Antineutrophil cytoplasmic antibodies preferentially engage Fc gammaRIIIb on human neutrophils

Antineutrophil cytoplasmic Abs (ANCA) are found in the circulation of many patients with systemic vasculitis. ANCA bind to ANCA target, such as proteinase 3 and myeloperoxidase, and activate neutrophils in an Fc gammaR-dependent manner. Human neutrophils constitutively express Fc gammaRIIa (CD32) and Fc gammaRIIIb (CD16), and there is clear in vitro experimental evidence of ANCA-mediated engagement of Fc gammaRIIa. However, direct experimental evidence of ANCA engagement of neutrophil Fc gammaRIIIb has been obscured by technical problems related to activation-induced receptor shedding and activation-induced expression of receptor on the surface of neutrophils. In this study, by blocking receptor shedding and using appropriate reporter anti-Fc gammaR mAb, we show that human cANCA and pANCA, and murine mAb with corresponding reactivities, can indeed engage Fc gammaRIIIb. Furthermore, our data suggest that Fc gammaRIIIb is preferentially engaged by ANCA relative to Fc gammaRIIa presumably due to the nearly 10-fold excess of Fc gammaRIIIb expression relative to Fc gammaRIIa expression. These results clearly demonstrate that the Fc region of ANCA bound to an ANCA target on the neutrophil surface engage Fc gammaRIIIb and indicate that Fc gammaRIIIb and Fc gammaRIIa may both be active participants in ANCA-induced neutrophil activation. However, given the low levels of ANCA target expression on neutrophils from patients with systemic vasculitis, Fc gammaRIIIb is likely to play a critical role in initiating and perpetuating ANCA-induced neutrophil activation.