Hematopoietic Stem Cells Are Active Contributors to Hematopoiesis in Steady State

Hematopoietic stem cell (HSC) function is typically evaluated using transplantation experiments, which offers quantitative and qualitative information on their self-renewal and multilineage differentiation potential. In this setting, potent long-term multilineage contribution can be observed from even single HSCs. However, emerging data has highlighted fundamental differences between hematopoiesis as seen after transplantation compared to that in steady state, with surprisingly little evidence of robust lymphomyeloid contribution from HSCs in native adult hematopoiesis (Busch et al., 2015; Sun et al., 2014). On the other hand, multilineage capacity has most often been defined as the ability to produce mature B-, T- and myeloid lineage cells, with evaluation of the capacity to generate erythroid cells and thrombocytes often neglected. Thus, the bulk amount of hematopoietic cells in need of continuous regeneration has been omitted from such analyses.

Here, we evaluated the kinetics of blood cell generation in adult hematopoiesis using Fgd5-Cre^ERT2 mediated cell tracing (Gazit et al., 2014), a model that can approach close to 100% label of adult HSCs in a highly specific manner. In agreement with that seen using an alternative model for HSC lineage tracing (Busch et al., 2015), HSC derived peripheral B and T lymphoid cells emerged with very slow kinetics (detection beginning at week 5 for B cells and at week 8 for T cells), while HSC derived granulocytes and monocytes emerged earlier (detectable by 3 weeks). NK cells were generated from HSCs with kinetics more resembling the kinetics for the myeloid lineages (detectable by 4 weeks), indicative of an intermediate developmental relationship of NK cells to myeloid and lymphoid cells. Finally, our ability to trace peripheral platelets revealed that this lineage was generated from HSCs not only in a highly robust manner, but also with the fastest kinetics (detectable by 2 weeks).

Consistent with the mature blood cell labeling kinetics, detailed kinetic evaluations of defined myeloerythroid progenitors in the bone marrow revealed that megakaryocyte progenitors acquire label earlier than other hematopoietic progenitor subsets while thymocytes acquired label slowly. Evaluation of label progression from HSCs to other subfractions of the immature Lineage negative, c-kit positive and Sca-1 (LSK) positive compartment revealed that a CD150+CD48+ subset acquired label faster than other LSK fractions, pointing to a close temporal relationship of cells with this phenotype and HSCs.

The demonstration that HSCs are potent and relatively rapid contributors to the megakaryocytic/platelet lineage in the settings evaluated strongly argues for their physiological importance not only upon transplantation, but also in steady state.