PD-1:PD-L1 Interactions Contribute to the Functional Suppression of Virus-Specific CD8 super(+) T Lymphocytes in the Liver

Mechanisms contributing to the development of chronic viral infections, including chronic hepatitis B virus (HBV) infections, are not well understood. We have shown recently that production of IFN- gamma , an important antiviral cytokine, by HBV-specific CTLs is rapidly induced when they enter the liver of HBV transgenic mice, and then rapidly suppressed, despite the continued presence of Ag. Suppression of IFN- gamma production by the CTLs coincides with the up-regulation of programmed cell death (PD)-1, a cell surface signaling molecule known to inhibit T cell function. To determine whether PD-1 plays a role in the functional suppression of IFN- gamma secretion by CTLs, we treated HBV transgenic mice with blocking Abs specific for PD ligand (PD-L)1, the most widely expressed PD-1 ligand, and adoptively transferred HBV-specific CTLs. Treatment with anti-PD-L1 Abs resulted in a delay in the suppression of IFN- gamma -producing CTLs and a concomitant increase in the absolute number of IFN- gamma -producing CTLs in the liver. These results indicate that PD-1:PD-L1 interactions contribute to the suppression of IFN- gamma secretion observed following Ag recognition in the liver. Blockade of inhibitory pathways such as PD-1:PD-L1 may reverse viral persistence and chronic infection in cases in which the CTL response is suppressed by this mechanism.