VE Cadherin Positive Endothelial Cells Regulate Hematopoietic Reconstitution In Vivo.

Abstract 3734

We have recently demonstrated that targeted deletion of the pro-apoptotic genes, Bak and Bax, in Tie2+ bone marrow endothelial cells (BM ECs) causes the protection of the BM sinusoidal vasculature, and the BM hematopoietic stem cell (BM HSC) pool following high dose total body irradiation (TBI). Since Tie2 is expressed on BM ECs and a small subset of quiescent BM HSCs, we developed a complementary model utilizing VE-cadherin Cre mice to more specifically confirm the function of BM ECs in regulating hematopoietic reconstitution in vivo. VE-cadherinCre;Bak−/−;BaxFl/− mice, which bear deletion of Bak and Bax in VE-cadherin+ ECs, and VE-cadherinCre;Bak−/−;BaxFl/+ control mice were generated and we compared the hematopoietic responses of these animals to high dose TBI. At steady state, these mice showed no difference in total BM cells, bone marrow ckit+sca+lineage- (KSL) progenitor cells, BM colony forming cells (CFCs), or BM colony forming unit spleen day 12 (CFU-S12) counts. At 2 hours after exposure to 500cGy TBI, the mice also did not demonstrate any significant differences in total BM cells, BM KSL cells, CFCs, or CFU-S12. However, 7 days after exposure to 500cGy TBI, VE-cadherin;Bak−/−;BaxFl/− mice displayed a 2-fold increase in total BM cells (p=0.006), a 3-fold increase in BM CFCs (p=0.0009), and 4-fold increase in BM CFU-S12 (p=0.079) compared to VE-cadherinCre;Bak−/−;BaxFl/+ control mice. Microscopic examination confirmed severe hypocellularity and corruption of the BM sinusoidal vasculature at day +7 post-TBI in VE-cadherinCre;Bak−/−;BaxFl/+ mice, whereas VE-cadherinCre;Bak−/−;BaxFl/− mice displayed nearly normal cellularity and preserved BM sinusoidal vessels. Taken together, these data demonstrate that VE-cadherin+ BM ECs regulate hematopoietic reconstitution in vivo and that targeted therapies aimed at augmentation of BM EC function can accelerate hematopoietic regeneration in vivo. Currently, we are comparing the cytokine production and gene expression profiles of VE-cadherinCre;Bak−/−;BaxFl/− BM ECs versus VE-cadherinCre;Bak−/−;BaxFl/+ BM ECs to identify candidate BM EC-derived proteins which are responsible for the protection of the BM stem/progenitor cell pool from radiation injury. VE-cadherin+ BM ECs represent an attractive mechanistic target for the identification of signaling pathways that regulate hematopoietic regeneration following injury.