Engraftment and reconstitution of hematopoiesis is dependent on VEGFR2-mediated regeneration of sinusoidal endothelial cells

Abstract

Myelosuppression damages the bone marrow (BM) vascular niche, but it is unclear how regeneration of bone marrow vessels contributes to engraftment of transplanted hematopoietic stem and progenitor cells (HSPCs) and restoration of hematopoiesis. We found that chemotherapy and sublethal irradiation induced minor regression of BM sinusoidal endothelial cells (SECs), while lethal irradiation induced severe regression of SECs and required BM transplantation (BMT) for regeneration. Within the BM, VEGFR2 expression specifically demarcated a continuous network of arterioles and SECs, with arterioles uniquely expressing Sca1 and SECs uniquely expressing VEGFR3. Conditional deletion of VEGFR2 in adult mice blocked regeneration of SECs in sublethally irradiated animals and prevented hematopoietic reconstitution. Similarly, inhibition of VEGFR2 signaling in lethally irradiated wild-type mice rescued with BMT severely impaired SEC reconstruction and prevented engraftment and reconstitution of HSPCs. Therefore, regeneration of SECs via VEGFR2 signaling is essential for engraftment of HSPCs and restoration of hematopoiesis.

Figure 1. BM SECs are phenotypically marked as VEGFR2⁺VEGFR3⁺Sca1⁻, while arterioles are VEGFR2⁺VEGFR3⁻Sca1⁺

The expression of VEGF receptors was determined by IHC of BM sections in WT mice and via genetic tracking in VEGFR1-lacZ and VEGFR2-GFP mice. A) Paraffin-embedded femoral sections from WT mice were stained with Abs for (a) MECA32, (b) VE-cadherin, (d) VEGFR2, (e) VEGFR3. High power image demonstrates the specificity of VEGFR3 expression to SECs and not arterioles (e, inset). (c) Femurs from VEGFR1-lacZ mice were processed for βgal activity. (f) Fixed-frozen BM sections were coimmunostained for Sca1 (red) and VE-cadherin (green). Arterioles are designated by yellow arrows, SECs by red arrows. B) Fluorescent microscopy of fixed-frozen femoral BM sections from VEGFR2-GFP mice stained for osteopontin (OPN, purple). Note VEGFR2-GFP⁺ vasculature (green). C–D) VEGFR2-GFP BM sections were coimmunostained for (C) CD31 (red) or (D) VEGFR3 (blue) and VE-cadherin (red), demonstrating selective expression of VEGFR2 on CD31⁺ and VE-cadherin⁺VEGFR3⁺ SECs. Nuclei are shown in blue. Micron bar = 50 µm.

Figure 2. VEGFR2 and VEGFR3 are primarily expressed on functional SECs

Whole BM from femurs/tibias of VEGFR2-GFP mice was prepared by disruption and enzymatic digestion. Mononuclear cells, excluding TER119⁺ cells, were analyzed by polyvariate flow cytometry. A) BM VEGFR2-GFP⁺ cells are CD45⁻ and co-express CD31. CD31⁺VEGFR2-GFP⁺ cells express varying levels of VE-cadherin and are CD45 negative (99.1%). B) Prior to BM isolation, WT mice were injected i.v. with Isolectin GS-IB4 to identify patent/functional vessels. Harvested BM cells were stained with for CD45, VEGFR3 and VE-cadherin. CD45 and VEGFR3 staining are mutually exclusive. Virtually all VEGFR3⁺ cells co-express VE-cadherin and bound Isolectin GS-IB4. C) Polyvariate analysis of BM from WT mice demonstrating that the majority of KLS population (HSPCs, blue) lacks VEGFR3 (highlighted in green within the dashed yellow line). D) Five thousand VEGFR2⁺GFP⁺Lin⁻BM cells harvested from β-actin transgenic mice were transplanted into groups of lethally irradiated mice (n=9) with a radioprotective dose of WT GFP⁻BM. After 4 weeks PB was assessed for the presence of GFP⁺ cells. No GFP⁺ cells were detected in the PB from any of the transplanted mice.

Figure 3. VEGFR2⁻VEGFR3⁻ OBs are preserved within the BM of irradiated Col2.3GFP mice

A) H&E and anti-VEGFR3 IHC of femurs from WT mice demonstrating the localization of VEGFR3⁺ SECs (S) in close apposition to osteoblasts (OBs) in both the diaphyseal and metaphyseal marrow. B–C) Co-IF for VE-cadherin (red) (B) and VEGFR3 (red) (C) in steady state Col2.3GFP mice BM showing the proximity of the SECs (white arrows) to GFP⁺ OBs (yellow arrows). D) Polyvariate flow cytometry of BM from steady state Col2.3GFP mice indicating that GFP⁺ OBs lack the expression of VEGFR2 and VEGFR3. E) Low power confocal micrographs of femurs from Col2.3GFP mice at steady state and at day 5 after 650 rad demonstrates persistence of GFP⁺ OBs after irradiation. F) IF for VE-cadherin (red) demonstrating alteration of the BM vasculature (white arrows) after irradiation, while the structural integrity of GFP⁺ OBs is maintained. White micron bar = 50 µm, yellow micron bar = 20 µm.

