Induction of immune tolerance to coagulation factor IX antigen by in vivo hepatic gene transfer

Role of FoxP3+ Regulatory T Cells in Modulating Immune Responses to Adeno-Associated Virus Gene Therapy

Adeno-associated virus (AAV) gene therapy is making rapid strides owing to its wide range of therapeutic applications. However, development of serious immune responses to the capsid antigen or the therapeutic transgene product hinders its full clinical impact. Immune suppressive (IS) drug treatments have been used in various clinical trials to prevent the deleterious effects of cytotoxic T cells to the viral vector or transgene, although there is no consensus on the best treatment regimen, dosage, or schedule. Regulatory T cells (Tregs) are crucial for maintaining tolerance against self or nonself antigens. Of importance, Tregs also play an important role in dampening immune responses to AAV gene therapy, including tolerance induction to the transgene product. Approaches to harness the tolerogenic effect of Tregs include the use of selective IS drugs that expand existing Tregs, and skew activated conventional T cells into antigen-specific peripherally induced Tregs. In addition, Tregs can be expanded ex vivo and delivered as cellular therapy. Furthermore, receptor engineering can be used to increase the potency and specificity of Tregs allowing for suppression at lower doses and reducing the risk of disrupting protective immunity. Because immune-mediated toxicities to AAV vectors are a concern in the clinic, strategies that can enhance or preserve Treg function should be considered to improve both the safety and efficacy of AAV gene therapy.

Characterization of a Bioengineered AAV3B Capsid Variant with Enhanced Hepatocyte Tropism and Immune Evasion

Capsid engineering of adeno-associated virus (AAV) can surmount current limitations to gene therapy such as broad tissue tropism, low transduction efficiency, or pre-existing neutralizing antibodies (NAb) that restrict patient eligibility. We previously generated an AAV3B combinatorial capsid library by integrating rational design and directed evolution with the aim of improving hepatotropism. A potential isolate, AAV3B-DE5, gained a selective proliferative advantage over five rounds of iterative selection in hepatocyte spheroid cultures. In this study, we reanalyzed our original dataset derived from the AAV3B combinatorial library and isolated variants from earlier (one to three) rounds of selection, with the assumption that variants with faster replication kinetics are not necessarily the most efficient transducers. We identified a potential candidate, AAV3B-V04, which demonstrated significantly enhanced transduction in mouse-passaged primary human hepatocytes as well as in humanized liver chimeric mice, compared to the parental AAV3B or the previously described isolate, AAV3B-DE5. Interestingly, the AAV3B-V04 capsid variant exhibited significantly reduced seroreactivity to pooled or individual human serum samples. Forty-four percent of serum samples with pre-existing NAbs to AAV3B had 5- to 20-fold lower reciprocal NAb titers to AAV3B-V04. AAV3B-V04 has only nine amino acid substitutions, clustered in variable region IV compared to AAV3B, indicating the importance of the loops at the top of the three-fold protrusions in determining both transduction efficiency and immunogenicity. This study highlights the effectiveness of rational design combined with targeted selection for enhanced AAV transduction via molecular evolution approaches. Our findings support the concept of limiting selection rounds to isolate the best transducing AAV3B variant without outgrowth of faster replicating candidates. We conclude that AAV3B-V04 provides advantages such as improved human hepatocyte tropism and immune evasion and propose its utility as a superior candidate for liver gene therapy.

IL-15 blockade and rapamycin rescue multifactorial loss of factor VIII from AAV-transduced hepatocytes in hemophilia A mice

Hepatic adeno-associated viral (AAV) gene transfer has the potential to cure the X-linked bleeding disorder hemophilia A. However, declining therapeutic coagulation factor VIII (FVIII) expression has plagued clinical trials. To assess the mechanistic underpinnings of this loss of FVIII expression, we developed a hemophilia A mouse model that shares key features observed in clinical trials. Following liver-directed AAV8 gene transfer in the presence of rapamycin, initial FVIII protein expression declines over time in the absence of antibody formation. Surprisingly, loss of FVIII protein production occurs despite persistence of transgene and mRNA, suggesting a translational shutdown rather than a loss of transduced hepatocytes. Some of the animals develop ER stress, which may be linked to hepatic inflammatory cytokine expression. FVIII protein expression is preserved by interleukin-15/interleukin-15 receptor blockade, which suppresses CD8+ T and natural killer cell responses. Interestingly, mice with initial FVIII levels >100% of normal had diminishing expression while still under immune suppression. Taken together, our findings of interanimal variability of the response, and the ability of the immune system to shut down transgene expression without utilizing cytolytic or antibody-mediated mechanisms, illustrate the challenges associated with FVIII gene transfer. Our protocols based upon cytokine blockade should help to maintain efficient FVIII expression.

Adding recombinant AAVs to the cancer therapeutics mix

Gene therapy is a powerful biological tool that is reshaping therapeutic landscapes for several diseases. Researchers are using both non-viral and viral-based gene therapy methods with success in the lab and the clinic. In the cancer biology field, gene therapies are expanding treatment options and the possibility of favorable outcomes for patients. While cellular immunotherapies and oncolytic virotherapies have paved the way in cancer treatments based on genetic engineering, recombinant adeno-associated virus (rAAV), a viral-based module, is also emerging as a potential cancer therapeutic through its malleability, specificity, and broad application to common as well as rare tumor types, tumor microenvironments, and metastatic disease. A wide range of AAV serotypes, promoters, and transgenes have been successful at reducing tumor growth and burden in preclinical studies, suggesting more groundbreaking advances using rAAVs in cancer are on the horizon.

Single-dose AAV vector gene immunotherapy to treat food allergy

Immunotherapies for patients with food allergy have shown some success in limiting allergic responses. However, these approaches require lengthy protocols with repeated allergen dosing and patients can relapse following discontinuation of treatment. The purpose of this study was to test if a single dose of an adeno-associated virus (AAV) vector can safely prevent and treat egg allergy in a mouse model. AAV vectors expressing ovalbumin (OVA) under an ubiquitous or liver-specific promoter were injected prior to or after epicutaneous sensitization with OVA. Mice treated with either AAV8-OVA vector were completely protected from allergy sensitization. These animals had a significant reduction in anaphylaxis mediated by a reduction in OVA-specific IgE titers. In mice with established OVA allergy, allergic responses were mitigated only in mice treated with an AAV8-OVA vector expressing OVA from an ubiquitous promoter. In conclusion, an AAV vector with a liver-specific promoter was more effective for allergy prevention, but higher OVA levels were necessary for reducing symptoms in preexisting allergy. Overall, our AAV gene immunotherapy resulted in an expansion of OVA-specific FoxP3⁺ CD4⁺ T cells, an increase in the regulatory cytokine IL-10, and a reduction in the IgE promoting cytokine IL-13.

