CD4+ YFP+ GFP+, CD4+ YFP+ GFP−, and CD4+ YFP− GFP− T cells were sort purified from the spleen, liver, and lungs of mice on day 7 of infection (Fig. 6B), and equal numbers of tissue-derived cells were stimulated in vitro for 24 h with anti-CD3 and anti-CD28, after which the supernatant was assayed for cytokine production. This evidence concerns the gene CD28 and infection.