Long-Term Clinical Follow-Up and Safety/Toxicity Analysis in 3 X-CGD Patients Treated by Gene Therapy and Non-Myeloablative Conditioning.

Gene transfer into hematopoietic stem cells has been successfully used to correct immunodeficiencies affecting the lymphoid compartment. However, similar results have not been reported for diseases affecting myeloid cells, mainly due to low engraftment levels of gene-modified cells observed in unconditioned patients. Here we report on two adult patients (P1 and P2, follow up >24 months) and one child (P3, 6 years, follow up 15 months) who received gene-transduced hematopoietic stem cells in combination with nonmyeloablative bone marrow conditioning for the treatment of X-linked Chronic Granulomatous Disease (X-CGD), a primary immunodeficiency caused by a defect in the oxidative antimicrobial activity of phagocytes. Therapeutically significant gene marking was detected in neutrophils of both adult patients (P1 and P2) leading to large numbers (up to 60%) of functionally corrected phagocytes 24 months after gene therapy. This high correction resulted from an unexpected but temporarily restricted expansion of gene transduced myeloid cells in vivo. In contrast gene marking and functionally reconstitution levels in P3 have been low (1–2%). Both adult patients suffered from active infections prior to gene therapy (P1 of bacterial liver abscesses and P2 of lung aspergillosis) and were free of severe bacterial and fungal infections until 24 months after transplantation. P3 suffered from an Aspergillus infection of the spinal cord with paraparesis before transplantation and recovered after gene therapy despite low numbers of functionally corrected cells in the peripheral blood. Large-scale mapping of retroviral integration site distribution revealed that activating insertions in the zinc finger transcription factor homologs MDS1/EVI1, PRDM16, or in SETBP1 have expanded gene-corrected long term myelopoiesis 3- to 4-fold in both adults, providing direct evidence in humans that these genes may influence regulation of normal long-term hematopoiesis. The hematopoietic repopulation in P1 was polyclonal until 18 months after therapy. P1 died of a severe bacterial sepsis after colon perforation 27 months after gene therapy. No evidence of malignant transformation was found in peripheral blood or bone marrow aspirates from this patient. Gene marking at death was still 60%; however the function of gene transduced cells, the number of corrected cell clones and the activity of a predominant clone was greatly decreased. P2 has been free of infections since transplantation (last monitoring: month 26). Hematopoietic repopulation was polyclonal in P2 until day 560. In conclusion, gene therapy in combination with bone marrow conditioning has provided a transitory therapeutic benefit for all 3 patients. Further improvements in vector design and conditioning regimes are under investigation to provide a stable and long term correction of the disease.