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 GPIIb alpha or GPIIIa were generated and tested for their ability to stimulate antigen-specific T-cell proliferation and anti-GPIIb-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 GPIIb alpha and GPIIIa (IIb alpha 18-259 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 IIb alpha 18-259 and IIIa22-262 were consistently detected, whereas those to other fragments were sometimes loaf. In vitro antigenic stimulation of peripheral blood mononuclear cells with IIb alpha 18-259 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 IIb alpha 18-259, IIIa22-262, or both, T-cell lines reactive to IIb alpha 18-259 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 GPIIb alpha and GPIIIa. (C) 2001 by The American Society of Hematology.