Differentiation-based model of hematopoietic stem cell functions and lineage pathways

Abstract

Advances in genetic labeling and barcoding of hematopoietic stem cells (HSCs) in situ now allow direct measurements of physiological HSC output, both quantitatively and qualitatively. Turning on a heritable label in HSCs and measuring the kinetics of label emergence in downstream compartments reveal rates of differentiation and self-renewal of HSCs and progenitor cells, whereas endogenous HSC barcoding probes physiological precursor-product relationships. Labels have been inserted at different stages of the hematopoietic differentiation hierarchy. Recent genetic and functional evidence suggests a phenotype (Tie2⁺ ) for tip HSCs. Fate mapping shows that many tip HSCs regularly feed into downstream stages, with individual cells contributing infrequently. Stem and progenitor cells downstream of tip HSCs serve as a major, nearly self-renewing source of day-to-day hematopoiesis, rendering the blood and immune system HSC-independent for extended periods of time. HSCs realize multilineage output, yet, fates restricted to several lineages or even a single lineage have also been observed. Single HSCs within a clone in the bone marrow that develop from a fetal HSC precursor have been observed to express clone-specific fates. Thus, the new tools probing HSC differentiation in situ are progressing beyond assays for HSC activity based on proliferation measurements and fates of transplanted stem cells, and the data challenge lineage interpretations of single-cell gene expression snapshots. Linking in vivo fate analyses to gene expression and other molecular determinants of cell fate will aid in unraveling the mechanisms of lineage commitment and the architecture of physiological hematopoiesis.

Figure 1.

Physiological hematopoiesis is fueled by a hierarchically organized, broad basis of (almost) self-renewing stem and progenitor cells. (A) Relative compartment sizes of HSCs (Lin⁻Sca⁺Kit⁺CD150⁺CD48⁻), ST-HSCs (Lin⁻Sca⁺Kit⁺CD150⁻CD48⁻), and MPPs (Lin⁻Sca⁺Kit⁺CD150⁻CD48⁺) (all phenotypes according to Oguro et al) are drawn to scale. Based on the measured output from Tie2⁺ HSCs, we estimated the frequencies of differentiating cells in each compartment, indicated by red circles (lower side, outgoing; upper side, incoming). The continuous but rare input from Tie2⁺ HSCs is greatly amplified in ST-HSCs and MPPs to sustain overall output (red arrows); to achieve this, the rates of self-renewing cell divisions also increase from HSCs to ST-HSCs to MPPs (black circle arrows). (B) Fate mapping data from Tie2^MeriCreMer knock-in mice, and the comprehensive functional characterization of Tie2⁺ vs Tie2⁻ HSCs in Tie2 reporter mice, indicate heterogeneity of the HSC compartment, with Tie2⁺ HSCs residing at the tip, differentiating, and self-renewing.

Figure 2.

The tracing of HSC barcodes introduced in situ supports a treelike model of hematopoiesis with major myeloerythroid and common lymphoid branches. (A) We introduced barcodes during fetal development in the mouse (embryonic day 9.5). Labeled clones of adult HSC (analyzed at ∼1 year of age) are of very different sizes. HSC clones typically yield multilineage progeny, or oligolineage progeny either on the myeloerythroid or on the lymphoid side. (B) When barcoding HSCs in adult mice, HSCs again give rise to multilineage or oligolineage (typically, myeloerythroid or lymphoid) progeny. In addition, HSC-derived barcodes are also found in more restricted sets of mature cells (bi- or unilineage). Overall, HSC-derived barcodes co-occur in mature lineages and in their respective progenitors. For example, barcodes enriched in erythroid progenitors (EryP), granulocytes (Gr), or monocytes (Mono) are likely to be enriched also in CMPs/GMPs, whereas barcodes prevalent in mature T and B cells are also prevalent in the respective progenitors but not in CMPs. Colors symbolize the genetic Polylox barcodes on which this figure is based. EryP, erythrocyte progenitor; preB, precursor B cell; preT, precursor T cell.