Gene-corrected human Munc13-4–deficient CD8+ T cells can efficiently restrict EBV-driven lymphoproliferation in immunodeficient mice

Precise CRISPR-Cas9 gene repair in autologous memory T cells to treat familial hemophagocytic lymphohistiocytosis

Familial hemophagocytic lymphohistiocytosis (FHL) is an inherited, often fatal immune deficiency characterized by severe systemic hyperinflammation. Although allogeneic bone marrow transplantation can be curative, more effective therapies are urgently needed. FHL is caused by inactivating mutations in proteins that regulate cellular immunity. Here, we used an adeno-associated virus-based CRISPR-Cas9 system with an inhibitor of nonhomologous end joining to repair such mutations in potentially long-lived T cells ex vivo. Repaired CD8 memory T cells efficiently cured lethal hyperinflammation in a mouse model of Epstein-Barr virus-triggered FHL2, a subtype caused by perforin-1 (Prf1) deficiency. Furthermore, repair of PRF1 and Munc13-4 (UNC13D)-whose deficiency causes the FHL subtype FHL3-in mutant memory T cells from two critically ill patients with FHL restored T cell cytotoxicity. These results provide a starting point for the treatment of genetic T cell immune dysregulation syndromes with repaired autologous T cells.

Gene editing of hematopoietic stem cells restores T-cell response in familial hemophagocytic lymphohistiocytosis

BACKGROUND

Hemophagocytic lymphohistiocytosis (HLH) is a hyperinflammatory disorder characterized by a life-threatening cytokine storm and immunopathology. Familial HLH type 3 (FHL3) accounts for approximately 30% of all inborn HLH cases worldwide. It is caused by mutations in the UNC13D gene that result in impaired degranulation of cytotoxic vesicles and hence compromised T-cell- and natural killer-cell-mediated killing. Current treatment protocols, including allogeneic hematopoietic stem cell (HSC) transplantation, still show high mortality.

OBJECTIVE

We sought to develop and evaluate a curative genome editing strategy in the preclinical FHL3 Jinx mouse model. Jinx mice harbor a cryptic splice donor site in Unc13d intron 26 and develop clinical symptoms of human FHL3 upon infection with lymphocytic choriomeningitis virus (LCMV).

METHODS

We employed clustered regularly interspaced short palindromic repeats (CRISPR)-Cas technology to delete the disease-causing mutation in HSCs and transplanted Unc13d-edited stem cells into busulfan-conditioned Jinx recipient mice. Safety studies included extensive genotyping and chromosomal aberrations analysis by single targeted linker-mediated PCR sequencing (CAST-Seq)-based off-target analyses. Cure from HLH predisposition was assessed by LCMV infection.

RESULTS

Hematopoietic cells isolated from transplanted mice revealed efficient gene editing (>95%), polyclonality of the T-cell receptor repertoire, and neither signs of off-target effects nor leukemogenesis. Unc13d transcription levels of edited and wild-type cells were comparable. While LCMV challenge resulted in acute HLH in Jinx mice transplanted with mock-edited HSCs, Jinx mice grafted with Unc13d-edited cells showed rapid virus clearance and protection from HLH.

CONCLUSIONS

Our study demonstrates that transplantation of CRISPR-Cas edited HSCs supports the development of a functional polyclonal T-cell response in the absence of genotoxicity-associated clonal outgrowth.

Advances in gene therapy for inborn errors of immunity

PURPOSE OF REVIEW

Provide an overview of the landmark accomplishments and state of the art of gene therapy for inborn errors of immunity (IEI).

RECENT FINDINGS

Three decades after the first clinical application of gene therapy for IEI, there is one market authorized product available, while for several others efficacy has been demonstrated or is currently being tested in ongoing clinical trials. Gene editing approaches using programmable nucleases are being explored preclinically and could be beneficial for genes requiring tightly regulated expression, gain-of-function mutations and dominant-negative mutations.

SUMMARY

Gene therapy by modifying autologous hematopoietic stem cells (HSCs) offers an attractive alternative to allogeneic hematopoietic stem cell transplantation (HSCT), the current standard of care to treat severe IEI. This approach does not require availability of a suitable allogeneic donor and eliminates the risk of graft versus host disease (GvHD). Gene therapy can be attempted by using a viral vector to add a copy of the therapeutic gene (viral gene addition) or by using programmable nucleases (gene editing) to precisely correct mutations, disrupt a gene or introduce an entire copy of a gene at a specific locus. However, gene therapy comes with its own challenges such as safety, therapeutic effectiveness and access. For viral gene addition, a major safety concern is vector-related insertional mutagenesis, although this has been greatly reduced with the introduction of safer vectors. For gene editing, the risk of off-site mutagenesis is a main driver behind the ongoing search for modified nucleases. For both approaches, HSCs have to be manipulated ex vivo, and doing this efficiently without losing stemness remains a challenge, especially for gene editing.

