Parameters affecting the immunogenicity of a liposome-associated synthetic hexapeptide antigen.

The effect of liposome association on the immunogenicity of the hexapeptide IRGERA was investigated. When administered in the absence of a carrier and adjuvant this peptide, which corresponds to a linear epitope located at the C-terminus of histone H3, was not immunogenic. When mice were immunized with the peptide covalently linked to the surface of small unilamellar vesicles containing monophosphoryl lipid A as adjuvant, a relatively long-lasting response with memory cell induction was observed. The anti-peptide antibodies raised in this way reacted with the cognate sequence in the native histone. In contrast, coupling of the peptide to the surface of large vesicles yielded both an IgM and IgG response of short duration whereas encapsulation of the free peptide in large vesicles was ineffective. These results indicate that with short synthetic peptides, liposomes provide a substitute for a carrier protein. However, an adjuvant has to be incorporated in the vesicles in order to obtain an efficient immune response. Such an approach may be useful for designing synthetic vaccines.