A large semi-synthetic single-chain Fv phage display library based on chicken immunoglobulin genes

Background

Antibody fragments selected from large combinatorial libraries have numerous applications in diagnosis and therapy. Most existing antibody repertoires are derived from human immunoglobulin genes. Genes from other species can, however, also be used. Because of the way in which gene conversion introduces diversity, the naïve antibody repertoire of the chicken can easily be accessed using only two sets of primers.

Results

With in vitro diagnostic applications in mind, we have constructed a large library of recombinant filamentous bacteriophages displaying single chain antibody fragments derived from combinatorial pairings of chicken variable heavy and light chains. Synthetically randomised complementarity determining regions are included in some of the heavy chains. Single chain antibody fragments that recognise haptens, proteins and virus particles were selected from this repertoire. Affinities of three different antibody fragments were determined using surface plasmon resonance. Two were in the low nanomolar and one in the subnanomolar range. To illustrate the practical value of antibodies from the library, phage displayed single chain fragments were incorporated into ELISAs aimed at detecting African horsesickness and bluetongue virus particles. Virus antibodies were detected in a competitive ELISA.

Conclusion

The chicken-derived phage library described here is expected to be a versatile source of recombinant antibody fragments directed against a wide variety of antigens. It has the potential to provide monoclonal reagents with applications in research and diagnostics. For in vitro applications, naïve phage libraries based on avian donors may prove to be useful adjuncts to the selectable antibody repertoires that already exist.

     

Chicken scFvs and bivalent scFv-CH fusions directed against HSP65 of Mycobacterium bovis

Two chicken single-chain variable region antibody fragments (scFvs) that recognised the 65 kDa heat-shock protein (HSP65) of Mycobacterium bovis were selected from a large semi-synthetic phage displayed library. Both recognised HSP65 in indirect enzyme-linked immunosorbent assay (ELISA) and immunoblots and retained their activity during storage. Neither, however, could function as the capture reagent in a sandwich ELISA when immobilised on polystyrene. To establish whether they could be engineered for general use in immunotests, the genes coding for these scFvs were subcloned in expression vectors that contained sequences encoding chicken IgY heavy-chain constant region domains. This resulted in larger bivalent constructs which more closely resembled IgY molecules. The engineered fragments were evaluated in ELISAs and gold-conjugated immunochromatographic tests (ICTs). In contrast to their previous behaviour as scFvs, the modified fragments (designated “gallibodies”) could be used for immunocapture in ELISA and could be readily conjugated to colloidal gold nanoparticles. A sandwich ICT that could detect recombinant HSP65 was also devised. Although converting the recombinant single-chain monomeric antibody fragments to bivalent immunoglobulin-like molecules did not entirely ‘standardise’ the behaviour of the scFvs, this approach remains potentially useful for developing practical, robust, immunodiagnostic reagents.