Antigen expression level threshold tunes the fate of CD8 T cells during primary hepatic immune responses

Optimized RNA interference therapeutics combined with interleukin-2 mRNA for treating hepatitis B virus infection

This study aimed to develop a pan-genotypic and multifunctional small interfering RNA (siRNA) against hepatitis B virus (HBV) with an efficient delivery system for treating chronic hepatitis B (CHB), and explore combined RNA interference (RNAi) and immune modulatory modalities for better viral control. Twenty synthetic siRNAs targeting consensus motifs distributed across the whole HBV genome were designed and evaluated. The lipid nanoparticle (LNP) formulation was optimized by adopting HO-PEG2000-DMG lipid and modifying the molar ratio of traditional polyethylene glycol (PEG) lipid in LNP prescriptions. The efficacy and safety of this formulation in delivering siHBV (tLNP/siHBV) along with the mouse IL-2 (mIL-2) mRNA (tLNP/siHBVIL2) were evaluated in the rAAV-HBV1.3 mouse model. A siRNA combination (terms "siHBV") with a genotypic coverage of 98.55% was selected, chemically modified, and encapsulated within an optimized LNP (tLNP) of high efficacy and security to fabricate a therapeutic formulation for CHB. The results revealed that tLNP/siHBV significantly reduced the expression of viral antigens and DNA (up to 3log10 reduction; vs PBS) in dose- and time-dependent manners at single-dose or multi-dose frequencies, with satisfactory safety profiles. Further studies showed that tLNP/siHBVIL2 enables additive antigenic and immune control of the virus, via introducing potent HBsAg clearance through RNAi and triggering strong HBV-specific CD4+ and CD8+ T cell responses by expressed mIL-2 protein. By adopting tLNP as nucleic acid nanocarriers, the co-delivery of siHBV and mIL-2 mRNA enables synergistic antigenic and immune control of HBV, thus offering a promising translational therapeutic strategy for treating CHB.

The lack of HBsAg secretion does neither facilitate induction of antiviral T cell responses nor Hepatitis B Virus clearance in mice

Immune tolerance to the hepatitis B virus (HBV) is crucial for developing chronic hepatitis B, and the HBV surface antigen (HBsAg) produced and secreted in high amounts is regarded as a key contributor. HBsAg is expressed in HBV-infected hepatocytes and those carrying an HBV integration. Whether either HBsAg secretion or the high antigen amount expressed in the liver determines its immunomodulatory properties, however, remains unclear. We, therefore, developed a novel HBV animal model that allowed us to study the role of secreted HBsAg. We introduced a previously described HBs mutation, C65S, abolishing HBsAg secretion into a replication-competent 1.3-overlength HBV genome and used adeno-associated virus vectors to deliver it to the mouse liver. The AAV-HBV established a carrier state of wildtype and C65S mutant HBV, respectively. We investigated antiviral B- and T-cell immunity in the HBV-carrier mice after therapeutic vaccination. Moreover, we compared the effect of a lacking HBsAg secretion with that of an antiviral siRNA. While missing HBsAg secretion allowed for higher levels of detectable anti-HBs antibodies after therapeutic vaccination, it did neither affect antiviral T-cell responses nor intrahepatic HBV gene expression, irrespective of the starting level. A treatment with HBV siRNA restricting viral antigen expression within hepatocytes, however, improved the antiviral efficacy of therapeutic vaccination, irrespective of the ability of HBV to secrete HBsAg. Our data indicate that clearing HBsAg from blood cannot significantly impact HBV persistence or T-cell immunity. This indicates that a restriction of hepatic viral antigen expression will be required to break HBV immunotolerance.

Preclinical evaluation of therapeutic vaccines for chronic hepatitis B that stimulate antiviral activities of T cells and NKT cells

Background & Aims

Liver diseases resulting from chronic HBV infection are a significant cause of morbidity and mortality. Vaccines that elicit T-cell responses capable of controlling the virus represent a treatment strategy with potential for long-term effects. Here, we evaluated vaccines that induce the activity of type I natural killer T (NKT) cells to limit viral replication and license stimulation of conventional antiviral T-cells.

Methods

Vaccines were prepared by conjugating peptide epitopes to an NKT-cell agonist to promote co-delivery to antigen-presenting cells, encouraging NKT-cell licensing and stimulation of T cells. Activity of the conjugate vaccines was assessed in transgenic mice expressing the complete HBV genome, administered intravenously to maximise access to NKT cell-rich tissues.

Results

The vaccines induced only limited antiviral activity in unmanipulated transgenic hosts, likely attributable to NKT-cell activation as T-cell tolerance to viral antigens is strong. However, in a model of chronic hepatitis B involving transfer of naive HBcAg-specific CD8+ T cells into the transgenic mice, which typically results in specific T-cell dysfunction without virus control, vaccines containing the targeted HBcAg epitope induced prolonged antiviral activity because of qualitatively improved T-cell stimulation. In a step towards a clinical product, vaccines were prepared using synthetic long peptides covering clusters of known HLA-binding epitopes and shown to be immunogenic in HLA transgenic mice. Predictions based on HLA distribution suggest a product containing three selected SLP-based vaccines could give >90 % worldwide coverage, with an average of 3.38 epitopes targeted per individual.

Conclusions

The novel vaccines described show promise for further clinical development as a treatment for chronic hepatitis B.

Impact and Implications

Although there are effective prophylactic vaccines for HBV infection, it is estimated that 350-400 million people worldwide have chronic hepatitis B, putting these individuals at significant risk of life-threatening liver diseases. Therapeutic vaccination aimed at activating or boosting HBV-specific T-cell responses holds potential as a strategy for treating chronic infection, but has so far met with limited success. Here, we show that a glycolipid-peptide conjugate vaccine designed to coordinate activity of type I NKT cells alongside conventional antiviral T cells has antiviral activity in a mouse model of chronic infection. It is anticipated that a product based on a combination of three such conjugates, each prepared using long peptides covering clusters of known HLA-binding epitopes, could be developed further as a treatment for chronic hepatitis B with broad global HLA coverage.

Retreatment with HBV siRNA Results in Additional Reduction in HBV Antigenemia and Immune Stimulation in the AAV-HBV Mouse Model

BACKGROUND AND AIMS

Treatment with siRNAs that target HBV has demonstrated robust declines in HBV antigens. This effect is also observed in the AAV-HBV mouse model, which was used to investigate if two cycles of GalNAc-HBV-siRNA treatment could induce deeper declines in HBsAg levels or prevent rebound, and to provide insights into the liver immune microenvironment.

METHODS

C57Bl/6 mice were transduced with one of two different titers of AAV-HBV for 28 days, resulting in stable levels of HBsAg of about 103 or 105 IU/mL. Mice were treated for 12 weeks (four doses q3wk) per cycle with 3 mg/kg of siRNA-targeting HBV or an irrelevant sequence either once (single treatment) or twice (retreatment) with an 8-week treatment pause in between. Blood was collected to evaluate viral parameters. Nine weeks after the last treatment, liver samples were collected to perform phenotyping, bulk RNA-sequencing, and immunohistochemistry.

RESULTS

Independent of HBsAg baseline levels, treatment with HBV-siRNA induced a rapid decline in HBsAg levels, which then plateaued before gradually rebounding 12 weeks after treatment stopped. A second cycle of HBV-siRNA treatment induced a further decline in HBsAg levels in serum and the liver, reaching undetectable levels and preventing rebound when baseline levels were 103 IU/mL. This was accompanied with a significant increase in inflammatory macrophages in the liver and significant upregulation of regulatory T-cells and T-cells expressing immune checkpoint receptors.

CONCLUSIONS

Retreatment induced an additional decline in HBsAg levels, reaching undetectable levels when baseline HBsAg levels were 3log10 or less. This correlated with T-cell activation and upregulation of Trem2.

A hepatic network of dendritic cells mediates CD4 T cell help outside lymphoid organs

While CD4+ T cells are a prerequisite for CD8+ T cell-mediated protection against intracellular hepatotropic pathogens, the mechanisms facilitating the transfer of CD4-help to intrahepatic CD8+ T cells are unknown. Here, we developed an experimental system to investigate cognate CD4+ and CD8+ T cell responses to a model-antigen expressed de novo in hepatocytes and reveal that after initial priming, effector CD4+ and CD8+ T cells migrate into portal tracts and peri-central vein regions of the liver where they cluster with type-1 conventional dendritic cells. These dendritic cells are locally licensed by CD4+ T cells and expand the number of CD8+ T cells in situ, resulting in larger effector and memory CD8+ T cell pools. These findings reveal that CD4+ T cells promote intrahepatic immunity by amplifying the CD8+ T cell response via peripheral licensing of hepatic type-1 conventional dendritic cells and identify intrahepatic perivascular compartments specialized in facilitating effector T cell-dendritic cell interactions.

