Quantification of platelet binding properties and length of VWF strings from FVIII-YFP and FVIII-YFP Y1680F-transduced BOECs. Fluorescence and phase-contrast tile scans were used to measure length of released VWF strings (A) and number of platelets per string (B) of at least 30 strings each from nontransduced, normal FVIII-YFP-, and FVIII-YFP Y1680F-transduced BOECs. (A) No significant differences were detected in the length of VWF strings released by nontransduced BOECs, FVIII-YFP-, and FVIII-YFP Y1680F-transduced BOECs, 360 ± 29 μm, 338 ± 23 μm (P = .57), and 297 ± 30 μm (P = .14), respectively. Values are mean ± SD. ns indicates not significant. (B) VWF strings released by FVIII-YFP- and FVIII-YFP Y1680F-transduced BOECs bind fewer platelets than strings released by nontransduced BOECs, 0.07 ± 0.02, 0.11 ± 0.04, and 0.18 ± 0.04, respectively (mean ± SD). ***P < .001. (C) Nontransduced BOECs were stimulated with 100μM histamine at 2.5 dyne/cm² for 10 minutes, incubated with SF medium (control) or 5 U/mL purified FVIII-YFP for 5 minutes followed by perfusion of washed human platelets for 5 minutes. Number of platelets per string was determined of 30 strings each. No difference was observed in platelet binding between strings covered with or without recombinant FVIII (P = .98).