Uncovering the mechanisms employed by latent Toxoplasma gondii within neurons to evade immune surveillance

IFN-γ-induced anti-microbial mechanisms are instrumental for parasite control and host protection during T. gondii (Tg) infection. Upon brain invasion by Tg, neurons upregulate surface MHC class I molecules, allowing them to present tachyzoite antigens to CD8+ T cells, which is key to avoid excessive parasite burden (Salvioni, Cell Rep 2019 PMID: 31189109). Neurons also restrict parasite growth in response to IFN-γ, although neurons may be less able to do so than other cells. Tg eventually persists in the brain throughout the latent phase of infection as bradyzoites within intra-neuronal cysts. The mechanisms by which Tg bradyzoite-infected neurons evade CD8+ T cell recognition, remain unclear.

Here we investigate to which extent Tg effectors that hijack IFN-γ responses in various cell types, also modulate the responses of infected neurons to IFN-γ. Using primary cultures of hippocampal neurons stimulated or not with IFN-γ, we assessed the nuclear translocation of IRF1 transcription factor and MHC I surface expression in neurons containing mostly tachyzoites (24 hpi) or bradyzoites (72 hpi). IRF1 was translocated in the host cell nucleus and surface expression of MHC I was upregulated after IFN-γ stimulation. Neurons stimulated by IFN-γ and infected by WT tachyzoites or bradyzoites failed to accumulate IRF1 in their nuclei. Conversely, neurons infected with TgIST-deficient parasites showed higher nuclear IRF1 intensity. Strikingly, neurons infected by tachyzoites upregulated surface MHC I similarly as uninfected ones, but neurons infected by bradyzoites showed a clear TgIST-dependent inhibition of surface MHC I upregulation.

These results suggest that the impairment of neuronal MHC I is TgIST-dependent but is delayed compared to the subversion of IRF1, suggesting that MHC I subversion may occur after or upon bradyzoite differentiation. Experiments are underway to explore the impact of bradyzoite-derived TgIST and other parasite effectors on the neuronal responses to IFN-γ, and ultimately, on the ability of CD8+ T cells to detect infected neurons and control the parasite in vivo.