Lentiviral vectors are the most frequently used tool to stably transfer and express genes in the context of gene therapy for monogenic diseases. The vast majority of clinical applications involves an ex vivo modality whereby lentiviral vectors are used to transduce autologous somatic cells, obtained from patients and re-delivered to patients after transduction. Examples are hematopoietic stem cells used in gene therapy for hematological or neurometabolic diseases or T cells for immunotherapy of cancer. We review the design and use of lentiviral vectors in gene therapy of monogenic diseases, with a focus on controlling gene expression by transcriptional or post-transcriptional mechanisms in the context of vectors that have already entered a clinical development phase.

Hematopoietic stem and progenitor cells (HSPCs) can establish a long‐lasting microglia‐like progeny in the central nervous system of properly myeloablated hosts. We exploited this approach to treat the severe CLN1 neurodegenerative disorder, which is the most aggressive form of neuronal ceroid lipofuscinoses due to palmitoyl‐protein thioesterase‐1 (PPT1) deficiency. We here provide the first evidence that (i) transplantation of wild‐type HSPCs exerts partial but long‐lasting mitigation of CLN1 symptoms; (ii) transplantation of HSPCs over‐expressing hPPT1 by lentiviral gene transfer enhances the therapeutic benefit of HSPCs transplant, with first demonstration of such a dose–effect benefit for a purely neurodegenerative condition like CLN1 disease; (iii) transplantation of hPPT1 over‐expressing HSPCs by a novel intracerebroventricular (ICV) approach is sufficient to transiently ameliorate CLN1‐symptoms in the absence of hematopoietic tissue engraftment of the transduced cells; and (iv) combinatorial transplantation of transduced HSPCs intravenously and ICV results in a robust therapeutic benefit, particularly on symptomatic animals. Overall, these findings provide first evidence of efficacy and feasibility of this novel approach to treat CLN1 disease and possibly other neurodegenerative conditions, paving the way for its future clinical application.

Ex vivo gene therapy (GT) is a promising treatment for inherited genetic diseases. An ideal transduction protocol should determine high gene marking in long-term self-renewing hematopoietic stem cells (HSCs), preserving their repopulation potential during in vitro manipulation. In the context of the improvement of a clinically applicable transduction protocol, we tested prostaglandin E₂ (PGE2) as a transduction enhancer (TE). The addition of PGE2 shortly before transduction of human CD34⁺ cells determined a significant transduction increase in the in vitro cell progeny paralleled by a significant reduction of their clonogenic potential. This effect increased with the duration of PGE2 exposure and correlated with an increase of CXCR4 expression. Blockage of CXCR4 with AMD3100 (plerixafor, Mozobil) did not affect transduction efficiency but partially rescued CD34⁺ clonogenic impairment in vitro. Once transplanted in vivo in a competitive repopulation assay, human CD34⁺ cells transduced with PGE2 contributed significantly less than cells transduced with a standard protocol to the repopulation of recipient mice, indicating a relative repopulation disadvantage of the PGE2-treated CD34⁺ cells and a counter-selection for the PGE2-treated cell progeny in vivo. In conclusion, our data indicate the need for risk/benefit evaluations in the use of PGE2 as a TE for clinical protocols of GT.

Autologous hematopoietic stem/progenitor cell (HSC)–gene therapy (GT) represents a promising therapeutic option for progranulin (PGRN)–related neurodegenerative diseases due to mutations in the PGRN encoding gene ( GRN ), such as frontotemporal dementia (FTD) and neuronal ceroid lipofuscinosis 11 (CLN11). These conditions are characterized by a deficiency in PGRN, have no cure, and represent an unmet medical need. We report on the efficacy and feasibility of an HSC GT approach that used a lentiviral vector encoding the human GRN complementary DNA to transduce HSCs that then were transplanted into a Grn ^−/− mouse model, which mirrors both FTD and CLN11 phenotypes. Two promoters, one with medium-low strength ( HLA-DRA gene–based promoter regulated by inflammation) and the other with medium-high strength [ubiquitous phosphoglycerate kinase (PGK) promoter], were compared for HSC transduction. Moreover, intravenous and intracerebroventricular HSC administration were compared. Under all tested conditions, a partial reconstitution of PGRN production by microglia-like cells (MLCs) derived from genetically corrected Grn ^−/− HSCs was observed, which uniformly led to a correction of lipid accumulation, reduced gliosis, and improved social recognition in Grn ^−/− mice. Therapeutic effects were similarly achieved with either of the promoters and administration routes and particularly also when the PGRN-expressing cells and their MLC progeny had engrafted exclusively in the central nervous system (CNS) after intracerebroventricular transplantation. These findings suggest that a durable yet modest restoration of PGRN expression in the CNS is sufficient to correct pathology.

