Role of Mitogen-Activated Protein Kinases in Activation of Human Neutrophils by Antineutrophil Cytoplasmic Antibodies

Abstract. Antineutrophil cytoplasmic antibodies (ANCA) may be important in the pathophysiology of necrotizing vasculitis. ANCA activate human neutrophils primed with tumor necrosis factor-α (TNF-α) in vitro . TNF-α priming results in translocation of ANCA antigens to the cell surface, where they are recognized by the antibodies. The signaling mechanisms involved in TNF-α priming and subsequent ANCA-induced activation were investigated. TNF-α-primed neutrophils were stimulated with monoclonal antibodies (MAb) to human myeloperoxidase (MPO) and proteinase 3 (PR3), and with preparations of human ANCA (three patients with PR3-ANCA and two patients with MPO-ANCA). Respiratory burst was measured with superoxide dismutase-inhibitable ferricytochrome C reduction and using dihydro-rhodamine-1,2,3. Phosphorylation of p38 mitogen-activated protein kinase (p38-MAPK) and the extracellular signal-regulated kinase (ERK) were assessed by immunoblotting. ANCA-antigen translocation was studied by flow cytometry. The tyrosine phosphorylation inhibitor genistein, but not calphostin or staurosporin, resulted in a significant dose-dependent superoxide generation inhibition (11.6 ± 1.7 nmol to 2.1 ± 0.5 for PR3-ANCA, and 16.0 ± 2.8 to 3.3 ± 1.3 for MPO-ANCA). The p38-MAPK inhibitor (SB202190) and the ERK inhibitor (PD98059) diminished PR3-ANCA-mediated superoxide production dose dependently (11.6 ± 1.7 nmol O 2 - to 1.9 ± 0.6 with 50 μM SB202190 and 4.0 ± 0.6 with 50 μM PD098059, respectively). For MPO-ANCA, the results were similar (16.0 ± 2.8 nmol to 0.9 ± 1.0 nmol with SB202190 and 6.4 ± 2.4 nmol with PD98059, respectively). Western blot showed phosphorylation of both p38-MAPK and ERK during TNF-α priming. The p38-MAPK inhibitor and the ERK inhibitor showed the strongest effect on respiratory burst when added before TNF-α priming, further supporting an important role for both signaling pathways in the priming process. Flow cytometry showed that p38-MAPK inhibition decreased the translocation of PR3 (by 93 ± 2%) and of MPO (by 64 ± 2%). In contrast, no such effect was seen when ERK was inhibited. Thus, p38-MAPK and ERK are important for the TNF-α-mediated priming of neutrophils enabling subsequent ANCA-induced respiratory burst. However, both pathways show differential effects, whereby p38-MAPK controls the translocation of ANCA antigens to the cell surface.