A previous study, conducted by others [35] using the BALB/c-background ICOS−/− mice, showed that whereas percentages of IFN-γ+ cells in CD4+ T cells in the spleens were reduced in ICOS−/− mice when compared to WT mice during the acute stage (day 7) of T. gondii infection, percentages of IFN-γ+ cells in CD8+ T cells did not differ between ICOS−/− and WT mice in their spleens during the early stage of infection and in their brains during a later stage (weeks 4–6) of infection. This evidence concerns the gene CD4 and infection.