A radiolabeled staphylococcal protein A assay for detection of anti-erythrocyte IgG in warm agglutinin autoimmune hemolytic anemia of dogs and man.

A cell wall protein from Staphylococcus aureus, Protein A, (SpA) has been shown to have the ability to bind the Fc region of most mammalian IgG molecules. This study uses this unusual property as the basis for a quantitative assay for erythrocyte (RBC) bound antibodies. Test serum is incubated in a suspension of normal RBC's. The cells are then washed and incubated with 125Iodine-labeled SpA (125I-SpA). After incubation cells are pelleted and bound radiolabeled SpA counted. This procedure has been performed using canine anti-goat RBC (DagRBC) serum and human anti-D serum (positive controls) to establish the kinetics of the SpA reaction in the above system. The results indicate that SpA binds to red blood cells as a function of membrane bound antibody. RBC's incubated with indirect Coombs positive sera bound 42.6% and 43.3% of the 125I-SpA, as compared to 19.2%, the upper limit of the 95% confidence interval (n = 9) for normal sera. Furthermore, significant binding was observed for certain indirect Coombs negative (direct Coombs positive) sera indicating that the SpA assay is more sensitive than the indirect Coombs test. The SpA system should provide the clinician with an inexpensive, sensitive, quantitative assay for the diagnosis of warm agglutinin autoimmune hemolytic anemia, as well as other autoimmune disorders involving membrane bound IgG.