IL-2 produced by HBV-specific T cells as a biomarker of viral control and predictor of response to PD-1 therapy across clinical phases of chronic hepatitis B

BACKGROUND

There are no immunological biomarkers that predict control of chronic hepatitis B (CHB). The lack of immune biomarkers raises concerns for therapies targeting PD-1/PD-L1 because they have the potential for immune-related adverse events. Defining specific immune functions associated with control of HBV replication could identify patients likely to respond to anti-PD-1/PD-L1 therapies and achieve a durable functional cure.

METHODS

We enrolled immunotolerant, HBeAg+ immune-active (IA+), HBeAg- immune-active (IA-), inactive carriers, and functionally cured patients to test ex vivo PD-1 blockade on HBV-specific T cell functionality. Peripheral blood mononuclear cells were stimulated with overlapping peptides covering HBV proteins +/-α-PD-1 blockade. Functional T cells were measured using a 2-color FluoroSpot assay for interferon-γ and IL-2. Ex vivo functional restoration was compared to the interferon response capacity assay, which predicts overall survival in cancer patients receiving checkpoint inhibitors.

RESULTS

Ex vivo interferon-γ+ responses did not differ across clinical phases. IL-2+ responses were significantly higher in patients with better viral control and preferentially restored with PD-1 blockade. Inactive carrier patients displayed the greatest increase in IL-2 production, which was dominated by CD4 T cell and response to the HBcAg. The interferon response capacity assay significantly correlated with the degree of HBV-specific T cell restoration.

CONCLUSIONS

IL-2 production was associated with better HBV control and superior to interferon-γ as a marker of T cell restoration following ex vivo PD-1 blockade. Our study suggests that responsiveness to ex vivo PD-1 blockade, or the interferon response capacity assay, may support stratification for α-PD-1 therapies.

What Is the Current Status of Hepatitis B Virus Viro-Immunology?

The natural history of hepatitis B virus (HBV) infection is closely dependent on the dynamic interplay between the host immune response and viral replication. Spontaneous HBV clearance in acute self-limited infection is the result of an adequate and efficient antiviral immune response. Instead, it is widely recognized that in chronic HBV infection, immunologic dysfunction contributes to viral persistence. Long-lasting exposure to high viral antigens, upregulation of multiple co-inhibitory receptors, dysfunctional intracellular signaling pathways and metabolic alterations, and intrahepatic regulatory mechanisms have been described as features ultimately leading to a hierarchical loss of effector functions up to full T-cell exhaustion.

Evaluation of RNAi therapeutics VIR-2218 and ALN-HBV for chronic hepatitis B: Results from randomized clinical trials

BACKGROUND & AIMS

Current therapy for chronic hepatitis B virus (cHBV) infection involves lifelong treatment. New treatments that enable HBV functional cure would represent a clinically meaningful advance. ALN-HBV and VIR-2218 are investigational RNA interference therapeutics that target all major HBV transcripts.

METHODS

We report on: i) the safety of single doses of VIR-2218 (modified from ALN-HBV by enhanced stabilization chemistry plus technology to reduce off-target, seed-mediated binding while maintaining on-target antiviral activity) and ALN-HBV in humanized mice; ii) a cross-study comparison of the safety of single doses of VIR-2218 and ALN-HBV in healthy human volunteers (n = 24 and n = 49, respectively); and iii) the antiviral activity of two doses of 20, 50, 100, 200 mg of VIR-2218 (total n = 24) vs. placebo (n = 8), given 4 weeks apart, in participants with cHBV infection.

RESULTS

In humanized mice, alanine aminotransferase (ALT) levels were markedly lower following administration of VIR-2218 compared with ALN-HBV. In healthy volunteers, post-treatment ALT elevations occurred in 28% of participants receiving ALN-HBV compared with none in those receiving VIR-2218. In participants with cHBV infection, VIR-2218 was associated with dose-dependent reductions in hepatitis B surface antigen (HBsAg). The greatest mean reduction of HBsAg at Week 20 in participants receiving 200 mg was 1.65 log IU/ml. The HBsAg reduction was maintained at 0.87 log IU/ml at Week 48. No participants had serum HBsAg loss or hepatitis B surface antibody seroconversion.

CONCLUSIONS

VIR-2218 demonstrated an encouraging hepatic safety profile in preclinical and clinical studies as well as dose-dependent HBsAg reductions in patients with cHBV infection. These data support future studies with VIR-2218 as part of combination regimens with a goal of HBV functional cure.

TRIAL REGISTRATION

ClinicalTrials.gov identifiers: NCT02826018 and NCT03672188.

IMPACT AND IMPLICATIONS

A significant unmet need exists for therapies for chronic HBV (cHBV) infection that achieve functional cure. We report clinical and non-clinical data on two investigational small-interfering RNAs that target HBx, ALN-HBV and VIR-2218, demonstrating that incorporation of enhanced stabilization chemistry plus technology in VIR-2218 reduces its propensity to cause ALT elevations relative to its parent compound, ALN-HBV. We also show that VIR-2218 reduces hepatitis B surface antigen levels in a dose-dependent manner in participants with cHBV infection. These studies support the continued development of VIR-2218 as part of therapeutic regimens for cHBV infection, with the goal of a functional cure, and are important for HBV researchers and physicians.

The multifaceted role of macrophages during acute liver injury

The liver is situated at the interface of the gut and circulation where it acts as a filter for blood-borne and gut-derived microbes and biological molecules, promoting tolerance of non-invasive antigens while driving immune responses against pathogenic ones. Liver resident immune cells such as Kupffer cells (KCs), a subset of macrophages, maintain homeostasis under physiological conditions. However, upon liver injury, these cells and others recruited from circulation participate in the response to injury and the repair of tissue damage. Such response is thus spatially and temporally regulated and implicates interconnected cells of immune and non-immune nature. This review will describe the hepatic immune environment during acute liver injury and the subsequent wound healing process. In its early stages, the wound healing immune response involves a necroinflammatory process characterized by partial depletion of resident KCs and lymphocytes and a significant infiltration of myeloid cells including monocyte-derived macrophages (MoMFs) complemented by a wave of pro-inflammatory mediators. The subsequent repair stage includes restoring KCs, initiating angiogenesis, renewing extracellular matrix and enhancing proliferation/activation of resident parenchymal and mesenchymal cells. This review will focus on the multifaceted role of hepatic macrophages, including KCs and MoMFs, and their spatial distribution and roles during acute liver injury.

Immunotolerance in liver transplantation: a primer for the clinician

The use of immunosuppressive medications for solid organ transplantation is associated with cardiovascular, metabolic, and oncologic complications. On the other hand, the development of graft rejection is associated with increased mortality and graft dysfunction. Liver transplant recipients can withdraw from immunosuppression without developing graft injury while preserving an adequate antimicrobial response - a characteristic known as immunotolerance. Immunotolerance can be spontaneously or pharmacologically achieved. Contrary to the classic dogma, clinical studies have elucidated low rates of true spontaneous immunotolerance (no serologic or histological markers of immune injury) among liver transplant recipients. However, clinical, serologic, and tissue biomarkers can aid in selecting patients in whom immunosuppression can be safely withdrawn. For those who failed an immunosuppression withdrawal trial or are at high risk of rejection, pharmacological interventions for immunotolerance induction are under development. In this review, we provide an overview of the mechanisms of immunotolerance, the clinical studies investigating predictors and biomarkers of spontaneous immunotolerance, as well as the potential pharmacological interventions for inducing it.

Intravital Imaging of Inflammatory Response in Liver Disease

The healthy liver requires a strictly controlled crosstalk between immune and nonimmune cells to maintain its function and homeostasis. A well-conditioned immune system can effectively recognize and clear noxious stimuli by a self-limited, small-scale inflammatory response. This regulated inflammatory process enables the liver to cope with daily microbial exposure and metabolic stress, which is beneficial for hepatic self-renewal and tissue remodeling. However, the failure to clear noxious stimuli or dysregulation of immune response can lead to uncontrolled liver inflammation, liver dysfunction, and severe liver disease. Numerous highly dynamic circulating immune cells and sessile resident immune and parenchymal cells interact and communicate with each other in an incredibly complex way to regulate the inflammatory response in both healthy and diseased liver. Intravital imaging is a powerful tool to visualize individual cells in vivo and has been widely used for dissecting the behavior and interactions between various cell types in the complex architecture of the liver. Here, we summarize some new findings obtained with the use of intravital imaging, which enhances our understanding of the complexity of immune cell behavior, cell–cell interaction, and spatial organization during the physiological and pathological liver inflammatory response.

