CD4 and infection: Although CD4+YFP+GFP− T cell–derived memory cells did not upregulate IL-10 expression within the liver during secondary infection (Fig. 4B, 4C), the large expansion of CD4+YFP+GFP− T cell–derived memory cells within the organ during challenge infection (Fig. 4A) led to a significant increase in the number of CD4+YFP+GFP+ T cells within the liver during secondary infection (Fig. 4D).