FOXP3 and infection: Additionally, since FoxP3 expression in CD4 T cells is driven by TGF‐beta (Xu et al., 2010), we quantified this and observed that while there was no difference in TGF‐beta in the BAL in uninfected young and aged mice, significantly higher levels of TGF‐beta were present in aged groups on days 5 and 7 post‐infection when compared to young mice.