Correction of X-linked immunodeficient mice by competitive reconstitution with limiting numbers of normal bone marrow cells.

Gene therapy for inherited disorders is more likely to succeed if gene-corrected cells have a proliferative or survival advantage compared with mutant cells. We used a competitive reconstitution model to evaluate the strength of the selective advantage that Btk normal cells have in Btk-deficient xid mice. Whereas 2,500 normal bone marrow cells when mixed with 497,500 xid cells restored serum IgM and IgG3 levels to near normal concentrations in 3 of 5 lethally irradiated mice, 25,000 normal cells mixed with 475,000 xid cells reliably restored serum IgM and IgG3 concentrations and the thymus-independent antibody response in all transplanted mice. Reconstitution was not dependent on lethal irradiation, because sublethally irradiated mice all had elevated serum IgM and IgG3 by 30 weeks postreconstitution when receiving 25,000 normal cells. Furthermore, the xid defect was corrected with as few as 10% of the splenic B cells expressing a normal Btk. When normal donor cells were sorted into B220(+)/CD19(+) committed B cells and B220(-)/CD19(-) cell populations, only the B220(-)/CD19(-) cells provided long-term B-cell reconstitution in sublethally irradiated mice. These findings suggest that even inefficient gene therapy may provide clinical benefit for patients with XLA.