Lentiviral Gene Transfer for Treatment of Children Older Than 2 Years of Age With X-Linked Severe Combined Immunodeficiency

This study will evaluate the safety and effectiveness of lentiviral gene transfer treatment at restoring immune function to participants with X-linked severe combined immunodeficiency (XSCID) who are 2 to 40 years of age, and have significant impairment of immunity. XSCID is a severe genetic condition of the immune system. Participants in this study will be treated at the National Institutes of Health (NIH) Clinical Center. Before the gene transfer treatment, a participant's own CD34+ hematopoietic stem cells (HSCs) will have been or will be collected from the participant's blood or bone marrow. When the participant has the required number of HSCs harvested, then the participant's HSCs will be grown in tissue culture and exposed to the lentiviral gene transfer vector containing the corrective gene (VSV-G pseudotyped CL20-4i-EF1α-hγc-OPT). These gene corrected HSCs will be administered by intravenous (IV) infusion to the participant. To increase engraftment of the corrected HSCs, participants will receive a chemotherapy drug called busulfan on 2 days before the gene transfer treatment. Participants will also receive palifermin, which helps prevent mucositis, which is the main side effect from the busulfan. After the gene transfer treatment, participants will be monitored to see if the treatment is safe and whether their immune system improves. Participants will be followed at frequent intervals for the first 2 years, and less frequently thereafter so that the effectiveness in restoration of immune function and the safety of the treatment can be evaluated. Additional safety follow-up may occur through Year 15.