Developing strategies to enhance and focus humoral immune responses using filamentous phage as a model antigen

Filamentous bacteriophage are commonly used as immunogenic carriers for peptides and proteins displayed on the phage surface. Previously, we showed that immunization with phage to which peptides had been chemically conjugated can elicit a focused anti-peptide antibody response compared with traditional carrier molecules bearing the same peptide, perhaps due to the low surface complexity of the phage. The regularity of its surface also gives the phage other advantages as a carrier, including immunological simplicity and thousands of well-defined sites for chemical conjugation. More recently, we showed that focusing of antibody responses against 'target' peptides was enhanced when the phage's molecular surface was simplified by removal of immunodominant B-cell epitopes present on the minor coat protein, pIII. The pIII-truncated variant elicits an antibody response that is largely restricted to the exposed N-terminus of the major coat protein, pVIII, and to phage-associated bacterial lipopolysaccharide, and a significant fraction of this response cross-reacts with a 12-residue peptide covering the surface-exposed region of pVIII. This allows one to track antibody responses against the phage (and any associated haptens) as they develop over time, and characterize them using a combination of serological, flow cytometric, cellular and immunogenetic assays. The filamentous phage thus provides an excellent model system for studying various aspects of the antibody response, all with the goal of targeting antibody production against weakly immunogenic peptides, proteins and carbohydrates.