Kinetic exclusion assay of monoclonal antibody affinity to the membrane protein Roundabout 1 displayed on baculovirus

The reliable assessment of monoclonal antibody (mAb) affinity against membrane proteins in vivo is a major issue in the development of cancer therapeutics. We describe here a simple and highly sensitive method for the evaluation of mAbs against membrane proteins by means of a kinetic exclusion assay (KinExA) in combination with our previously developed membrane protein display system using budded baculovirus (BV). In our BV display system, the membrane proteins are displayed on the viral surface in their native form. The BVs on which the liver cancer antigen Roundabout 1 (Robo1) was displayed were adsorbed onto magnetic beads without fixative (BV beads). The dissociation constant (Kd, ∼10−11 M) that was measured on the Robo1 expressed BV beads correlated well with the value from a whole cell assay (the coefficient of determination, R2 = 0.998) but not with the value for the soluble extracellular domains of Robo1 (R2 = 0.834). These results suggest that the BV–KinExA method described here provides a suitably accurate Kd evaluation of mAbs against proteins on the cell surface.