IFN-λ1 does not affect T-cell proliferation. Naive and memory CD4⁺ T cells were labeled with CFSE and cultured under Th2 (anti-CD2/3/28 beads + 10 ng/mL IL-4) or control (10 ng/mL IL-4) conditions for 6 days in the absence (left) or presence (middle) of IFN-λ1. The cells were stained with PECy5.5-labeled anti-CD4 antibody and analyzed by flow cytometry to assess cell division. Histogram overlays (right) compare CFSE staining in IFN-λ1–treated (bold line) to untreated (gray filled) cells. One representative experiment of 3 is shown.