Too much of a good thing: a review of primary immune regulatory disorders

Primary immune regulatory disorders (PIRDs) are inborn errors of immunity caused by a loss in the regulatory mechanism of the inflammatory or immune response, leading to impaired immunological tolerance or an exuberant inflammatory response to various stimuli due to loss or gain of function mutations. Whilst PIRDs may feature susceptibility to recurrent, severe, or opportunistic infection in their phenotype, this group of syndromes has broadened the spectrum of disease caused by defects in immunity-related genes to include autoimmunity, autoinflammation, lymphoproliferation, malignancy, and allergy; increasing focus on PIRDs has thus redefined the classical 'primary immunodeficiency' as one aspect of an overarching group of inborn errors of immunity. The growing number of genetic defects associated with PIRDs has expanded our understanding of immune tolerance mechanisms and prompted identification of molecular targets for therapy. However, PIRDs remain difficult to recognize due to incomplete penetrance of their diverse phenotype, which may cross organ systems and present to multiple clinical specialists prior to review by an immunologist. Control of immune dysregulation with immunosuppressive therapies must be balanced against the enhanced infective risk posed by the underlying defect and accumulated end-organ damage, posing a challenge to clinicians. Whilst allogeneic hematopoietic stem cell transplantation may correct the underlying immune defect, identification of appropriate patients and timing of transplant is difficult. The relatively recent description of many PIRDs and rarity of individual genetic entities that comprise this group means data on natural history, clinical progression, and treatment are limited, and so international collaboration will be needed to better delineate phenotypes and the impact of existing and potential therapies. This review explores pathophysiology, clinical features, current therapeutic strategies for PIRDs including cellular platforms, and future directions for research.

Gene Therapy for Inborn Errors of Immunity

In the early 1990s, gene therapy (GT) entered the clinical arena as an alternative to hematopoietic stem cell transplantation for forms of inborn errors of immunity (IEIs) that are not medically manageable because of their severity. In principle, the use of gene-corrected autologous hematopoietic stem cells presents several advantages over hematopoietic stem cell transplantation, including making donor searches unnecessary and avoiding the risks for graft-versus-host disease. In the past 30 years or more of clinical experience, the field has witnessed multiple examples of successful applications of GT to a number of IEIs, as well as some serious drawbacks, which have highlighted the potential genotoxicity of integrating viral vectors and stimulated important progress in the development of safer gene transfer tools. The advent of gene editing technologies promises to expand the spectrum of IEIs amenable to GT to conditions caused by mutated genes that require the precise regulation of expression or by dominant-negative variants. Here, we review the main concepts of GT as it applies to IEIs and the clinical results obtained to date. We also describe the challenges faced by this branch of medicine, which operates in the unprofitable sector of human rare diseases.

Gene therapy for inborn error of immunity - current status and future perspectives

PURPOSE OF REVIEW

Development of hematopoietic stem cell (HSC) gene therapy (GT) for inborn errors of immunity (IEIs) continues to progress rapidly. Although more patients are being treated with HSC GT based on viral vector mediated gene addition, gene editing techniques provide a promising new approach, in which transgene expression remains under the control of endogenous regulatory elements.

RECENT FINDINGS

Many gene therapy clinical trials are being conducted and evidence showing that HSC GT through viral vector mediated gene addition is a successful and safe curative treatment option for various IEIs is accumulating. Gene editing techniques for gene correction are, on the other hand, not in clinical use yet, despite rapid developments during the past decade. Current studies are focussing on improving rates of targeted integration, while preserving the primitive HSC population, which is essential for future clinical translation.

SUMMARY

As HSC GT is becoming available for more diseases, novel developments should focus on improving availability while reducing costs of the treatment. Continued follow up of treated patients is essential for providing information about long-term safety and efficacy. Editing techniques have great potential but need to be improved further before the translation to clinical studies can happen.

