Regulation of the Intrahepatic CD8+ T Lymphocyte

The liver is a tolerogenic environment exploited by persistent infections such as hepatitis B (HBV) and C (HCV) viruses. Using murine models of intravenous (IV) adenovirus and mouse cytomegalovirus (MCMV) infections, I establish that antiviral CD8+ T cells primed directly in the liver acquire a dysfunctional phenotype characterized by a decreased ability to produce pro-inflammatory cytokines and cytolytically kill target cells. During the onset of intrahepatic viral infection, CD8+ T cell infiltration/expansion are accompanied by the upregulation of three core immunoregulatory pathways: IL-10, PD-1/PD-L1, and Tim-3. To examine the role of hepatic myeloid PD-L1 expression during the early phase of viral infection, I administered PD-L1 siRNA encapsulated in lipidoid nanoparticles (LNP) in mice. My studies indicate that Kupffer cells (KC) preferentially engulfed PD-L1 LNP within a short period of time and silenced Pdl1 during adenovirus and MCMV infections leading to enhanced natural killer (NK) and CD8+ T cell intrahepatic accumulation, effector function, CD8+ T cell-mediated viral clearance, and memory. Without application of PD-L1 LNP, if the dysfunctional liver-primed CD8+ T cells were allowed to expand in the natural setting, these cells also acquired a late phase T regulatory (Treg) cell function on the in vitro and in vivo priming of naïve CD8+ OT-I T cells. Liver-primed CD8+ T cell (herein renamed CD8+ Treg cells) regulatory activity was independent of PD-1/PD-L1 interaction, IL-10 production, and expression of the canonical Tim-3 ligand, Gal-9. I discovered that CD8+ Treg cell surface Tim-3 controls the expansion of antiviral CD8+ Teff cells by binding to a novel ligand, HMGB-1. HMGB-1, originally identified as a DNA binding protein, may therefore act as a potent cytokine controlling the outcome of acute and chronic viral infections in the liver. Although PD-L1 did not appear to directly play a functional role in CD8+ Treg cell suppression at the late phase antiviral immune response, PD-1/PD-L1 signaling from KCs during the early phase promoted the development of CD8+ Treg cells. Overall, KC specific PD-1/PD-L1 negative signaling was central in diminishing the early antiviral immune response, whereas Tim-3 limited late phase CD8+ T cell antiviral immunity in the liver. The presence of IL-10-producing, PD-1+PD-L1+Tim-3+CD8+ T cells in the livers of chronic HCV patients raises the possibility that liver-primed CD8+ T cells could play a pivotal role in dampening hepatic T cell responses in the local tissue microenvironment.