Enhanced In Vivo Regenerative Potential of HOXB4-Transduced Hematopoietic Stem Cells With Regulation of Their Pool Size

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Abstract
After bone marrow transplantation (BMT), there is a rapid regeneration to normal pretransplantation levels in the number of hematopoietic progenitors and mature end cells, whereas hematopoietic stem cell (HSC) numbers recover to only 5% to 10% of normal levels. This suggests that HSC are significantly restricted in their self-renewal behavior and hence in their ability to repopulate the host stem cell compartment. Previously, we have reported that HSC engineered to overexpress the homeobox transcription factor HOXB4 have a large repopulation advantage over untransduced cells as assessed at 4 months in a murine transplantation model (Sauvageau et al, Genes Dev 9:1753, 1995). This phenomenon has now been examined in detail for periods extending to 12 months in cohorts of mice transplanted with various numbers of HOXB4-transduced HSC. In all mice analyzed, HOXB4-transduced HSC were capable of fully reconstituting the HSC compartment, resulting, on average, in some 14-fold greater numbers of HSC than observed when transplanting control, non–HOXB4-transduced bone marrow cells. These data indicate that HOXB4 is a limiting factor in the regeneration of HSC to normal levels after BMT. Furthermore, we show thatHOXB4-transduced HSC did not expand above levels normally observed in unmanipulated mice, indicating that its overexpression does not override the regulatory mechanisms that maintain the HSC pool size within normal limits.