Hematopoietic Stem Cells Mediate the Repair of Damaged Endothelium

Recent studies indicate that vascular endothelium is an important component of the hematopoietic niche. As endothelial cells (ECs) are sensitive to radiation-induced damage, we evaluated the potential role of hematopoietic stem cells to enhance EC proliferation and repair. To test this hypothesis, lethally irradiated mice were transplanted with either 200–500 c-kit+, Sca+, lineage- (KSL) cells or an equivalent dose of unfractionated bone marrow (BM) cells (1×10⁶ cells). Control groups included irradiated, non-transplanted, and non-irradiated, non-transplanted mice. Immediately after irradiation, all recipients were maintained on 0.8mg/ml Bromodeoxyuridine (BrdU) -containing water. Eleven days following irradiation, liver tissue was harvested and the fraction of proliferating BrdU+ ECs in the portal vein was assessed by immunostaining using both light and fluorescence microscopy. In irradiated, non-transplanted mice, 0.95% ± 0.17 SEM of portal vein ECs demonstrated the incorporation of BrdU. Transplantation of KSL cells increased the frequency of proliferating endothelial cells 2.5-fold to 2.5% ± 0.20 (p<0.0006). The transplantation of an equivalent number of unfractionated BM cells further increased the frequency of proliferating ECs by more than 3.5-fold (3.75% ± 0.21; p<0.0005). In non-transplanted, non-irradiated mice, BrdU+ ECs were detected at an intermediate level (2.30% ± 0.24) that is significantly higher than irradiated nontransplant recipients (p<0.006). To gain a better understanding of how hematopoietic stem cells (HSCs) influence the label retention capacity of ECs, we performed a BrdU pulse-chase experiment. Lethally irradiated mice were transplanted with 200 KSL cells, allowed 4 weeks for recovery, and then maintained on BrdU drinking water for 4 weeks. Consistent with our findings from the short term experiment described above, significantly more BrdU+ ECs were detected in the portal veins of KSL transplanted mice (15.36% ± 2.07) compared to those in non-transplanted, non-irradiated mice (8.68% ± 0.54; p<0.04) at the start of the chase period. During the first 24 weeks of the washout phase, BrdU+ ECs declined at a greater rate in the KSL recipients than in controls, indicating increased EC turnover. Interestingly, however, in both experimental groups, BrdU retention plateaued at 24 weeks and remained constant through 36 weeks. Taken together, our results indicate that radiation damage suppresses the incorporation of BrdU into ECs compared to steady state conditions and that this suppression can be reversed by the transplantation of either hematopoietic stem cells or unfractionated bone marrow. The extent to which BM derived ECs contribute to the proliferating EC pool will be addressed in future studies.