Potentiation of immune response against the RESA peptides of Plasmodium falciparum by incorporating a universal T-cell epitope (CS-T3) and an immunomodulator (polytuftsin), and delivery through liposomes

Synthetic peptides representing repeat sequences of ring-infected erythrocyte surface antigen (RESA) of Plasmodium falciparum have shown poor immunogenicity and protection. In this study, the RESA peptides [(EENVEHDA)(2) and (DDEHVEEPTVA)(2)] were chemically linked to a universal T-cell determinant, CS.T3, derived from the CS protein of P. falciparum. Polytuftsin (TKPR)(40), a polymer of naturally occurring immunomodulator "tuftsin," was physically mixed with these conjugates, These preparations in alum and liposomes mere immunized in four inbred strains of mice with different genetic backgrounds to study the humoral response, In the case of liposome-entrapped preparations, a 10 mu g dose of antigen showed the optimum antibody response. Mice immunized with liposome containing RESA peptide(s)-CS.T3 conjugate along with polytuftsin showed the highest antibody levels in all the strains, whereas the RESA peptide(s) alone, adsorbed on alum or entrapped in liposomes, showed either poor or moderate antibody levels. The antibodies raised against liposome-entrapped preparations in both high-responder strain (SJL/J H-2(s)) and low-responder strain (FVB/J H-2(q)) showed 2-4-fold lower Kd values as compared to the alum adsorbed preparations, suggestive of high affinity antibodies. All the antigen preparations predominantly induced IgG2a and IgG2b isotype response, suggesting that the T-helper response involved is of the CD4(+) Th1 type. The in vitro merozoite reinvasion inhibition assay showed 50-92% inhibition with sera raised against different antigen formulations. The highest percentage inhibition was observed with the RESA peptide-CS.T3 conjugate containing polytuftsin in liposomes, Thus, the incorporation of peptide antigens inside liposomes not only reduced the antigen dose by 5-fold but also elicited a high titre with high affinity antibodies and the inhibition of merozoites to RBC in vitro. Therefore, we conclude that the incorporation of these synthetic constructs in liposomes could be a useful strategy for the development of a subunit immunogen against malaria.