Effects of IL-1a, IL-17A, IL-23, and IL-27 on PD-L1 expression by human immune cells. Enriched cell populations were cultured in the presence of cytokines for 2 days; then, PD-L1 cell-surface protein expression was detected by flow cytometry (see “Methods”). (A) Monocytes were exposed to cytokines in the absence or presence of IFN-γ. IL-1a- and IL-27–enhanced PD-L1 expression in the absence of IFN-γ. IL-1a, but not IL-27, further increased PD-L1 expression in the presence of IFN-γ. In contrast, no cytokine significantly affected expression of CD86 (B7.2) on monocytes. Results are representative of 4 of 4 normal donors. (B) DCs were exposed to cytokines in the absence or presence of IFN-γ. IL-27 increased PD-L1 expression in the absence of IFN-γ, but did not further augment expression in the presence of IFN-γ. In contrast, no effects were seen on CD86 expression in the same experiment. Results are representative of 2 of 2 donors. (C) CD3⁺ T cells that were resting or activated with anti-CD3/CD28 were exposed to cytokines. IL-27 increased PD-L1 expression on both resting and activated T cells. Expression of CD69, an early T-cell activation marker, was also increased by exposure to IL-27. Results are representative of 2 of 2 donors. MAX, % of maximum cell count; MFI, mean fluorescence of intensity; ∆MFI, MFI of isotype control was subtracted from that of specific staining.