Reduced immunogenicity of DNA vaccine plasmids in mixtures.

We measured the ability of nine DNA vaccine plasmids encoding candidate malaria vaccine antigens to induce antibodies and interferon-gamma responses when delivered alone or in a mixture containing all nine plasmids. We further examined the possible immunosuppressive effect of individual plasmids, by assessing a series of mixtures in which each of the nine vaccine plasmids was replaced with a control plasmid. Given alone, each of the vaccine plasmids induced significant antibody titers and, in the four cases for which appropriate assays were available, IFN-gamma responses. Significant suppression or complete abrogation of responses were seen when the plasmids were pooled in a nine-plasmid cocktail and injected in a single site. Removal of single genes from the mixture frequently reduced the observed suppression. Boosting with recombinant poxvirus increased the antibody response in animals primed with either a single gene or the mixture, but, even after boosting, responses were higher in animals primed with single plasmids than in those primed with the nine-plasmid mixture. Boosting did not overcome the suppressive effect of mixing for IFN-gamma responses. Interactions between components in a multiplasmid DNA vaccine may limit the ability to use plasmid pools alone to induce responses against multiple targets simultaneously.