Antineutrophil Cytoplasmic Autoantibodies Interact with Primary Granule Constituents on the Surface of Apoptotic Neutrophils in the Absence of Neutrophil Priming

The pathogenic role of antineutrophil cytoplasmic autoantibodies (ANCA) remains controversial because of the difficulty in explaining how extracellular ANCA can interact with intracellular primary granule constituents. It has been postulated that cytokine priming of neutrophils (PMN), as may occur during a prodromal infection, is an important trigger for mobilization of granules to the cell surface, where they may interact with ANCA. We show by electron microscopy that apoptosis of unprimed PMN is also associated with the translocation of cytoplasmic granules to the cell surface and alignment just beneath an intact cell membrane. Immunofluorescent microscopy and FACS^® analysis demonstrate reactivity of ANCA-positive sera and antimyeloperoxidase antibodies with apoptotic PMN, but not with viable PMN. Moreover, we show that apoptotic PMN may be divided into two subsets, based on the presence or absence of granular translocation, and that surface immunogold labeling of myeloperoxidase occurs only in the subset of PMN showing translocation. These results provide a novel mechanism that is independent of priming, by which ANCA may gain access to PMN granule components during ANCA-associated vasculitis.