Elevated Placental Growth Factor Levels after Allogeneic Hematopoietic Cell Transplantation: Clinical and Peripheral Blood Mononuclear Cell Receptor Expression Correlates
Authors: Newell Laura , Meyers Gabrielle , Holtan Shernan G. 
Published: October 2019

Abstract
Background: Placental growth factor-1 (PlGF) is a member of the vascular endothelial growth factor (VEGF) family of angiogenic factors possessing inflammatory properties as well as a chemotactic effect for monocytes. Additionally, circulating PlGF levels are associated with disease severity in hypoxic conditions including sickle cell disease and coronary artery disease. We have recently identified that circulating levels of PlGF are significantly elevated in recipients of allogeneic hematopoietic cell transplantation (HCT), and most significantly at the onset of acute graft versus host disease [(aGVHD), Holtan, SG et al, ASH 2014]. It is currently unknown whether PlGF levels are associated with an alteration of expression levels of angiogenic factor receptors on mononuclear cells subsets or other clinical factors post-allogeneic HCT. Thus, as PlGF could play an immunomodulatory role on the developing donor-derived immune system, we sought to characterize peripheral blood mononuclear cell (PBMC) subset expression of the PlGF receptor (VEGFR1) using HCT donor cells collected prior to stem cell collection compared to HCT recipient PBMC collected at 3 months post-HCT. We also correlated clinical factors with PlGF levels and VEGF receptor expression on PBMCs at 3 months post-HCT.
Patients and methods: Plasma and PBMCs from a cohort of 14 adult patients undergoing allogeneic HCT at Oregon Health & Science University were collected at 3 months post-HCT and compared to samples obtained from their matched sibling donors (MSD) prior to stem cell donation. Plasma levels of PlGF were quantified by ELISA. Lymphocyte and monocyte expression of VEGFR1 was determined by multiparameter flow cytometry. Comparison of donor and recipient plasma PlGF levels and PBMC phenotypes were performed by Wilcoxon signed rank. Correlations were determined using Spearman's rho. Survival curves generated using Kaplan Meier estimates with differences between curves determined by log rank.
Results: PlGF and clinical correlates: PlGF levels were 3.5-fold higher (25.1 versus 7.1 pg/mL, p=0.006) in recipients at 3 months post-HCT compared to their MSD. Donor PlGF levels were not associated with donor age or donor sex. Allogeneic HCT recipient PlGF levels were not associated with donor age, donor sex, recipient age, recipient sex, sex mismatch, conditioning intensity, disease risk index, or disease type. PlGF levels were not associated with the development of acute GVHD prior to the 3-month post-HCT blood draw (p=0.4); however, PlGF levels were negatively associated with steroid dose as a continuous variable (Spearman's rho -0.6, p=0.02). Recipient PlGF levels >25.1 pg/ml at 3 months post-HCT were associated with improved overall survival. PBMC subsets: Allogeneic HCT recipients had significantly higher levels of circulating monocytes (15.4 vs 8.1%, p=0.05) and approximately double the percentage of CD8+ T-cell (1.5 vs 0.6%, p=0.009) and CD3-CD11b+DRlo/- subsets (6.0 vs 3.3%, p=0.01) expressing VEGFR1 as compared to their donors. CD16+ NK cells (r=0.62, p=0.02), CD14+16+ monocytes (r=0.64, p=0.01), and CD14+16+VEGFR1+ monocytes (r=0.61, p=0.02) correlated with PlGF levels in recipients.
Conclusion: Elevated PlGF in HCT recipients is associated with an increase in monocyte percentage and an approximate doubling of VEGFR1-expressing lymphoid and myeloid cells compared to their MSD. Despite published association of elevated PlGF levels and chronic inflammatory states, in our small series, higher PlGF levels were associated with improved early survival. Studies are ongoing to correlate PlGF levels with later time points. Further research into the direct angiogenic and immune regulatory effects of PlGF in allogeneic HCT recipients, as well as larger scale studies of the impact of PlGF in late complications of HCT, are indicated.
Figure. Figure. Kaplan-Meier estimate of PlGF levels at 3 months post-HCT and overall survival.

Disclosures
No relevant conflicts of interest to declare.