Antigen Load and T Cell Function: A Challenging Interaction in HBV Infection

Current treatment for chronic HBV infection is mainly based on nucleos(t)ide analogues, that in most cases need to be administered for a patient’s lifetime. There is therefore a pressing need to develop new therapeutic strategies to shorten antiviral treatments. A severe dysfunction of virus-specific T cell responses contributes to virus persistence; hence, immune-modulation to reconstitute an efficient host antiviral response is considered a potential approach for HBV cure. In this perspective, a detailed understanding of the different causes of T cell exhaustion is essential for the design of successful functional T cell correction strategies. Among many different mechanisms which are widely believed to play a role in T cell dysfunction, persistent T cell exposure to high antigen burden, in particular HBsAg, is expected to influence T cell differentiation and function. Definitive evidence of the possibility to improve anti-viral T cell functions by antigen decline is, however, still lacking. This review aims at recapitulating what we have learned so far on the complex T cell–viral antigen interplay in chronic HBV infection.

Transplant Tolerance, Not Only Clonal Deletion

The quest to understand how allogeneic transplanted tissue is not rejected and how tolerance is induced led to fundamental concepts in immunology. First, we review the research that led to the Clonal Deletion theory in the late 1950s that has since dominated the field of immunology and transplantation. At that time many basic mechanisms of immune response were unknown, including the role of lymphocytes and T cells in rejection. These original observations are reassessed by considering T regulatory cells that are produced by thymus of neonates to prevent autoimmunity. Second, we review "operational tolerance" induced in adult rodents and larger animals such as pigs. This can occur spontaneously especially with liver allografts, but also can develop after short courses of a variety of rejection inhibiting therapies. Over time these animals develop alloantigen specific tolerance to the graft but retain the capacity to reject third-party grafts. These animals have a "split tolerance" as peripheral lymphocytes from these animals respond to donor alloantigen in graft versus host assays and in mixed lymphocyte cultures, indicating there is no clonal deletion. Investigation of this phenomenon excludes many mechanisms, including anti-donor antibody blocking rejection as well as anti-idiotypic responses mediated by antibody or T cells. This split tolerance is transferred to a second immune-depleted host by T cells that retain the capacity to effect rejection of third-party grafts by the same host. Third, we review research on alloantigen specific inhibitory T cells that led to the first identification of the CD4+CD25+T regulatory cell. The key role of T cell derived cytokines, other than IL-2, in promoting survival and expansion of antigen specific T regulatory cells that mediate transplant tolerance is reviewed. The precise methods for inducing and diagnosing operational tolerance remain to be defined, but antigen specific T regulatory cells are key mediators.

Current laboratory tests for diagnosis of hepatitis B virus infection

BACKGROUND

Hepatitis B virus (HBV) has a long history in human infectious diseases. HBV infection can progress chronically, leading to cancer. After introduction of a vaccine, the overall incidence rate of HBV infection has decreased, although it remains a health problem in many countries.

PURPOSE

The aim of this review was to summarise current diagnostic efforts for HBV infection and future HBV diagnosis perspectives.

METHODS

We reviewed and summarised current laboratory diagnosis related with HBV infection in clinical practice.

RESULTS

There have been various serologic- and molecular-based methods to diagnose acute or chronic HBV infection. Since intrahepatic covalently closed circular DNAs (cccDNAs) function as robust HBV replication templates, cure of chronic HBV infection is limited. Recently, new biomarkers such as hepatitis B virus core-related antigen (HBcrAg) and HBV RNA have emerged that appear to reflect intrahepatic cccDNA status. These new biomarkers should be validated before clinical usage.

CONCLUSION

An effective diagnostic approach and current updated knowledge of treatment response monitoring are important for HBV infection management. Brand new ultrasensitive and accurate immunologic methods may pave the way to manage HBV infection in parallel with immunotherapy era.

The Immune System in Duchenne Muscular Dystrophy Pathogenesis

Growing evidence demonstrates the crosstalk between the immune system and the skeletal muscle in inflammatory muscle diseases and dystrophic conditions such as Duchenne Muscular Dystrophy (DMD), as well as during normal muscle regeneration. The rising of inflammation and the consequent activation of the immune system are hallmarks of DMD: several efforts identified the immune cells that invade skeletal muscle as CD4+ and CD8+ T cells, Tregs, macrophages, eosinophils and natural killer T cells. The severity of muscle injury and inflammation dictates the impairment of muscle regeneration and the successive replacement of myofibers with connective and adipose tissue. Since immune system activation was traditionally considered as a consequence of muscular wasting, we recently demonstrated a defect in central tolerance caused by thymus alteration and the presence of autoreactive T-lymphocytes in DMD. Although the study of innate and adaptive immune responses and their complex relationship in DMD attracted the interest of many researchers in the last years, the results are so far barely exhaustive and sometimes contradictory. In this review, we describe the most recent improvements in the knowledge of immune system involvement in DMD pathogenesis, leading to new opportunities from a clinical point-of-view.

Towards tolerance in liver transplantation

Life-long immunosuppression has always been considered the key in managing liver graft protection from recipient rejection. However, it is associated with severe adverse effects that lead to increased morbidity and mortality, including infections, cardiovascular diseases, kidney failure, metabolic disorders and de novo malignancies. This explains the great interest that has developed in the concept of tolerance in recent years. The liver, thanks to its marked tolerogenicity, is to be considered a privileged organ: up to 60% of selected patients undergoing liver transplantation could safely withdraw immunosuppression.

Lymph Node-Targeted Synthetically Glycosylated Antigen Leads to Antigen-Specific Immunological Tolerance

Inverse vaccines that tolerogenically target antigens to antigen-presenting cells (APCs) offer promise in prevention of immunity to allergens and protein drugs and treatment of autoimmunity. We have previously shown that targeting hepatic APCs through intravenous injection of synthetically glycosylated antigen leads to effective induction of antigen-specific immunological tolerance. Here, we demonstrate that targeting these glycoconjugates to lymph node (LN) APCs under homeostatic conditions leads to local and increased accumulation in the LNs compared to unmodified antigen and induces a tolerogenic state both locally and systemically. Subcutaneous administration directs the polymeric glycoconjugate to the draining LN, where the glycoconjugated antigen generates robust antigen-specific CD4+ and CD8+ T cell tolerance and hypo-responsiveness to antigenic challenge via a number of mechanisms, including clonal deletion, anergy of activated T cells, and expansion of regulatory T cells. Lag-3 up-regulation on CD4+ and CD8+ T cells represents an essential mechanism of suppression. Additionally, presentation of antigen released from the glycoconjugate to naïve T cells is mediated mainly by LN-resident CD8+ and CD11b+ dendritic cells. Thus, here we demonstrate that antigen targeting via synthetic glycosylation to impart affinity for APC scavenger receptors generates tolerance when LN dendritic cells are the cellular target.

Liver-Directed AAV8 Booster Vaccine Expressing Plasmodium falciparum Antigen Following Adenovirus Vaccine Priming Elicits Sterile Protection in a Murine Model

Hepatocyte infection by malaria sporozoites is a bottleneck in the life-cycle of Plasmodium spp. including P. falciparum, which causes the most lethal form of malaria. Therefore, developing an effective vaccine capable of inducing the strong humoral and cellular immune responses necessary to block the pre-erythrocytic stage has potential to overcome the spatiotemporal hindrances pertaining to parasite biology and hepatic microanatomy. We recently showed that when combined with a human adenovirus type 5 (AdHu5)-priming vaccine, adeno-associated virus serotype 1 (AAV1) is a potent booster malaria vaccine vector capable of inducing strong and long-lasting protective immune responses in a rodent malaria model. Here, we evaluated the protective efficacy of a hepatotropic virus, adeno-associated virus serotype 8 (AAV8), as a booster vector because it can deliver a transgene potently and rapidly to the liver, the organ malaria sporozoites initially infect and multiply in following sporozoite injection by the bite of an infected mosquito. We first generated an AAV8-vectored vaccine expressing P. falciparum circumsporozoite protein (PfCSP). Intravenous (i.v.) administration of AAV8-PfCSP to mice initially primed with AdHu5-PfCSP resulted in a hepatocyte transduction rate ~2.5 times above that seen with intramuscular (i.m.) administration. This immunization regimen provided a better protection rate (100% sterile protection) than that of the i.m. AdHu5-prime/i.m. AAV8-boost regimen (60%, p < 0.05), i.m. AdHu5-prime/i.v. AAV1-boost (78%), or i.m. AdHu5-prime/i.m. AAV1-boost (80%) against challenge with transgenic PfCSP-expressing P. berghei sporozoites. Compared with the i.m. AdHu5-prime/i.v. AAV1-boost regimen, three other regimens induced higher levels of PfCSP-specific humoral immune responses. Importantly, a single i.v. dose of AAV8-PfCSP recruited CD8⁺ T cells, especially resident memory CD8⁺ T cells, in the liver. These data suggest that boost with i.v. AAV8-PfCSP can improve humoral and cellular immune responses in BALB/c mice. Therefore, this regimen holds great promise as a next-generation platform for the development of an effective malaria vaccine.

