Epitope specificity of monoclonal antibodies against Newcastle disease virus: competitive fluorogenic enzyme immunoassay.

A test to determine the epitope specificity of monoclonal antibodies (MCA) was developed for hybridoma clones producing antibodies against Newcastle disease virus (NDV). The virus was first immobilized on nitrocellulose membranes of Millititer HA plates. Dilutions of MCA were then added, singly, or simultaneously in pairs, and bound antibody was quantitated with alkaline phosphatase-labelled detector antibody and a fluorogenic substrate, 4-methylumbelliferyl phosphate (4-MUP). Fluorescence count was measured fluorometrically. Additivity indices were calculated and plotted against dilutions of paired MCA. Antibodies that recognized identical epitopes displayed non-additivity at saturating antibody dilutions, followed by partial additivity and by total additivity at low, non-saturating dilutions. In contrast, MCA that recognized distinct epitopes exhibited total additivity throughout the curve. MCA that exhibited partial additivity were interpreted as competing for overlapping shared epitopes, or, distinct epitopes in close proximity, resulting in steric hinderance.