TGF-β-dependent induction of CD4⁺CD25⁺Foxp3 ⁺ Tregs by liver sinusoidal endothelial cells

Background & Aims CD4⁺ CD25⁺ Foxp3⁺ regulatory T cells (Tregs) have a profound ability to control immune responses. We have previously shown that the liver is a major source of peripherally induced Tregs. Here, we investigate the liver cell types and molecular mechanisms responsible for hepatic Treg induction. Methods To assess the Treg-inducing potential of liver resident antigen-presenting cell types, we studied the conversion of Foxp3^- non-Tregs into Foxp3⁺ Tregs induced by liver dendritic cells (DCs), liver sinusoidal endothelial cells (LSECs), or Kupffer cells (KCs). The dependency of Treg induction on TGF-β was tested in Treg conversion assays using T cells with reduced TGF-β sensitivity. The suppressive potential of liver cell-induced Tregs was assessed by an in vitro suppression assay and in vivo, in the model of experimental autoimmune encephalomyelitis (EAE). Results All tested liver cell types were capable of inducing Foxp3⁺ Tregs; however, LSECs were most efficient in inducing Tregs. Treg-induction was antigen-specific and depended on TGF-β. LSECs featured membrane-bound LAP/TGF-β and the anchor molecule GARP, which is required for tethering LAP/TGF-β to the cell membrane. LSEC-induced Tregs suppressed proliferation and cytokine secretion of effector T cells in vitro. LSEC-induced Tregs were also functional suppressors in vivo, as neuroantigen-specific Tregs induced by LSECs were able to suppress EAE. Conclusions We demonstrate that LSECs are the major liver cell type responsible for TGF-β dependent hepatic Treg induction. The extraordinary capacity of LSECs to induce Tregs was associated with their unique ability to tether TGF-β to their membrane.