Dynamics of genetically engineered hematopoietic stem and progenitor cells after autologous transplantation in humans

Hematopoietic stem and progenitor cells (HSPC) are endowed with the role of generating and maintaining lifelong the extremely diverse pool of blood cells¹. Clinically, transplantation of human HSPC from an allogeneic healthy donor or infusion of autologous gene-corrected HSPC can effectively replenish defective blood cell production caused by congenital or acquired disorders^2-9. However, due to methodological and ethical constraints that have limited the study of human HSPC primarily to in vitro assays¹⁰ or xenotransplantation models^11,12, the in vivo activity of HSPC has to date remained relatively unexplored in humans^13-16. Here we report a comprehensive study of the frequencies, dynamics and output of seven HSPC subtypes in humans that was performed by tracking 148,093 individual clones in six patients treated with lentiviral gene therapy using autologous HSPC transplantation and followed for up to 5 years. We discovered that primitive multipotent progenitor and hematopoietic stem cell (HSC) populations have distinct roles during the initial reconstitution after transplant, compared with subsequent steady-state phases. Furthermore, we showed that a fraction of in vitro-activated HSC are resilient and undergo a defined delayed activation period upon transplant. Finally, our data support the concept that early lymphoid-biased progenitors might be capable of long-term survival, such that they can be maintained independently of their continuous production from HSC. Overall, this study provides comprehensive data on HSPC dynamics after autologous transplantation and gene therapy in humans.