Temporal definition of haematopoietic stem cell niches in a large animal model of in utero stem cell transplantation

Abstract

The fetal sheep model has served as a biologically relevant and translational model to study in utero haematopoietic stem cell transplantation (IUHSCT), yet little is known about the ontogeny of the bone marrow (BM) niches in this model. Because the BMmicroenvironment plays a critical role in the outcome of haematopoietic engraftment, we have established the correlation between the fetal-sheep and fetal-human BM niche ontogeny, so that studies addressing the role of niche development at the time of IUHSCT could be accurately performed. Immunofluorescence confocal microscopic analysis of sheep fetal bone from gestational days (gd) 25-68 showed that the BM microenvironment commences development with formation of the vascular niche between 25-36 gd in sheep; correlating with the events at 10-11 gestational weeks (gw) in humans. Subsequently, between 45-51 gd in sheep (~14 gw in humans), the osteoblastic/endosteal niche started developing, the presence of CD34+CD45+ cells were promptly detected, and their number increased with gestational age. IUHSCT, performed in sheep at 45 and 65 gd, showed significant haematopoietic engraftment only at the later time point, indicating that a fully functional BM microenvironment improved engraftment. These studies show that sheep niche ontogeny closely parallels human, validating this model for investigating niche influence/manipulation in IUHSCT engraftment.

Preterm Cord Blood Contains a Higher Proportion of Immature Hematopoietic Progenitors Compared to Term Samples

“…The prevalence of the erythroid lineage commitment within the CD34 + cell population and the capacity of these progenitors to growin secondary generation, is suggestive for the presence in cord blood of an immature progenitor devoid to transport oxygen, carbon dioxide and to facilitate vascular remodeling. These progenitor cells disappear very soon after birth[ 31 , 8 ].…”

“…UCB from full term newborns exhibit a higher WBC count than UCB from preterm, but a lower HP concentration in keeping with a previous study [ 11 ].This difference is inversely correlated with the gestational age, thus suggesting that circulating CD34 + cells do not substantially expand from the second to the third trimester of the fetal life or that this population undergoes expansion but leaves the circulation and progressively reaches the hematopoietic niches in the bone marrow. In fact, it has been demonstrated in the humans, that the vascular microenvironment reaches relative maturity by 20 th gestational week and that osteoblastic/endosteal niche is completed later, when mesenchymal (endochondral) tissue became ossified[ 8 ]. Thus, it is possible that, a population of immature HP can persist into the blood stream of the preterm neonates before homing into a transient endothelial niche and finally into the trabecular bone, as demonstrated in the zebrafish model [ 32 ].…”

“…The relative contribution of each of the above sites to the final pool of adult Hematopoietic Stem Cells (HSC) remains largely unknown and it is believed that none of these sites is accompanied by de novo HSC generation, rather, than these niches support the expansion of migrating HSCs capable of hematopoietic reconstitution, at least starting from AGM region [ 8 ].…”

Influence of a dual-injection regimen, plerixafor and CXCR4 on in utero hematopoietic stem cell transplantation and engraftment with use of the sheep model

“…The optimal age for IUHSCT in the sheep model is between 55-65 days in gestation and engraftment dwindles after day 75 (6, 49). The engraftment of MSCs, however, has shown to occur as late in gestation as day 85, likely due to their immunomodulatory characteristics (33).…”

Hemophilia A: an ideal disease to correct in utero

“…Firstly, sheep share many important physiological and developmental characteristics with humans. As a result, they have been used extensively in the study of mammalian fetal physiology, and the results obtained with this model have been directly applicable to the understanding of human fetal growth and development (Jeanblanc et al, 2014 ). In contrast to dogs, pigs, and many other large animals which tend to have large litters of offspring, sheep, like humans, typically give birth to only one or two offspring in each pregnancy.…”