The development of synthetic epitope-based vaccines is often hampered during manufacture by synthesis of long or difficult peptide sequences resulting in poor vaccine quality and yields. Therefore we have investigated the feasibility of building lipopeptide-based vaccines in sections or modules as a means to overcome inherent synthesis difficulties. In addition, this approach increases vaccine quality control and speed of production while also allowing for the inclusion of proteins and/or conformational determinants into vaccine design.
We have constructed two modules both consisting of a single peptide epitope coupled to the self-adjuvanting lipid moiety S-[2,3-bis(palmitoyloxy)propyl]cysteine (Pam2Cys), the difference being that in the case of Module 1 a T helper epitope is chosen while for Module 2 a target epitope is used. Both modules include a chemical group with complementary chemical properties to either a target epitope (Module 1) or a source of CD4+ T cell help (Module 2) that allows for chemoselective ligation to form a complete vaccine.
Our results demonstrate that the choice of ligation chemistry can significantly impact vaccine immunogenicity. Vaccines linked by a more biologically unusual thioether bond elicit high antibody titers regardless of whether they are produced via the Module 1 or 2 methodologies. Interestingly utilizing the more natural disulfide chemistry, we find that Module 1 vaccines abrogate antibody responses however simply changing the orientation of the target and T helper epitope in relation to the bond is enough to significantly improve the humoral immune response. These findings shed insights into the way the immune system responds to modular lipopeptide-based vaccines and highlight the importance of choosing the correct chemistry for producing the desired immune response.