Chronic infection with؉ T cells does not require their production of interferon-␥, the major mediator to prevent proliferation of tachyzoites during acute infection, but does require perforin. These results suggest that CD8 ؉ T cells induce elimination of T. gondii cysts through their perforin-mediated cytotoxic activity. Our findings provide a new mechanism of the immune system to fight against chronic infection with T. gondii and suggest a possibility of developing a novel vaccine to eliminate cysts from patients with chronic infection and to prevent the establishment of chronic infection after a newly acquired infection.

Chronic infection with Toxoplasma gondii is one of the most common parasitic infections in humans. Formation of tissue cysts is the basis of persistence of the parasite in infected hosts and this cyst stage has generally been regarded as untouchable. Here we provide the first evidence that the immune system can eliminate T. gondii cysts from the brains of infected hosts when immune T cells are transferred into infected immunodeficient animals that have already developed large numbers of the cysts. This T cell-mediated immune process was associated with accumulation of microglia and macrophages around tissue cysts. CD8+ immune T cells possess a potent activity to remove the cysts. The initiation of this process by CD8+ T cells does not require their production of IFN-γ, the major mediator to prevent proliferation of tachyzoites during acute infection, but does require perforin. These results suggest that CD8+ T cells induce elimination of T. gondii cysts through their perforin-mediated cytotoxic activity. Our findings provide a new mechanism of the immune system to fight against chronic infection with T. gondii and suggest a possibility of developing a novel vaccine to eliminate cysts from patients with chronic infection and to prevent the establishment of chronic infection after a newly acquired infection.