Fab assembly and enrichment in a monovalent phage display system

We have developed a system that allows the expression of a single copy of an antibody Fab molecule on the surface of the filamentous bacteriophage M13 and demonstrate the utility of this system for enrichment of specific "Fab phage". A "humanized" version of antibody 4D5 (h4D5) directed against the extracellular domain of the HER2 (neu) receptor, was used as prototype to assess the assembly of Fab molecules on the phage and to determine the power of the enrichment system. The h4D5 Fab phage showed specific binding to the extracellular domain of the receptor and exhibited an affinity for its antigen virtually identical to the Fab itself. By virtue of its antigen binding, the h4D5 Fab phage could be sorted from a million-fold excess of non-cognate Fab phage in only two rounds of sorting. Further experiments demonstrated that the h4D5 Fab phage could be preferentially enriched from mixtures of variant Fab phages that had lower affinities for the HER2 receptor.