Pathogenetic Mechanisms of T Cell Dysfunction in Chronic HBV Infection and Related Therapeutic Approaches

Abstract

A great effort of research has been devoted in the last few years to developing new anti-HBV therapies of finite duration that also provide effective sustained control of virus replication and antigen production. Among the potential therapeutic strategies, immune-modulation represents a promising option to cure HBV infection and the adaptive immune response is a rational target for novel therapeutic interventions, in consideration of the key role played by T cells in the control of virus infections. HBV-specific T cells are severely dysfunctional in chronic HBV infection as a result of several inhibitory mechanisms which are simultaneously active within the chronically inflamed liver. Indeed, the liver is a tolerogenic organ harboring different non-parenchymal cell populations which can serve as antigen presenting cells (APC) but are poorly efficient in effector T cell priming, with propensity to induce T cell tolerance rather than T cell activation, because of a poor expression of co-stimulatory molecules, up-regulation of the co-inhibitory ligands PD-L1 and PD-L2 upon IFN stimulation, and production of immune regulatory cytokines, such as IL10 and TGF-β. They include resident dendritic cells (DCs), comprising myeloid and plasmacytoid DCs, liver sinusoidal endothelial cells (LSECs), Kupffer cells (KCs), hepatic stellate cells (HSCs) as well as the hepatocytes themselves. Additional regulatory mechanisms which contribute to T cell attrition in the chronically infected liver are the high levels of soluble mediators, such as arginase, indoleamine 2,3-dioxygenase (IDO) and suppressive cytokines, the up-regulation of inhibitory checkpoint receptor/ligand pairs, the expansion of regulatory cells, such as CD4+FOXp3+ Treg cells, myeloid-derived suppressor cells and NK cells. This review will deal with the interactions between immune cells and liver environment discussing the different mechanisms which contribute to T cell dysfunction in chronic hepatitis B, some of which are specifically activated in HBV infection and others which are instead common to chronic inflammatory liver diseases in general. Therapeutic interventions targeting dysregulated pathways and cellular functions will be also delineated.

Keywords: T cell exhaustion; chronic HBV infection; immune-therapy; immunoregulatory mechanisms; liver environment.

FIGURE 1

Relationship between antigen persistence and anti-viral T cell responses in the liver. Priming of naïve CD8 T cells can occur in lymph nodes (LNs) or within the liver. In the lymph nodes naïve CD8 T cells differentiate into functional effectors; after migration to the liver, if the majority of hepatocytes are infected and express high antigen levels, virus-specific CD8 T cells undergo functional impairment or physical deletion. Some specific features of exhausted HBV-specific CD8 T cells are the over-expression of multiple inhibitory receptors and pro-apoptotic molecules, CD3ζ chain down-regulation and various metabolic alterations (left panel). If only a minor proportion of liver cells express limited amounts of viral antigens, virus specific CD8 cells can maintain efficient anti-viral activity and can promote virus control and antigen clearance (right panel). This model of T cell activation derives form studies performed in mouse models of HBV infection, but no definitive evidence is available in human infection to confirm that induction of functionally efficient effector CD8 T cells is driven by the amount of antigen (number of infected hepatocytes and amount of antigen expressed by individual liver cells) and that decline of antigen can allow restoration of functionally efficient HBV-specific CD8 T cell responses.

FIGURE 2

Immunosuppressive circuits in the liver. Within the infected liver, persistent expression of viral antigens in concert with different immunoregulatory pathways drive T cell differentiation toward exhaustion. Upon inflammation, IFN-γ stimulation activates immunosuppressive mechanisms that can be mediated by soluble factors or require cell to cell contact. (A) Soluble factors: immunosuppressive cytokines, such as TGF-β and IL-10 can be produced by expanded regulatory T cells (Treg) as well as by stellate cells (HSCs), dendritic cells (DCs) and Kupffer cells (KCs). In addition, a number of liver infiltrating cells, such as monocytes, macrophages and dendritic cells, can release the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO), which causes either tryptophan depletion with consequent dampening of T cell proliferation and function or the generation of toxic catabolites, such as kynurenines, which can cause T cell apoptosis and CD4 differentiation in Treg cells. Another key soluble mediator is represented by arginase, released by damaged hepatocytes and myeloid-derived suppressor cells (MDSC), inducing arginine depletion. Lack of arginine determines CD3ζ downregulation and suppression of T cells proliferation. Moreover, the enzyme cyclooxygenase-2 produced by activated KCs participates in the synthesis of the immunosuppressive prostaglandin E2 (PGE-2). (B) Contact-dependent mechanisms: antigen-specific CD8 T cells can be killed by NK cells through NKG2D- and TRAIL-dependent lysis. Over-expression of inhibitory ligands (e.g., PD-L1, Galectin-9) on liver endothelial sinusoidal cells (LSECs), dendritic cells, KCs, and stellate cells facilitates the triggering of T cell inhibitory pathways. In addition, inflammatory monocytes recruited into the liver through ICAM-1 expression on LSECs can differentiate into myeloid-derived suppressor cells by a CD44-dependent mechanism driven by activated stellate cells. Finally, Treg cell expansion and IL-10 production can be caused by pDCs and mDCs via the ICOS/ICOSL-mediated interaction or IDO secretion.