High Vector Doses Research Articles

Redundancy in Innate Immune Pathways That Promote CD8+ T-Cell Responses in AAV1 Muscle Gene Transfer.

While adeno-associated viral (AAV) vectors are successfully used in a variety of in vivo gene therapy applications, they continue to be hampered by the immune system. Here, we sought to identify innate and cytokine signaling pathways that promote CD8+ T-cell responses against the transgene product upon AAV1 vector administration to murine skeletal muscle. Eliminating just one of several pathways (including DNA sensing via TLR9, IL-1 receptor signaling, and possibly endosomal sensing of double-stranded RNA) substantially reduced the CD8+ T-cell response at lower vector doses but was surprisingly ineffective at higher doses. Using genetic, antibody-mediated, and vector engineering approaches, we show that blockade of at least two innate pathways is required to achieve an effect at higher vector doses. Concurrent blockade of IL-1R1 > MyD88 and TLR9 > MyD88 > type I IFN > IFNaR pathways was often but not always synergistic and had limited utility in preventing antibody formation against the transgene product. Further, even low-frequency CD8+ T-cell responses could eliminate transgene expression, even in MyD88- or IL-1R1-deficient animals that received a low vector dose. However, we provide evidence that CpG depletion of vector genomes and including TLR9 inhibitory sequences can synergize. When this construct was combined with the use of a muscle-specific promoter, transgene expression in muscle was sustained with minimal local or systemic CD8+ T-cell response. Thus, innate immune avoidance/blockade strategies by themselves, albeit helpful, may not be sufficient to prevent destructive cellular responses in muscle gene transfer because of the redundancy of immune-activating pathways.

Single cell and TCR analysis of immune cells from AAV gene therapy-dosed Duchenne muscular dystrophy patients

Clinical trials for Duchenne muscular dystrophy (DMD) are assessing the therapeutic efficacy of systemically delivered adeno-associated virus (AAV) carrying a modified DMD transgene. High vector doses (>1E14 vg/kg) are needed to globally transduce skeletal muscles; however, such doses trigger immune-related adverse events. Mitigating these immune responses is crucial for widespread application of AAV-based therapies. We used single-cell RNA-sequencing and T-cell receptor (TCR)-sequencing on peripheral blood mononuclear cells from five participants prior to, and after, dosing. One subject in the high-dose cohort experienced thrombotic microangiopathy (TMA). Few changes in cell frequencies occurred after treatment; however, differential gene expression demonstrated induction of interferon response genes in most T-cell types. T-cell clonotype and clumping analysis showed the expansion or appearance of groups of related TCR sequences in the post-treatment samples. Three of these expanded clumps could be assigned to prior Human Herpesvirus infections, two of which were present in the participant that exhibited TMA. These data provide insight on the mechanistic basis of human immune-AAV interactions and lay a foundation for improved understanding of why TMA arises in some patients and not others.

Human umbilical cord mesenchymal stem cell-based gene therapy for hemophilia B using scAAV-DJ/8-LP1-hFIXco transduction

BackgroundHemophilia B is an X-linked bleeding disorder caused by a mutation in the gene responsible for encoding coagulation factor IX (FIX). Gene therapy offers promising potential for curing this disease. However, the current method of relatively high dosage of virus injection carries inherent risks. The purpose of this study was to introduce a novel scAAV-DJ/8-LP1-hFIXco vector transduced human umbilical cord blood derived mesenchymal stem cells (HUCMSCs) as an alternative cell-based gene therapy to conventional gene therapy for Hemophilia B.MethodsThe LP1-hFIXco gene structure was designed by us through searching the literature from NCBI and the scAAV-DJ/8-LP1-hFIXco vector was constructed by a commercial company. The HUCMSCs were cultivated in routine approach and transduced with scAAV-DJ/8-LP1-hFIXco vector. The human FIX activation system was employed for detection of hFIXco activity. The RNA and protein expression levels of the hFIXco were evaluated using PCR and western blot techniques. In animal studies, both NSG and F9-KO mice were used for the experiment, in which clotting time was utilized as a parameter for bleeding assessment. The immunohistochemical analysis was used to assess the distribution of HUCMSCs in mouse tissue sections. The safety for tumorigenicity of this cell-based gene therapy was evaluated by pathological observation after hematoxylin-eosin staining.ResultsThe transduction of HUCMSCs with the scAAV-DJ/8-LP1-hFIXco vector results in consistent and sustainable secretion of human FIXco during 5 months period both in vitro and in mouse model. The secretion level (hFIXco activity: 97.1 ± 2.3% at day 7 to 48.8 ± 4.5% at 5 months) was comparable to that observed following intravenous injection with a high dose of the viral vector (hFIXco activity: 95.2 ± 2.2% to 40.8 ± 4.3%). After a 5-month observation period, no clonal expansions of the transduced cells in tissues were observed in any of the mice studied.ConclusionsWe have discovered a novel and safer HUCMSCs mediated approach potentially effective for gene therapy in hemophilia B.

