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

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          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.

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          Most cited references133

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          CD8 T Cell Exhaustion During Chronic Viral Infection and Cancer

          Exhausted CD8 T (Tex) cells are a distinct cell lineage that arise during chronic infections and cancers in animal models and humans. Tex cells are characterized by progressive loss of effector functions, high and sustained inhibitory receptor expression, metabolic dysregulation, poor memory recall and homeostatic self-renewal, and distinct transcriptional and epigenetic programs. The ability to reinvigorate Tex cells through inhibitory receptor blockade, such as αPD-1, highlights the therapeutic potential of targeting this population. Emerging insights into the mechanisms of exhaustion are informing immunotherapies for cancer and chronic infections. However, like other immune cells, Tex cells are heterogeneous and include progenitor and terminal subsets with unique characteristics and responses to checkpoint blockade. Here, we review our current understanding of Tex cell biology, including the developmental paths, transcriptional and epigenetic features, and cell intrinsic and extrinsic factors contributing to exhaustion and how this knowledge may inform therapeutic targeting of Tex cells in chronic infections, autoimmunity, and cancer.
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            Diversity, Mechanisms, and Significance of Macrophage Plasticity

            Macrophages are a diverse set of cells present in all body compartments. This diversity is imprinted by their ontogenetic origin (embryonal versus adult bone marrow–derived cells); the organ context; by their activation or deactivation by various signals in the contexts of microbial invasion, tissue damage, and metabolic derangement; and by polarization of adaptive T cell responses. Classic adaptive responses of macrophages include tolerance, priming, and a wide spectrum of activation states, including M1, M2, or M2-like. Moreover, macrophages can retain long-term imprinting of microbial encounters (trained innate immunity). Single-cell analysis of mononuclear phagocytes in health and disease has added a new dimension to our understanding of the diversity of macrophage differentiation and activation. Epigenetic landscapes, transcription factors, and microRNA networks underlie the adaptability of macrophages to different environmental cues. Macrophage plasticity, an essential component of chronic inflammation, and its involvement in diverse human diseases, most notably cancer, is discussed here as a paradigm.
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              Epigenetic stability of exhausted T cells limits durability of reinvigoration by PD-1 blockade.

              Blocking Programmed Death-1 (PD-1) can reinvigorate exhausted CD8 T cells (TEX) and improve control of chronic infections and cancer. However, whether blocking PD-1 can reprogram TEX into durable memory T cells (TMEM) is unclear. We found that reinvigoration of TEX in mice by PD-L1 blockade caused minimal memory development. After blockade, reinvigorated TEX became reexhausted if antigen concentration remained high and failed to become TMEM upon antigen clearance. TEX acquired an epigenetic profile distinct from that of effector T cells (TEFF) and TMEM cells that was minimally remodeled after PD-L1 blockade. This finding suggests that TEX are a distinct lineage of CD8 T cells. Nevertheless, PD-1 pathway blockade resulted in transcriptional rewiring and reengagement of effector circuitry in the TEX epigenetic landscape. These data indicate that epigenetic fate inflexibility may limit current immunotherapies.
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                Author and article information

                Contributors
                Journal
                Front Immunol
                Front Immunol
                Front. Immunol.
                Frontiers in Immunology
                Frontiers Media S.A.
                1664-3224
                12 May 2020
                2020
                : 11
                : 849
                Affiliations
                [1] 1Laboratory of Viral Immunopathology, Unit of Infectious Diseases and Hepatology, Azienda Ospedaliero-Universitaria di Parma , Parma, Italy
                [2] 2Department of Medicine and Surgery, University of Parma , Parma, Italy
                Author notes

                Edited by: Seung Kew Yoon, The Catholic University of Korea, South Korea

                Reviewed by: Claudia Feriotti, Queen’s University Belfast, United Kingdom; Kyong-Mi Chang, University of Pennsylvania, United States

                *Correspondence: Carolina Boni, cboni@ 123456ao.pr.it

                These authors have contributed equally to this work

                This article was submitted to Viral Immunology, a section of the journal Frontiers in Immunology

                Article
                10.3389/fimmu.2020.00849
                7235343
                32477347
                ba04056d-a001-4563-9ac5-1354b848db04
                Copyright © 2020 Fisicaro, Barili, Rossi, Montali, Vecchi, Acerbi, Laccabue, Zecca, Penna, Missale, Ferrari and Boni.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

                History
                : 12 November 2019
                : 14 April 2020
                Page count
                Figures: 2, Tables: 0, Equations: 0, References: 192, Pages: 16, Words: 0
                Funding
                Funded by: Regione Emilia-Romagna 10.13039/501100009879
                Funded by: Ministero della Salute 10.13039/501100003196
                Categories
                Immunology
                Review

                Immunology
                chronic hbv infection,t cell exhaustion,immune-therapy,liver environment,immunoregulatory mechanisms

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