Transplantation of vascular endothelial cells mediates the hematopoietic recovery and survival of lethally irradiated mice

Abstract

Flk-1 ؉ endothelial progenitors contribute critically to the definitive onset of hematopoiesis during embryogenesis. Recent studies have suggested that adult sources of endothelial cells also possess hematopoietic activity. In this study, we sought to determine whether transplantation of primary vascular endothelial cells (ECs) could enhance the hematopoietic recovery and survival of irradiated mice. C57Bl6 mice were exposed to sublethal and lethal doses of irradiation and were subsequently given transplants of either primary murine brain-derived ECs (MBECs) or fetal blood-derived ECs (FBECs). Mice that received a transplant with MBECs alone demonstrated accelerated BM cellular recovery, radioprotection of BM c-kit ؉ sca-1 ؊ lin ؊ progenitors and enhanced regeneration of c-kit ؉ sca-1 ؉ lin ؊ (KSL) stem/progenitor cells following irradiation compared with controls. MBEC transplantation also facilitated the recovery of circulating white blood cell and platelet counts following radiation exposure. Remarkably, 57% of mice that received a transplant with MBECs alone survived long term following 1050 cGy exposure, which was 100% lethal in control mice. FBEC transplantation was also associated with increased survival compared with controls, although these mice did not survive in the long term. These data suggest that reestablishment of endothelial cell activity can improve the hematopoietic recovery and survival of irradiated mice. IntroductionStudies have shown that hematopoietic stem cells (HSCs) reside in close association with osteoblasts within the BM niche, and this association contributes to the maintenance of the HSC pool in vivo. 1,2 HSCs also reside in intimate anatomic proximity to vascular endothelial cells (ECs) from the earliest stages of embryonic development, 3 through their migration to the fetal liver, 4 and ultimately throughout their adult residence within the BM compartment. 5 During embryogenesis, mice lacking flk-1 ϩ endothelial precursor cells fail to initiate normal hematopoiesis, 3 and gene marking studies suggest that hematopoietic and vascular endothelial cells derive from a common precursor cell, the hemangioblast. 6 Similarly, bone marrow-, umbilical vein-, and yolk sac-derived endothelial cells elaborate growth factors that support the proliferation of myeloid, erythroid, and megakaryocytic progenitors in vitro. [7][8][9] Our laboratory has shown that primary human brain-derived endothelial cells produce a soluble hematopoietic activity that supports a 1 to 2 log expansion of the most primitive assayable human hematopoietic cell, which is capable of long-term repopulation in nonobese diabetic/severe combined immunodeficient mice. [10][11][12] Li et al 13 have also confirmed that murine brain-and heart-derived endothelial cells can support the maintenance of colony-forming unit-spleen day-8 colonies in vitro. Taken together, these data implicate vascular endothelial cells as a source of proliferative and regenerative signals for hematopoietic stem and progenitor cells.Although these stud...

The vascular niche: home for normal and malignant hematopoietic stem cells

“…Our laboratory subsequently showed that systemic infusion of syngenic ECs promoted the acceleration of both BM vascular regeneration, hematologic recovery and improved survival in mice following TBI (Figure 4). 18 Allogeneic endothelial progenitor cell infusions were also shown to induce hematologic recovery and a quantitative increase in long-term repopulating HSC content compared with control mice following TBIinduced myelosuppression, suggesting that ECs positively regulated HSC reconstitution in vivo. 53 Interestingly, we found that transplanted ECs did not engraft directly in the BM vasculature, but rather engrafted temporarily in the lungs of recipient mice, suggesting that soluble factors were elaborated by infused ECs, which promoted vascular and HSC regeneration.…”

“…The BM sinusoidal vasculature is radiosensitive but regenerates and reorganizes within 3-4 weeks following sublethal exposure. 18 However, at doses at or above 20 Gy, complete repair and regeneration of the BM sinusoidal vasculature does not occur. 37 Importantly, Rafii et al 38,39 showed that primary human BM endothelial cells (ECs) supported the proliferation and differentiation of human CD34 þ cells in culture and produced several hematopoietic cytokines.…”

“…In vitro and in vivo studies have also clearly demonstrated that ECs regulate the self-renewal and regeneration of adult HSCs (Table 1). 16,18,19,[40][41][42]45,52,53,55,[104][105][106] Although mechanistic pathways such as Notch and CXCR4-SDF1 signaling have been shown to contribute to BM endosteal and reticular cell regulation of HSC fate in vivo, 4,7 the mechanisms through which BM ECs regulate HSC maintenance and regeneration remain less well defined. 3 In parallel, phenotypic and functional characterization of LSCs has been demonstrated, coupled with the demonstration of homing and engraftment of LSCs in vascular niches in vivo.…”

“…Recently, genetic knockout studies have begun to demonstrate the functional role of specific cells within the BM microenvironment in regulating hematopoiesis. [3][4][5]7,[15][16][17][18][19][20] Interestingly, certain niches within the BM may uniquely regulate HSC homeostasis in vivo while other niches may be more relevant to HSC regeneration following injury. However, it remains unknown whether there is meaningful 'cross-talk' between distinct microenvironmental cells in regulating HSC fate in vivo.…”

Endothelial progenitor cell infusion induces hematopoietic stem cell reconstitution in vivo

“…[6][7][8] Since ECs critically regulate the onset of hematopoiesis during embryogenesis 9,10 and since adult sources of ECs support the expansion and repair of BM stem/progenitor cells in vitro, [11][12][13] we hypothesized that BM ECs regulate hematopoietic reconstitution in vivo. Since BM ECs are profoundly sensitive to radiation and chemotherapy-induced toxicity, 14,15 we propose that systemic administration of endothelial progenitor cells (EPCs) could replenish endogenous EC activity and augment hematopoietic regeneration following myelotoxic conditioning. Herein, we demonstrate that infusion of EPCs into irradiated mice causes the accelerated reconstitution of BM HSCs, peripheral blood (PB) counts, and BM sinusoidal vessels.…”