Activation of Syk in neutrophils by antineutrophil cytoplasm antibodies occurs via Fcgamma receptors and CD18.

Antineutrophil cytoplasm antibodies (ANCA) activate TNF-alpha-primed neutrophils to undergo a respiratory burst. The intracellular signals that mediate activation have not been studied extensively but could increase the understanding of the pathogenesis small vessel vasculitis. It was demonstrated that ANCA-IgG induced phosphorylation of the tyrosine kinase Syk in TNF-alpha-primed neutrophils from healthy donors. Syk was not phosphorylated in response to ANCA F(ab')(2). Furthermore, Syk phosphorylation was attenuated by blockade of both low-affinity Fcgamma receptors and CD18. Similarly, low-affinity Fcgamma receptor blockade reduced ANCA-induced superoxide production. In patient-derived neutrophils, the high-affinity Fcgamma receptor FcgammaRI was also demonstrated to be involved in ANCA-induced superoxide production. However, Syk phosphorylation was not attenuated by blockade of the FcgammaRI, present on neutrophils from vasculitis patients. The tyrosine kinase inhibitor 4-Amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine inhibited the ANCA-induced respiratory burst and Syk phosphorylation, suggesting that Src kinases lie upstream of Syk activation but downstream of ANCA engagement of Fcgamma receptors. Piceatannol, another tyrosine kinase inhibitor, also inhibited ANCA-induced Syk phosphorylation and the ANCA-stimulated respiratory burst, supporting the proposed functional role for Syk in ANCA signaling. ANCA-induced phosphorylation of Cbl and intracellular calcium transients, potential downstream mediators of Syk activation, were also blocked by tyrosine kinase inhibitors. While it has previously been shown that pertussis toxin diminishes the ANCA-induced respiratory burst, indicating heterotrimeric G protein involvement, Syk phosphorylation and calcium transients were unaffected by pertussis toxin. Collectively, these data show that Syk phosphorylation is induced during ANCA-triggered neutrophil activation.