Expression of VEGFA by placental macrophages has been documented [52] , and whereas macrophages have been shown to be involved in angiogenesis, they require stimulation by activating factors [53] . We observed that PSG1 induced significant secretion of VEGFA and TGFB1 in primary monocyte/macrophages, with a small variation in the concentration of PSG1 required depending on the donor. The cells used in our in vitro studies most likely do not exactly replicate the phenotype of placental macrophages, but we propose that PSGs could be one of the placental products, which increases FIG. 5. In vitro endothelial tube formation assay on three-dimensional collagen gel in the presence or absence of PSG1 and VEGFA. Human dermal microvascular endothelial cells were seeded on the top of polymerized collagen gel and cultured until they achieved subconfluence. The cells were then stimulated with PSG1 alone or in combination with VEGF. VEGF induces endothelial tubes (A, arrows) as expected, whereas no tubes were seen in the control when the cells were exposed to basal medium only (B). Addition of PSG1-Fc to basal media induces endothelial tubes (C, arrows) and also supports VEGF-induced tube formation (D, arrows). Original magnification 3450.