Adoptive T cell therapy cures mice from active hemophagocytic lymphohistiocytosis (HLH)

Primary hemophagocytic lymphohistiocytosis (HLH) is a hyperinflammatory syndrome caused by impaired lymphocyte cytotoxicity. First-line therapeutic regimens directed against activated immune cells or secreted cytokines show limited efficacy since they do not target the underlying immunological problem: defective lymphocyte cytotoxicity causing prolonged immune stimulation. A potential rescue strategy would be the adoptive transfer of ex vivo gene-corrected autologous T cells. However, transfusion of cytotoxicity-competent T cells under conditions of hyperinflammation may cause more harm than benefit. As a proof-of-concept for adoptive T cell therapy (ATCT) under hyperinflammatory conditions, we transferred syngeneic, cytotoxicity-competent T cells into mice with virally triggered active primary HLH. ATCT with functional syngeneic trigger-specific T cells cured Jinx mice from active HLH without life-threatening side effects and protected Perforin-deficient mice from lethal HLH progression by reconstituting cytotoxicity. Cured mice were protected long-term from HLH relapses. A threshold frequency of transferred T cells with functional differentiation was identified as a predictive biomarker for long-term survival. This study is the first proof-of-concept for ATCT in active HLH.

Gene Edited T Cell Therapies for Inborn Errors of Immunity

Inborn errors of immunity (IEIs) are a heterogeneous group of inherited disorders of the immune system. Many IEIs have a severe clinical phenotype that results in progressive morbidity and premature mortality. Over 450 IEIs have been described and the incidence of all IEIs is 1/1,000–10,000 people. Current treatment options are unsatisfactory for many IEIs. Allogeneic haematopoietic stem cell transplantation (alloHSCT) is curative but requires the availability of a suitable donor and carries a risk of graft failure, graft rejection and graft-versus-host disease (GvHD). Autologous gene therapy (GT) offers a cure whilst abrogating the immunological complications of alloHSCT. Gene editing (GE) technologies allow the precise modification of an organisms’ DNA at a base-pair level. In the context of genetic disease, this enables correction of genetic defects whilst preserving the endogenous gene control machinery. Gene editing technologies have the potential to transform the treatment landscape of IEIs. In contrast to gene addition techniques, gene editing using the CRISPR system repairs or replaces the mutation in the DNA. Many IEIs are limited to the lymphoid compartment and may be amenable to T cell correction alone (rather than haematopoietic stem cells). T cell Gene editing has the advantages of higher editing efficiencies, reduced risk of deleterious off-target edits in terminally differentiated cells and less toxic conditioning required for engraftment of lymphocytes. Although most T cells lack the self-renewing property of HSCs, a population of T cells, the T stem cell memory compartment has long-term multipotent and self-renewal capacity. Gene edited T cell therapies for IEIs are currently in development and may offer a less-toxic curative therapy to patients affected by certain IEIs. In this review, we discuss the history of T cell gene therapy, developments in T cell gene editing cellular therapies before detailing exciting pre-clinical studies that demonstrate gene editing T cell therapies as a proof-of-concept for several IEIs.

Lentiviral Gene Therapy for Familial Hemophagocytic Lymphohistiocytosis Type 3, Caused by UNC13D Genetic Defects

Familial hemophagocytic lymphohistiocytosis type 3 (FHL3) is a rare disease caused by mutations to the UNC13D gene and the subsequent absence or decreased activity of the Munc13-4 protein. Munc13-4 is essential for the exocytosis of perforin and granzyme containing granules from cytotoxic cells. Without it, these cells are able to recognize an immunological insult but are unable to execute their cytotoxic functions. The result is a hyperinflammatory state that, if left untreated, is fatal. At present, the only curative treatment is hematopoietic stem cell transplantation (HSCT), but eligibility and response to this treatment are largely dependent on the ability to control inflammation before HSCT. In this study, we describe an optimized lentiviral vector that can restore Munc13-4 expression and degranulation capacity in both transduced FHL3 patient T cells and transduced hematopoietic stem cells from the FHL3 (Jinx) disease model.

Gene Therapy for Primary Immunodeficiency

Over the past 3 decades, there has been significant progress in refining gene therapy technologies and procedures. Transduction of hematopoietic stem cells ex vivo using lentiviral vectors can now create a highly effective therapeutic product, capable of reconstituting many different immune system dysfunctions when reinfused into patients. Here, we review the key developments in the gene therapy landscape for primary immune deficiency, from an experimental therapy where clinical efficacy was marred by adverse events, to a commercialized product with enhanced safety and efficacy. We also discuss progress being made in preclinical studies for challenging disease targets and emerging gene editing technologies that are showing promising results, particularly for conditions where gene regulation is important for efficacy.

