The use and limitation of labeled staphylococcal protein A for study of antineutrophil antibodies

Antineutrophil antibodies can be detected following their attachment to neutrophils by employing labeled staphylococcal protein A (SPA). Radiolabeled SPA provides a sensitive means for identifying the presence of IgG restricted to subclasses IgG1, IgG2, and IgG4 that will specifically bind to neutrophils and that are found in the serum of patients with isoimmune and autoimmune neutropenia. However, SPA bound to the Fc region of IgG does not interfere with the attachment of IgG to the Fc domain of the neutrophil. Fluorescein-labeled SPA, in turn is useful in monitoring the functional consequences of antibody attachment to the surface of polymorphonuclear leukocytes (PMN). Both heterologous and isoimmune antisera induced lateral movement of surface antigens into polar-capped pseudopodia. The formation of such pseudopodia may facilitate leukoagglutination and the subsequent removal of sensitized cells from the circulation.