FLT-1 function in ALL cells. (A) Proliferation assays to test the mitogenic effects of PlGF stimulation of the 4 ALL cell lines used in this study. Results show cells that were stimulated for 24 hours with PlGF (10 ng/mL) in the presence of heparin and counted in triplicate experiments (using the trypan blue exclusion test). ^*PlGF induces a significant increase in cell proliferation in the 697 cell line (P < .05). (B) Transwell migration assay to test the chemotactic effects of PlGF on the 4 ALL cell lines used in this study. Results show the ratio of PlGF-induced cell migration/control cells (calculated from the mean cell number in 6 high power fields; HPF, × 400 magnification) after 4 hours of PlGF (10 ng/mL) stimulation. ^*697 migrated significantly more in the presence of PlGF (3 independent experiments; P < .01 for 697 cells). (C) RS4;11 cells were transfected with full-length FLT-1 to determine the result of FLT-1 overexpression in vitro and in vivo. FLT-1 mRNA expression, as determined by real-time PCR. Note the increased expression of FLT-1 mRNA in FLT-1-transfected cells (RS4;11 FLT-1T). (D) Transwell migration assay was performed with nontransfected RS4;11 and those transfected with full-length FLT-1, in the presence of VEGF (30 ng/mL) for 4 hours. Results are shown as the mean cell number in 6 HPF (× 400 magnification) and demonstrate that RS4;11 FLT-1T cells migrate significantly more in the presence of VEGF than their untransfected counterparts (^*3 independent experiments; P < .05). (E) Migration assay to demonstrate the transfected full-length FLT-1 modulates cell migration. Results show RS4;11 FLT-1-transfected cells exposed to VEGF (30 ng/mL) for 4 hours, alone or in the presence of the FLT-1-neutralizing Ab, 6.12 (1 μg/mL). ^*VEGF-induced RS4;11 FLT-1T migration is significantly reduced in the presence of the 6.12 Ab (3 independent experiments, P < .05), demonstrating the transfected receptor is functional and modulates cell migration. Error bars depict the standard error of the mean.