On the role of the stem cell niche in hematopoietic development

Abstract

Hematopoietic stem cells (HSCs) emerge from a population of specialized endothelial cells in the aorta during embryogenesis. These cells then migrate to a temporary niche where they reside and proliferate for the rest of fetal development. This niche supplies growth factors and cytokines to the newly formed HSCs and consists largely of sinusoidal vasculature, stromal cells, and primitive immune cells. While it is generally understood that HSCs proliferate in the fetal niche, precise analysis of changes to individual HSCs during this time has proven difficult due to the inaccessibility of the tissue in mammals and low cell numbers. Here, I leveraged the advantages of zebrafish embryos to address these problems and analyze changes to HSCs induced by the early niche. I developed new transgenic tools to enable magnetic enrichment of rare embryonic populations and applied this to study the transcriptional profile of HSCs over the first days of development. I also performed high-resolution time-lapse imaging of HSCs as they colonized the niche. I discovered intimate and specific cell-cell interactions between primitive macrophages and HSCs. Macrophage interactions frequently led to either removal of cytoplasmic material and stem cell division, or complete engulfment and stem cell death. Macrophage interactions were stimulated by the presentation of Calreticulin on the surface of HSCs. Surface Calreticulin was induced as a result of cellular stress and reactive oxygen species accumulation in emerging HSCs. Stem cells with high stress displayed higher levels of surface Calreticulin and were fully engulfed by macrophages. Stem cells with low-to-moderate stress levels displayed moderate levels of surface Calreticulin and were stimulated to divide by macrophage-produced IL1β. Using cellular barcoding, I found that Calreticulin knock-down or embryonic macrophage depletion reduced the number of stem cell clones that established adult hematopoiesis. My work indicates that the fetal niche serves to both supply proliferative factors for the emerging HSC pool, but also monitor stem cell quality and shape the clonal architecture of the stem cell population through interactions with primitive macrophages. This mechanism could be manipulated to therapeutically target specific cells in other high-stress environments, as in hematologic disease or tissue regeneration.