The Expression of PD-1 Ligands and Their Involvement in Regulation of T Cell Functions in Acute and Chronic Woodchuck Hepatitis Virus Infection

BACKGROUND The programmed cell death 1 (PD-1)/programmed death-1 ligand 1 (PD-L1) system may play a role in the negative regulation of T cell functions in hepatitis B virus (HBV) infection. Thus, it is important to study its role in the widely used animal model for HBV infection of woodchucks with woodchuck hepatitis virus (WHV). METHODS Woodchuck PD-L1 (wPD-L1) and -L2 (wPD-L2) were cloned and characterized. The levels of wPD-L1 expression in primary woodchuck hepatocytes (PWH), peripheral blood mononuclear cells (PBMCs), and liver tissue of naive and WHV-infected woodchucks were examined by real time reverse transcription (RT)-PCR and flow cytometry. Using antibodies against wPD-L1 and -L2, the effect of blocking PD-1/PD-L1/PD-L2 interaction on the proliferation and degranulation of woodchuck PBMCs was examined. PRINCIPAL FINDINGS Both wPD-L1 and -L2 showed a high homology to their counterparts of other mammalian species and humans. WPD-L1 expression in PWH and PBMCs of naive animals was low but could be stimulated by Toll-like receptor (TLR) ligands and interferons (IFN). WPD-L1 expression in liver tissue was significantly higher than that measured in PWHs and was slightly elevated during acute and chronic WHV infection. However, wPD-1 and wPD-L1 expression on PBMCs was strongly up-regulated during acute and chronic infection. In vitro blockade with antibodies against wPD-L1 and -L2 partially enhanced proliferation and degranulation of PBMCs from WHV-infected woodchucks. CONCLUSIONS Our results demonstrated that wPD-1/wPD-L1 expression in hepatocytes and PBMCs can be induced by different inflammatory stimuli and is up-regulated mainly on PBMCs during WHV infection. A blockade of the woodchuck PD-1/PD-L pathway could partially enhance T cell functions in WHV infection.