Summary
We have observed that naturally occurring serum antibodies generated a 30 Kd band
in a platelet immunoblot assay. The target protein had the same molecular weight (30
Kd) under nonreduced and reduced electrophoretic conditions, and could be immunoblotted
from either autologous or homologous platelet lysates. Also, the 30 Kd reactive autoantibodies
could be totally adsorbed by platelet cytoskeletons. From these data one likely candidate
for the autoantibody target was the intracellular platelet protein tropomyosin. Indeed,
a commercially available monoclonal antitropomyosin antibody reacted with proteins
comigrating with this 30 Kd band; affinity purified human platelet tropomyosin was
bound by the antibodies that recognized the 30 Kd protein. This body of evidence conclusively
demonstrated that naturally occurring serum autoantibodies reacted with the platelet
cytoskeleton protein - tropomyosin. These tropomyosin specific antibodies were found
in roughly the same percentage of sera from patients with chronic idiopathic thrombocytopenic
purpura (ITP) as from normal individuals.
