In contrast to the genetically resistant BALB/c mice, mice with genetically susceptible C57BL/6-background showed that the blocking of ICOS signaling by anti-ICOSL mAbs or the genetic deletion of ICOS increases numbers of CD4+ and CD8+ T cells and IFN-γ+ CD8+ T cells in the spleens and brains 5–6 weeks after infection, but significantly greater numbers of T. gondii cysts were detected in the brains of the infected ICOS−/− than WT mice [36,37]. The gene discussed is ICOSLG; the disease is infection.