Soluble N-Acetylgalactosamine-Modified Antigens Enhance Hepatocyte-Dependent Antigen Cross-Presentation and Result in Antigen-Specific CD8+ T Cell Tolerance Development

Hepatocytes compose up to 80% of the total liver and have been indicated as important players in the induction of immunologic tolerance in this organ. We show that hepatocytes possess the molecular machinery required for the cross-presentation of extracellular antigens. Using a derivative of the model antigen ovalbumin (OVA) covalently modified with a polymer containing multiple N-acetylgalactosamine residues (pGal-OVA) that enhance extracellular antigen uptake by mimicking the glycome of apoptotic debris, we show efficient hepatocyte-dependent induction of cross-tolerance of both adoptively transferred OT-I cells and endogenous OVA-specific CD8+ T lymphocytes, for example inducing tolerance to OVA-expressing skin transplants. Our study confirms that hepatocytes are capable of inducing peripheral tolerogenesis and provides proof of concept that they may be a valuable candidate for in vivo targeted tolerogenic treatments.

Harnessing liver-resident memory T cells for protection against malaria

INTRODUCTION

Tissue-resident memory T cells (T_RM cells) are powerful mediators of protracted adaptive immunity to infection in peripheral organs. Harnessing T_RM cells through vaccination hence promises unprecedented potential for protection against infection. A paramount example of this is malaria, a major infectious disease for which immunity through traditional vaccination strategies remains challenging. Liver T_RM cells appear to be highly protective against malaria, and recent developments in our knowledge of the biology of these cells have defined promising, novel strategies for their induction.

AREAS COVERED

Here, we describe the path that led to the discovery of T_RM cells and discuss the importance of liver T_RM cells in immunity against Plasmodium spp. infection; we summarize current knowledge on T_RM cell biology and discuss the current state and potential of T_RM-based vaccination against malaria.

EXPERT OPINION

T_RM based vaccination has emerged as a promising means to achieve efficient protection against malaria. Recent advances provide a solid basis for continuing the development of this area of research. Deeper understanding of the mechanisms that mediate T_RM formation and maintenance and identification of immunogenic and protective target epitopes suitable for human vaccination remain the main challenges for translation of these discoveries.

Understanding, predicting and achieving liver transplant tolerance: from bench to bedside

In the past 40 years, liver transplantation has evolved from a high-risk procedure to one that offers high success rates for reversal of liver dysfunction and excellent patient and graft survival. The liver is the most tolerogenic of transplanted organs; indeed, immunosuppressive therapy can be completely withdrawn without rejection of the graft in carefully selected, stable long-term liver recipients. However, in other recipients, chronic allograft injury, late graft failure and the adverse effects of anti-rejection therapy remain important obstacles to improved success. The liver has a unique composition of parenchymal and immune cells that regulate innate and adaptive immunity and that can promote antigen-specific tolerance. Although the mechanisms underlying liver transplant tolerance are not well understood, important insights have been gained into how the local microenvironment, hepatic immune cells and specific molecular pathways can promote donor-specific tolerance. These insights provide a basis for the identification of potential clinical biomarkers that might correlate with tolerance or rejection and for the development of novel therapeutic targets. Innovative approaches aimed at promoting immunosuppressive drug minimization or withdrawal include the adoptive transfer of donor-derived or recipient-derived regulatory immune cells to promote liver transplant tolerance. In this Review, we summarize and discuss these developments and their implications for liver transplantation.

Antigen presentation, autoantibody production, and therapeutic targets in autoimmune liver disease

The liver is an important immunological organ that controls systemic tolerance. The liver harbors professional and unconventional antigen-presenting cells that are crucial for tolerance induction and maintenance. Orchestrating the immune response in homeostasis depends on a healthy and well-toned immunological liver microenvironment, which is maintained by the crosstalk of liver-resident antigen-presenting cells and intrahepatic and liver-infiltrating leukocytes. In response to pathogens or autoantigens, tolerance is disrupted by unknown mechanisms. Intrahepatic parenchymal and nonparenchymal cells exhibit unique antigen-presenting properties. The presentation of microbial and endogenous lipid-, metabolite- and peptide-derived antigens from the gut via conventional and nonconventional mechanisms can educate intrahepatic immune cells and elicit effector responses or tolerance. Perturbation of this balance results in autoimmune liver diseases, such as autoimmune hepatitis, primary biliary cholangitis, and primary sclerosing cholangitis. Although the exact etiologies of these autoimmune liver diseases are unknown, it is thought that the disruption of tolerance towards self-antigens and microbial metabolites and lipids, as well as alterations in bile acid composition, may result in changes in effector cell activation and polarization and may reduce or impair protective anti-inflammatory regulatory T and B cell responses. Additionally, the canonical and noncanonical transmission of antigens and antigen:MHC complexes via trogocytosis or extracellular vesicles between different (non) immune cells in the liver may play a role in the induction of hepatic inflammation and tolerance. Here, we summarize emerging aspects of antigen presentation, autoantibody production, and the application of novel therapeutic approaches in the characterization and treatment of autoimmune liver diseases.

Live Imaging of Innate and Adaptive Immune Responses in the Liver

Immune response in the liver is determined by the spatial organization and cellular dynamics of hepatic immune cells. The liver vasculature accommodates abundant tissue-resident innate immune cells, such as Kupffer cells, natural killer cells, and natural killer T cells, to ensure efficient intravascular immunosurveillance. The fenestrated sinusoids also allow direct contact between circulating T cells and non-canonical antigen-presenting cells, such as hepatocytes, to instruct adaptive immune responses. Distinct cellular behaviors are exploited by liver immune cells to exert proper functions. Intravital imaging enables real-time visualization of individual immune cell in living animals, representing a powerful tool in dissecting the spatiotemporal features of intrahepatic immune cells during steady state and liver diseases. This review summarizes current advances in liver immunology prompted by in vivo imaging, with a particular focus on liver-resident innate immune cells and hepatic T cells.

Induction Phase of Spontaneous Liver Transplant Tolerance

The liver has long been known to possess tolerogenic properties. Early experiments in liver transplantation demonstrated that in animal models, hepatic allografts could be accepted across MHC-mismatch without the use of immunosuppression, and that transplantation of livers from the same donor was capable of inducing tolerance to other solid organs that would normally otherwise be rejected. Although this phenomenon is less pronounced in human liver transplantation, lower levels of immunosuppression are nevertheless required for graft acceptance than for other solid organs, and in a minority of individuals immunosuppression can be discontinued in the longer term. The mechanisms underlying this unique hepatic property have not yet been fully delineated, however it is clear that immunological events in the early period post-liver transplant are key to generation of hepatic allograft tolerance. Both the hepatic parenchyma and the large number of donor passenger leukocytes contained within the liver allograft have been demonstrated to contribute to the generation of donor-specific tolerance in the early post-transplant phase. In particular, the unique nature of hepatic-leukocyte interactions appears to play a crucial role in the ability of the liver to silence the recipient alloimmune response. In this review, we will summarize the evidence regarding the potential mechanisms that mediate the critical early phase in the generation of hepatic allograft tolerance.

