Corticosteroids but not cyclosporine a inhibit cytokine-induced upregulation of inducible costimulator ligand (ICOS-L) on human endothelial cells

Abstract

The recently described accessory T cell receptor “inducible costimulator” (ICOS) mediates prolonged costimulation of human T-lymphocytes resulting in proliferation and cytokine production. The ICOS pathway contributes to chronic tissue inflammation; in transplantation, animal studies suggest a crucial role in acute allograft rejection and potentially in transplant vasculopathy. While expression of the costimulatory ligand for ICOS (ICOS-L) has been demonstrated on B cells and macrophages its expression on human endothelial cells has not been studied. In the present study, constitutive expression of ICOS-L was demonstrated on the surface of human umbilical vein endothelial cells (HUVEC) by flow cytometry and immunofluorescence staining using an ICOS-Ig fusion protein and confirmed by western blotting and reverse transcription (RT)-PCR. The expression of ICOS-L on HUVEC was further regulated by cytokines in a dose- and time-dependent manner: Interleukin-1 (IL-1) and TNF-α rapidly induced a more than 3-fold upregulation of ICOS-L expression persisting for > 72h; IL-4 led to a similar increase in expression but peak expression was only reached after 72h. Stimulation with IFN-γ caused a transient, 2-fold increase in expression whereas IL-10 had no effect. 12h preincubation of HUVEC with prednisolone caused a dose-dependent decrease in basal expression of ICOS ligand (maximum: -50% at 10 μM). Prednisolone also completely abolished IL-4-induced upregulation of ICOS-L and inhibited the increase in expression after stimulation with TNF-α and IL-1 by more than 50%. In contrast, pre-incubation of HUVEC with cyclosporine had no effect upon basal or cytokine-induced expression of ICOS-L. Human endothelial cells constitutively express the novel costimulator ICOS-L on the cell surface. Several proinflammatory cytokines markedly enhance the expression of ICOS-L on endothelial cells, underscoring the potential role of the endothelium in vascular inflammation in the allograft setting. Corticosteroids appear to downregulate basal and stimulated ICOS-L ligand expression on the endothelium adding further to the immunomodulatory spectrum of these immunosuppressive agents.