PD-L1 is induced in hepatocytes by viral infection and by interferon-α and -γ and mediates T cell apoptosis
Introduction
The liver is constantly exposed to bacterial products and food-derived antigens. These antigens and microbiologically derived molecules provide a unique hepatic microenvironment and require unique immunological properties of the liver which often induces peripheral tolerance rather than immunity [1], [2]. Hepatic tolerance may also contribute to the common ineffectiveness of immune responses against hepatitis viruses such as HBV and HCV which often results in chronic persistence of viral infection [3]. The molecular mechanisms underlying these observations have not yet been elucidated. However, it has been hypothesized that there are unusual interactions within the hepatic milieu and immune cells [1].
B7-like molecules and their cognate receptors constitute important costimulatory pathways that control and fine-tune immune responses. In recent years, an array of new members of the B7 family have been identified, including B7-H1 (PD-L1) and B7-DC (PD-L2) [3]. Both are ligands for programmed death-1 (PD-1), which is expressed on activated T and B cells [4], [5], [6]. PD-L1 has been described to be inducibly expressed in a variety of organs [3], [7], [8], [9], [10], [11], [12], [13]. An overwhelming number of studies supported the role of PD-L1 as a negative regulator of T cell responses and suggest that PD-L1 promotes peripheral tolerance [14].
Recently, hepatic accumulation and impaired apoptosis in CD8+ T cells have been observed in experimental autoimmune hepatitis in PD-L1-deficient mice leading to accelerated hepatocyte damage [15]. Similarly, adenoviral infection of PD-1-deficient mice resulted in increased proliferation of effector T cells in the liver and more rapid clearance of the virus compared to wild-type mice, leading to a more severe but more transient hepatitis [16].
Hepatic PD-L1 expression was reported recently in murine non-parenchymal liver cells [16], [17]. Here, we analyzed PD-L1 expression in human liver cells. We report that – in addition to non-parenchymal liver cells – also hepatocytes constitutively express PD-L1 and provide data suggesting a novel bidirectional interaction between hepatocytes and lymphocytes modulated by PD-L1 expression in hepatocytes.
Section snippets
Cell isolation and cell culture
Isolation and culture of hepatocytes and non-parenchymal liver cells were performed as described [18], [19].
HepG2 and PLC cells were purchased from the American Type Culture Collection (Rockville, MD). HepG2.2.15 cells with integrated full-length HBV genome were kindly provided by Dr. Jilg (Institute of Medical Microbiology and Hygiene, University of Regensburg). For some experiments, cells were stimulated with recombinant IFN-α2a (R&D Systems, Wiesbaden, Germany) or IFN-γ (Sigma, Deisenhofen,
PD-L1 expression in primary liver cells
Initially, we analyzed PD-L1 mRNA expression in fractionally isolated human liver cells. In accordance with previous studies analyzing murine liver cells [16], [17] we detected PD-L1 mRNA expression in Kupffer cells and endothelial cells. Further, a faint band was also visible in freshly isolated hepatic stellate cells (HSC) while a strong band was found in HSC activated in vitro. In addition, PD-L1 mRNA expression was clearly detectable in primary human hepatocytes (PHH) (Fig. 1A).
Accordingly,
Discussion
We demonstrated for the first time that hepatocytes constitutively express low levels of PD-L1, while its expression is strongly enhanced by activated T cells and viral infection, and markedly augmented by further stimulation with type I or type II interferons. Moreover, PD-L1 expression on hepatocytes induces apoptosis in T cells.
The detection of constitutive PD-L1 expression in human hepatocytes in addition to non-parenchymal liver cells is novel and surprising, since a recent study analyzing
Acknowledgements
We are indebted to Dr. Wolfgang Jilg (Institute of Medical Microbiology and Hygiene, University of Regensburg, Germany) for providing HepG2.2.15 and to Gertrud Lallinger and Dagmar Halbritter for excellent technical assistance. This work was supported by grants from the Else-Kröner Fresenius Stiftung to C.H., Deutsche Krebshilfe to C.B., and Wilhelm Sander-Stiftung to M.F.
References (46)
- et al.
Local control of the immune response in the liver
Immunol Rev
(2000) - et al.
Neighborhood politics: the immunoregulatory function of organ-resident liver endothelial cells
Trends Immunol
(2001) - et al.
Fas ligand-induced apoptosis as a mechanism of immune privilege
Science
(1995) - et al.
B7-H1, a third member of the B7 family, co-stimulates T-cell proliferation and interleukin-10 secretion
Nat Med
(1999) - et al.
Engagement of the PD-1 immunoinhibitory receptor by a novel B7 family member leads to negative regulation of lymphocyte activation
J Exp Med
(2000) - et al.
PD-L2 is a second ligand for PD-1 and inhibits T cell activation
Nat Immunol
(2001) - et al.
Induction of tolerance to autoimmune diabetes with islet antigens
J Exp Med
(1992) - et al.
The expression of B7-H1 on keratinocytes in chronic inflammatory mucocutaneous disease and its regulatory role
Immunol Lett
(2004) - et al.
Probing the mechanism of TNF-alpha(cachectin)- and TNF-beta(lymphotoxin)-induced pancreatic inflammation with transgenic mice
Res Immunol
(1993) - et al.
Costimulator B7-1 confers antigen-presenting-cell function to parenchymal tissue and in conjunction with tumor necrosis factor alpha leads to autoimmunity in transgenic mice
Proc Natl Acad Sci USA
(1994)
Differential induction of adhesion molecule and chemokine expression by LTalpha3 and LTalphabeta in inflammation elucidates potential mechanisms of mesenteric and peripheral lymph node development
J Immunol
BLC expression in pancreatic islets causes B cell recruitment and lymphotoxin-dependent lymphoid neogenesis
Immunity
Accessory cell function of airway epithelial cells
Am J Physiol Lung Cell Mol Physiol
The balance of immune responses: costimulation verse coinhibition
J Mol Med
B7-H1 determines accumulation and deletion of intrahepatic CD8(+) T lymphocytes
Immunity
PD-1 inhibits antiviral immunity at the effector phase in the liver
J Exp Med
Inhibition of T-cell responses by hepatic stellate cells via B7-H1-mediated T-cell apoptosis in mice
Hepatology
Expression of intracellular adhesion molecule 1 by activated hepatic stellate cells
Hepatology
Activated hepatic stellate cells express keratinocyte growth factor in chronic liver disease
Am J Pathol
Mature but not immature Fas ligand (CD95L)-transduced human monocyte-derived dendritic cells are protected from Fas-mediated apoptosis and can be used as killer APC
J Immunol
Blockade of programmed death-1 ligands on dendritic cells enhances T cell activation and cytokine production
J Immunol
Program death-1 engagement upon TCR activation has distinct effects on costimulation and cytokine-driven proliferation: attenuation of ICOS, IL-4, and IL-21, but not CD28, IL-7, and IL-15 responses
J Immunol
B7-H1 is expressed by human endothelial cells and suppresses T cell cytokine synthesis
J Immunol
Cited by (324)
Early application of IFNγ mediated the persistence of HBV in an HBV mouse model
2024, Antiviral ResearchPreclinical development and clinical studies of targeted JAK/STAT combined Anti-PD-1/PD-L1 therapy
2024, International ImmunopharmacologyA dual-functional oncolytic adenovirus ZD55-aPD-L1 scFv armed with PD-L1 inhibitor potentiates its antitumor activity
2024, International Immunopharmacology
- ♯
Both authors contributed equally to this work.