Contributions
Abstract: S248
Type: Oral Presentation
Session title: Stem cell biology and microenvironment
Background
Hematopoietic stem cells (HSCs) are at the top of the hierarchy of the blood system. In the adult, they reside in the bone marrow, where they are mostly quiescent, only dividing to maintain the stem cell pool. During embryonic development, HSCs colonize the fetal liver where they expand. The fetal liver microenvironment is assumed to support HSC proliferation and is the only organ that provides the conditions for physiological HSC expansion. HSC transplantation is a widely used cell therapy in numerous disorders, including immunodeficiencies and leukemias. However, this therapy is still associated with high mortality rates, mainly due to infection, graft-versus-host disease (GvHD), and organ dysfunction, urging the need to improve the therapeutics. Robust and reproducible conditions to expand HSCs ex vivo would allow attaining sufficient numbers to reconstitute hematopoiesis in adult patients.
Aims
Understand the complexity of the HSC microenvironment in the fetal liver that promotes HSC expansion and differentiation into distinct lineages, to devise efficient strategies for HSC expansion ex vivo.
Methods
We characterized the stromal compartment of the fetal liver by spectral flow cytometry, and identified cytokine-producing cells by single-cell gene expression analysis. To localize the stromal cells in the tissue we used confocal microscopy in thin and thick sections, allowing a 3D reconstruction of the tissue microanatomy. For lineage tracing of yolk-sac erythromyeloid progenitors or HSCs, we used transgenic mice models.
Results
Characterization of fetal liver from E10-newborn revealed that the non-hematopoietic compartment represents only around 5% of total cells. Analysis of hepatoblasts, endothelial, perivascular, and mesothelial cells showed that all subsets express relevant but distinct patterns of hematopoietic cytokines. The chemoattractant Cxcl12, produced by perivascular cells, is the most expressed factor across liver development, while other cytokines such as Epo and Il7 are expressed at low-levels by hepatoblasts. In situ analysis showed that from the early stages of liver development, c-Kit+ hematopoietic cells are abundant close to the mesothelium and distinct hematopoietic lineages accumulate around blood vessels at E16.5. Lineage tracing experiments showed that, opposite to previous views, HSCs do not contribute to erythrocyte production up until birth, indicating that HSC differentiation in the embryo is skewed. We demonstrate that the low-levels of erythropoietin in the fetal liver confer a selective advantage of yolk sac-derived erythroid progenitors over HSC-derived progeny in the production of red blood cells.
Conclusion
Altogether, our work supports the premise that embryonic hematopoiesis is influenced by distinct niche cells that cooperate to provide the signals required for expansion and/or differentiation of hematopoietic stem and progenitor cells.
Keyword(s): Cytokine, Erythropoieisis, Hematopoietic stem and progenitor cells, Microenvironment
Abstract: S248
Type: Oral Presentation
Session title: Stem cell biology and microenvironment
Background
Hematopoietic stem cells (HSCs) are at the top of the hierarchy of the blood system. In the adult, they reside in the bone marrow, where they are mostly quiescent, only dividing to maintain the stem cell pool. During embryonic development, HSCs colonize the fetal liver where they expand. The fetal liver microenvironment is assumed to support HSC proliferation and is the only organ that provides the conditions for physiological HSC expansion. HSC transplantation is a widely used cell therapy in numerous disorders, including immunodeficiencies and leukemias. However, this therapy is still associated with high mortality rates, mainly due to infection, graft-versus-host disease (GvHD), and organ dysfunction, urging the need to improve the therapeutics. Robust and reproducible conditions to expand HSCs ex vivo would allow attaining sufficient numbers to reconstitute hematopoiesis in adult patients.
Aims
Understand the complexity of the HSC microenvironment in the fetal liver that promotes HSC expansion and differentiation into distinct lineages, to devise efficient strategies for HSC expansion ex vivo.
Methods
We characterized the stromal compartment of the fetal liver by spectral flow cytometry, and identified cytokine-producing cells by single-cell gene expression analysis. To localize the stromal cells in the tissue we used confocal microscopy in thin and thick sections, allowing a 3D reconstruction of the tissue microanatomy. For lineage tracing of yolk-sac erythromyeloid progenitors or HSCs, we used transgenic mice models.
Results
Characterization of fetal liver from E10-newborn revealed that the non-hematopoietic compartment represents only around 5% of total cells. Analysis of hepatoblasts, endothelial, perivascular, and mesothelial cells showed that all subsets express relevant but distinct patterns of hematopoietic cytokines. The chemoattractant Cxcl12, produced by perivascular cells, is the most expressed factor across liver development, while other cytokines such as Epo and Il7 are expressed at low-levels by hepatoblasts. In situ analysis showed that from the early stages of liver development, c-Kit+ hematopoietic cells are abundant close to the mesothelium and distinct hematopoietic lineages accumulate around blood vessels at E16.5. Lineage tracing experiments showed that, opposite to previous views, HSCs do not contribute to erythrocyte production up until birth, indicating that HSC differentiation in the embryo is skewed. We demonstrate that the low-levels of erythropoietin in the fetal liver confer a selective advantage of yolk sac-derived erythroid progenitors over HSC-derived progeny in the production of red blood cells.
Conclusion
Altogether, our work supports the premise that embryonic hematopoiesis is influenced by distinct niche cells that cooperate to provide the signals required for expansion and/or differentiation of hematopoietic stem and progenitor cells.
Keyword(s): Cytokine, Erythropoieisis, Hematopoietic stem and progenitor cells, Microenvironment