vWF-dependent ristocetin-induced CHO-GPIbβ/IX cell aggregation. (A) Aggregation of CHO-GPIbβ/IX (▪), CHO-GPIbβ/IX (□), or CHOdhfr⁻ (▴) cells was performed in the presence of various concentrations of vWF (0 to 40 nmol/L) by adding 1.4 mg/mL ristocetin. (B) CHO-GPIbβ/IX cell aggregation was performed in the absence (□) or in the presence (▪) of 10 nmol/L vWF and various concentrations of ristocetin (0 to 2 mg/mL). After 20 minutes of shaking at 6 cycles per second, microphotographs were taken to measure the percentages of aggregated cells relative to the total number of cells. The mean ± SEM of 4 separate experiments was reported. Cell aggregation was dependent on the concentration of vWF and ristocetin and required the GPIb subunit.