Neonatal gene therapy achieves sustained disease rescue of maple syrup urine disease in mice

Maple syrup urine disease (MSUD) is a rare recessively inherited metabolic disorder causing accumulation of branched chain amino acids leading to neonatal death, if untreated. Treatment for MSUD represents an unmet need because the current treatment with life-long low-protein diet is challenging to maintain, and despite treatment the risk of acute decompensations and neuropsychiatric symptoms remains. Here, based on significant liver contribution to the catabolism of the branched chain amino acid leucine, we develop a liver-directed adeno-associated virus (AAV8) gene therapy for MSUD. We establish and characterize the Bckdha (branched chain keto acid dehydrogenase a)^−/− mouse that exhibits a lethal neonatal phenotype mimicking human MSUD. Animals were treated at P0 with intravenous human BCKDHA AAV8 vectors under the control of either a ubiquitous or a liver-specific promoter. BCKDHA gene transfer rescued the lethal phenotype. While the use of a ubiquitous promoter fully and sustainably rescued the disease (long-term survival, normal phenotype and correction of biochemical abnormalities), liver-specific expression of BCKDHA led to partial, though sustained rescue. Here we show efficacy of gene therapy for MSUD demonstrating its potential for clinical translation.

Maple syrup urine disease (MSUD) is a rare inborn error of metabolism, which is currently treated with life-long low-protein diet that can be challenging to maintain. Here the authors develop an AAV8-directed gene therapy providing sustainable disease rescue in a mouse model of MSUD.

Harnessing the liver to induce antigen-specific immune tolerance

Autoimmune diseases develop when the adaptive immune system attacks the body’s own antigens leading to tissue damage. At least 80 different conditions are believed to have an autoimmune aetiology, including rheumatoid arthritis, type I diabetes, multiple sclerosis or systemic lupus erythematosus. Collectively, autoimmune diseases are a leading cause of severe health impairment along with substantial socioeconomic costs. Current treatments are mostly symptomatic and non-specific, and it is typically not possible to cure these diseases. Thus, the development of more causative treatments that suppress only the pathogenic immune responses, but spare general immunity is of great biomedical interest. The liver offers considerable potential for development of such antigen-specific immunotherapies, as it has a distinct physiological capacity to induce immune tolerance. Indeed, the liver has been shown to specifically suppress autoimmune responses to organ allografts co-transplanted with the liver or to autoantigens that were transferred to the liver. Liver tolerance is established by a unique microenvironment that facilitates interactions between liver-resident antigen-presenting cells and lymphocytes passing by in the low blood flow within the hepatic sinusoids. Here, we summarise current concepts and mechanisms of liver immune tolerance, and review present approaches to harness liver tolerance for antigen-specific immunotherapy.

Genome wide CRISPR/Cas9 screen identifies the coagulation factor IX (F9) as a regulator of senescence

During this last decade, the development of prosenescence therapies has become an attractive strategy as cellular senescence acts as a barrier against tumour progression. In this context, CDK4/6 inhibitors induce senescence and reduce tumour growth in breast cancer patients. However, even though cancer cells are arrested after CDK4/6 inhibitor treatment, genes regulating senescence in this context are still unknown limiting their antitumour activity. Here, using a functional genome-wide CRISPR/Cas9 genetic screen we found several genes that participate in the proliferation arrest induced by CDK4/6 inhibitors. We find that downregulation of the coagulation factor IX (F9) using sgRNA and shRNA prevents the cell cycle arrest and senescent-like phenotype induced in MCF7 breast tumour cells upon Palbociclib treatment. These results were confirmed using another breast cancer cell line, T47D, and with an alternative CDK4/6 inhibitor, Abemaciclib, and further tested in a panel of 22 cancer cells. While F9 knockout prevents the induction of senescence, treatment with a recombinant F9 protein was sufficient to induce a cell cycle arrest and senescence-like state in MCF7 tumour cells. Besides, endogenous F9 is upregulated in different human primary cells cultures undergoing senescence. Importantly, bioinformatics analysis of cancer datasets suggest a role for F9 in human tumours. Altogether, these data collectively propose key genes involved in CDK4/6 inhibitor response that will be useful to design new therapeutic strategies in personalised medicine in order to increase their efficiency, stratify patients and avoid drug resistance.

Genomic Designs of rAAVs Contribute to Pathological Changes in the Livers and Spleens of Mice

Recombinant AAV (rAAV) gene therapy is being investigated as an effective therapy for several diseases including hemophilia B. Reports of liver tumor development in certain mouse models due to AAV treatment and genomic integration of the rAAV vector has raised concerns about the long-term safety and efficacy of this gene therapy. To investigate whether rAAV treatment causes cancer, we utilized two mouse models, inbred C57BL/6 and hemophilia B Balb/C mice (HemB), to test if injecting a high dose of various rAAV8 vectors containing or lacking hFIX transgene, a Poly-A sequence, or the CB or TTR promoter triggered liver fibrosis and/or cancer development over the course of the 6.5-month study. We observed no liver tumors in either mouse cohort regardless of rAAV treatment through ultrasound imaging, gross anatomical assessment at sacrifice, and histology. We did, however, detect differences in collagen deposition in C57BL/6 livers and HemB spleens of rAAV-injected mice. Pathology reports of the HemB mice revealed many pathological phenomena, including fibrosis and inflammation in the livers and spleens across different AAV-injected HemB mice. Mice from both cohorts injected with the TTR-hFIX vector demonstrated minimal adverse events. While not tumorigenic, high dose of rAAVs, especially those with incomplete genomes, can influence liver and spleen health negatively that could be problematic for cementing AAVs as a broad therapeutic option in the clinic.

Modulating immune responses to AAV by expanded polyclonal T-regs and capsid specific chimeric antigen receptor T-regulatory cells

Immune responses to adeno-associated virus (AAV) capsids limit the therapeutic potential of AAV gene therapy. Herein, we model clinical immune responses by generating AAV capsid-specific chimeric antigen receptor (AAV-CAR) T cells. We then modulate immune responses to AAV capsid with AAV-CAR regulatory T cells (Tregs). AAV-CAR Tregs in vitro display phenotypical Treg surface marker expression, and functional suppression of effector T cell proliferation and cytotoxicity. In mouse models, AAV-CAR Tregs mediated continued transgene expression from an immunogenic capsid, despite antibody responses, produced immunosuppressive cytokines, and decreased tissue inflammation. AAV-CAR Tregs are also able to bystander suppress immune responses to immunogenic transgenes similarly mediating continued transgene expression, producing immunosuppressive cytokines, and reducing tissue infiltration. Taken together, AAV-CAR T cells and AAV-CAR Tregs are directed and powerful immunosuppressive tools to model and modulate immune responses to AAV capsids and transgenes in the local environment.

Coagulation factor IX gene transfer to non-human primates using engineered AAV3 capsid and hepatic optimized expression cassette

Hepatic gene transfer with adeno-associated viral (AAV) vectors shows much promise for the treatment of the X-linked bleeding disorder hemophilia B in multiple clinical trials. In an effort to further innovate this approach and to introduce alternative vector designs with potentially superior features into clinical development, we recently built a vector platform based on AAV serotype 3 because of its superior tropism for human hepatocytes. A vector genome with serotype-matched inverted terminal repeats expressing hyperactive human coagulation factor IX (FIX)-Padua was designed for clinical use that is optimized for translation using hepatocyte-specific codon-usage bias and is depleted of immune stimulatory CpG motifs. Here, this vector genome was packaged into AAV3 (T492V + S663V) capsid for hepatic gene transfer in non-human primates. FIX activity within or near the normal range was obtained at a low vector dose of 5 × 1011 vector genomes/kg. Pre-existing neutralizing antibodies, however, completely or partially blocked hepatic gene transfer at that dose. No CD8+ T cell response against capsid was observed. Antibodies against the human FIX transgene product formed at a 10-fold higher vector dose, albeit hepatic gene transfer was remarkably consistent, and sustained FIX activity in the normal range was nonetheless achieved in two of three animals for the 3-month duration of the study. These results support the use of this vector at low vector doses for gene therapy of hemophilia B in humans.

