ENGINEERED TOPOGRAPHIC DETERMINANTS WITH ALPHA-BETA, BETA-ALPHA-BETA,AND BETA-ALPHA-BETA-ALPHA TOPOLOGIES SHOW HIGH-AFFINITY BINDING TO NATIVE PROTEIN ANTIGEN (LACTATE DEHYDROGENASE-C(4))

The use of peptides has attracted much interest in the development of synthetic vaccines. Although our current understanding of peptide anti gens as immunogens has been greatly advanced recently, there still rem ain many obstacles. The B cell response elicited by a peptide antigen is governed by a number of poorly understood events such as epitope st ructure, T cell dependency and major histocompatibility complex restri ction, adjuvancy, route of immunization, and immunogen stability. In t his paper, we extend our previous studies on the problem of the topogr aphical nature of antigenic sites on native protein antigens, in terms of how much molecular mimicry must be maintained in an antigenic dete rminant for the induction of high affinity antibodies specific for nat ive protein. We show here that an antigenic epitope from the model con traceptive vaccine candidate lactate dehydrogenase (LDH-C4) can be rat ionally engineered into a highly structured conformation that mimics t he corresponding site in the native three-dimensional protein. Our str ategy is based on the selection of an antigenic segment which exhibits certain secondary structural properties and by design principles is f ixed in three dimensions by appropriate grafting onto a supersecondary structural motif such as alphabeta, betaalphabeta, or betaalphabetaal pha. The biophysical data are consistent with the proposed secondary s tructures, and antibodies raised to the various construct show high af finity for the native protein. These studies lend further credence to the conformational nature of peptide epitopes and provide a basis for the rational design of peptide vaccines.