Time course of IL-6 inhibition on the cleavage of ULVWF strings. The rates of cleavage of ULVWF strings by normal plasma, plasma pretreated with 100 ng/mL IL-6, or plasma from patients with congenital TTP were measured by quantitating the numbers of ULVWF-platelet strings formed on HUVECs over a period of 15 minutes. Normal plasma completely cleaved and released ULVWF strings from endothelial cells within 2 minutes after the initiation of perfusion. Cleavage by plasma treated with IL-6 was significantly delayed with maximal cleavage achieved 15 minutes after perfusion initiation. TTP plasma failed to cleave ULVWF strings within 15 minutes of perfusion. Figures are mean ± SEM, n = 4.