Placental Growth Factor (PlGF) in Allogeneic Hematopoietic Cell Transplantation (HCT): Clinical, Immunologic, and Pathologic Correlates

Background:  Placental growth factor (PlGF) is a member of the vascular endothelial growth factor (VEGF) family, with a role in inflammation and monocyte chemotaxis.  Pathologically high levels of PlGF have been described in the setting of inflammatory diseases, and upregulation of the PlGF receptor (VEGFR1) can be induced by vascular injury and inflammation.  In a pilot study, we identified that circulating levels of PlGF are significantly elevated in recipients of allogeneic HCT.  Thus, we hypothesized that PlGF may play an immunomodulatory role post-HCT, and sought to quantify differences in circulating PlGF levels between HCT donors and recipients, characterize peripheral blood mononuclear cell (PBMC) expression of VEGFR1, and determine GVHD target organ expression of PlGF. 

Patients and methods:  Plasma and PBMCs from a cohort of 14 adult patients undergoing allogeneic HCT were collected at 3 months post-HCT and compared to samples obtained from their matched sibling donors (MSD).  Plasma PlGF levels were quantified by ELISA; PBMC expression of VEGFR1 was determined by flow cytometry.  Immunohistochemical (IHC) staining for PlGF was performed on biopsies of aGVHD target organs. 

Results:  PlGF levels and VEGFR1 expression:  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 and negatively associated with steroid dose (r=-0.6, p=0.02).  Allogeneic HCT recipients had significantly higher levels of circulating monocytes (15.4 vs 8.1%, p=0.05) and approximately double the percentage of both CD8⁺ T-cell (1.5 vs 0.6%, p=0.009) and CD3^-CD11b⁺DR(lo) myeloid-derived suppressor cell (6.0 vs 3.3%, p=0.01) subsets expressing VEGFR1, compared to MSDs.  PlGF tissue expression: Seven of 9 skin biopsies were consistent with aGVHD by pathology review.  PlGF was present at 2-3+ staining intensity in all 7 cases of skin aGVHD, while the 2 cases without aGVHD, as well as normal skin, had little to no PlGF staining (0-1+).  In contrast, control GI tissues showed higher PlGF staining (2-3+).  Fourteen of 15 GI biopsies were consistent with aGVHD.  PlGF expression in patients with GI aGVHD was less intense than control GI biopsies in 11/15 cases, with lowest scores observed in two patients who died of steroid refractory GI aGVHD.

Conclusion:  Elevated PlGF in HCT recipients is associated with an increase in monocytes and VEGFR1-expressing lymphoid and myeloid cells compared to their MSD.  Additionally, increased tissue expression of PlGF by IHC was seen in skin biopsies of aGVHD, while GI aGVHD tissue demonstrated pathologically low PlGF expression.  To our knowledge, these results provide the first evidence of altered circulating and tissue expression of PlGF occurring in the HCT setting.  Studies are ongoing to determine the role of PlGF in neovascularization and tissue repair in aGVHD, and on the direct angiogenic and immune regulatory effect of PlGF in HCT.