Functional Interactions in the Hematopoietic Stem Cell Niche: Stem Cells, Endothelial Cells, and Macrophages

Abstract

The hematopoietic stem cell (HSC) niche is a specialized environment that supports the maintenance and regulation of hematopoietic stem and progenitor cells (HSPCs). The various cell types that comprise the niche and the molecular mechanisms underlying their supportive functions remain incompletely understood. During zebrafish development, HSCs engraft in a vascular plexus in the tail called the caudal hematopoietic tissue (CHT), which provides a niche for the HSCs. This tissue, analogous to the mammalian fetal liver, provides a model to study interactions between HSCs and their microenvironment. To study the vascular niche, a gain-of-function screen was performed by overexpressing genes differentially expressed in endothelial cells and assaying for HSCs in the CHT by in situ hybridization. Overexpression of the receptor CXCR1 or its ligand CXCL8 were both found to independently increase HSC colonization of the CHT. Using parabiotic animals overexpressing CXCR1 in one animal and fluorescently labeled HSCs circulating from the other animal, CXCR1 was also demonstrated to act cell autonomously to the sinusoidal vascular niche rather than the HSC. In parallel experiments, live cell imaging of macrophages and HSCs in the CHT revealed frequent intimate cell-cell interactions between the two populations. Quantification of the data showed that approximately 17% of HSPCs in the CHT are interacting with a macrophage at any point in time during development. A macrophage-HSPC co-culture system re-capitulated the result that roughly 15-20% of HSPCs are interacting with a macrophage at steady state. To test whether these interactions may play a functional role, macrophages were depleted from the zebrafish embryo using clodronate liposomes, and HSC numbers were assayed, revealing a significantly decreased number of HSCs in the CHT in the setting of macrophage loss. Together, these findings suggest a role for the CXCL8/CXCR1 signaling pathway in the HSC vascular niche as well as a functional role for macrophage-HSC interactions in the CHT, which may have important implications for the design of new therapeutic approaches for improving transplantation-based treatment of hematopoietic disorders.