Evaluation by quantitative acid elution and radioimmunoassay of multiple classes of immunoglobulins and serum albumin associated with platelets in idiopathic thrombocytopenic purpura

Immunoglobulins (Igs) and serum albumin were eluted from normal platelets and platelets from patients with idiopathic thrombocytopenic purpura (ITP) with a quantitative acid elution procedure followed by solid-phase radioimmunoassay (SPRIA). Acid elution was shown to release a reproducible fraction of platelet-associated Igs, and the amounts released per platelet were independent of the platelet concentration over a wide range of concentrations. This procedure is suitable for sensitive, reproducible, and specific quantitation of large numbers of samples. Washed platelets from 13 normal donors contained the following components (expressed in femtograms per platelet, mean +/- 2 SEM): IgG, 1.40 +/- 0.26; IgA, 0.72 +/- 0.36; IgM 0.078 +/- 0.036; albumin 7.7 +/- 1.5. Immunoglobulins and albumin eluted from the platelets of ten ITP patients (two in remission), expressed as femtograms per platelet, mean (range), were: IgG 104 (0.3 to 750); IgA 90 (0.9 to 715); IgM 162 (1.2 to 1,300); and albumin 34 (6.8 to 199). All platelet-associated Igs from thrombocytopenic ITP patients were found to be elevated twofold to 2,300-fold with one Ig class occasionally elevated 50-fold to 100-fold higher than the others. A similar group of ten thrombocytopenic ITP patients was found to have twofold to 26-fold elevations of platelet- associated albumin. This demonstration of increases in multiple classes of Igs as well as serum albumin associated with platelets from ITP patients suggests that some nonimmune process may be contributing to the phenomenon of increased platelet-associated proteins in ITP.