Lentiviral Gene Therapy with Low Dose Busulfan for Infants with X-Linked Severe Combined Immune Deficiency (XSCID) results in the Development of a Normal and Sustained Immune System: Interim Results of an ongoing Phase I/II Clinical Study

Abstract BACKGROUND Allogeneic hematopoietic stem-cell transplantation for X-linked severe combined immunodeficiency (SCID-X1) often fails to reconstitute immunity associated with T cells, B cells, and natural killer (NK) cells when matched sibling donors are unavailable unless high-dose chemotherapy is given. In previous studies, autologous gene therapy with γ-retroviral vectors failed to reconstitute B-cell and NK-cell immunity and was complicated by vector-related leukemia. METHODS We performed a dual-center, phase 1–2 safety and efficacy study of a lentiviral vector to transfer IL2RG complementary DNA to bone marrow stem cells after low-exposure, targeted busulfan conditioning in eight infants with newly diagnosed SCID-X1. RESULTS Eight infants with SCID-X1 were followed for a median of 16.4 months. Bone marrow harvest, busulfan conditioning, and cell infusion had no unexpected side effects. In seven infants, the numbers of CD3+, CD4+, and naive CD4+ T cells and NK cells normalized by 3 to 4 months after infusion and were accompanied by vector marking in T cells, B cells, NK cells, myeloid cells, and bone marrow progenitors. The eighth infant had an insufficient T-cell count initially, but T cells developed in this infant after a boost of gene-corrected cells without busulfan conditioning. Previous infections cleared in all infants, and all continued to grow normally. IgM levels normalized in seven of the eight infants, of whom four discontinued intravenous immune globulin supplementation; three of these four infants had a response to vaccines. Vector insertion-site analysis was performed in seven infants and showed polyclonal patterns without clonal dominance in all seven. CONCLUSIONS Lentiviral vector gene therapy combined with low-exposure, targeted busulfan conditioning in infants with newly diagnosed SCID-X1 had low-grade acute toxic effects and resulted in multilineage engraftment of transduced cells, reconstitution of functional T cells and B cells, and normalization of NK-cell counts during a median follow-up of 16 months. (Funded by the American Lebanese Syrian Associated Charities and others; LVXSCID-ND ClinicalTrials.gov number, NCT01512888.) X-linked severe combined immunodeficiency (SCID-X1) is a rare, life-threatening disorder caused by mutations in the gene that encodes the common γ-chain (IL2RG [GenBank accession number, D11086.1]), which is shared by multiple cytokine receptors necessary for the development and function of lymphocytes.1-3 Unless the condition is detected by newborn screening before the appearance of symptoms or identified on the basis of a positive family history, affected infants present with severe opportunistic infections during the first months of life because of defects in both cellular and humoral immunity. Laboratory studies typically show the lack of T cells, natural killer (NK) cells, and functional B cells. Procedures to restore immunity include either allogeneic hematopoietic stem-cell transplantation or autologous gene therapy.4-8 Hematopoietic stem-cell transplants from a matched sibling donor are effective but available for less than 20% of patients, and transplants from alternative donors are associated with an increased risk of graft-versus-host disease and incomplete immune reconstitution.9-14 Gene therapy is an experimental treatment that inserts a normal copy of the coding region of IL2RG into the genome of a patient’s own hematopoietic stem cells. Previous studies of gene therapy showed that first-generation γ-retroviral vectors restored T-cell immunity but resulted in vector-induced leukemia.15,16 Second-generation γ-retroviral vectors had an improved safety profile but did not restore humoral immunity or durable NK-cell production when used without conditioning chemotherapy.17 Our new lentiviral vector gene therapy combined with nonmyeloablative busulfan conditioning has been successful in restoring immunity in five patients 7 to 23 years of age in whom a previous allogeneic hematopoietic stem-cell transplantation for SCID-X1 had failed; two of the five patients gained normal B-cell function and independence from immune globulin infusions.18 We hypothesized that the combination of this lentiviral vector and low-exposure busulfan administered by means of pharmacokinetic dose targeting would be safe and effective as the primary treatment in infants with newly diagnosed SCID-X1. To test this hypothesis, we developed the Lentiviral Vector SCID-X1 Newly Diagnosed (LVXSCID-ND) dual-center, phase 1–2 safety and efficacy trial. We report the patient characteristics, vector marking (vector copy number per cell) in blood and bone marrow cells, immune reconstitution, and vector insertion-site patterns in the first eight consecutive patients.