The IL-7–induced proliferation of adult CD31⁺ naive CD4⁺ T cells is dependent on the PI3K pathway. CD31⁺ and CD31⁻ naive CD4⁺ T cells were purified from adult peripheral blood and CB, cultured in the presence of IL-7 with or without the PI3K inhibitor LY294002 or the MEK-ERK inhibitor PD98059 as indicated, and harvested at day 7 of culture. DMSO was used as a vehicle control. Representative examples of the 6 adults and 4 CBs studied are shown. (A) Assessment of proliferation using Ki67 in an adult sample. Representative analysis of a CB (1 of 4) is also shown illustrating the blocking effects of LY294002 on whole naive CD4⁺ T-cell subset proliferation as assessed by CFSE labeling. CD31 staining is shown on the y-axis. (B) IL-7Rα and (C) Bcl-2 expression analyzed at day 0 within CD31⁺ (gray filled histograms) and CD31⁻ cells (black line). Analysis at day 7 within CD31⁺ (red line) and CD31⁻ (green line) purified populations cultured in the presence of IL-7 and the indicated inhibitors are also shown. (D) Evaluation of apoptosis by annexin V and PI staining after 7 days of culture of the purified CD31⁺ and CD31⁻ naive subsets.