Gene therapy for primary immunodeficiencies

Primary immunodeficiencies (PIDs) are a group of rare inherited disorders of the immune system. Many PIDs are devastating and require a definitive therapy to prevent progressive morbidity and premature mortality. Allogeneic haematopoietic stem cell transplantation (alloHSCT) is curative for many PIDs, and while advances have resulted in improved outcomes, the procedure still carries a risk of mortality and morbidity from graft failure or graft-versus-host disease (GvHD). Autologous haematopoietic stem cell gene therapy (HSC GT) has the potential to correct genetic defects across haematopoietic lineages without the complications of an allogeneic approach. HSC GT for PID has been in development for the last two decades and the first licensed HSC-GT product for adenosine deaminase-deficient severe combined immunodeficiency (ADA-SCID) is now available. New gene editing technologies have the potential to circumvent some of the problems associated with viral gene-addition. HSC GT for PID shows great promise, but requires a unique approach for each disease and carries risks, notably insertional mutagenesis from gamma-retroviral gene addition approaches and possible off-target toxicities from gene-editing techniques. In this review, we discuss the development of HSC GT for PID and outline the current state of clinical development before discussing future developments in the field.

Gene therapy for primary immunodeficiencies: up-to-date

INTRODUCTION

Primary immunodeficiencies (PIDs) are monogenic disorders of the immune system associated with increased susceptibility to life-threatening infection. Curative treatment has been limited to hematopoietic stem cell transplant (HSCT), however toxic immunosuppression, graft failure, and graft versus host disease greatly reduce overall survival rates. Gene therapy is a targeted curative therapy that reduces these risks by utilizing autologous hematopoietic stem cells. The treatment has found significant success and is anticipated to become the standard of care in a number of PIDs.

AREAS COVERED

This review is a summary of the developments in gene therapy, gene editing, and current gene therapy approaches in specific PIDs.

EXPERT OPINION

The field of gene therapy has rapidly developed over the last three decades, with the first licensed pharmaceutical gene therapy product now available. After initial clinical trials discovered serious adverse events in the form of insertional oncogenesis, significant improvements in vector design have made the treatment a viable curative therapy. Cryopreservation has expanded the scope of gene therapy by increasing accessibility of the product to wider geographic locations. Targeted gene editing using engineered nucleases, while still in early stages of development, will further add to the repertoire of potential treatments available for PIDs.

T cell gene therapy to treat immunodeficiency

The application of therapeutic T cells for a number of conditions has been developed over the past few decades with notable successes including donor lymphocyte infusions, virus-specific T cells and more recently CAR-T cell therapy. Primary immunodeficiencies are monogenetic disorders leading to abnormal development or function of the immune system. Haematopoietic stem cell transplantation and, in specific candidate diseases, haematopoietic stem cell gene therapy has been the only definitive treatment option so far. However, autologous gene-modified T cell therapy may offer a potential cure in conditions primarily affecting the lymphoid compartment. In this review we will highlight several T cell gene addition or gene-editing approaches in different target diseases with a focus on what we have learnt from clinical experience and promising preclinical studies in primary immunodeficiencies. Functional T cells are required not only for normal immune responses to infection (affected in CD40 ligand deficiency), but also for immune regulation [disrupted in IPEX syndrome (immune dysregulation, polyendocrinopathy, enteropathy, X-Linked) due to dysfunctional FOXP3 and CTLA4 deficiency] or cytotoxicity [defective in X-lymphoproliferative disease and familial haemophagocytic lymphohistiocytosis (HLH) syndromes]. In all these candidate diseases, restoration of T cell function by gene therapy could be of great value.

Gene therapy for severe combined immunodeficiencies and beyond

This review describes how gene therapy of severe combined immunodeficiency became a reality, primarily based on the expected selective advantage conferred by transduction of hematopoietic progenitor cells. Thus, it resulted in a progressive extension to the treatment of other primary immunodeficiencies.

