Neutralizing human Fab fragments against measles virus recovered by phage display

Five human recombinant Fab fragments (Fabs) specific for measles virus (MV) proteins were isolated from three antibody phage display libraries generat ed from RNAs derived from bone marrow or splenic lymphocytes from three MV- immune individuals. All Fabs reacted in an enzyme-linked immunosorbent assa y with MV antigens. In radioimmunoprecipitation assays two of the Fabs, MV1 2 and MT14, precipitated an approximate to 80-kDa protein band correspondin g to the hemagglutinin (H) protein from MV-infected Vero cell cultures, whi le two other Fabs, MT64 and GL29, precipitated an approximate to 60-kDa pro tein corresponding the nucleocapsid (N) protein. In competition studies wit h MV fusion, H- and N protein-specific monoclonal antibodies (MAbs), the H- specific Fabs predominantly blocked the binding of H-specific MAbs, while t he N-specific Fabs blocked MAbs to N. In addition, N-specific Fabs bound to denatured MV N protein in Western blotting. The specificity of the fifth F ab, MV4, could not be determined. By plaque reduction assays, three of the five Fabs, MV4, MV12, and MT14, exhibited neutralizing activity (80% cutoff ) against MV (LEC-KI strain) at concentrations ranging between approximate to2 and 7 mug ml(-1). Neutralization capacity against MV strains Edmonston and Schwarz was also detected, albeit at somewhat higher Fab concentrations . In conclusion, three neutralizing Fabs were isolated, two of them reactiv e against the H glycoprotein of MV and another reactive against an undefine d epitope. This is the first study in which NW-neutralizing human recombina nt Fab antibodies have been isolated from phage display libraries.