Toxoplasma gondii and the central nervous system

Anita Koshy, Stanford University
Basic-Applied Clinical

Toxoplasma gondii, the etiologic agent of toxoplasmic encephalitis (TE), is a geographically ubiquitous, intracellular parasite that has a wide range of mammalian hosts, including humans. In humans, it usually causes an asymptomatic infection after ingestion of the parasite. During the initial, acute stage of the infection, the rapidly dividing stage of the parasite, the tachyzoite, is able to disseminate to many tissues in the host, including the central nervous system (CNS.) Although the immune response is usually able to contain the acute stage, it is unable to eradicate the infection which allows the slowly dividing form, the bradyzoite, to encyst and viably persist, especially in the CNS. The latent infection can reactivate in the brain in the setting of severe T-cell immunodeficiency, as seen in AIDS patients, where TE can produce devastating neurologic consequences. Very little is known about how Toxoplasma accomplishes either dissemination or persistence. This proposal plans to exploit the mouse model to determine how Toxoplasma enters the CNS during the acute infection and where Toxoplasma resides during the chronic infection. My hypothesis is that the tachyzoite spreads to the CNS as a free parasite, breaches the CNS from parenchymal capillaries, and then rapidly enter neuronal and glial cells. As the immune system responds to the infection, I hypothesize that the parasites then transition to the bradyzoite/cyst stage, remain intracellularly and preferentially in neurons, thereby evading the immune response. To test my hypotheses, I will infect mice with a strain of Toxoplasma that has been genetically engineered to constitutively express firefly luciferase. Use of this parasite will allow the infection to be followed from its start within the gastrointestinal tract through to eventual infection of the CNS by in vivo bioluminescence imaging (BLI.) These images will be used to determine appropriate times to sacrifice the animals to capture the initial acute CNS infection, as well as determine when the parasite is in the chronic stage. Thin-sections of the infected brains will be processed by immunohistochemistry and reviewed by confocal microscopy to determine how the parasite enters the brain and where it persists in the chronic stage. Ultimately, understanding how Toxoplasma migrates to the CNS and persists in a quiescent form could lead to treatments that eradicate the infection, and thus prevent TE in those with AIDS.