Antineutrophil cytoplasmic antibodies preferentially engage Fc gamma RIIIbon human neutrophils

Antineutrophil cytoplasmic Abs (ANCA) are found in the circulation of many patients with systemic vasculitis. ANCA bind to ANCA target, such as protei nase 3 and myeloperoxidase, and activate neutrophils in an Fc gamma R-depen dent manner. Human neutrophils constitutively express Fc gamma RIIa (CD32) and Fc gamma RIIIb (CD16), and there is clear in vitro experimental evidenc e of ANCA-mediated engagement of Fc gamma RIIa, However, direct experimenta l evidence of ANCA engagement of neutrophil Fc gamma RIIIb has been obscure d by technical problems related to activation-induced receptor shedding and activation-induced expression of receptor on the surface of neutrophils. I n this study, by blocking receptor shedding and using appropriate reporter anti-Fc gamma R mAb, we show that human cANCA and pANCA, and murine mAb wit h corresponding reactivities, can indeed engage Fc gamma RIIIb, Furthermore , our data suggest that Fc gamma RIIIb is preferentially engaged by ANCA re lative to Fc gamma RIIa presumably due to the nearly 10-fold excess of Fc g amma RIIIb expression relative to Fc gamma RIIa expression. These results c learly demonstrate that the Fc region of ANCA bound to an ANCA target on th e neutrophil surface engage Fc gamma RIIIb and indicate that Fc gamma RIIIb and Fc gamma RIIa may both be active participants in ANCA-induced neutroph il activation. However, given the low levels of ANCA target expression on n eutrophils from patients with systemic vasculitis, Fc gamma RIIIb is likely to play a critical role in initiating and perpetuating ANCA-induced neutro phil activation.