Antineutrophil cytoplasm autoantibodies with specificity for proteinase 3 (PR3) are thought to play a major role in the pathogenesis of Wegener's granulomatosis (WG), presumably by their potential to activate neutrophils. In patients with WG, high expression of PR3 on the surface of nonprimed neutrophils is associated with an increased incidence and rate of relapse. In this study, we analyzed the functional significance of constitutive PR3 expression for neutrophil activation as induced by anti-PR3 antibody. Therefore, primed and nonprimed neutrophils were stimulated with the monoclonal anti-PR3 antibody PR3G-3. Activation was measured as actin polymerization by the phalloidin assay as an early, detectable activation event and oxidative burst by the dihydrorhodamine assay, as a late, detectable activation event. In contrast to the oxidative burst, we found that anti-PR3 antibody-induced actin polymerization could be triggered in neutrophils without priming with tumor necrosis factor alpha (TNF-alpha). In addition, a correlation was found between the level of PR3 expression on the surface of these nonprimed neutrophils and the degree of actin polymerization. However, after priming with TNF-alpha, no correlation was found between membrane expression of PR3 and the level of actin polymerization or respiratory burst as induced by anti-PR3 antibody. These data suggest that the presence of PR3 on the surface of nonprimed neutrophils has consequences for their susceptibility to the initial activation step by anti-PR3 antibodies. These data may be relevant in view of the observed relation between membrane expression of PR3 on nonprimed neutrophils of patients with WG and their susceptibility for relapses.