Homodimeric Minimal Factor H: In Vivo Tracking and Extended Dosing Studies in Factor H Deficient Mice

C3 glomerulopathy (C3G) is associated with dysregulation of the alternative pathway (AP) of complement and treatment options remain inadequate. Factor H (FH) is a potent regulator of the AP. An in-depth analysis of FH-related protein dimerised minimal (mini)-FH constructs has recently been published. This analysis showed that addition of a dimerisation module to mini-FH not only increased serum half-life but also improved complement regulatory function, thus providing a potential treatment option for C3G. Herein, we describe the production of a murine version of homodimeric mini-FH [mHDM-FH (mFH1-5^18-20^R1-2)], developed to reduce the risk of anti-drug antibody formation during long-term experiments in murine models of C3G and other complement-driven pathologies. Our analysis of mHDM-FH indicates that it binds with higher affinity and avidity to WT mC3b when compared to mouse (m)FH (mHDM-FH KD=505 nM; mFH KD=1370 nM) analogous to what we observed with the respective human proteins. The improved binding avidity resulted in enhanced complement regulatory function in haemolytic assays. Extended interval dosing studies in CFH-/- mice (5mg/kg every 72hrs) were partially effective and bio-distribution analysis in CFH-/- mice, through in vivo imaging technologies, demonstrates that mHDM-FH is preferentially deposited and remains fixed in the kidneys (and liver) for up to 4 days. Extended dosing using an AAV- human HDM-FH (hHDM-FH) construct achieved complete normalisation of C3 levels in CFH-/- mice for 3 months and was associated with a significant reduction in glomerular C3 staining. Our data demonstrate the ability of gene therapy delivery of mini-FH constructs to enhance complement regulation in vivo and support the application of this approach as a novel treatment strategy in diseases such as C3G.

In Vivo Mouse Models for Hepatitis B Virus Infection and Their Application

Despite the availability of effective vaccination, hepatitis B virus (HBV) infection continues to be a major challenge worldwide. Research efforts are ongoing to find an effective cure for the estimated 250 million people chronically infected by HBV in recent years. The exceptionally limited host spectrum of HBV has limited the research progress. Thus, different HBV mouse models have been developed and used for studies on infection, immune responses, pathogenesis, and antiviral therapies. However, these mouse models have great limitations as no spread of HBV infection occurs in the mouse liver and no or only very mild hepatitis is present. Thus, the suitability of these mouse models for a given issue and the interpretation of the results need to be critically assessed. This review summarizes the currently available mouse models for HBV research, including hydrodynamic injection, viral vector-mediated transfection, recombinant covalently closed circular DNA (rc-cccDNA), transgenic, and liver humanized mouse models. We systematically discuss the characteristics of each model, with the main focus on hydrodynamic injection mouse model. The usefulness and limitations of each mouse model are discussed based on the published studies. This review summarizes the facts for considerations of the use and suitability of mouse model in future HBV studies.

Improving Therapeutic Vaccination against Hepatitis B-Insights from Preclinical Models of Immune Therapy against Persistent Hepatitis B Virus Infection

Chronic hepatitis B affects more than 250 million individuals worldwide, putting them at risk of developing liver cirrhosis and liver cancer. While antiviral immune responses are key to eliminating hepatitis B virus (HBV) infections, insufficient antiviral immunity characterized by failure to eliminate HBV-infected hepatocytes is associated with chronic hepatitis B. Prophylactic vaccination against hepatitis B successfully established protective immunity against infection with the hepatitis B virus and has been instrumental in controlling hepatitis B. However, prophylactic vaccination schemes have not been successful in mounting protective immunity to eliminate HBV infections in patients with chronic hepatitis B. Here, we discuss the current knowledge on the development and efficacy of therapeutic vaccination strategies against chronic hepatitis B with particular emphasis on the pathogenetic understanding of dysfunctional anti-viral immunity. We explore the development of additional immune stimulation measures within tissues, in particular activation of immunogenic myeloid cell populations, and their use for combination with therapeutic vaccination strategies to improve the efficacy of therapeutic vaccination against chronic hepatitis B.

Adeno-Associated Viruses (AAV) and Host Immunity - A Race Between the Hare and the Hedgehog

Adeno-associated viruses (AAV) have emerged as the lead vector in clinical trials and form the basis for several approved gene therapies for human diseases, mainly owing to their ability to sustain robust and long-term in vivo transgene expression, their amenability to genetic engineering of cargo and capsid, as well as their moderate toxicity and immunogenicity. Still, recent reports of fatalities in a clinical trial for a neuromuscular disease, although linked to an exceptionally high vector dose, have raised new caution about the safety of recombinant AAVs. Moreover, concerns linger about the presence of pre-existing anti-AAV antibodies in the human population, which precludes a significant percentage of patients from receiving, and benefitting from, AAV gene therapies. These concerns are exacerbated by observations of cellular immune responses and other adverse events, including detrimental off-target transgene expression in dorsal root ganglia. Here, we provide an update on our knowledge of the immunological and molecular race between AAV (the “hedgehog”) and its human host (the “hare”), together with a compendium of state-of-the-art technologies which provide an advantage to AAV and which, thus, promise safer and more broadly applicable AAV gene therapies in the future.

Hepatic expression of GAA results in enhanced enzyme bioavailability in mice and non-human primates

Pompe disease (PD) is a severe neuromuscular disorder caused by deficiency of the lysosomal enzyme acid alpha-glucosidase (GAA). PD is currently treated with enzyme replacement therapy (ERT) with intravenous infusions of recombinant human GAA (rhGAA). Although the introduction of ERT represents a breakthrough in the management of PD, the approach suffers from several shortcomings. Here, we developed a mouse model of PD to compare the efficacy of hepatic gene transfer with adeno-associated virus (AAV) vectors expressing secretable GAA with long-term ERT. Liver expression of GAA results in enhanced pharmacokinetics and uptake of the enzyme in peripheral tissues compared to ERT. Combination of gene transfer with pharmacological chaperones boosts GAA bioavailability, resulting in improved rescue of the PD phenotype. Scale-up of hepatic gene transfer to non-human primates also successfully results in enzyme secretion in blood and uptake in key target tissues, supporting the ongoing clinical translation of the approach.

