PROJECT SUMMARY Toxoplasma gondii is an obligate intracellular parasite that causes severe disease in immunocompromised individuals (e.g., AIDS patients) and fetuses. Its abilities to proliferate inside all nucleated cells and establish an infection in almost all warm-blooded animals make the parasite an ideal model to study the mechanisms involving in host-pathogen interaction. The outcome of Toxoplasma infection varies between host species. For example, mice generally succumb to acute Toxoplasma infection while most non-murine hosts, such as rats and humans, usually do not display symptoms but the chronic infection is established. As one of the cell types determining the infection outcome, macrophages play an essential role in the early innate immune response against Toxoplasma and coordinate the adaptive immune response. To successfully establish infection, Toxoplasma co-opts host macrophages via parasite effectors secreted from its unique secretory organelles (e.g., rhoptry and dense granule), named ROPs and GRAs, respectively. The role of Toxoplasma effectors has been predominantly studied in murine macrophages, however, the effectors contributing to the infection in rat and human macrophages are mostly unknown. My previous studies, together with preliminary data of this project, identified several Toxoplasma effectors that specifically modulate the innate immune response in rat but not murine macrophages. My central hypothesis is that Toxoplasma secretes a different set of parasite effectors involved in the modulation of various innate immune responses to establish an infection in macrophages from non-murine hosts. The goals during the K99 mentored phase are: 1) to understand the mechanism of Toxoplasma effector-mediated activation of the NLRP1 inflammasome in rat macrophages; 2) to mechanistically determine the role of Toxoplasma effectors that are required for parasite proliferation in naïve rat macrophages. In the independent R00 phase, I will apply the training from the mentored phase to study the interaction between Toxoplasma effectors and the innate immune response in human macrophages. Specifically, I will identify fitness-conferring Toxoplasma secreted effectors and mainly determine parasite effector-regulated host transcriptional responses in human macrophages. To accomplish these goals, valuable training from a highly complementary mentor team with scientific and mentoring skills will guide my path to an independent researcher. Furthermore, I will develop my leadership skills and strengthen my technical skills through the proposed training. Collectively, the completion of this project will enhance our understanding of the molecular mechanisms controlling host susceptibility to Toxoplasma infection. Also, knowing the parasite effectors important for host modulation will provide better drug targets against toxoplasmosis.