ICOS and infection: Ag-expCD4+ T cell effector function continued to deteriorate between days 9 and 15 of infection, with cells on day 15 of infection, expressing low levels of Ki67, ICOS, and CD25 and producing very low levels of TNF, IFN-γ, IL-2, and IL-10 (Fig. 2A, 2B, Supplemental Fig. 1A).