Definitive hematopoietic stem cells minimally contribute to embryonic hematopoiesis.

Hematopoietic stem cells (HSCs) are rare cells that arise in the embryo and sustain adult hematopoiesis. Although the functional potential of nascent HSCs is detectable by transplantation, their native contribution during development is unknown, in part due to the overlapping genesis and marker gene expression with other embryonic blood progenitors. Using single-cell transcriptomics, we define gene signatures that distinguish nascent HSCs from embryonic blood progenitors. Applying a lineage-tracing approach to selectively track HSC output in situ, we find significantly delayed lymphomyeloid contribution. An inducible HSC injury model demonstrates a negligible impact on larval lymphomyelopoiesis following HSC depletion. HSCs are not merely dormant at this developmental stage, as they showed robust regeneration after injury. Combined, our findings illuminate that nascent HSCs self-renew but display differentiation latency, while HSC-independent embryonic progenitors sustain developmental hematopoiesis. Understanding these differences could improve de novo generation and expansion of functional HSCs.

Advances in preclinical hematopoietic stem cell models and possible implications for improving therapeutic transplantation.

Hematopoietic stem cell transplantation (HSCT) is a treatment for many malignant, congenital, and acquired hematologic diseases. Some outstanding challenges in the HSCT field include the paucity of immunologically-matched donors, our inability to effectively expand hematopoeitic stem cells (HSCs) ex vivo, and the high infection risk during engraftment. Scientists are striving to develop protocols to generate, expand, and maintain HSCs ex vivo, however these are not yet ready for clinical application. Given these problems, advancing our understanding of HSC specification, regulation, and differentiation in preclinical models is essential to improve the therapeutic utility of HSCT. In this review, we link biomedical researchers and transplantation clinicians by discussing the potential therapeutic implications of recent fundamental HSC research in model organisms. We consider deficiencies in current HSCT practice, such as problems achieving adequate cell dose for successful and rapid engraftment, immense inflammatory cascade activation after myeloablation, and graft-vs-host disease. Furthermore, we discuss recent advances in the field of HSC biology and transplantation made in preclinical models of zebrafish, mouse, and nonhuman primates that could inform emerging practice for clinical application.

A novel conditioning-free hematopoietic stem cell transplantation model in zebrafish.

Transplantation is the most common assay for measuring the in vivo functionality of hematopoietic stem cells (HSCs). Although various HSC transplantation strategies have been developed in zebrafish, they are underutilized because of challenges related to immune matching and preconditioning toxicity. To circumvent these limitations, we developed a simple and robust transplantation model using HSC-deficient hosts. Homozygous runx1W84X mutants are devoid of definitive hematopoietic cells, including HSCs and adaptive immune cells; thus, they require no preconditioning regimen for transplantation. Marrow cell transplantation into runx1-mutant zebrafish 2 days after fertilization significantly improved their survival to adulthood and resulted in robust, multilineage, long-lasting, serially repopulating engraftment. Furthermore, we demonstrated that engraftment into runx1 homozygous mutants was significantly higher than into runx1 heterozygotes, demonstrating that the improved transplantation success is attributable to the empty HSC niche in mutants and not just the embryonic environment. Competitive transplantation of marrow cells into runx1 mutants revealed a stem cell frequency similar to that of murine marrow cells, which demonstrates the utility of this model for quantifying HSC function. The streamlined approach and robustness of this assay will help broaden its feasibility for future high-throughput transplantation experiments in zebrafish and will enable further novel discoveries in the biology of HSCs.