Innate Immune Sensing of Adeno-Associated Virus Vectors.

Adeno-associated virus (AAV) based viral vectors are widely used in human gene therapy and form the basis of approved treatments for several genetic diseases. Immune responses to vector and transgene products, however, substantially complicate these applications in clinical practice. The role of innate immune recognition of AAV vectors was initially unclear, given that inflammatory responses early after vector administration were typically mild in animal models. However, more recent research continues to identify innate immune pathways that are triggered by AAV vectors and that serve to provide activation signals for antigen-presenting cells and initiation of adaptive immune responses. Sensing of the AAV genome by the endosomal DNA receptor toll-like receptor 9 (TLR9) promotes early inflammatory response and interferon expression. Thus, activation of the TLR9>MyD88 pathway in plasmacytoid dendritic cells (pDCs) leads to the conditioning of antigen cross-presenting DCs through type I interferon (IFN-I) and ultimately CD8+ T cell activation. Alternatively, pDCs may also promote CD8+ T cell responses in a TLR9-independent manner by the production of IL-1 cytokines, thereby activating the IL-1R1>MyD88 signaling pathway. AAV can induce cytokine expression in monocyte-derived DCs, which in turn increases antibody formation. Binding of AAV capsid to complement components likely further elevates B cell activation. At high systemic vector doses in humans and in non-human primates, AAV vectors can trigger complement activation, with contributions by classical and alternative pathways, leading to severe toxicities. Finally, evidence for activation of TLR2 by the capsid and of additional innate receptors for nucleic acids has been presented. These observations show that AAV vectors can initiate several and likely redundant innate immune pathways resulting in an exaggerated adaptive immune response.

How safe is gene therapy? : Second death after Duchenne therapy

Duchenne muscular dystrophy (DMD) is asevere monogenic hereditary disease with early manifestation and aprogressive course. Treatment options have so far been limited. Gene therapy opens up new options for DMD patients. Against the background of afurther death following DMD gene therapy, the side effects and risks of the gene therapeutics already approved or undergoing clinical trials will be evaluated and alternative gene therapeutics will be described. Based thereon, the future of DMD gene therapy will be discussed. For the first time, in June 2023, delandistrogene moxeparvovec (SRP-9001), agene replacement therapy based on an adeno-associated virus (AAV) vector, was approved in the USA for children aged 4-5years with DMD. Other promising gene therapies are in preclinical development or clinical trials, including CRISPR/Cas9-mediated strategies to restore dystrophin expression. Two deaths following DMD gene therapy with high-dose AAV vectors were attributed to AAV-mediated immune responses. The pre-existing disease underlying the therapy is most likely involved in the fatal AAV toxicity. Although gene therapy applications of AAV vectors are generally considered safe, the systemic administration of high vector doses can lead to severe side effects with apotentially fatal outcome in individual patients, especially after activation of the immune system. In the future, new methods for immunosuppression, reduction of AAV dose and alternative vectors will therefore increasingly come to the fore.

CRISPR-Based Gene Therapies: From Preclinical to Clinical Treatments.

