Conserved Lineage Development in Human and Nonhuman Primate Hematopoiesis

For more than 30 years, hematopoietic stem cell (HSC) research has been performed according to the classical model of human hematopoiesis suggesting an early segregation of lymphoid and erythro-myeloid potentials. However, several studies have recently proposed a great variety of models for the human blood hierarchy showing alternative lineage relationships and read-outs for multipotent HSCs. While the debate about hematopoietic lineage relationships is still ongoing, consequences of these findings and the challenges they pose for the development of treatment strategies for hematological diseases and malignancies are rarely discussed.

A critical factor for the development of stem cell therapies is the availability of a reliable and robust read-out for multipotent HSCs/MPPs (multipotent progenitor cells) supporting the development of all blood cell lineages. Unfortunately, the current gold standard, NOD/SCID mouse xenograft repopulation assay does not support erythrocyte and megakaryocyte development and, thus, does not fully read-out multi-lineage potential of human HSCs/MPP. In addition, development of therapeutic approaches in the mouse model is not possible due to differences in cell surface marker expression, physiology, life span, and the demand on stem cell self-renewal and differentiation compared to humans.

The pigtail macaque (PM; Macaca nemestrina) and the rhesus macaque (RM: Macaca mulatta) share a close evolutionary relationship with humans and have been used as a pre-clinical model system to study basic HSC biology or to develop specific HSC gene therapy approaches. Surprisingly, however, a comparison of hematopoietic subpopulations and the hierarchical organization of defined lineages has not been performed between NHPs and humans. This will be a critical factor for a better understanding of the newly defined blood lineage associations and hierarchies, as well as the development of treatment approaches based on these lineages.

Here, we comprehensively analyzed all known markers of human hematopoiesis in the NHP to identify subpopulations of candidate NHP hematopoietic stem and progenitor cells (HSPCs) and then validated HSPC phenotypes of these fractions with functional in vitro read outs. We further evaluated and compared lineage relationships between these subpopulations to recently proposed models of human hematopoiesis to determine whether conservation of hematopoiesis exists with the goal of informing studies evaluating treatments for hematological diseases in the NHP model.

We show for the first time a phenotypic mapping strategy in NHP hematopoietic cells predicting a revised model of hematopoiesis. Similar to humans, NHP HSCs give rise to multipotent progenitors (MPPs), followed by a segregation of lympho-myeloid, erythro-myeloid, and megakaryocytic lineages (see figure). Conservation of hematopoietic lineage relationships was confirmed by RNA expression analysis of corresponding subpopulations.

In summary, we identified corresponding human and NHP hematopoietic subpopulations, which share phenotypical, functional and transcriptional properties in both species, validating the NHP as an excellent pre-clinical model system for HSC biology and the development of novel HSC-based treatment approaches.

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