Inherent hepatocytic heterogeneity determines expression and retention of edited F9 alleles post-AAV/CRISPR infusion

Infusing CRISPR/donor-loaded adeno-associated viral vectors (AAV/CRISPR) could enable in vivo hepatic gene editing to remedy hemophilia B (HB) with inherited deficiency of clotting factor IX (FIX). Yet, current regimens focus on correcting HB with simple mutations in the coding region of the F9, overlooking those carrying complicated mutations involving the regulatory region. Moreover, a possible adverse effect of treatment-related inflammation remains unaddressed. Here we report that a single DNA cutting-mediated long-range replacement restored the FIX-encoding function of a mutant F9 (mF9) carrying both regulatory and coding defects in a severe mouse HB model, wherein incorporation of a synthetic Alb enhancer/promoter-mimic (P2) ensured FIX elevation to clinically meaningful levels. Through single-cell RNA sequencing (scRNA-seq) of liver tissues, we revealed that a subclinical hepatic inflammation post-AAV/CRISPR administration regulated the vulnerability of the edited mF9-harboring host cells to cytotoxic T lymphocytes (CTLs) and the P2 activity in a hepatocytic subset-dependent manner via modulating specific sets of liver-enriched transcription factors (LETFs). Collectively, our study establishes an AAV/CRISPR-mediated gene-editing protocol applicable to complicated monogenetic disorders, underscoring the potentiality of improving therapeutic benefits through managing inflammation.

B cell-activating factor modulates the factor VIII immune response in hemophilia A

Inhibitors of factor VIII (FVIII) remain the most challenging complication of FVIII protein replacement therapy in hemophilia A (HA). Understanding the mechanisms that guide FVIII-specific B cell development could help identify therapeutic targets. The B cell-activating factor (BAFF) cytokine family is a key regulator of B cell differentiation in normal homeostasis and immune disorders. Thus, we used patient samples and mouse models to investigate the potential role of BAFF in modulating FVIII inhibitors. BAFF levels were elevated in pediatric and adult HA inhibitor patients and decreased to levels similar to those of noninhibitor controls after successful immune tolerance induction (ITI). Moreover, elevations in BAFF levels were seen in patients who failed to achieve FVIII tolerance with anti-CD20 antibody-mediated B cell depletion. In naive HA mice, prophylactic anti-BAFF antibody therapy prior to FVIII immunization prevented inhibitor formation and this tolerance was maintained despite FVIII exposure after immune reconstitution. In preimmunized HA mice, combination therapy with anti-CD20 and anti-BAFF antibodies dramatically reduced FVIII inhibitors via inhibition of FVIII-specific plasma cells. Our data suggest that BAFF may regulate the generation and maintenance of FVIII inhibitors and/or anti-FVIII B cells. Finally, anti-CD20/anti-BAFF combination therapy may be clinically useful for ITI.

Cas9-directed immune tolerance in humans-a model to evaluate regulatory T cells in gene therapy?

The dichotomic nature of the adaptive immune response governs the outcome of clinical gene therapy. On the one hand, neutralizing antibodies and cytotoxic T cells can have a dramatic impact on the efficacy and safety of human gene therapies. On the other hand, regulatory T cells (Treg) can promote tolerance toward transgenes thereby enabling long-term benefits of in vivo gene therapy after a single administration. Pre-existing antibodies and T cell immunity has been a major obstacle for in vivo gene therapies with viral vectors. As CRISPR-Cas9 gene editing advances toward the clinics, the technology's inherent immunogenicity must be addressed in order to guide clinical treatment decisions. This review summarizes the recent evidence on Cas9-specific immunity in humans-including early results from clinical trials-and discusses the risks for in vivo gene therapies. Finally, we focus on solutions and highlight the potential role of Cas9-specific Treg cells to promote immune tolerance. As a "beneficial alliance" beyond Cas9-immunity, antigen-specific Treg cells may serve as a living and targeted immunosuppressant to increase safety and efficacy of gene therapy.

Emerging Immunogenicity and Genotoxicity Considerations of Adeno-Associated Virus Vector Gene Therapy for Hemophilia

Adeno-associated viral (AAV) vector gene therapy has shown promise as a possible cure for hemophilia. However, immune responses directed against AAV vectors remain a hurdle to the broader use of this gene transfer platform. Both innate and adaptive immune responses can affect the safety and efficacy of AAV vector-mediated gene transfer in humans. These immune responses may be triggered by the viral capsid, the vector's nucleic acid payload, or other vector contaminants or excipients, or by the transgene product encoded by the vector itself. Various preclinical and clinical strategies have been explored to overcome the issues of AAV vector immunogenicity and transgene-related immune responses. Although results of these strategies are encouraging, more efficient approaches are needed to deliver safe, predictable, and durable outcomes for people with hemophilia. In addition to durability, long-term follow-up of gene therapy trial participants will allow us to address potential safety concerns related to vector integration. Herein, we describe the challenges with current methodologies to deliver optimal outcomes for people with hemophilia who choose to undergo AAV vector gene therapy and the potential opportunities to improve on the results.

Effect of CpG Depletion of Vector Genome on CD8+ T Cell Responses in AAV Gene Therapy

Adeno associated viral (AAV) vectors have emerged as a preferred platform for in vivo gene replacement therapy and represent one of the most promising strategies to treat monogenetic disorders such as hemophilia. However, immune responses to gene transfer have hampered human gene therapy in clinical trials. Over the past decade, it has become clear that innate immune recognition provides signals for the induction of antigen-specific responses against vector or transgene product. In particular, TLR9 recognition of the vector's DNA genome in plasmacytoid dendritic cells (pDCs) has been identified as a key factor. Data from clinical trials and pre-clinical studies implement CpG motifs in the vector genome as drivers of immune responses, especially of CD8+ T cell activation. Here, we demonstrate that cross-priming of AAV capsid-specific CD8+ T cells depends on XCR1+ dendritic cells (which are likely the main cross-presenting cell that cooperates with pDCs to activate CD8+ T cells) and can be minimized by the elimination of CpG motifs in the vector genome. Further, a CpG-depleted vector expressing human coagulation factor IX showed markedly reduced (albeit not entirely eliminated) CD8+ T cell infiltration upon intramuscular gene transfer in hemophilia B mice when compared to conventional CpG+ vector (comprised of native sequences), resulting in better preservation of transduced muscle fibers. Therefore, this deimmunization strategy is helpful in reducing the potential for CD8+ T cell responses to capsid or transgene product. However, CpG depletion had minimal effects on antibody responses against capsid or transgene product, which appear to be largely independent of CpG motifs.

Advancements in AAV-mediated Gene Therapy for Pompe Disease

Pompe disease (glycogen storage disease type II) is caused by mutations in acid α-glucosidase (GAA) resulting in lysosomal pathology and impairment of the muscular and cardio-pulmonary systems. Enzyme replacement therapy (ERT), the only approved therapy for Pompe disease, improves muscle function by reducing glycogen accumulation but this approach entails several limitations including a short drug half-life and an antibody response that results in reduced efficacy. To address these limitations, new treatments such as gene therapy are under development to increase the intrinsic ability of the affected cells to produce GAA. Key components to gene therapy strategies include the choice of vector, promoter, and the route of administration. The efficacy of gene therapy depends on the ability of the vector to drive gene expression in the target tissue and also on the recipient's immune tolerance to the transgene protein. In this review, we discuss the preclinical and clinical studies that are paving the way for the development of a gene therapy strategy for patients with early and late onset Pompe disease as well as some of the challenges for advancing gene therapy.