In recent years, clustered regularly interspaced short palindromic repeats (CRISPRs) and CRISPR-associated (Cas) protein have emerged as a revolutionary gene editing tool to treat inherited disorders affecting different organ systems, such as blood and muscles. Both hematological and neuromuscular genetic disorders benefit from genome editing approaches but face different challenges in their clinical translation. The ability of CRISPR/Cas9 technologies to modify hematopoietic stem cells ex vivo has greatly accelerated the development of genetic therapies for blood disorders. In the last decade, many clinical trials were initiated and are now delivering encouraging results. The recent FDA approval of Casgevy, the first CRISPR/Cas9-based drug for severe sickle cell disease and transfusion-dependent β-thalassemia, represents a significant milestone in the field and highlights the great potential of this technology. Similar preclinical efforts are currently expanding CRISPR therapies to other hematologic disorders such as primary immunodeficiencies. In the neuromuscular field, the versatility of CRISPR/Cas9 has been instrumental for the generation of new cellular and animal models of Duchenne muscular dystrophy (DMD), offering innovative platforms to speed up preclinical development of therapeutic solutions. Several corrective interventions have been proposed to genetically restore dystrophin production using the CRISPR toolbox and have demonstrated promising results in different DMD animal models. Although these advances represent a significant step forward to the clinical translation of CRISPR/Cas9 therapies to DMD, there are still many hurdles to overcome, such as in vivo delivery methods associated with high viral vector doses, together with safety and immunological concerns. Collectively, the results obtained in the hematological and neuromuscular fields emphasize the transformative impact of CRISPR/Cas9 for patients affected by these debilitating conditions. As each field suffers from different and specific challenges, the clinical translation of CRISPR therapies may progress differentially depending on the genetic disorder. Ongoing investigations and clinical trials will address risks and limitations of these therapies, including long-term efficacy, potential genotoxicity, and adverse immune reactions. This review provides insights into the diverse applications of CRISPR-based technologies in both preclinical and clinical settings for monogenic blood disorders and muscular dystrophy and compare advances in both fields while highlighting current trends, difficulties, and challenges to overcome.

Complement System Response to Adeno-Associated Virus Vector Gene Therapy.

Adeno-associated virus (AAV) vectors represent a novel tool for the delivery of genetic therapeutics and enable the treatment of a wide range of diseases. Success of this new modality is challenged, however, by cases of immune-related toxicities that complicate the clinical management of patients and potentially limit the therapeutic efficacy of AAV gene therapy. While significant progress has been made to manage immune-related liver enzyme elevations following systemic AAV delivery in humans, recent clinical trials utilizing high vector doses have highlighted a new challenge to AAV gene transfer-activation of the complement system. While current in vitro models implicate AAV-specific antibodies in the initiation of the classical complement pathway, evidence from in vivo pre-clinical and clinical studies suggests that the alternative pathway also contributes to complement activation. A convergence of AAV-specific, environmental, and patient-specific factors shaping complement responses likely contributes to differential outcomes seen in clinical trials, from priming of the adaptive immune system to serious adverse events such as hepatotoxicity and thrombotic microangiopathy. Research focused on the interplay of patient-specific and AAV-related factors driving complement activation is needed to understand and identify critical components in the complement cascade to target and devise strategies to mitigate vector-related immune responses.

Single intranasal immunization with a high dose of influenza vector protects against infection with heterologous influenza virus and SARS-CoV-2 in ferrets and hamsters

BACKGROUND: The challenge of vaccine effectiveness against viruses that undergo constant antigenic changes during evolution is currently being addressed by updating vaccine formulations to match circulating strains. However, this approach proves ineffective if a virus undergoes antigenic drift and shift, or if a new virus, such as SARS-CoV-2, emerges and enters circulation. Hence, there is a pressing need to develop universal vaccines that elicit a T-cell immune response targeting conserved antigenic determinants of pathogens. OBJECTIVE: To develop a vaccine candidate against influenza virus and coronavirus based on an attenuated influenza vector. METHODS: In pursuit of this objective, we developed a recombinant influenza vector named FluCoV-N. It incorporates attenuating modifications in the ns1 and nep genes and expresses the N-terminal half of the N protein (N1-209) of the SARS-CoV-2 virus. To assess the vector’s protective efficacy against influenza, ferrets were infected with heterologous influenza A/Austria/1516645/2022 (H3N2) virus on the 25th day after a single immunization with 9.4 log10EID50 of the studied vector. To test protection against coronavirus, hamsters were immunized once with the vector at a dose of 8.2 log10EID50 and challenged with SARS-CoV-2 virus 21 days later. RESULTS: As a result of modifications to the NS genomic segment, the constructed vector acquired a temperature-sensitive (ts) phenotype and demonstrated a heightened ability to induce type 1 interferons. It was harmless to animals when administered intranasally at high doses exceeding 8.0 log10EID50. In ferrets, a single intranasal immunization with FluCoV-N accelerated the resolution of infection caused by heterologous influenza H3N2 virus. Similar immunization in hamsters led to a 10,000-fold reduction in SARS-CoV-2 viral titers in the lungs on the second day after challenge and reduced pathology in the lungs of animals. CONCLUSION: A single intranasal immunization with the FluCoV-N vector protected from heterologous influenza or SARS-CoV-2 viruses in ferrets and hamsters.

