Identification of a human heavy chain antibody fragment directed against human platelet alloantigen la by phage display library

Abstract: The human platelet alloantigen HPA-la (Pl^A1) is responsible for most cases of post-transfusion purpura and neonatal alloimmune thrombo-cytopenia in the Caucasian population. HPA-la and HPA-lb are two allelic forms of the platelet membrane glycoprotein IIIa (GPIIIa) gene that differ by a single amino acid. In this report, we describe the development of a recom-binant heavy chain antibody fragment capable of distinguishing between the homozygous forms of HPA-la and HPA-lb. This antibody fragment was isolated from the lymphocytes of an immunized individual through the use of a phage display library system. The recombinant antibody fragment reacted with human platelet lysates from HPA-la homozygous donors, the HPA-la form of recombinant N-terminal GPIIIa and intact HPA-la platelets, but did not react with platelet lysate from HPA-1b homozygous donors, reduced HPA-la form of platelet GPIIIa or other platelet glycoproteins. This HPA-la specific human antibody fragment works well in common laboratory assays such as ELISA and flow cytometry, which can assist in identifying HPA-lb homozygous individuals who are known to have a higher risk for developing neonatal alloimmmune thrombocytopenia and post-transfusion purpura. Thus, selection of recombinant antibody fragment using phage display offers a promising alternative to hybridoma technology for the production of human antibodies against human alloantigens and holds potential as a technique in therapeutic applications.