A rational mouse model to detect on-target, off-tumor CAR T cell toxicity

Off-tumor targeting of human antigens is difficult to predict in preclinical animal studies and can lead to serious adverse effects in patients. To address this, we developed a mouse model with stable and tunable human Her2 (hHer2) expression on normal hepatic tissue and compared toxicity between affinity-tuned Her2 chimeric antigen receptor T cells (CARTs). In mice with hHer2-high livers, both the high-affinity (HA) and low-affinity (LA) CARTs caused lethal liver damage due to immunotoxicity. In mice with hHer2-low livers, LA-CARTs exhibited less liver damage and lower systemic levels of IFN-γ than HA-CARTs. We then compared affinity-tuned CARTs for their ability to control a hHer2-positive tumor xenograft in our model. Surprisingly, the LA-CARTs outperformed the HA-CARTs with superior antitumor efficacy in vivo. We hypothesized that this was due, in part, to T cell trafficking differences between LA and HA-CARTs and found that the LA-CARTs migrated out of the liver and infiltrated the tumor sooner than the HA-CARTs. These findings highlight the importance of T cell targeting in reducing toxicity of normal tissue and also in preventing off-tumor sequestration of CARTs, which reduces their therapeutic potency. Our model may be useful to evaluate various CARTs that have conditional expression of more than 1 single-chain variable fragment (scFv).

The Role of Diverse Liver Cells in Liver Transplantation Tolerance

Liver transplantation is the ideal treatment approach for a variety of end-stage liver diseases. However, life-long, systemic immunosuppressive treatment after transplantation is required to prevent rejection and graft loss, which is associated with severe side effects, although liver allograft is considered more tolerogenic. Therefore, understanding the mechanism underlying the unique immunologically privileged liver organ is valuable for transplantation management and autoimmune disease treatment. The unique hepatic acinus anatomy and a complex cellular network constitute the immunosuppressive hepatic microenvironment, which are responsible for the tolerogenic properties of the liver. The hepatic microenvironment contains a variety of hepatic-resident immobile non-professional antigen-presenting cells, including hepatocytes, liver sinusoidal endothelial cells, Kupffer cells, and hepatic stellate cells, that are insufficient to optimally prime T cells locally and lead to the removal of alloreactive T cells due to the low expression of major histocompatibility complex (MHC) molecules, costimulatory molecules and proinflammatory cytokines but a rather high expression of coinhibitory molecules and anti-inflammatory cytokines. Hepatic dendritic cells (DCs) are generally immature and less immunogenic than splenic DCs and are also ineffective in priming naïve allogeneic T cells via the direct recognition pathway in recipient secondary lymphoid organs. Although natural killer cells and natural killer T cells are reportedly associated with liver tolerance, their roles in liver transplantation are multifaceted and need to be further clarified. Under these circumstances, T cells are prone to clonal deletion, clonal anergy and exhaustion, eventually leading to tolerance. Other proposed liver tolerance mechanisms, such as soluble donor MHC class I molecules, passenger leukocytes theory and a high-load antigen effect, have also been addressed. We herein comprehensively review the current evidence implicating the tolerogenic properties of diverse liver cells in liver transplantation tolerance.

The unique immune microenvironment of liver metastases: Challenges and opportunities

Liver metastases from gastrointestinal and non-gastrointestinal malignancies remain a major cause of cancer-related mortality and a major clinical challenge. The liver has unique properties that facilitate metastatic expansion, including a complex immune system that evolved to dampen immunity to neoantigens entering the liver from the gut, through the portal circulation. In this review, we describe the unique microenvironment encountered by cancer cells in the liver, focusing on elements of the innate and adaptive immune response that can act as a double-edge sword, contributing to the elimination of cancer cells on the one hand and promoting their survival and growth, on the other. We discuss this microenvironment in a clinical context, particularly for colorectal carcinoma, and highlight how a better understanding of the role of the microenvironment has spurred an intense effort to develop novel and innovative strategies for targeting liver metastatic disease, some of which are currently being tested in the clinic.

Transplant Tolerance Induction: Insights From the Liver

A comparison of pre-clinical transplant models and of solid organs transplanted in routine clinical practice demonstrates that the liver is most amenable to the development of immunological tolerance. This phenomenon arises in the absence of stringent conditioning regimens that accompany published tolerizing protocols for other organs, particularly the kidney. The unique immunologic properties of the liver have assisted our understanding of the alloimmune response and how it can be manipulated to improve graft function and survival. This review will address important findings following liver transplantation in both animals and humans, and how these have driven the understanding and development of therapeutic immunosuppressive options. We will discuss the liver's unique system of immune and non-immune cells that regulate immunity, yet maintain effective responses to pathogens, as well as mechanisms of liver transplant tolerance in pre-clinical models and humans, including current immunosuppressive drug withdrawal trials and biomarkers of tolerance. In addition, we will address innovative therapeutic strategies, including mesenchymal stem cell, regulatory T cell, and regulatory dendritic cell therapy to promote liver allograft tolerance or minimization of immunosuppression in the clinic.

HBV X protein-based therapeutic vaccine accelerates viral antigen clearance by mobilizing monocyte infiltration into the liver in HBV carrier mice

Background

Hepatitis B virus (HBV) persistently infected about 250 million people worldwide, and a curative treatment remains an unmet medical need. Among many approaches to treat chronic hepatitis B (CHB), therapeutic vaccines have been developed for two decades, but none have yielded promising results in clinical trials. Therefore, dissection of HBV clearance mechanisms during therapeutic vaccination in appropriate models, which could give rise to new curative therapies, is urgently needed. Growing evidence indicates that prolonged and intensive exposure of antigen-specific T cells to viral antigens is a major cause of T cell exhaustion, and decreases anti-HBV immunity efficacy of therapeutic vaccination. HBV X protein (HBx) is expressed at low levels, and the understanding of its immunogenicity and potential in therapeutic CHB vaccines is limited.

Methods

HBV genome sequences from CHB patients were cloned into a pAAV plasmid backbone and transfected into immunocompetent mouse hepatocytes through hydrodynamic injection. Mice carrying > 500 IU/mL serum HBV surface antigen (HBs) for more than 4 weeks were considered HBV carriers mimicking human CHB and received 3 doses of weekly HBx vaccine by subcutaneous immunization. Serum HBV clearance was evaluated by monitoring serum HBs and HBV-DNA titers. Residual HBV in the liver was evaluated by western blotting for HBV core antigen. The splenic antigen-specific T cell response was quantified by a 15-mer overlapping peptide-stimulated interferon-γ enzyme-linked immunospot assay. Blood and hepatic immune cells were quantified by flow cytometric analysis.

Results

Our HBx-based vaccine induced systemic HBx-specific CD4⁺ and CD8⁺ T cell responses in HBV carrier mice and demonstrated significant HBs and HBV-DNA elimination. The protective effect persisted for at least 30 days without additional booster immunization. Different infiltrating myeloid cell subsets, each with distinctive roles during immune-mediated HBV clearance, were found in the liver of vaccinated mice. During vaccine therapy, inflammatory monocyte depletion resulted in sustained HBV clearance inhibition, whereas phagocytic monocyte-derived macrophage and Kupffer cell elimination resulted in only transient inhibition of vaccine-induced HBV clearance.

Conclusions

We report the potential role of HBx as a major immunogen in an HBV therapeutic vaccine and the significance of a liver-infiltrating monocyte subset during hepatic viral clearance.

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

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.

Priming of Antiviral CD8 T Cells without Effector Function by a Persistently Replicating Hepatitis C-Like Virus

Immune-competent animal models for the hepatitis C virus (HCV) are nonexistent, impeding studies of host-virus interactions and vaccine development. Experimental infection of laboratory rats with a rodent hepacivirus isolated from Rattus norvegicus (RHV) is a promising surrogate model due to its recapitulation of HCV-like chronicity. However, several aspects of rat RHV infection remain unclear, for instance, how RHV evades host adaptive immunity to establish persistent infection. Here, we analyzed the induction, differentiation, and functionality of RHV-specific CD8 T cell responses that are essential for protection against viral persistence. Virus-specific CD8 T cells targeting dominant and subdominant major histocompatibility complex class I epitopes proliferated considerably in liver after RHV infection. These populations endured long term yet never acquired antiviral effector functions or selected for viral escape mutations. This was accompanied by the persistent upregulation of programmed cell death-1 and absent memory cell formation, consistent with a dysfunctional phenotype. Remarkably, transient suppression of RHV viremia with a direct-acting antiviral led to the priming of CD8 T cells with partial effector function, driving the selection of a viral escape variant. These data demonstrate an intrinsic abnormality within CD8 T cells primed by rat RHV infection, an effect that is governed at least partially by the magnitude of early virus replication. Thus, this model could be useful in investigating mechanisms of CD8 T cell subversion, leading to the persistence of hepatotropic pathogens such as HCV.IMPORTANCE Development of vaccines against hepatitis C virus (HCV), a major cause of cirrhosis and cancer, has been stymied by a lack of animal models. The recent discovery of an HCV-like rodent hepacivirus (RHV) enabled the development of such a model in rats. This platform recapitulates HCV hepatotropism and viral chronicity necessary for vaccine testing. Currently, there are few descriptions of RHV-specific responses and why they fail to prevent persistent infection in this model. Here, we show that RHV-specific CD8 T cells, while induced early at high magnitude, do not develop into functional effectors capable of controlling virus. This defect was partially alleviated by short-term treatment with an HCV antiviral. Thus, like HCV, RHV triggers dysfunction of virus-specific CD8 T cells that are vital for infection resolution. Additional study of this evasion strategy and how to mitigate it could enhance our understanding of hepatotropic viral infections and lead to improved vaccines and therapeutics.