Stable Therapeutic Transgenic FIX Levels for More Than 10 Years in Subjects with Severe Hemophilia B Who Received scAAV2/8-LP1-Hfixco Adeno-Associated Virus Gene Therapy

Background: Previously, we reported data from a first-in-human, dose-escalation, Phase 1 gene therapy trial demonstrating a dose-dependent increase in transgenic factor IX (FIX) expression in 10 adults with severe hemophilia B (FIX activity ≤1%). This was achieved following a single intravenous infusion of a self-complementary adeno-associated virus (scAAV) vector containing the wild-type FIX gene (codon-optimized), under the control of a synthetic liver-specific promoter and pseudotyped AAV8 capsid (scAAV2/8-LP1-hFIXco; ClinicalTrials.gov:NCT00979238). We now report durable efficacy and long-term safety over at least 10 years of follow-up in these participants. Methods: FIX activity, annualized bleeding rate (ABR), use of FIX concentrates, safety (specifically transaminitis and oncogenesis), and FIX expression kinetics were evaluated in the 10 participants following a bolus infusion of scAAV2/8-LP1-hFIXco at a dose of 2x10 11vg/kg (N=2), 6x10 11vg/kg (N=2), or 2x10 12vg/kg (N=6), between 2010 and 2012. Results: As of December 31, 2022, the median follow-up was 10.7 years (range 4-12 years). A total of 11 treatment-related adverse events occurred in 10 participants, including transient elevation of liver transaminases within 3 months of vector infusion in 4 of 6 patients treated with the high vector dose, without any recurrence. Two new serious adverse events were reported: (1) non-mucinous lung adenocarcinoma in situ, identified incidentally following a bullectomy for spontaneous pneumothorax 5 years after gene therapy in a 48-year-old participant considered not related to gene therapy following molecular studies, and (2) adenocarcinoma of the prostate in a 72-year-old participant 12 years after gene therapy. Tissue evaluation of the latter case is ongoing. No FIX inhibitors, thrombosis, persistent transaminitis or deaths were observed. Transgenic FIX expression has remained stable, with mean (±SD) FIX activity (one-stage) for the three dose cohorts of 1.7±0.9, 2.3±0.9 and 4.9±2.2 IU/dL, respectively (Figure 1). The mean and median ABR over the 10-year period following gene therapy of all 10 participants were 1.95 and 1.6 compared to 16.5 and 14, respectively, before gene therapy. This represents an 8.5-fold reduction in bleeding events (Wilcoxon signed rank test p=0.002). In the 6 high-dose participants, the ABR was 21-fold lower (mean=1.16, median 1, paired t-test, p<0.008). Mean and median FIX concentrate usage before gene therapy in the 10 participants were 2869 and 2526 IU/kg dropping to 945 and 274 IU/kg, respectively, over a 10-year period after gene therapy (p=0.0003). All participants developed a persistent, high-titer polyclonal anti-AAV8 capsid-specific antibody response after administration of scAAV2/8-LP1-hFIXco. Conclusion: Expression of transgenic FIX has remained stable over a period of 10 years following systemic administration of scAAV2/8-LP1-hFIXco resulting in sustained clinical benefit, with substantial reduction in ABR and FIX concentrate use.

Readministration of high-dose adeno-associated virus gene therapy vectors enabled by ImmTOR nanoparticles combined with B cell-targeted agents.

Tolerogenic ImmTOR nanoparticles encapsulating rapamycin have been demonstrated to mitigate immunogenicity of adeno-associated virus (AAV) gene therapy vectors, enhance levels of transgene expression, and enable redosing of AAV at moderate vector doses of 2 to 5E12 vg/kg. However, recent clinical trials have often pushed AAV vector doses 10-fold to 50-fold higher, with serious adverse events observed at the upper range. Here, we assessed combination therapy of ImmTOR with B cell-targeting drugs for the ability to increase the efficiency of redosing at high vector doses. The combination of ImmTOR with a monoclonal antibody against B cell activation factor (aBAFF) exhibited strong synergy leading to more than a 5-fold to 10-fold reduction of splenic mature B cells and plasmablasts while increasing the fraction of pre-/pro-B cells. In addition, this combination dramatically reduced anti-AAV IgM and IgG antibodies, thus enabling four successive AAV administrations at doses up to 5E12 vg/kg and at least two AAV doses at 5E13 vg/kg, with the transgene expression level in the latter case being equal to that observed in control animals receiving a single vector dose of 1E14 vg/kg. Similar synergistic effects were seen with a combination of ImmTOR and a Bruton's tyrosine kinase inhibitor, ibrutinib. These results suggest that ImmTOR could be combined with B cell-targeting agents to enable repeated vector administrations as a potential strategy to avoid toxicities associated with vector doses above 1E14 vg/kg.

