Friday, 29/08/2008
Phage Display
Phage Display is a discovery technique pioneered and optimised at CAT, from which libraries containing billions (1011) of different human antibodies are created and from which in turn, antibodies to target molecules can be rapidly isolated.
Bacteriophage or phage are viruses that infect and reproduce within bacterial cells but are harmless to humans, and are routinely used in molecular biology techniques. Phage Display exploits a particular characteristic of phage that allows the expression of foreign proteins, including antibodies, on the phage surface. To achieve this, the DNA encoding the proteins of interest are engineered to link to the DNA that encodes one of the phage coat proteins. A protein ‘displayed’ in this manner can still interact normally with the other molecules.
A full-length antibody is a molecule which is too large to be presented in this way, so DNA sequences are usually limited to just those fragments that encode for the antibody variable regions, known as scFvs, which are responsible for antigen binding.
We have developed vast libraries of antibody genes extracted from human cells. The genes are copied by PCR and inserted into a phage population so that each phage carries a different antibody sequence and thus displays a different antibody variable region on its surface – a Phage Display library.
Click on the icon below for a diagram of human DNA being inserted in a Phage.
MedImmune Cambridge isolates antibodies to any target antigen by ‘selecting’ the entire Phage Display library against it. The target is typically immobilised on a solid surface, such as the wells of a plastic testing tray and then incubated with the library. The relatively few antibodies that bind with highest affinity to the target remain attached, while the remaining billions that do not bind can be washed away.
A key feature of Phage Display is the direct linkage of the antibody with its DNA sequence. This allows further manipulation of the antibody sequence allowing the development of antibodies against both foreign and self-antigens with very high affinity.
Antibodies were the first proteins to be successfully displayed on the surface of phage and CAT scientists were involved in the early development of this technique. MedImmune continues to be at the forefront of evolving technology and in developing other display techniques such as Ribosome Display.