Activation of the Gi Heterotrimeric G Protein by ANCA IgG F(ab′)2 Fragments Is Necessary but not Sufficient to Stimulate the Recruitment of Those Downstream Mediators Used by Intact ANCA IgG

Abstract

ABSTRACT. Anti-neutrophil cytoplasm autoantibodies (ANCA) are implicated in the pathogenesis of systemic vasculitis. Intact ANCA IgG activate superoxide generation in cytokine-primed neutrophils after binding their antigens and co-engaging Fcγ receptors (FcγR). The contribution of antigen binding via ANCA F(ab′)2 fragments to signaling has been unclear. This study shows that both ANCA IgG and F(ab′)2 fragments of ANCA IgG induce significant GTPase activity, which could be blocked with pertussis toxin and anti-Gi protein antibodies. Pertussis toxin inhibited ANCA IgG-induced superoxide generation but was without effect on superoxide production after conventional FcγR ligation. ANCA F(ab′)2 fragments did not induce superoxide generation. ANCA IgG activated PI 3-kinase–generating PIP3, activated protein kinase B (PKB), and p21ras; activation of each mediator was inhibited with pertussis toxin, but PI3K and PKB were not activated by ANCA IgG F(ab′)2 fragments. Intact ANCA IgG induced tyrosine phosphorylation, whereas F(ab′)2 fragments did not, and ANCA IgG-mediated superoxide generation was inhibited with genistein. Both genistein and pertussis toxin together completely abrogated the ANCA-induced oxidative burst. Genistein also inhibited ANCA IgG-induced PIP3 generation and p21ras activation. These data implicate a novel ANCA IgG stimulated signaling pathway that involves both F(ab′)2-mediated antigen binding and Fc-mediated FcγR ligation in cooperative interactions between Gi proteins and tyrosine kinases that facilitates activation of downstream mediators. E-mail: C.O.S.Savage@bham.ac.uk.