It takes 2 to thrombopoies in the vascular niche

In this issue of Blood, Pitchford et al demonstrate that intravenous administration of VEGF-A promotes megakaryocyte (Mk) maturation and localization to bone marrow sinusoidal endothelial cells (BMECs), stimulating thrombopoiesis.

Using specific agonists of VEGFR1 and VEGFR2, the authors show VEGF-A acts upstream of VEGFR1 to up-regulate CXCR4 on Mks and stimulate their migration from the endosteal region to the vascular niche.¹  These intriguing results implicate VEGFR1 activation as a trigger for reactive thrombocytosis in the context of hypoxia and angiogenesis.

While platelet production was once thought to be a product of systemically acting cytokines, our group and others have firmly established a critical role for the vascular niche in Mk maturation, platelet production, and recovery of thrombopoiesis after bone marrow injury.^2-5  Genetic studies using thrombopoietin (TPO)– and TPO-receptor (c-Mpl)–deficient mice show that TPO is not solely responsible for Mk development. Chemokine-mediated interactions between Mk progenitors and bone marrow niche cells, including BMECs, have been shown to augment Mk maturation, endomitosis, and platelet production. For instance, intravascular delivery of FGF-4 and SDF-1 is sufficient to increase circulating platelet counts in Tpo^−/− and Mpl^−/− mice by enhancing survival and maturation of Mks, promoting redistribution of Mks adjacent to BMECs, and stimulating the surface expression of adhesion molecules that induce transendothelial migration of pro-platelets.²  We have previously shown that the bone marrow of mice treated with 5-fluorouracil (5-FU) had a decrease in megakaryocytic lineage cells after administration of VEGFR1 blocking antibodies.³  Further, selective inhibition of platelet recovery was observed after coadministration of myelosuppressive doses of 5-FU with a vascular disrupting agent (combretastatin-A4-phosphate), demonstrating that BMEC integrity is essential for thrombopoiesis.⁴  Indeed, the supportive role of the bone marrow niche on thrombopoiesis is reciprocal—platelets deploy angiogenic factors (VEGF, FGF, SDF-1, etc) that promote vascular activation and angiogenesis in the context of tissue injury and malignancy,^6,7  further reinforcing vascular niche support of thrombopoiesis (see figure).

The hematopoietic effects of VEGF-A are not only mediated indirectly through its angiogenic activity, but also directly via receptors (eg, VEGFR1) expressed on the hematopoietic progenitors.³  Thus, this 1 signal can potentially orchestrate complex hematopoietic processes such as bone marrow regeneration after cytotoxic damage. Pitchford et al make a significant contribution by elucidating how VEGF-A promotes Mk migration toward the vascular niche—a critical step in the reconstitution of the bone marrow and initiation of thrombopoiesis after myeloablation. The authors demonstrate that VEGF-A, acting via VEGFR1, up-regulates CXCR4, resulting in redistribution of Mks to the vascular niche. Although the number of bone marrow Mks is not affected by VEGF-A, the redistribution is associated with Mk maturation and, apparently, platelet shedding. Notably, activation of VEGFR1 but not VEGFR2 appears to account for all these effects because a selective VEGFR1 agonist (PlGF) phenocopies, and a VEGFR1 blocking antibody neutralizes, the activities of VEGF-A. Up-regulation of CXCR4 is essential for Mk localization to the sinusoidal ECs because blockade with a specific CXCR4 antagonist prevents localization to the vasculature and thrombocytosis.

While these novel results implicate VEGFR1 activation upstream of CXCR4 expression and subsequent Mk redistribution, this report poses several questions. For instance, it is unclear that these effects are due to direct stimulation of VEGFR1 on Mks or indirectly via other niche components. Because vascular cells and myeloid cells also express VEGFR1, it is possible that VEGFR1 signaling may be induced in multiple cell types that each could, in turn, secrete paracrine/cytokine signals leading to heretofore undefined combinatorial effects that contribute to up-regulation of CXCR4 and increased thrombopoiesis. In this regard, delineation of the cell type–specific contribution of VEGFR1 would advance our understanding of this process. In addition, the molecular mechanism mediating CXCR4 up-regulation by VEGFR1 also needs to be elucidated. Whether VEGFR1 stimulation increases CXCR4 expression levels or prevents receptor internalization could also provide insight into possible cross-talk between 2 signaling pathways, especially during platelet production.

Although SDF-1 and CXCR4 signaling is essential for vascular niche–supported hematopoiesis,⁸  the cellular source of SDF-1 is not known. Pitchford et al note that despite the prominent role of the spleen during regenerative hematopoiesis in the mouse, VEGF-A does not induce Mk changes in this organ. This may point to the fact that organ-specific vascular niche cells, including the specialized VEGFR3⁺ sinusoidal ECs and perivascular cells, might play a decisive role in this response.

The clinical implications of this study are potentially significant. Treatment-related thrombocytopenia contributes to the morbidity and mortality associated with cytotoxic chemotherapy and hematopoietic stem cell transplantation. While treatment of noniatrogenic thrombocytopenias with c-Mpl agonists has been effective, Mk-active cytokines have not been approved to decrease the latency of platelet recovery after cytotoxic and myeloablative chemotherapies.^9,10  We have previously shown that megakaryopoiesis promotes endothelial regeneration after myelosuppressive therapy.³  This study highlights the mutually supportive role of Mks and the marrow's vasculature and suggests that cytotoxic damage to the vascular niche renders megakaryopoiesis refractory to TPO analogs because Mk precursors cannot relocalize to the sinusoids. New strategies to promote the association of Mks with BMECs could potentially enhance platelet recovery and help restore the integrity of the bone marrow vascular niche and its supportive role in thrombopoiesis.