Effect of RAM.1 on adhesion of K562-GPIb/V/IX cells containing deletions in the intracellular domain of GPIbα.

Adhesion of K562 cells expressing GPIb/V/IX containing wild-type GPIbα (WT) or GPIbα with deletions of the entire intracellular domain (Δ518-610) or of residues 535-568 (Δ535-568) or 569-610 (Δ569-610) was followed in microcapillaries coated with 1% HSA (♦) or 25 μg/mL bovine VWF (▪,■). The cells (1 × 10⁶/mL) were perfused through the capillaries for 10 minutes at a shear rate of 150 s^-1 after preincubation for 10 minutes with 10 μg/mL RAM.1 (■) or control rat IgG1 (▪). Adherent cells were counted off-line at the indicated times and results are expressed as the mean ± SEM of 4 separate experiments. None of the deletions of the GPIbα intracellular region affected the kinetics and levels of adhesion as compared with cells containing the wild-type complex. However, all 3 mutants were resistant to treatment with RAM.1, unlike cells expressing the wild-type GPIbα sequence.