Chronic viral infection

In contrast to acute viral infection in which robust immunities against viral pathogens are rapidly elicited and control the viral replication and spread, chronic viral infection is considered a process of equilibrium between virus and host (1). Two fundamental events are required for the establishment of chronic viral infection. First, the invading viruses have to escape from antiviral immunities that eliminate the pathogens. Second, the strength of immune responses must be adjusted to tolerate the presence of foreign antigens without the induction of excessive inflammation, which can cause severe damage to infected tissues. The functionally impaired immunities are generally

observed in patients of chronic viral infection, such as human immunodeficiency virus (HIV), hepatitis B virus (HBV) and hepatitis C virus (HCV), in comparison to the antiviral immune responses in patients with acute infection.

During self-limited acute infection, the innate immune responses, including natural killer (NK) cell activity and interferon (IFN) production, are generated at early stage of infection and act as the first line of defense that limits the spread of virus. Thereafter, adaptive immunities, such as B cells, helper T cells (CD4 T cells), and cytotoxic T lymphocytes (CTL, CD8 T cells), are induced and activated through antigen-presenting cells (APCs). The effective adaptive immune responses contribute to the viral clearance by eliminating infected cells and neutralizing free-form viruses. The immunological situation in chronic viral infection is altered that the hosts fail to generate potent immunity against the invading pathogens. The failure of viral clearance is generally attributed to ineffective adaptive immunity, which could result from an active inhibition of immune activation by the virus or a lack of pathogen-associated molecular patterns

(PAMPs) that are required for the induction of immune activation (2).

1.1.1 T cell exhaustion

Researches in chronic viral infection mostly focus on the dysfunction of T cells, which are the main effector in terms of both cellular and humoral immunities against viral infection. T cell dysfunction is generally classified into T cell anergy and T cell exhaustion. Both of types are characterized with the loss of proliferative and cytolytic activity, the incapability of cytokine production, as well as the susceptibility to apoptosis (1, 3). Nevertheless, anergy arises at the time of first encounter of cognate antigens with the stimulation of MHC-peptide complexes (signal 1) in the absence of costimulatory molecules (signal 2), such as B7.1 and B7.2 (3). In contrast, T cell

exhaustion is not decided at the first time of antigen exposure but a progressive progress in the context of persistent existence of cognate antigens, which is a common phenotype of chronic viral infection. Exhausted T cells that are virus-specific in chronic carriers of HIV, HBV, and HCV are found to react poorly or even unresponsive to antigen

stimulation.

Several regulatory mechanisms that cause T cell exhaustion are found in chronic viral infection: immature APCs that constantly present antigens in the absence of

costimulatory signals, inhibitory cytokines such as IL-10 and TGF-β, regulatory cell subsets like regulatory T cells (Tregs), and inhibitory receptors expressed on T cells that generate regulatory signals and counteract T cell activation. Blocking these inhibitory mechanisms could prevent T cell exhaustion and restore the functions of T cells against virus. They are potential targets of immunotherapies for chronic infection in the future (2).

1.1.2 Inhibitory receptors

Recent studies show that in chronic viral infection, inhibitory receptors play critical roles in regulating the immune tolerance, especially in T cell exhaustion (4).

Programmed death-1 (PD-1, CD279) is the first inhibitory molecule found essential to T cell exhaustion, as blocking PD-1 signaling restores antiviral functions of virus-specific T cells and help the clearance of persistent virus in chronic lymphocytic

choriomeningitis virus (LCMV) infection (5) and in clinical studies of HIV (6). Upon activation, T cells upregulate the expression of PD-1 and thus prevent excessive activation (7). In self-limited infection, the expression of PD-1 is downregulated days after activation. In contrast, the expression is prolonged on T cells in chronic infection, which could be a consequence of constant exposure to antigens (1). When engaging with its ligands, PD-L1 (also as known as B7-H1) and PD-L2 (B7-DC), PD-1 is

phosphorylated on its two intracellular, and then binds phosphatases, SHP-1 and SHP-2, that downregulate antigen receptor signaling by dephosphorylation of signaling

intermediates (7). PD-L1 has a wider range of expression in parenchymal tissues and hematopoietic cells than that of PD-L2. Blocking PD-1 signaling in vivo has not only been shown to have therapeutic effect on animal models of chronic viral infection such as simian immunodeficiency virus (SIV) (8), but also lead to surprising responsive rate in cancer patients of several different types of tumors (9-12).

Other inhibitory receptors, including Lymphocyte activation gene-3 (LAG-3, CD223), Natural killer cell receptor 2B4 (CD244), T cell immunoglobulin (Tim-3) have been identified in LCMV studies and other human chronic viral infection (4, 13-16). They interact with their ligands (LAG-3:MHC II; 2B4:CD48; Tim-3:Galectin-9) and

transduce repressive signals that inhibit T cell activation. Individual blockades of these receptors using mAb show positive effects on the rescue of exhausted T cells and the

augmentation of antiviral immunities (4). A synergistic effect of combinatorial blockade of PD-1 and LAG-3 accelerate clearance of chronic pathogens, such as LCMV and plasmodium sp. in animal models (17, 18).

1.1.3 Effects of viral burdens on T cell exhaustion

Studies in LCMV clearly demonstrate that higher viral loads have negative effects on antiviral immune responses. Sustained exposure to high amount of cognate antigens results in the functional impairment of virus-specific CD8 T cells which thus fail to eliminate viral infection (19). In addition, mice with low viral load respond better to therapeutic vaccination (20). High antigen load also can lead to quick loss of memory CD8 T cells (21). In other human disease of chronic viral infection, the negative

correlation of viral loads and the antiviral functions of T cells are mostly determined by in vitro assays using peripheral blood mononuclear cells (PBMCs) (22, 23), whereas in vivo studies are rarely done.