Origin and Characterization of CD166+ Cells During Human Marrow Ontogeny

Abstract 1252

The development of hematopoiesis during embryonic and fetal life has served as a valuable model for characterizing putative HSC markers, as well as for studying the ontogeny of cells comprising the hematopoietic niche, and defining the nature of the interactions of hematopoietic stem/progenitor cells with these microenvironmental cells that drive HSC maintenance and/or differentiation. Alcam (CD166) has been shown to be expressed during ontogeny by the endothelium within the yolk sac and dorsal aorta, playing a key role in capillary tube formation and differentiation, but the adult aorta is devoid of this molecule. In the adult bone marrow, CD166 has been shown to be expressed by both highly primitive HSC and mesenchymal cells of the microenvironment, and to mediate homophilic adhesion between these two cell types. Here, we investigated the origin of the CD166+ cells during ontogeny and extensively characterized their phenotype as the marrow developed to better understand their function. To this end, we used flow cytometry and confocal microscopy to analyze human bone marrow (BM) during the period of development from 10 to 20 gestational weeks. CD166 expression was not detected on any cell population within the BM until week 18 of gestation, a time point when hematopoiesis is already well established within this tissue, as determined by the presence of CD34+CD45+ cells. CD166+ cells were rare at week 18, localized with other circulating cells in hematopoietic areas/spaces, and co-expressed FLK-1, CD44, N-cadherin, and Stro-1. By 20 weeks of gestation, the CD166+ population had increased significantly in number, and could be separated based on phenotype and spacial localization. CD166+/Stro-1+/CD44+/N-cadherin+/FLK-1+ cells increased approximately 4-fold in number between weeks 18 and 20, and they continued to be localized within the hematopoietic zones. In contrast, the more numerous CD166+/Stro-1+/CD44+/FLK-1- population was confined to the perivascular niche, with some of these cells integrating within the forming/maturing vasculature. In conclusion, we found that CD166 expression defines two distinct populations during development; one that seemingly associates with the perivascular niche, and another that localized with circulating hematopoietic cells. Interestingly, even though some CD166+ cells co-expressed N-cadherin, these cells did not localize to the osteoblastic niche. Studies are currently underway to further characterize these two populations with respect to their ability to support and/or give rise to hematopoiesis.