Significance of Antibody Orientation Unraveled: Well-Oriented Antibodies Recorded High Binding Affinity

To investigate the effect of antibody orientation on its immunological activities, we developed a novel and versatile platform consisting of a well-defined phospholipid polymer surface on which staphylococcal protein A (SpA) was site-selectively immobilized. The application of a biocompatible phospholipid-based platform ensured minimal denaturation of immobilized antibodies, and the site-selective immobilization of SpA clarified the effect of antibody orientation on immunological activities. The phospholipid polymer platform was prepared on silicon substrates using the surface-initiated atom transfer radical polymerization (SI-ATRP) technique. An enzymatic reaction was performed for orientation-selective coupling of SpA molecules to the polymer brush surface. Orientation-controlled antibodies were achieved using enzymatic reactions, and these antibodies captured 1.8 ± 0.1 antigens on average, implying that at least 80% of immobilized antibodies reacted with two antigens. Theoretical multivalent binding analysis further revealed that orientation-controlled antibodies had antigen−antibody reaction equilibrium dissociation constants (Kd) as low as 8.6 × 10−10 mol/L, whereas randomly oriented and partially oriented antibodies showed Kd values of 2.0 × 10−7 and 1.2 × 10−7 mol/L, respectively. Strict control of antibody orientation not only formed an approximately 100-fold stronger antigen−antibody complex than the controls but also sustained the native antibody Kd (10−10−10−9 mol/L). These findings support the significance of antibody orientation because controlling the orientation resulted in high reactivity and theoretical binding capacity.