Selection and application of antibodies modifying the function of β-lactamase

Nine monoclonal antibodies directed against calss A β-lactamases were detected and selected by a novel screening procedure based on assaying the modifications in the catalytic and stability properties of β-lactamase in solution. Unlike conventional screening, e.g., ELISA or immunoprecipitation, the present method does not depend on firm binding and thus favors detection of low affinity antibodies. Individual antibodies were found to affect the enzymatic activity in various ways including stimulation, neutralization, protection and stabilization. Class A β-lactamases show only 20% sequence homologies, and previously anti-enzyme polyclonal antibodies failed to detect cross-reactivities among membets of this class. In contrast, two of our monoclonal antibodies cross-reacted with different β-lactamases and thus demonstrate the presence of shared structural epitopes in this class of enzymes. One of the cross-reacting antibodies was elicited by sequential immunization with two different β-lactamases. Taken together, our findings stress the importance of the screening method in antibody selection and illustrate the use of 'functional' monoclonal antibodies in the study of the structure-function relationship in an enzyme.