CD4 and infection: To further explore the mechanisms underpinning the age-related loss of Treg cell prorepair function after influenza infection, we performed gene expression profiling using RNA-Seq on flow cytometry–sorted lung CD3ε+CD4+CD25hiFR4+ Treg cells during the naive state or late recovery phase from influenza (day 60 after infection; Supplemental Figure 3D, Figure 4A, and ref. 23).