Toxoplasma gondii: the role of parasite surface and secreted proteins in host cell invasion.

The potential role of the 5 surface proteins of Toxoplasma gondii tachyzoites in host cell invasion was investigated using an in vitro neutralization assay. Supporting earlier findings, TG05.54, a monoclonal antibody recognizing the major surface protein SAG 1, was shown to cause a consistent and significant blockade of invasion into bovine kidney cells, indicating a functional role for this protein in host cell invasion. The neutralizing effect was only seen with certain anti-SAG 1 monoclonal antibodies, suggesting the presence of a functional ligand within the molecule. A second surface protein, SAG 2 was also shown to be involved in the invasion process. Anti-SAG 2 antibodies prevented parasite reorientation, leaving zoites immobilized on the host cell membrane and resulting in increased internalization of tachyzoites. Antibodies recognizing other surface, rhoptry, dense granule and microneme molecules had no effect on invasion.