Factor XI assembly and activation on human umbilical vein endothelial cells in culture

Biotin-FXI optimally bound to HUVEC in the presence of 40 nM high molecular weight kininogen (HK) and greater than or equal to7 muM Zn2+. There was li ttle specific FXI binding in the absence of added HK and at concentrations of Zn2+ <15 <mu>M. FXI and prekallikrein, but not prothrombin, blocked biot in-FXI binding to HUVEC in the presence of HK with an IC50 of 18 nM and 180 nM, respectively. Monoclonal antibody HKL16 and peptide SDD31 also inhibit ed biotin-XI binding in the presence of HK with an IC50 of 4.7 nM and 50 mu M, respectively. Alternatively, peptide T-249-F-260 of FXI's apple domain 3 and heparin monosulfate were weak inhibitors of FXI binding to HUVEC. FXI bound to HUVEC with an apparent K-d of 6.9 +/- 3.0 nM and B-max of 13 +/- 2 .6 x 10(6) sites/cell. FXI bound to HK on HUVEC, but not prothrombin, becam e converted to FXIa. FXI activation on HUVEC resulted from tissue culture m edia bovine factor XIIa. HUVEC grown in human factor XII-deficient serum di d not support FXI activation. FXI binding to HUVEC in culture was mostly me diated by HK and FXI activation on HUVEC is dependent on cell-associated fa ctor XIIa.