X-linked severe combined immunodeficiency (SCID-X1), caused by a defect of the cytokine receptor common\r\ngamma chain (?c), has been successfully treated by gene therapy in the clinic. However, the occurrence of leukemia\r\nin several patients preceded by loss of oligoclonality revealed that treatment is associated with a risk inherent to\r\nthe genetic modification of hematopoietic stem cells. In this study, we developed a safety approach that allows\r\nthe specific elimination of gene-modified cells. For this, a small peptide sequence (myc-tag) was introduced into\r\nthe murine ?c protein. Cells expressing the modified chain can be detected with a myc-specific antibody by flow\r\ncytometry and are effectively depleted in vitro in the presence of complement factors. Further, thymic-derived T\r\ncells from mice reconstituted with myc-tagged ?c-transduced bone marrow stem cells can be depleted by antibody\r\nadministration in vivo. Similarly, specific complement-mediated lysis was observed for human T cells expressing\r\nthe human myc-tagged ?c. In a cell proliferation assay, the modified cytokine receptor chain showed no functional\r\nimpairment compared to the wild-type chain. In sum, we show proof-of-principle of a safety mechanism for SCID-X1\r\ngene therapy that would allow elimination of gene-corrected cells in a patient upon observation of monoclonal\r\noutgrowth.
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