Blocking effect of mFlt-1-hIgG1 on the growth of Flk-1⁺ cells on OP9 feeder layer. (A) The preparation of mFlt-1-hIgG1 and assessment of its binding to M-CSF, VEGF, or PlGF were performed as described in Materials and Methods. The bars show the mean (±SD) of triplicate samples. (B, C, and D) Sorted Flk-1⁺ cells were cultured on OP9 feeder layer in the presence of 100 ng/mL mFlt-1-hIgG1 (B), 100 ng/mL mFlt-1-hIgG1 and 3 ng/mL VEGF (C), or 100 ng/mL mFlt-1-hIgG1 and 100 ng/mL PlGF (D). After 3 days of culture, immunostaining with MoAb for Flk-1 was performed as described in Materials and Methods. Scale bar = 500 μm.