Consistently, a marked reduction of cyst numbers and upregulation of ICOS, CXCR3, CXCR6, IL-18R1, and Chil3 mRNA levels were also detected when the perforin-sufficient CD8+ immune T cells were transferred to infected SCID mice at 6 weeks after infection, indicating that the CD8+ T cell-mediated protective immunity is capable of eliminating mature T. gondii cysts. This evidence concerns the gene ICOS and cyst.