Impaired Bone Marrow Endothelial Progenitor Cells May Contribute to Acute Graft-Versus-Host Disease after Allotransplant

Background: Graft-versus-host disease (GVHD) is a major complication after allogeneic hematopoietic stem cell transplantation (allo-HSCT). While traditionally known as an immune cells-mediated immune injury, GVHD is also associated with endothelial injury, which determines the severity of GVHD. Moreover, emerging clinical and physiopathologic evidences suggest that vascular endothelium can be a target of aGVHD in the early phase and the increased circulating endothelial cells (ECs) represents a surrogate marker of endothelial damage. Bone marrow (BM) endothelial progenitor cells (EPCs), although the origin is not completely understood, are believed to be the precursors of ECs. Protective roles of EPCs in cardiovascular disorders have been extensively testified while their potential benefits in HSCT have rarely addressed. Moreover, BM suppression without infection is often observed in patients with acute GVHD (aGVHD) after allo-HSCT and the exact mechanism remains unclear. Therefore, it is imperative to evaluate whether the number and function of BM EPCs and hematopoietic stem cells (HSCs) in allo-HSCT patients with aGVHD differed from those without aGVHD. Furthermore, given that reactive oxygen species(ROS) exhibits the important role in maintaining endothelial function and vascular integrity, it will also be necessary to investigate whether aberrant levels of ROS in BM EPCs and HSCs play a role in the pathogenesis of aGVHD after allo-HSCT.

Aims: To evaluate whether the number and function of BM EPCs and HSCs in allo-HSCT patients with aGVHD differed from those without aGVHD. Furthermore, to investigate whether aberrant levels of ROS in BM EPCs and HSCs play a role in the pathogenesis of aGVHD after allo-HSCT.

Methods: In the current prospective case-pair study, 59 patients with aGVHD, 118 matched patients without aGVHD after allo-HSCT and 15 health donors as normal controls were enrolled. BM EPCs were cultured as previous reported. The number and function of BM EPCs were evaluated by cell counting, DiI-Ac-LDL and FITC-lectin-UEA-1 double staining, migration, tube formation, levels of ROS and apoptosis at 30 days, 60 days and 90 days after allo-HSCT. The number and function of BM HSCs were evaluated by cell counting, levels of ROS and apoptosis. Meanwhile, the quantity and function of synchronous peripheral blood (PB) ECs and HSCs were measured in patients with aGVHD at 15 days, 30 days, 60 days and 90 days after allo-HSCT.

Results: Human BM EPCs were demonstrated as the spindle shape and the similar expression of CD34, VEGFR2 and CD133 at day 7 of cultivation among the enrolled subjects. Reduced and dysfunctional BM EPCs, characterized by decreased capacities of migration and angiogenesis, as well as higher levels of ROS and apoptosis, were observed in patients with the onset of aGVHD compared with those without aGVHD. Moreover, reduced and dysfunctional BM HSCs, characterized by higher levels of ROS and apoptosis, were identified in patients with aGVHD compared with those without aGVHD. In PB, increased frenquency of ECs and decreased HSCs with higher levels of ROS and apoptosis were detected in patients with aGVHD compared to those without aGVHD. In addition, the frequencies of BM EPCs and HSCs in patients with the onset of aGVHD ≥grade 2 were significantly lower than those with aGVHD <grade 2, while the levels of ROS and apoptosis in BM EPCs and HSCs were remarkably higher. At 30 days, 60 days and 90 days after allo-HSCT, the percentages of BM EPCs and HSCs were significant lower, whereas ROS levels in BM EPCs and HSCs were significant higher in patients with aGVHD than those without aGVHD. At 15 days, 30 days, 60 days and 90 days after allo-HSCT, the percentages of PB ECs and their ROS levels in patients with aGVHD were higher than those without aGVHD, while the percentages PB HSCs were lower than those without aGVHD at the different time points.

Summary / Conclusion: These findings suggest that reduced and dysfunctional BM EPCs may be involved in the pathogenesis of aGVHD. Meanwhile, reduced and dysfunctional BM HSCs were also found in patients with aGVHD. Moreover, the impairment of EPCs and HSCs in BM and PB was associated with the elevated levels of ROS in patients with aGVHD. Therefore, our data indicate that it would be of value to investigate whether the improvement of BM EPCs would possibly ameliorate aGVHD, thereby providing a novel clinical intervention for aGVHD in the future.