CD8+ T cell recognition of cyst-derived antigen during chronic CNS infection with Toxoplasma gondii

Toxoplasma gondii establishes persistent infection within neurons by forming cysts that harbor bradyzoites and it was previously thought that cysts were protected from immune recognition. However, current evidence suggests that CD8+ T cells can both recognize cyst-derived antigens and potentially participate in cyst control – though it remains unclear how cyst antigens would become available to enter the MHC-I pathway for presentation to CD8+ T cells. To explore if cyst-derived antigens can be transported beyond the cyst wall for direct presentation by infected neurons, we generated a BAG1-GRA6-SIINFEKL (BAG1-GRA6-SIIN) parasite line in which there is bradyzoite-specific expression of the SIINFEKL CD8+ T cell epitope linked to GRA6, a secreted dense granule protein hypothesized to be transported across the cyst wall into the host cell cytoplasm. In C57BL/6 mice infected with BAG1-GRA6-SIIN, SIINFEKL-specific CD8+ T cell responses were generated yet only detectable primarily in the brain during the chronic phase, confirming both the stage-specific expression of the construct and its availability to enter the MHC-I pathway. To assess if SIINFEKL-specific CD8+ T cells encounter their cognate antigen in the CNS, TCR reporter Nur77-GFP-OT-Is were transferred into and then recovered from the brains of BAG1-GRA6-SIIN-infected mice. A proportion of CNS-derived OT-Is were found to express Nur77-GFP, indicating that these T cells had recently encountered cyst-derived antigen. The use of bone marrow chimeras to restrict the expression of MHC-I expression suggested that CNS-resident cells (including neurons) were not required for local antigen presentation of cyst-derived antigens. Future studies will identify which cells present cyst-derived antigen in the CNS and determine if antigen can be transported beyond the cyst wall for direct presentation by neurons.