IFN-λ1 directly inhibits Th2 cytokine production by activated memory T cells and Th2 polarization of naive T cells. (A) IL-13 production by naive and memory CD4⁺ T cells. Purified populations of each were stimulated under neutral (anti-CD2/3/28 beads only) or Th2 conditions (anti-CD2/3/28 beads + 10 ng/mL IL-4) in 24-well plates, at a concentration of 10⁶ cells/mL for 1, 3, and 6 days. Supernatants (SNs) were harvested and analyzed for the presence of IL-13 by ELISA. The effect of IFN-λ1 on production of Th2 cytokines was determined by culturing purified naive (top) or memory CD4⁺ T cells (bottom) under neutral or Th2 conditions for 1 or 3 days. Where indicated, cells were cultured in the presence or absence of 100 ng/mL IFN-λ1. SNs were harvested from memory T cells at day 1 and from naive T cells at day 3, and tested for IL-13 (B) and IL-5 (C) by ELISA. The results from 5 individual donors are presented as means ± SDs.