Development of a Clinical Candidate AAV3 Vector for Gene Therapy of Hemophilia B

Although recombinant adeno-associated virus serotype 8 (AAV8) and serotype 5 (AAV5) vectors have shown efficacy in Phase 1 clinical trials for gene therapy of hemophilia B, it has become increasingly clear that these serotypes are not optimal for transducing primary human hepatocytes. We have previously reported that among the 10 most commonly used AAV serotypes, AAV serotype 3 (AAV3) vectors are the most efficient in transducing primary human hepatocytes in vitro as well as in "humanized" mice in vivo, and suggested that AAV3 vectors expressing human coagulation factor IX (hFIX) may be a more efficient alternative for clinical gene therapy of hemophilia B. In the present study, we extended these findings to develop an AAV3 vector incorporating a compact yet powerful liver-directed promoter as well as optimized hFIX cDNA sequence inserted between two AAV3 inverted terminal repeats. When packaged into an AAV3 capsid, this vector yields therapeutic levels of hFIX in hemophilia B and in "humanized" mice in vivo. Together, these studies have resulted in an AAV3 vector predicted to achieve clinical efficacy at reduced vector doses, without the need for immune-suppression, for clinical gene therapy of hemophilia B.

B Cell Depletion Eliminates FVIII Memory B Cells and Enhances AAV8-coF8 Immune Tolerance Induction When Combined With Rapamycin

Hemophilia A is an inherited coagulation disorder resulting in the loss of functional clotting factor VIII (FVIII). Presently, the most effective treatment is prophylactic protein replacement therapy. However, this requires frequent life-long intravenous infusions of plasma derived or recombinant clotting factors and is not a cure. A major complication is the development of inhibitory antibodies that nullify the replacement factor. Immune tolerance induction (ITI) therapy to reverse inhibitors can last from months to years, requires daily or every other day infusions of supraphysiological levels of FVIII and is effective in only up to 70% of hemophilia A patients. Preclinical and recent clinical studies have shown that gene replacement therapy with AAV vectors can effectively cure hemophilia A patients. However, it is unclear how hemophilia patients with high risk inhibitor F8 mutations or with established inhibitors will respond to gene therapy, as these patients have been excluded from ongoing clinical trials. AAV8-coF8 gene transfer in naïve BALB/c-F8e16^−/Y mice (BALB/c-HA) results in anti-FVIII IgG1 inhibitors following gene transfer, which can be prevented by transient immune modulation with anti-mCD20 (18B12) and oral rapamycin. We investigated if we could improve ITI in inhibitor positive mice by combining anti-mCD20 and rapamycin with AAV8-coF8 gene therapy. Our hypothesis was that continuous expression of FVIII protein from gene transfer compared to transient FVIII from weekly protein therapy, would enhance regulatory T cell induction and promote deletion of FVIII reactive B cells, following reconstitution. Mice that received anti-CD20 had a sharp decline in inhibitors, which corresponded to FVIII memory B (B_mem) cell deletion. Importantly, only mice receiving both anti-mCD20 and rapamycin failed to increase inhibitors following rechallenge with intravenous FVIII protein therapy. Our data show that B and T cell immune modulation complements AAV8-coF8 gene therapy in naïve and inhibitor positive hemophilia A mice and suggest that such protocols should be considered for AAV gene therapy in high risk or inhibitor positive hemophilia patients.

A Highly Efficacious PS Gene Editing System Corrects Metabolic and Neurological Complications of Mucopolysaccharidosis Type I

Our previous study delivered zinc finger nucleases to treat mice with mucopolysaccharidosis type I (MPS I), resulting in a phase I/II clinical trial (ClinicalTrials.gov: NCT02702115). However, in the clinical trial, the efficacy needs to be improved due to the low transgene expression level. To this end, we designed a proprietary system (PS) gene editing approach with CRISPR to insert a promoterless α-l-iduronidase (IDUA) cDNA sequence into the albumin locus of hepatocytes. In this study, adeno-associated virus 8 (AAV8) vectors delivering the PS gene editing system were injected into neonatal and adult MPS I mice. IDUA enzyme activity in the brain significantly increased, while storage levels were normalized. Neurobehavioral tests showed that treated mice had better memory and learning ability. Also, histological analysis showed efficacy reflected by the absence of foam cells in the liver and vacuolation in neuronal cells. No vector-associated toxicity or increased tumorigenesis risk was observed. Moreover, no off-target effects were detected through the unbiased genome-wide unbiased identification of double-stranded breaks enabled by sequencing (GUIDE-seq) analysis. In summary, these results showed the safety and efficacy of the PS in treating MPS I and paved the way for clinical studies. Additionally, as a therapeutic platform, the PS has the potential to treat other lysosomal diseases.

Liver-Targeted AAV8 Gene Therapy Ameliorates Skeletal and Cardiovascular Pathology in a Mucopolysaccharidosis IVA Murine Model

Mucopolysaccharidosis type IVA (MPS IVA) is due to the deficiency of GALNS (N-acetylgalactosamine 6-sulfate sulfatase) and is characterized by systemic skeletal dysplasia. We have evaluated adeno-associated virus 8 (AAV8) vectors expressing different forms of human GALNS under a liver-specific promoter. The vectors were delivered intravenously into 4-week-old MPS IVA knockout (KO) and immune tolerant (MTOL) mice at a dose of 5 × 10¹³ genome copies (GC)/kg. These mice were monitored for 12 weeks post-injection. GALNS enzyme activity was elevated significantly in plasma of all treated mice at 2 weeks post-injection. The activity observed was 4- to 19-fold higher than that in wild-type mice and was maintained throughout the monitoring period. Treatment with AAV vectors resulted in a reduction of keratan sulfate (KS) levels in plasma to normal levels 2 weeks post-injection, which were maintained until necropsy. Both vectors reduced the storage in articular cartilage, ligaments, and meniscus surrounding articular cartilage and growth plate region as well as heart muscle and valves. Our results suggest that the continuous presence of high levels of circulating enzyme increases the penetration into bone and heart and reduces the KS level, thereby improving storage in these regions. The current data support a strategy for developing a novel treatment to address the bone and heart disease in MPS IVA using AAV gene therapy.

Towards the clinical translation of optogenetic skeletal muscle stimulation

Paralysis is a frequent phenomenon in many diseases, and to date, only functional electrical stimulation (FES) mediated via the innervating nerve can be employed to restore skeletal muscle function in patients. Despite recent progress, FES has several technical limitations and significant side effects. Optogenetic stimulation has been proposed as an alternative, as it may circumvent some of the disadvantages of FES enabling cell type–specific, spatially and temporally precise stimulation of cells expressing light-gated ion channels, commonly Channelrhodopsin2. Two distinct approaches for the restoration of skeletal muscle function with optogenetics have been demonstrated: indirect optogenetic stimulation through the innervating nerve similar to FES and direct optogenetic stimulation of the skeletal muscle. Although both approaches show great promise, both have their limitations and there are several general hurdles that need to be overcome for their translation into clinics. These include successful gene transfer, sustained optogenetic protein expression, and the creation of optically active implantable devices. Herein, a comprehensive summary of the underlying mechanisms of electrical and optogenetic approaches is provided. With this knowledge in mind, we substantiate a detailed discussion of the advantages and limitations of each method. Furthermore, the obstacles in the way of clinical translation of optogenetic stimulation are discussed, and suggestions on how they could be overcome are provided. Finally, four specific examples of pathologies demanding novel therapeutic measures are discussed with a focus on the likelihood of direct versus indirect optogenetic stimulation.

