IL-4 enhances IFN-λ1 by PBMCs, but not purified pDCs. (A) Peripheral blood mononuclear cells (PBMCs) were partially depleted of plasmacytoid dendritic cells (pDCs) using magnetic beads. PBMCs or pDC-depleted PBMCs were incubated overnight with the indicated stimuli. Supernatants (SNs) were harvested and IFN-λ1 was measured by enzyme-linked immunosorbent assay (ELISA; n = 3; mean ± SD is shown). The effect of interleukin-4 (IL-4) on IFN-λ1 production by PBMCs (B) or pDCs (C) was determined by stimulating either population overnight with HSV in the presence or absence of IL-4. SNs were harvested and assayed as in panel A. IL-4 was shown to significantly increased IFN-λ1 production in HSV-stimulated PBMCs (n = 18; P < .001) but not pDCs (n = 7; P = .35) as determined by the Wilcoxon signed-rank test.