ICOS and infection: However, a recrudescent, low burden infection persists in WT C57BL/6 mice for several months, only resolving fully between 90–100 days p.i. Following control of the acute infection, a relatively small frequency of TCM cells were present in the spleen of WT and Icos-/- mice at day 35 p.i. and their numbers were comparable to that seen in naïve mice (Fig 1A–1C).