Alessandro Aiuti,¹ Shimon Slavin,² Memet Aker,² Francesca Ficara,¹ Sara Deola,¹ Alessandra Mortellaro,¹ Shoshana Morecki,² Grazia Andolfi,¹ Antonella Tabucchi,³ Filippo Carlucci,³ Enrico Marinello,³ Federica Cattaneo,¹ Sergio Vai,¹ Paolo Servida,⁴ Roberto Miniero,⁵ Maria Grazia Roncarolo,^16* Claudio Bordignon^16*

Hematopoietic stem cell (HSC) gene therapy for adenosine deaminase (ADA)-deficient severe combined immunodeficiency (SCID) has shown limited clinical efficacy because of the small proportion of engrafted genetically corrected HSCs. We describe an improved protocol for gene transfer into HSCs associated with nonmyeloablative conditioning. This protocol was used in two patients for whom enzyme replacement therapy was not available, which allowed the effect of gene therapy alone to be evaluated. Sustained engraftment of engineered HSCs with differentiation into multiple lineages resulted in increased lymphocyte counts, improved immune functions (including antigen-specific responses), and lower toxic metabolites. Both patients are currently at home and clinically well, with normal growth and development. These results indicate the safety and efficacy of HSC gene therapy combined with nonmyeloablative conditioning for the treatment of SCID.

¹ San Raffaele Telethon Institute for Gene Therapy (HSR-TIGET), Milan, Italy.
² Departments of Bone Marrow Transplantation and Pediatrics, Hadassah University Hospital, Jerusalem, Israel.
³ Institute of Biochemistry and Enzymology, University of Siena, Siena, Italy.
⁴ BMT and Gene Therapy Program, Scientific Institute H. S. Raffaele;
⁵ Department of Biological and Clinical Science, University of Turin, Turin, Italy.
⁶ Università Vita-Salute San Raffaele, Milan, Italy.
^*   These authors contributed equally to this work.

   To whom correspondence should be addressed. E-mail: claudio.bordignon{at}hsr.it