3H9 inhibits baboon ADAMTS13. (A) Enzyme activity was measured using FRETS-VWF73 as a substrate and baboon plasma as a source of ADAMTS13 in the presence of increasing concentrations of either 3H9 (●) or 5C11 (■). Change in fluorescence in function of time is shown in function of the mAb concentration. (B) Binding of rADAMTS13 to immobilized 3H9 (●) and 5C11 (■) is shown as an increase in absorbance (A, 490 nm) in function of free rADAMTS13 concentration (n = 3). Nonlinear regression to the quadratic binding equation was performed (solid line). Error bars represent SD. (C) Baboons were injected with boluses of 600 μg/kg of mAb at the time points indicated by the arrow. Blood was sampled at the time points indicated by the vertical dashed lines. (D) Concentration of mAb was determined ex vivo in citrated plasma samples from subject (●, n = 6) and control baboons (■, n = 5) and expressed as mean ± SD in function of time. The dashed lines indicate the rise in murine immunoglobulin titer after the second injection. Arrows indicate moments of injection.