Co-selection of cognate antibody-antigen pairs by selectively-infective phages.

We have developed a chloramphenicol resistant derivative of fd phage with which cognate pairs of antibodies and antigens can be selected. The phage genome encodes a fusion of single-chain antibody to the C-terminal domain of gIIIp, rendering the phage non-infective. The antigen fused to the N-terminal domains of gIIIp is encoded in the same phage genome. Antigen and antibody fusion interact with each other in the periplasm of the phage-producing cell, restoring infectivity. This system has a very low background and will allow simultaneous randomisation of antibody and antigen.