Figure 4. Magnitude of myelosuppression correlates with the extent of damaged Type I and Type II SECs

WT mice were treated with 250 mg/kg 5FU i.v. 650 rad, 950 rad and allowed to recover. A cohort of 950 rad treated mice were transplanted with 5×10⁵ whole BM cells (950 rad + BMT). Femurs were harvested at day 7, 10 and 14 and were processed in paraffin for IHC with BM EC markers, including VE-cadherin & VEGFR3. A) Day 14 H&E and anti-VE-cadherin IHC of the models of myelosuppression. B) Prototypical examples of day 14 VE-cadherin⁺ normal, discontinuous or hemorrhagic (Type I), and hemorrhagic/discontinuous and EC denuded (Type II/regressed) SECs. The yellow arrows represent the discontinuity in the vessel wall of the injured and regressed SECs, while the red star highlights the areas of profuse hemorrhage detected adjacent to damaged SECs. C) Quantitation of Type I and Type II vessels in femoral BM of 5FU, 650 rad, 950 rad and 950 rad + BMT mice analyzed at day 7 and 10 post myelosuppressive insult. Y-axis represents the percent of normal, Type I, and Type II vessels quantitated in three 200X fields. Micron bar = 50 µm.

Figure 5. Impaired regeneration of SECs after 650 rad irradiation results in failure of hematopoietic reconstitution

The consequence of 650 rad in the reconstitution of hematopoiesis was evaluated in WT (A–F) and VEGFR2 conditional knockdown (VEGFR2^flox/−) mice (G–J). A–F) WT mice were treated with 650 rad with and without neutralizing mAb to VEGFR2 (DC101, 800 µg/dose thrice weekly). At day 14, PB, spleens and BM were harvested for evaluation of (A) platelets, (B) CFU-spleen forming units (CFU-S), (C) total BM mononuclear cells, and (D) IHC quantification of Type I VE-cadherin⁺ vessels. E–F) BM mononuclear cells were evaluated for KLS population (E), and colony forming units (CFU-C) (F). G–J) Groups of ROSA-CreER^T2VEGFR2^e3loxP/− mice and control WT and ROSA-CreER^T2VEGFR2^e3loxP/+ mice were treated with tamoxifen and allowed to recover. Mice were given 650 rad, and hematopoietic recovery and vessel integrity were monitored. Ga) Steady state untreated femurs from VEGFR2^flox/− mice were immunostained with anti-VEGFR3 Abs and vascular integrity of SECs (normal versus Type I/II vessels) was assessed. Virtually all vessels in VEGFR2^flox/− mice prior to irradiation are normal SECs. Gb-c) WBC (Gb) and platelets (Gc) were counted at designated time points. At steady state conditions after tamoxifen treatment, there is no difference between the WBC and platelets counts as compared to non irradiated controls. H–J) At day 10, femurs were harvested and SEC status was evaluated using H&E (H) and anti-VEGFR3 IHC (I). J) VEGFR3 stained BM sections were assessed for Type I and Type II SECs. Although the control animals consistently show signs of SEC recovery, VEGFR2 knockdown animals have a profound persistence in Type II regressed SECs. Micron bar = 50 µm.

Figure 6. Engraftment and replenishment of HSPCs require VEGFR2 dependent regeneration of regressed SECs

WT mice were treated with 950 rad + BMT in combination with mAb to VEGFR2 (DC101) at high dose (800 µg/dose) or low dose (400 µg/dose), or control IgG, thrice weekly. At day 13, PB, BM and spleen were processed for analyses. A) Survival plot for mice given 950 rad + BMT in combination with anti-VEGFR2 mAb treatment (low and high, blue and orange, respectively) vs. control (grey) demonstrating that 100% of mice treated with high dose mAb succumbed to hematopoietic failure even after BMT (n=10, P<0.001). B) Platelet and WBC counts are decreased in mice treated with high dose (n=6, P<0.006 as compared to low dose, orange circles), but not low dose, mAb to VEGFR2 (blue circles) or IgG control (gray circles). C) As compared to IgG control mice, irradiated 950 rad + BMT treated mice injected with high dose, but not low dose, anti-VEGFR2 mAb showed a marked decrease in splenic mass (n=6, P<0.001). D) IHC for BM vessels using VE-cadherin (Da) reveals a predominance of Type II VE-cadherin⁺ regressed BM vessels (Db) (n=5, P<0.04). E) Total number of SECs was quantified by polyvariate flow cytometry. Calculating the total number of VEGFR3⁺VEGR2⁺VE-cadherin⁺CD31⁺CD45⁻CD11b⁻Sca1⁻ vessels demonstrated a reduction in SECs in high dose mAb treated mice vs. control (n=3 P<0.04). F–H) Treatment of 950 rad ⁺ BMT irradiated mice with high dose or low dose mAb to VEGFR2, but not control IgG, resulted in a statistically significant decrease in total BM mononuclear cells (F, P<0.007 for high dose, P<0.05 for low dose), decrease in Lin⁻ cells (G, P<0.02 for high dose, P<0.03 for low) and virtually absent KLS (H, P<0.004) in high dose and diminished KLS (P<0.01) in low dose treated mice. Micron bar = 50 µm.

Figure 7. Schema depicting the effect of VEGFR2 and VEGFR3 inhibition in the generation of Type I and Type II vessels and reconstitution of HSPCs

Interfering with VEGFR2 signaling in mice treated with potent myelosuppressive insults, such as 650 to 950 rad results in significant impairment in the reconstruction of regressed SECs, while inhibition of VEGFR3 results in a defect in patterning of SECs. VEGFR2 inhibition impairs BMT induced regeneration of SECs resulting in the failure of engraftment and reconstitution of HSPCs. Regeneration of SECs after 5FU may be driven by VEGFR2 independent but VEGFR1 and Tie2 dependent regeneration of hemangiogenic cells (Hattori et al., 2002; Kopp et al., 2005).