Immunodominant, protective response to the parasite Toxoplasma gondii requires antigen processing in the endoplasmic reticulum

The Toxoplasma gondii peptides recognized by protective CD8+ T cells remain uncharacterized. Shastri and colleagues identify an immunodominant T. gondii peptide generated by a mechanism dependent on the endoplasmic reticulum aminopeptidase ERAAP. The parasite Toxoplasma gondii replicates in a specialized intracellular vacuole and causes disease in many species. Protection from toxoplasmosis is mediated by CD8+ T cells, but the T. gondii antigens and host genes required for eliciting protective immunity are poorly defined. Here we identified GRA6, a polymorphic protein secreted in the parasitophorous vacuole, as the source of the immunodominant and protective decapeptide HF10 presented by the H-2Ld major histocompatibility complex class I molecule. Presentation of the HF10–H-2Ld ligand required proteolysis by ERAAP, the endoplasmic reticulum aminopeptidase associated with antigen processing. Consequently, expansion of protective CD8+ T cell populations was impaired in T. gondii–infected ERAAP-deficient mice, which were more susceptible to toxoplasmosis. Thus, endoplasmic reticulum proteolysis is critical for eliciting protective immunity to a vacuolar parasite.