HUMAN MONOCLONAL FAB FRAGMENTS RECOVERED FROM A COMBINATORIAL LIBRARYBIND SPECIFICALLY TO THE PLATELET HPA-1A ALLOANTIGEN ON GLYCOPROTEIN IIB-IIIA

Background and objectives: Certain clinical conditions are related to the presence of platelet-specific alloantibodies in the patient's seru m. We studied the molecular diversity of HPA-1a antibodies to analyze some peculiarities of this antibody response. Materials and methods: H uman antibody Fab fragments that bind to the platelet alloantigen HPA- 1a on glycoprotein IIb-IIIa (GPIIbIIIa) were generated by using a reco mbinant phage display system. We established an immunoglobulin G1, kap pa combinatorial library from the peripheral blood lymphocytes of a pe rson undergoing a severe posttransfusion purpura. Results: Characteriz ation of Fab clones selected from the fifth round of antigen-specific panning of this library demonstrates a highly specific reactivity to t he HPA-1a alloantigen. The nucleotide sequence analysis of representat ive HPA-1a-specific clones reveals at least 3 distinct V-L and 3V(H) g ene segments that present an extensive degree of mutation as demonstra ted by comparison of gene usage and homologies to the nearest germline genes. Conclusions: These human HPA-1a-specific Fab reagents should a llow us to better understand the molecular mechanism involved in HPA-1 a alloimmunization.