Ex vivo retrovirally mediated gene therapy has been shown within the last 20 yr to correct the T cell immunodeficiency caused by γc-deficiency (SCID X1) and adenosine deaminase (ADA) deficiency. The rationale was brought up by the observation of the revertant of SCIDX1 and ADA deficiency as a kind of natural gene therapy. Nevertheless, the first attempts of gene therapy for SCID X1 were associated with insertional mutagenesis causing leukemia, because the viral enhancer induced transactivation of oncogenes. Removal of this element and use of a promoter instead led to safer but still efficacious gene therapy. It was observed that a fully diversified T cell repertoire could be generated by a limited set (<1,000) of progenitor cells. Further advances in gene transfer technology, including the use of lentiviral vectors, has led to success in the treatment of Wiskott–Aldrich syndrome, while further applications are pending. Genome editing of the mutated gene may be envisaged as an alternative strategy to treat SCID diseases.

Alternative UNC13D Promoter Encodes a Functional Munc13-4 Isoform Predominantly Expressed in Lymphocytes and Platelets

Autosomal recessive mutations in genes required for cytotoxicity are causative of a life-threatening, early-onset hyperinflammatory syndrome termed familial hemophagocytic lymphohistiocytosis (FHL). Mutations in UNC13D cause FHL type 3. UNC13D encodes Munc13-4, a member of the Unc13 protein family which control SNARE complex formation and vesicle fusion. We have previously identified FHL3-associated mutations in the first intron of UNC13D which control transcription from an alternative transcriptional start site. Using isoform specific antibodies, we demonstrate that this alternative Munc13-4 isoform with a unique N-terminus is preferentially expressed in human lymphocytes and platelets, as compared to the conventional isoform that was mostly expressed in monocytes and neutrophils. The distinct N-terminal of the two isoforms did not impact on Munc13-4 localization or trafficking to the immunological synapse of cytotoxic T cells. Moreover, ectopic expression of both isoforms efficiently restored exocytosis by FHL3 patient-derived Munc13-4 deficient T cells. Thus, we demonstrate that the conventional and alternative Munc13-4 isoforms have different expression pattern in hematopoietic cell subsets, but display similar localization and contribution to T cell exocytosis. The use of an alternative transcriptional starting site (TSS) in lymphocytes and platelets could be selected for increasing the overall levels of Munc13-4 expression for efficient secretory granule release.

Recent advances in primary immunodeficiency: from molecular diagnosis to treatment

The technological advances in diagnostics and therapy of primary immunodeficiency are progressing at a fast pace. This review examines recent developments in the field of inborn errors of immunity, from their definition to their treatment. We will summarize the challenges posed by the growth of next-generation sequencing in the clinical setting, touch briefly on the expansion of the concept of inborn errors of immunity beyond the classic immune system realm, and finally review current developments in targeted therapies, stem cell transplantation, and gene therapy.

Gene therapy for primary immunodeficiency

Gene therapy is now being trialled as a therapeutic option for an expanding number of conditions, based primarily on the successful treatment over the past two decades of patients with specific primary immunodeficiencies (PIDs) including severe combined immunodeficiency and Wiskott–Aldrich syndrome and metabolic conditions such as leukodystrophy. The field has evolved from the use of gammaretroviral vectors to more sophisticated lentiviral platforms that offer an improved biosafety profile alongside greater efficiency for hematopoietic stem cells gene transfer. Here we review more recent developments including licensing of gene therapies, use of gene corrected autologous T cells as an alternative strategy for some PIDs and the potential of targeted gene correction using various gene editing platforms. Given the promising results of recent clinical trials, it is likely that autologous gene therapies will become standard of care for a number of devastating diseases in the coming decade.

Retroviral UNC13D Gene Transfer Restores Cytotoxic Activity of T Cells Derived from Familial Hemophagocytic Lymphohistiocytosis Type 3 Patients In Vitro