Topoisomerase Inhibitors Increase Episomal DNA Expression by Inducing the Integration of Episomal DNA in Hepatic Cells.

Gene therapy is a promising strategy to treat and cure most inherited metabolic liver disorders. Viral vectors such as those based on adeno-associated viruses (AAVs) and lentiviruses (LVs) are used as vehicles to deliver functional genes to affected hepatocytes. Adverse events associated with the use of high vector doses have motivated the use of small molecules as adjuvants to reduce the dose. In this study, we showed that a one-hour treatment with topoisomerase inhibitors (camptothecin and etoposide) prior to viral transduction is enough to increase AAV and LV reporter expression in non-dividing hepatic cells in culture. Topoisomerase inhibitors increased both integration-competent (ICLV) and integration-deficient (IDLV) LV-derived expression, with a much stronger increase in the IDLV transduction system. In agreement with that, topoisomerase inhibitors increased viral genome integration in both strains, with a greater impact on the IDLV strain, supporting the idea that topoisomerase inhibitors increased episomal DNA integration, especially when viral integrase activity is abolished. These effects correlated with an increase in the DNA damage response produced by the treatments. Our study highlights the need to monitor DNA damage and undesired integration of viral episomal DNAs into the host genome when studying chemical compounds that increase viral transduction.

Sildenafil increases AAV9 transduction after a systemic administration and enhances AAV9-dystrophin therapeutic effect in mdx mice.

Adeno-associated virus (AAV) vectors have been successfully used to deliver genes for treating rare diseases. However, the systemic administration of high AAV vector doses triggers several adverse effects, including immune response, the asymptomatic elevation of liver transaminase levels, and complement activation. Thus, improving AAV transduction and reducing AAV dosage for treatment is necessary. Recently, we found that a phosphodiesterase-5 inhibitor significantly promoted AAV9 transduction in vitro by regulating the caveolae and macropinocytosis pathways. When AAV9-Gaussian luciferase (AAV9-Gluc) and AAV9-green fluorescent protein (AAV9-GFP) were injected intravenously into mice pre-treated with sildenafil, the expressions of Gluc in the plasma and GFP in muscle tissues significantly increased (P < 0.05). Sildenafil also improved Evans blue permeation in tissues. Additionally, we found that sildenafil promoted Treg proliferation, inhibited B-cell activation, and decreased anti-AAV9 IgG levels (P < 0.05). Furthermore, sildenafil significantly promoted Duchenne muscular dystrophy gene therapy efficacy using AAV9 in mdx mice; it increased micro-dystrophin gene expression, forelimb grip strength, and time spent on the rotarod test, decreased serum creatine kinase levels, and ameliorated histopathology by improving muscle cell morphology and reducing fibrosis (P < 0.05). These results show that sildenafil significantly improved AAV transduction, suppressed the levels of anti-AAV9 IgG, and enhanced the efficacy of gene therapy.

Every little bit helps: A single-residue switch in a vascular AAV enables blood-brain barrier penetration

Every little bit helps: A single-residue switch in a vascular AAV enables blood-brain barrier penetration

Assessment of Pre-Clinical Liver Models Based on Their Ability to Predict the Liver-Tropism of Adeno-Associated Virus Vectors.

The liver is a prime target for in vivo gene therapies using recombinant adeno-associated viral vectors. Multiple clinical trials have been undertaken for this target in the past 15 years; however, we are still to see market approval of the first liver-targeted adeno-associated virus (AAV)-based gene therapy. Inefficient expression of the therapeutic transgene, vector-induced liver toxicity and capsid, and/or transgene-mediated immune responses reported at high vector doses are the main challenges to date. One of the contributing factors to the insufficient clinical outcomes, despite highly encouraging preclinical data, is the lack of robust, biologically and clinically predictive preclinical models. To this end, this study reports findings of a functional evaluation of 6 AAV vectors in 12 preclinical models of the human liver, with the aim to uncover which combination of models is the most relevant for the identification of AAV capsid variant for safe and efficient transgene delivery to primary human hepatocytes. The results, generated by studies in models ranging from immortalized cells, iPSC-derived and primary hepatocytes, and primary human hepatic organoids to in vivo models, increased our understanding of the strengths and weaknesses of each system. This should allow the development of novel gene therapies targeting the human liver.

