Prostaglandin E2 Promotes the Sequential Recovery of Bone Marrow Vasculature and the Megakaryocyte Lineage Following Radiation Injury

The sinusoidal vascular niche is the site of terminal megakaryopoiesis and platelet production in the bone marrow. The association of megakaryocytes (MKs) with the sinusoidal endothelium and microenvironmental cues from the vascular niche are critical for efficient thrombopoiesis.  Thrombocytopenia is an important clinical problem that can cause significant morbidity for patients receiving genotoxic treatments such as chemotherapy and radiation therapy.  Following myeloablative bone marrow injury, the recovery of both hematopoietic stem and progenitor cells (HSPCs) and the MK lineage is dependent on marrow vascular recovery (Hooper et al. Cell Stem Cell 2009, Avecilla et al. Nature 2004).  Accordingly, we reasoned that improving recovery of the vasculature might serve as a rational therapeutic strategy for thrombocytopenia stemming from cytotoxic treatments.  We focused our efforts on the arachidonic acid derivative prostaglandin E2 (PGE2), which regulates numerous physiologic processes including microvascular endothelial cell proliferation and angiogenesis.  Within the context of the bone marrow, PGE2 also has diverse roles in HSPC expansion, survival, and trafficking.  Importantly, systemic administration of the long-acting analogue 16,16-dimethyl PGE2 (dmPGE2) improves hematopoietic recovery and enhances overall survival following total body irradiation (TBI) in mice (Hoggatt et al. Blood Cells 2013).  Intriguingly, following sublethal TBI, dmPGE2-induced recovery of peripheral platelets precedes restoration of red blood cells and neutrophils (Porter et al. Stem Cells 2013).  We hypothesized that the preferential recovery of platelets following dmPGE2 administration is mediated by accelerated vascular recovery in the marrow. We tested this in a murine model of sublethal radiation injury in which we had previously characterized MK lineage kinetics following 4Gy TBI (Niswander et al. Blood 2014).  First, the kinetics of MK progenitors (MKPs), maturing MKs, and platelets were determined at days 5, 7, and 10 in mice receiving 6mg/kg dmPGE2 or vehicle at 48 and 72 hours post-4Gy TBI. Mice receiving dmPGE2 had an early decrease in both MKPs and platelets, with no change in MKs at day 5 post-TBI. However, by day 10, dmPGE2-treated mice demonstrated significantly enhanced recovery of both MKs and circulating platelets compared to controls, but no difference in the number of immunophenotypic HSPCs.  Having confirmed enhanced recovery of the MK lineage with dmPGE2 treatment, we next sought to characterize vascular recovery.  Marked injury and dilation of the marrow sinusoidal vasculature occurs rapidly following TBI.  Immunohistochemistry with morphometric analysis revealed no change in vascular dilation at day 5 post-TBI with dmPGE2 treatment.  However, mice receiving dmPGE2 exhibited a more than 25% improvement in vascular dilation that was first evident at day 7 and was sustained at day 10 post-TBI.  While vascular dilation post-TBI has been posited to correlate with loss of surrounding hematopoietic cells, improvement in vascular dilation with dmPGE2 treatment is contrastingly evident in the setting of slightly fewer (day 7) or equivalent (day 10) numbers of nucleated marrow cells compared to irradiated control mice.  These data suggest that dmPGE2-mediated recovery of the vasculature coincides temporally with the accelerated onset of MK lineage recovery.  In addition, immunohistochemical morphometric analyses indicate that dmPGE2-treated mice had an 80% increase in maturing MKs in physical contact with the vascular endothelium, the site of platelet production, at day 10 post-TBI. Taken together, these data support the concept that dmPGE2-mediated marrow vascular recovery following radiation injury enhances the recovery of the MK lineage and accelerates the production of circulating platelets.  Modulation of vascular recovery with dmPGE2 may be a promising therapeutic strategy for thrombocytopenia secondary to myeloablative bone marrow injury.