We have generated recombinant adenoviruses encoding three genetically modified
surface antigens (SAGs) of the parasite Toxoplasma gondii, that is, AdSAG1, AdSAG2, and AdSAG3. Modifications included the removal of their
glycosylphosphatidylinositol (GPI) anchoring motifs and, in some cases, the exchange of the native
signal peptide for influenza virus
hemagglutinin signal sequence. Adenovirus immunization of BALB/c mice elicited potent antibody responses against each
protein, displaying a significant bias toward a helper T cell type 1 (Th1) profile in animals vaccinated with AdSAG1. Furthermore, the presence of parasite-specific IFN-gamma-producing T cells was analyzed by proliferation assays and
enzyme-linked immunospot assays in the same animals. Splenocytes from immunized mice secreted IFN-gamma after in vitro stimulation with tachyzoite lysate
antigen or with a fraction enriched for membrane-purified
GPI-anchored proteins (F3) from the T. gondii tachyzoite surface.
Epitopes recognized by CD8+ T cells were identified in SAG1 and SAG3, but not SAG2, sequences, although this
protein also induced a specific response. We also tested the capacity of the immune responses detected to protect mice against a challenge with live T. gondii parasites. Although no protection was observed against tachyzoites of the highly virulent RH strain, a significant reduction in
cyst loads in the brain was observed in animals challenged with the P-Br strain. Thus, up to 80% of the
cysts were eliminated from animals vaccinated with a mixture of the three recombinant viruses. Because adenoviruses seemed capable of inducing Th1-biased protective immune responses against T. gondii
antigens, other parasite
antigens should be tested alone or in combination with those described here to further develop a protective
vaccine against
toxoplasmosis.