Lipophilic multiple antigen peptide system for peptide immunogen and synthetic vaccine.

We describe the development and structural requirements of a new lipophilic multiple antigen peptide (lipoMAP) system for immunogens that contains a built-in lipophilic adjuvant and has the ability to elicit cytotoxic T-lymphocytes (CTLs). In addition to the peptide antigens of choice at the amino terminus, the basic lipoMAP design consists of three components: a tetravalent symmetrical core matrix containing two levels of branching beta-alanyl-lysine as a building unit, a hydrophilic Ser-Ser dipeptide linker, and at the carboxyl terminus, palmitoyl lysines (PL) with alternating chirality. An 18-residue peptide from the third variable region in the gp120 of HIV-1 was used as antigen in eight models for a structure-function study. Alternating palmitoyl lysine (PL) was introduced as the lipid anchor and built-in adjuvant because D and L Lys (Pal) was found via molecular modeling to best mimic phosphatidylcholine and thus provide the most stable peptide antigens on the ordered lipid membranes. The requirements of the palmitoyl lysines and the L-Ser-L-Ser linker were crucial, since replacement with palmitoyl serines or L-Ser-D-Ser linkers led to a marked decrease in immune response. The stoichimetric ratio of PL vs MAP was also important. Multiple antigen peptide (MAP) constructs without the lipophilic PLs, those that were underlipidated and contained one PL, or those that were overlipidated containing four PLs, were ineffective. LipoMAPs containing three palmitic acids elicited significant humoral responses in oil-based emulsion and liposomes, but not in water or alum formulations. LipoMAP containing only two PLs was found best to be incorporated in liposomes and elicited a significant immune response and cytotoxic T-lymphocytes (CTLs). These models were compared favorably with a preparation using tripalmitoyl-S-glyceryl cysteine (P3C) as the lipid anchor. We also developed a modular synthesis of MAP-P3C that incorporated P3C as a premade unit containing a thiopyridine, which simplified the overall scheme and minimized oxidation during stepwise peptide synthesis. This lipoMAP model is a new addition to the design of our macromolecular assemblage approach mimicking peptide antigens on the surface of micro-organisms. It may be a potentially useful approach to the design of a synthetic vaccine for humans.