The parasite that causes Toxoplasmosis, Toxoplasma gondii, is common and may be found in undercooked or contaminated meat, and cat feces. So most people who have a littler box at home have been exposed to the parasite. While it doesn't usually cause harm, it can lead to serious problems in fetal development, and may also cause blindness or death in immunocompromised adults.
Scientists have now learned how dormant Toxoplasmosis parasites, called bradyzoites, can infect neurons in the brain and muscle cells and interfere with their function, but extend the life of the parasite. Thus, the parasite is able to hide from the immune system, awaiting for an opportune time to awaken and cause disease. The findings have been reported in Cell Host & Microbe.
In this work, the researchers showed that the parasites can disrupt the ability of the host cell to alert the immune system to the infection. With imaging tools and genetic techniques, the scientists determined that the parasite causes a protein known as inhibitor of STAT1 transcription (IST) to be released from the host cell, which supressess signaling to the immune system.
The immune system has to be able to trigger cell death to fight parasitic infections. But a Toxoplasmosis parasite host cell is shielded from the process, because IST limits interferon signaling in bradyzoites, explained study co-author and WEHI Associate Professor Chris Tonkin. “Interferon is the beacon molecule of the immune system. It gives off a blinking sign to notify the immune system that the body has been infected with a range of diseases, including Toxoplasma,” he said.
The parasite aims to survive, so while it's dormant in a host cell, it deactivates interferon, a crucial partner in the cell's fight against disease. “So, it’s essentially a tug-of-war, a molecular battle," Tonkin added.
Researchers still have to figure out how and why the parasite reawakens to trigger disease, but there seems to be a link between brain lesions and immune deficiency. In recent years, Toxoplamsosis has also been linked to neuropsychiatric conditions, like bipolar disorder, schizophrenia, and some neurodegenerative disease, noted Tonkin. But, a causative connection has not yet been established.
“The way this parasite resides in our brains during chronic infection and the way that it shuts down our innate immune system to survive is quite unique," Tonkin said.