Mechanistic mathematical representation of CD8+ T cell mediated drug-induced liver injury (DILI) Part 1: initial development using experimental antigen, ovalbumin (OVA)

Idiosyncratic DILI (iDILI), rare liver injury not dependent on drug dose, is poorly understood and costly to patients and drug developers. Some iDILI events appear immune-mediated. This work builds on the established hepatocyte life cycle and intrinsic injury models in DILIsym, utilizing quantitative systems toxicology methods to incorporate direct activation of liver CD8+ T cells by OVA-expressing hepatocytes.

Simulated hepatocyte OVA expression leads to activation of naïve OVA-specific CD8+ T cells. Activated T cells differentiate to effector, memory, or exhausted T cells. T cell cytotoxicity induces apoptosis in antigen-presenting hepatocytes, with resultant ALT release. By design, the model reproduces a differential CD8+ T cell response to low vs. high levels of hepatocyte OVA expression consistent with data 1,2. Mechanistically, the differential response is driven by effector T cell mediated clearance of low antigen vs. persistent high antigen and differentiation of exhausted T cells3. Reducing T cell functional avidity by 70x, as described for OVA altered peptide ligands4, reduced IFN-γ production and T cell cytotoxicity consistent with data5.

By reproducing these results, DILIsym establishes proof-of-concept in the quantitative relationships among antigen-expressing hepatocytes, antigen-specific T cell number, cytotoxicity, and ALT release. These quantitative relationships set the stage for translation to drug-mediated CD8+ T cell cytotoxicity6.

Human Immune Responses to Adeno-Associated Virus (AAV) Vectors

Recombinant adeno-associated virus (rAAV) vectors are one of the most promising in vivo gene delivery tools. Several features make rAAV vectors an ideal platform for gene transfer. However, the high homology with the parental wild-type virus, which often infects humans, poses limitations in terms of immune responses associated with this vector platform. Both humoral and cell-mediated immunity to wild-type AAV have been documented in healthy donors, and, at least in the case of anti-AAV antibodies, have been shown to have a potentially high impact on the outcome of gene transfer. While several factors can contribute to the overall immunogenicity of rAAV vectors, vector design and the total vector dose appear to be responsible of immune-mediated toxicities. While preclinical models have been less than ideal in predicting the outcome of gene transfer in humans, the current preclinical body of evidence clearly demonstrates that rAAV vectors can trigger both innate and adaptive immune responses. Data gathered from clinical trials offers key learnings on the immunogenicity of AAV vectors, highlighting challenges as well as the potential strategies that could help unlock the full therapeutic potential of in vivo gene transfer.

AAV Vector Immunogenicity in Humans: A Long Journey to Successful Gene Transfer

Gene therapy with adeno-associated virus (AAV) vectors has demonstrated safety and long-term efficacy in a number of trials across target organs, including eye, liver, skeletal muscle, and the central nervous system. Since the initial evidence that AAV vectors can elicit capsid T cell responses in humans, which can affect the duration of transgene expression, much progress has been made in understanding and modulating AAV vector immunogenicity. It is now well established that exposure to wild-type AAV results in priming of the immune system against the virus, with development of both humoral and T cell immunity. Aside from the neutralizing effect of antibodies, the impact of pre-existing immunity to AAV on gene transfer is still poorly understood. Herein, we review data emerging from clinical trials across a broad range of gene therapy applications. Common features of immune responses to AAV can be found, suggesting, for example, that vector immunogenicity is dose-dependent, and that innate immunity plays an important role in the outcome of gene transfer. A range of host-specific factors are also likely to be important, and a comprehensive understanding of the mechanisms driving AAV vector immunogenicity in humans will be key to unlocking the full potential of in vivo gene therapy.

Definition of the Chemical and Immunological Signals Involved in Drug-Induced Liver Injury

Idiosyncratic drug-induced liver injury (iDILI), which is rare and often recognized only late in drug development, poses a major public health concern and impediment to drug development due to its high rate of morbidity and mortality. The mechanisms of DILI are not completely understood; both non-immune- and immune-mediated mechanisms have been proposed. Non-immune-mediated mechanisms including direct damage to hepatocytes, mitochondrial toxicity, interference with transporters, and alteration of bile ducts are well-known to be associated with drugs such as acetaminophen and diclofenac; whereas immune-mediated mechanisms involving activation of both adaptive and innate immune cells and the interactions of these cells with parenchymal cells have been proposed. The chemical signals involved in activation of both innate and adaptive immune responses are discussed with respect to recent scientific advances. In addition, the immunological signals including cytokine and chemokines that are involved in promoting liver injury are also reviewed. Finally, we discuss how liver tolerance and regeneration can have profound impact on the pathogenesis of iDILI. Continuous research in developing in vitro systems incorporating immune cells with liver cells and animal models with impaired liver tolerance will provide an opportunity for improved prediction and prevention of immune-mediated iDILI.

Liver-Mediated Adaptive Immune Tolerance

The liver is an immunologically tolerant organ that is uniquely equipped to limit hypersensitivity to food-derived antigens and bacterial products through the portal vein and can feasibly accept liver allografts. The adaptive immune response is a major branch of the immune system that induces organ/tissue-localized and systematic responses against pathogens and tumors while promoting self-tolerance. Persistent infection of the liver with a virus or other pathogen typically results in tolerance, which is a key feature of the liver. The liver's immunosuppressive microenvironment means that hepatic adaptive immune cells become readily tolerogenic, promoting the death of effector cells and the “education” of regulatory cells. The above mechanisms may result in the clonal deletion, exhaustion, or inhibition of peripheral T cells, which are key players in the adaptive immune response. These tolerance mechanisms are believed to be responsible for almost all liver diseases. However, optimal protective adaptive immune responses may be achieved through checkpoint immunotherapy and the modulation of hepatic innate immune cells in the host. In this review, we focus on the mechanisms involved in hepatic adaptive immune tolerance, the liver diseases caused thereby, and the therapeutic strategies needed to overcome this tolerance.

Single-Dose Vaccination with a Hepatotropic Adeno-associated Virus Efficiently Localizes T Cell Immunity in the Liver with the Potential To Confer Rapid Protection against Hepatitis C Virus

Hepatitis C virus (HCV) is a significant contributor to the global disease burden, and development of an effective vaccine is required to eliminate HCV infections worldwide. CD4+ and CD8+ T cell immunity correlates with viral clearance in primary HCV infection, and intrahepatic CD8+ tissue-resident memory T (TRM) cells provide lifelong and rapid protection against hepatotropic pathogens. Consequently, we aimed to develop a vaccine to elicit HCV-specific CD4+ and CD8+ T cells, including CD8+ TRM cells, in the liver, given that HCV primarily infects hepatocytes. To achieve this, we vaccinated wild-type BALB/c mice with a highly immunogenic cytolytic DNA vaccine encoding a model HCV (genotype 3a) nonstructural protein (NS5B) and a mutant perforin (pVAX-NS5B-PRF), as well as a recombinant adeno-associated virus (AAV) encoding NS5B (rAAV-NS5B). A novel fluorescent target array (FTA) was used to map immunodominant CD4+ T helper (TH) cell and cytotoxic CD8+ T cell epitopes of NS5B in vivo, which were subsequently used to design a KdNS5B451-459 tetramer and analyze NS5B-specific T cell responses in vaccinated mice in vivo The data showed that intradermal prime/boost vaccination with pVAX-NS5B-PRF was effective in eliciting TH and cytotoxic CD8+ T cell responses and intrahepatic CD8+ TRM cells, but a single intravenous dose of hepatotropic rAAV-NS5B was significantly more effective. As a T-cell-based vaccine against HCV should ideally result in localized T cell responses in the liver, this study describes primary observations in the context of HCV vaccination that can be used to achieve this goal.IMPORTANCE There are currently at least 71 million individuals with chronic HCV worldwide and almost two million new infections annually. Although the advent of direct-acting antivirals (DAAs) offers highly effective therapy, considerable remaining challenges argue against reliance on DAAs for HCV elimination, including high drug cost, poorly developed health infrastructure, low screening rates, and significant reinfection rates. Accordingly, development of an effective vaccine is crucial to HCV elimination. An HCV vaccine that elicits T cell immunity in the liver will be highly protective for the following reasons: (i) T cell responses against nonstructural proteins of the virus are associated with clearance of primary infection, and (ii) long-lived liver-resident T cells alone can protect against malaria infection of hepatocytes. Thus, in this study we exploit promising vaccination platforms to highlight strategies that can be used to evoke highly functional and long-lived T cell responses in the liver for protection against HCV.