Evading the AAV Immune Response in Mucopolysaccharidoses

The humoral immune response elicited by adeno-associated virus (AAV)-mediated gene therapy for the treatment of mucopolysaccharidoses (MPS) poses a significant challenge to achieving therapeutic levels of transgene expression. Antibodies targeting the AAV capsid as well as the transgene product diminish the production of glycosaminoglycan (GAG)-degrading enzymes essential for the treatment of MPS. Patients who have antibodies against AAV capsid increase in number with age, serotype, and racial background and are excluded from the clinical trials at present. In addition, patients who have undergone AAV gene therapy are often excluded from the additional AAV gene therapy with the same serotype, since their acquired immune response (antibody) against AAV will limit further efficacy of treatment. Several methods are being developed to overcome this immune response, such as novel serotype design, antibody reduction by plasmapheresis and immunosuppression, and antibody evasion using empty capsids and enveloped AAV vectors. In this review, we examine the mechanisms of the anti-AAV humoral immune response and evaluate the strengths and weaknesses of current evasion strategies in order to provide an evidence-based recommendation on evading the immune response for future AAV-mediated gene therapies for MPS.

Variability in Cardiac miRNA-122 Level Determines Therapeutic Potential of miRNA-Regulated AAV Vectors

Systemically delivered adeno-associated viral vector serotype 9 (AAV9) effectively transduces murine heart, but provides transgene expression also in liver and skeletal muscles. Improvement of the selectivity of transgene expression can be achieved through incorporation of target sites (TSs) for miRNA-122 and miRNA-206 into the 3′ untranslated region (3′ UTR) of the expression cassette. Here, we aimed to generate such miRNA-122- and miRNA-206-regulated AAV9 vector for a therapeutic, heart-specific overexpression of heme oxygenase-1 (HO-1). We successfully validated the vector functionality in murine cell lines corresponding to tissues targeted by AAV9. Next, we evaluated biodistribution of transgene expression following systemic vector delivery to HO-1-deficient mice of mixed C57BL/6J × FVB genetic background. Although AAV genomes were present in the hearts of these animals, HO-1 protein expression was either absent or significantly impaired. We found that miRNA-122, earlier described as liver specific, was present also in the hearts of C57BL/6J × FVB mice. Various levels of miRNA-122 expression were observed in the hearts of other mouse strains, in heart tissues of patients with cardiomyopathy, and in human induced pluripotent stem cell-derived cardiomyocytes in which we also confirmed such posttranscriptional regulation of transgene expression. Our data clearly indicate that therapeutic utilization of miRNA-based regulation strategy needs to consider inter-individual variability.

Translational Potential of Immune Tolerance Induction by AAV Liver-Directed Factor VIII Gene Therapy for Hemophilia A

Hemophilia A (HA) is an X-linked bleeding disorder due to deficiencies in coagulation factor VIII (FVIII). The major complication of current protein-based therapies is the development of neutralizing anti-FVIII antibodies, termed inhibitors, that block the hemostatic effect of therapeutic FVIII. Inhibitors develop in about 20-30% of people with severe HA, but the risk is dependent on the interaction between environmental and genetic factors, including the underlying F8 gene mutation. Recently, multiple clinical trials evaluating adeno-associated viral (AAV) vector liver-directed gene therapy for HA have reported promising results of therapeutically relevant to curative FVIII levels. The inclusion criteria for most trials prevented enrollment of subjects with a history of inhibitors. However, preclinical data from small and large animal models of HA with inhibitors suggests that liver-directed gene therapy can in fact eradicate pre-existing anti-FVIII antibodies, induce immune tolerance, and provide long-term therapeutic FVIII expression to prevent bleeding. Herein, we review the accumulating evidence that continuous uninterrupted expression of FVIII and other transgenes after liver-directed AAV gene therapy can bias the immune system toward immune tolerance induction, discuss the current understanding of the immunological mechanisms of this process, and outline questions that will need to be addressed to translate this strategy to clinical trials.

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.

The Immune Response to the fVIII Gene Therapy in Preclinical Models

Neutralizing antibodies to factor VIII (fVIII), referred to as "inhibitors," remain the most challenging complication post-fVIII replacement therapy. Preclinical development of novel fVIII products involves studies incorporating hemophilia A (HA) and wild-type animal models. Though immunogenicity is a critical aspect of preclinical pharmacology studies, gene therapy studies tend to focus on fVIII expression levels without major consideration for immunogenicity. Therefore, little clarity exists on whether preclinical testing can be predictive of clinical immunogenicity risk. Despite this, but perhaps due to the potential for transformative benefits, clinical gene therapy trials have progressed rapidly. In more than two decades, no inhibitors have been observed. However, all trials are conducted in previously treated patients without a history of inhibitors. The current review thus focuses on our understanding of preclinical immunogenicity for HA gene therapy candidates and the potential indication for inhibitor treatment, with a focus on product- and platform-specific determinants, including fVIII transgene sequence composition and tissue/vector biodistribution. Currently, the two leading clinical gene therapy vectors are adeno-associated viral (AAV) and lentiviral (LV) vectors. For HA applications, AAV vectors are liver-tropic and employ synthetic, high-expressing, liver-specific promoters. Factors including vector serotype and biodistribution, transcriptional regulatory elements, transgene sequence, dosing, liver immunoprivilege, and host immune status may contribute to tipping the scale between immunogenicity and tolerance. Many of these factors can also be important in delivery of LV-fVIII gene therapy, especially when delivered intravenously for liver-directed fVIII expression. However, ex vivo LV-fVIII targeting and transplantation of hematopoietic stem and progenitor cells (HSPC) has been demonstrated to achieve durable and curative fVIII production without inhibitor development in preclinical models. A critical variable appears to be pre-transplantation conditioning regimens that suppress and/or ablate T cells. Additionally, we and others have demonstrated the potential of LV-fVIII HSPC and liver-directed AAV-fVIII gene therapy to eradicate pre-existing inhibitors in murine and canine models of HA, respectively. Future preclinical studies will be essential to elucidate immune mechanism(s) at play in the context of gene therapy for HA, as well as strategies for preventing adverse immune responses and promoting immune tolerance even in the setting of pre-existing inhibitors.