Big stride in gene therapy for hemophilia B in China.

Big stride in gene therapy for hemophilia B in China.

Significance of Preexisting Vector Immunity and Activation of Innate Responses for Adenoviral Vector-Based Therapy.

An adenoviral (AdV)-based vector system is a promising platform for vaccine development and gene therapy applications. Administration of an AdV vector elicits robust innate immunity, leading to the development of humoral and cellular immune responses against the vector and the transgene antigen, if applicable. The use of high doses (1011-1013 virus particles) of an AdV vector, especially for gene therapy applications, could lead to vector toxicity due to excessive levels of innate immune responses, vector interactions with blood factors, or high levels of vector transduction in the liver and spleen. Additionally, the high prevalence of AdV infections in humans or the first inoculation with the AdV vector result in the development of vector-specific immune responses, popularly known as preexisting vector immunity. It significantly reduces the vector efficiency following the use of an AdV vector that is prone to preexisting vector immunity. Several approaches have been developed to overcome this problem. The utilization of rare human AdV types or nonhuman AdVs is the primary strategy to evade preexisting vector immunity. The use of heterologous viral vectors, capsid modification, and vector encapsulation are alternative methods to evade vector immunity. The vectors can be optimized for clinical applications with comprehensive knowledge of AdV vector immunity, toxicity, and circumvention strategies.

Immunogenicity and toxicity of AAV gene therapy.

Gene transfer using adeno-associated viral (AAV) vectors has made tremendous progress in the last decade and has achieved cures of debilitating diseases such as hemophilia A and B. Nevertheless, progress is still being hampered by immune responses against the AAV capsid antigens or the transgene products. Immunosuppression designed to blunt T cell responses has shown success in some patients but failed in others especially if they received very high AAV vectors doses. Although it was initially thought that AAV vectors induce only marginal innate responses below the threshold of systemic symptoms recent trials have shown that complement activation can results in serious adverse events. Dorsal root ganglia toxicity has also been identified as a complication of high vector doses as has severe hepatotoxicity. Most of the critical complications occur in patients who are treated with very high vector doses indicating that the use of more efficient AAV vectors to allow for dose sparing or giving smaller doses repeatedly, the latter in conjunction with antibody or B cell depleting measures, should be explored.

Pharmacokinetic analysis identifies a factor VIII immunogenicity threshold after AAV gene therapy in hemophilia A mice

AbstractAdvances in the development of novel treatment options for hemophilia A are prevalent. However, the anti–factor VIII (FVIII) neutralizing antibody (inhibitor) response to existing FVIII products remains a major treatment challenge. Although some novel products are designed to function in the presence of inhibitors, they do not specific address the immunogenicity risk or mechanistic causes of inhibitor development, which remain unclear. Furthermore, most preclinical studies supporting clinical gene therapy programs have reported immunogenicity signals in animal models, especially at higher vector doses and sometimes using multiple vector designs. In these settings, immunogenicity risk factor determination, comparative immunogenicity of competing vector designs, and the potential for obtaining meaningful prognostic data remain relatively unexplored. Additionally, there remains the opportunity to investigate clinical gene therapy as an alternative to standard immune tolerance induction therapy. The current study was designed to address these issues through longitudinal dose-response evaluation of 4 adeno-associated viral (AAV) vector candidates encoding 2 different FVIII transgenes in a murine model of hemophilia A. Plasma FVIII activity and anti-FVIII antibody data were used to generate a pharmacokinetic model that (1) identifies initial AAV-FVIII product expression kinetics as the dominant risk factor for inhibitor development, (2) predicts a therapeutic window where immune tolerance is achieved, and (3) demonstrates evidence of gene therapy–based immune tolerance induction. Although there are known limitations to the predictive value of preclinical immunogenicity testing, these studies can uncover or support the development of design principles that can guide the development of safe and effective genetic medicines.

In vivo prime editing of a metabolic liver disease in mice.

