CD4 and infection: We found that that the frequency of IFN-γ expression byCD4+ T cells in the lungs varied with the time of infection(Figure 1B).IFN-γ+ CD4+ cells were undetectable inthe lungs at day 14, increased in frequency beginning by day 21 to a peak at day35 post-infection, and then markedly decreased afterward; no more than 7%of the bulk population of CD4+ T cells expressed IFN-γ atany time point after infection, and fewer than 1% expressed IFN-γduring the chronic phase.