Single-domain antibodies targeting antithrombin reduce bleeding in hemophilic mice with or without inhibitors

Novel therapies for hemophilia, including non-factor replacement and in vivo gene therapy, are showing promising results in the clinic, including for patients having a history of inhibitor development. Here, we propose a novel therapeutic approach for hemophilia based on llama-derived single-domain antibody fragments (sdAbs) able to restore hemostasis by inhibiting the antithrombin (AT) anticoagulant pathway. We demonstrated that sdAbs engineered in multivalent conformations were able to block efficiently AT activity in vitro, restoring the thrombin generation potential in FVIII-deficient plasma. When delivered as a protein to hemophilia A mice, a selected bi-paratopic sdAb significantly reduced the blood loss in a model of acute bleeding injury. We then packaged this sdAb in a hepatotropic AAV8 vector and tested its safety and efficacy profile in hemophilic mouse models. We show that the long-term expression of the bi-paratopic sdAb in the liver is safe and poorly immunogenic, and results in sustained correction of the bleeding phenotype in hemophilia A and B mice, even in the presence of inhibitory antibodies to the therapeutic clotting factor.

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.

Structure of the AAVhu.37 capsid by cryoelectron microscopy

Adeno-associated viruses (AAVs) are used as in vivo gene-delivery vectors in gene-therapy products and have been heavily investigated for numerous indications. Over 100 naturally occurring AAV serotypes and variants have been isolated from primate samples. Many reports have described unique properties of these variants (for instance, differences in potency, target cell or evasion of the immune response), despite high amino-acid sequence conservation. AAVhu.37 is of interest for clinical applications owing to its proficient transduction of the liver and central nervous system. The sequence identity of the AAVhu.37 VP1 to the well characterized AAVrh.10 serotype, for which no structure is available, is greater than 98%. Here, the structure of the AAVhu.37 capsid at 2.56 Å resolution obtained via single-particle cryo-electron microscopy is presented.

Muscle-Directed Delivery of an AAV1 Vector Leads to Capsid-Specific T Cell Exhaustion in Nonhuman Primates and Humans

With the US Food and Drug Administration (FDA) and European Medicines Agency (EMA) approvals for Zolgensma, Luxturna, and Glybera, recombinant adeno-associated viruses (rAAVs) are considered efficient tools for gene transfer. However, studies in animals and humans demonstrate that intramuscular (IM) AAV delivery can trigger immune responses to AAV capsids and/or transgenes. IM delivery of rAAV1 in humans has also been described to induce tolerance to rAAV characterized by the presence of capsid-specific regulatory T cells (Tregs) in periphery. To understand mechanisms responsible for tolerance and parameters involved, we tested 3 muscle-directed administration routes in rhesus monkeys: IM delivery, venous limb perfusion, and the intra-arterial push and dwell method. These 3 methods were well tolerated and led to transgene expression. Interestingly, gene transfer in muscle led to Tregs and exhausted T cell infiltrates in situ at both day 21 and day 60 post-injection. In human samples, an in-depth analysis of the functionality of these cells demonstrates that capsid-specific exhausted T cells are detected after at least 5 years post-vector delivery and that the exhaustion can be reversed by blocking the checkpoint pathway. Overall, our study shows that persisting transgene expression after gene transfer in muscle is mediated by Tregs and exhausted T cells.

Immune Responses to Viral Gene Therapy Vectors

Several viral vector-based gene therapy drugs have now received marketing approval. A much larger number of additional viral vectors are in various stages of clinical trials for the treatment of genetic and acquired diseases, with many more in pre-clinical testing. Efficiency of gene transfer and ability to provide long-term therapy make these vector systems very attractive. In fact, viral vector gene therapy has been able to treat or even cure diseases for which there had been no or only suboptimal treatments. However, innate and adaptive immune responses to these vectors and their transgene products constitute substantial hurdles to clinical development and wider use in patients. This review provides an overview of the type of immune responses that have been documented in animal models and in humans who received gene transfer with one of three widely tested vector systems, namely adenoviral, lentiviral, or adeno-associated viral vectors. Particular emphasis is given to mechanisms leading to immune responses, efforts to reduce vector immunogenicity, and potential solutions to the problems. At the same time, we point out gaps in our knowledge that should to be filled and problems that need to be addressed going forward.

Immune Response Mechanisms against AAV Vectors in Animal Models

Early preclinical studies in rodents and other species did not reveal that vector or transgene immunity would present a significant hurdle for sustained gene expression. While there was early evidence of mild immune responses to adeno-associated virus (AAV) in preclinical studies, it was generally believed that these responses were too weak and transient to negatively impact sustained transduction. However, translation of the cumulative success in treating hemophilia B in rodents and dogs with an AAV2-F9 vector to human studies was not as successful. Despite significant progress in recent clinical trials for hemophilia, new immunotoxicities to AAV and transgene are emerging in humans that require better animal models to assess and overcome these responses. The animal models designed to address these immune complications have provided critical information to assess how vector dose, vector capsid processing, vector genome, difference in serotypes, and variations in vector delivery route can impact immunity and to develop approaches for overcoming pre-existing immunity. Additionally, a comprehensive dissection of innate, adaptive, and regulatory responses to AAV vectors in preclinical studies has provided a framework that can be utilized for development of immunomodulatory therapies to overcome or bypass immune responses and for developing strategic approaches toward engineering stealth AAV vectors that can circumvent immunity.

Adeno-associated viral vector-mediated immune responses: Understanding barriers to gene delivery

Adeno-associated viral (AAV) vectors have emerged as the leading gene delivery platform for gene therapy and vaccination. Three AAV-based gene therapy drugs, Glybera, LUXTURNA, and ZOLGENSMA were approved between 2012 and 2019 by the European Medicines Agency and the United States Food and Drug Administration as treatments for genetic diseases hereditary lipoprotein lipase deficiency (LPLD), inherited retinal disease (IRD), and spinal muscular atrophy (SMA), respectively. Despite these therapeutic successes, clinical trials have demonstrated that host anti-viral immune responses can prevent the long-term gene expression of AAV vector-encoded genes. Therefore, it is critical that we understand the complex relationship between AAV vectors and the host immune response. This knowledge could allow for the rational design of optimized gene transfer vectors capable of either subverting host immune responses in the context of gene therapy applications, or stimulating desirable immune responses that generate protective immunity in vaccine applications to AAV vector-encoded antigens. This review provides an overview of our current understanding of the AAV-induced immune response and discusses potential strategies by which these responses can be manipulated to improve AAV vector-mediated gene transfer.

Adeno-associated virus vector as a platform for gene therapy delivery

Adeno-associated virus (AAV) vectors are the leading platform for gene delivery for the treatment of a variety of human diseases. Recent advances in developing clinically desirable AAV capsids, optimizing genome designs and harnessing revolutionary biotechnologies have contributed substantially to the growth of the gene therapy field. Preclinical and clinical successes in AAV-mediated gene replacement, gene silencing and gene editing have helped AAV gain popularity as the ideal therapeutic vector, with two AAV-based therapeutics gaining regulatory approval in Europe or the United States. Continued study of AAV biology and increased understanding of the associated therapeutic challenges and limitations will build the foundation for future clinical success.

