p32, a platelet autoantigen recognized by an SLE-derived autoantibody that inhibits platelet aggregation

We have previously described an SLE-derived human hybridoma autoantibody, 9604, which binds to activated but not to resting platelets, inhibits platelet aggregation, and immunoprecipitates a surface-labelled poly-peptide of 32,000 molecular weight (MW) (p32). In the present study, using a murine monoclonal anti-p32 antibody (8E8), we show that p32 is responsible for the binding of 9604 to activated platelets. First, 8E8 bound to the surface of activated but not resting platelets, as detected by flow cytometry, showing saturation with 1900 antibody molecules bound/platelet and a Kd of 72 nM. Second, complete inhibition of the binding of 9604 to ADP-activated platelets by 8E8 and inhibition of the reactivity of 8E8 with p32 by 9604 demonstrated that 8E8 and 9604 recognize the same 32,000 MW platelet polypeptide. Isolation of platelet proteins, using an 8E8-affinity column, showed high MW bands corresponding to multimers of p32 under non-reducing conditions, and isolated p32 tended to self-associate even under reducing conditions. High performance liquid chromatography (HPLC) demonstrated that the native p32-containing protein has an approximate MW of 450,000. A comparison of the functional properties of murine monoclonal 8E8 and human monoclonal 9604 revealed that both antibodies are cytotoxic to platelets in 51Cr-release assays. In contrast, 8E8, unlike 9604, did not affect platelet aggregation, suggesting that different epitopes may be recognized by the two antibodies. These findings demonstrate that p32 is a subunit of an activation marker that is expressed on the surface of activated platelets and recognized by an SLE-derived anti-platelet autoantibody. Our data also indicate that murine monoclonal antibodies produced against platelet autoantigens may not mimic the functional activities of spontaneously occurring autoantibodies, probably due to the recognition of different epitopes.