Domain structure of growth signalobodies critically affects the outcome of antibody library selection

Wide applications of antibodies have demanded rapid and easy methods for isolating high-affinity antibodies. We recently developed an antibody screening system in mammalian cells using a growth signalobody, which is a single-chain Fv (scFv) library/cytokine receptor chimera that can transduce a growth signal in response to a target oligomeric antigen. However, we have never investigated how the domain structure of signalobodies affects the outcome of library screening. In this study, we screened naïve scFv library-inserted signalobodies having distinct extracellular and transmembrane (TM) domains. Although the previously constructed signalobody with the extracellular D1/D2 domains of erythropoietin receptor had recovered the clones with high affinity against a target antigen and with low background cell growth, its D1/D2-deficient variant which was tested in this study recovered the clones with low affinity against a target antigen and with considerable background cell growth. In addition, mutagenesis in the TM domain lowered the level of the background cell growth. These results suggest that the D1/D2 domains increase a threshold to activate signalobodies, thereby selecting clones with high affinity against a target antigen and that the TM domain could be engineered to minimize background growth signalling.