IDENTIFICATION OF A HUMAN HEAVY-CHAIN ANTIBODY FRAGMENT DIRECTED AGAINST HUMAN PLATELET ALLOANTIGEN 1A BY PHAGE DISPLAY LIBRARY

The human platelet alloantigen HPA-la (Pl(A1)) is responsible for most cases of post-transfusion purpura and neonatal alloimmune thrombocyto penia in the Caucasian population. ;HPA-1a and ;HPA-1b are two allelic forms of the platelet membrane glycoprotein ma (GPIIIa) gene that dif fer by a single amino acid. In this report, we describe the developmen t of a recombinant heavy chain antibody fragment capable of distinguis hing 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 ant ibody fragment reacted with human platelet lysates from HPA-la homozyg ous donors, the HPA-la form of recombinant N-terminal GPIIIa and intac t HPA-la platelets, but did not react with platelet lysate from HPA-lb homozygous donors, reduced HPA-la form of platelet GPIIIa or other pl atelet glycoproteins. This HP;S-la specific human antibody fragment wo rks 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 Molecular Life Science, developing ne onatal alloimmmune thrombocytopenia and posttransfusion 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.