Gene Therapy for Artemis-SCID and Artemis-Leaky-SCID Patients: Preliminary Results of the French Artegene Phase I/II Clinical Trial

Introduction: Genetic deficiency of the endonuclease DCLRE1C/Artemis, a key factor for the Non-Homologous End-Joining (NHEJ) mechanism, causes Severe Combined Immunodeficiency (SCID) or leaky-SCID characterized by a lack of T and B-cells associated with hypersensitivity to ionizing radiations. Allogeneic Hematopoietic Stem and progenitor Cells (HSPCs) transplantation is the gold standard for treating Artemis deficiency. However, graft-versus-host disease and graft rejection, which can lead to post-transplant complications and mortality, are increased in case of HLA incompatibility between the donor and recipient. In this context, gene addition for autologous HSPCs was presented as an alternative therapeutic strategy, and a clinical trial of gene therapy is already ongoing in the United States. Using a different Lentiviral Vector (LV) containing a short intron-less EF1α promoter regulating the expression of human DCLRE1C cDNA, we recently opened a European phase I/II clinical trial (NCT05071222) at the Necker hospital in Paris. Methods: Two SCID phenotypes (P1, P3) and one leaky-SCID form (P2) have been included in this trial with 6 to 18 months follow-up. P1 had a severe and heavily treated maternal GvHD (steroids, ciclosporin, Ruxolitinib, Thymoglobulin) with a pretransplant history of infection due to immune deficiency, including treatment for pneumocystis, post-rotavirus vaccination gastroenteritis, and undocumented hepatitis. At the same time, P3 was asymptomatic and diagnosed after newborn screening. P2, aged 41 months, was diagnosed in the context of IBD, CMV disease, and hepatitis. We processed and transduced autologous CD34+ HSPCs from bone marrow and mobilized peripheral blood to increase the quantity of gene-corrected HSPCs. Each patient received between 1.8 to 3.7 10e6 CD34/kg corrected HSPC after a BU-based conditioning regimen with an AUC of 10000 microM.min. Results: For all patients, we could detect the engraftment of gene-corrected HSPC at the hematological recovery by quantifying the transgene integration in CD15+ circulating cells. Both SCID patients had progressively increased VCN in the PBMC and restored T-cell and B-cell compartments after transplant. By testing the corrected HSPCs' functionality, we also demonstrated that transduced HSPCs initiate T-cell rearrangements in vitro and acquire the capacity to restore T-cell differentiation with polyclonal repertoires in vivo. P1 rapidly controlled his pretransplant maternal GvHD and post-rotavirus vaccination gastroenteritis. However, for the leaky phenotype (P2), despite sufficient HSPC-corrected cells in the drug product and the engraftment of 20% corrected CD34+ cells in bone marrow, the VCN in the PBMC never exceeded 0.2 copies/cells. This was insufficient to initiate the T-cells and B-cell reconstitution after seven months of follow-up. Conclusion/discussion: Our data confirme the success of the ARTEGENE gene transfer protocol, which has restored a functional T- and B-cell compartment in Artemis-deficient SCID phenotypes with low toxicity. However, the reason why gene therapy failed in the patient with the leaky phenotype requires further analysis. The detailed clinical outcomes of all patients will be presented at the upcoming congress. These results need to be confirmed by including more patients.