CEACAM1 and infection: However, in Ceacam1–/– mice these CD8+ T cell functions were impaired by infection with either amount of LCMV-Docile PFUs, because the ratio of the percentage of IFN-γ+ T cells to the percentage of Tet-GP33+ CD8+ T cells was reduced and was almost equally independent of antigen load, a finding suggesting that the lack of CEACAM1 renders CD8+ T cells sensitive to exhaustion (Fig. 1f).