X-linked severe combined immunodeficiency (SCID-X1) is caused by mutations in the gene encoding the interleukin-2 receptor γ chain (IL2RG), and is characterized by lack of response to common γ-chain-dependent cytokines and profound defects in T-, B- and NK-cell functions. Previous gene therapy clinical trials based on autologous transplantation of hematopoietic stem/progenitor cells (HSPCs) genetically corrected with MLV-derived retroviral vectors expressing IL2RG in non-conditioned patients showed long-lasting restoration of T-cell immunity in most cases but resulted in vector-induced leukemia through enhancer-mediated insertional mutagenesis in 25% of patients. The use of an enhancer-less MLV vector to deliver the IL2RG gene caused no adverse events while retaining a significant clinical benefit. To increase the efficacy of gene therapy and reduce at the same time the probability of genotoxic insertional events, we developed a third-generation lentiviral vector carrying a codon-optimized human IL2RG cDNA under the control of the human EF1α-S promoter. The vector was VSV-G-pseudotyped and produced by transient transfection of 293T cells, followed by ion-exchange chromatography, tangential-flow filtration and formulation in HSC growth medium. Replacement of the native IL2RG open reading frame by a codon-optimized sequence resulted in a 3-fold increase in mRNA expression and a 1.5-fold increase in IL2RG protein expression per integrated vector copy. The performance of the vector was demonstrated in vitro by the restoration of a normal level of IL2RG mRNA or protein in a human IL2RG-deficient T-cell line at a VCN of 1 to 3, and by high efficiency (>80%) transduction of human mobilized CD34⁺ HSPCs and transgene expression with no impact on viability or clonogenic capacity. An in vitro immortalization assay (IVIM) showed a safe genotoxic profile, while the in vivo safety and efficacy of the vector was tested it in a preclinical model of SCID-X1 gene therapy based on transplantation of genetically corrected Lin^- cells from IL2rg^-/- donor mice into lethally-irradiated IL2rg^-/--Rag2^-/- recipients. The study showed biodistribution of the transgene in hematopoietic organs only, restoration of T, B and NK cell counts in bone marrow and peripheral blood, normalization of lymphoid organs (thymus and spleen) and a low frequency of hematopoietic malignancies, comparable to that of untreated animals, six months after transplantation. An extensive insertion site analysis was carried out in bone marrow, thymus and peripheral blood of individual or groups of animals to detect the presence of any clonal abnormality or skewing, and any deviation from a normal lentiviral integration pattern. The study showed the expected genomic integration profile and no signs of clonal dominance of transduced cells. Interestingly, analysis of >100,000 integration sites in pre- and post-transplant murine cells showed that lentiviral vectors target at high frequency a substantially different set of genes compared to human CD34⁺ cells, uncovering the need for appropriate controls and at the same time the limits in the predictive power of mouse-based genotoxicity studies, particularly those addressing proto-oncogene activation and clonal dominance. These studies will enable a multicenter phase-I/II clinical trial aimed at establishing the safety and clinical efficacy of lentiviral vector-mediated gene therapy for SCID-X1. The clinical protocol is based on transplantation of autologous CD34⁺ HSPCs transduced with the EF1a-IL2RGco lentiviral vector in infants after non-myeloablative conditioning, and aims at stable and sustained restoration of both T- and B-cell immunity.

Lysosomal storage diseases (LSDs) are >70 devasting diseases causing progressive neurodegeneration and systemic symptoms. Hematopoietic Stem Cell (HSC) Gene Therapy (GT), i.e. the autologous transplantation of genetically-corrected HSCs by lentiviral transduction, has shown transformative efficacy in multiple neurodegenerative LSDs so far. We developed a translational HSC GT approach for GM1-gangliosidosis, a neurodegenerative LSD due to β-galactosidase (β-gal) deficiency, based on a lentiviral vector expressing either the human or the murine therapeutic enzyme. Cells were administered by a standard intravenous injection, or by an innovative, combined systemic and local cell delivery to enhance the GT efficacy. In the murine model of the disease, GT broadly restored enzymatic activity and reduced the lysosomal storage in the brain, ameliorates the neuromuscular phenotype and increased animal survival. Combined cell-delivery GT with the human enzyme, or standard intravenous GT with the murine β-gal were similarly effective in mitigating disease phenotype, indicating a substantial therapeutic potential of the approach for a future clinical translation.