Abstract
Placental growth factor (PlGF) is a member of the vascular endothelial growth factor (VEGF) family which signals through VEGF receptor-1. In mice, administration of an adenoviral vector expressing human PlGF accelerates bone marrow recovery following myelosuppression and mobilizes PBPCs. By injecting either murine or human PlGF alone, we failed to detect any PBPC mobilization in mice, whereas the combined injection of PlGF plus G-CSF resulted in a 2- to 4-fold increase of PBPCs. Due to the relevant clinical impact of any procedure capable of improving PBPC mobilization, we tested the mobilizing activity of PlGF (Geymonat S.p.A., Anagni, Italy) as an adjunct to G-CSF in a nonhuman primate model. A cohort of Rhesus Monkeys (n = 4) was initially mobilized with G-CSF alone (100 μg/kg/day, SC, for 5 days) (cycle 1), and after a 6-week wash-out period, received a second mobilization therapy consisting of PlGF (130 μg/kg, IV, for 5 days) plus G-CSF (cycle 2). After an additional 6-week wash-out period, a third mobilization cycle consisting of PlGF (260 μg/kg, IV, for 5 days) plus G-CSF (cycle 3) was administered. Hematopoietic mobilization was evaluated by white blood cell counts (WBCs), committed colony-forming cells (CFCs), high-proliferative potential CFCs (HPP-CFCs), and long-term culture-initiating cells (LTC-ICs). Mobilization parameters were analyzed daily during treatment (days 1 to 5), and 3 and 5 days post-cessation of therapy. As compared to baseline values, a 5-day administration of G-CSF alone induced an average 5-fold increment of the mean (±SD) WBC counts (8,708±2,458 vs 43,523±13,790, P ≤.005). As compared to G-CSF alone, the peak values of WBCs were slightly increased by adding PlGF at 130 μg/kg (60,040±9,508) or 260 μg/kg (49,048±7,120). As compared to pretreatment values, the absolute numbers of circulating CFCs per ml blood were increased on average by 85-, 335-, and 358-fold under G-CSF (11,406±4,093, P ≤.005), G-CSF/PlGF 130 μg/kg (46,283±8,287, P ≤.005), and G-CSF/PlGF 260 μg/kg (60,777±8,563, P ≤.005), respectively. At cycles 2 and 3, the peak levels of CFCs were increased by 4- and 5-fold over cycle 1 (G-CSF alone). As compared to pretreatment values, the absolute numbers of circulating HPP-CFCs per ml blood were increase on average by 17-, 158, and 284-fold after under G-CSF (1,593±405, P ≤.005), G-CSF/PlGF 130 μg/kg (8,557±1,142, P ≤.005), and G-CSF/PlGF 260 μg/kg (12,205±2,172, P ≤.005), respectively. At cycles 2 and 3, the levels of day-5 HPP-CFCs were increased by 5- and 8-fold over cycle 1. Under G-CSF alone, the absolute numbers of circulating LTC-ICs were increased by 53-fold as compared to baseline values (211±41 vs 4±7, P≤.005). The combined G-CSF/PlGF (130 μg/kg) treatment increased LTC-ICs by 389-fold as compared to pretreatment values (3,115±988 vs 8±5, P≤.005), with a 15-fold increase over G-CSF alone. In conclusion, our data demonstrate that in nonhuman primates PlGF strongly synergizes with G-CSF for the mobilization of primitive and committed PBPCs.
A Placental Growth Factor Variant Unable to Recognize Vascular Endothelial Growth Factor (VEGF) Receptor-1 Inhibits VEGF-Dependent Tumor Angiogenesis via Heterodimerization