Characterization of platelet glycoproteins and platelet/endothelial cell antibodies in patients with thrombotic thrombocytopenic purpura

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Abstract
Platelets and sera from 12 patients with thrombotic thrombocytopenic purpura (TTP) and 12 healthy normal control subjects were examined. As determined by quantitative flow cytometry, prior to plasma exchange therapy platelet surface glycoprotein (GP) Ib levels were similar in TTP patients and normal controls (mean 20 188 and 20 226 molecules/platelet, respectively). Platelets from patients with TTP did, however, have significantly reduced levels of GPIIb/IIIa prior to plasmapheresis (mean 36 348 v 52 505 molecules/platelet in controls; P = 0.0004) and of GPIV (mean 13 321 v 26 212 molecules/platelet in controls; P = 0.0002). An increase in activated platelets, as determined by CD62 expression, was observed in 82% of patients. Increased platelet‐associated immunoglobulins and/or complement was also seen in approximately 60% of the patients. In general, with return of platelet counts to normal levels following seven plasmaphereses, the above abnormalities were reversed, although often not to normal levels. Western blot analysis indicated the presence of antibodies reactive to platelet GPIV (88 kD) in 70% of pretreatment sera from patients with TTP; a similar band was observed in 80% of patient sera against microvascular endothelial cells. Immunofluorescence microscopic examination indicated the presence of antibody in pretreatment sera from patients with TTP to microvascular (73%) and large vessel (36%) endothelial cells. As measured by an indirect flow cytometric assay, pretreatment sera from 55% of patients with TTP were reactive with large vessel endothelial cells and 100% reacted with microvascular endothelial cells; reactivity was significantly greater against the microvascular endothelial cells (P = 0.0048) and was reduced following plasma exchange therapy. These results indicate abnormalities in platelet glycoprotein expression in TTP and suggest that anti‐platelet and anti‐endothelial cell antibodies play a role in the thrombocytopenia and vasculitis characteristic of this disorder.