Toxoplasma gondii establishes an important chronic infection capable of causing life-threatening toxoplasmic encephalitis in AIDS patients and other immunocompromised individuals. The basis of persistence of the infection is the tissue cyst, which remains largely quiescent for the life of the host, but can reactivate and cause disease. This stage of the parasite is not affected by any of the current drug treatments and it has been generally regarded as untouchable. However, our recent studies revealed that the immune T cells are able to eliminate T. gondii cysts from the brains of infected hosts when the T cells are transferred into infected immunodeficient animals that have already developed large numbers of the cysts. This T cell-mediated immune process is associated with accumulation of mononuclear cells, primarily microglia and macrophages morphologically, around tissue cysts. Since the accumulated phagocytes penetrate within the cyst, these cells appear to be the main effector cells that destroy the cysts and eliminate them from the brain after initiation of this process by immune T cells. CD8+ immune T cells possess a potent activity to initiate this anti-cyst immune process, and the protective activity of the T cells requires perforin. The overall goal of this research project is to determine the molecular mechanisms by which perforin-mediated activity of CD8+ immune T cells cooperates with the phagocytes to remove T. gondii cysts from the brain. In the first aim, we will determine if CD8+ immune T cells accumulate and attach to the cyst-containing cells in the brain. We will also determine if the CD8+ T cells lyse cyst-containing cells in a perforin-dependent manner in vitro and whether the perforin- mediated activity of CD8+ T cells induces the accumulation of phagocytes around the cysts in the brain. In the second aim, we will determine the MHC class I molecule(s) critical for recognition of cyst-containing cells by CD8+ T cells. We will then determine the T. gondii cyst epitope(s) presented by the identified MHC class I molecule for recognition by the CD8+ T cells by screening predicted bradyzoite epitope peptides and/or bradyzoite cDNA library.
The third aim i s to determine the mechanisms by which phagocytes eliminate T. gondii cysts after initiation of the immune process by CD8+ T cells. We will examine the roles of inducible nitric oxide synthase and autophagy in killing of bradyzoites and elimination of cysts using mice lacking a molecule required for each mechanism. The information generated from the studies in these three specific aims all together will provide the essential basis for developing a novel method(s) to activate CD8+ T cells and phagocytes to eliminate T. gondii cysts that can reactivate and cause toxoplasmic encephalitis.

Public Health Relevance

Reactivation of chronic infection with Toxoplasma gondii causes life-threatening toxoplasmic encephalitis in AIDS patients and other immunocompromised individuals. The proposed studies are to analyze how T cells and phagocytes cooperate to eliminate the persisting cyst stage of T. gondii, which can reactivate and cause the disease. These studies will provide the information essential for developing a novel method(s) to eliminate the persisting cyst stage of the parasite and prevent or reduce the risk of developing toxoplasmic encephalitis.

National Institute of Health (NIH)
National Institute of Allergy and Infectious Diseases (NIAID)
Research Project (R01)
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AIDS-associated Opportunistic Infections and Cancer Study Section (AOIC)
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Wali, Tonu M
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University of Kentucky
Schools of Medicine
United States
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Sa, Qila; Tiwari, Ashish; Ochiai, Eri et al. (2018) Inducible nitric oxide synthase in innate immune cells is important for restricting cyst formation of Toxoplasma gondii in the brain but not required for the protective immune process to remove the cysts. Microbes Infect 20:261-266
Sa, Qila; Ochiai, Eri; Tiwari, Ashish et al. (2017) Determination of a Key Antigen for Immunological Intervention To Target the Latent Stage of Toxoplasma gondii. J Immunol 198:4425-4434
Ochiai, Eri; Sa, Qila; Perkins, Sara et al. (2016) CD8(+) T cells remove cysts of Toxoplasma gondii from the brain mostly by recognizing epitopes commonly expressed by or cross-reactive between type II and type III strains of the parasite. Microbes Infect 18:517-22
Sa, Qila; Ochiai, Eri; Tiwari, Ashish et al. (2015) Cutting Edge: IFN-? Produced by Brain-Resident Cells Is Crucial To Control Cerebral Infection with Toxoplasma gondii. J Immunol 195:796-800
Ochiai, Eri; Sa, Qila; Brogli, Morgan et al. (2015) CXCL9 is important for recruiting immune T cells into the brain and inducing an accumulation of the T cells to the areas of tachyzoite proliferation to prevent reactivation of chronic cerebral infection with Toxoplasma gondii. Am J Pathol 185:314-24
Sa, Qila; Ochiai, Eri; Sengoku, Tomoko et al. (2014) VCAM-1/?4?1 integrin interaction is crucial for prompt recruitment of immune T cells into the brain during the early stage of reactivation of chronic infection with Toxoplasma gondii to prevent toxoplasmic encephalitis. Infect Immun 82:2826-39
Sa, Qila; Woodward, Jerold; Suzuki, Yasuhiro (2013) IL-2 produced by CD8+ immune T cells can augment their IFN-? production independently from their proliferation in the secondary response to an intracellular pathogen. J Immunol 190:2199-207
Sullivan, Adam M; Zhao, Xiaopeng; Suzuki, Yasuhiro et al. (2013) Evidence for finely-regulated asynchronous growth of Toxoplasma gondii cysts based on data-driven model selection. PLoS Comput Biol 9:e1003283
Hester, James; Mullins, Jeremi; Sa, Qila et al. (2012) Toxoplasma gondii antigens recognized by IgG antibodies differ between mice with and without active proliferation of tachyzoites in the brain during the chronic stage of infection. Infect Immun 80:3611-20
Suzuki, Yasuhiro; Sa, Qila; Gehman, Marie et al. (2011) Interferon-gamma- and perforin-mediated immune responses for resistance against Toxoplasma gondii in the brain. Expert Rev Mol Med 13:e31