priming and lost on removal of the Fc parts of the antibodies. Similar results were obtained when the neutrophils were activated with antibodies against known membrane antigens without major changes in the expression of the target antigens. The TNF-induced enhancement of the neutrophil activation was not observed when adherence of the cells was prevented by continuous stirring of the suspension or by the addition of CD18 antibodies before TNF exposure. Hence, our results indicate that engagement of both FcγRlla and β₂ integrins is instrumental in neutrophil activation induced by ANCA.