Correction of Murine SCID-X1 by Lentiviral Gene Therapy Using a Codon-optimized IL2RG Gene and Minimal Pretransplant Conditioning

Clinical trials have demonstrated the potential of ex vivo hematopoietic stem cell gene therapy to treat X-linked severe combined immunodeficiency (SCID-X1) using retroviral vectors, leading to immune system functionality in the majority of treated patients without pretransplant conditioning. The success was tempered by insertional oncogenesis in a proportion of the patients. To reduce the genotoxicity risk, a self-inactivating (SIN) lentiviral vector (LV) with improved expression of a codon optimized human interleukin-2 receptor gamma gene (IL2RG) cDNA (co gamma c), regulated by its 1.1 kb promoter region (gamma cPr), was compared in efficacy to the viral spleen focus forming virus (SF) and the cellular phosphoglycerate kinase (PGK) promoters. Pretransplant conditioning of IL2rg(-/-) mice resulted in long-term reconstitution of T and B lymphocytes, normalized natural antibody titers, humoral immune responses, ConA/IL-2 stimulated spleen cell proliferation, and polyclonal T-cell receptor gene rearrangements with a clear integration preference of the SF vector for proto-oncogenes, contrary to the PGK and gamma cPr vectors. We conclude that SIN lentiviral gene therapy using co gamma c driven by the gamma cPr or PGK promoter corrects the SCID phenotype, potentially with an improved safety profile, and that low-dose conditioning proved essential for immune competence, allowing for a reduced threshold of cell numbers required. Received 23 February 2011; accepted 31 May 2011; published online 12 July 2011. doi:10.1038/mt.2011.127