Targeted disruption of the glycosylphosphatidylinositol-anchored surface antigen SAG3 gene in Toxoplasma gondii decreases host cell adhesion and drastically reduces virulence in mice

The protozoan parasite Toxoplasma gondii is able to invade a broad range of cells within its mammalian hosts through mechanisms that are not yet fully understood. Several glycosylphosphatidyl inositol-anchored antigens found in the parasite membrane are considered as major determinants in the critic al interactions with the host cell. We have discovered that two of these su rface antigens, SAG1 and SAG3, share significant identity, with considerabl e similarities in structure, suggesting an overall conserved topology. To i nvestigate their physiological roles further, we have generated T. gondii m utants deficient in SAG3 through gene disruption. The disrupted strains dis play at least a twofold reduction in host cell invasion when compared with wild-type parasites. This correlated with a similar decrease in host cell a dhesion in the SAG3 null mutants. Importantly, the null SAG3 mutants show a ttenuated infectivity, with a markedly reduced capacity to cause mortality in mice, whereas both wild-type and complemented mutants that re-expressed SAG3 were lethal at the same doses. Taken together, our results indicate th at SAG3 is one member of the redundant system of T. gondii receptors that a ct as ligands mediating host cell recognition and attachment.