FVIII binding to VWF results in formation of VWF TIL’-TIL3 disulfides. (A) Comparison of the redox states of the VWF TIL’-TIL3 disulfides in 3 healthy human donors (2 males, 1 female) (purple symbols), the plasma VWF bound to human recombinant full-length FVIII (red symbols) and the unbound VWF in plasma following removal of VWF-FVIII complexes (orange symbols). The bars and errors are mean ± SD. Parametric unpaired t test was used to evaluate differences between groups. Significant difference (P < .05) between VWF and VWF-FVIII groups is indicated by ∗. There was no significant difference between the VWF and VWF – VWF-FVIII groups. (B) Model for disulfide-dependent interaction of VWF with FVIII. We suggest that the different disulfide-bonded forms of the TIL’-TIL3 domains of VWF and the inherent conformational flexibility therein facilitate initial interactions with FVIII, and subsequent disulfide formation in VWF results in a productive interaction.