TLR1/2 Ligand–Stimulated Mouse Liver Endothelial Cells Secrete IL-12 and Trigger CD8+ T Cell Immunity In Vitro

Abstract

Liver sinusoidal endothelial cells (LSECs) are unique organ-resident APCs capable of Ag cross-presentation and subsequent tolerization of naive CD8+ T cells. Under certain conditions, LSECs can switch from a tolerogenic to an immunogenic state and promote the development of T cell immunity. However, little is known about the mechanisms of LSECs to induce T cell immunity. In this study, we investigated whether functional maturation of LSECs can be achieved by TLR ligand stimulation and elucidated the mechanisms involved in LSEC-induced T cell immunity. We demonstrate that pretreatment of LSECs with palmitoyl-3-cysteine-serine-lysine-4 (P3C; TLR1/2 ligand) but not poly(I:C) (TLR3 ligand) or LPS (TLR4 ligand) reverted their suppressive properties to induce T cell immunity. Importantly, P3C stimulation caused functional maturation of Ag-presenting LSECs and enabled them to activate virus-specific CD8+ T cells. The LSEC-mediated CD8+ T cell immunity was initiated by soluble mediators, one of which was IL-12 secreted at a low but sustained level after P3C stimulation. P3C stimulation did not induce programmed death ligand 1 expression on LSECs, thereby favoring T cell proliferation and activation instead of suppression. Our data suggest that LSECs undergo maturation exclusively in response to TLR1/2 ligand stimulation and that the immunological status of LSECs was dependent upon the balance between programmed death ligand 1 and IL-12 expression. These results have implications for our understanding of liver-specific tolerance and autoimmunity and for the development of strategies to overcome T cell tolerance in situations such as chronic viral liver infections or liver cancer.

Role of Toll-Like Receptors in Immune Activation and Tolerance in the Liver

“…An appropriate T cell response is required to eradicate HBV and HCV, while exhausted HCV-specific T cells with inhibitory immune receptors, such as PD-1 and CTLA-4, account for persistent viral infection within the liver (3, 4, 81, 82). LSECs with up-regulated PD-L1 expression were reported to induce antigen-specific T cell tolerance (40), and recent reports indicated that stimulation of LSECs with TLR1/2 ligands, but not TLR3 or TLR4 ligands could overcome liver-specific tolerance (83). Further study is required to clarify the effect of TLR1/2 ligands on the function of tolerant HBV- and HCV-specific T cells.…”

Contribution of Toll-like receptors to the control of hepatitis B virus infection by initiating antiviral innate responses and promoting specific adaptive immune responses

“…59 Recently, we examined functional maturation of LSECs by TLR ligand stimulation, demonstrating that pretreatment of LSECs with P3C (TLR1/2 ligand) but not poly(I:C) (TLR3 ligand) or LPS (TLR4 ligand) reverted their suppressive properties to induce specific T-cell immunity. 60 IL-12, which was produced at a low but sustainable level after TLR2 stimulation, was identified as one essential mediator for LSEC-mediated CD8 1 Tcell immunity. 60 In chronically HBV-infected patients, it was demonstrated that IL-12 but not IFN-a is capable of rescuing the anti-viral function of exhausted HBV-specific CD8 1 T cells.…”

“…60 IL-12, which was produced at a low but sustainable level after TLR2 stimulation, was identified as one essential mediator for LSEC-mediated CD8 1 Tcell immunity. 60 In chronically HBV-infected patients, it was demonstrated that IL-12 but not IFN-a is capable of rescuing the anti-viral function of exhausted HBV-specific CD8 1 T cells. 61 Therefore, we speculate that activation of the TLRmediated signaling pathway may lead to an enhancement of HBV-specific T-or B-cell responses in the liver.…”

Hepatitis C Virus Infection Induces Autocrine Interferon Signaling by Human Liver Endothelial Cells and Release of Exosomes, Which Inhibits Viral Replication

“…Prior work in LSECs has largely focused on Toll-like receptor (TLR)-4 and its ligand, lipopolysaccaride (LPS), which is constantly present in the portal venous blood 67 . However, the precise role of LSECs in sensing other TLR ligands, such as viral products, is just beginning to emerge 6, 8, 9 . Moreover, in addition to the TLR system, retinoic acid inducible gene-I (RIG-I)-like receptors (RLRs) have been identified as cytosolic receptors for intracellular double-stranded RNA sensing 10 .…”