The endothelial tube formation assay was carried out using Purecol, a threedimensional type I collagen gel (Nutacon) prepared in 48-well cluster tissue culture dishes as previously described [27] . After polymerization of the gel at 378C, wild-type HDMECs were seeded onto solidified gels at a concentration of 6 3 10 4 cells/well in 300 ll of endothelial growth medium MV containing 5% fetal calf serum and SupplementMix (containing heparin, epidermal growth factor, and hydrocortisone; Promocell). At confluence, the medium was replaced by basal medium containing 5% heat-inactivated fetal calf serum without further supplements. After 24 h, cells were treated with PSG1-Fc or FLAG-Fc (100 and 500 ng/ml) alone or in combination with VEGFA (25 ng/ ml). This treatment was repeated every 3 days, and photographs were taken with a phase-contrast microscope (Carl Zeiss, Inc.) equipped with a digital camera (ProgRes C10 plus ; Jenoptik AG).