Using an improved gene-transfer protocol, Aiuti and colleagues report in Science the correction of adenosine deaminase (ADA)-deficient severe combined immunodeficiency (SCID) by gene therapy in combination with non-myeloablative conditioning.

ADA-SCID patients have a purine metabolic defect that results in impaired lymphocyte development and function, as well as additional non-immunological abnormalities. Previous gene-therapy trials for this disease have shown that peripheral-blood lymphocytes (PBLs) can be corrected genetically by transducing these cells with a retrovirus that encodes the ADA gene, which results in the long-term survival of functional T cells. However, in these trials, patients were also given polyethylene-glycol-conjugated ADA (PEG–ADA) enzyme, so that it was difficult to assess the contribution of the gene therapy to immune reconstitution.

In this study, CD34+ haematopoietic stem cells (HSCs) from two patients for whom PEG–ADA was unavailable were transduced with ADA and infused back into the patients. The patients also received non-myeloablative conditioning — a conditioning regimen to achieve immunosuppression and prevent graft rejection without the complete ablation of host haematopoiesis — to provide the transduced cells with a developmental advantage. After transient myelosuppression, haematopoiesis was recovered in these patients — the neutrophil count returned to normal, and the number of PBLs, T cells, B cells and natural killer cells was increased. T-cell development and thymic activity were restored, the T-cell-receptor repertoire was normal, and T cells had normal proliferative and cytotoxic responses. B-cell functions also seemed to recover. In addition, gene therapy completely restored intracellular ADA activity in PBLs and bone-marrow cells. The two patients are clinically well and remain off enzyme therapy.

The authors carried out real-time PCR to assess the proportion of vector-containing cells in the peripheral blood and bone marrow of the patients. Both sites contained genetically corrected cells of many lineages, including granulocytic, erythroid, megakaryocytic and lymphoid subsets. It was also shown formally that the genetically corrected HSCs retained their repopulation and differentiation capacities.

Therefore, this study establishes that HSC gene therapy combined with non-myeloablative conditioning is effective at restoring lymphoid development and functions, and at correcting the metabolic defect of ADA-SCID, which results in complete reversal of the clinical phenotype in the absence of enzyme-replacement therapy.