Enhancement of binding avidity by bivalent binding enables PrP(Sc)-specific detection by anti-PrP monoclonal antibody 132

Anti-prion protein (PrP) monoclonal antibody 132, which recognizes mouse PrP amino acids 119–127, enables us to reliably detect abnormal isoform prion protein (PrP(Sc)) in cells or frozen tissue sections by immunofluorescence assay, although treatment with guanidinium salts is a prerequisite. Despite the benefit of this mAb, the mechanism of PrP(Sc)-specific detection remains unclear. Therefore, to address this mechanism, we analyzed the reactivities of mono- and bivalent mAb 132 to recombinant mouse PrP (rMoPrP) by enzyme-linked immunosorbent assay (ELISA) and surface plasmon resonance (SPR). In ELISA, binding of the monovalent form was significantly weaker than that of the bivalent form, indicating that bivalent binding confers a higher binding stability to mAb 132. Compared with other anti-PrP mAbs tested, the reactivity of bivalent mAb 132 was easily affected by a decrease in antigen concentration. The binding kinetics of mAb 132 assessed by SPR were consistent with the results of ELISA. The dissociation constant of the monovalent form was approximately 260 times higher than that of the bivalent form, suggesting that monovalent binding is less stable than bivalent binding. Furthermore, the amount of mAb 132 that bound to rMoPrP decreased if the antigen density was too low to allow bivalent binding. If two cellular PrP (PrP(C)) are close enough to allow bivalent binding, mAb 132 binds to PrP(C). These results indicate that weak monovalent binding to monomeric PrP(C) diminishes PrP(C) signals to background level, whereas after exposure of the epitope, mAb 132 binds stably to oligomeric PrP(Sc) in a bivalent manner.