Familial hemophagocytic lymphohistiocytosis (FHL) is a group of life-threatening, autosomal recessive disorders of severe hyperinflammation. FHL type 3 (FHL-3) accounts for about 30% of FHL cases. It is characterized by mutations in the UNC13D gene that give rise to functionally impaired or absent Munc13-4 protein, resulting in impaired secretion of lytic granules by cytotoxic lymphocytes. Etoposide-based therapy is currently used as the standard of care that results in around 60% 5-year survival, illustrating the need for novel treatment approaches. Key problems include treatment toxicity and failure to induce or maintain remission of the hyperinflammation. Instead of immunosuppression, transplantation of autologous gene-corrected T cells can be envisaged as an approach to restore the impaired immune reaction. This study established a protocol that enabled hyperactivated, FHL-3 patient-derived T cells to be cultured and a codon-optimized UNC13D expression cassette to be delivered by either alpha- or gamma-retroviral gene transfer. The data demonstrate that the established protocol can be applied to FHL-3 patient cells with various genetic backgrounds and that gamma-retroviral UNC13D transfer restored expression of functional Munc13-4, as well as degranulation capacity and cell-mediated cytotoxicity of those patient-derived CD8⁺ T cells. Furthermore, the study shows that the co-introduction of a truncated low-affinity nerve growth factor receptor coding sequence enabled the therapeutic effect to be optimized by enriching transduced cells in a Good Manufacturing Practice-compliant manner. In conclusion, this study lays the foundation for an adaptive immune cell therapy approach aiming at immunological stabilization of FHL-3 patients with autologous, immune-competent T cells prior to hematopoietic stem-cell transplantation.

Haematopoietic Stem Cell Transplantation for Primary Haemophagocytic Lymphohistiocytosis

Haematopoietic stem cell transplantation currently remains the only curative treatment of primary forms of haemophagocytic lymphohistiocytosis (HLH). Rapid diagnosis, efficient primary treatment of hyperinflammation, and conditioning regimens tailored to this demanding condition have substantially improved prognosis in the past 40 years. However, refractory hyperinflammation, central nervous system (CNS) involvement, unavailability of matched donors, susceptibility to conditioning-related toxicities, and a high frequency of mixed chimaerism remain a challenge in a substantial proportion of patients. Gene therapeutic approaches for several genetic defects of primary HLH are being developed at pre-clinical and translational levels.

A Nontoxic Transduction Enhancer Enables Highly Efficient Lentiviral Transduction of Primary Murine T Cells and Hematopoietic Stem Cells

Lentiviral vectors have emerged as an efficient, safe therapeutic tool for gene therapy based on hematopoietic stem cells (HSCs) or T cells. However, the monitoring of transduced cells in preclinical models remains challenging because of the inefficient transduction of murine primary T cells with lentiviral vectors, in contrast to gammaretroviral vectors. The use of this later in preclinical proof of concept is not considered as relevant when a lentiviral vector will be used in a clinical trial. Hence, there is an urgent need to develop an efficient transduction protocol for murine cells with lentiviral vectors. Here, we describe an optimized protocol in which a nontoxic transduction enhancer (Lentiboost) enables the efficient transduction of primary murine T cells with lentiviral vectors. The optimized protocol combines low toxicity and high transduction efficiency. We achieved a high-level transduction of murine CD4⁺ and CD8⁺ T cells with a VSV-G-pseudotyped lentiviral vector with no changes in the phenotypes of transduced T cells, which were stable and long-lived in culture. This enhancer also increased the transduction of murine HSCs. Hence, use of this new transduction enhancer overcomes the limitations of lentiviral vectors in preclinical experiments and should facilitate the translation of strategies based on lentiviral vectors from the bench to the clinic.

Gene transfer into hematopoietic stem cells reduces HLH manifestations in a murine model of Munc13-4 deficiency

Patients with mutations in the UNC13D gene (coding for Munc13-4 protein) suffer from familial hemophagocytic lymphohistiocytosis type 3 (FHL3), a life-threatening immune and hyperinflammatory disorder. The only curative treatment is allogeneic hematopoietic stem cell (HSC) transplantation, although the posttreatment survival rate is not satisfactory. Here, we demonstrate the curative potential of UNC13D gene correction of HSCs in a murine model of FHL3. We generated a self-inactivating lentiviral vector, used it to complement HSCs from Unc13d-deficient (Jinx) mice, and transplanted the cells back into the irradiated Jinx recipients. This procedure led to complete reconstitution of the immune system (ie, to wild-type levels). The recipients were then challenged with lymphocytic choriomeningitis virus to induce hemophagocytic lymphohistiocytosis (HLH)-like manifestations. All the clinical and biological signs of HLH were significantly reduced in mice having undergone HSC UNC13D gene correction than in nontreated animals. This beneficial effect was evidenced by the correction of blood cytopenia, body weight gain, normalization of the body temperature, decreased serum interferon-γ level, recovery of liver damage, and decreased viral load. These improvements can be explained by the restoration of the CD8⁺ T lymphocytes' cytotoxic function (as demonstrated here in an in vitro degranulation assay). Overall, our results demonstrate the efficacy of HSC gene therapy in an FHL-like setting of immune dysregulation.