Immunodominant epitopes on glycoprotein IIb-IIIa recognized by autoreactive T cells in patients with immune thrombocytopenic purpura

It was recently reported that autoreactive CD4⁺ T cells to glycoprotein IIb-IIIa (GPIIb-IIIa) mediate antiplatelet autoantibody production in patients with immune thrombocytopenic purpura (ITP). To further examine the antigenic specificity of the GPIIb-IIIa-reactive T cells, 6 recombinant fragments encoding different portions of GPIIbα or GPIIIa were generated and tested for their ability to stimulate antigen-specific T-cell proliferation and antiGPIIb-IIIa antibody production in vitro. T cells from the peripheral blood of 25 patients with ITP and 10 healthy donors proliferated in response to recombinant GPIIb-IIIa fragments in various combinations. The amino-terminal portions of both GPIIbα and GPIIIa (IIbα18-259 and IIIa22-262) were frequently recognized (60% and 64%, respectively) compared with other fragments (4%-28%) in patients with ITP, but this tendency was not detected in healthy donors. In subsequent analyses in patients with ITP, T-cell reactivities to IIbα18-259 and IIIa22-262 were consistently detected, whereas those to other fragments were sometimes lost. In vitro antigenic stimulation of peripheral blood mononuclear cells with IIbα18-259 or IIIa22-262 promoted the synthesis of anti-GPIIb-IIIa antibodies in patients with ITP, but not in healthy donors. Of 15 CD4⁺ T-cell lines specific for platelet-derived GPIIb-IIIa generated from 5 patients with ITP, 13 lines recognized IIbα18-259, IIIa22-262, or both. T-cell lines reactive to IIbα18-259 or IIIa22-262 promoted the production of anti-GPIIb-IIIa antibodies that were capable of binding to normal platelet surfaces. These results indicate that the immunodominant epitopes recognized by pathogenic CD4⁺ T cells in patients with ITP are located within the amino-terminal portions of both GPIIbα and GPIIIa.