Gene Therapy for Pompe Disease: The Time is now

Pompe disease (PD) is caused by the deficiency of the lysosomal enzyme acid α-glucosidase (GAA), resulting in systemic pathological glycogen accumulation. PD can present with cardiac, skeletal muscle, and central nervous system manifestations, as a continuum of phenotypes among two main forms: classical infantile-onset PD (IOPD) and late-onset PD (LOPD). IOPD is caused by severe GAA deficiency and presents at birth with cardiac hypertrophy, muscle hypotonia, and severe respiratory impairment, leading to premature death, if not treated. LOPD is characterized by levels of residual GAA activity up to ∼20% of normal and presents both in children and adults with a varied severity of muscle weakness and motor and respiratory deficit. Enzyme replacement therapy (ERT), based on repeated intravenous (i.v.) infusions of recombinant human GAA (rhGAA), represents the only available treatment for PD. Upon more than 10 years from its launch, it is becoming evident that ERT can extend the life span of IOPD and stabilize disease progression in LOPD; however, it does not represent a cure for PD. The limited uptake of the enzyme in key affected tissues and the high immunogenicity of rhGAA are some of the hurdles that limit ERT efficacy. GAA gene transfer with adeno-associated virus (AAV) vectors has been shown to reduce glycogen storage and improve the PD phenotype in preclinical studies following different approaches. Here, we present an overview of the different gene therapy approaches for PD, focusing on in vivo gene transfer with AAV vectors and discussing the potential opportunities and challenges in developing safe and effective gene therapies for the disease. Based on emerging safety and efficacy data from clinical trials for other protein deficiencies, in vivo gene therapy with AAV vectors appears to have the potential to provide a therapeutically relevant, stable source of GAA enzyme, which could be highly beneficial in PD.

Role of Regulatory T Cell and Effector T Cell Exhaustion in Liver-Mediated Transgene Tolerance in Muscle

The pro-tolerogenic environment of the liver makes this tissue an ideal target for gene replacement strategies. In other peripheral tissues such as the skeletal muscle, anti-transgene immune response can result in partial or complete clearance of the transduced fibers. Here, we characterized liver-induced transgene tolerance after simultaneous transduction of liver and muscle. A clinically relevant transgene, α-sarcoglycan, mutated in limb-girdle muscular dystrophy type 2D, was fused with the SIINFEKL epitope (hSGCA-SIIN) and expressed with adeno-associated virus vectors (AAV-hSGCA-SIIN). Intramuscular delivery of AAV-hSGCA-SIIN resulted in a strong inflammatory response, which could be prevented and reversed by concomitant liver expression of the same antigen. Regulatory T cells and upregulation of checkpoint inhibitor receptors were required to establish and maintain liver-mediated peripheral tolerance. This study identifies the fundamental role of the synergy between Tregs and upregulation of checkpoint inhibitor receptors in the liver-mediated control of anti-transgene immunity triggered by muscle-directed gene transfer.

A dynamical motif comprising the interactions between antigens and CD8 T cells may underlie the outcomes of viral infections

Some viral infections culminate in very different outcomes in different individuals. They can be rapidly cleared in some, cause persistent infection in others, and cause mortality from immunopathology in yet others. The conventional view is that the different outcomes arise as a consequence of the complex interactions between a large number of different factors (virus, different immune cells, and cytokines). Here, we identify a simple dynamical motif comprising the essential interactions between antigens and CD8 T cells and posit it as predominantly determining the outcomes. Viral antigen can activate CD8 T cells, which in turn, can kill infected cells. Sustained antigen stimulation, however, can cause CD8 T-cell exhaustion, compromising effector function. Using mathematical modeling, we show that the motif comprising these interactions recapitulates all of the outcomes observed. The motif presents a conceptual framework to understand the variable outcomes of infection. It also explains a number of confounding experimental observations, including the variation in outcomes with the viral inoculum size, the evolutionary advantage of exhaustion in preventing lethal pathology, the ability of natural killer (NK) cells to act as rheostats tuning outcomes, and the role of the innate immune response in the spontaneous clearance of hepatitis C. Interventions that modulate the interactions in the motif may present routes to clear persistent infections or limit immunopathology.

Interferon at the cellular, individual, and population level in hepatitis C virus infection: Its role in the interferon-free treatment era

The advent of powerful direct-acting antiviral agents (DAAs) has revolutionized the treatment of hepatitis C. DAAs cure nearly all patients with short duration, oral treatments. Significant efforts are now underway to optimize DAA-based treatments. We discuss the potential role of interferon in this optimization. Clinical studies present compelling evidence that DAAs perform better in treatment-naive individuals than in individuals who previously failed treatment with interferon, a surprising correlation because interferon and DAAs are thought to act independently. Recent mathematical models explore a mechanistic hypothesis underlying this correlation. The hypothesis invokes the action of interferon at the cellular, individual, and population levels. Strong interferon responses prevent the productive infection of cells, reduce viral replication, and impede the development of resistance to DAAs in infected individuals and improve cure rates elicited by DAAs in treated populations. The models develop descriptions of these processes, integrate them into a comprehensive framework, and capture clinical data quantitatively, providing a successful test of the hypothesis. Individuals with strong endogenous interferon responses thus present a promising subpopulation for reducing DAA treatment durations. This review discusses the conceptual advances made by the models, highlights the new insights they unravel, and examines their applicability to optimize DAA-based treatments.

Immune network for viral hepatitis B: Topological representation

The liver is a well-known immunotolerogenic environment, which provides the adequate setting for liver infectious pathogens persistence such as the hepatitis B virus (HBV). Consequently, HBV infection can derive in the development of chronic disease in a proportion of the patients. If this situation persists in time, chronic hepatitis B (CHB) would end in cirrhosis, hepatocellular carcinoma and eventually, the death of the patient. It is thought that this immunotolerogenic environment is the result of complex interactions between different elements of the immune system and the viral biology. Therefore, the purpose of this work is to unravel the mechanisms implied in the development of CHB and to design a tool able to help in the study of adequate therapies. Firstly, a conceptual framework with the main components of the immune system and viral dynamics was constructed providing an overall insight on the pathways and interactions implied in this disease. Secondly, a review of the literature was performed in a modular fashion: (i) viral dynamics, (ii) innate immune response, (iii) humoral and (iv) cellular adaptive immune responses and (v) tolerogenic aspects. Finally, the information collected was integrated into a single topological representation that could serve as the plan for the systems pharmacology model architecture. This representation can be considered as the previous unavoidable step to the construction of a quantitative model that could assist in biomarker and target identification, drug design and development, dosing optimization and disease progression analysis.

Dual muscle-liver transduction imposes immune tolerance for muscle transgene engraftment despite preexisting immunity

Immune responses to therapeutic transgenes are a potential hurdle to treat monogenic muscle disorders. These responses result from the neutralizing activity of transgene-specific B cells and cytotoxic T cells recruited upon gene transfer. We explored here how dual muscle-liver expression of a foreign transgene allows muscle transgene engraftment after adenoassociated viral vector delivery. We found in particular that induction of transgene-specific tolerance is imposed by concurrent muscle and liver targeting, resulting in the absence of CD8+ T cell responses to the transgene. This tolerance can be temporally decoupled, because transgene engraftment can be achieved in muscle weeks after liver transduction. Importantly, transgene-specific CD8+ T cell tolerance can be established despite preexisting immunity to the transgene. Whenever preexisting, transgene-specific CD4+ and CD8+ memory T cell responses are present, dual muscle-liver transduction turns polyclonal, transgene-specific CD8+ T cells into typically exhausted T cells with high programmed cell death 1 (PD-1) expression and lack of IFN-γ production. Our results demonstrate that successful transduction of muscle tissue can be achieved through liver-mediated control of humoral and cytotoxic T cell responses, even in the presence of preexisting immunity to the muscle-associated transgene.

