Orientation of epitopes influences the immunogenicity of synthetic peptide dimers.

The immunogenicity of synthetic peptide dimers based on epitope sequences derived from the mycobacterial 65-kDa antigen and the foot and mouth disease virus (FMDV) VP1 protein was examined in inbred mice. The analysis was directed towards the potential helper role of a T cell stimulatory mycobacterial epitope (65-85) with respect to poorly immunogenic sites either from the same molecule (422-436) or from VP1 (141-160). The 65-85 repeat homodimer induced an antibody response in CBA/ca but not in C57BL/6 mice, both nonresponders to the 65-85 monomer, and amplified the antibody response in BALB/c, monomer-responder mice. Analysis of the immunogenicity of hybrid dimers in BALB/c mice showed that the orientation of peptides within the dimer is critical for the extent of the produced antibody response. Only the 422-436/65-85 but not the 65-85/422-436 induced antibodies binding to the 422-436 sequence which was nonimmunogenic when injected either as a monomer or dimer. Despite the striking difference in immunogenicity, both tested hybrid dimers reacted equally in the solid-phase immunoassay with antisera raised to 65-85-dimer or 422-436/65-85 peptides or with a monoclonal antibody to the 422-436 epitope. The described differences in antibody responsiveness also cannot be attributed merely to the extent of T cell stimulation since the proliferative responses were uniformly expressed for all relevant combinations of peptides. Antisera to 65-85 dimer and 422-436/65-85 hybrid also reacted with the native 65-kDa protein. Furthermore, the production of FMDV-neutralizing antibodies in response to the 141-160 (VP1-derived)/65-85 hybrid peptide in 141-160 nonresponder B10.D2 mice also confirmed the helper activity of the 65-85 epitope. Thus, combining heterologous peptides with the N-terminal of the mycobacterial 65-85 sequence may be generally applicable for the potentiation of peptide vaccines.