Abstract Text: Interleukin-15 (IL-15) is a potent cytokine that augments lymphocyte-mediated killing and is basally produced at low levels by many cell types, including endothelial cells (ECs). The regulation of IL-15 is extremely complex with multifaceted control of transcription, translation, protein trafficking, and proteolysis. In response to pro-inflammatory signals including tissue necrosis factor (TNF) and IL-1β, ECs can display IL-15 on the cell surface as a heterodimeric complex with the alpha chain of the IL-15 receptor (IL-15Rα), where it can signal to lymphocytes in a process called trans-presentation. Trans-presented IL-15 contributes to the activation of circulating alloreactive CD8+ and CD4+ effector memory T cells. In cultured human ECs, interferon-γ (IFN-γ) increases IL-15 and IL-15Rα transcripts, but surface IL-15 is not expressed. IL-1β has a much smaller effect on IL-15 and IL-15Rα transcript levels, but results in expression of surface IL-15/IL-15Rα complexes, with or without IFN-γ. Small interfering RNA (siRNA) targeting IL-15 and IL-15Rα prevent IL-1-induced surface complex expression. Surface expression also depends on protein translation and on both activation and nuclear import of the transcription factor p65/RelA (i.e. canonical NF-κB). However, inhibition of RNA polymerase II by DRB, assessed as inhibiting E-selectin, does not prevent the surface expression of IL-15 by ECs, suggesting NF-κB regulation of translation, perhaps through non-coding RNA. Dicer knockdown by siRNA, preventing miRNA processing, produces surface IL-15/IL-15Rα complexes, in the absence of cytokine treatment. These data suggest tonic inhibition of EC surface IL-15 by an unknown miRNA(s) that is reduced in response to canonical NF-κB stimulation.