Toward DNA-Based T-Cell Mediated Vaccines to Target HIV-1 and Hepatitis C Virus: Approaches to Elicit Localized Immunity for Protection

Human immunodeficiency virus (HIV)-1 and hepatitis C virus (HCV) are major contributors to the global disease burden with many experts recognizing the requirement of an effective vaccine to bring a durable end to these viral epidemics. The most promising vaccine candidates that have advanced into pre-clinical models and the clinic to eliminate or provide protection against these chronic viruses are viral vectors [e.g., recombinant cytomegalovirus, Adenovirus, and modified vaccinia Ankara (MVA)]. This raises the question, is there a need to develop DNA vaccines against HIV-1 and HCV? Since the initial study from Wolff and colleagues which showed that DNA represents a vector that can be used to express transgenes durably in vivo, DNA has been regularly evaluated as a vaccine vector albeit with limited success in large animal models and humans. However, several recent studies in Phase I-IIb trials showed that vaccination of patients with recombinant DNA represents a feasible therapeutic intervention to even cure cervical cancer, highlighting the potential of using DNA for human vaccinations. In this review, we will discuss the limitations and the strategies of using DNA as a vector to develop prophylactic T cell-mediated vaccines against HIV-1 and HCV. In particular, we focus on potential strategies exploiting DNA vectors to elicit protective localized CD8⁺ T cell immunity in the liver for HCV and in the cervicovaginal mucosa for HIV-1 as localized immunity will be an important, if not critical component, of an efficacious vaccine against these viral infections.

Towards multiscale modeling of the CD8+ T cell response to viral infections

The CD8⁺ T cell response is critical to the control of viral infections. Yet, defining the CD8⁺ T cell response to viral infections quantitatively has been a challenge. Following antigen recognition, which triggers an intracellular signaling cascade, CD8⁺ T cells can differentiate into effector cells, which proliferate rapidly and destroy infected cells. When the infection is cleared, they leave behind memory cells for quick recall following a second challenge. If the infection persists, the cells may become exhausted, retaining minimal control of the infection while preventing severe immunopathology. These activation, proliferation and differentiation processes as well as the mounting of the effector response are intrinsically multiscale and collective phenomena. Remarkable experimental advances in the recent years, especially at the single cell level, have enabled a quantitative characterization of several underlying processes. Simultaneously, sophisticated mathematical models have begun to be constructed that describe these multiscale phenomena, bringing us closer to a comprehensive description of the CD8⁺ T cell response to viral infections. Here, we review the advances made and summarize the challenges and opportunities ahead. This article is categorized under: Analytical and Computational Methods > Computational Methods Biological Mechanisms > Cell Fates Biological Mechanisms > Cell Signaling Models of Systems Properties and Processes > Mechanistic Models.

TCR Retrogenic Mice as a Model To Map Self-Tolerance Mechanisms to the Cancer Mucosa Antigen GUCY2C

Characterizing self-tolerance mechanisms and their failure is critical to understand immune homeostasis, cancer immunity, and autoimmunity. However, examination of self-tolerance mechanisms has relied primarily on transgenic mice expressing TCRs targeting well-characterized, but nonphysiologic, model Ags, such as OVA and hemagglutinin. Identifying TCRs directed against bona fide self-antigens is made difficult by the extraordinary diversity of TCRs and the low prevalence of Ag-specific clones (<10-100 naive cells per organism), limiting dissection of tolerance mechanisms restricting immunity to self-proteins. In this study, we isolated and characterized TCRs recognizing the intestinal epithelial cell receptor and colorectal cancer Ag GUCY2C to establish a model to study self-antigen-specific tolerance mechanisms. GUCY2C-specific CD4+ effector T cells were isolated from immunized, nontolerant Gucy2c -/- mice. Next-generation sequencing identified GUCY2C-specific TCRs, which were engineered into CD4+ T cells in vitro to confirm TCR recognition of GUCY2C. Further, the generation of "retrogenic" mice by reconstitution with TCR-transduced hematopoietic stem cells resulted in normal CD4+ T cell development, responsiveness to immunization, and GUCY2C-induced tolerance in recipient mice, recapitulating observations in conventional models. This retrogenic model can be employed to define self-tolerance mechanisms restricting T and B cell responses to GUCY2C to optimize colorectal cancer immunotherapy without autoimmunity.

Can Patrolling Liver-Resident T Cells Control Human Malaria Parasite Development?

Recently, a population of non-recirculating, tissue-resident memory CD8⁺ T cells has been identified; cells that seems to act as key sentinels for invading microorganisms with enhanced effector functions. In malaria, the liver represents the first site for parasite development before a definite infection is established in circulating red blood cells. Here, we discuss the evidence obtained from animal models on several diseases and hypothesize that liver-resident memory CD8⁺ T cells (hepatic T_RM) play a critical role in providing protective liver-stage immunity against Plasmodium malaria parasites. Although observations in human malaria trials are limited to peripheral blood, we propose recommendations for the translation of some of these findings to human malaria research.

Multiple receptors converge on H2-Q10 to regulate NK and γδT-cell development

Class Ib major histocompatibility complex (MHC) is an extended family of molecules, which demonstrate tissue-specific expression and presentation of monomorphic antigens. These characteristics tend to imbue class Ib MHC with unique functions. H2-Q10 is potentially one such molecule that is overexpressed in the liver but its immunological function is not known. We have previously shown that H2-Q10 is a ligand for the natural killer cell receptor Ly49C and now, using H2-Q10-deficient mice, we demonstrate that H2-Q10 can also stabilize the expression of Qa-1b. In the absence of H2-Q10, the development and maturation of conventional hepatic natural killer cells is disrupted. We also provide evidence that H2-Q10 is a new high affinity ligand for CD8αα and controls the development of liver-resident CD8αα γδT cells. These data demonstrate that H2-Q10 has multiple roles in the development of immune subsets and identify an overlap of recognition within the class Ib MHC that is likely to be relevant to the regulation of immunity.

Outcome of Antiviral Immunity in the Liver Is Shaped by the Level of Antigen Expressed in Infected Hepatocytes

The liver bears unique immune properties that support both immune tolerance and immunity, but the mechanisms responsible for clearance versus persistence of virus-infected hepatocytes remain unclear. Here, we dissect the factors determining the outcome of antiviral immunity using recombinant adenoviruses that reflect the hepatropism and hepatrophism of hepatitis viruses. We generated replication-deficient adenoviruses with equimolar expression of ovalbumin, luciferase, and green fluorescent protein driven by a strong ubiquitous cytomegalovirus (CMV) promoter (Ad-CMV-GOL) or by 100-fold weaker, yet hepatocyte-specific, transthyretin (TTR) promoter (Ad-TTR-GOL). Using in vivo bioluminescence to quantitatively and dynamically image luciferase activity, we demonstrated that Ad-TTR-GOL infection always persists, whereas Ad-CMV-GOL infection is always cleared, independent of the number of infected hepatocytes. Failure to clear Ad-TTR-GOL infection involved mechanisms acting during initiation as well as execution of antigen-specific immunity. First, hepatocyte-restricted antigen expression led to delayed and curtailed T-cell expansion-10,000-fold after Ad-CMV-GOL versus 150-fold after Ad-TTR-GOL-infection. Second, CD8 T-cells primed toward antigens selectively expressed by hepatocytes showed high PD-1/Tim-3/LAG-3/CTLA-4/CD160 expression levels similar to that seen in chronic hepatitis B. Third, Ad-TTR-GOL but not Ad-CMV-GOL-infected hepatocytes escaped being killed by effector T-cells while still inducing high PD-1/Tim-3/LAG-3/CTLA-4/CD160 expression, indicating different thresholds of T-cell receptor signaling relevant for triggering effector functions compared with exhaustion. Conclusion: Our study identifies deficits in the generation of CD8 T-cell immunity toward hepatocyte-expressed antigens and escape of infected hepatocytes expressing low viral antigen levels from effector T-cell killing as independent factors promoting viral persistence. This highlights the importance of addressing both the restauration of CD8 T-cell dysfunction and overcoming local hurdles of effector T-cell function to eliminate virus-infected hepatocytes.