Characterization of Antibody Responses to a Plasmodium falciparum Blood-Stage Antigen Induced by a DNA Prime/Protein Boost Immunization Protocol

The humoral immune responses elicited by priming with a DNA plasmid and boosting with either the plasmid or the corresponding recombinant protein in alum adjuvant were compared. The plasmid DNA encoded a sequence (M3) derived from the Plasmodium falciparum antigen Pf155/RESA, and the recombinant protein consisted of the identical malarial sequence fused to an albumin-binding region (BB) of streptococcal protein G. Mice of different genetic backgrounds (CBA, Balb/c and C57Bl/6) were primed with plasmid DNA and boosted with either plasmid or recombinant protein. In all strains of mice, boosting with protein elicited higher anti-M3 antibody levels than obtained by boosting with plasmid, yet the kinetics and longevity of the secondary responses were comparable. Antiserum obtained after protein boosting displayed an immunoglobulin (Ig)G subclass profile skewed to the IgG1 isotype, regardless of the mouse strain. In contrast, mice receiving a second injection with plasmid responded with a more mixed IgG subclass profile. Inclusion of a P. falciparum circumsporozoite protein-derived T-helper epitope (CS.T3) in the immunization plasmid as well as in the fusion protein, did not significantly change the humoral responses to M3. The results show the potential of DNA vaccination for the purpose of priming an antibody response against the malarial blood-stage antigen Pf155/RESA. When combined with a protein boost, this DNA priming results in high-titred and long-lasting anamnestic responses.