Improving the copy numbers of antibody fragments expressed on the major coat protein of bacteriophage M13.

Background: Antibody fragments have been expressed on the major coat protein of filamentous phage using a gene VIII expression system, but with low copy numbers (averaging 0.2 Fab/phage).

Objectives: As a general strategy to increase copy number, the phage vector was optimized by site directed mutagenesis.

Study Design: One mutation was the introduction of a random six amino acid tether between the heavy chain and pseudo gene VIII to form a phage library. The other mutation was the removal of two cysteine residues which form a disulfide bond between the heavy and light chains. An assay was developed to measure Fab concentrations and used to calculate the average number of copies displayed on phage.

Results: Phage libraries containing random tethers were panned, and clones containing a proline rich motif were extracted. Removing the interchain disulfide had a greater effect on copy number and soluble Fab concentrations in the periplasmic space of the bacterial cultures.

Conclusion: A tenfold increase in the copy number was achieved using the optimized vector. Incorporation of these vector mutations may be a general strategy for optimizing Fab display on the major coat protein of bacteriophage M13.