Antibodies elicited by immunization with short peptides containing antigenic determinants have been shown, in general, to bind poorly, with greatly reduced affinity to the corresponding region in the native proteins. Thus, such peptides have not proven to be effective immunogens in generating high affinity neutralizing or protective antibodies capable of binding to the native structures and consequently have appeared in many cases to be poor prospects for vaccines. In all likelihood this is because the binding of antibodies to antigen is highly specific and exquisitely sensitive to the three-dimensional structure of the protein. Peptides which are assumed to be disordered and in random coil, cannot preserve the conformational features essential for the antigenic determinant. The long range goal of this research proposal is to design and synthesize peptides which mimic the critical conformational features of native antigens and thus can be used as immunogens generating antibodies with enhanced affinity and specificity for the corresponding native antigen. The model antigen to be used in this study is the immunologically and biochemically well defined protein lactate dehydrogenase C4 (LDH-C4). The three-dimensional structure of LDH-C4 has been determined to a resolution of 2.9 A and the B cell antigenic determinants have been extensively mapped. LDH- C4 is expressed only in sperm and testis and is immunogenic in females. The immune response to LDH-C4 is of medical interest because immunization of females with LDH-C4 has been shown to reduce fertility and is thus a good candidate for a contraceptive vaccine. The proposed experiments are designed to produce a rational population of conformational peptides. This will be achieved by grafting a linear segment (20x30A) of the surface of the LDH-C4 protein, displaying certain structural features such as alpha helix, beta sheet and beta loop, onto a folding unit (beta alpha beta, beta alpha, alpha beta, beta alpha beta alpha beta). These peptides will be synthesized by solution and solid phase peptide chemistry and characterized by CD, FT-IR, NMR, and x- ray diffraction. Their efficacy as an immunogen will be assessed in inducing high affinity antibodies and stimulating LDH-C4 specific T cells. The goal of this research is to point to a general strategy for tailoring peptide vaccine with more useful antigenic and immunogenic characteristics.
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