Stochastic fate decisions of HSCs after transplantation: early contribution, symmetric expansion, and pool formation.

Hematopoietic stem cells (HSCs) are assumed to be rare, infrequently dividing, long-lived cells not involved in immediate recovery after transplantation. Here, we performed unprecedented high-density clonal tracking in nonhuman primates and found long-term persisting HSC clones to actively contribute during early neutrophil recovery, and to be the main source of blood production as early as 50 days after transplantation. Most surprisingly, we observed a rapid decline in the number of unique HSC clones, while persisting HSCs expanded, undergoing symmetric divisions to create identical siblings and formed clonal pools ex vivo as well as in vivo. In contrast to the currently assumed model of hematopoietic reconstitution, we provide evidence for contribution of HSCs in short-term recovery as well as symmetric expansion of individual clones into pools. These findings provide novel insights into HSC biology, informing the design of HSC transplantation and gene therapy studies.

CD90-targeted lentiviral vectors for HSC gene therapy.

Hematopoietic stem cell (HSC) gene therapy is currently performed on CD34+ hematopoietic stem and progenitor cells containing less than 1% true HSCs and requiring a highly specialized infrastructure for cell manufacturing and transplantation. We have previously identified the CD34+CD90+ subset to be exclusively responsible for short- and long-term engraftment. However, purification and enrichment of this subset is laborious and expensive. HSC-specific delivery agents for the direct modification of rare HSCs are currently lacking. Here, we developed novel targeted viral vectors to specifically transduce CD90-expressing HSCs. Anti-CD90 single chain variable fragments (scFvs) were engineered onto measles- and VSV-G-pseudotyped lentiviral vectors that were knocked out for native targeting. We further developed a custom hydrodynamic titration methodology to assess the loading of surface-engineered capsids, measure antigen recognition of the scFv, and predict the performance on cells. Engineered vectors formed with minimal impairment in the functional titer, maintained their ability to fuse with the target cells, and showed highly specific recognition of CD90 on cells ex vivo. Most important, targeted vectors selectively transduced human HSCs with secondary colony-forming potential. Our novel HSC-targeted viral vectors have the potential to significantly enhance the feasibility of ex vivo gene therapy and pave the way for future in vivo applications.