Prime editing is a highly versatile CRISPR-based genome editing technology that works without DNA double-strand break formation. Despite rapid technological advances, in vivo application for the treatment of genetic diseases remains challenging. Here, we developed a size-reduced SpCas9 prime editor (PE) lacking the RNaseH domain (PE2ΔRnH) and an intein-split construct (PE2 p.1153) for adeno-associated virus-mediated delivery into the liver. Editing efficiencies reached 15% at the Dnmt1 locus and were further elevated to 58% by delivering unsplit PE2ΔRnH via human adenoviral vector 5 (AdV). To provide proof of concept for correcting a genetic liver disease, we used the AdV approach for repairing the disease-causing Pahenu2 mutation in a mouse model of phenylketonuria (PKU) via prime editing. Average correction efficiencies of 11.1% (up to 17.4%) in neonates led to therapeutic reduction of blood phenylalanine, without inducing detectable off-target mutations or prolonged liver inflammation. Although the current in vivo prime editing approach for PKU has limitations for clinical application due to the requirement of high vector doses (7 × 1014 vg/kg) and the induction of immune responses to the vector and the PE, further development of the technology may lead to curative therapies for PKU and other genetic liver diseases.

1014 – CELLULAR SENESCENCE: AN UNINTENDED TWIST FOR GENE AND CELL THERAPY APPLICATIONS

The success of Hematopoietic Stem and Progenitor Cell (HSPC) gene therapies critically depends on the capacity to genetically engineer HSPCs without compromising their functional properties. Emerging evidence, including our own, indicates that HSPC exposure to currently available gene transfer and gene editing technologies –which require prolonged ex-vivo culture, high viral vector doses and nuclease-induced DNA double strand breaks¬– unexpectedly converge on the DNA damage response (DDR), a signaling cascade leading to cell cycle arrest. Protracted DDR impairs the hematopoietic reconstitution of gene-modified cells upon transplantation; instead, its transient inhibition significantly improves their functionality. This experimental evidence uncovers a previously unappreciated interplay between viral vector sensing and the host cell DDR machinery in human HSPCs. Yet, we have just scratched the surface of a plethora of still unexplored cellular programs potentially hampering the clonal composition and the dynamics of reconstitution upon transplantation of this primitive cell compartment. We build on the innovative concept that current gene engineering protocols may inadvertently trigger the activation of a cellular senescence program in HSPCs with both cell-autonomous and paracrine short- and long-term consequences on engineered human hematopoiesis. By employing quantitative imaging and cutting-edge genomic technologies and taking advantage of uniquely available gene therapy platforms and primary patient samples, we aim to identify the molecular determinants that promote senescence in HSPCs and design innovative hypothesis-driven strategies to mitigate senescence barriers for more effective, safer and clinically broader gene therapy applications. Our findings will unveil a new set of mechanisms controlling the biology of gene-engineered HSPCs and open novel scientific horizons for the development of innovative gene and cell therapies. The success of Hematopoietic Stem and Progenitor Cell (HSPC) gene therapies critically depends on the capacity to genetically engineer HSPCs without compromising their functional properties. Emerging evidence, including our own, indicates that HSPC exposure to currently available gene transfer and gene editing technologies –which require prolonged ex-vivo culture, high viral vector doses and nuclease-induced DNA double strand breaks¬– unexpectedly converge on the DNA damage response (DDR), a signaling cascade leading to cell cycle arrest. Protracted DDR impairs the hematopoietic reconstitution of gene-modified cells upon transplantation; instead, its transient inhibition significantly improves their functionality. This experimental evidence uncovers a previously unappreciated interplay between viral vector sensing and the host cell DDR machinery in human HSPCs. Yet, we have just scratched the surface of a plethora of still unexplored cellular programs potentially hampering the clonal composition and the dynamics of reconstitution upon transplantation of this primitive cell compartment. We build on the innovative concept that current gene engineering protocols may inadvertently trigger the activation of a cellular senescence program in HSPCs with both cell-autonomous and paracrine short- and long-term consequences on engineered human hematopoiesis. By employing quantitative imaging and cutting-edge genomic technologies and taking advantage of uniquely available gene therapy platforms and primary patient samples, we aim to identify the molecular determinants that promote senescence in HSPCs and design innovative hypothesis-driven strategies to mitigate senescence barriers for more effective, safer and clinically broader gene therapy applications. Our findings will unveil a new set of mechanisms controlling the biology of gene-engineered HSPCs and open novel scientific horizons for the development of innovative gene and cell therapies.