Immunotherapy in hepatocellular carcinoma: the complex interface between inflammation, fibrosis, and the immune response

Hepatocellular carcinoma (HCC) is the third leading cause of cancer deaths worldwide and confers a poor prognosis. Beyond standard systemic therapy with multikinase inhibitors, recent studies demonstrate the potential for robust and durable responses from immune checkpoint inhibition in subsets of HCC patients across disease etiologies. The majority of HCC arises in the context of chronic inflammation and from within a fibrotic liver, with many cases associated with hepatitis virus infections, toxins, and fatty liver disease. Many patients also have concomitant cirrhosis which is associated with both local and systemic immune deficiency. Furthermore, the liver is an immunologic organ in itself, which may enhance or suppress the immune response to cancer arising within it. Here, we explore the immunobiology of the liver from its native state to chronic inflammation, fibrosis, cirrhosis and then to cancer, and summarize how this unique microenvironment may affect the response to immunotherapy.

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.

Liver induced transgene tolerance with AAV vectors

Immune tolerance is a vital component of immunity, as persistent activation of immune cells causes significant tissue damage and loss of tolerance leads to autoimmunity. Likewise, unwanted immune responses can occur in inherited disorders, such as hemophilia and Pompe disease, in which patients lack any expression of protein, during treatment with enzyme replacement therapy, or gene therapy. While the liver has long been known as being tolerogenic, it was only recently appreciated in the last decade that liver directed adeno-associated virus (AAV) gene therapy can induce systemic tolerance to a transgene. In this review, we look at the mechanisms behind liver induced tolerance, discuss different factors influencing successful tolerance induction with AAV, and applications where AAV mediated tolerance may be helpful.

Progress and challenges of gene therapy for Pompe disease

Pompe disease (PD) is a monogenic disorder caused by mutations in the acid alpha-glucosidase gene (Gaa). GAA is a lysosomal enzyme essential for the degradation of glycogen. Deficiency of GAA results in a severe, systemic disorder that, in its most severe form, can be fatal. About a decade ago, the prognosis of PD has changed dramatically with the marketing authorization of an enzyme replacement therapy (ERT) based on recombinant GAA. Despite the breakthrough nature of ERT, long-term follow-up of both infantile and late-onset Pompe disease patients (IOPD and LOPD, respectively), revealed several limitations of the approach. In recent years several investigational therapies for PD have entered preclinical and clinical development, with a few next generation ERTs entering late-stage clinical development. Gene therapy holds the potential to change dramatically the way we treat PD, based on the ability to express the Gaa gene long-term, ideally driving enhanced therapeutic efficacy compared to ERT. Several gene therapy approaches to PD have been tested in preclinical animal models, with a handful of early phase clinical trials started or about to start. The complexity of PD and of the endpoints used to measure efficacy of investigational treatments remains a challenge, however the hope is for a future with more therapeutic options for both IOPD and LOPD patients.

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.

TLR9-Activating CpG-B ODN but Not TLR7 Agonists Triggers Antibody Formation to Factor IX in Muscle Gene Transfer

Innate immune signals that promote B cell responses in gene transfer are generally ill-defined. In this study, we evaluate the effect of activating endosomal Toll-like receptors 7, 8, and 9 (TLR7, TLR7/8, and TLR9) on antibody formation during muscle-directed gene therapy with adeno-associated virus (AAV) vectors. We examined whether activation of endosomal TLRs, by adenine analog CL264 (TLR7 agonist), imidazolquinolone compound R848 (TLR7/8 agonist), or class B CpG oligodeoxynucleotides ODN1826 (TLR9 agonist), could augment antibody formation upon intramuscular administration of AAV1 expressing human clotting factor IX (AAV1-hFIX) in mice. The TLR9 agonist robustly enhanced antibody formation by the 1st week, thus initially eliminating systemic hFIX expression. By contrast, the TLR7 and TLR7/8 agonists did not markedly promote antibody formation, or significantly reduce circulating hFIX. We concurrently investigated the effects of these TLR agonists during muscle gene transfer on mature B cells and dendritic cells (DCs) in the draining lymph nodes including conventional DCs (CD11b+ or CD8α+ cDCs), monocyte-derived dendritic cells (moDCs), and plasmacytoid dendritic cells (pDCs). Only TLR9 stimulation caused a striking increase in the frequency of moDCs within 24 h. The TLR7/8 and TLR9 agonists activated pDCs, both subsets of cDCs, and mature B cells, whereas the TLR7 agonist had only mild effects on these cells. Thus, these TLR ligands have distinct effects on DCs and mature B cells, yet only the TLR9 agonist enhanced the humoral immune response against AAV-expressed hFIX. These new findings indicate a unique ability of certain TLR9 agonists to stimulate B cell responses in muscle gene transfer through enrichment of moDCs.

In Vivo Model Systems for Hepatitis B Virus Research

Hepatitis B virus (HBV) affects more than 257 million people globally, resulting in progressively worsening liver disease, manifesting as fibrosis, cirrhosis, and hepatocellular carcinoma. The exceptionally narrow species tropism of HBV restricts its natural hosts to humans and non-human primates, including chimpanzees, gorillas, gibbons, and orangutans. The unavailability of completely immunocompetent small-animal models has contributed to the lack of curative therapeutic interventions. Even though surrogates allow the study of closely related viruses, their host genetic backgrounds, immune responses, and molecular virology differ from those of HBV. Various different models, based on either pure murine or xenotransplantation systems, have been introduced over the past years, often making the choice of the optimal model for any given question challenging. Here, we offer a concise review of in vivo model systems employed to study HBV infection and steps in the HBV life cycle or pathogenesis.

Immunogenicity of Protein Pharmaceuticals

Protein therapeutics have drastically changed the landscape of treatment for many diseases by providing a regimen that is highly specific and lacks many off-target toxicities. The clinical utility of many therapeutic proteins has been undermined by the potential development of unwanted immune responses against the protein, limiting their efficacy and negatively impacting its safety profile. This review attempts to provide an overview of immunogenicity of therapeutic proteins, including immune mechanisms and factors influencing immunogenicity, impact of immunogenicity, preclinical screening methods, and strategies to mitigate immunogenicity.

Five Years of Successful Inducible Transgene Expression Following Locoregional Adeno-Associated Virus Delivery in Nonhuman Primates with No Detectable Immunity

Anti-transgene immune responses elicited after intramuscular (i.m.) delivery of recombinant adeno-associated virus (rAAV) have been shown to hamper long-term transgene expression in large-animal models of rAAV-mediated gene transfer. To overcome this hurdle, an alternative mode of delivery of rAAV vectors in nonhuman primate muscles has been described: the locoregional (LR) intravenous route of administration. Using this injection mode, persistent inducible transgene expression for at least 1 year under the control of the tetracycline-inducible Tet-On system was previously reported in cynomolgus monkeys, with no immunity against the rtTA transgene product. The present study shows the long-term follow-up of these animals. It is reported that LR delivery of a rAAV2/1 vector allows long-term inducible expression up to at least 5 years post gene transfer, with no any detectable host immune response against the transactivator rtTA, despite its immunogenicity following i.m. gene transfer. This study shows for the first time a long-term regulation of muscle gene expression using a Tet-On-inducible system in a large-animal model. Moreover, these findings further confirm that the rAAV LR delivery route is efficient and immunologically safe, allowing long-term skeletal muscle gene transfer.