Immunogenicity and antigenicity in rabbits of a repeated sequence of Plasmodium falciparum antigen Pf155/RESA fused to two immunoglobulin G-binding domains of staphylococcal protein A.
A synthetic gene encoding a tetramer of the repeated subunit EENVEHDA of the Plasmodium falciparum antigen Pf155/RESA was expressed in a dual-expression system. The resulting fusion proteins, designated ZZ-M1 and BB-M1, comprised the EENVEHDA repeats and either two immunoglobulin G-binding domains from staphylococcal protein A or the human serum albumin-binding domains from streptococcal protein G, respectively. The soluble fusion proteins were affinity purified to homogeneity in one-step procedures. ZZ-M1 was used for immunization of rabbits. The rabbit antisera reacted with BB-M1 in an enzyme-linked immunosorbent assay and with Pf155/RESA in immunofluorescence of infected erythrocytes and immunoblotting. Inhibition studies revealed that the antibodies mainly recognized epitopes formed by two or more EENVEHDA subunits and were remarkably specific for Pf155/RESA. Importantly, the antibodies also inhibited P. falciparum merozoite reinvasion in vitro efficiently, indicating that they reacted with biologically important epitopes exposed on the native antigen. Immunization with Freund complete adjuvant resulted in high levels of specific immunoglobulin G antibodies over a 1-year period, whereas the antibody response obtained after immunization without adjuvant was generally weaker, immunoglobulin G and M mediated, and not sustained for longer periods. However, these titers were restored after booster injection. Taken together, the results support the usefulness of recombinant gene constructs of this type as immunogens for malaria vaccines.